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Densely packed organic nanocrystals ultrathin film using a liquid–liquid interface
Synthetic Metals 159 (2009) 847–850
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Densely packed organic nanocrystals ultrathin film using a liquid–liquid interface J. Matsui a,b,∗ , T. Shibata a , K. Yamamoto a , T. Yokoyama a , A. Masuhara a , H. Kasai a,b , H. Oikawa a , T. Miyashita a a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi 332-001, Japan b
a r t i c l e
i n f o
Article history: Received 15 November 2008 Received in revised form 14 January 2009 Accepted 14 January 2009 Available online 12 February 2009 Keywords: Self-assembly Polydiacetylene nanocrystal Liquid–liquid interface
a b s t r a c t We report a fabrication of densely packed polydiacetylene nanocrystal film using a liquid–liquid interface. Polydiacetylene (pDCHD) nanocrystals was dispersed into water solution by reprecipitation method from a good solvent. Then, hexane was added to the water dispersion to create the liquid–liquid interface. pDCHD nanocrystals were assembled at the interface, when ethanol was added to the pDCHD water dispersion–hexane solution. The assembled film was transferred onto a solid substrate and the film morphology was observed by scanning electron microscope (SEM). With addition of 10 vol% of ethanol to 2.5 mM of pDCHD water dispersion–hexane solution, a densely packed pDCHD film is fabricated. Optical and electrical properties of the pDCHD film are discussed by UV–vis spectroscopy and current–voltage measurement. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the past decade, research for organic materials, that are applicable to optical and electronic devices have aroused considerable interest because of the unique advantages provided by organic materials [1]. These advantages include low fabrication cost, high flexibility and versatility of the chemical structure. In this context, there are many reports for constructing organic active devices such as organic field-effect transistor [2], light emitting diodes [3], etc. Recently, organic crystals have attached much interest because of their high carrier mobility [4]. Usually, preparation of organic crystals is a time consuming process, therefore efficient and large-scale fabrication of organic devices using organic crystals is difficult. On the other hand, organic nanocrystals with several crystal structures can be fabricated using the “reprecipitation method” [5], which is a simple and efficient method. Application of the nanocrystals to organic electronic devices requires tailor-made assembly of the nanocrystals. Several techniques such as layer-by-layer technique [6], and electropholic deposition [7] have been used to assemble organic nanocrystals. However, these techniques require polymer electrolyte or are only applicable to electrodes. A more versatile technique for assembling organic
∗ Corresponding author at: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 217 5639; fax: +81 22 217 5639. E-mail address: jun [email protected] (J. Matsui). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.01.025
nanocrystal should be developed for its real device application. Recently, liquid–liquid interface have applied to assemble inorganic nanoparticles [8,9]. We have reported that not only a spherical nanoparticle but also a tubular nanomaterials such as carbon nanotubes can be assembled using the liquid–liquid interface [10–12]. In this paper, we applied the liquid–liquid interfacial assembling technique to prepare a densely packed monoparticle film of a polydiacetylene nanocrystal. We observed that the nanocrystals were self-assembled at the interface with controlling the surface energy of polydiacetylene nanocrystal by adding ethanol. The assembly formed at the liquid–liquid interface was transferred onto a solid substrate by immersing the substrate into the interface. The transferred film was observed using scanning electron microscope (SEM). Moreover, the optical and electrical properties of the film were observed by UV–vis spectra and I–V characteristics measurements.
2. Experimental section Nanocrystals of poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (pDCHD) with square-shape (∼30 nm in width and length) and wire-shape (∼30 nm in width and 0.5–1 m in length) were fabricated by reprecipitation method as reported previously [13]. The -potential analysis of the nanocrystal was performed at 20 ◦ C with an electrophoretic light scattering spectrometer (ELS-8000; Otsuka Electronics). The surface morphology of the nanocrystal films was
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observed by environmental scanning electron microscope (ESEM, XL30; Nikon Corp). The absorption spectra of pDCHD were measured using a UV–vis spectrophotometer (U-3000; Hitachi Ltd.). Current–voltage characteristics of the transferred film were measured using a semiconductor parameter analyzer (4155c; Agilent). 3. Results and discussion 3.1. Reduction of pDCHD surface potential by ethanol The key point for assembling nanomaterials at a liquid–liquid interface is controlling its surface wettability [14]. It was reported that ethanol can control a surface wettability of gold nanoparticle and carbon nanotube, those were dispersed in water [9,11,12]. We measured the -potential of pDCHD while changing the amount of ethanol added. The -potential of pDCHD initially shows a high negative value. With addition of ethanol, the negative -potential of pDCHD gradually decreases (Fig. 1). The detail of the decrement is not fully understood at the present time. The smaller dielectric constant of ethanol compared to that of water would decrease the amount of static charge at the pDCHD surface. 3.2. Fabrication of densely packed pDCHD films using a liquid–liquid interface To fabricate pDCHD film using a liquid–liquid interface, aqueous dispersion of the square-shape nanocrystal was placed into a glass vessel and then hexane was added to create the liquid–liquid interface. The boundary tension of the hexane–water interface is 50.8 mN/m. We observed that pDCHD nanocrystal was assembled at the liquid–liquid interface by adding 10 vol% of ethanol to the vessel. Different concentration of pDCHD water dispersion were used to fabricate pDCHD films, and the films formed at the interface was
Fig. 1. Values of -potential for the aqueous dispersion of pDCHD nanocrystal at the different ethanol concentration.
transferred onto a solid substrate by immersing a substrate into the interface at a rate of 10 mm min−1 . Fig. 2 shows SEM image of the transferred pDCHD film prepared from different concentrations of pDCHD water dispersion. The pDCHD film prepared from a 1.25 mM of pDCHD dispersion as a water phase shows several voids in the film. On the other hand, a densely packed pDCHD nanocrystal film was fabricated when 2.5 mM of pDCHD water dispersion was used as a water phase. The film is uniform throughout the SEM image area. Moreover, the digital camera image shows that uniformity of the film persist more than 1.5 cm2 . However, large aggregates were observed in the film when the pDCHD concentration increased to 5.0 mM. This results show that 2.5 mM of pDCHD water dispersion is appropriate to form a densely packed and uniform pDCHD film.
Fig. 2. SEM and digital camera image of pDCHD film fabricated using different concentration of pDCHD water dispersion as a water phase: (a) 1.25 mM, (b)–(d) 2.5 mM and (e) 5.0 mM.
J. Matsui et al. / Synthetic Metals 159 (2009) 847–850
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Fig. 3. A water droplet covered with n-hexane resting on (a) pDCHD cast film and (b) densely packed pDCHD film prepared using the liquid–liquid interface.
The contact angle of the film was measured to confirm the assembling process. It is known that a nanoparticle with a contact angle close to 90◦ is preferred to adsorb at a liquid–liquid interface [14]. It is, however, difficult to measure the contact angle of one pDCHD nanocrystal directly at the liquid–liquid interface. Therefore, the contact angle of a pDCHD nanocrystal at the interface was estimated by measuring the contact angle of pDCHD films. The contact angle of a pDCHD film prepared using a liquid–liquid interface shows a contact angle of 90.8◦ with the water–n-hexane interface, whereas the contact angle of a pDHCD cast film shows 31◦ (Fig. 3). The contact angle measurement supports that surface wettability of pDCHD is reduced by added ethanol. 3.3. Optical and electrical property of pDCHD film Fig. 4 shows UV–vis spectra of wire-shape pDCHD nanocrystal dispersed in water (Fig. 4(a)) and the nanocrystal’s film fabricated using the liquid–liquid interface (Fig. 4(b)). The wire-shape pDCHD nanocrystal in water dispersion state shows a sharp excitonic absorption peak around 650 nm and phonon sideband peaks around 600 nm. The pDCHD nanocrystal film prepared using the liquid–liquid interface shows broadening of these peaks, which indicates – interactions between the pDCHD nanocrystals. The – interactions are produced from closed packed structure of the film. Wire-shape pDCHD nanocrystal film formed at the liquid–liquid interface was transferred onto a glass substrate with gold electrode to examine the electrical properties of the pDCHD nanocrystal film. Fig. 5 shows the pDCHD nanocrystal film’s current–voltage characteristics. The pDCHD film initially shows low conductivity (1.8 × 10−8 S/cm). However, the conductivity increased with I2 doping and became 2.8 × 10−5 S/cm after 30 min doping of I2 (Fig. 5(b)). The value is closed to the reported bulk state [15,16]. The higher conductivity will be attainable by using much strong dopant [17].
Fig. 4. UV–vis absorption spectra of (a) pDCHD nanocrystal dispersed in water and (b) densely packed pDCHD nanocrystal film. Wire-shape pDCHD nanocrystals were used. Inset: SEM image of the nanocrystal film.
4. Conclusion Densely packed pDCHD film was fabricated using a liquid–liquid interface. SEM image of the film indicates that densely packed and uniform pDCHD film was fabricated by adding 10 vol% of ethanol to the 2.5 mM of pDCHD water dispersion–hexane solution. The wire-based pDCHD nanocrystal film shows initially a conductivity of 1.8 × 10−8 S/cm and the conductivity increased to 2.5 × 10−5 S/cm by I2 doping. The assembling technique using a liquid–liquid interface will be applicable to other organic nanocrys-
Fig. 5. (a) I–V characteristics of pDCHD nanocrystal film with different I2 doping time. (b) Effect of I2 doping time to the conductivity of pDCHD nanocrystal film.
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tals. Hetero-assembly of different organic nanocrystals is under in progress. Acknowledgments This work was partially supported by a Grant-in-Aid for Scientific Research in a Priority Area “Super-Hierarchical Structures” from the Ministry of Education, Culture, Sports, Science and Technology, Japan and PRESTO from the Japan Science and Technology Agency. References [1] H. Klaruk (Ed.), Organic Electronics: Materials, Manufacturing, and Applications., Wiley–VCH, Weinheim, 2006. [2] H. Sirringhaus, Adv. Mater. 17 (2005) 2411. [3] K. Müllen, U. Scherf (Eds.), Organic Light Emitting Devices: Synthesis, Properties And Applications, Wiley–VCH, Weinheim, 2006. [4] V.C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R.L. Willett, T. Someya, M.E. Gershenson, J.A. Rogers, Science 303 (2004) 1644. [5] H. Kasai, H.S. Nalwa, S. Okada, H. Oikawa, H. Nakanishi, in: H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nanotechnology, Academic Press, San Diego, 1999.
[6] A. Masuhara, H. Kasai, T. Kato, S. Okada, H. Oikawa, Y. Nozue, S.K. Tripathy, H. Nakanishi, J. Macromol. Sci. (2001) 1371. [7] G. Zhao, T. Ishizaka, H. Kasai, H. Oikawa, H. Nakanishi, Mol. Cryst. Liq. Cryst. 464 (2007) 613. [8] H.W. Duan, D.A. Wang, D.G. Kurth, H. Mohwald, Angew. Chem. Int. Edit. 43 (2004) 5639. [9] F. Reincke, S.G. Hickey, W.K. Kegel, D. Vanmaekelbergh, Angew. Chem. Int. Edit. 43 (2004) 458. [10] J. Matsui, M. Iko, N. Inokuma, H. Orikasa, M. Mitsuishi, T. Kyotani, T. Miyashita, Chem. Lett. 35 (2006) 42. [11] J. Matsui, K. Yamamoto, N. Inokuma, H. Orikasa, T. Kyotani, T. Miyashita, J. Mater. Chem. 17 (2007) 3806. [12] J. Matsui, K. Yamamoto, T. Miyashita, Mater. Res. Soc. Symp. Proc. 1057 (2008) 1057. [13] K. Baba, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, Jpn. J. Appl. Phys. 146 (2007) 7558. [14] B.P. Binks, T.S. Horozov, in: B.P. Binks, T.S. Horozov (Eds.), Colloidal Particles at Liquid Interfaces, Cambridge University Press, Cambridge, 2006. [15] N. Ferrer-Anglada, D. Bloor, F.I. Chalmers, G.I. Hunt, D.R. Hercliffe, J. Mater. Sci. Lett. 4 (1985) 83. [16] H. Nakanishi, H. Matsuda, M. Kato, Mol. Cryst. Liq. Cryst. 105 (1984) 77. [17] K. Baba, H. Kasai, Y. Shinohara, S. Okada, H. Oikawa, H. Matsuda, H. Nakanishi, Jpn. J. Appl. Phys. 247 (2008) 3769.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Densely packed organic nanocrystals ultrathin film using a liquid–liquid interface J. Matsui a,b,∗ , T. Shibata a , K. Yamamoto a , T. Yokoyama a , A. Masuhara a , H. Kasai a,b , H. Oikawa a , T. Miyashita a a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi 332-001, Japan b
a r t i c l e
i n f o
Article history: Received 15 November 2008 Received in revised form 14 January 2009 Accepted 14 January 2009 Available online 12 February 2009 Keywords: Self-assembly Polydiacetylene nanocrystal Liquid–liquid interface
a b s t r a c t We report a fabrication of densely packed polydiacetylene nanocrystal film using a liquid–liquid interface. Polydiacetylene (pDCHD) nanocrystals was dispersed into water solution by reprecipitation method from a good solvent. Then, hexane was added to the water dispersion to create the liquid–liquid interface. pDCHD nanocrystals were assembled at the interface, when ethanol was added to the pDCHD water dispersion–hexane solution. The assembled film was transferred onto a solid substrate and the film morphology was observed by scanning electron microscope (SEM). With addition of 10 vol% of ethanol to 2.5 mM of pDCHD water dispersion–hexane solution, a densely packed pDCHD film is fabricated. Optical and electrical properties of the pDCHD film are discussed by UV–vis spectroscopy and current–voltage measurement. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the past decade, research for organic materials, that are applicable to optical and electronic devices have aroused considerable interest because of the unique advantages provided by organic materials [1]. These advantages include low fabrication cost, high flexibility and versatility of the chemical structure. In this context, there are many reports for constructing organic active devices such as organic field-effect transistor [2], light emitting diodes [3], etc. Recently, organic crystals have attached much interest because of their high carrier mobility [4]. Usually, preparation of organic crystals is a time consuming process, therefore efficient and large-scale fabrication of organic devices using organic crystals is difficult. On the other hand, organic nanocrystals with several crystal structures can be fabricated using the “reprecipitation method” [5], which is a simple and efficient method. Application of the nanocrystals to organic electronic devices requires tailor-made assembly of the nanocrystals. Several techniques such as layer-by-layer technique [6], and electropholic deposition [7] have been used to assemble organic nanocrystals. However, these techniques require polymer electrolyte or are only applicable to electrodes. A more versatile technique for assembling organic
∗ Corresponding author at: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 217 5639; fax: +81 22 217 5639. E-mail address: jun [email protected] (J. Matsui). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.01.025
nanocrystal should be developed for its real device application. Recently, liquid–liquid interface have applied to assemble inorganic nanoparticles [8,9]. We have reported that not only a spherical nanoparticle but also a tubular nanomaterials such as carbon nanotubes can be assembled using the liquid–liquid interface [10–12]. In this paper, we applied the liquid–liquid interfacial assembling technique to prepare a densely packed monoparticle film of a polydiacetylene nanocrystal. We observed that the nanocrystals were self-assembled at the interface with controlling the surface energy of polydiacetylene nanocrystal by adding ethanol. The assembly formed at the liquid–liquid interface was transferred onto a solid substrate by immersing the substrate into the interface. The transferred film was observed using scanning electron microscope (SEM). Moreover, the optical and electrical properties of the film were observed by UV–vis spectra and I–V characteristics measurements.
2. Experimental section Nanocrystals of poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (pDCHD) with square-shape (∼30 nm in width and length) and wire-shape (∼30 nm in width and 0.5–1 m in length) were fabricated by reprecipitation method as reported previously [13]. The -potential analysis of the nanocrystal was performed at 20 ◦ C with an electrophoretic light scattering spectrometer (ELS-8000; Otsuka Electronics). The surface morphology of the nanocrystal films was
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observed by environmental scanning electron microscope (ESEM, XL30; Nikon Corp). The absorption spectra of pDCHD were measured using a UV–vis spectrophotometer (U-3000; Hitachi Ltd.). Current–voltage characteristics of the transferred film were measured using a semiconductor parameter analyzer (4155c; Agilent). 3. Results and discussion 3.1. Reduction of pDCHD surface potential by ethanol The key point for assembling nanomaterials at a liquid–liquid interface is controlling its surface wettability [14]. It was reported that ethanol can control a surface wettability of gold nanoparticle and carbon nanotube, those were dispersed in water [9,11,12]. We measured the -potential of pDCHD while changing the amount of ethanol added. The -potential of pDCHD initially shows a high negative value. With addition of ethanol, the negative -potential of pDCHD gradually decreases (Fig. 1). The detail of the decrement is not fully understood at the present time. The smaller dielectric constant of ethanol compared to that of water would decrease the amount of static charge at the pDCHD surface. 3.2. Fabrication of densely packed pDCHD films using a liquid–liquid interface To fabricate pDCHD film using a liquid–liquid interface, aqueous dispersion of the square-shape nanocrystal was placed into a glass vessel and then hexane was added to create the liquid–liquid interface. The boundary tension of the hexane–water interface is 50.8 mN/m. We observed that pDCHD nanocrystal was assembled at the liquid–liquid interface by adding 10 vol% of ethanol to the vessel. Different concentration of pDCHD water dispersion were used to fabricate pDCHD films, and the films formed at the interface was
Fig. 1. Values of -potential for the aqueous dispersion of pDCHD nanocrystal at the different ethanol concentration.
transferred onto a solid substrate by immersing a substrate into the interface at a rate of 10 mm min−1 . Fig. 2 shows SEM image of the transferred pDCHD film prepared from different concentrations of pDCHD water dispersion. The pDCHD film prepared from a 1.25 mM of pDCHD dispersion as a water phase shows several voids in the film. On the other hand, a densely packed pDCHD nanocrystal film was fabricated when 2.5 mM of pDCHD water dispersion was used as a water phase. The film is uniform throughout the SEM image area. Moreover, the digital camera image shows that uniformity of the film persist more than 1.5 cm2 . However, large aggregates were observed in the film when the pDCHD concentration increased to 5.0 mM. This results show that 2.5 mM of pDCHD water dispersion is appropriate to form a densely packed and uniform pDCHD film.
Fig. 2. SEM and digital camera image of pDCHD film fabricated using different concentration of pDCHD water dispersion as a water phase: (a) 1.25 mM, (b)–(d) 2.5 mM and (e) 5.0 mM.
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Fig. 3. A water droplet covered with n-hexane resting on (a) pDCHD cast film and (b) densely packed pDCHD film prepared using the liquid–liquid interface.
The contact angle of the film was measured to confirm the assembling process. It is known that a nanoparticle with a contact angle close to 90◦ is preferred to adsorb at a liquid–liquid interface [14]. It is, however, difficult to measure the contact angle of one pDCHD nanocrystal directly at the liquid–liquid interface. Therefore, the contact angle of a pDCHD nanocrystal at the interface was estimated by measuring the contact angle of pDCHD films. The contact angle of a pDCHD film prepared using a liquid–liquid interface shows a contact angle of 90.8◦ with the water–n-hexane interface, whereas the contact angle of a pDHCD cast film shows 31◦ (Fig. 3). The contact angle measurement supports that surface wettability of pDCHD is reduced by added ethanol. 3.3. Optical and electrical property of pDCHD film Fig. 4 shows UV–vis spectra of wire-shape pDCHD nanocrystal dispersed in water (Fig. 4(a)) and the nanocrystal’s film fabricated using the liquid–liquid interface (Fig. 4(b)). The wire-shape pDCHD nanocrystal in water dispersion state shows a sharp excitonic absorption peak around 650 nm and phonon sideband peaks around 600 nm. The pDCHD nanocrystal film prepared using the liquid–liquid interface shows broadening of these peaks, which indicates – interactions between the pDCHD nanocrystals. The – interactions are produced from closed packed structure of the film. Wire-shape pDCHD nanocrystal film formed at the liquid–liquid interface was transferred onto a glass substrate with gold electrode to examine the electrical properties of the pDCHD nanocrystal film. Fig. 5 shows the pDCHD nanocrystal film’s current–voltage characteristics. The pDCHD film initially shows low conductivity (1.8 × 10−8 S/cm). However, the conductivity increased with I2 doping and became 2.8 × 10−5 S/cm after 30 min doping of I2 (Fig. 5(b)). The value is closed to the reported bulk state [15,16]. The higher conductivity will be attainable by using much strong dopant [17].
Fig. 4. UV–vis absorption spectra of (a) pDCHD nanocrystal dispersed in water and (b) densely packed pDCHD nanocrystal film. Wire-shape pDCHD nanocrystals were used. Inset: SEM image of the nanocrystal film.
4. Conclusion Densely packed pDCHD film was fabricated using a liquid–liquid interface. SEM image of the film indicates that densely packed and uniform pDCHD film was fabricated by adding 10 vol% of ethanol to the 2.5 mM of pDCHD water dispersion–hexane solution. The wire-based pDCHD nanocrystal film shows initially a conductivity of 1.8 × 10−8 S/cm and the conductivity increased to 2.5 × 10−5 S/cm by I2 doping. The assembling technique using a liquid–liquid interface will be applicable to other organic nanocrys-
Fig. 5. (a) I–V characteristics of pDCHD nanocrystal film with different I2 doping time. (b) Effect of I2 doping time to the conductivity of pDCHD nanocrystal film.
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tals. Hetero-assembly of different organic nanocrystals is under in progress. Acknowledgments This work was partially supported by a Grant-in-Aid for Scientific Research in a Priority Area “Super-Hierarchical Structures” from the Ministry of Education, Culture, Sports, Science and Technology, Japan and PRESTO from the Japan Science and Technology Agency. References [1] H. Klaruk (Ed.), Organic Electronics: Materials, Manufacturing, and Applications., Wiley–VCH, Weinheim, 2006. [2] H. Sirringhaus, Adv. Mater. 17 (2005) 2411. [3] K. Müllen, U. Scherf (Eds.), Organic Light Emitting Devices: Synthesis, Properties And Applications, Wiley–VCH, Weinheim, 2006. [4] V.C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R.L. Willett, T. Someya, M.E. Gershenson, J.A. Rogers, Science 303 (2004) 1644. [5] H. Kasai, H.S. Nalwa, S. Okada, H. Oikawa, H. Nakanishi, in: H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nanotechnology, Academic Press, San Diego, 1999.
[6] A. Masuhara, H. Kasai, T. Kato, S. Okada, H. Oikawa, Y. Nozue, S.K. Tripathy, H. Nakanishi, J. Macromol. Sci. (2001) 1371. [7] G. Zhao, T. Ishizaka, H. Kasai, H. Oikawa, H. Nakanishi, Mol. Cryst. Liq. Cryst. 464 (2007) 613. [8] H.W. Duan, D.A. Wang, D.G. Kurth, H. Mohwald, Angew. Chem. Int. Edit. 43 (2004) 5639. [9] F. Reincke, S.G. Hickey, W.K. Kegel, D. Vanmaekelbergh, Angew. Chem. Int. Edit. 43 (2004) 458. [10] J. Matsui, M. Iko, N. Inokuma, H. Orikasa, M. Mitsuishi, T. Kyotani, T. Miyashita, Chem. Lett. 35 (2006) 42. [11] J. Matsui, K. Yamamoto, N. Inokuma, H. Orikasa, T. Kyotani, T. Miyashita, J. Mater. Chem. 17 (2007) 3806. [12] J. Matsui, K. Yamamoto, T. Miyashita, Mater. Res. Soc. Symp. Proc. 1057 (2008) 1057. [13] K. Baba, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, Jpn. J. Appl. Phys. 146 (2007) 7558. [14] B.P. Binks, T.S. Horozov, in: B.P. Binks, T.S. Horozov (Eds.), Colloidal Particles at Liquid Interfaces, Cambridge University Press, Cambridge, 2006. [15] N. Ferrer-Anglada, D. Bloor, F.I. Chalmers, G.I. Hunt, D.R. Hercliffe, J. Mater. Sci. Lett. 4 (1985) 83. [16] H. Nakanishi, H. Matsuda, M. Kato, Mol. Cryst. Liq. Cryst. 105 (1984) 77. [17] K. Baba, H. Kasai, Y. Shinohara, S. Okada, H. Oikawa, H. Matsuda, H. Nakanishi, Jpn. J. Appl. Phys. 247 (2008) 3769.