Liquid Crystalline Colloids of Nanoparticles

SOLID STATE PHYSICS, VOL. 62

Liquid Crystalline Colloids of Nanoparticles: Preparation, Properties, and Applications

Y URIY A. G ARBOVSKIY Center for Innovations in Biophysics & Energy Research, Advanced Technologies & Optical Materials, CU BioFrontiers Institute & Department of Physics, University of Colorado at Colorado Springs, Colorado Springs, Colorado, USA

A NATOLIY V. G LUSHCHENKO Center for Innovations in Biophysics & Energy Research, Advanced Technologies & Optical Materials, CU BioFrontiers Institute & Department of Physics, University of Colorado at Colorado Springs, Colorado Springs, Colorado, USA I. Introduction: Toward Liquid Crystalline Nanoscience . . . . . . . . . . . . . . . . . . . II. Doped Liquid Crystals and Liquid Crystal Colloidal Dispersions: A Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Nanoparticles in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Filled Liquid Crystals—Silica Nanoparticles (Aerosil) Dispersed in Liquid Crystals 2. Dielectric and Semiconductor Nanoparticles Dispersed in Liquid Crystals . . . . . . 3. Carbon Nanotubes in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Metallic Nanoparticles in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . 5. Ferroelectric Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Magnetic Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Organic Nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Applications of Liquid Crystalline Nanocolloids . . . . . . . . . . . . . . . . . . . . . . 8. Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Nondisplay Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 6 10 10 15 18 22 27 49 64 65 65 68 73 73

I. Introduction: Toward Liquid Crystalline Nanoscience

Liquid crystals were discovered due to a close collaboration between Friedrich Reinitzer, a professional botanist, and Otto Lehmann, a physicist.1 Since that time, liquid crystal science has become truly interdisciplinary, crossing 1

D. Dunmur and T. Sluckin, Soap, Science, and Flat-Screen TVs: A History of Liquid Crystals, Oxford University Press, USA (2010). 1 ISBN 978-0-12-374293-3 ISSN 0081-1947/11

#

2011 Elsevier Inc. (USA) All rights reserved.

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Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

the boundaries of many fundamental scientific disciplines including physics,2,2a,b chemistry,3,3a medicine,4 biology2b, material science,5 nanoscience, and nanotechnology.6 All liquid crystals are divided into two large classes—thermotropic and lyotropic. Thermotropic liquid crystals exist in a certain temperature interval, and all phase transitions in them occur mainly due to temperature changes. Lyotropic liquid crystals exist at a certain ratio of the components concentration; all phase transitions in them occur due to either changes of the components concentration or temperatures. Simultaneously, the soft, mobile, and ordered states of liquid crystals allow ‘‘mature’’ liquid crystal science to keep being relevant and an idea-generating platform for testing and introducing novel scientific concepts. As an example, Figure I.1 illustrates the diversity of liquid crystal phases and their connections with a variety of modern research directions. More specifically, during the past decade, liquid crystal science has made enormous contributions to nanoscience and nanotechnology.7 At this point, the relationship between nanoscience and liquid crystals is so deep that a new term ‘‘liquid crystal nanoscience’’ has come into common use.8 Mobile at the molecular level, liquid crystalline states are self-organized at the nanoscale into one-, two-, or threedimensional nanostructures. In addition, because liquid crystals are composed of anisotropic molecules (rod-like, disk-like, bent-core9), liquid crystals are very sensitive to external physical fields (electric, magnetic, acoustic, temperature). Thus their spatial ordering as well as their physical properties can be tuned in desirable ways. In addition, liquid crystals are very welcoming to other materials which are mixed with them, or are perfect to be imbedded into other materials/confinements. This creates an opportunity for the construction of a whole new world of composite materials. The most widely used approaches to create liquid crystalline nanomaterials include:

2

P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, Clarendon, Oxford (1993); (a) S. Chandrasekhar, Liquid Crystals, 2nd ed. Cambridge University Press, Cambridge (1992); (b) A. G. Petrov, The Lyotropic State of Matter: Molecular Physics and Living Matter Physics, Gordon and Breach Science Publishers, Amsterdam (1999). 3 P. J. Collings and M. Hird, Introduction to Liquid Crystals: Chemistry and Physics (Liquid Crystals Book Series), CRC Press, Philadelphia, PA, USA (1997); (a) S. Kumar, Chemistry of Discotic Liquid Crystals: From Monomers to Polymers (Liquid Crystals Book Series), 1st ed. CRC Press, Boca Raton, FL (2010). 4 S. J. Woltman, G. D. Jay, and G. P. Crawford, Liquid Crystals: Frontiers in Biomedical Applications, World Scientific Publishing Company, New York (2007). 5 T. Kato, Liquid Crystalline Functional Assemblies and Their Supramolecular Structures, Springer Verlag Berlin Heidelberg (2008). 6 J. W. Goodby, I. M. Saez, S. J. Cowling, V. Gortz, M. Draper, A. W. Hall, S. Sia, G. Cosquer, S.-E. Lee, and E. P. Raynes, Transmission and amplification of information and properties in nanostructured liquid crystals, Angew. Chem., Int. Ed. 47(9), 2754–2787 (2008). 7 T. Kato, N. Mizoshita, and K. Kishimoto, Angew. Chem. Int. Ed. 45(1), 38–68 (2006). 8 H. K. Bisoyi and S. Kumar, Chem. Soc. Rev. 40(1), 306–319 (2011). 9 H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys. 45(2A), 597–625 (2006).

Self-organizing soft matter

Calamitic liquid crystals

Plastic crystals Smectics Membranes

Nematics

Drug delivery Photonics optical networks

Cell and artificial

Discotic liquid crystals

Nematic discotics

Light Retardation modulators films

Displays switches

LIQUID CRYSTALS Molecular Shape Dependency

Columnar discotics

Lyotropic liquid crystals

Lamellar lyotropics

Surfactants Soaps Detergents Templates catalysts Hexagonal lyotropics

Biological structures DNA

Gene therapy

SUPERMOLECULES

SPHERES RODS

Rheology EL polymers transistors

DISCS AMPHIPHILES

High performance polymers

Main chain polymer liquid crystals

Ferroelectrics Pyroelectrics Sensors

Side chain polymer liquid crystals

Artificial skin muscle Liquid crystal elastomers

Self-assembled Systems

Monolayer

Dendrimers

LB - Multilayers

FIG. I.1. Classes of liquid crystals, and their applications (drawn after Ref. 6).

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Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

(1) Nanomaterials (with sizes limited to be between 1 and 100 nm in at least one dimension) can be embedded into a liquid crystalline host matrix. In this case, the nanomaterial (e.g., a nanoparticle) is prepared in advance, and then such a nanoguest is mixed with a liquid crystalline host in accordance with a special protocol (which can be different for various nanodopants). Materials prepared in such a way are called liquid crystalline nanocolloids and they are the subject of the current review. (2) Nanomaterials (nanodots, nanorods, nanoplates, nanobelts) are synthesized chemically inside liquid crystals. In this case, a liquid crystalline matrix is utilized as a nanoreactor (a concept of template synthesis). Lyotropic liquid crystals are preferable for template synthesis since their lattice constants range from several nanometers to tens of nanometers. Lyotropic liquid crystal templates (lamellar, hexagonal, cubic phases) confine the reactant in limited dimensions, and the structure and sizes of these dimensions determine the morphology of the nanomaterial to be synthesized. For more details, readers are referred to topical reviews.10,11 (3) Nanostructured liquid crystals are synthesized by using the concepts of supramolecular chemistry—self-assembly and self-organization. This extremely active topic of research introduces and tests a simple idea that a liquid crystalline state can be formed by supramolecular mesogens of nonconventional molecular topologies—pyramids, cones, rings, shuttle-cock, and dendritic molecules (Figure I.2), for example. Such self-organizing systems can be achieved through the successful combination of specific intermolecular interactions, nanosegregation behavior, and a molecular shape. As a result, these systems combine unusual physical properties. For example, the material can be both an electronic and an ionic conductor, and it can exhibit electrochromic properties. A detailed treatment of this subject can be found in the reviews.6–9,12 (4) Liquid crystals are composed of nanoparticles (or inorganic liquid crystals,13 mineral liquid crystals,14 colloidal liquid crystals15)—it was found that dispersions of anisometric nanoparticles can form different mesophases. The type of mesophases (uniaxial or biaxial, nematic or smectic)

C. Wang, D. Chen, and X. Jiao, Sci. Technol. Adv. Mater. 10(2), 023001, 11 (2009). T. Hegmann, H. Qi, and V. M. Marx, J. Inorg. Organomet. Polym. Mater. 17(3), 483–508 (2007). 12 M. Antonietti, Philos. Trans. R. Soc. A 364(1847), 2817–2840 (2006). 13 A. S. Sonin, J. Mater. Chem. 8(12), 2557–2574 (1998). 14 P. Davidson and J.-C.h.P. Gabriel, Curr. Opin. Colloid Interface Sci. 9(6), 377–383 (2005). 15 E. van den Pol, D. M. E. Thies-Weesie, A. V. Petukhov, D. V. Byelov, and G. J. Vroege, Liq. Cryst. 37(6–7), 641–651 (2010). 10 11

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

Rod

5

Disc

Cone

Ring

Shuttlecock Rod-coil or rod-dendron

Bent-core

FIG. I.2. Molecular shapes of liquid-crystal molecules (drawn after Ref. 7).

strongly depends on the size, shape, and concentration of the nanoparticles. We welcome interested readers to the extensive review papers written in this rapidly growing field,13–16 as well as to the papers published recently in Ref. 17,17a–e. In addition to liquid crystalline phases, ensembles of nanoparticles can also form ordered nanoparticle superstructures, so-called mesocrystals which were reviewed recently in Ref. 18. (5) Liquid crystal nanoparticles such as liquid crystal nanodroplets or liquid crystal nanoemulsions. Micron-sized and nanosized droplets of nematic liquid crystals in polymers (PDLC—polymer dispersed liquid crystals) have V. A. Davis, J. Mater. Res. 26(2), 140–153 (2011). U. Agarwal and F. A. Escobedo, Nat. Mater. 10, 230–235 (2011); (a) S. H. Aboutalebi, M. M. Gudarzi, Q. B. Zheng, and J.-K. Kim, Adv. Funct. Mater. 21(15), 2978–2988 (2011); (b) N. Puech, Ch. Blanc, E. Grelet, C. Zamora-Ledezma, M. Maugey, C. Zakri, E. Anglaret, and Ph. Poulin, J. Phys. Chem. C 115(8), 3272–3278 (2011); (c) A. A. Verhoeff, R. P. Brand, and H. N. W. Lekkerkerker, Mol. Phys. 109(7–10), 1363–1371 (2011); (d) M. Zorn, M. N. Tahir, B. Bergmann, W. Tremel, Ch. Grigoriadis, G. Floudas, and R. Zentel, Macromol. Rapid Commun. 31, 1101–1107 (2010); (e) M. Wojcik, W. Lewandowski, J. Matraszek, J. Mieczkowski, J. Borysiuk, D. Pociecha, and E. Gorecka, Angew. Chem., Int. Ed. 48(28), 5167–5169 (2009). 18 R. Q. Song and H. Co¨lfen, Adv. Mater. 22(12), 1301–1330 (2010). 16 17

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long been studied as electro-optical materials with numerous applications.19 While these materials typically involve micron-sized droplets embedded in a solid polymer matrix, liquid crystal nanoemulsions consisting of droplets in a continuous liquid solvent have become the next important step in understanding the nature of liquid crystal emulsions. In fact, the physics of nanosized droplets of liquid crystals in a host liquid or rigid matrix20,20a,b is still a rapidly developing field, being the subfield of a more general research direction—the physics and chemistry of nanoemulsions (more details can be found in review papers 21,21a). The focus of this review is multifold. In the first section, we look at doped liquid crystals and liquid crystal colloidal dispersions from a historical perspective. In the second section, we make an attempt to classify all the types of liquid crystalline nanocolloids and highlight the most interesting experimental and theoretical results for each class. The third section considers the most exciting applications of liquid crystal colloidal dispersions. In addition, throughout the review, we make an attempt to indicate current problems and possible future directions. II. Doped Liquid Crystals and Liquid Crystal Colloidal Dispersions: A Historical Perspective

This chapter is not intended to be a complete historical overview of liquid crystals doped with different particles (molecular, nano-, micro-) but rather simply outlines the most important stages of this development. Historically, the concept of doped liquid crystals was born due to the demands of a growing display industry by means of a ‘‘guest–host’’ effect demonstration.22,22a Pleochroic dyes were the first ‘‘guest’’ dopants to a liquid crystal ‘‘host’’ which served as a tunable ordering matrix: anisometric molecules of organic dyes were aligned along a preferred orientation of liquid crystalline

19

P. S. Drzaic, Liquid Crystal Dispersions (Series on Advances in Mathematics for Applied Sciences), World Scientific Publishing Company, Singapore (1998). 20 O. Tongcher, R. Sigel, and K. Landfester, Langmuir 22(10), 4504–4511 (2006); (a) R. Berardi, A. Costantini, L. Muccioli, S. Orlandi, and C. Zannoni, J. Chem. Phys. 126(4), 044905, 8 (2007); (b) W. M. Brown, M. K. Petersen, S. J. Plimpton, and G. S. Grest, J. Chem. Phys. 130(4), 044901, 7 (2009). 21 T. G. Mason, J. N. Wilking, K. Meleson, C. B. Chang, and S. M. Graves, J. Phys. Condens. Matter 18, R635–R666 (2006). (a) D. J. McClements and J. Rao, Crit. Rev. Food Sci. Nutr. 51(4), 285–330 (2011). 22 G. H. Heilmeier and L. A. Zanoni, Appl. Phys. Lett. 13(3), 91–92 (1968); (a) G. H. Heilmeier, J. A. Castellano, and L. A. Zanoni, Mol. Cryst. Liq. Cryst. 8, 293–304 (1969).

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

7

molecules (a director). Observed in Ref. 22,22a, the electro-optic guest–host effect is rather simple: external electric or magnetic fields drive liquid crystals (the host), and the liquid crystals reorient embedded organic dyes (the guest), thus changing the absorbance of such a guest–host system. Using different dyes allowed controlling the absorbance (in a desired wavelength interval) of an electro-optical guest–host cell with external electric or magnetic fields. This idea to dope liquid crystals with organic dyes stimulated active research throughout the world, and it is appropriate to note that the ‘‘dye–liquid crystal’’ system still continues to amaze human imagination with novel physical phenomena, especially in the fields of linear and nonlinear optics.23 Dyes were molecular additives—their sizes are comparable with the sizes of liquid crystalline molecules. Since the time when the concept of the ‘‘dye–liquid crystal’’ mixture was introduced, this approach has been often used in order to modify the physical properties of liquid crystals by using different types of dopants. Attempts to dope liquid crystals with particles larger than molecular size have given birth to very important concepts in liquid crystal science. The most important one is the concept of liquid crystal colloidal dispersions. A very detailed historic account on this subject can be found in the review, Ref. 24 Nevertheless, we would like to mention a couple of the most important milestones in the development of this research area. In 1970, Brochard and de Gennes25 published a theoretical paper on the magnetic properties of nematic liquid crystals doped with magnetic small particles. The first experimental work was done simultaneously by Rault et al.26 The idea of these two seminal papers was to increase the sensitivity of liquid crystals to an external magnetic field via coupling between ferromagnetic particles and liquid crystal molecule orientations: ferromagnetic particles transfer the magnetic orientational effect onto the underlying liquid crystal matrix. These landmark papers launched the era of ferroliquid crystals, or liquid crystals doped with magnetic particles. The term ‘‘ferronematic’’ is the most popular one to refer to these materials. After Ref. 25, further experimental attempts to investigate ferroliquid crystals were completed27,28; these investigations utilized magnetic particles of different types and sizes (mostly microparticles), and a plethora of papers was published. We review many of them later in this chapter.

L. Lucchetti, M. Gentili, L. Tifi, and F. Simoni, Mol. Cryst. Liq. Cryst. 489(1), 280/[606]–290/[616] (2008). 24 H. Stark, Phys. Rep. 351, 387–474 (2001). 25 F. Brochard and P. G. de Gennes, J. Phys. (Paris) 31, 691–708 (1970). 26 J. Rault, P. E. Cladis, and J. P. Burger, Phys. Lett. 32A(3), 199–200 (1970). 27 C. F. Hayes, Mol. Cryst. Liq. Cryst. 36(3–4), 245–253 (1976). 28 Sh.-H. Chen and N. M. Amer, Phys. Rev. Lett. 51(25), 2298–2301 (1983). 23

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In 1976,29 Hilsum proposed doping liquid crystals with micron-sized soda glass spheres in order to fabricate a light scattering device based on a mismatching the refractive indices between the liquid crystals and the glass microparticles. It is interesting to note that later a light scattering device based on a similar physical mechanism was developed by using polymer dispersed liquid crystals— droplets of liquid crystals dispersed in a rigid polymer matrix.30 In 1991, Eidenschink and de Jeu described a bistable liquid crystal device based on filled nematic liquid crystals.31 As materials, nematic liquid crystals doped with solid nanoparticles of pyrogenic silanized silica (Aerosil R812 with diameter of spheres below 30 nm) were utilized. This paper31 stimulated the appearance of many additional publications on the subject of ‘‘filled liquid crystals’’ which will be discussed in the next chapter. Liquid crystal colloidal dispersions can be divided into two groups depending on the sizes R of the particles dispersed in the liquid crystal: (1) colloids of microparticles and (2) colloids of nanoparticles. However, a proper characterization requires additional physical data: the polar anchoring energy W, the work needed to reorient the director away from the preferred direction by 90
, calculated per unit area, and the Frank elastic constant K of a liquid crystal. If the size of a particle is larger than some critical radius Rc ~ K/W, then it will create director distortions (defects) in the surrounding liquid crystal. The type of defect depends on the boundary conditions.32 When R, the size of the particle, is smaller than the critical radius mentioned above (R  K/W), such a particle can be considered as a small one. A small particle does not distort the director field. These two cases are schematically shown in Figure II.1 from Ref. 33. As a rule, microparticles embedded into liquid crystals always cause defect formations similar to the one shown in Figure II.1b. They can be detected optically (e.g., by optical microscopy). In the case of embedded nanoparticles, both cases (3a) and (3b) are possible. However, defect formations around nanoparticles require more sophisticated experimental tools in order to identify them. The following few papers originated a very extensive field of research—liquid

29

Hilsum, U.K. Patent 1,442,360, July 14, 1976. J. W. Doane, N. A. Vaz, B. G. Wu, and S. Zumer, Appl. Phys. Lett. 48(4), 269–271 (1986). 31 R. Eidenschink and W. H. De Jeu, Electron. Lett. 27(13), 1195–1196 (1991). 32 M. V. Kurik and O. D. Lavrentovich, Sov. Phys. Usp. 31, 196–224 (1988). 33 O. P. Pishnyak, S. Tang, J. R. Kelly, S. V. Shiyanovskii, and O. D. Lavrentovich, Ukr. J. Phys. 54(1–2), 101–108 (2009). 30

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

(a) R <<

K W

(b) R >>

9

K W

FIG. II.1. Schematics of a colloidal particle in a liquid crystal: (a) a small particle does not disturb the liquid crystal director; (b) a particle larger than some critical radius Rc  K/W creates a satellite defect, a hyperbolic hedgehog (drawn after Ref. 33).

crystal mediated interactions between colloidal particles, defect formation, and self-organized colloidal structures.11,34–41 Advances in nanoparticle preparation have launched a new stage in the development of liquid crystal colloidal dispersions. Almost simultaneously, several papers were published on the subject of liquid crystals doped with carbon-based nanoparticles (fullerenes,42 and nanotubes43), metallic nanoparticles,44,44a and ferroelectric nanoparticles.45,45a These first papers highlighted a nonsynthetic P. Poulin, Curr. Opin. Colloid Interface Sci. 4(1), 66–71 (1999). J. C. Loudet, Liq. Cryst. Today 14(1), 1–14 (2005). 36 B. Lev and H. Yokoyama, Mol. Cryst. Liq. Cryst. 435(1), 21/[681]–43/ [703] (2005). 37 I. Musevic, Liq. Cryst. 36(6–7), 639–647 (2009). 38 I. Musˇevicˇ and M. Sˇkarabot, Soft Matter 4, 195–199 (2008). 39 I. Musevic, Liq. Cryst. Today 19(1), 2–12 (2010). 40 I. I. Smalyukh, Proc. Natl. Acad. Sci. USA 107(9), 3945–3946 (2010). 41 R. P. Trivedi, D. Engstrom, and I. I. Smalyukh, J. Optics 13, 044001, 19 (2011). 42 M. Suzuki, H. Furue, and Sh. Kobayashi, Mol. Cryst. Liq. Cryst. 368, 191–196 (2001). 43 M. D. Lynch and D. L. Patrick, Nano Lett. 2(11), 1197–1201 (2002). 44 Yu. Shiraishi, N. Toshima, K. Maeda, H. Yoshikawa, Ju. Xu, and Sh. Kobayashi, Appl. Phys. Lett. 81(15), 2845–2847 (2002); (a) J. Muller, C. Sonnichsen, H. von Poschinger, G. von Plessen, T. A. Klar, and J. Feldmann, Appl. Phys. Lett. 81(1), 171–173 (2002). 45 Yu. Reznikov, O. Buchnev, O. Tereshchenko, V. Reshetnyak, A. Glushchenko, and J. West, Appl. Phys. Lett. 82(12), 1917–1919 (2003); (a) E. Ouskova, O. Buchnev, V. Reshetnyak, Yu. Reznikov, and H. Kresse, Liq. Cryst. 30(10), 1235–1239 (2003). 34 35

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way to modify the properties of liquid crystals, as well as the enormous potential of this approach. Once this idea was brought forth, a great number of papers were published in the field of liquid crystal nanocolloids, describing many unique effects. Since different nanoparticles influence the properties of liquid crystals in different ways, the research community working with such systems is multidisciplinary. It includes specialists in ferroelectric, ferromagnetic, photorefractive, and semiconductor materials. We summarize the most important stages in the historical development of doped liquid crystals and liquid crystal colloidal dispersions in Table II.1. III. Nanoparticles in Liquid Crystals

1. F ILLED L IQUID C RYSTALS —S ILICA N ANOPARTICLES (A EROSIL ) D ISPERSED IN L IQUID C RYSTALS As mentioned in the previous chapter, silica nanoparticles (aerosil) were the first type of inorganic dielectric nonmagnetic nanoparticles embedded in liquid crystals. Aerosil particles are small spheres of amorphous silicon dioxide. They exist in a variety of sizes (from 7 to 40 nm) and surface properties (hydrophilic and hydrophobic). As an example, Figure III.1 shows the structure of AerosilÒ711 and AerosilÒ7200; the surfaces of the particles are functionalized with methacrylic groups.56 We outline below some of the major research results for filled liquid crystals: The structure of filled liquid crystals was analyzed by means of vibrational spectra in Ref. 46, by using AFM in Ref. 47,47a–c, by acoustic methods in Ref. 48,48a,b, and by using NMR in Ref. 49. The agglomeration of silica nanoparticles in filled nematic liquid crystals was studied in Ref. 50.

46 A. V. Glushchenko, G. A. Puchkovskaya, Yu.A. Reznikov, and O. V. Yaroshchuk, SPIE Int. Soc. Opt. Eng. 2795, 106–113 (1996). 47 A. Hauser, H. Kresse, and O. Yaroshchuk, Mol. Cryst. Liq. Cryst. 324(1), 51–56 (1998); (a) A. Hauser, O. Yaroshchuk, and H. Kresse, Mol. Cryst. Liq. Cryst. 330(1), 407–414 (1999); (b) A. Hauser, H. Kresse, A. Glushchenko, and O. Yaroshchuk, Liq. Cryst. 26(11), 1603–1607 (1999); (c) A. Hauser, A. Glushchenko, O. Yaroshchuk, and H. Kresse, Proc. SPIE Int. Soc. Eng. 4147, 172–175 (2000). 48 A. Glushchenko, V. Sperkach, and A. Gvozdenko, Ukr. J. Phys. 41(10), 924–926 (1996); (a) A. Glushchenko, V. Sperkach, and O. Yaroshchuk, Acoust. Bull. 3(3), 26–31 (2000); (b) V. Sperkach, A. Glushchenko, and O. Yaroshchuk, Mol. Cryst. Liq. Cryst. 367, 463–467 (2001). 49 T. Jin and D. Finotello, Phys. Rev. Lett. 86(5), 818–821 (2001). 50 S. Lee and C. Park, Mol. Cryst. Liq. Cryst. 333, 123–134 (1999).

TABLE II.1. MOST IMPORTANT STAGES

IN THE

HISTORICAL DEVELOPMENT OF DOPED LIQUID CRYSTALS COLLOIDAL DISPERSIONS Size of the guest materials

Year

Effect

1968

Guest–host effect (by Heilmeier and Zanoni)

˚ < 100 A

1970

Concept of anisotropic ferromagnetic fluids introduced by F. Brochard and P. de Gennes (theory) and by Rault, Cladis, and Burger (experiment) Concept of filled liquid crystals by Hilsum Experimental verification of the Brochard—de Gennes theory by Hayes (1976) and Chen and Amer (1983) Filled nematics by Eidenschink and de Jeu

From micro- to nanometers

1976 1976 and 1983

1991

2002–2003

Liquid crystals doped with metallic nanoparticles, fullerenes, carbon nanotubes, ferroelectric nanoparticles

Micrometers Micrometers, nanometers

Nanometers

Nanometers

AND

LIQUID CRYSTAL

Impact on science development

References

Development of nonlinear—optical materials with colossal optical nonlinearity Development of ferroliquid crystals, and liquid crystal colloidal dispersions

22,22a,23

Liquid crystal colloidal dispersions Experimental research in the field of ferronematics

Beginning of the field of liquid crystal nanocolloids Rapid development of the field of liquid crystal nanocolloids

24–26

29 27,28

31

42–45,45a

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Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

O

(C

)3 H2

O

OH Si O Si O

HO Si

O

O

Si

Si ) (CH 2 3 O

Si

O

Si

O

Si

O

HO

O

OH

O

(CH2)3

O

O OH

®

Aerosil 711

O

)3 CH 2

O

O

(

)3 H2

O

(C

OH Si O Si O Si

O Si

O ) (CH 2 3

O

Si

O

O

Si

H 2) 3

(C

O O

Si (CH ) 2 3

O OH O

O

O

Si

O (CH2)3

O

Aerosil®7200

O

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

13

Sophisticated experimental techniques were applied in Ref. 51,51a in order to measure simultaneously optical turbidity, specific heat, and latent heat of such colloids. The effect of silica nanoparticles on the viscosity of liquid crystals was studied in Ref. 52. Ordering–disordering effects were reported in Ref. 53. The effects of dispersed hydrophobic or hydrophilic aerosil nanoparticles on the order and dynamics of the 5CB liquid crystal were demonstrated in Ref. 54. The kinetics of nanocolloids in an aligned domain of octylcyanobiphenyl and aerosil dispersion were measured in Ref. 55. The morphology and electro-optical properties of nematic liquid crystal/aerosil nanoparticle composites were reported in Ref. 56. Electro-optic properties of colloidal silica filled nematics were reviewed in more detail in Ref. 57. First-order isotropic–smectic-A transitions in liquid crystal–aerosil gels were studied in Ref. 58. Nematic–smectic-A and smectic-A–nematic transitions in liquid crystal–aerosil gels were reported in Refs. 59,60, and isotropic–nematic phase transitions in the system ‘‘aerosil–liquid crystals’’ were studied in Ref. 61. FIG. III.1. AerosilÒ711 and AerosilÒ7200.64 Aerosil is a registered trademark for a fumed silica product developed in 1942 by Degussa AG (currently Evonik Industries) in Germany. It is pure silicon dioxide, made from vaporized silicon tetrachloride oxidized in a high-temperature flame with hydrogen and oxygen. Aggregated amorphous nanosized primary particles give free flow to powder materials. It gives a thickening effect and thixotropy when dispersed into liquid materials. Standard hydrophilic products are made of primary particles from 7 to 40 nm. The surface of nonmodified particles contains about 5 OH groups/nm2; modification reduced this number to 0.29 OH groups/nm2 thus greatly reducing the ability of these particles to aggregate.

S. Paoloni, F. Mercuri, M. Marinelli, and U. Zammit, Phys. Rev. E 78, 042701, 4 (2008); (a) S. Paoloni, F. Mercuri, M. Marinelli, U. Zammit, F. Scudieri, C. Neamtu, and D. Dadarlat, Mol. Cryst. Liq. Cryst. 516(1), 202–210 (2010). 52 J. Janik-Kokoszka and J. K. Mos´cicki, Mol. Cryst. Liq. Cryst. 545(1), 29/ [1253]–35/[1259] (2011). 53 J. Leys, Ch. Glorieux, and J. Thoen, J. Non Cryst. Solids 356(597–601), (2010). 54 G. Venditti and C. Zannoni, Chem. Phys. Lett. 396(4–6), 433–441 (2004). 55 D. Sharma, Liq. Cryst. 35(10), 1215–1224 (2008). 56 S. Mormile, G. De Filpo, G. Chidichimo, and F. P. Nicoletta, Liq. Cryst. 35(9), 1095–1100 (2008). 57 N. J. Jr Diorio, M. R. Fisch, and J. W. West, Liq. Cryst. 29(4), 589–596 (2002). 58 M. K. Ramazanoglu, P. S. Clegg, R. J. Birgeneau, C. W. Garland, M. E. Neubert, and J. M. Kim, Phys. Rev. E 69, 061706, 8 (2004). 59 P. S. Clegg, R. J. Birgeneau, S. Parkand, C. W. Garland, G. S. Iannacchione, R. L. Leheny, and M. E. Neubert, Phys. Rev. E 68, 031706, 7 (2003). 60 M. Ramazanoglu, S. Larochelle, C. W. Garland, and R. J. Birgeneau, Phys. Rev. E 77, 031702, 10 (2008). 61 S. Aya, Yu. Sasaki, F. Araoka, K. Ishikawa, K. Ema, and H. Takezoe, Phys. Rev. E 83, 061714, 5 (2011). 51

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The electro-optics and structure of ternary systems ‘‘liquid crystal–silica– photopolymer’’ and ‘‘liquid crystal–silica–polymer’’ were studied in Refs. 62,63. A multistable alignment of liquid crystals doped with aerosil was demonstrated in Ref. 64. Silica nanoparticles added to liquid crystals are the cause of the so-called memory effect65–67—the initial scattering of the colloid can be decreased by applying an electric field. At sufficiently large fields, the light transmittance T (%) reaches 100% in some cases; then, when the electric field is reduced, a partial transparency can remain, yielding a memory parameter M defined as M = (Tm  T0)/(Tsat  T0), where T0 is the initial light transmittance, Tm is the residual transmittance, and Tsat is the maximum transmittance at the applied voltage Usat. High values of the memory parameter M and the contrast ratio K = Tsat/T0 enable the use of these heterogeneous nanocomposites for display applications. In order to get high values of K and M, aerosol nanoparticles should be chemically modified, that is, covered with hydrophobic surfactants. It was also found that the memory effect depends on many factors such as the surface of the nanoparticles (hydrophilic or hydrophobic), their concentration, temperature, etc.68–70,70a–c One of the most beautiful applications of this effect was realized in the development of a bistable liquid crystal display (LCD) device. Recently, the dielectric properties71 and the memory effect were also measured for ferroelectric liquid crystals doped with silica.72 Also, the stability of the

62 A. V. Kovalchuk, S. S. Zakrevskaa, O. V. Yaroshchuk, and U. Maschke, Mol. Cryst. Liq. Cryst. 368(1), 129–136 (2001). 63 O. V. Yaroshchuk and L. O. Dolgov, Opt. Mater. 29, 1097–1102 (2007). 64 A. Glushchenko and O. Yaroshchuk, Mol. Cryst. Liq. Cryst. 330, 1659–1666 (1999). 65 M. Kreuzer, Th. Tschudi, and R. Eidenschink, Mol. Cryst. Liq. Cryst. 223(1), 219–227 (1992). 66 M. Kreuzer, Th. Tschudi, W. H. de Jeu, and R. Eidenschink, Appl. Phys. Lett. 62, 1712–1714 (1993). 67 M. Bittner and M. Kreuzer, Mol. Cryst. Liq. Cryst. 282(1), 373–386 (1996). 68 A. Glushchenko, G. Guba, Yu. Reznikov, N. Lopukhovich, V. Ogenko, V. Reshetnyak, and O. Yaroshchuk, Mol. Cryst. Liq. Cryst. 262, 1399–1406 (1995). 69 A. Glushchenko, H. Kresse, and O. Yaroshchuk, Cryst. Res. Technol. 30(3), K32–K36 (1995); (a) A. Glushchenko, G. Guba, Yu. Reznikov, N. Lopukhovich, V. Ogenko, V. Reshetnyak, and O. Yaroshchuk, Funct. Mater. 2(2), 253–257 (1995). 70 A. Glushchenko, G. Puchkovska, Yu. Reznikov, A. Yakubov, and O. Yaroshchuk, J. Mol. Struct. 381(1–3), 133–139 (1996); (a) A. Glushchenko, G. Puchkovska, Yu. Reznikov, A. Yakubov, and O. Yaroshchuk, J. Mol. Struct. 404(121–128) (1997); (b) A. V. Glushchenko, Properties and structure of heterogeneous aerosil—liquid crystal systems, Ph.D. thesis. Press of the Institute of Physics, National Academy of Sciences of Ukraine, Kyiv (1997); (c) A. Glushchenko, H. Kresse, V. Reshetnyak, Yu. Reznikov, and O. Yaroshchuk, Liq. Cryst. 23(2), 241–246 (1997). 71 Neeraj and K. K. Raina, Phase Transitions 83(8), 615–626 (2010). 72 P. Malik, A. Chaudhary, R. Mehra, and K. K. Raina, Mol. Cryst. Liq. Cryst. 541(1), 243/[481]–251/ [489] (2011).

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

15

memory state was investigated for the silica nanoparticle-doped hybrid aligned nematic cell.73 Silica nanoparticle-doped nematic displays with multistable and dynamic modes were demonstrated in Ref. 74. In Ref. 75, silica nanoparticles were utilized in order to improve the parameters of holographic polymer dispersed liquid crystals. In Ref. 76, they were used to create a dual operation mode liquid crystal lens. In addition, silica-doped (~ 0.3–1.0 wt.%) nematics were used to fabricate an initially twisted p-cell77—a nontrivial configuration of liquid crystal molecules used in many display and nondisplay applications. So far, the articles cited above have dealt with thermotropic liquid crystals doped with silica nanoparticles. There are also a few interesting papers describing the properties of lyotropic liquid crystals doped with silica nanoparticles,78,79 but the use of lyotropic liquid crystals is only starting to be explored. 2. D IELECTRIC AND S EMICONDUCTOR N ANOPARTICLES D ISPERSED IN L IQUID C RYSTALS Inspired by the study of liquid crystals doped with silica nanoparticles, extensive research was performed considering many alternative inorganic dielectric and semiconductor nanoparticles as nanodopants: organoclay (clay minerals),80–88 73

C.-Y. Huang, J.-H. Chen, Ch.-T. Hsieh, H.-Ch. Song, Yu.-Wu. Wang, L. Horng, Ch.-J. Tian, and Sh.-J. Hwang, J. Appl. Phys. 109, 023505, 4 (2011). 74 Ch.-Y. Huang, Ch.-Ch. Lai, Y.-H. Tseng, Y.-T. Yang, Ch.-J. Tien, and K.-Y. Lo, Appl. Phys. Lett. 92, 221908, 3 (2008). 75 J. D. Busbee, A. T. Juhl, L. V. Natarajan, V. P. Tongdilia, T. J. Bunning, R. A. Vaia, and P. V. Braun, Adv. Mater. 21, 3659–3662 (2009). 76 C.-Y. Huang, Y.-J. Huang, and Y.-H. Tseng, Opt. Express 17(23), 20861–20865 (2009). 77 Ch.-W. Chang, Ch.-Y. Huang, and H.-Ch. Song, Opt. Express 19(14), 13307–13311 (2011). 78 G. Salamat and E. W. Kaler, Langmuir 15, 5414–5421 (1999). 79 E. Venugopal, S. K. Bhat, J. J. Vallooran, and R. Mezzenga, Langmuir 27, 9792–9800 (2011). 80 T. Bezrodna, I. Chashechnikova, L. Dolgov, G. Puchkovska, Y. Shaydyuk, N. Lebovka, V. Moraru, J. Baran, and H. Ratajczak, Liq. Cryst. 32(8), 1005–1012 (2005). 81 T.-Y. Tsai, Yu.-P. Huang, H.-Yu. Chen, W. Lee, Yu.-M. Chang, and W.-K. Chin, Nanotechnology 16, 1053–1057 (2005). 82 T. Bezrodna, I. Chashechnikova, G. Puchkovska, A. Tolochko, E. Shaydyuk, N. Lebovka, J. Baran, M. Drozd, and H. Ratajczak, Liq. Cryst. 33(10), 1113–1119 (2006). 83 J. Baran, L. Dolgov, T. Gavrilko, L. Osinkina, G. Puchkovska, H. Ratajczak, Y. Shaydyuk, and A. Hauser, Philos. Mag. 87(28), 4273–4285 (2007). 84 T. Bezrodna, I. Chashechnikova, T. Gavrilko, G. Puchkovska, Y. Shaydyuk, A. Tolochko, J. Baran, and M. Drozd, Liq. Cryst. 35(3), 265–274 (2008). 85 Yu.-P. Huang, Yu.-M. Chang, T.-Y. Tsai, and W. Lee, Mol. Cryst. Liq. Cryst. 512(1), 167/[2013]– 178/[2024]. (2009). 86 T. Bezrodna, I. Chashechnikova, V. Nesprava, G. Puchkovska, Ye. Shaydyuk, Yu. Boyko, J. Baran, and M. Drozd, Liq. Cryst. 37(3), 263–270 (2010).

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metal oxides,89–104and metal selenides/arsenides/sulfides.105–118 The structure,80–88 electrical,87,88 optical, and electro-optical performance80–88 of Y. Shaydyuk, G. Puchkovska, A. Goncharuk, and N. Lebovka, Liq. Cryst. 38(2), 155–161 (2011). T. Bezrodna, I. Chashechnikova, V. Melnyk, and V. Nesprava, Liq. Cryst. 38(8), 957–962 (2011). 89 L. Dolgov and O. Yaroshchuk, Mol. Cryst. Liq. Cryst. 409(1), 77–89 (2004). 90 T. Bezrodna, G. Puchkovska, V. Shimanovska, and J. Baran, Mol. Cryst. Liq. Cryst. 413(1), 71–80 (2004). 91 T. Gavrilko, O. Kovalchuk, V. Nazarenko, G. Puchkovska, V. Shymanovska, A. Hauser, and H. Kresse, Ukr. J. Phys 49(12), 1167–1173 (2004). 92 O. V. Yaroshchuk, L. O. Dolgov, and A. D. Kiselev, Phys. Rev. E 2, 051715, 11 (2005). 93 A. Tercjak, J. Gutierrez, C. Ocando, and I. Mondragon, Langmuir 26(6), 4296–4302 (2010). 94 W.-T. Chen, P.-Sh. Chen, and Ch.-Yu. Chao, Mol. Cryst. Liq. Cryst. 507(1), 253–263 (2009); (a) P.-Sh. Chen, Ch.-Ch. Huang, Yu.-W. Liu, and Ch.-Y. Chao, Mol. Cryst. Liq. Cryst. 507(1), 202–208 (2009). 95 W.-K. Lee, J.-H. Choi, H.-J. Na, J.-H. Lim, J.-M. Han, J.-Y. Hwang, and D.-Sh. Seo, Opt. Lett. 34(23), 3653–3655 (2009). 96 S. S. Shim, J. Y. Woo, H. M. Jeong, and B. K. Kim, Soft Mater. 7(2), 93–104 (2009). 97 B.-J. Liang, D.-G. Liu, Ch.-Yu. Chang, and W.-Y. Shie, Jpn. J. Appl. Phys. 50, 055002 (2011). 98 T. Joshi, J. Prakash, A. Kumar, J. Gangwar, A. K. Srivastava, S. Singh, and A. M. Biradar, J. Phys. D Appl. Phys. 44, 315404, 7 (2011). 99 M. Zorn and R. Zentel, Macromol. Rapid Commun. 29, 922–927 (2008). 100 Liu-Shiaun Li and Jung Y. Huang, J. Phys. D Appl. Phys. 42, 125413, 5 (2009). 101 L. J. Martı´nez-Miranda, K. M. Traister, I. Mele´ndez-Rodrı´guez, and L. Salamanca-Riba, Appl. Phys. Lett. 97, 223301, 3 (2010). 102 T. Joshi, A. Kumar, J. Prakash, and A. M. Biradar, Appl. Phys. Lett. 96, 253109, 3 (2010). 103 H. Ch. Lin, M. D. Jiang, L. Yu. Wang, W. H. Chen, S. F. Chen, and Ch.N. Mo, J. Chin. Inst. Eng. 33 (7), 1069–1074 (2010). 104 R. Manohar, S. P. Yadav, A. Kumar Misra, and K. Kumar Pandey, Mol. Cryst. Liq. Cryst. 534(1), 57–68 (2011). 105 K.-J. Wu, K.-Ch. Chu, Ch.-Y. Chao, Y.-F. Chen, Ch.-W. Lai, Ch.-Ch. Kang, Ch.-Y. Chen, and P.-T. Chou, Nano Lett. 7(7), 1908–1913 (2007). 106 V. V. Danilov, M. V. Artem’ev, A. V. Baranov, G. M. Ermolaeva, N. A. Utkina, and A. I. Khrebtov, Opt. Spectrosc. 105(2), 306–309 (2008). 107 S. Acharya, S. Kundu, J. P. Hill, G. J. Richards, and K. Ariga, Adv. Mater. 21, 989–993 (2009). 108 R. Basu and G. S. Iannacchione, Phys. Rev. E 80, 010701, 4 (2009). 109 B. Kinkead and T. Hegmann, J. Mater. Chem. 20, 448–458 (2010). 110 S. Kundu, J. P. Hill, G. J. Richards, K. Ariga, A. H. Khan, U. Thupakula, and S. Acharya, Appl. Mater. Interfaces 2(10), 2759–2766 (2010). 111 L. S. Hirst, J. Kirchhoff, R. Inmana, and S. Ghosh, Proc. SPIE 7618 76180F1 (2010). 112 A. Kumar, J. Prakash, M. T. Khan, S. K. Dhawan, and A. M. Biradar, Appl. Phys. Lett. 97, 163113, 3 (2010). 113 H. L. Lee, I. A. Mohammed, M. Belmahi, M. Badreddine Assouar, H. Rinnert, and M. Alnot, Materials 3, 2069–2086 (2010). 114 Y. K. Verma, R. H. Inman, C. G. L. Ferri, H. Mirafzal, S. N. Ghosh, D. F. Kelley, L. S. Hirst, and S. Ghosh, Phys. Rev. B 82, 163113, 5 (2010). 87 88

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LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

nanocomposites based on thermotropic nematic liquid crystals (5CB,80,82,84, ZLI-2293,83 E781) and organoclay nanoparticles as a function of nanoparticle concentration, modification of nanoparticles with different organic compounds (surfactants), and interactions between liquid crystals and nanoparticles have been studied by different authors. It has been found that nematic liquid crystals doped with chemically modified montmorillonite nanoparticles also exhibit the memory effect. Once again, in order to get high values of K and M, montmorillonite smectite-like nanoparticles should be chemically modified, that is, covered with hydrophobic surfactants. Montmorillonite nanoparticles are nanosized smectite-like quasi-crystallites which are composed of a certain number of aluminosilicate platelets with interplanar distance within the range of about 1–2 nm.80,81 Chemical modification of organoclay nanoparticles affects this interplanar distance, as shown in Table III.1, and changes their hydrophobicity/hydrophility.80–85 It has been found that surfactants also can modify the properties and the electro-optical performance of such nanocomposites in different ways (see Table III.1). A light scattering liquid crystal device based on nematic liquid crystals filled with nanoparticles of Sb2O5 (sizes ~ 7–11 nm) was proposed in Ref. 89. It has been shown that the insertion of Sb2O5 nanoparticles (up to 25% by weight) in the nematic E7 leads to a drastic increase of light scattering, which can be controlled by an external electric field. Such systems do not exhibit the memory effect as in the case of nematic filled with aerosil or montmorillonite; they also degrade over time. 85,87,88

TABLE III.1. PROPERTIES OF NANOPARTICLES AND ELECTRO-OPTICAL PERFORMANCE OF THE NANOCOMPOSITES (4.5 WT.%) BASED ON NEMATICS AND MONTMORILLONITE NANOPARTICLES84,86

Sample

Pure MMT MMT modified with B2 MMT modified with B3

Interplanar distance (nm)

Nanocomposite: liquid crystal/ T0 surfactant (%)

1.24 1.8 1.9

Tsat (%)

Tm (%)

Usat, (V)

M (%)

K (%)

5CB/B2 5CB/B3

4 75 0.2 55

58 4

55 38

76 7

18 300

5CB/B4

20

67

17

69

5

88

115 E. Karatairi, B. Rozˇicˇ, Z. Kutnjak, V. Tzitzios, G. Nounesis, G. Cordoyiannis, J. Thoen, Ch. Glorieux, and S. Kralj, Phys. Rev. E 81, 041703, 5 (2010). 116 T. Hegmann, B. Melissa Kinkead, Planar nematic Liquid Crystal cells doped with nanoparticles and methods of inducing Freedericksz transition, US patent 0302470 A1, 2010. 117 A. Bobrovsky, K. Mochalov, V. Oleinikov, and V. Shibaev, Liq. Cryst. 38(6), 737–742 (2011). 118 S. Sarathi Bhattacharyya, A. Mukherjee, S. L. Wu, S. H. Lee, and B. K. Chaudhuri, Mol. Cryst. Liq. Crys. 541(1), 270/[508]–275/[513] (2011).

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The addition of a third component (a polymer, up to 50% by weight) stabilizes the system. The electro-optics and the structural peculiarities of such ‘‘liquid crystal–nanoparticle (Sb2O5, SiO2, TiO2)–polymer’’ composites were studied in Ref. 92. Interactions of TiO2 nanoparticles and mesogenic molecules were studied in Refs. 90,91,93 by means of IR spectroscopy,90 dielectric spectroscopy,91 and luminescence.93 The effect of TiO2 nanoparticles on the electro-optical performance of nematic liquid crystals was reported in Refs. 94,94a,95. It was found that the addition of 2 wt.% TiO2 nanoparticles to liquid crystals can reduce the threshold voltage by a factor of 5 (2.5 V for pure nematic, and 0.5 V for nematics doped with TiO2 nanoparticles). In addition, these nanoparticles can trap ions in a nematic host, thus improving the overall electro-optical performance of the liquid crystal device. Similar ‘‘ion purification’’ effects have been found for nematic liquid crystals doped with diamond nanoparticles, Si3N4, ZnO,94,94a,210 for nematics doped with tin-doped indium oxide nanoparticles,97 and for ferroelectric liquid crystals doped with alumina nanoparticles.98 Nematic101,103,104 and ferroelectric liquid crystals100,102 doped with ZnO nanoparticles have also been studied. It was found that ZnO nanoparticles improve the performance of liquid crystal devices in terms of faster switching times, lower threshold voltage, and higher optical contrast. The unique properties of semiconductor nanoparticles and liquid crystals can be combined by the preparation of suspensions of semiconductor nanoparticles in liquid crystals. Elongated (nanorods made of CdS,105 CdSe/ZnS,106 ZnS,107 PbS110) and sphere-like (quantum dots: CdS,108,111,113,118 CdSe109 and CdTe,109,112 GaSe114) semiconductor nanoparticles have been used as dopants to liquid crystals. Electrically controlled polarized light emission,105–107,117 self-assembly phenomena,108,111,114 memory effect,112 optical, and electrooptical response109,110,113 have been studied for such systems, revealing their promise for different optoelectronic applications. 3. C ARBON N ANOTUBES

IN

L IQUID C RYSTALS

Carbon nanotubes (CNTs) have been the subject of very strong interest since their discovery in the 1990s.119,119,a These highly anisometric rigid-like particles are very promising materials for various applications due to their unusual mechanical, electronic, and electromechanical properties. The integration of nanometerlong liquid crystal molecules and micrometer-long CNTs results in a unique self-organization of a composite governed by the highly anisotropic excludedvolume interactions. The first experimental paper43 reported on the orientational ordering of single- and multiwall CNTs dispersed in nematic liquid crystal solvents. One year later,120 the formation of a nematic mesophase was found 119

R. Saito, G. Dresselhaus, and M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London (2001); (a) J. Ch. Charlier, X. Blase, and S. Roche, Rev. Mod. Phys. 79, 677–732 (2007). 120 R. A. Mrozek, B.-S. Kim, V. C. Holmberg, and T. A. Taton, Nano Lett. 3(12), 1665–1669 (2003).

Number of published peer-reviewed papers

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

19

16 14 12 10 8 6 4 2 0 2000

2002

2004

2006 Year

2008

2010

2012

FIG. III.2. Number of papers published per year on the subject ‘‘CNTs in liquid crystals.’’

for the dispersion of CNTs. Since then, a great number of interesting papers have been published in the field. Figure III.2 shows the growth in publications of CNT in liquid crystals as a function of time.121–198 Several comprehensive review papers199–203 have been published describing the properties of dispersions of CNTs in liquid crystals. Below, we highlight a few of the most important results. S. Courty, J. Mine, A. R. Tajbakhshand, and E. M. Terentjev, Europhys. Lett. 64(5), 654–660 (2003). W. Lee, Ch.-Y. Wang, and Y.-C.h. Shih, Appl. Phys. Lett. 85(4), 513–515 (2004). 123 I. Dierking, G. Scalia, P. Morales, and D. LeClere, Adv. Mater. 16, 865–869 (2004). 124 J. M. Russell, S. Oh, I. LaRue, O. Zhou, and E. T. Samulski, Thin Solid Films 509, 53–57 (2005). 125 W. Lee, J.-S. Gau, and H.-Y. Chen, Appl. Phys. B 81, 171–175 (2005). 126 I. Dierking, G. Scalia, and P. Morales, J. Appl. Phys. 97, 044309, 5 (2005). 127 H. Duran, B. Gazdecki, A. Yamashita, and Th. Kyu, Liq. Cryst. 32(7), 815–821 (2005). 128 C.-Y. Huang, C.-Y. Hu, H.-C. Pan, and K.-Y. Lo, Jpn. J. Appl. Phys. 44, 8077–8081 (2005). 129 J. P. F. Lagerwall, G. Scalia, M. Haluska, U. Dettlaff-Weglikowska, F. Giesselmann, and S. Roth, Phys. Status Solidi B 246(13), 3046–3049 (2006). 130 G. Scalia, J. P. F. Lagerwall, M. Haluska, U. Dettlaff-Weglikowska, F. Giesselmann, and S. Roth, Phys. Status Solidi B 246(13), 3238–3241 (2006). 131 H.-Y. Chen and W. Lee, Appl. Phys. Lett. 88, 222105, 3 (2006). 132 C.-Y. Huang, H.-C. Pan, and C.-T. Hsieh, Jpn. J. Appl. Phys. 45, 6392–6394 (2006). 133 S. Y. Jeon, S. H. Shin, S. J. Jeong, S. H. Lee, S. H. Jeong, Y. H. Lee, H. Ch. Choi, and K. J. Kim, Appl. Phys. Lett. 90, 121901, 3 (2007). 134 K. A. Park, S. M. Lee, S. H. Lee, and Y. H. Lee, J. Phys. Chem. C 111, 1620–1624 (2007). 135 K.-X. Yang and W. Lee, Mol. Cryst. Liq. Cryst. 475, 201–208 (2007). 136 S. J. Jeong, K. A. Park, S. H. Jeong, H. J. Jeong, K. H. An, Ch.W. Nah, D. Pribat, S. H. Lee, and Y. H. Lee, Nano Lett. 7(8), 2178–2182 (2007). 137 S. Kumar, Metal Org. Nano Metal Chem. 37, 327–331 (2007). 138 J. P. F. Lagerwall, R. Dabrowski, and G. Scalia, J. Non Cryst. Solids 353, 4411–4417 (2007). 121 122

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It has been shown that the orientational order of liquid crystals can be transmitted to CNTs, which results in a high level of nematic order transferred to the 139

J. Lagerwall, G. Scalia, M. Haluska, U. Dettlaff-Weglikowska, S. Roth, and F. Giesselmann, Adv. Mater. 19, 359–364 (2007). 140 H.-Y. Chen, W. Lee, and N. A. Clark, Appl. Phys. Lett. 90, 033510, 3 (2007). 141 S. Y. Jeon, S. H. Shin, J. H. Lee, S. H. Lee, and Y. H. Lee, Jpn. J. Appl. Phys. 46, 7801–7802 (2007). 142 S. Y. Jeon, K. A. Park, I.-S. Baik, S. J. Jeong, S. H. Jeong, K. H. An, S. H. Lee, and Y. H. Lee, NANO Brief Rep. Rev. 2, 41–49 (2007). 143 W. Gwizdaa, K. Gorny, and Z. Gburski, J. Mol. Struct. 887, 148–151 (2008). 144 S. E. San, M. Okutan, O. Kaoysal, and Y. Yerli, Chin. Phys. Lett. 25, 212–215 (2008). 145 I. Dierking, K. Casson, and R. Hampson, Jpn. J. Appl. Phys. 47, 6390–6393 (2008). 146 C.-Y. Huang, Y.-G. Lin, and Y.-J. Huang, Jpn. J. Appl. Phys. 47, 6407–6409 (2008). 147 R. Dhar, A. Shanker Pandey, M. Bhushan Pandey, S. Kumar, and R. Dabrowski, Appl. Phys. Express 1, 121501 (2008). 148 S. Y. Lu and L. C. Chien, Opt. Express 16, 12777 (2008). 149 R. Basu and G. S. Iannacchione, Appl. Phys. Lett. 93, 183105, 3 (2008). 150 H. J. Shah, A. K. Fontecchio, D. Mattia, and Yu. Gogotsi, J. Appl. Phys. 103, 064314 (2008). 151 Y. S. Suleiman, S. Ghosh, M. E. Abbasov, and G. O. Carlisle, J. Mater. Sci. Mater. Electron. 19, 662–668 (2008). 152 M. E. Abbasov and G. O. Carlisle, J. Nanophotonics 2, (2008). 153 L. Dolgov, O. Yaroshchuk, and M. Lebovka, Mol. Cryst. Liq. Cryst. 496, 212–229 (2008). 154 N. Lebovka, T. Dadakova, L. Lysetskiy, O. Melezhyk, G. Puchkovska, T. Gavrilko, J. Baran, and M. Drozd, J. Mol. Struct. 877, 135–143 (2008). 155 O. Trushkevych, N. Collings, T. Hasan, V. Scardaci, A. C. Ferrari, T. D. Wilkinson, W. A. Crossland, W. I. Milne, J. Geng, B. F. G. Johnson, and S. Macaulay, J. Phys. D Appl. Phys. 41, 125106 (2008). 156 G. Scalia, C. von Bu¨hler, C. Ha¨gele, S. Roth, F. Giesselmann, and J. P. F. Lagerwall, Soft Matter 4, 570–576 (2008). 157 R. Basua and G. S. Iannacchione, Appl. Phys. Lett. 95, 173113, 3 (2009). 158 L. A. Dolgov, N. I. Lebovka, and O. V. Yaroshchuk, Colloid J. 71(5), 603–611 (2009). 159 F. Akkurt, N. Kaya, and A. Alicilar, Fullerenes Nanotubes Carbon Nanostruct. 17, 616–624 (2009). 160 R. Basu and G. S. Iannacchione, J. Appl. Phys. 106, 124312. (2009). 161 A. I. Goncharuk, N. I. Lebovka, L. N. Lisetski, and S. S. Minenko, J. Phys. D Appl. Phys. 42, 165411, 8 (2009). 162 L. N. Lysetski, S. S. Minenko, A. P. Fedoryako, and N. I. Lebovka, Physics E 41, 431–435 (2009). 163 E. M. Jo, A. Kumar Srivastava, J. Jun Bae, M. Kim, M.-H. Lee, H. K. Lee, S.-E. Lee, S. H. Lee, and Y. H. Lee, Mol. Cryst. Liq. Cryst. 498, 74–82 (2009). 164 L. N. Lisetski, S. S. Minenko, A. V. Zhukov, P. P. Shtifanyuk, and N. I. Lebovka, Mol. Cryst. Liq. Cryst. 510, 43[1177]–50[1184] (2009). 165 P. Arora, A. Mikulko, F. Podgornov, and W. Haase, Mol. Cryst. Liq. Cryst. 502, 1–8 (2009). 166 S. Lopatnikov, J. M. Deitzel, and J. W. Gillespie, Jr., Mol. Cryst. Liq. Cryst. 506, 87–106 (2009). 167 J. Prakash, A. Choudhary, D. S. Mehta, and A. M. Biradar, Phys. Rev. E 80, 012701, 4 (2009). 168 W. Zhao, J. Wang, J. He, L. Zhang, X. Wang, and R. Li, Appl. Surf. Sci. 255, 6589–6592 (2009). 169 S. Schymura, M. Ku¨hnast, V. Lutz, S. Jagiella, U. Dettlaff-Weglikowska, S. Roth, F. Giesselmann, C. Tschierske, G. Scalia, and J. Lagerwall, Adv. Funct. Mater. 20, 3350–3357 (2010). 170 R. Basu, K. A. Boccuzzi, S. Ferjani, and Ch. Rosenblatt, Appl. Phys. Lett. 97, 121908, 3 (2010). 171 M. Zhao, Y. Gao, and L. Zheng, Colloids Surf. A Physicochem. Eng. Asp. 369, 95–100 (2010).

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

21

organization of nanotubes.123 However, the liquid crystal media may be strongly anchored to the nanotube surface125,157 and enhanced liquid crystal alignment due to the presence of nanotubes can also be observed.126,175 V. Popa-Nita and S. Kralj, J. Chem. Phys. 132(2), 024902, 8 (2010). A. Matsuyama, J. Chem. Phys. 132(21), 214902, 10 (2010). 174 S. K. Shriyan and A. K. Fontecchio, Opt. Express 18(24), 24842–24852 (2010). 175 R. Basuand and G. S. Iannacchione, Phys. Rev. E. 81(5), 051705, 5 (2010). 176 V. Ponevchinskya, A. I. Goncharukb, V. I. Vasil’eva, N. I. Lebovkab, and M. S. Soskin, Proc. SPIE 7613, 761306-1–761306-11 (2010). 177 V. V. Ponevchinsky, A. I. Goncharuk, V. I. Vasil’ev, N. I. Lebovka, and M. S. Soskin, JETP Lett. 91(5), 241–244 (2010). 178 M. E. Abbasov, S. Ghosh, A. Quach, and G. O. Carlisle, J. Mater. Sci. Mater. Electron. 21, 854–859 (2010). 179 G. Scalia, ChemPhysChem 11(2), 333–340 (2010). 180 R. K. Shukla, K. K. Raina, V. Hamplova, M. Kaspar, and A. Bubnov, Phase Transitions 84(9–10), 850–857 (2011) 181 M. E. Abbasov and G. O. Carlisle, J. Mater. Sci. Mater. Electron. (2011). 10.1007/s10854-0110477-8. 182 K. P. Sigdela and G. S. Iannacchione, Eur. Phys. J. E 34(4), 9 (2011). 183 N. Kaya, F. Akkurt, and A. Alicilar, Fullerenes Nanotubes Carbon Nanostruct. 19, 262–270 (2011). 184 Neeraj and K. K. Raina, Integr. Ferroelectr. 125(1), 104–110 (2011). 185 A. Y. G. Fuhand and K. Yu-Ch. Huang, Modell. Simul. Mater. Sci. Eng. 19, 025006, 16 (2011). 186 R. Basu, Ch.L. Chen, and Ch. Rosenblatt, J. Appl. Phys. 109(8), 083518, 4 (2011). 187 L. N. Lisetski, N. I. Lebovka, S. V. Naydenov, and M. S. Soskin, J. Mol. Liq. (2011). 10.1016/j. molliq.2011.04.020. 188 S. Kumar Gupta, A. Kumar, A. Kumar Srivastava, and R. Manohar, J. Non Cryst. Solids 357, 1822– 1826 (2011). 189 N. Puech, Ch. Blanc, E. Grelet, C. Zamora-Ledezma, M. Maugey, C. Zakri, E. Anglaret, and Ph. Poulin, J. Phys. Chem. C 115, 3272–3278 (2011). 190 N. Kaya, Liq. Cryst. 38(1), 1–7 (2011). 191 A. Matsuyama, Liq. Cryst. 38(7), 885–891 (2011). 192 L. N. Lisetski, S. S. Minenko, V. V. Ponevchinsky, M. S. Soskin, A. I. Goncharuk, and N. I. Lebovka, Materwiss. Werksttech. 42(1), 5–14 (2011). 193 J. Prakash, A. Kumar, T. Joshi, D. S. Mehta, A. M. Biradar, and W. Haase, Mol. Cryst. Liq. Cryst. 541, 166[404]–176[414] (2011). 194 Sh. Ghosh, P. Nayek, S. Kr. Roy, R. Gangopadhyay, M. Rahaman Molla, and R. Dabrowski, Mol. Cryst. Liq. Cryst. 545, 22/[1246]–28/[1252] (2011). 195 R. Basu, R. G. Petschek, and C. Rosenblatt, Phys. Rev. E. 83(4), 041707, 4 (2011). 196 O. Buluy, S. Nepijko, V. Reshetnyak, E. Ouskova, V. Zadorozhnii, A. Leonhardt, M. Ritschel, G. Schonhense, and Yu. Reznikov, Soft Matter 7(2), 644–649 (2011). 197 Z. Mitro´ova´, N. Tomasˇovicˇova´, M. Timko, M. Koneracka´, J. Kova´cˇ, J. Jadzyn, I. Va´vra, N. E´ber, T. To´th-Katona, E. Beaugnon, X. Chaud, and P. Kopcˇansky´, New J. Chem. 35(6), 1260–1264 (2011). 198 S. Schymura, S. Dolle, J. Yamamoto, and J. Lagerwall, Soft Matter 7(6), 2663–2667 (2011). 199 C. Zakri, Liq. Cryst. Today 16, 1–11 (2007). 200 J. P. F. Lagerwall and G. Scalia, J. Mater. Chem. 18(25), 2890–2898 (2008). 201 H. Qi and T. Hegmann, J. Mater. Chem. 18(28), 3288–3294 (2008). 202 Sh. Zhang and S. Kumar, Small 4(9), 1270–1283 (2008). 203 M. Rahmanand and W. Lee, J. Phys. D Appl. Phys. 42, 063001, 12 (2009). 172 173

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Noticeable changes in the nematic–isotropic phase transition temperatures were reported,127 and the possibility of controlling such a transition by external electric fields was demonstrated.150 The liquid crystal alignment can be easily controlled by means of shear or by electric and magnetic fields.126–130,132 Other studies were related to the percolation effects in the electrical conductivity,154,161,162,193 as well as to peculiar features of optical transmittance162,164,176,177 and dielectrical properties.180,188 Electrically or magnetically steered switches based on nanotube–liquid crystal composites were implemented.132,145,146 Recently, new applications of nanotubes–liquid crystal composites in memory devices have been demonstrated by studies of electro-optical and electromechanical effects.140–142,149,153,158 Nanotube–liquid crystal composites are promising for conventional LCD applications as well. They demonstrated improved electro-optic response times with a reduced display driving voltage and the elimination of image sticking.131,133,142 In addition to CNTs, fullerenes,204–208 diamond,209–211 and graphene oxide nanoparticles212 have been embedded into liquid crystals. The experimental results presented in these papers are very similar to the results described for CNTs in liquid crystals. 4. M ETALLIC N ANOPARTICLES

IN

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During the past decade, and after Pendry’s famous paper on superresolution using a super lens,213 there have been a large number of works on metamaterials. Metamaterials are artificial materials that exhibit strong magnetic and electric response in a certain range of frequencies that may result in a negative refractive

M. Suzuki, H. Furue, and Sh. Kobayashi, Mol. Cryst. Liq. Cryst. 368, 191–196 (2001). O. Koysal and S. Eren San, Synth. Met. 158(13), 527–531 (2008). 206 A. Kumar Srivastava, M. Kim, S. M. Kim, M. K. Kim, K. Lee, Y. H. Lee, M. H. Lee, and S. H. Lee, Phys. Rev. E 80, 051702, 6 (2009). 207 O. Trushkevych, P. Ackerman, W. A. Crossland, and I. I. Smalyukh, Appl. Phys. Lett. 97, 201906, 3 (2010). 208 F. Yao, Y. Pei, Y. Zhang, Ch. Ren, X. Sun, and Zh. Lu¨, Liq. Cryst. 38(7), 907–910 (2011). 209 P. Sh. Chen, Ch.Ch. Huang, Y. W. Liu, and Ch. Yu Chao, Appl. Phys. Lett. 90(21), 211111, 3 (2007). 210 S. Tomylko, O. Yaroshchuk, O. Kovalchuk, U. Maschke, and R. Yamaguchi, Mol. Cryst. Liq. Cryst. 541(1), 35/[273]–43/[281] (2011). 211 P.-S.h. Chen, Ch.-C.h. Huang, Yu.-W. Liu, and Ch.-Y.u. Chao, Mol. Cryst. Liq. Cryst. 507(1), 202–208 (2009). 212 A. Malik, A. Choudhary, P. Silotia, A. M. Biradar, V. K. Singh, and N. Kumar, J. Appl. Phys. 108 (12), 124110, 6 (2010). 213 J. B. Pendry, Phys. Rev. Lett. 85(18), 3966–3969 (2000). 204 205

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

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index.214 This leads to new optical phenomena such as negative refraction and superlensing and can lead to numerous possible applications such as cloaking. A lot of work has been dedicated to making such a metamaterial practical, low cost, and tunable. Liquid crystals seem to be a perfect candidate for developing tunable metamaterials. As a result, a whole subfield has been formed—metallic nanoparticles in liquid crystals.215–266 V. G. Veselago, Sov. Phys. Usp. 10, 509–514 (1968). Yu. Shiraishi, N. Toshima, K. Maeda, H. Yoshikawa, J. Xu, and Sh. Kobayashi, Appl. Phys. Lett. 81 (15), 2845–2847 (2002). 216 J. Muller, C. Sonnichsen, H. von Poschinger, G. von Plessen, T. A. Klar, and J. Feldmann, Appl. Phys. Lett. 81(1), 71–173 (2002). 217 M. Perez-Mendez, R. Marsal-Berenguel, and S. Sanchez-Cortez, Appl. Spectrosc. 58(5), 562–569 (2004). 218 S. Yong Parka and D. Stroud, Appl. Phys. Lett. 85(14), 2920–2922 (2004). 219 T. Miyama, H. Shiraki, Y. Sakai, T. Masumi, S. Kundu, Y. Shiraishi, N. Toshima, and S. Kobayashi, Mol. Cryst. Liq. Cryst. 433, 29–40 (2005). 220 M. Mitov, F. de Guerville, and Ch. Bourgerette, Mol. Cryst. Liq. Cryst. 435, 13/[673]–19/[679]. (2005). 221 A. Terheiden, B. Rellinghaus, M. Acet, and C. Mayer, Phase Transitions 78(1–3), 25–34 (2005). 222 P. A. Kossyrev, A. Yin, S. G. Cloutier, D. A. Cardimona, D. Huang, P. M. Alsing, and J. M. Xu, Nano Lett. 5(10), 1978–1981 (2005). 223 S. Yong Park and D. Stroud, Phys. Rev. Lett. 94(21), 217401, 4 (2005). 224 E. B. Barmatov, D. A. Pebalk, and M. V. Barmatova, Liq. Cryst. 33(9), 1059–1063 (2006). 225 G. Zhang, X. Chen, J. Zhao, Y. Chai, W. Zhuang, and L. Wang, Mater. Lett. 60(23), 2889–2892 (2006). 226 I. C. Khoo, D. H. Werner, X. Liang, A. Diaz, and B. Weiner, Opt. Lett. 31(17), 2592–2594 (2006). 227 S. Krishna Prasad, K. L. Sandhya, G. G. Nair, U. S. Hiremath, C. V. Yelamaggad, and S. Sampath, Liq. Cryst. 33(10), 1121–1125 (2006). 228 S. Kobayashi, T. Miyama, N. Nishida, Y. Sakai, H. Shiraki, Y. Shiraishi, and N. Toshima, J. Disp. Technol. 2(2), 121–129 (2006). 229 S. Kobayashi, T. Miyama, N. Nishida, Y. Sakai, H. Shiraki, Y. Shiraishi, and N. Toshima, J. Disp. Technol. 2(4), 418 (2006). 230 V. Ajay Mallia, P. Kumar Vemula, G. John, A. Kumar, and P. M. Ajayan, Angew. Chem. 119(18), 3333–3338 (2007). 231 S. Kaur, S. P. Singh, and A. M. Biradar, Appl. Phys. Lett. 91(2), 023120, 3 (2007). 232 L. Cseh and G. H. Mehl, J. Mater. Chem. 17(4), 311–315 (2007). 233 P. R. Evans, G. A. Wurtz, W. R. Hendren, R. Atkinson, W. Dickson, A. V. Zayats, and R. J. Pollard, Appl. Phys. Lett. 91(4), 043101, 3 (2007). 234 N. Nishida, Yu. Shiraishi, Sh. Kobayashi, and N. Toshima, J. Phys. Chem. C 112(51), 20284– 20290 (2008). 235 V. K. S. Hsiao, Yu.B. Zheng, B. Krishna Juluri, and T. J. Huang, Adv. Mater. 20(18), 3528–3532 (2008). 236 Sh.Yu. Huang, Ch.Ch. Peng, Li-W. Tu, and Ch.T. Kuo, Mol. Cryst. Liq. Cryst. 507, 301–306 (2009). 214 215

24

Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

237 X. Zeng, F. Liu, A. G. Fowler, G. Ungar, L. Cseh, G. H. Mehl, and J. E. Macdonald, Adv. Mater. 21 (17), 1746–1750 (2009). 238 M. Ojima, N. Numata, Y. Ogawa, K. Murata, H. Kubo, A. Fujii, and M. Ozaki, Appl. Phys. Express 2, 086001 (2009). 239 A. Kumar, J. Prakash, D. S. Mehta, A. M. Biradar, and W. Haase, Appl. Phys. Lett. 95(2), 023117, 3 (2009). 240 H. Qi, B. Kinkead, V. M. Marx, H. R. Zhang, and T. Hegmann, ChemPhysChem 10(8), 1211–1218 (2009). 241 D. Vijayaraghavan and S. Kumar, Mol. Cryst. Liq. Cryst. 508, 101/ [463]–114/[476] (2009). 242 R. Pratibha, K. Park, I. I. Smalyukh, and W. Park, Opt. Express 17(22), 19459–19469 (2009). 243 M. Dridi and A. Vial, Opt. Lett. 34(17), 2652–2654 (2009). 244 W. Ch. Hung, I. M. Jiang, M. Sh. Tsai, P. Yeh, W. H. Cheng. Tunable cholesteric liquid crystal diffraction grating based on the effect of localized surface plasmons, OSA/CLEO/QELS (2010). 245 P. Kopcansky´, N. Tomasˇovicova´, M. Koneracka´, M. Timko, Z. Mitro´ova´, V. Za´visˇova´, N. E´ber, K. Fodor-Csorba, T. To´th-Katona, A. Vajda, J. Jadzyn, E. Beaugnonand, and X. Chaud, Acta Phys. Pol. A 118, 988–989 (2010). 246 H. Yoshida, K. Kawamoto, H. Kubo, T. Tsuda, A. Fujii, S. Kuwabata, and M. Ozaki, Adv. Mater. 22(5), 622–626 (2010). 247 F. V. Podgornov, A. V. Ryzhkova, and W. Haase, Appl. Phys. Lett. 97(2), 212903, 3 (2010). 248 D. Hartono, H. Kun-Lin Yang, and Yung Lin-Yue Lanry, Biomaterials 31, 3008–3015 (2010). 249 Y. Lin, Y. Zou, D. Ke, J. Namkung, R. G. Lindquist, Enhanced sensitivity using liquid crystals for optical fiber-based localized surface plasmon resonance sensor, OSA/CLEO/QELS (2010). 250 R. Pratibha, W. Park, and I. I. Smalyukh, J. Appl. Phys. 107(6), 063511, 5 (2010). 251 S. Khatua, P. Manna, W.-S.h. Chang, A. Tcherniak, E. Friedlander, E. R. Zubarev, and S. Link, J. Phys. Chem. C 114(16), 7251–7257 (2010). 252 M. Dridi and A. Vial, J. Phys. D Appl. Phys. 43, 415102, 10 (2010). 253 M. Urbanskia, B. Kinkead, T. Hegmann, and H. S. Kitzerow, Liq. Cryst. 37(9), 1151–1156 (2010). 254 T. Joshi, A. Kumar, J. Prakash, and A. M. Biradar, Liq. Cryst. 37(11), 1433–1438 (2010). 255 S. Torgova, E. Pozhidaev, A. Lobanov, M. Minchenko, and B. Khlebtsov, Mol. Cryst. Liq. Cryst. 525, 176–183 (2010). 256 Q. Liu, Y. Cui, D. Gardner, X. Li, S. He, and I. I. Smalyukh, Nano Lett. 10(4), 1347–1353 (2010). 257 G. Pawlik, M. Jarema, W. Walasik, A. C. Mitus, and I. C. Khoo, J. Opt. Soc. Am. B 27(3), 567–576 (2010). 258 R. Aylo, P. P. Banerjee, and G. Nehmetallah, Proc. SPIE 7754, 775409-1–775409-8 (2010). 259 E. Spinozzi and A. Ciattoni, Opt. Mater. Express 1(4), 732–741 (2011). 260 E. Ouskova, D. Lysenko, Yu. Reznikov, S. Ksiondzyk, L. Cseh, G. Mehl, and V. Reshentnyak, Gratings in liquid crystal golden nanosuspensions due to thermal nonlinearity, IEEE Conference 2011, (2011). 261 G. Nehmetallah, R. Aylo, and P. P. Banerjee, J. Nanophotonics 5, 051603-1–0516031-5 (2011). 262 N. Toshima, Macromol. Symp 304, 24–32 (2011). 263 Y. J. Liu, Yu.B. Zheng, J. Liou, I.-K. Chiang, I. Ch. Khoo, and T. J. Huang, J. Phys. Chem. C 115 (15), 7717–7722 (2011). 264 K. K. Vardanyan, E. D. Palazzo, and R. D. Walton, Liq. Cryst. 38(6), 709–715 (2011). 265 K. Oishi and K. Kajikawa, Opt. Commun. 284, 3445–3448 (2011). 266 J. Li, Y. Ma, Y. Gu, I. Ch. Khoo, and Q. Gong, Appl. Phys. Lett. 98(21), 213101, 3 (2011).

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It has been shown experimentally that light scattered by single metallic nanoparticles embedded in nematic liquid crystals can be electrically controlled.216 In addition, the observed shift in the particle plasmon resonance opens up the possibility of tuning the particle plasmonic resonances by the application of an electric field. An exact calculation of the optical properties of a dilute suspension of metallic nanoparticles in nematic liquid crystals has been described in Refs. 218,223. It was demonstrated that, in such systems, the surface plasmon resonance of the metal nanoparticles splits into two polarized resonances—parallel and perpendicular to the nematic director. The calculations were based on the Maxwell–Garnett approximation (MGA) which is suitable either for higher concentrations of particles in a nematic liquid crystal host or for a single nanoparticle coated with nematogenic molecules. The accuracy of the MGA has been confirmed by the method of the discrete dipole approximation (DDA) for nematic liquid crystals-coated spheres. In addition to metallic nanoparticles embedded in liquid crystals,226,233,235,258,259 more complicated plasmonic structures have been studied: plasmonic nanodots arrays222 and nanorods embedded in liquid crystals233; Au nanodisk arrays and photoresponsive liquid crystals235 and metallic grating with twisted nematic liquid crystals.238 Theoretical calculations and modeling have been done for metallic nanostructures embedded in liquid crystals, predicting both a narrow plasmon resonance and its large spectral tunability.243,266 Additionally, finite difference time domain (FDTD)252 and Monte-Carlo simulations257 have been used to model metallic nanostructures embedded in a liquid crystal. Lyotropic nematic liquid crystals have been utilized for the alignment of gold nanorods over a relatively large surface area in Ref. 256 showing the potential of such systems as a reconfigurable component of tunable metamaterials. The distribution of gold nanospheres in smectic liquid crystals has been tested in Ref. 250 by analyzing the spectra of the surface plasmon resonance. The properties of binary and core–shell nanoparticles dispersed in liquid crystals which are suitable for metamaterial development were reported in Ref. 261, and a plasmonic all-optical bistable device based on nematic liquid crystals in Ref. 265. The field of tunable liquid crystal metamaterials is growing so rapidly that several review papers are already available.267–269 In addition to the topic of tunable metamaterials in liquid crystals, there are many other papers which show how metallic nanoparticles affect the properties 267

I. Ch. Khoo, A. Diaz, J. Liou, M. V. Stinger, J. Huang, and Yi. Ma, IEEE J. Sel. Top. Quantum Electron. 16(2), 410–417 (2010). 268 A. Diaz and I. C. Khoo, Compr. Nanosci. Technol. 3, 225–261 (2010). 269 M. Draper, I. M. Saez, S. J. Cowling, P. Gai, B. Heinrich, B. Donnio, D. Guillon, and J. W. Goodby, Adv. Funct. Mater 21, 1260–1278 (2011).

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of liquid crystals.215,219,224,227–229,231,239,241,244,245,247,250,254,260,262,264 Nematic liquid crystals doped with metallic nanoparticles exhibit a frequencymodulated electro-optical response which can be used for novel display modes.215,219,228,229 Silver nanoparticles enhance the order parameter of polymer liquid crystals224 and the photoluminescence (around 55%) of nematic liquid crystals. Gold nanoparticles embedded in nematic liquid crystals drastically enhance their birefringence,264 thermal nonlinear-optical response,260 and electrical conductivity,227 reduce threshold voltage,246 and increase their sensitivity to external magnetic fields.245 All these influences may lead to the improved optical, nonlinear-optical, electro- and magneto-optical performance of nematics.262 The electro-optical performance of ferroelectric liquid crystals also can be improved by doping them with metallic nanoparticles.231,239,247,254 Ferroelectric liquid crystals doped with gold nanoparticles exhibit two times faster electrooptical response as compared to the pure ferroelectric liquid crystal231,247 and enhanced photoluminescence.239 Gold nanoparticles also affect elastic constants (38.3 pN for pure liquid crystal and 13.4–18.2 pN for the doped one) and dielectric properties (domination of the low frequency relaxation mode due to the increased electrical conductivity) of ferroelectric liquid crystals.247,254 Interactions between different metallic nanoparticles as well as nanoparticle–liquid crystal interactions can result in segregation and/or selfassembly. Segregation of platinum nanoparticles embedded in cholesteric liquid crystals with the formation of periodic ribbons has been reported in Ref. 220. Self-assembled periodic superlattices of gold nanoparticles in discotic liquid crystals (DLCs) have been studied in Ref. 241. It has been found that smaller (~ 1.2 nm) gold nanoparticles randomly occupy positions in the liquid crystalline matrix within the columns as well as in between the columns. The bigger nanoparticles (~ 2.6 and 4.6 nm) form a two-dimensional intercalated hexagonal structure with the disk molecules. Assemblies of gold nanoparticles have resulted in about an increase in the DC conductivity of about two to three orders in the isotropic and mesophases with respect to those of the pure discotics. Liquid crystals also can be considered as a model of cell membranes. A number of biophysics experiments considering the interactions of gold nanoparticles with such model systems have been studied in Refs. 221,248. Lyotropic liquid crystals have been found to be the most suitable systems for modeling of living objects.2,2a,b,4 Recently, a lyotropic mesophase has been used as a selective filter for gold nanoparticles,270 giving new insight toward the understanding of complicated biological structures.

270 B. Pansu, A. Lecchi, D. Constantin, M. Imperor-Clerc, M. Veber, and I. Dozov, J. Phys. Chem. C 115(36), 17682–17687 (2011).

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5. F ERROELECTRIC N ANOPARTICLES In 1988, Bachmann and Birner found that the sensitivity of isotropic liquids to an applied electric field can be increased by doping them with ultra-fine (less than 1 mm in size) ferroelectric BaTiO3 particles.271 They showed that a long milling process of ferroelectric BaTiO3 particles (with a spontaneous polarization of 0.26 mC/cm2) in the presence of a surfactant (oleic acid) results in a stable suspension of ultra-fine particles of BaTiO3 in heptane. These particles consist of ferroelectric single crystals and have an average radius of about 10 nm. The birefringence of the suspension, which is impossible to achieve in a pure heptane matrix, was controlled by applying an electric field. According to Bachmann and Birner, it was a first successful step to transfer the solid state property of ferroelectricity to a fluid. This result and the idea to increase the sensitivity of liquid crystals to external magnetic fields by doping them with magnetic fine particles (proposed by Brochard and de Gennes in 197025) have inspired scientists to use ferroelectric nanoparticles as ‘‘active’’ dopants to liquid crystals.272–322 R. Bachmann and K. Birner, Solid State Commun. 68(9), 865–869 (1988). Yu. Reznikov, O. Buchnev, O. Tereshchenko, V. Reshetnyak, A. Glushchenko, and J. West, Appl. Phys. Lett. 82(12), 1917–1919 (2003). 273 E. Ouskova, O. Buchnev, V. Reshetnyak, Yu. Reznikov, and H. Kresse, Liq. Cryst. 30(10), 1235–1239 (2003). 274 O. Buchnev, A. Glushchenko, Yu. Reznikov, V. Reshetnyak, O. Tereshchenko, and J. West, Proc. SPIE (Nonlinear Opt. Liq. Photorefract. Cryst.) 5257, 7–12 (2003). 275 O. Buchnev, E. Ouskova, Yu. Reznikov, V. Reshetnyak, H. Kresse, and A. Grabar, Mol. Cryst. Liq. Cryst. 422(1), 47–55 (2004); (a) V. Reshetnyak, Mol. Cryst. Liq. Cryst. 421(1), 219–224 (2004). 276 O. Buchnev, C. I. Cheon, A. Glushchenko, Yu. Reznikov, and J. L. West, J. SID 13, 9 (2005). 277 Yu. Reznikov, O. Buchnev, V. Reshetnyak, O. Tereshchenko, A. Glushchenko, J. West, Photonics West Conference, Jan. 22-27, San Jose Convention Center, USA. 278 Yu. Reznikov, A. Glushchenko, V. Reshetnyak, J. West, World Patent 2003060598, Liquid Crystal Cell Comprising Ferroelectric Particle Suspensions, World patent issued in 2005. 279 Yu. Reznikov, O. Buluy, O. Tereshchenko, A. Glushchenko, and J. West, Proc. SPIE 5741, 171 (2005). 280 C. I. Cheon1, L. Li, A. Glushchenko, J. L. West, Yu. Reznikov, J. S. Kim, and D. H. Kim, SID Tech. Dig. 2, 45 (2005). 281 F. Li, O. Buchnev, Ch. Cheon, A. Glushchenko, V. Reshetnyak, Yu. Reznikov, T. J. Sluckin, and J. L. West, Phys. Rev. Lett. 97(14), 147801, 4 (2006). 282 A. Glushchenko, Ch. Cheon, J. West, F. Li, E. Bu¨yu¨ktanir, Yu. Reznikov, and A. Buchnev, Mol. Cryst. Liq. Cryst. 453(1), 227–237 (2006). 283 V. Yu. Reshetnyak, S. M. Shelestiuk, and T. J. Sluckin, Mol. Cryst. Liq. Cryst. 454(1), 201/ [603]–206/[608] (2006). 284 F. Li, J. West, A. Glushchenko, Ch. Cheon, and Yu. Reznikov, J. SID 14(6), 523–527 (2006). 285 F. Li, O. Buchnev, Ch. Cheon, A. Glushchenko, V. Reshetnyak, Yu. Reznikov, T. J. Sluckin, and J. L. West, Phys. Rev. Lett. 99(21), 219901 (2007). 271 272

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We use the term ‘‘active’’ in order to emphasize that such a particle exhibits a permanent dipole moment; however, the reader should be careful since a very

286

A. Glushchenko, O. Buchnev, A. Iljin, O. Kurochkin, and Yu. Reznikov, SID Symp. Dig. Tech. Pap. 38(1), 1086–1089 (2007). 287 A. Glushchenko, Ch. Cheon, J. West, and Yu. Reznikov, Proc. SPIE 6487 (2007) 6487-0T. 288 G. Cook, V. Reshetnyak, A. V. Glushchenko, M. A. Saleh, D. R. Evans, Nanoparticle doped organic–inorganic hybrid photorefractives, OSA/PR (2007). 289 M. Kaczmarek, A. Dyadyusha, G. D’Alessandro, O. Buchnev, Hybrid liquid crystal nanomaterials with improved photorefractive response, OSA/PR (2007). 290 J. L. West, F. Li, K. Zhang, H. M. Atkuri, and A. V. Glushchenko, SID Symp. Dig. Tech. Pap. 38(1), 1090–1092 (2007). 291 O. Buchnev, A. Dyadyusha, M. Kaczmarek, V. Reshetnyak, and Yu. Reznikov, J. Opt. Soc. Am. B 24(7), 1512–1516 (2007). 292 ˇ opicˇ, A. Mertelj, O. Buchnev, and Yu. Reznikov, Phys. Rev. E 76(1), 011702, 5 (2007). M. C 293 G. Cook, A. Glushchenko, V. Reshetnyak, E. Beckel, M. Saleh, and D. Evans, IEEE/LEOS Winter Top. Meet. Ser., 129–130 (2008). 294 G. Cook, A. V. Glushchenko, V. Reshetnyak, A. T. Griffith, M. A. Saleh, and D. R. Evans, Opt. Express 16(6), 4015–4022 (2008). 295 M. Kaczmarek, O. Buchnev, and I. Nandhakumar, Appl. Phys. Lett. 92(10), 103307, 3 (2008). 296 H. Atkuri, G. Cook, D. R. Evans, A. Glushchenko, V. Reshetnyak, Yu. Reznikov, J. West, and K. Zhang, J. Opt. A Pure Appl. Opt. 11, 024006, 5 (2009). 297 A. Mikulko, P. Arora, A. Glushchenko, A. Lapanik, and W. Haase, Europhys. Lett. 87(2), 27009, 4 (2009). 298 O. Kurochkin, O. Buchnev, A. Iljin, S. K. Park, S. B. Kwon, O. Grabar, and Yu. Reznikov, J. Opt. A Pure Appl. Opt 11, 024003, 5 (2009). 299 H. M. Atkuri, K. Zhang, and J. L. West, Mol. Cryst. Liq. Cryst. 508(1), 183/[545]–190/[552] (2009). 300 M. Akimoto, S. Kundu, K. Isomura, I. Hirayama, Sh. Kobayashi, and K. Takatoh, Mol. Cryst. Liq. Cryst. 508(1), 1/[363]–13/[375] (2009). 301 L. Scolari, S. Gauza, H. Xianyu, L. Zhai, L. Eskildsen, Th. Tanggaard Alkeskjold, Sh.-T. Wu, and A. S. Bjarklev, Opt. Express 17(5), 3754–3764 (2009). 302 D. R. Evans, G. Cook, and M. A. Saleh, Opt. Mater. 31, 1059–1060 (2009). 303 L. M. Lopatina and J. V. Selinger, Phys. Rev. Lett. 102(19), 197802, 4 (2009). 304 O. Kurochkin, H. Atkuri, O. Buchnev, A. Glushchenko, O. Grabar, R. Karapinar, V. Reshetnyak, J. West, and Yu. Reznikov, Condens. Matter Phys. 13(3), 33701, 9 (2010). 305 G. Cook, J. L. Barnes, S. A. Basun, D. R. Evans, R. F. Ziolo, A. Ponce, V. Yu. Reshetnyak, A. Glushchenko, and P. P. Banerjee, J. Appl. Phys. 108(6), 064309, 5 (2010). 306 G. Cook, V. Yu. Reshetnyak, A. Ponce, R. F. Ziolo, S. A. Basun, D. R. Evans, Improved holographic beam coupling through selective harvesting of single domain ferroelectric nanoparticles, OSA/BIOMED/DH (2010). 307 M. S. S. Pereira, A. A. Canabarro, I. N. de Oliveira, M. L. Lyra, and L. V. Mirantsev, Eur. Phys. J. E. 31, 81–87 (2010). 308 J. F. Blach, S. Saitzek, C. Legrand, L. Dupont, J. F. Henninot, and M. Warenghem, J. Appl. Phys. 107(7), 074102-1–074102-7 (2010). 309 M. Gupta, I. Satpathy, A. Roy, and R. Pratibha, J. Colloid Interface Sci. 352, 292–298 (2010).

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similar term ‘‘active soft matter’’ became popular in soft matter science to denote different complex soft building elements of living organisms.323 The concept of a diluted suspension of ferroelectric nanoparticles in a liquid crystal was launched by Reznikov and coauthors45,45a: a liquid crystal material was doped with stabilized (i.e., covered with a surfactant) ferroelectric nanoparticles of a very low volume concentration (~ 0.3%). These dilute suspensions are stable because the nanoparticles do not significantly perturb the director field in the liquid crystal (similar to Figure II.1a), and interactions between the particles are weak. Importantly, the nanoparticles share their intrinsic properties with the liquid crystal matrix due to the alignment within the liquid crystal and the interaction with the host matrix molecules. This very first paper in the field45,45a reported on the main intriguing features of such colloids: (1) enhanced dielectric anisotropy, by a factor of 2; (2) lower threshold voltage by a factor of 1.7; (3) linear electro-optical response—the sensitivity to the sign of an applied electric field, not only to its direction (see Figure III.3, Ref. 45,45a), a property intrinsic to ferroelectric liquid crystals rather than to nematics.

310

V. Domenici, B. Zupancic, V. V. Laguta, A. G. Belous, O. I. V’yunov, M. Remskar, and B. Zalar, J. Phys. Chem. C 114, 10782–10789 (2010). 311 H. H. Liang, Ya-Zh. Xiao, F. J. Hsh, Ch.Ch. Wu, and J. Y. Lee, Liq. Cryst. 37(3), 255–261 (2010). 312 M. R. Herrington, O. Buchnev, M. Kaczmarek, and I. Nandhakumar, Mol. Cryst. Liq. Cryst. 527, 72/[228]–79/[235] (2010). 313 G. Cook, V. Yu. Reshetnyak, R. F. Ziolo, S. A. Basun, P. P. Banerjee, and D. R. Evans, Opt. Express 18(16), 17339–17345 (2010). 314 A. Meneses-Franco, V. H. Trujillo-Rojo, and E. A. Soto-Bustamante, Phase Transitions 83(10–11), 1037–1047 (2010). 315 J. L. West, Ch. I. I. Cheon, A. V. Glushchenko, Yu. Reznikov, F. Li, Non-synthetic method for modifying properties of liquid crystals, US Patent 7,758,773 B2, July 20 (2010). 316 S. N. Paul, R. Dhar, R. Verma, Sh. Sharma, and R. Dabrowski, Mol. Cryst. Liq. Cryst. 545(1), 105/ [1329]–111/[1335]. (2011). 317 I. Coondoo, P. Goel, A. Malik, and A. M. Biradar, Integr. Ferroelectr. 125(1), 81–88 (2011). 318 S. N. Paul, R. Dhar, R. Verma, Sh. Sharma, and R. Dabrowski, Mol. Cryst. Liq. Cryst. 545(1), 105/ [1329]–111/[1335] (2011). 319 S. A. Basun, G. Cook, V. Yu. Reshetnyak, A. V. Glushchenko, and D. R. Evans, Phys. Rev. B 84(2), 024105, 8 (2011). 320 S. M. Shelestiuk, V. Yu. Reshetnyak, and T. J. Sluckin, Phys. Rev. E 83(4), 041705, 13 (2011). 321 L. M. Lopatina, J. V. Selinger, Maier-Saupe-type theory of ferroelectric nanoparticles in nematic liquid crystals, arXiv:1105.3428v1 [cond-matt.soft], 17 May (2011) . 322 L. M. Lopatina, Statistical mechanics of nanoparticles suspensions and granular materials, Ph.D. Thesis, Kent State University, August (2011). 323 M. E. Cates and F. C. MacKintosh, Soft Matter 7, 3050–3051 (2011). See also articles in the same themed issue: Soft Matter, 7, March, (2011).

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(a)

(b) E=0

E=0

FIG. III.3. Schematic representation of the ferroelectric particles (arrow indicates permanent dipole moment) in a liquid crystal host: (a) without an external electric field ferroelectric nanoparticles are equally distributed in the plus and minus directions; (b) DC-electric field aligns the particle dipoles along the field, breaking the plus–minus symmetry.

After this initial discovery, the field of liquid crystalline colloids of ferroelectric nanoparticles became one of the hottest topics of modern liquid crystal research, and more than 100 papers were published within the time frame of a few years.272–322 Different thermotropic liquid crystals were tested to prepare such colloids: nematics, cholesterics, smectics, and ferroelectric smectics. As ferroelectric particles, both nanoparticles of BaTiO3 (in most cases) and Sn2P2S6 were utilized (see Table III.3). The basic parameters of bulk ferroelectric materials used as raw materials for nanoparticle preparation are summarized in Table III.2.324–328 So far, only thermotropic liquid crystals are in the game, leaving lyotropic liquid crystals yet to be explored. However, recently, PbTiO3 nanoparticles were 324

Yu. Garbovskiy, V. Weiss, A. Glushchenko, D. Evans, G. Cook, S. Basun, A. Grabar, B. Wechsler, and V. Reshetnyak, Seventh International Photorefractive Workshop, 5, July 13–17, Destin, Florida, (2009). 325 M. B. Klein, Photorefractive properties of BaTiO3, In Photorefractive Materials and Their Applications SpringerSeries in Optical Sciences. Vol. 2, 241–284 (2007). 326 A. A. Grabar, M. Jazbinsˇek, A. N. Shumelyuk, Yu.M. Vysochanskii, G. Montemezzani, and P. Gu¨nter, Photorefractive Effects in Sn2P2S6, In Photorefractive Materials and Their Applications SpringerSeries in Optical Sciences. Ed. By P. Gunter and J.-P. Huignard, Vol. 2, 327–362 Springer Science þ Business Media, LCC, New York (2007). 327 R. Johannes and W. Haas, Appl. Opt. 6(6), 1059–1061 (1967); (a) S. Surthi, S. Kotru, and R. K. Pandey, J. Mater. Sci. Lett. 22, 591–593 (2003). 328 A. Simon, J. Ravez, V. Maisonneuve, C. Payen, and V. B. Cajipe, Chem. Mater. 6, 1575–1580 (1994); (a) V. Maisonneuve, V. B. Cajipe, A. Simon, R. Von Der Muhll, and J. Ravez, Phys. Rev. B 56 (7), 10860–10868 (1997).

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TABLE III.2. BASIC PARAMETERS OF TYPICAL FERROELECTRIC MACROMATERIALS (PS—SPONTANEOUS POLARIZATION, TCURIE—CURIE TEMPERATURE, e—DIELECTRIC CONSTANT, REF.—REFERENCES) Ferroelectrics

Ps (C/m2)

TCurie (K)

e

References

BaTiO3 Sn2P2S6 SbSI CuInP2S6

0.15–0.28 0.14 0.023–0.30 0.0255

403 339 292–338 315

200–700 200–9000 4800 419

325 326 327,327a 328,328a

embedded in a liquid crystalline elastomer matrix,310 boosting the exploration of alternative liquid crystalline materials and ferroelectric nanoparticles.

a. Preparation of Ferroelectric Nanoparticles The reliability of the experimental results strongly depends on the correct preparation of the colloids of ferroelectric nanoparticles in liquid crystals. The most crucial steps are: (1) preparation of the ferroelectric nanoparticles and (2) dispersion of these ferroelectric nanoparticles into a liquid crystal host. There are many ways to produce ferroelectric nanoparticles. For example, BaTiO3 can be prepared using coprecipitation,329 sol–gel processing,330 hydrothermal synthesis,331 reactions in molten salts,332 processing from polymeric precursors,333 oxalate,334 and citrate335 routes. Nonetheless, the method of wet grinding has become the most widely used one when nanoparticles are produced for liquid crystalline nanocolloids. Nanoparticles made using this technique are almost always ferroelectric.296,305,319 Ferroelectric nanoparticles which are synthesized chemically do not maintain ferroelectricity even if their sizes are larger than the critical size. A detailed search through the available literature shows that the critical size below which the crystal structure changes from a tetragonal phase to a cubic one with no spontaneous polarization has been reported to be from 9 to 110 nm.296 The wet grinding production technology of ferroelectric nanoparticles for liquid crystal applications has passed through several important stages. The first approaches resulted in more or less coarse particles and with a broad size T. T. Fang, H. B. Lin, and J. B. Hwang, J. Am. Ceram. Soc. 73(11), 3363–3367 (1990). P. Phule and S. H. Risbud, Adv. Ceram. Mater. 3, 183–185 (1988). 331 K. Fukai, K. Hikada, M. Aoki, and K. Abe, Ceram. Int. 16, 285–290 (1990). 332 Y. Ito, S. Shimada, and M. Inagaki, J. Am. Ceram. Soc. 78, 2695–2699 (1995). 333 N. G. Eror and H. U. Anderson, Mater. Res. Soc. Symp. Proc. 73, 571–578 (1986). 334 H. S. Potdar, P. Singh, S. B. Deshpande, P. D. Godboleand, and S. K. Date, Mater. Lett. 10, 112–117 (1990). 335 J. P. Coutures, P. Odier, and C. Proust, J. Mater. Sci. 27, 1849–1856 (1992). 329 330

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distribution. First, the following materials were used: a single-component nematic liquid crystal 5CB, a multicomponent nematic mixture ZLI 4801, and thiohypodiphosphate Sn2P2S6 as a ferroelectric dopant.45,45a Microparticles (~ 1 mm) of Sn2P2S6 were mixed with a solution of a dispersing agent (oleic acid, a surfactant needed to cover ferroelectric particles to prevent their agglomeration) in a carrier liquid (heptane) in a weight ratio of 1:2:10, respectively. It should be noted that, without a surfactant, the ferroelectric particles can decompose chemically because of overheating during the milling. The molecules of the dispersing agent (oleic acid) attach their polar groups to the ferroelectric particle surface while the motion of their nonpolar tails builds up a repulsive force between the particles.336 The particles were ground in a vibration mill for 120 h, and then the resulting ferroelectric suspension was mixed with liquid crystals ultrasonically. After the heptane was evaporated, the mixture was ultrasonically dispersed for 5 min. Due to the large sizes of the particles in this initial recipe, the majority of the particles aggregated and stopped influencing the properties of the liquid crystalline host. Nevertheless, the quality of the samples was sufficient to observe some initial results. After this success, a great amount of work was done to optimize the production of ferroelectric nanoparticles and the way they are introduced into a liquid crystal. Two sets of important technical questions were considered in great detail. The first set is about nanoparticle preparation: (1) how to choose a liquid carrier and stabilizing agent (surfactant); (2) how to determine the right weight ratio ‘‘liquid carrier: surfactant:ferroelectric particles’’; (3) how the sizes and size distribution of the nanoparticles depend upon the milling time and the parameters of the milling machine; and (4) how to harvest nanoparticles of the same size or of a very narrow size distribution. The second set of questions is about the preparation of liquid crystalline colloids of ferroelectric nanoparticles: (1) how to disperse ferroelectric nanoparticles into a liquid crystal host and (2) how to evaporate the liquid carrier. Heptane was suggested as the liquid carrier in Ref. 45,45a. It is nonpolar and does not dissolve either the dispersing agent (oleic acid) or the liquid crystal host. Heptane can be easily evaporated and it does not contaminate the liquid crystalline colloid of ferroelectric nanoparticles. A surfactant should cover the surfaces of the ferroelectric particles completely (and, of course, both liquid carrier and liquid crystals should not dissolve this surfactant). The best surfactant (oleic acid) for ferroelectric particles was found by the classical method of trial and error. The weight ratio ‘‘liquid carrier:surfactant:ferroelectric particles’’ strongly depends on the milling time and the properties of the milling machine. Descriptions of milling equipment available to produce micro- and nanopowders are given in

336

L. Zhang, R. He, and H. C. Gu, Appl. Surf. Sci. 253, 2611–2617 (2006).

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Refs. 337,338,296. In particular, Ref. 296 presents a complete description of a two-station PM200 planetary ball mill from Retsch used to produce ferroelectric nanoparticles in the reported experiments. Figure III.4 shows this planetary ball mill, describes schematically how it operates, and demonstrates the dependence of the particles’ size upon milling time. Assuming a spherical shape of the particles and a uniform size distribution, the ratio of the total particle mass to the oleic acid mass was found in Ref. 296 to acid ¼ R3 h rracid e R1 where racid is the oleic acid density, rparticle is be given by MMparticle particle the particle density, h is the thickness of the oleic acid layer at the particle surface, and R is the particle radius. Since 1/R ~ Tgrinding (Figure III.4), it means that the concentration of the oleic acid should be proportional to the milling time.

FIG. III.4. (Continued)

337

C. Suryanarayana, Powder Metal Technologies and Applications, ASM Handbook. Vol. 7. ASM International, Materials Park, OH, 80 (1998). 338 C. Suryanarayana, Prog. Mater. Sci. 46, 1184 (2001).

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Average particle diameter (nm)

30

20

10

0 0

5

10

15

20

25

Grinding time (hours) FIG. III.4. Two-station PM200 planetary ball mill from Retsch on the previous page; milling ball motions inside a jar of a planetary ball mill (top on this page); average particles’ diameter as a function of grinding time, obtained by TEM analysis bottom on this page.296

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The authors of paper 296 concluded that the preparation of ferroelectric nanoparticles is a complex process and hence involves the optimization of a number of variables to achieve the desired sizes of nanoparticles. Besides the type of milling machine, the most important parameters that influence the experimental results are: milling container; milling speed; milling time; type, size, and size distribution of the grinding medium; ball- to-powder weight ratio; extent of filling the jar; process control agent; and temperature of milling. All these process variables are not completely independent.338 The dependence shown in Figure III.4 does not reflect the size distribution of the milled particles: in fact, as the output of such a grinding process, we have a plethora of nanoparticles, aggregates of nanoparticles, and microparticles of different sizes. The distribution function of such ensembles of particles is quite complicated. Attempts to harvest nanoparticles of the same sizes (or at least, of the same ferroelectric properties) have resulted in the development of an experimental technique which allows harvesting single ferroelectric domain nanoparticles, and, additionally, involved the concept of stressed ferroelectric nanoparticles.305,306,313,319 The harvesting concept305 is based on the fact that dipoles experience a translational force only when exposed to a field gradient, in which case the net translational force vector F is given by F = (p r)E, where p is the net average dipole moment of the nanoparticle and E is the electric field. For a given linear field gradient, and assuming a single ferroelectric domain, the net translational force on a dipole scales proportionally with the particle size. Also, the Brownian motion effects become progressively more pronounced at smaller particle sizes and so the required field strength for successful separation scales nonlinearly as the particle size is reduced. There are two main techniques for harvesting nanoparticles (Figure III.5). For the gas-phase harvesting technique, a large Van de Graaff generator was constructed (Figure III.5, top). It produces an open circuit potential of approximately +2.7 MV, with a charging belt current of approximately 60 mA. A remote control is required because of the extreme potential and the large stored energy of the Van de Graaff collection sphere (approximately 25 J). The gas-phase method of harvesting ferroelectric nanoparticles is rapid (the separation process takes just a few seconds to complete) but requires careful dried nanoparticle preparation and is best suited to larger scale productions (a few grams of powder per operation). The gaseous aerosol of nanoparticles is introduced into the extreme electric field gradients. This process creates a translational force sufficient to overcome the Brownian motion mixing the aerosol. This technique separates nanoparticles containing a single ferroelectric domain from those which may contain multiple ferroelectric domains. For laboratory needs, it is more convenient to harvest nanoparticles directly from the liquid dispersion used in the mechanical grinding fabrication process— liquid phase nanoparticle harvesting. This is a much slower procedure than

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FIG. III.5. Gas-phase nanoparticle harvesting (top), and liquid phase nanoparticle harvesting (bottom).305

gas-phase harvesting. It requires 30–60 min per operation and is suitable only for very small sample preparations (a few milligrams of nanoparticles per batch). The solvent used to perform liquid phase harvesting should be nonionic and nonconducting, and concentrations of nanoparticles should be smaller than 0.08 wt.% in order to avoid flocculation when the electric field (~ 20,000 V for a vial of diameter < 20 mm) is applied.305 The liquid phase harvesting system is shown in Figure III.5 (bottom). A small sealed glass vial is fitted internally with a narrow-gage wire axial electrode and an external radial foil electrode (which is not shown in the figure). The inner wire electrode is supported within a thinwalled sealed glass capillary tube. In this case, both the inner and outer electrodes are separated by glass from the fluid harvesting medium to prevent any possibility of direct charge injection into the fluid and to avoid any electrochemical

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reactions (electrolysis). A number of insulating poly-tetrafluoroethylene (PTFE) disks are distributed along the inner axial electrode to catch the accumulated nanoparticles as they fall from the electrode when the electric field is removed. A comparison of the harvesting methods is given in Table III.3. Both gas-phase and liquid-phase harvesting methods were tested successfully, and single ferroelectric monodomain nanoparticles as small as 9 nm from mechanically ground nanoparticles were selectively harvested.305 In contrast to many reports on the lack of ferroelectricity for nanoparticles below 10 nm, the harvested nanoparticles do maintain ferroelectricity. The ferroelectric response of such tiny nanoparticles was attributed to the existence of an induced surface strain as a result of the grinding process. The lack of a mechanically induced strain in similarly sized but chemically produced nanoparticles accounts for the absence of ferroelectricity in these materials. The concept of stress-induced ferroelectricity was verified in Ref. 319. Even more, the authors found out that the spontaneous polarization of the nanoparticles is four to five times larger than the spontaneous polarization of the bulk raw materials (see Table III.2). To obtain this result, the following two conditions must be satisfied: (1) a nonpolar solvent for the nanoparticle suspension and (2) nonaggregated nanoparticles. Under these conditions, for 9 nm nanoparticles, the values of 100–120 mC/cm2 and 8.9  10 23 C cm have been measured for the spontaneous polarization and dipole moment, respectively. Aggregation of ferroelectric nanoparticles masks the ferroelectric response due to the partial compensation of the dipole moments of the individual particles. This was experimentally detected by noting the decrease of the transferred charge density versus concentration of BaTiO3 in the suspension (Figure III.6). Recent advances in the production of uniform, monodomain, highly ferroelectric nanoparticles indicate that this field has reached its maturity. The particles can be prepared to certain specifications, with certain characteristics and they are reliable. TABLE III.3. METHODS Method of Required harvesting potentiala

TO

Scale production

HARVEST FERROELECTRIC NANOPARTICLES Time per operation

Special requirements

~ seconds Carefully dried particles Gas Megavolts ~ grams phase (~2– 3 MV) Liquid Kilovolts ~ milligrams ~ 30–60 min Dielectric liquid carrier, phase (~20 kV) concentration of the particles < 0.08 wt.% a

Assuming the same distance between electrodes.

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Transferred charge density (µC/cm2)

Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

9 nm 12 nm 31 nm

100

50

0

0.01

0.1 1 Concentration (wt. %)

10

FIG. III.6. Transferred charge density (normalized to the volume fraction of BaTiO3) measured through the integration of the displacement current on the cells for different concentrations of BaTiO3 in the suspensions.319

b. Liquid Crystal Colloids of Ferroelectric Nanoparticles i. Preparation. One of the most complete descriptions of how to produce liquid crystal colloids of ferroelectric nanoparticles can be found in Refs. 304,308 (although a brief mention of this very important procedure also occurred in Ref. 45,45a). After a certain amount of ferroelectric particles dispersed in heptane is added to a liquid crystal, the liquid carrier evaporates at room temperature after a few days. The mixture needs to be periodically shaken in order to prevent aggregation of the ferroelectric nanoparticles. We would like to emphasize that the carrier evaporation process really matters.281,285 It is possible under different conditions: vacuum evaporation, evaporation at room temperature, evaporation at elevated temperature, etc. In fact, all these conditions can be utilized successfully, but certain precautions should be taken into consideration. For example, if a liquid crystal is a multicomponent system, then vacuum evaporation can lead to changes in the liquid crystal composition due to the evaporation of some liquid crystal components. Such scenarios took place in Ref. 281 where a remarkable increase (40 C) of the ‘‘nematic–isotropic’’ phase transition temperature was reported and attributed to the effects of ferroelectric nanoparticles. Further studies285 revealed that the reported effect of a colossal shift of TNI was caused by the changes in the liquid crystal composition. The actual increase of TNI was found to be ~ 9 C which is still a very astonishing result.

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

39

ii. Physical properties (experiment). Ferroelectric nanoparticles embedded into a liquid crystal host strongly interact with the surrounding mesogenic molecules due to the strong permanent electric field of this particle. These interactions can affect the basic physical parameters of liquid crystals: dielectric permittivity e, birefringence Dna, viscosity g, elastic constants Kii (i = 1, 2, 3), temperatures of phase transitions T, electrical conductivity s, etc. Most of these physical quantities depend on the order parameter S (e.g., e ~ S, Kii ~ S2), therefore, direct experimental measurements of S can contribute significantly to our understanding of the physics behind these phenomena. We summarize the experimental data in Table III.4. We believe this table will be used by researchers and engineers considering the use of nanoparticle–liquid crystal colloids for various optics, photonics, and biophysics applications. iii. Summary of the main experimental results. References 272,273: The addition of ferroelectric nanoparticles created a number of distinct changes: dielectric anisotropy was increased by a factor of about 2; threshold voltage was decreased by a factor of about 2 (it was attributed to the increase in the dielectric anisotropy since the pretilt angle was almost the same for both the pure liquid crystals and the colloids); in addition, the sensitivity to the sign of the applied electric field was demonstrated; electrical conductivity was increased by two orders of magnitude; the temperature of phase transition TNI was almost not affected. Reference 282: the cell with particles demonstrated lower driving voltages, more profiled dependence of the reflectance versus applied voltage and better contrast. In addition, it reported lowering the driving voltage by a factor of 2 for a smectic liquid crystal doped with Sn2P2S6. Reference 291: doping liquid crystal 18,523 with particles of S2P2S6 leads to more than a twofold increase in the values of Dn and De, TNI increased by 12
C, the threshold voltage decreased by half. A strong enhancement of two-beam coupling in the ferroelectric nanoparticle–liquid crystal colloid was found. References 292,295: A substantial increase in the nematic order parameter in agreement with other measurements in similar systems was found. In Ref. 295 enhancement in dielectric anisotropy, lowering the threshold voltage, and an increase of TNI were reported. The experimental investigation demonstrated the nonsynthetic method of optimization of nematic liquid crystals (particularly, their optical performance) via functionalization with inorganic nanoparticles. In Ref. 293,294, two-beam coupling gain coefficients of up to 1400 cm 1 were achieved in the Bragg regime for a hybrid photorefractive cell.

TABLE III.4. PUBLISHED EXPERIMENTAL DATA

Liquid crystal host

Ferroelectric dopant/surfactant

Nematic liquid crystals ZLI-4801 Sn2P2S6 5CB <200 nm in diameters ~ 0.3% volume concentration/oleic acid MLC 6609

BaTiO3 50–100 nm ~ 0.2% by weight/ oleic acid

Preparation method Mechanical wet grinding in heptane, ultrasonic dispersion Mechanical wet grinding in heptane, ultrasonic dispersion

Observed effects Pure liquid crystal

Liquid crystal colloid

eLC, Ι Ι ¼ 7:8 eLC, ? ¼ 2:9 sAC; LC  1010 Sm=m TNI; LC ¼ 92:3
C

ecolloid, Ι Ι ¼ 17:1 ecolloid, ? ¼ 4:2 sAC; colloid  5  109 Sm=m TNI; colloid ¼ 92:6
C Sensitivity to the sign of applied electric field

K22 ¼ 9:6 pN K33 ¼ 16:5 pN TNI, LC ¼ 91:5
C

K22 ¼ 9:69 pN

Comments

References

All data were taken at room temperature for ZLI-4801

272,273

Uth; LC Uth; colloid

¼ 1:9

K33 ¼ 19:5 pN

Scolloid ¼ 1:2ðat 30
CÞ SLC

TNI; colloid ¼ 130
C

Decolloid ¼ 1:44 DeLC

281,284,285

Uth; LC ¼ 1:8 Uth; colloid TNI; colloid  101
C ðrevised dataÞ LC 18523

LC 18523

Sn2P2S6 ~ 100 nm in diameters ~ 0.3% volume concentration/ oleic acid BaTiO3 and Sn2P2S6 ~ 100 nm in diameters ~ 1% volume concentration/oleic acid

Mechanical wet grinding in heptane, ultrasonic dispersion

eLC, ΙΙ ¼ 7:2 eLC, ? ¼ 4:2 DnLC  0:05 GLC  100 cm1 TNI; LC ¼ 58
C

ecolloid, ΙΙ ¼ 12:7 ecolloid, ? ¼ 5:2 Dncolloid  0:11 Gcolloid  480 cm1 TNI; LC ¼ 70
C

Decolloid ¼ 2:5 DeLC Uth; LC ¼ 2:2 SLC  Scolloid Uth; colloid

TNI; LC ¼ 63:5
Cðdopant BaTiO3 Þ TNI; LC ¼ 61
Cðdopant Sn2 P2 S6 Þ

h i1

291

h i Mechanical wet grinding in heptane, ultrasonic dispersion




TNI, LC = 55 C 292

DeLC ¼ 2:1

K2 g

K2 g1

LC

 1:6

colloid

DnLC ¼ 0:05

292 Decolloid ¼ 4:8

Decolloid ¼ 6:2

TNI; LC ¼ 57
C

Dncolloid ¼ 0:09

ncolloid ¼ 0:10

Uth ¼ 1:95 V

TNI; colloid ¼ 68

TNI; colloid ¼ 69

295

Uth ¼ 1:05 V

Uth ¼ 0:85 V

295

Sn2P2S6 295

292,295

LC 1550

BaTiO3 (5–40 nm) andSn2P2S6 (5–80 nm) diameters ~ 0.3% volume concentration/oleic acid

Mechanical wet grinding in heptane, ultrasonic dispersion

DeLC ¼ 2:38

Decolloid ¼ 3:36

DnLC ¼ 0:05

Dncolloid ¼ 0:071

TNI; LC ¼ 79

TNI, colloid ¼ 84

Uth ¼ 1:45 V

Uth ¼ 0:95 V (for BaTiO3) Decolloid ¼ 4:46

295

Dncolloid ¼ 0:09 TNI; colloid ¼ 86 Uth ¼ 0:75 V TL205

BaTiO3 ~ 10 nm 0.5% by weight/ oleic acid

TL205

BaTiO3 ~ 9 nm 0.3% by weight/ oleic acid

TL205

BaTiO3 12 3 nm (B) 15 3 nm (A) 18 3 nm (C) 0.3% by weight/ oleic acid

MLC6815

BaTiO3 12 3 nm (B) 15 3 nm (A) 18 3 nm (C) 0.3% by weight/ oleic acid

Mechanical wet grinding in heptane, ultrasonic dispersion Mechanical wet grinding in heptane, liquid phase harvesting Mechanical wet grinding in heptane, ultrasonic dispersion Mechanical wet grinding in heptane, ultrasonic dispersion

GLC  400 cm

1

(for Sn2P2S6)

1

Gcolloid  1100 cm

Two-beam coupling experimental scheme, Bragg regime

293,294

GLC  400 cm1 Uth; LC ¼ 2 V

Gcolloid  1100 cm1 Uth; colloid ¼ 1 V

Two-beam coupling experimental scheme, Bragg regime

306,313

DeLC ¼ 3:09 DnLC ¼ 0:21

Decolloid ðAÞ ¼ 5:5 Decolloid ðBÞ ¼ 4:48 Decolloid ðCÞ ¼ 6:01 Dncolloid ðAÞ ¼ 0:25 Dncolloid ðBÞ ¼ 0:23 Dncolloid ðCÞ ¼ 0:26 Decolloid ðAÞ ¼ 3:6 Decolloid ðBÞ ¼ 3:24

DC-electric field was used to drive liquid crystal cells

312

DC-electric field was used to drive liquid crystal cells

312

DeLC ¼ 1:46 DnLC ¼ 0:05

Decolloid ðCÞ ¼ 3:74 Dncolloid ðAÞ ¼ 0:07 Dncolloid ðBÞ ¼ 0:06 Dncolloid ðCÞ ¼ 0:08

(continued)

TABLE III.4 (continued) Liquid crystal host

Ferroelectric dopant/surfactant

E7

Is not specified BaTiO3 ~ 10 nm 0.02% by weight/ oleic acid Sn2P2S6 Mechanical wet grinding in ~ 20 nm in diameters heptane, ~ 0.3% volume ultrasonic concentration/oleic dispersion acid

5CB

Preparation method

Observed effects Pure liquid crystal

Liquid crystal colloid

Comments

Performance of solid-corephotonic crystal fibers filled with doped liquid crystals was studied

TNI; LC ¼ 34:5
C Uth; LC ¼ 1:84 V

DTNI; colloid ¼ TNI; colloid  TNI; LC 2:8
References 301

Nanoparticles as molecular additives

304

Switching times are measured for cell gap 23 mm

308

DTNI; colloid <11:1
C Uth; colloid ¼ 1:87 V

5CB

BaTiO3 ~ 150 nm 1–4% by volume/ oleic acid

Mechanical wet grinding in heptane, ultrasonic dispersion

Uth; LC ¼ 0:79 V tON; LC ¼ 450 ms tOFF; LC ¼ 5:26 s

Uth, colloid ¼ 0:54 V tON, colloid ¼ 300 ms tOFF;colloid ¼ 7:75 s Scolloid ¼ 0:60 TNI, colloid ¼ 36:6
C

SLC ¼ 0:55 TNI; LC ¼ 35:2
C 8OCB

BaTiO3 ~ 30 nm 1–4% by volume/ oleic acid Stearic acid

Particles were chemically synthesized (from Sigma Aldrich)

TNI; LC ¼ 79:6
C TNSmA; LC ¼ 67
C TCrSmA; LC ¼ 54
C

Threshold voltages, order parameter, dielectric permittivity, anisotropy of permittivity of colloids were decreased as compared to the same quantities of pure liquid crystals. In addition, defects formation and particle aggregation was observed for such systems.

309

6CHTB

BaTiO3 ~ 30–50 nm 0.54% by weight/ oleic acid

No information how nanoparticles were prepared, ultrasonic dispersion

TNI; LC ¼ 41:7  42:1
C

TNI; colloid ¼ 41:6  41:7
C

Uth; LC ¼ 0:7 V

Uth; colloid ¼ 1:3 V

eLC,

ecolloid,

ΙΙ

¼ 10:6

eLC, ? ¼ 4:1 K11; LC ¼ 0:029  1010 N Ea; LC ¼ 71 kJ=mole fR; LC ¼ 2:3 MHz

ΙΙ

at 27 C

318

Cholesteric bistable cell was tested

282

Two-beam coupling experimental scheme, Bragg regime

293

¼ 9:1

ecolloid, ? ¼ 3:9 K11; colloid ¼ 0:079  1010 N Ea; colloid ¼ 68 kJ=mole fR; colloid ¼ 2:4 MHz DHIN; LC ¼ 1:62 J=g

DHIN; LC ¼ 2:13 J=g Cholesteric liquid crystals Wet grinding in BL118 Sn2P2S6 heptane < 200 nm in diameters ~ 1.0% by weight/ oleic acid Mechanical wet BL118 BaTiO3 grinding in 9–50 nm heptane, Up to 1% by weight/ ultrasonic oleic acid dispersion BaTiO3 Mechanical ZLI 4801wet grinding doped < 70 nm in heptane, with 35 wt. 0.2% by volume/ ultrasonic % of a oleic acid dispersion chiral agent R-811

Uth, LC = 10 and 37 V

GLC  400 cm

1

beLC, ΙΙ ¼ 6:2 eLC, ? ¼ 2:8 DnLC ¼ 0:09 TNI; LC ¼ 91:5 (for nematic) Uth, LC = 48 V

Uth, colloid = 13 and 45 V

Gcolloid  1400 cm

1

Uth; colloid ¼ 33 V TNI;colloid ¼ 89:4
C

Dncolloid  1:25 DnLC Pcolloid  0:9 PLC

298

Decolloid  2:6 DeLC

(continued)

TABLE III.4 (continued) Liquid crystal host

Ferroelectric dopant/surfactant

Ferroelectric liquid crystals LAHS9 BaTiO3 ~ 28 nm 1.0% by weight/ oleic acid

Preparation method Mechanical wet grinding in heptane, ultrasonic dispersion

Observed effects Pure liquid crystal

Liquid crystal colloid

Comments

References

Ps; LC  58 nC=cm2 tON; LC  55 ms eLC  210

Ps; colloid  50 nC=cm2 tON; colloid  35 ms

Layer spacing

297

ecolloid  140

˚ dcolloid  dLC  30:7 A gLC >1 gcolloid Ea; LC  1:1 Ea; colloid

FLC CS1024

SCE13

Mechanical wet BaTiO3 grinding in ~ 39.9 nm tetrahydro1.0% by weight/ furan (THF), polymeric surfactant ultrasonic dispersion

BaTiO3 ~ 30–50 nm oleic acid

Smectic liquid crystals 8CB BaTiO3 and Sn2P2S6 <200 nm in diameters ~ 0.36% by weight/ oleic acid

Ps; LC  46:9 nC=cm2 tON; LC  75 ms

Ps; LC  80:0 nC=cm2 tON; colloid  30 ms

Ps; LC  65:0 nC=cm2 tON; colloid  100 ms

tOFF; LC  360 ms

tOFF; colloid  280 ms

tOFF; colloid  370 ms

TSmC SmA; LC ¼ 61:15
C

TSmC SmA;LC ¼ 61:01
C

TSmC SmA; LC ¼ 56:78
C

TSmAN ; LC ¼ 82:58
C

TSmAN ; LC ¼ 82:35
C

TSmAN , LC ¼ 81:89
C

TN I; LC ¼ 90:35
C

TN I; LC ¼ 90:12
C

TN I; LC ¼ 89:68
C

eLC  20

ecolloid  20

ecolloid  40

2

(for 0.1 wt.% of BaTiO3)

Mechanical wet grinding in heptane, ultrasonic dispersion

Ps, LC  27 nC/cm

Ps, colloid  36 nC/cm

Wet grinding in heptane

Uth, LC  30 V

Uth, colloid  15 V

2

311

(for 1.0 wt.% of BaTiO3) 317

No effects was observed for the BaTiO3

282

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

45

In Ref. 297, electro-optic studies showed a faster response time for the nanocomposite as compared to the pure ferroelectric liquid crystal mixture. However, the values of the spontaneous polarization and the dielectric permittivity were lower for the nanocomposite materials. This difference can result from the antiparallel dipole–dipole correlation between the molecules of the ferroelectric liquid crystal mixture and the barium titanate nanoparticles. Reference 298: the introduction of ferroelectric nanoparticles into a cholesteric matrix results in a strong change in the mesogenic, optical, and electro-optic properties of the cholesterics, keeping the optical quality and the director field in the cell undisturbed. The most impressive impact of the ferroelectric nanoparticles is the 45% decrease of the driving voltage, caused by a strong (2.5 times) increase of the effective dielectric anisotropy of the nematic matrix, which was the basic component of the cholesteric compound. In Ref. 301, a device based on a photonic crystal fiber infiltrated with liquid crystals doped with BaTiO3 nanoparticles was demonstrated. Compared to similar devices based on undoped liquid crystals, new interesting features appear, such as a frequency modulation response and a transmission spectrum with tunable attenuation on the short wavelength side of the bandgap. In Ref. 304: it was experimentally found that ferroelectric nanoparticles, in common with other molecular additives, shift the clearing temperature, TNI, extending the two-phase coexistent region and changing the average order parameter of the single-component nematics. An increase of up to 11 C or a smaller decrease by 3 C of TNI, and corresponding changes of the order parameter were observed. For both of these cases, the order parameter of the colloid showed the universal temperature behavior, which is a characteristic of liquid crystals with molecular additives. In Ref. 312, the effect of the size of BaTiO3 nanoparticles on the electro-optic properties of nematic liquid crystals was reported. The authors used different experimental methods to determine the sizes of the nanoparticles. A DC-electric field was used to drive the liquid crystal cells. References 305,306,313 reported the improved holographic beam coupling through selective harvesting of single-domain ferroelectric nanoparticles. Reference 308 reported a decrease in the Freedericksz threshold, a decrease in the switching time-ON, and an increase in the restoring switching time-OFF for nematic 5CB doped with BaTiO3 nanoparticles. In Ref. 309, a rigorous grinding procedure was not adopted in order to avoid a reduction of the Curie temperature to below the temperature at which 8OCB is liquid crystalline. As a result, liquid crystal defect formation and the absence of effects reported previously for similar systems were found. Reference 311 reported an enhancement of the electro-optical properties of ferroelectric liquid crystals by doping them with ferroelectric nanoparticles.

46

Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

In Ref. 317, the authors reported on the enhancement of the spontaneous polarization of the ferroelectric liquid crystals doped with BaTiO3 nanoparticles. Data in Ref. 318 suggested a decrease of the order parameter due to the dispersion of BaTiO3. iv. Theoretical explanations. By analyzing both Table III.4 and the summary of the main experimental results, the following conclusions can be drawn: experimental data272,273,281,282,284,285,291– report that ferroelectric nanoparticles embedded in liquid crystals at low concentrations (1) enhance dielectric permittivity (both e|| and e?), dielectric anisotropy De = e||  e?, and optical birefringence Dn = n||  n?; (2) lower switching voltage Uth for the Freedericksz transition; (3) increase the orientational order parameter S and the isotropic–nematic transition temperature TNI; (4) reduce switching times needed to reorient liquid crystals by an external electric field. In all these cases, the method of wet grinding was used to prepare the ferroelectric nanoparticles. (2) A smaller number of papers309,318 claim results opposite to the ones published in the references in the previous paragraph. These papers report (1) a decrease of the dielectric permittivity anisotropy; (2) lower values of the order parameter; and (3) a decrease of the isotropic–nematic phase transition temperature. Conclusions (1) and (2) are entirely contradictory. Reasonable explanations for this contradiction can be found in the comparison of the methods used to prepare the ferroelectric nanoparticles. In the previous references, wet grinding of the nanoparticles was utilized and the nanoparticles retained their ferroelectricity after such a preparation. In, 309,318 the nanoparticles were used as purchased, without any additional mechanical milling. As a result, such nanoparticles could be simply nonferroelectric. (3) The effects of ferroelectric nanoparticles on the physical properties of liquid crystals are much more pronounced when additional experimental procedures to select nanoparticles are applied, for example, harvesting of the monodomain ferroelectric nanoparticles with enhanced ferroelectric characteristics

(1) Most

of the

published

295,297,298,304–306,308,310–313,317

To summarize, a strong ferroelectric response of ferroelectric nanoparticles is a key factor leading to all the ‘‘positive’’ effects mentioned above. There are several approaches to develop a theory for ferroelectric nanoparticles in nematic liquid crystals272,275,275a,281,283,303,307,320–322:

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

47

–. Theoretical models based on generalized Maxwell–Garnett theory and minimization of the free energy of the mixture272,275, 275a,281,283,320

Ferroelectric nanoparticles are considered as guests in the host liquid crystal matrix, forming a homogenous material.272 The Maxwell–Garnett theory gives the following expression for the dielectric anisotropy De of such a guest–host system: De ¼ ð1  fv ÞDeLC þ fv DeFNP ; where DeLC and DeFNP are the dielectric permittivity anisotropies of a liquid crystal (host) and of a ferroelectric nanoparticle (guest), respectively; fv is a volume fraction of nanoparticles. This model is valid only for particles with a dielectric constant close to the dielectric constant of the liquid crystal (eLC  eFNP). In the case where eLC  eFNP, the Maxwell–Garnett theory gives the following expression for the dielectric permittivity275,275a:   3 2 eLC 3fv eeFNP þe FNP LC  5; e ¼ eLC 41 þ eFNP eLC 1  fv eFNP þeLC which simplifies for diluted colloids as e = eLC[1 + 3fv]. It means that dielectric permittivity e of the suspension of low concentrated spherical particles is mostly determined by the permittivity of the liquid crystal and does not really differ from eLC. In order to explain the experimental results, Reshetnyak modified the Maxwell–Garnett theory and derived effective dielectric functions for ferroelectric liquid crystal suspensions (for spheroid-like particles).275,275a,283,320 In Ref. 275,275a, he considered a suspension of ferroelectric ellipsoidal particles oriented in a nematic liquid crystal matrix along a local director, suggesting the permanent polarization P of each particle to be parallel or antiparallel to the director. It was also assumed that the main axes of the polarisability tensor of the particles coincided with the particles’ long axis and local liquid crystal director. These assumptions are allowed for the following expressions for the dielectric constant of the ferroelectric suspension: fv Tjj eFNP, jj þ ð1  fv ÞeLC, jj þ fv e0PkBvT heLC iþljj ðhheeLCFNPi iheLC iÞ 2

ejj ¼

1  fv þ fv Tjj

e? ¼

fv T? eFNP, ? þ ð1  fv ÞeLC, ? 1  fv þ fv T?

48

Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

(where lI are the depolarization factors). According to these expressions, both components e? and ek of dielectric permittivity can be increased, and numerical estimations give values which are close to the experimental data.275,275a

–. Minimization of the free energy of the mixture Minimization of the free energy of the mixture was done taking into account the coupling of liquid crystals with the ferroelectric nanoparticles’ intrinsic field.283 In this model, no interactions between ferroelectric nanoparticles were considered due to the particles’ low concentration. The effective dielectric anisotropy is De = DeLC + fvP2vbl(1 + l), where DeLC is the dielectric anisotropy of the liquid crystal (host), fv is the volume fraction of nanoparticles, P is the polarization of the particle, v is the volume of the particle, l is the local field factor, b = 1/kT. A generalization of the approaches published in Refs. 275,275a,283 was done in Ref. 320. The theory takes into account the particles’ shape, dielectric susceptibility, and local field effects. All these calculations suggest, in qualitative agreement with the experimental data, that doping a nematic liquid crystal with ferroelectric particles, even at very low particle concentrations, can significantly decrease the electric Freedericksz threshold field. –. Computer simulations—molecular dynamics307 In Ref. 307, Pereia et al. used molecular dynamic simulations to calculate the density of liquid crystal molecules, the orientational order parameter, and the polar and azimuthal angle profiles as a function of the distance from the center of the immersed nanoparticle. Simulations were done for different temperatures of the system. It was found that the electric field created by the ferroelectric nanoparticles can maintain the orientational order in such guest–host systems at temperatures sufficiently (about 5%) higher than the nematic–isotropic phase transition temperature. The concentration of the ferroelectric nanoparticles in the system under consideration was relatively high (one ferroelectric nanoparticle per about 6000 mesogenic molecules) as compared to the concentrations used in experiments which is about two orders of magnitude lower. This fact makes comparison of such simulations and experimental data too difficult. However, the simulations performed in Ref. 307 confirm the general theoretical picture— aligned ferroelectric nanoparticles with a strong permanent electric dipole can polarize liquid crystal molecules, hence increasing the intermolecular interactions and causing substantial enhancement of the liquid crystal order. –. Statistical mechanics of ferroelectric nanoparticles in liquid crystals303,321,322 A theory proposed for the statistical mechanics of ferroelectric nanoparticles in liquid crystals was proposed in Ref. 303 and is based on the orientational distribution of the nanoparticle dipole moments. This distribution is characterized by an orientational order parameter, which interacts

LIQUID CRYSTALLINE COLLOIDS OF NANOPARTICLES

49

with the orientational order of the liquid crystals and stabilizes the nematic phase. This theory predicts the enhancement in the isotropic–nematic transition temperature (in good agreement with experimental data). This enhancement occurs even when electrostatic interactions are partially screened by moderate concentrations of ions in the liquid crystal—the closest case to reality. Additionally, this paper demonstrated the coupling of nanoparticles with macroscopic orientational order and provided an opportunity to improve the properties of liquid crystals without chemical synthesis. All calculations in Ref. 303 were performed using a Landau theory, based on coupled orientational order parameters for the liquid crystal and the nanoparticles. This model has one important limitation: like all Landau theories, it involves an expansion of the free energy in powers of the order parameters, and hence it overestimates the order parameters that occur in the low-temperature phase. For that reason, a new Maier–Saupe-type model was proposed in Refs. 321,322, which explicitly shows the low-temperature saturation of the order parameters. This model reduces to the Landau theory in the limit of high temperature or weak coupling but shows a different behavior in the opposite limit. In general, the concept of coupled orientational distribution functions should be useful for many other systems besides ferroelectric nanoparticles in liquid crystals. For example, it applies to any type of nonspherical colloidal particles, such as CNTs, in a liquid crystal solvent. It also applies to two distinct species of nonspherical colloids suspended in an isotropic solvent, which could have a coupled ordering transition.

c. Applications of Ferroelectric Nanoparticles/Liquid Crystal Colloids Ferroelectric nanoparticles can modify the intrinsic properties of liquid crystal materials without time-consuming and expensive chemical synthesis. Experimental data on the enhancement of optical, electro-optical, and nonlinear-optical responses of such materials show the strong potential of ferroelectric nanoparticles for improving the ‘‘practical’’ properties of liquid crystals, especially for those materials where the method of chemical synthesis has reached its limit. Such modified materials are very attractive and suitable for use in displays, switchable lenses, beam steering, as well as other light-controlling devices. 6. M AGNETIC N ANOPARTICLES Most display applications of liquid crystals are based on the reorientation of liquid crystalline molecules in external electric or magnetic fields. Despite the fact that early experimental and theoretical works addressed magnetic

50

Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

reorientation, in practice, the majority of liquid crystal devices are driven by electric fields. This is because the sensitivity of the liquid crystals to an electric field is very high: driving voltages of only a few volts are needed to trigger the orientational and optical response of liquid crystals. In contrast, the sensitivity of liquid crystals to magnetic fields (except the very special case of so-called magnetic liquid crystals339) is very low. The diamagnetic permeability anisotropy is approximately 10 7 cgs units, and as a result, a rather large magnetic field (~ 1–10 kG) must be applied to get a magneto-optical response comparable to its electro-optical analog. The idea to increase the sensitivity of liquid crystals to magnetic fields (Brochards and de Genes25) by means of ferromagnetic inclusions launched long-term research programs throughout the world.340–432 The first theoretical scenario (for ferronematics, i.e., nematic liquid crystals doped with magnetic 339 P. Day, A. E. Underhill, K. Binnemans, D. W. Bruce, S. R. Collinson, R. Van Deun, Yu. G. Galyametdinov, and F. Martin, Towards magnetic liquid crystals, 206–220 in Metal - Organic and Inorganic Molecular Magnets, edited by P. Day and A. E. Underhill, Royal Society of Chemistry, Cambridge, UK (1999) 340 L. Liebert and A. Martinet, J. Phys. Lett. 40, 363–368 (1979). 341 Yu.L. Raikher, S. V. Burylov, and A. N. Zakhlevnykh, Sov. Phys. JETP 64(2), 319–324 (1986). 342 A. M. Figueiredo Neto and M. M. F. Saba, Phys. Rev. A. 34(4), 3483–3485 (1986). 343 T. Kroin and A. M. Figueiredo Neto, Phys. Rev. A 36(6), 2987–2990 (1987). 344 Sh.-H. Chen and B. J. Liang, Opt. Lett. 13(9), 716–718 (1988). 345 M.-H. Lu and Ch. Rosenblatt, Appl. Phys. Lett. 56(6), 590–592 (1990); (a) P. B. Sunil Kumara and G. S. Ranganatha, Mol. Cryst. Liq. Cryst. 177(1), 123–130 (1989); (b) P. B. Sunil Kumara and G. S. Ranganatha, Mol. Cryst. Liq. Cryst. 196(1), 27–37 (1991). 346 B. J. Liang and Sh.H. Chen, Phys. Rev. A 39(3), 1441–1446 (1989). 347 J. B. Hayter, R. Pynn, S. Charles, A. T. Skjeltorp, J. Trewhella, G. Stubbs, and P. Timmins, Phys. Rev. Let. 62(14), 1667–1670 (1989). 348 S. V. Burylov and Yu.L. Raikher, J. Magn. Magn. Mater. X5 (1990), 74–76. 349 J. C. Dabadie, P. Fabre, M. Veyssi, V. Cabuil, and R. Massart, J. Phys. Condens. Matter 2, SA291–SA294 (1990). 350 S. V. Burylov and Yu.L. Raikher, Phys. Lett. A 149, 5,6 (1990), pp. 279–283. 351 P. Fabre, C. Casagrande, M. Veyssie, V. Cabuil, and R. Massart, Phys. Rev. lett. 64(5), 539–542 (1990). 352 P. Fabre, C. Quilliet, M. Veyssie´, F. Nallet, D. Roux, V. Cabuil, and R. Massart, Europhys. Lett. 20 (31), 229–234 (1992); (a) V. Ponsinet, P. Fabre, M. Veyssie, and L. Auvray, J. Phys. II France 3(7), 1021–1039 (1993). 353 S. V. Burylov and Yu.L. Raikher, J. Magn. Magn. Mater. 122, 62–65 (1993). 354 J. C. Bacri and A. M. Figueiredo Neto, Phys. Rev. E 50(5), 3860–3864 (1994). 355 S. V. Burylov and Yu.L. Raikher, Phys. Rev E 50(1), 358–367 (1994); (a) A. N. Zakhlevnykh and P. A. Sosnin, Int. J. Polym. Mater. 27(1–2), 89–99 (1994). 356 S. V. Burylov and Yu.L. Raikher, Braz. J. Phys. 25(2), 148–173 (1995); (a) S. V. Burylov and Y. L. Raikher, Mol. Cryst. Liq. Cryst. 258(1), 107–122 (1995); (b) S. V. Burylov and Yu. L. Raikher, Mol. Cryst. Liq. Cryst. 258(1), 123–141 (1995). 357 V. Ponsinet, P. Fabre, and M. Veyssie, Europhys. Lett. 30(5), 277–282 (1995); (a) V. Ponsinet and P. Fabre, J. Phys. II France 6(7), 955–960 (1996).

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51

grains) suggested the following picture25: (a) the magnetic moments of the particles in ferronematics are aligned by the external magnetic field; (b) the

A. N. Zakhlevnykh and P. A. Sosnin, J. Magn. Magn. Mater. 146, 103–110 (1995). A. M. Figueiredo Neto, SPIE 2372, 208–211 (1995). 360 M. Koneracka, V. Zavisova, P. Kopcansky, J. Jadzyn, G. Czechowski, and B. Zywucki, J. Magn. Magn. Mater. 157/158, 589–590 (1996). 361 S. Fontanini, A. L. Alexe-Ionescu, G. Barbero, and A. M. Figueiredo Neto, J. Chem. Phys. 106(14), 6187–6193 (1997). 362 V. Berejnov, J.-C. Bacri, V. Cabuil, R. Perzynski, and Yu. Raikher, Europhys. Lett. 41(5), 507–512 (1998). 363 V. Berejnov, V. Cabuil, R. Perzynski, and Yu. Raikher, J. Phys. Chem. B 102, 7132–7138 (1998). 364 P. I. C. Teixeira, Liq. Cryst. 25(6), 721–726 (1998). 365 Yu.L. Raikher and V. I. Stepanov, J. Magn. Magn. Mater. 201, 182–185 (1999). 366 S. I. Bastrukov and P. Y. Lai, J. Phys. Condens. Matter 11, L205–L208 (1999). 367 A. Zakhlevnykh and V. Shavkunov, Mol. Cryst. Liq. Cryst. 330(1), 593–599 (1999). 368 C. Y. Matuo and A. M. Figueiredo Neto, Phys. Rev. E 60(2), 1815–1820 (1999). 369 D. Spoliansky, V. Ponsinet, J. Ferre, and J.-P. Jamet, Eur. Phys. J. E. 1, 227–235 (2000). 370 T. S. Perova, P. C. Fannin, and P. A. Perov, Mol. Cryst. Liq. Cryst. 352(1), 141–148 (2000). 371 E. Petrescu and C. Motoc, J. Magn. Magn. Mater. 234, 142–147 (2001). 372 H. Pleiner, E. Jarkova, H.-W. Muller, and H. R. Brand, Magnetohydrodynamics 37(3), 254–260 (2001); (a) E. Jarkova, H. Pleiner, H.-W. Muller, A. Fink, and H. R. Brand, Eur. Phys. J. E. 5, 583–588 (2001). 373 V. S. Shavkunov and A. N. Zakhlevnykh, Mol. Cryst. Liq. Cryst. 367(1), 175–182 (2001). 374 C. Y. Matuo, F. A. Tourinho, M. H. Souza, J. Depeyrot, and A. M. Figueiredo Neto, Braz. J. Phys. 32(2B), 458–463 (2002). 375 R.-E. Bena and E. Petrescu, J. Magn. Magn. Mater. 248, 336–340 (2002). 376 H. Pleiner, E. Jarkova, H.-W. Muller, and H. R. Brand, J. Magn. Magn. Mater. 252, 147–149 (2002). 377 I. Potocova, P. Kopcansky, M. Koneracka, L. Tomco, M. Timko, J. Jadzyn, and G. Czechowski, J. Magn. Magn. Mater. 252, 150–152 (2002). 378 S. V. Burylov, V. I. Zadorozhnii, I. P. Pinkevich, V. Yu. Reshetnyak, and T. J. Sluckin, J. Magn. Magn. Mater. 252, 153–155 (2002). 379 O. Buluy, E. Ouskova, Yu. Reznikov, A. Glushchenko, J. West, and V. Reshetnyak, Mol. Cryst. Liq. Cryst. 375(1), 81–87 (2002). 380 S. V. Burylov, V. I. Zadorozhnii, I. P. Pinkevich, V. Yu. Reshetnyak, and T. J. Sluckin, Mol. Cryst. Liq. Cryst. 375(1), 525–534 (2002). 381 K. I. Morozov, Phys. Rev. E 66(1), 011704, 4 (2002). 382 E. Jarkov, H. Pleiner, and H.-W. Muller, J. Chem. Phys. 118(5), 2422–2430 (2003). 383 M. Hirabayashi, Yu. Chen, and H. Ohashi, Mol. Cryst. Liq. Cryst. 401(1), 87–95 (2003). 384 B. J. Lemaire, P. Davidson, J. Ferre, J. P. Jamet, D. Petermann, P. Panine, I. Dozov, and J. P. Jolivet, Eur. Phys. J. E. 13, 291–308 (2004). 385 B. J. Lemaire, P. Davidson, D. Petermann, P. Panine, I. Dozov, D. Stoenescu, and J. P. Jolivet, Eur. Phys. J. E. 13, 309–319 (2004). 386 V. I. Zadorozhnii, I. P. Pinkevich, V. Yu. Reshetnyak, S. V. Burylov, and T. J. Sluckin, Mol. Cryst. Liq. Cryst. 409(1), 285–292 (2004). 358 359

52

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coupling between the orientations of the ferroparticles and the liquid crystal molecules then transfers the magnetic orientational effect onto the underlying E. Petrescu, C. Motoc, and C. Petrescu, Mol. Cryst. Liq. Cryst. 415(1), 197–210 (2004). B. J. Lemaire, P. Davidson, P. Panine, and J. P. Jolivet, Phys. Rev. Lett. 93(26), 267801, 4 (2004). 389 P. Kopansky, J. Potocova´, M. Koneracka´, M. Timko, J. Jadzyn, G. Czechowski, and A. M. G. Jansen, Proc. SPIE 5565, 263–269 (2004). 390 C. Scherer and A. M. Figueiredo Neto, Braz. J. Phys. 35(3A), 718–727 (2005). 391 C. Lapointe, N. Cappallo, D. H. Reich, and R. L. Leheny, J. Appl. Phys. 97(10), 10Q304, 6 (2005). 392 D. Walton, S. M. Shibli, M. L. Vega, and E. A. Oliveira, J. Magn. Magn. Mater. 292, 310–316 (2005). 393 P. R. G. Fernandes, H. Mukai, and I. M. Laczkowski, J. Magn. Magn. Mater. 289, 115–117 (2005). 394 L. J. Martinez-Miranda, K. Mccarthy, L. K. Kurihara, and A. Noel, Mol. Cryst. Liq. Cryst. 435(1), 87/[747]–93/[753] (2005). 395 V. I. Zadorozhnii, I. P. Pinkevich, V. Yu. Reshetnyak, and M. P. Allen, Mol. Cryst. Liq. Cryst. 437, 243/[1487]–250/[1494] (2005). 396 L. J. Martı´nez-Miranda, K. McCarthy, L. K. Kurihara, J. J. Harry, and A. Noel, Appl. Phys. Lett. 89 (16), 161917, 3 (2006). 397 V. I. Zadorozhnii, A. N. Vasilev, V. Yu. Reshetnyak, K. S. Thomas, and T. J. Sluckin, Europhys. Lett. 73(3), 408–414 (2006). 398 N. Tomasˇovicˇova, M. Koneracka, P. Kopcˇansky, M. Timko, V. Zavisˇova, and J. Jadzyn, Phase Transitions 79(6–7), 595–603 (2006). 399 H. M. Song, J. Ch. Kim, J.-H. Hong, Y. B. Lee, J. Choi, J. I. Lee, W. S. Kim, J. H. Kim, and N. H. Hur, Adv. Funct. Mater. 17, 2070–2076 (2007). 400 Y. Xiang, T. Li, L. Zi-Yang, and L. Jie, J. Appl. Phys. 101(3), 036109, 3 (2007). 401 V. I. Zadorozhnii, V. Yu. Reshetnyak, A. V. Kleshchonok, T. J. Sluckin, and K. S. Thomas, Mol. Cryst. Liq. Cryst. 475(1), 221–231 (2007). 402 A. N. Zakhlevnykh and D. V. Makarov, Mol. Cryst. Liq. Cryst. 475(1), 233–245 (2007). 403 M. C. Calderer, A. De Simone, D. Golovaty, and A. Panchenko, Proc. Appl. Math. Mech. 7, 1130401–1130402 (2007). 404 V. I. Zadorozhnii, T. J. Sluckin, V. Yu. Reshetnyak, and K. S. Thomas, SIAM J. Appl. Math. 68(6), 1688–1716 (2008). 405 D. V. Makarov and A. N. Zakhlevnykh, J. Magn. Magn. Mater. 320, 1312–1321 (2008). 406 N. Tomasovicova, P. Kopcansky, M. Koneracka, L. Tomco, V. Zavisova, M. Timko, N. Eber, K. Fodor-Csorba, T. Toth-Katona, A. Vajda, and J. Jadzyn, J. Phys. Condens. Matter 20, 204123, 5 (2008). 407 K. Beneut, D. Constantin, P. Davidson, A. Dessombz, and C. Chaneac, Langmuir 24(15), 8205–8209 (2008). 408 C. P. Lapointe, D. H. Reich, and R. L. Leheny, Langmuir 24(19), 11175–11181 (2008). 409 P. Kopcˇansky, N. Tomasˇovicˇova, M. Koneracka, V. Zavisˇova, M. Timko, A. Dzˇarova, A. Sˇprincova, N. Eber, K. Fodor-Csorba, T. Toth-Katona, A. Vajda, and J. Jadzyn, Phys. Rev. E 78 (1), 011702, 5 (2008). 410 P. Kopcansky, N. Tomasovicova, M. Timko, M. Koneracka, V. Zavisova, L. Tomco, and J. Jadzyn, J. Phys. Conf. Ser. 200, 072055 (2010). 411 E. Petrescu and E.-R. Bena, J. Magn. Magn. Mater. 321, 2757–2762 (2009). 412 G. Cordoyiannis, L. K. Kurihara, L. J. Martinez-Miranda, Ch. Glorieux, and J. Thoen, Phys. Rev. E 79(1), 011702, 5 (2009). 387 388

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liquid crystal matrix; (c) such a coupling results in a matrix with collective ferromagnetic behavior which occurs at a critical concentration of well-dispersed magnetic particles. In the first experiments,26–28 thermotropic nematic liquid crystals (MBBA) were utilized as a matrix to which magnetic grains (g-Fe2O3 monodomain needles, ~ 40 nm in diameter, and ~400 nm in length) were added. The magnetic needles were covered with a surfactant to prevent particle aggregation. As a result of these experimental attempts, the sensitivity of doped liquid crystals to magnetic fields was increased by factor of about 500–1000, and the predicted collective behavior was demonstrated. However, further experimental progress had to overcome the crucial problem of the time instability of such systems caused by particle aggregation.

C. Sa´tiro, Phys. Rev. E 80(4), 042701, 4 (2009). D. Constantin, P. Davidson, and C. Chan, Langmuir 26(7), 4586–4589 (2010). 415 W. Hu, H. Zhao, L. Shan, L. Song, H. Cao, Zh. Yang, Z. Cheng, Ch. Yan, S. Li, H. Yang, and L. Guo, Liq. Cryst. 37(5), 563–569 (2010). 416 E. Ouskova, O. Buluy, C. Blanc, H. Dietsch, and A. Mertelj, Mol. Cryst. Liq. Cryst. 525(1), 104–111 (2010). 417 V. I. Zadorozhnii, K. V. Bashtova, V. Yu. Reshetnyak, and T. J. Sluckin, Mol. Cryst. Liq. Cryst. 526 (1), 38–45 (2010). 418 P. Kopcansky´, A. Koval’chuk, O. Gornitska, V. Vovk, T. Koval’chuk, N. Tomasovicova´, M. Koneracka´, M. Timko, V. Za´visova´, J. Jadzyn, N. E´ber, and I. Studenyak, Phys. Procedia 9, 36–40 (2010). 419 F. R. Arantes, A. M. Figueiredo Neto, and D. R. Cornejo, Phys. Procedia 9, 2–5 (2010). 420 J. B. Rovner, C. P. Lapointe, D. H. Reich, and R. L. Leheny, Phys. Rev. Lett. 105, 22 (2010), 228301, p. 4. 421 D. V. Makarov and A. N. Zakhlevnykh, Phys. Rev. E 81(5), 051710, 9 (2010). 422 O. Buluy, S. Nepijko, V. Reshetnyak, E. Ouskova, V. Zadorozhnii, A. Leonhardt, M. Ritschel, G. Schonhense, and Yu. Reznikov, Soft Matter 7(2), 644–649 (2011). 423 J. J. Vallooran, S. Bolisetty, and R. Mezzenga, Adv. Mater. 23(34), 3932–3937 (2011). 424 B. Rozˇicˇ, M. Jagodicˇ, S. Gyergyek, M. Drofenik, S. Kralj, G. Lahajnar, Z. Jaglicˇic´, and Z. Kutnjak, Ferroelectrics 410(1), 37–41 (2010). 425 P. Kopcansky´, N. Tomasˇovicova´, M. Koneracka´, M. Timko, V. Za´visˇova´, A. Dzˇarova´, J. Jadzyn, E. Beaugnon, and X. Chaud, Int. J. Thermophys. 32, 807–817 (2011). 426 M. R. Hakobyan and R. S. Hakobyan, J. Contemp. Phys. (Armenian Acad. Sci) 46(3), 116–118 (2011). 427 A. V. Kleshchonok, V. Yu. Reshetnyak, and V. A. Tatarenko, J. Mol. Liq. (2011) 10.1016/j. physletb.2003.10.071 in press. 428 E. van den Pol, A. A. Verhoeff, A. Lupascu, M. A. Diaconeasa, P. Davidson, I. Dozov, B. W. M. Kuipers, D. M. E. Thies-Weesie, and G. J. Vroege, J. Phys. Condens. Matter 23, 194108, 10 (2011). 429 A. N. Zakhlevnykh and O. R. Semenova, Mol. Cryst. Liq. Cryst. 540(1), 219–226 (2011). 430 A. N. Zakhlevnykh and D. V. Makarov, Mol. Cryst. Liq. Cryst. 540(1), 135–144 (2011). 431 B. RozI¨icI¨, M. JagodicI¨, S. Gyergyek, M. Drofenik, S. Kralj, G. Cordoyiannis, and Z. Kutnjak, Mol. Cryst. Liq. Cryst. 545(1), 99/[1323]–104/[1328] (2011). 432 N. Podoliak, O. Buchnev, O. Buluy, G. D’Alessandro, M. Kaczmarek, Yu. Reznikov, and T. J. Sluckin, Soft Matter 7(10), 4742–4749 (2011). 413 414

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The problem of instability for thermotropic liquid crystals doped with magnetic fine particles still remains a challenge. However, advances in the field of ferrofluids (colloidal suspensions of single-domain magnetic particles, with typical dimensions of about 10 nm, dispersed in a liquid carrier390) found very successful application in the development of stable ferrolyotropic liquid crystals. Using small, spherical ferrofluid particles, Liebert and Martinet340 showed for the first time that a micellar lyotropic nematic can be oriented by using a relatively low magnetic field. Early successes in the ferrofluid-enhanced orientation of lyotropic mesophases stimulated the appearance of lyotropic ferronematics, lyotropic ferrosmectics, lyotropic cholesterics (see Tables III.5 and III.6). Experimental realization of the stable ferrolyotropic liquid crystals and the instability of the analogous thermotropic systems can be understood by comparing the structure of these systems. Magnetic nanoparticles used to dope thermotropic liquid crystals are much bigger than the size of liquid crystalline molecules. In the case of lyotropic liquid crystals, magnetic dopants are of the same size as the micelles. Moreover, the relatively rigid structure of lyotropic liquid crystals prevents magnetic dopants from aggregating. The doping of lyotropic liquid crystals (nematic, cholesteric, and lamellar phases) permits alignment of such systems by a relatively low magnetic field and allows the investigation of the physical (both static and dynamical) properties of those systems. In addition to the many works on the physics of liquid crystals doped with magnetic nanoparticles, the ability of magnetic nanoparticles to form mesophases has been studied in Refs. 384,385,388,428, and magnetic nanoparticles decorated by liquid crystals were prepared in Ref. 399, but these works are beyond the scope of our review.

a. Summary of the Main Experimental Results for Magnetic Nanoparticles in Thermotropic Liquid Crystals (Table III.5) Reference 387: 0.1% yttrium iron garnet nanoparticles were embedded into a nematic 4799 (Merck) liquid crystal. Mean diameter 30 nm, and a volume fraction of f = 10 4. Reference 391: magnetic nickel nanowires were suspended in a nematic liquid crystal 5CB: static and dynamic properties were measured. Reference 408: the dynamics of magnetic Ni nanowires in 5CB were measured. Reference 420: the Stokes drag on magnetic nanowires suspended in a nematic liquid crystal (5CB) was studied.

TABLE III.5.

Thermotropic liquid crystals Nematics MBBA

MBBA

PUBLISHED EXPERIMENTAL DATA

Magnetic nanoparticles

Observed effects

References

g-Fe2O3, needle-like (~70  500 nm), filling factor f = 1.89  10 6. Needles are coated with dimethylcatadecyl aminopropyl trimethoxysilylchloride (DMOAP). g-Fe2O3, 10 nm

Authors have shown that the variable pretilt angle of a ferronematic film can be achieved by a weak magnetic field. No threshold voltage. Quantitative measurements show that, in the low field regime, the square root of the phase retardation varies linearly with the square of the applied field.

344,346

A decrease in the threshold field is observed when the magnetic particle concentration is increased, contrary to the Burylov–Raikher theory of thermotropic ferronematics.

360

+ Magnetic nanoparticle Nematic liquid crystal

Ferronematic

(continued)

TABLE III.5 (continued) Thermotropic liquid crystals

Magnetic nanoparticles

Fe3O4, needle-like particles covered with surfactant; hematite particles b-Fe2O3, carbon nanotubes filled with a-Fe. This data clearly demonstrates that, during the past decade, scientific efforts have been directed at producing stable colloids of magnetic nanoparticles in thermotropic liquid crystals by reducing the particles’ concentration (by a few orders of magnitude). 6CHBT (4-trans-4-n- Fe3O4 particles (diameter d  10 nm, standard hexyl-cyclohexyldeviation s = 0.28) coated isothiocyanatowith oleic acid as a benzene), surfactant, volume nematic concentration 10 3–10 5. ZLI1695 (negative magnetic anisotropy of diamagnetic susceptibility wa = xII  x? is 2.55  10 8)

5CB

Observed effects

References

Magnetic nanoparticles’ concentration, wt.% Reorientation angle, deg. (magnetic field is 3 kGs) Magnetic nanoparticles’ concentration, wt.% Freedericksz transition threshold, mT Magnetic nanoparticles’ volume concentration Phase lag, rad. (magnetic field is 300 G)

0

2

3

4

5

0

15

33

38

45

0

0.15

0.21

0.46

138

130

128

122

0

2  10 5

1  10 4

2  10 4

~0

~0.05

0.2

0.3

379

416

379,416, 422,432

406,410 m n

B=0

E=0

m n

m n

B=0

E=0

Ferronematic in external crossed magnetic and electrical fields. As a rule, for pure liquid crystals reorientation effects caused by electric fields are much stronger when compared to the reorientations caused by magnetic fields. Liquid crystal colloids of magnetic nanoparticles are so sensitive to magnetic fields that these fields can even beat the effects of electric fields.

B=0

E=0

Smectics 8CB liquid crystal with the general formula CH3(CH2)7 (C6H4)2 CN 7CP5BOC 8CB 6CHBT

Cholesterics ChLC Discotics Hexa-decanoyloxyrufigallol (Aq6nlO) discotic liquid crystal

Experimental data in359 have not supported theoretical predictions made by Burylov and Raikher. However, further studies made by the same authors have resolved these contradictions—the experimental verification of Burylov–Raikher theory has been done and experimental results have been found to be in good agreement with theory.

377,389

394,396,412

Fe3O4 spherical (12 nm); rod-like (80 nm); chain-like (~500 nm).

Effects of different surfactants have been studied with 11 nm particles covered with polyethelene glycol (PEG) 3000. It has been found that the reduction of the critical magnetic field by doping thermotropic liquid crystals with magnetic nanoparticles depends on both the size and shape of the particles. The critical magnetic field reduction increases as the shape of the nanoparticles changes from spherical to chain-like and to rod-like.

Fe3O4 modified with oleic acid.

New type of reflective color M-paper based on cholesterics doped with magnetic nanoparticles was shown.

415

First report on the preparation of discotic liquid crystals doped with magnetic nanoparticles.

370

The magnetic susceptibility and heat capacity measurements show that besides the disordering effect of magnetic nanoparticles on the director field, the orientation of magnetic nanoparticles is directly coupled to the liquid crystal molecular director field. Such a coupling between the direction of liquid crystal molecules and the direction of magnetic nanoparticles allows the possibility of indirect coupling between the magnetic and ferroelectric order, thus making these mixtures candidates for soft indirect magnetoelectrics.

424,431

Magnetic Fe3O4 particles (mean diameter ~11 nm, standard deviation s = 0.28) were coated with the surfactant (oleic acid) for suppressing their aggregation. FeCo, 11 nm in size.

An isopar-M solution of Co–Fe and Ni–Zn spheroidal magnetic particles with an average particle size of 9.2 nmfor Co–Fe and 6.7 nm for Ni–Zn. Ferroelectric liquid crystals SCE9 ferroelectric Magnetite (g-Fe2O3) particles of 20 nm diameter that liquid crystal were covered with oleic acid and dispersed in toluene.

409,418,425

TABLE III.6.

Lyotropic liquid crystals

Magnetic nanoparticles

PUBLISHED EXPERIMENTAL DATA

Observed effects

Nematics and cholesterics Lyotropic systems, Ionic ferrofluid is composed Optical birefringence of lyotropic ferronematic as a potassium laurate function of magnetic field and time was measured. of grains of (g-Fe2O3) with typical dimensions (LK) decanol (De Experimental results (characteristic times, rotational of 10 nm (electrically OH)—water and viscosity) give insight to the dynamics of such systems. charged) and dispersed in potassium water. laurate/ decilamonium chloride (DaC1)—water, doped with ionic ferrofluids. Experimental measurements and theoretical calculations Lyotropic micellar Magnetic nanoparticles were done for the ferrodiscotic phase. The obtained results system: ((g-Fe2O3) ~ 6 nm) up to 1 vol.%. indicate a decrease of magnetic Freedericksz transition by potassium two orders of magnitude for magnetic liquid crystal colloids laurate/1-decanol/ as compared to the same value of undoped liquid crystal. water (PLDW). A quaternary Microspheres of Reorientation of doped lamellar phases by relatively low mixture, ferromagnetic material magnetic field (~20 G) was shown. Such a reorientation consisting of oil (g-Fe2O3) are dispersed is strongly anisotropic: nothing happens when the magnetic (cyclohexane), in oil (cyclohexane) and field is parallel to the lamellae, and the reorientations are water, surfactant stabilized by adsorbed observed only for the perpendicular direction of the magnetic field. (sodium dodecyl organophosphorated sulfate), and molecules. The average cosurfactant diameter of the particles (pentanol). is on the order of 10 nm. The particle volume fraction of this ferrofluid may be varied from 0.1% to 10%.

References 354

362

349

Smectics The lyotropic Microspheres of system is a ferromagnetic material quaternary one, (g-Fe2O3). The particle volume fraction is from composed of 0.1% to 6%. water, cyclohexane, SDS (sodium dodecyle sulfate) as surfactant, and 1-pentanol as cosurfactant. The lamellar phase The colloid is a dispersion consists of a fourin cyclohexane of iron component oxide particles (g-Fe2O3), stabilized against Van der solution where Waals attractions by inverse surfaction. Particle size is bilayers—sodium 3.3 nm. The ferrofluid is dodecyl sulfate stable up to a volume (SDS, surfactant), fraction in magnetic 1-pentanol material of 10%. (cosurfactant) and water—are swollen with cyclohexane.

Volume fraction, % 0.15 1.5 6

˚ The oil-layer thickness, A 110 160 Aggregates Aggregates Aggregates Aggregates Phase Phase separation separation

351 200 Aggregates Stable Phase separation

250 Aggregates Stable Phase separation

The quasi-elastic light scattering experiments demonstrate the anisotropy of the Brownian diffusion 352,352a of submicron particles in ferrosmectic phases. More precisely, it was found that the diffusion coefficient perpendicular to the layers is equal to zero within the experimental accuracy, while the diffusion in the plane of the layers is reduced, but not drastically, compared to the isotropic case. We conclude that the surfactant membranes efficiently confine the particles in the quasitwo-dimensional space between them, whereas their presence just slightly slows down the Brownian diffusion in the layers.

(continued)

TABLE III.6 (continued) Lyotropic liquid crystals

Magnetic nanoparticles

The lamellar phase The colloid is a dispersion consists of a fourin cyclohexane of iron component oxide particles (g-Fe2O3), stabilized against van der solution where Waals attractions by inverse surfaction. Particle size is bilayers— 3.3 nm. The ferrofluid is sodiumdo stable up to a volume decylsulfate fraction in magnetic (SDS, surfactant), material 10%. 1-pentanol (cosurfactant) and water—are swollen with cyclohexane

Observed effects

References 357,357a

d

Pronounced anisotropy of the Brownian diffusion of the magnetic nanoparticles in ferrosmectic phases has been studied by the method of the quasi-elastic light scattering. It has been shown that the diffusion coefficient perpendicular to the smectic layers is equal to zero (within the experimental accuracy), and magnetic nanoparticles (confined within quasi-two-dimensional space) can move only along the smectic layers. Two types of In the ferrosmectic, some of magnetic liquid the particles are adsorbed crystals, on the lamellae and ferrosmectics and exhibit a small ferrohexagonal anisometry. Their phases. The moments are former is built preferentially oriented in from a lyotropic the layer plane and have lamellar phase in an obviously slower which the oil rotation dynamics than solvent was the main population of replaced by a free particles.

369

ferrofluid, which is a suspension of maghemite (g-Fe2O3) nanoparticles in cyclohexane.

Dilute lamellar phase of the nonionic surfactant C12EO5 (the matrix is the C12EO5/hexanol/ H2O system).

Linear birefringence dynamic measurements on ferrohexagonal phases confirm that no particles are adsorbed to the micellar cylinders. Experimental results can be accounted for by a model where the orientation of the magnetic particles is limited to a cone around the axis of the cylinders.

Schematic representation of particles adsorbed on the lamellae with their moment oriented along the smectic plane.

jmax

Schematic representation of particles restricted in a cone of angle ’max around the axis of the micellar cylinders. Goethite (iron oxide, A nonionic lamellar phase doped with large magnetic nanorods (a-FeOOH)) nanorods up (in comparison with the interlayer distance) was formulated. to a fraction of 5 vol.%. The inclusions experience an attractive interaction under confinement, a feature absent in simple aqueous solutions of similar concentration or in systems of confined silica spheres. Under even higher confinement (membrane concentration), the nanorods aggregate.

407,414

62

Y. A. GARBOVSKIY AND A. V. GLUSHCHENKO

b. Summary of the Main Experimental Results for Magnetic Nanoparticles in Lyotropic Liquid Crystals (Table III.6) Reference 349: has a couple of interesting results: (1) In magnetic lyotropic phases, the existence of lamellar phases, which may be swollen by oil while keeping constant the thickness of the water layer, in a wide range of sizes from 4 to 40 nm. (2) In a ferronematic, the existence of two isotropic phases, adjacent to the lamellar domain; one of them is a ‘‘sponge’’ phase, characterized by a bicontinuous topology; the other one is a water-in-oil microemulsion. It is important to emphasize that one may go from the sponge phase to the lamellar one and, ultimately, to the microemulsion just by increasing the cosurfactant (alcohol) content. Reference 359: a brief overview of the most important results in the field of lyotropic ferronematics and ferrocholesterics up to the year 1995. Reference 361: measurements of the elastic constants of lyotropic ferronematics. Reference 363: synthesis of lyotropic mixtures of potassium laurate, 1-decanol, and water, and the phase diagram of this ternary system is investigated in the vicinity of the nematic region. Reference 368: the dynamical behavior of lyotropic nematic liquid crystals doped with ionic and surfacted magnetic fluids—ferronematics—is studied using a linear optical technique. The response of these mesophases to a combination of a static and a pulsed magnetic field is investigated by measuring the relaxation times as a function of the pulse width. A reversible modification of the magnetic grain concentration in the bulk of the samples and a secondary aggregation process due to the presence of a field gradient introduced by the pulsed field is discussed. Reference 374: the properties of lyotropic ferronematic liquid crystals based on new Ni, Cu, and Zn ionic magnetic fluids are discussed. References 392,393: the lyotropic liquid crystals employed are a mixture of potassium laurate/1-decanol/water(KL/DeOH/H2O), with concentrations in weight percent of 28.90/7.10/64.0, respectively. The ferrofluid added to this material was purchased from Ferrotec Corporation and consisted of nanoparticles of magnetite (Fe3O4) suspended in water. The concentration of ferrofluid added to the liquid crystal was about 1013 magnetic particles/cm3. The size distribution of the magnetic particles, determined magnetically, obeyed a log-normal distribution with a mean size of 13 nm and a full-width at half-maximum of 8 nm. Magneto-optical properties were studied in Ref. 393.

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c. Theory of Liquid Crystals Doped with Magnetic Nanoparticles Since the first theoretical work on ferroliquid crystals (colloids of magnetic particles in liquid crystals), there have been substantial developments in this field.348,350,353,355,358,364–367,371–373,375,376,378,380–383,386,395,397,401–405,411, 413,417,421,426,427,429,430 The classical continuum theory developed by Brochard and de Gennes was modified by Raikher, Burylov, and Zakhlevnykh in order to take into account the finiteness of anchoring of the nematic molecules on the particle surfaces. According to their calculations (based on the assumption of weak anchoring at the magnetic surface particles), the Freedericksz transition in such systems has a zero threshold. It means that ferronematics can be notably affected even by small magnetic fields, and the magnetic field of the Earth should be taken into consideration while performing experimental measurements. Advances in the theory of thermotropic ferronematics before 1995 were summarized by Burylov and Raikher in their review articles.356,356,a,b At first, they considered the effect of a single solid anisometric particle inside a uniform nematic. For the case of finite anchoring energy, they calculated the distortion energy. Then the orientational interactions of the particle assembly with the nematic matrix were analyzed. Based on the assumption of weak anchoring, the following results were obtained: (1) magnetization states of ferronematics; (2) free energy of ferronematics; (3) bonding equation and segregation effects in a ferronematic; (4) diamagnetic Freedericksz transition in a strong bias field; (5) magnetic field-induced birefringence; (6) a formula for the optical phase lag; (7) Freedericksz transition in the electric field; (8) birefringence of a ferronematic in the crossed magnetic and electric fields; and (9) the orientation and concentration distributions in the ferronematic cell. By considering a number of particular cases encountered in the experiment, Burylov and Raikher came to the conclusion that their theoretical model is capable of providing a noncontradictory interpretation of the existing experimental data. Additionally, they noticed that there is a great need to evaluate the material parameters of ferronematics (effective Frank constant, the amplitude of the anchoring energy, the true concentration of single-domain magnetic grains) using properly defined methods. Further development of the theory of ferronematics can be found in the works of Burylov, Zadorozhnii, Pinkevich, Reshetnyak, and Sluckin. One of the possible ways to make ferronematics stable is to reduce the concentration of magnetic nanoparticles. For this reason, ultrasmall concentrations (~10 5) of anisometric ferromagnetic nanoparticles in liquid crystals have been studied both theoretically and experimentally.432 Zadorozhnii et al. showed that the magneto-optics of liquid crystals is strongly affected by such ultrasmall concentrations of magnetic nanoparticles; the reorientation of nanoparticles by a magnetic field results in a nonthreshold reorientation of liquid crystals, and the monitoring of the magnetic fields of about 10 mT is easily achieved.

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Recent advances in the theory of ferronematics can be found in review paper 404. Other important topics include magnetically induced bistability,380 adsorption phenomena in ferronematics,386 laser-induced optical nonlinearities in ferronematics (both theory and experiment387), and the inverse Freedericksz effect.401 One of the most intriguing predictions is related to the inverse Freedericksz effect when the nematic reorientation angle is reduced and then disappeared continuously at a critical magnetic field.401 Monte-Carlo simulations of ferronematics were reported in Ref. 395. These simulations demonstrated that, depending on the nematic-particle anchoring and magnetic field strengths, the ferronematic system can display unexpected physical properties. For intermediate anchoring strengths, a three-dimensional reorientation of the ferroparticles and the nematic director were observed. In moderate magnetic fields, the nematic director can decouple from the ferroparticles. We would like to mention two review papers where the theory of thermotropic ferronematics is well developed and described.356,356a,b,404 However, so far, no essential progress in the development of stable thermotropic ferronematics has been made and this direction is still progressing. A theoretical treatment of lyotropic liquid crystals (both pure and doped with magnetic nanoparticles) can be found in a monograph.445 7. O RGANIC N ANOPARTICLES In addition to inorganic nanoparticles, recently, nanoparticles made of organic materials have been synthesized and embedded in a liquid crystal host.433–443 This section will give a short review of some of the most interesting results in this area. Ch.-W. Kuo, Sh.-Ch. Jeng, H.-L. Wang, and Ch.-Ch. Liao, Appl. Phys. Lett. 91(14), 141103, 3 (2007). A. Kumar, J. Prakash, P. Goel, T. Khan, S. K. Dhawan, P. Silotia, and A. M. Biradar, Europhys. Lett. 88, 26003, 6 (2009). 435 Sh.-J. Hwang, Sh.-Ch. Jeng, Ch.-Yu. Yang, Ch.-W. Kuo, and Ch.-Ch. Liao, J. Phys. D Appl. Phys. 42, 025102, p. 6 (2009). 436 Sh.-Ch. Jeng, Sh.-J. Hwang, and Ch.-Yu. Yang, Opt. Lett. 34(4), 455–457 (2009). 437 W.-Zh. Chen, Y.-T. Tsai, and T.-H. Lin, Opt. Lett. 34(17), 2545–2547 (2009). 438 Sh. Ghosh, P. Nayek, S. Kr, R. Roy, M. R. Gangopadhyay, and R. D. Molla, Appl. Phys. Lett. 96(7), 073101, 3 (2010). 439 A. Kumar, P. Silotia, and A. M. Biradar, J. Appl. Phys. 108(2), 024107, 7 (2010). 440 Sh.-Ch. Jeng, Sh.-J. Hwang, Yu.-H. Hung, and Sh.-Ch. Chen, Opt. Express 18(21), 22572–22577 (2010). 441 Sh.-J. Hwang, Sh.-Ch. Jeng, and I.-M. Hsieh, Opt. Express 18(16), 16507–16512 (2010). 442 S. Ghosh, P. Nayek, S. K. Roy, R. Gangopadhyay, M. Rahaman Molla, and T. P. Majumder, Eur. Phys. J. E 34(4), 35, 6 (2011). 443 A. Y.-G. Fuh, Ch.-Yu. Huang, Ch.K. Liu, Yu.-Di. Chen, and Ko.-Ti. Cheng, Opt. Express 19(12), 11825–11831 (2011). 433 434

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H N R

Si

NH2

O Si

Si

O R

65

O

O O

R O

Si

CH3 R=

O O

Si

O Si

R

Si CH2

CH3

R

Si

O R

R

FIG. III.7. The structure of polyhedral oligomeric silsesquioxanes (POSS) nanoparticles.435

A new method to align liquid crystals vertically (nanoparticle-induced vertical alignment (NIVA)) was proposed in Refs. 433–443. Polyhedral oligomeric silsesquioxane (POSS) nanoparticles were dispersed in nematics,433,435–437 cholesterics,440 and ferroelectric liquid crystals434,439 (Figure III.7). By varying the POSS concentration from 0% to 2% (by weight), the pretilt angle can be tuned from 0
to 90
.436 Nanotubes made of conducting polymer PEDOT added to nematic liquid crystals 5CB have reduced the threshold voltage (from 1.38 to 0.82 V), reduced the effective elastic constant (from 18.01 to 5.21 pN), and reduced the dielectric anisotropy (from 9.28 to 8.64) as compared to the same values of pure liquid crystal.438 The same nanoparticles embedded in ferroelectric liquid crystals442 affect the spontaneous polarization, tilt angle, rotational viscosity, switching times, and dielectric permittivity. Summarizing, POSS and PEDOT nanoparticles can improve the performance of liquid crystal devices. IV. Applications of Liquid Crystalline Nanocolloids

8. D ISPLAY A PPLICATIONS The development of LCDs still continues. Liquid crystals are known to the general public primarily as one of the typical display methods. According to Ref. 1, in 2007, LCD televisions (TV) surpassed cathode ray tube TV units in worldwide sales for the first time, and in 2008, LCD TVs became the majority, with a 50% market share of the 200 million TVs forecast to be shipped globally. Historically, the idea of doping liquid crystals with various particles of different sizes (from molecules to nano- and microparticles) has been boosted by the growing demands

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of the display market. Nanoparticles in liquid crystals can benefit display development in many ways which can be classified into two large groups: (1) Development of new display modes and (2) Performance improvement of already existing display modes by modifying the basic components of a LCD (properties of liquid crystals and alignment layers).

a. Display Modes Filled nematics (nematic liquid crystals doped with silica nanoparticles, inorganic clay nanoparticles, ternary systems liquid crystal–polymer–nanoparticles) have found application in the development of bistable scattering-type displays.31 The proposed display prototypes are polarizer-free and can be operated thermally, ultrasonically, electrically, optically, and even electrophoretically. The intensive light scattering of filled nematics (field OFF state) can be switched to a transparent state by applying an electric field (field ON state). A specific property of filled nematics, the so-called memory effect mentioned before, is one of the most interesting features. It allows for the realization of multistable and dynamical modes of displays.31,50,57,64–67 Nematic liquid crystals doped with metallic nanoparticles (Pd) exhibit frequency-modulated electro-optical response215,219,228,229: the transmittance of a twisted nematic cell depends upon the frequency f of the applied electric field. Thus, such a cell can be switched between a transparent and dark state by changing f. In addition, standard switching by an external voltage can also be used. Electronic paper and flexible displays are expected to be among the most strongly demanded directions of modern display technologies. Liquid crystals doped with nanoparticles form the material base for such applications. For example, a new type of reflective color M-paper with magnetically controllable characteristics based on cholesteric thermotropic liquid crystals doped with magnetic nanoparticles was shown in Ref. 415. Recent advances both in theoretical understanding and in the preparation of the stable liquid crystal colloids of magnetic nanoparticles allow us to anticipate the emergence of magneto-optical LCDs.

b. Alignment Controllable and uniform alignment of liquid crystals is the key factor in LCD manufacturing. Recent experimental data obtained in different labs throughout the world suggest that certain types of nanoparticles (silica nanoparticles,64 TiO2

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nanoparticles,95 metallic nanoparticles,240,444 CNTs,124,146 and organic POSS nanoparticles433–437,440,441,443 can strongly affect the alignment of thermotropic liquid crystals, thus improving the performance of liquid crystal devices. NIVA of nematic and ferroelectric liquid crystals were reported in Ref. 433. This method is suitable for fabricating a flexible display due to the reduction of the temperature, necessary to process the alignment layers on polymer films for flexible displays.434 In addition, it has been found that nanoparticles doped into the alignment layers can tune the pretilt angle of liquid crystals441 and induce bulk alignment of liquid crystals.124 Multiple alignment modes for nematic liquid crystals doped with gold nanoparticles, highly potential for display applications, were reported in Ref. 444. Since the LCD industry utilizes thermotropic liquid crystals, so far we have not discussed to the possibility of aligning lyotropic liquid crystals by means of nanoparticles. Doping lyotropic liquid crystals with magnetic nanoparticles (in fact, by using ferrofluids, the dispersion of magnetic nanoparticles in a liquid carrier) in order to align them has become a standard experimental procedure in research laboratories since the early 1980s.445 Strong magnetic fields (> 1 T) are needed to align pure lyotropic liquid crystals, but lyotropics doped with magnetic ferrofluids can be aligned by relatively small magnetic fields (~ 100 mT). Macroscopic alignment of lyotropic liquid crystals by magnetic nanoparticles has been reported in numerous papers.423

c. Modification of Physical Properties During the past decade, a nonsynthetic approach to modify the properties of liquid crystals by dispersing low concentrations of nanoparticles has become one of the most rapidly growing directions in modern liquid crystal science. Papers published during this time clearly demonstrate the potential of this method to improve the general performance of liquid crystal devices. Important properties (the dielectric constants, the birefringence, elastic constants, viscosity, the threshold voltage, switching times, contrast ratio, and the temperatures of phase transitions) of various liquid crystals can be tailored just by changing the concentration and type of nanoparticles. Since these effects have been presented in previous chapters as well as in review,201 we limit ourselves to discussing another interesting property of nanoparticles in thermotropic liquid crystals— their ability to trap ions, thus allowing purification of a liquid crystal host. As has H. Qi and T. Hegmann, Appl. Mater. Interfaces 1(8), 1731–1738 (2009). A. M. Figueiredo Neto and S. R. A. Salinas, The Physics of Lyotropic Liquid Crystals: Phase Transitions and Structural Properties (Monographs on the Physics & Chemistry of Materials), Oxford University Press, USA (2005).

444 445

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been shown, different nanoparticles (diamond,94,94a,209–211 tin-doped indium oxide,97,446 alumina,98 zinc oxide,102,446 Si3N4,446 montmorillonite,447 zirconium dioxide448) can trap ions, allowing the purification of liquid crystals. Purification phenomenon have been suggested to be due to the guest dopants (nanoparticles) whose interaction with ion impurities decreases the concentration of the free ionic impurities and eliminates the so-called screening effect in liquid crystals. It is interesting to note that the suppression of field screening in nematic liquid crystals by CNTs has been reported in Ref. 131, but liquid crystals doped with CNTs have a low value of the voltage holding ratio (~ 37% for nematics doped with CNTs vs. ~ 97% for the same nematics doped with diamond nanoparticles).209 9. N ONDISPLAY A PPLICATIONS In addition to the well-known display applications,1 liquid crystals doped with nanoparticles have a number of nondisplay applications. Some of these nondisplay applications already have a long history (polarization and adaptive optics, holography and pattern recognition, and tunable lasers), but many need to be reinvestigated (biophysics and medicine) or are still in the beginning of their development (photonic crystals and metamaterials, photovoltaics 2–4). The fluid origin of liquid crystals allows them to be used as tunable fillers for photonic crystals and metamaterials. The long-range orientational order and tunability of liquid crystals enable both polarization and intensity control of the light passed through or emitted by such an anisotropic system, thus giving rise to the development of novel liquid crystal-based nanomaterials for optoelectronics, photonics, and optical communications. The similarity between the structure of liquid crystals and biological living cells boosts the development of various application soft liquid crystals doped with nanoparticles in biophysics and medicine. In fact, nanoparticles in liquid crystals can mimic some important biological processes.2,2a,b,4

a. Optoelectronics and Photonics Zero, one, and two-dimensional semiconductor nanostructures (quantum dots, quantum rods, and quantum disks) exhibit unique thermal, electric, optical, and

W. T. Chen, P. Sh. Chen, and Ch.Yu. Chao, Jpn. J. Appl. Phys. 48, 015006, 5 (2009). H. H. Liu and W. Lee, Appl. Phys. Lett. 97(2), 023510, 3 (2010). 448 H.-J. Kim, Y.-G. Kang, H.-G. Park, K.-M. Lee, S. Yang, H.-Y. Jung, and D.-Sh. Seo, Liq. Cryst. 38 (7), 871–875 (2011). 446 447

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mechanical properties which have potential applications in novel solar cell architectures, nanolasers, communication, optoelectronics, and biological science. Once synthesized, semiconductor nanostructures could be integrated into optoelectronic devices allowing further practical use of the unique properties of such nanomaterials. A classical scheme of a ‘‘guest (semiconductor nanoparticle)–host (liquid crystals)’’ has been applied to develop novel tunable materials for optoelectronics. Nematic liquid crystals doped with semiconductor nanorods were studied in Refs. 105–107,117. Optoelectronic devices which allow manipulation of the polarization of the emission from semiconductor nanorods by an external bias have been proposed in Refs. 105,106. This device consists of a composite of liquid crystals and semiconductor nanorods (CdS in Ref. 105 and CdSe/ZnS in Ref. 106). The ability to control the polarization of the emission from an optoelectronic device is very important because it offers an excellent opportunity to manipulate the communication of information and images. The nematic liquid crystal host provides a unidirectional alignment of semiconductor nanorods, which can be tuned by external field. Cholesteric liquid crystals used as a host matrix, instead of nematics, bring additional opportunities to manipulate circularly polarized light.117 This general approach opens new possibilities for semiconductor nanoparticles in smart optoelectronic applications, including electochromatic gadgets, optical switches, and integrated photonic devices, in the near future. Liquid crystals exhibit colossal optical nonlinearities and doping them can significantly enhance the nonlinear-optical response.23 For example, the nonlinear-optical response of liquid crystal colloids of ferroelectric nanoparticles is enhanced by a factor of about 5 (see Table III.4), and such systems can be used for all-optical switching.

b. Photonic Crystals and Metamaterials The world of liquid crystals is very rich and offers phases with different levels of molecular ordering: from the simplest nematics (only long-range orientational ordering) to phases which are spatially periodic in one (smectics and cholesterics), two (columnar phases), and three (blue phases and cubic phases) dimensions. Since the period of such spatial ordering is often in the range of tens of nanometers to a few micrometers, these liquid crystalline materials are promising candidates for developing photonic crystals and metamaterials. A photonic crystal is a periodic structure that modulates the refractive index at the scale of the wavelength of light.449 This periodic structure is designed to form

449 J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. Princeton University Press, Princeton, New Jersey (2008).

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so-called ‘‘photonic bandgaps’’ (i.e., a range of wave vectors for which light cannot propagate). To provide the full bandgap in all directions, a photonic crystal should be structured in three dimensions, which represents a challenge. Additionally, a photonic crystal often needs to have structural defects, such as points to trap light or dislocations to guide light. Liquid crystals, due to their phase variety, seem to be perfect materials from which to develop photonic crystals.450 Cholesteric liquid crystals are an example of one-dimensional photonic crystals. Experimental results indicate that a small amount of nanoparticles added to cholesteric liquid crystals can tune the basic parameters of these onedimensional photonic crystals, thus allowing extra control of such systems. For example, semiconductor nanoparticles change the helical pitch P of cholesterics, thus shifting the spectral position lmax of the band gap. At the same time, a cholesteric liquid crystal host allows for the control of the polarization properties of the light emitted in this system.117 Liquid crystal blue phases are an example of self-assembled three-dimensional photonic crystals with a period of about 100 nm. So far, the most crucial parameters limiting the technological potential of the blue phases are the relatively high temperature and very narrow temperature range at which they appear. Different methods have been used to widen the temperature range of blue phases as well as to shift them toward room temperature. Doping blue phase materials with nanoparticles (metallic particles such as gold451 and semiconductor nanoparticles such as CdSe115) have proved to be a promising experimental technique allowing both a stabilization and a widening (up to 20 K for liquid crystal CE6) of the blue phases temperature range. In both papers, the same physical mechanism of stabilizing the cholesteric blue phases has been suggested: the nanoparticles accumulate in the lattice disclinations, stabilizing the overall cholesteric blue structure. An elementary ‘‘meta-atom’’ of metamaterials is an artificial combination of dielectric and metal elements, such as split-ring resonators or paired metal nanorods with a typical size of 10–100 nm.450,452 There are two serious challenges to overcome: (1) manufacturing of metamaterials on a relatively large scale (because of submicron periodicity such materials are hard to manufacture) and (2) making the metamaterials to be reconfigurable/switchable. Liquid crystals, doped with metallic nanoparticles, can satisfy both of these demands. They can be aligned over a large scale, and liquid crystals are entirely O. D. Lavrentovich, Proc. Natl. Acad. Sci. USA 108(13), 5114–5144 (2011). H. Yoshida, Yu. Tanaka, K. Kawamoto, H. Kubo, T. Tsuda, A. Fujii, S. Kuwabata, H. Kikuchi, and M. Ozaki, Appl. Phys. Express 2, 121501, 3 (2009). 452 F. Capolino (ed.), (2009). Metamaterials Handbook—Two Volume Slipcase Set: Theory and Phenomena of Metamaterials, CRC Press, Boca Raton, FL (2009). 450 451

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tunable materials—their properties can be tuned by means of external physical fields (magnetic, electric, temperature, pressure, etc.). Numerous recent reports on the subject are reviewed in Refs. 267–269 and prove the promising opportunities of liquid crystals doped with metallic nanoparticles for metamaterials development.

c. Biophysics The structure of cell walls (i.e., membranes) in living creatures is similar to smectic lyotropic liquid crystals. Therefore, this type of liquid crystal can be a model system to study interactions between nanoparticles and membranes, how membranes behave when nanoparticles parameters are changed. Hierarchical transport of nanoparticles in a lyotropic lamellar phase has been reported in Ref. 453 (more particularly, a hierarchical structure is found in the dynamics of the particles). This kind of hierarchical dynamics is ubiquitous in soft complex fluids and living systems, and the experimental methods used in this work are powerful tools to investigate these systems. The liquid crystal-based system is often easier to observe visually than that of a cell membrane. As a result, they can be used to study the biophysical interactions between protein-coated gold nanoparticles and liquid crystals as a model for cell membrane interactions.248

d. Photovoltaics At this point, the vast majority of the solar cells are fabricated from inorganic semiconductors, typically from polycrystalline silicon. The production technologies of silicon-based solar panels are energy consuming; the panels are heavy and often very expensive. An alternative type of solar cells are organic thin-film solar cells, production of which is potentially much cheaper due to the possibility of applying an efficient roll-to-roll-technology which has been well developed for fabrication of e-paper, flexible LCDs, and organic electronics. The organic solar films are very light and can be easily utilized at curved surfaces which seriously extends the range of the solar panel applications. The organic solar cells consist of two electrode substrates with a heterojunction of organic molecules in between. Absorption of light produces neutral excitons localized at the organic molecules. The excitons diffuse to the heterojunction region where they are separated into holes and electrons resulting in the

Y. Kimura, T. Mori, A. Yamamoto, and D. Mizuno, J. Phys. Condens. Matter 17, S2937–S2942 (2005).

453

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photovoltaic effect. The efficiency of the charge separation increases with the charge carriers’ mobility and a longer diffusion length for the excitons. Until the beginning of twenty-first century, the photovoltaic effect in organic materials did not look very exciting due to the low mobility of charge carriers and strong trapping of the excitons before their separation in the heterojunction. This resulted in rather small external quantum efficiency (EQE) and energy conversion (< 1%). The situation changed drastically after the publication of SchmidtMende et al., 454 who used a discotic liquid crystal (DLC) as a hole-transport material to make an organic solar film. The disk-shape molecules of DLC are stacked one on top of another and form molecular columns in a mesophase (columnar phase), along which the charge transfer is very effective. Actually, the molecular columns work as perfect one-dimensional conductors with a very high mobility of charge carriers, that is, m = 1 cm2 V 1 s 1 and extremely long excitons diffusion length Lc > 70 nm. It leads to the excellent transport properties of DLC and effective charge separation at the heterojunction. Schmidt-Mende et al. demonstrated the photovoltaic device having EQE of 34% and a power efficiency up to 2% in a cell with a heterojunction formed by DLC hexadodecylphenylhexabenzocoroneno (HBC) as a hole transporter and a crystalline dye of a perylene as an electron acceptor.454 The distinctive feature of this cell was a one-step fabrication technology; the spin-coating of the ingredients in a chloroform solution resulted in vertical separation of HBC and the dye and producing of the heterojunction with a highly developed surface. These results and their utilization for solar elements clearly show the great potential of DLCs as the basic materials for thin organic solar films. The horizons of mass-production technology development and wide applications would be even clearer if well-oriented DLCs were used in the films, since this should essentially increase both the mobility of charges and the diffusion exciton length. An additional improvement of the operation characteristics can be achieved by introducing nanoparticles to the DLC and heterojunction region of the solar cells. It is important that nanoparticles do not deteriorate the macroscopic alignment of DLCs. Recently, liquid crystals doped with ZnO nanoparticles have been studied in terms of their photovoltaic properties.101 Since this field is still in the beginning of its development, we just can make a general remark on the promising potential to modify DLCs for photovoltaics by doping them with nanoparticles.

454

L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons, R. H. Friend, and J. D. MacKenzie, Science 293, 1119-1122 (2001).

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V. Conclusions

Liquid crystals doped with nanoparticles fall in the following categories: 1. Diluted suspensions when nanoparticles do not ‘‘feel’’ each other but affect interactions between mesogenic molecules. These systems are homogeneous and macroscopically behave as anisotropic fluids with modified physical properties. Changes in the physical properties of such colloids strongly depend on the nature of both the nanoparticles and the liquid crystals. ‘‘Active’’ nanoparticles (with a permanent electric and/or magnetic dipole) enhance interactions between mesogenic molecules causing the increase of the liquid crystals order parameter. In addition, both size and shape of nanoparticles do matter: spherical nanoparticles can decrease the order parameter while elongated particles fit into liquid crystalline ordering and promote long-range molecular interactions. So far, no unified theory has been developed to describe all types of liquid crystal diluted colloids because of their intrinsic complexity and variety. However, for ferroelectric and ferromagnetic nanoparticles embedded in nematic liquid crystals promising theoretical calculations which are in agreement with experiments have been done. 2. In highly concentrated liquid crystal colloids of nanoparticles, embedded nanoparticles interact both directly and through the liquid crystal. In this case, different phenomena can take place—from simple aggregation and phase separation to self-assembly and self-organization. The resulting systems can be heterogeneous or highly structured at the nanoscale level. There are no well-defined borders between these two categories of liquid crystal colloids. In fact, in many cases, initially homogeneous colloids are unstable over time, or can be transformed into homogeneous ones by applying external fields (electric, magnetic, temperature, pressure, etc.). There are still many challenges to be overcome—both in experiment and in the theory of liquid crystal colloids of nanoparticles. Nevertheless, it is clear that this area of research is one of the hottest topics of modern soft condensed matter. We will not be far from the truth in saying that this is probably the most outstanding areas of science where nanoparticles have already proved their power. Acknowledgments

The authors are very grateful to all coauthors of the papers related to the topic of liquid crystalline colloids of nanoparticles. We would like to extend special thanks to Yuri Reznikov, Victor Reshetnyak, and Oleg Yaroshchuk who

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introduced A. G. to the world of nanoparticles in the early 1990s and with whom a great number of wonderful papers have been published on all types of nanoparticles. We are grateful to Dean Evans and his group (Gary Cook and Serguey Basun) for many years of fruitful collaboration and support. Special thanks to John West for many years of mentorship, friendship, and joint work related to the topic of this review. A. G. would like to mention Chae Il Cheon for introducing the authors to the first nanoparticle production techniques. All the people mentioned here are among the founders of ‘‘nanoparticles in liquid crystals’’ topic, and without their ideas and work, this review would not be possible. Special thanks to Robert Camley for his encouragement to write this review and to Caroline Camley for her patience and careful proofreading of the manuscript. A great portion of the work described in this review has been supported by the grants from the NSF: #1010508 ‘‘STTR Phase I: Design, Fabrication and Characterization of Ferroelectric Nanoparticle Doped Liquid Crystal/Polymer Composites’’ and #1102332 ‘‘Liquid Crystal Signal Processing Devices for Microwave and Millimeter Wave Operation.’’ The authors are also grateful to the Air Force Research Lab for many years of financial support for the development of nanoparticles.