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Langmuir Blodgett films of arachidic acid and a nematic liquid crystal: Characterization and use in homeotropic alignment
Thin Solid Films 496 (2006) 601 – 605 www.elsevier.com/locate/tsf
Langmuir Blodgett films of arachidic acid and a nematic liquid crystal: Characterization and use in homeotropic alignment John Collins 1, Denis Funfschilling 2, Michael Dennin * Department of Physics and Astronomy, University of California at Irvine, Irvine, California 92697-4575, USA Received 16 May 2005; received in revised form 2 September 2005; accepted 2 September 2005 Available online 19 October 2005
Abstract We study the behavior of a mixed Langmuir monolayer consisting of a fatty acid and a nematic liquid crystal. We demonstrate that the mixed monolayer successfully transfers as a Langmuir – Blodgett film and characterize the transferred film using UV spectroscopy. An important application of Langmuir – Blodgett films is in the alignment of liquid crystals for electro-optical applications, such as displays. We show that including the liquid crystal in the Langmuir – Blodgett film produces homeotropic alignment for a system which fails to align by other standard techniques. D 2005 Elsevier B.V. All rights reserved. Keywords: Langmuir – Blodgett films; Optical properties; Homeotropic alignment; Liquid crystal
1. Introduction The importance of liquid crystals in a wide range of applications relies on the ability to produce liquid crystal devices with macroscopically uniform alignment [1]. The two basic types of alignment are planar and homeotropic [2]. In planar alignment, the director is aligned parallel to the boundaries of interest. In homeotropic alignment, the director is aligned perpendicular to the boundary. (The director is the axis along which the molecules of the nematic liquid crystal are aligned on average). In addition to these two basic types of alignments, there are a number of variations, depending on the application. In general, all of the alignment techniques are based on some thin film technology. For planar alignment, one is generally interested in obtaining alignment in a particular direction in the plane. Therefore, planar alignment techniques often involve treating a thin film so that steric effects play a role, such as rubbing a polymer to induce ‘‘grooves’’. The grooves select a direction of interest. Homeotropic alignment
* Corresponding author. E-mail address: [email protected] (M. Dennin). 1 Current address: Department of Biomedical Engineering, University of California at Irvine, Irvine, California, USA 92697-4575. 2 Current address: Department of Physics and Astronomy, University of California at Santa Barbara, Santa Barbara, California, USA 93106-4170. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.087
techniques tend to be more straightforward. Usually, a simple surfactant coating is used that induces the liquid crystal to prefer a perpendicular alignment. A challenge in any alignment technique is the fact that there are usually both chemical and steric effects, and no one technique is guaranteed to work for all liquid crystals [2]. One motivation for this work was our need to produce homeotropic alignment in a particular liquid crystal: liquid crystal Merck N4 (referred to as N4). Several techniques exist for producing homeotropic cells [2]. Most of the common techniques are based on coating the glass surface with a surfactant. Typically, the slide is immersed in a solution of the surfactant and then dried or baked to remove any solvent and fix the surfactant to the surface. Common surfactants are lecithin (egg yolk), dimethyloctadecyl[3-trimethoxysilyl)-propyl] ammonium chloride, and N-methyl-3-aminopropyltrimethoxysilane. As these techniques failed to align N4, an alternative approach was needed. A promising development in homeotropic alignment is the use of techniques based on Langmuir– Blodgett deposition (LB film) of fatty acid films [3 – 7]. Successful alignment of standard liquid crystals has been achieved with LB films of pure fatty acids [3,5] and fatty acids mixed with a liquid crystal (5CB) [4]. The later experiments motivated our approach of using a mixed film of fatty acids and N4. The mixed film increases the interaction between the LB film and the liquid
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J. Collins et al. / Thin Solid Films 496 (2006) 601 – 605
crystal by taking advantage of the aligning properties of the nematic itself. The liquid crystal molecules that are trapped inside the LB film are oriented in the same direction as the fatty acid, i.e., perpendicular to the surface of the substrate. The expectation is that this orientation is transmitted to the bulk liquid crystal in the cell. Because a LB film is a layer by layer transfer of material from a Langmuir monolayer to a solid substrate, the experiments reported on in this paper can be divided into three steps. First, we characterized the mixed monolayer of N4 and a fatty acid. (A Langmuir monolayer is a single layer of molecules at the air –water interface). Second, we confirm that the monolayer can be transferred to the appropriate solid substrate, which in this case is glass that is coated with a transparent conductor. Finally, we test the alignment and electro-optical response of the a liquid cell that is made from the treated glass. On the technology side, homeotropic alignment has been identified as a potentially useful arrangement for displays (see for example, [8 –10]). This is true for the case of so-called vertical cells and mixed alignment displays. The interest in homeotropic displays stems from the potential advantages in terms of speed of response and viewing angle. For N4, our main interest stems from the role liquid crystals have played in the study of pattern formation in anisotropic systems [11,12]. The general problem of pattern formation refers to the study of transitions in system subjected to external driving [13]. Traditionally, a system is prepared in a spatially uniform state. Above a critical value of the driving, transitions to states with periodic structures (patterns) are observed. Among the advantages of using liquid crystals is the ability to control the alignment to select varying degrees of anisotropy. One common pattern forming system is electroconvection in nematic liquid crystals [11,14,15]. This utilizes materials with a negative dielectric anisotropy in a planar alignment between two parallel glass plates. (The dielectric anisotropy is the difference between the dielectric constant when the material is aligned parallel to an electric field and when it is aligned perpendicular to an electric field). When driven with an ac electric voltage, there is a transition from the uniform conducting state to a periodic convecting state. An interesting variation on electroconvection is to start with a homeotropic sample. In this case, a material with a negative dielectric anisotropy typically undergoes the Free´dericksz transition at some initial critical voltage, as it prefers to align perpendicular to the electric field. This is a transition from homeotropic alignment to planar alignment. Then, at a higher critical voltage, there is a transition to electroconvection [16 – 18]. Therefore, the ability to produce homeotropic samples of nematic liquid crystals with negative dielectric anisotropy for studies of electroconvection is useful. Though a detailed study of electroconvection in homeotropic N4 is outside the scope of this paper, it is this example of electro-optical response on which we report in this paper as a test of the alignment of the liquid crystal samples. (There have also been studies of homeotropic samples in which there is a direct transition to electroconvection, e.g., see [19], but these cases involve a positive dielectric anisotropy).
2. Materials and characterization techniques We used arachidic acid obtained from Sigma-Aldrich with a quoted purity of 99%. It was used without further purification. The arachidic acid was dissolved in spectral grade chloroform obtained from EM Science at a concentration of 1 mg/ml. The liquid crystal N4 is a eutectic mixture of the two isomers of 4-methoxy-4V-n-butylazoxybenzene (CH3O-C6H4NON-C6H4-C4H9 and CH3OC6H4-NNO-C6H4-C4H9). It was obtained from EM Industries (a Merck company), now EMD Chemicals Inc. [20]. The N4 was used without further purification. The monolayers were formed on an ionic subphase composed of 1 mM CaCl2 solution in purified water. The water was deionized water with a resistivity of 18.2 MVcm. Two different methods were used to include the N4 in the arachidic acid monolayers. First, an arachidic acid monolayer was formed by placing drops of the arachidic acid/chloroform solution on the subphase. Subsequently, a solution of 3.95 10 3 mol/l of N4 in chloroform was added to the surface. The second method involved placing different mixtures of arachidic acid and N4 (described later) directly on the surface. The most basic characterization of Langmuir monolayers is the measurement of the surface pressure versus area isotherms (or isotherms, for short) [21,22]. For monolayers, the surface pressure (P) is defined as the surface tension of pure water (c w ) minus the surface tension of the water– monolayer system (c), i.e., P = c w c. We made isotherm measurements using a KSV 5000 LB trough of maximum area 150 475 mm2. The monolayer is compressed by moving a single barrier at a rate of 10 mm/min (or 15 cm2/min). The surface pressure is monitored during the compression. Phase transitions within the monolayer are identified by the presence of plateaus (first order transitions) or kinks (second order transitions) in the isotherm. The isotherms were measured at a temperature of 22 -C. In addition to surface-pressure area isotherms, we also characterized the composition of the LB films using UV-visible absorption spectroscopy recorded on a Hewlett-Packard 8453 diode array spectrophotometer. The LB films are deposited on a CaF2 substrate for this characterization. The LB films were made by passing the substrate through the Langmuir monolayer in a vertical orientation at a constant rate of 10 mm/min. The number of layers transferred to the substrate was set by the number of times the substrate was passed through the monolayer, with one layer transferred during each lowering and raising of the substrate through the monolayer. 3. Results and discussion Fig. 1 shows the isotherms for the pure arachidic acid system in which 50 Al of the 1 mg/ml arachidic acid solution was placed on the trough. Fig. 1 also shows the arachidic acid systems subjected to the addition of N4 in 10 Al increments of the 3.95 10 3 mol/l N4 solution each time. The pure arachidic acid isotherms are consistent with the behavior expected from previous measurements of this system. Because
J. Collins et al. / Thin Solid Films 496 (2006) 601 – 605
Fig. 1. Plot of isotherms for arachidic acid (solid black curve), and arachidic acid with the addition of N4 (dashed curves). Reading from left to right, each dashed curve represents an addition of 10 Al of the N4 solution, as described in the text. All curves are plotted as a function of the area per molecule for the arachidic acid molecules. Each isotherm shows a single compression and expansion. For the pure arachidic acid case, the compression and expansion are essentially identical. For the cases with N4, the direction of each curve is indicated by the arrows.
we remain below the collapse pressure, the measured isotherm is reproducible in compression/expansion, as can be seen by the overlap of the solid and dashed line. We are able to identify the three phases of arachidic acid that are expected at room ˚ 2/molecules, temperature. Up to an area per molecule of 20.5 A the phase is gaseous. At this point (the ‘‘liftoff point’’), we observed the initial rise in the isotherm that corresponds to the L2 phase of the monolayer. This exists until the observed kink at a surface pressure of 22.5 mN/m. This corresponds to the transition to the LV2 phase. As expected, there are no observed plateaus in the isotherm. As the N4 is added to the monolayer with the arachidic acid in the gas phase, it is expected that the N4 inserts into the monolayer. This expectation is confirmed by the changes in the liftoff point and the existence of a plateau in the isotherm at 12 mN/m. The isotherms are still being plotted as a function of the area/molecule for the number of arachidic acid molecules present in the monolayer. Therefore, the fact that the liftoff point occurs at a higher apparent area per molecule is due to the space occupied by the N4 molecules. Using the isotherms in Fig. 1, one can compute the effective liftoff area as a function of the amount of N4 added to the system. Using the fact that the areas are in terms of the initial amount of arachidic acid, one can estimate the area per molecule occupied by N4 at liftoff based on the shift. For example, for the 10 Al addition, there are 2.4 1016 N4 molecules. The additional area at liftoff is 50.3 cm3. This corresponds to each N4 molecule occupying an area of ˚ 2. Similar calculations for the 20 approximately 21 T .0.1 A ˚ 2. This Al and 30 Al additions also yield an area of 21 T .0.1 A suggests that the N4 is occupying a similar area as the arachidic acid molecules at liftoff, and one can expect a vertical orientation for the N4 molecules in the L2 phase of the monolayer. The next issue is the nature of the plateau. It is known that other liquid crystal systems are able to form multilayers at the
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Fig. 2. Isotherms for three examples of the arachidic acid/N4 mixtures. The mixtures are (by mole percent) 43% (solid curve), 60% (dashed curve), and 69% (dotted curve) N4.
air –water interface under compression. The plateau region is indicative of the formation of a multilayer of N4. The fact that the isotherms ultimately converge on the pure arachidic acid isotherm suggests that the N4 lays on top of a dense arachidic acid monolayer. The subsequent expansion illustrates that most of the N4 is recovered, as the isotherms returns to the compression isotherm at high areas. Though not directly related to the focus of this paper, this collapse mechanisms is interesting and requires further study. An alternate method of formation for the monolayer is to make a mixture of arachidic acid and N4 and directly form a Langmuir monolayer from the mixture. The results for isotherms from this solution are given in Fig. 2. Mixtures by mole percent of 43 – 57%, 60 –40% and 69– 31% of N4 and arachidic acid, respectively are premixed before introduction on the trough. A plateau region at approximately 13 mN/m is observed for these systems. In this case, the shift in liftoff area is also consistent with the amount of N4 present. However, it is interesting that in this case, the expansion curves do not ever recover the compression curves. This suggests a loss of
Fig. 3. Ultraviolet-visible absorption spectra for bulk N4 (dashed line), blank substrate (dotted line), arachidic acid/N4 film transferred at 10 mN/m (lower solid line) and arachidic acid/N4 film transferred at 20 mN/m (upper solid curve). The optical density is given in arbitrary units, as the relative intensity is the important factor. The bulk film is relative to the left axis, with all other cases relative to the right axis. The key feature is the N4 peak at 350 nm that is visible in both LB films.
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Fig. 4. Critical voltage (V c ) for the onset of the Free´dericksz transition (squares) and electroconvection (circles) as a function of applied frequency. As expected, the Free´dericksz transition is independent of frequency, and the transition to electroconvection increases with increasing frequency.
material from this system. It will be interesting to explore the differences between the two film formation mechanisms in future work. However, for the purposes of obtaining LB films with N4 molecules orientated perpendicular to the surface, both types of film suggest that the best conditions for transfer are just before the plateau region. In this region, the N4 is still fully incorporated into the arachidic acid monolayer, and it is oriented perpendicular to the surface. To test that the N4 remains incorporated during the formation of the LB film, the monolayer is transferred at 10 mN/m on to a CaF2 substrate. A UV-visible absorption spectrum of LB films with different numbers of layers is carried out. A typical result is presented in Fig. 3. For comparison, we show the spectra for the plain substrate and bulk N4. The peak for the N4 molecules is clearly observable at 350 nm in all of the LB examples, with the expected increase in amplitude of the peak as the number of layers is increased. To test for homeotropic alignment, a liquid crystal cell was made using the following procedure. An indium – tin oxide (ITO) coated glass slide was washed sequentially in water, in NH3 solution, and in an ultrasonic bath with a solution of 350 ml distilled water, 50 ml sodium hydroxide, and 40 ml liquinox. We selected the mixture of 43% N4 and 57% arachidic acid diluted in chloroform and formed a monolayer
(a)
(b)
that was compressed to 10 mN/m. The pressure was held constant while the ITO glass is coated with 10 layers of the Langmuir monolayers. After the coating of the surface, the glass is baked in an oven at a temperature of 50 -C. A 25 Am mylar spacer is placed between the two ITO glasses. Two opposite sides of the cell are sealed with epoxy. The cell is filled with N4 by capillary action. The two remaining sides of the cell are sealed with 5 minute epoxy. The alignment was checked by observing the samples between crossed polarizers. This confirmed the general homeotropic alignment. As the ultimate goal is to produce samples useful for the study of electroconvection in homeotropic samples, we further tested the alignment by measuring the response of the N4 samples to an applied ac electric field as a function of both applied voltage and frequency. As discussed, N4 has a negative dielectric anisotropy. Therefore, at each frequency, we expect to observe the Free´dericksz transition followed by electroconvection. To measure the response, the applied ac voltage was increased in steps of 0.05 V. A waiting time of 120 s was used at each step. The temperature was maintained at 30 -C. The system was placed between cross polarizers and imaged from above. In the undistorted state, the image is uniformly black. Above the Free´dericksz transition, as the director tilts, the total intensity of the image increased continuously as a function of the applied voltage. This increase in intensity can be used to determine the onset voltage for the Free´dericksz transition. Finally, at the critical value for electroconvection, one observes periodic patterns in the image. For the purposes of this paper, we made a quantitative measure of the Free´dericksz transition as a test of the alignment characterization and report on some qualitatively features of the subsequent electroconvection. A more detailed study of the electroconvection will be the subject of future work. The response to the applied electric field is summarized in Fig. 4. As expected, the Free´dericksz transition is independent of the applied frequency, but the onset to convection is frequency dependent. For strong alignment, one expects the Free´dericksz transition to occur at V cF = (K 33 / (e a e o ))1 / 2, where e a is the difference between the dielectric constant parallel and perpendicular to the electric field (e a = e || e –), K 33 is the bend elastic constant, and e o = 8.85 10 12 C2 / Nm2. For our
(c)
Fig. 5. Three images of the sample covering an area of 3.05 3.05 mm2. The scale bar represents 1 mm. The images were taken with an applied frequency of 5000 Hz and a voltage of (a) 9.05 V (b) 9.55 V and (c) 14.5 V.
J. Collins et al. / Thin Solid Films 496 (2006) 601 – 605
(a)
(b)
(c)
(d)
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quantitative characterization of these transitions is beyond the scope of this paper and will be the subject of future work. In summary, we have demonstrated that one can form monolayers of the liquid crystal N4 within an arachidic acid monolayer, and we have characterized the phase behavior of this mixed system at a single temperature. This system can be transferred to ITO coated substrates using standard LB techniques. The resulting surface provides homeotropic alignment for N4 in a standard electroconvection cell when other standard alignment techniques failed. The cells were tested both by imaging the initial state with cross polarizers and by measuring the response to an applied electric field. Acknowledgments This work was supported by NSF grant DMR-9975479 and PRF 39070-AC9. The authors thank Wytze Van Der Veer for his help in using the UCI Laser Facility.
Fig. 6. Four images of the sample covering an area of 3.05 3.05 mm2. The scale bar represents 1 mm. The images were taken with an applied frequency of 5000 Hz and a voltage of (a) 14.55 V (b) 14.60 V (c) 14.79 V and (d) 14.95 V.
measured value of V cF = 9.3 V and the reported value of e a = 0.2 [20], we get K 33 = 15.5 10 12 N. It is worth briefly commenting on the nature of the patterns that are observed above the transition to electroconvection. Typical images are summarized in Figs. 5 and 6. All of the images in Figs. 5 and 6 cover 3.05 3.05 mm2. It should be noted that both figures focus on the case for higher frequencies. For frequencies lower than 1500 Hz, the initial electroconvection state is chaotic, and single images are not particularly useful. Fig. 5 illustrates the two main transitions. Fig. 5a is below the Free´dericksz transition; hence, it is uniformly dark. Fig. 5b illustrates a state above the Free´dericksz, but it is below the transition to electroconvection. Finally, Fig. 5c shows the initial state above the transition to electronconvection for high values of the applied frequency. In this regime, the orientation of the rolls is essentially uniform throughout the cell, and independent of the underlying Free´dericksz domain. (The Free´dericksz domains are distinguished by different degrees of overall intensity due to the crossed polarizers). Fig. 6 illustrates the secondary transition exhibited by the convecting state as the voltage is increased. In Fig. 6a, the the orientation of the rolls has begun to align with the Free´dericksz domains. As the voltage is increased, even more ‘‘solid looking’’ boundaries develop between the roll orientations and the domains of electroconvection shrink in size (Fig. 6b– d). It is interesting to note that this series of patterns is different from that reported in Ref. [16] for a homeotropic cell containing the liquid crystal 4-methoxybenzylidene-4V-butylaniline. The
References [1] P.G. de Gennes, J. Prost, The Physics of Liquid Crystals, Clarendon Press, Oxford, 1993. [2] J. Cognard, Aignment of Nematic Liquid Crystals and Their Mixtures, Gordon and Breach, New York, 1982. [3] L. Komitov, B. Stebler, G. Gabrielli, M. Puggelli, A. Sparavigna, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A Mol. Cryst. Liq. Cryst. 243 (1994) 107. [4] U. Kuhnau, B. Madler, S. Wurlitzer, G. Rapp, H. Schmiedel, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A Mol. Cryst. Liq. Cryst. 304 (1997) 171. [5] V.S.U. Fazio, F. Nannelli, L. Komitov, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 63 (2001) 061712. [6] S. Peralta, F. Hapiot, Y. Barbaux, Liq. Cryst. 30 (2003) 463. [7] Y. Zong, J. Ruhe, W. Knoll, Thin Solid Films 477 (2005) 203. [8] S.-T. Wu, J. Appl. Phys. 76 (1994) 5975. [9] P.J. Hamelinck, W.T.S. Huck, J. Mater. Chem. 15 (2005) 381. [10] Y. Suzuki, N. Mizoshita, K. Hanabusa, T. Kato, J. Mater. Chem. 13 (2003) 2870. [11] L. Kramer, W. Pesch, Annu. Rev. Fluid Mech. 27 (1995) 515. [12] A. Buka, L. Kramer (Eds.), Pattern Formation in Liquid Crystals, Springer-Verlag, New York, 1996. [13] M.C. Cross, P.C. Hohenberg, Rev. Mod. Phys. 65 (1993) 851. [14] E. Bodenschatz, W. Zimmermann, L. Kramer, J. Phys. (France) 49 (1988) 1875. [15] I. Rehberg, B.L. Winkler, M. de la Torre Jua´rez, S. Rasenat, W. Scho¨pf, Festko¨rperprobleme 29 (1989) 35. [16] P. To´th, A. Buka, J. Peinke, L. Kramer, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 58 (1998) 1983. [17] J.-H. Huh, Y. Hidaka, S. Kai, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 58 (1998) 7355. [18] A.G. Rossberg, N. E´ber, A. Buka, L. Kramer, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 61 (2000) R25. [19] A. Buka, B. Dressel, L. Kramer, W. Pesch, Chaos 14 (2004) 793. [20] EMD Chemicals Inc., Hawthorne, New York, 10532-2156. [21] C.M. Knobler, R.C. Desai, Annu. Rev. Phys. Chem. 43 (1992) 207. [22] V.M. Kaganer, H. Mohwald, P. Dutta, Rev. Mod. Phys. 71 (1999) 779.
Langmuir Blodgett films of arachidic acid and a nematic liquid crystal: Characterization and use in homeotropic alignment John Collins 1, Denis Funfschilling 2, Michael Dennin * Department of Physics and Astronomy, University of California at Irvine, Irvine, California 92697-4575, USA Received 16 May 2005; received in revised form 2 September 2005; accepted 2 September 2005 Available online 19 October 2005
Abstract We study the behavior of a mixed Langmuir monolayer consisting of a fatty acid and a nematic liquid crystal. We demonstrate that the mixed monolayer successfully transfers as a Langmuir – Blodgett film and characterize the transferred film using UV spectroscopy. An important application of Langmuir – Blodgett films is in the alignment of liquid crystals for electro-optical applications, such as displays. We show that including the liquid crystal in the Langmuir – Blodgett film produces homeotropic alignment for a system which fails to align by other standard techniques. D 2005 Elsevier B.V. All rights reserved. Keywords: Langmuir – Blodgett films; Optical properties; Homeotropic alignment; Liquid crystal
1. Introduction The importance of liquid crystals in a wide range of applications relies on the ability to produce liquid crystal devices with macroscopically uniform alignment [1]. The two basic types of alignment are planar and homeotropic [2]. In planar alignment, the director is aligned parallel to the boundaries of interest. In homeotropic alignment, the director is aligned perpendicular to the boundary. (The director is the axis along which the molecules of the nematic liquid crystal are aligned on average). In addition to these two basic types of alignments, there are a number of variations, depending on the application. In general, all of the alignment techniques are based on some thin film technology. For planar alignment, one is generally interested in obtaining alignment in a particular direction in the plane. Therefore, planar alignment techniques often involve treating a thin film so that steric effects play a role, such as rubbing a polymer to induce ‘‘grooves’’. The grooves select a direction of interest. Homeotropic alignment
* Corresponding author. E-mail address: [email protected] (M. Dennin). 1 Current address: Department of Biomedical Engineering, University of California at Irvine, Irvine, California, USA 92697-4575. 2 Current address: Department of Physics and Astronomy, University of California at Santa Barbara, Santa Barbara, California, USA 93106-4170. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.087
techniques tend to be more straightforward. Usually, a simple surfactant coating is used that induces the liquid crystal to prefer a perpendicular alignment. A challenge in any alignment technique is the fact that there are usually both chemical and steric effects, and no one technique is guaranteed to work for all liquid crystals [2]. One motivation for this work was our need to produce homeotropic alignment in a particular liquid crystal: liquid crystal Merck N4 (referred to as N4). Several techniques exist for producing homeotropic cells [2]. Most of the common techniques are based on coating the glass surface with a surfactant. Typically, the slide is immersed in a solution of the surfactant and then dried or baked to remove any solvent and fix the surfactant to the surface. Common surfactants are lecithin (egg yolk), dimethyloctadecyl[3-trimethoxysilyl)-propyl] ammonium chloride, and N-methyl-3-aminopropyltrimethoxysilane. As these techniques failed to align N4, an alternative approach was needed. A promising development in homeotropic alignment is the use of techniques based on Langmuir– Blodgett deposition (LB film) of fatty acid films [3 – 7]. Successful alignment of standard liquid crystals has been achieved with LB films of pure fatty acids [3,5] and fatty acids mixed with a liquid crystal (5CB) [4]. The later experiments motivated our approach of using a mixed film of fatty acids and N4. The mixed film increases the interaction between the LB film and the liquid
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J. Collins et al. / Thin Solid Films 496 (2006) 601 – 605
crystal by taking advantage of the aligning properties of the nematic itself. The liquid crystal molecules that are trapped inside the LB film are oriented in the same direction as the fatty acid, i.e., perpendicular to the surface of the substrate. The expectation is that this orientation is transmitted to the bulk liquid crystal in the cell. Because a LB film is a layer by layer transfer of material from a Langmuir monolayer to a solid substrate, the experiments reported on in this paper can be divided into three steps. First, we characterized the mixed monolayer of N4 and a fatty acid. (A Langmuir monolayer is a single layer of molecules at the air –water interface). Second, we confirm that the monolayer can be transferred to the appropriate solid substrate, which in this case is glass that is coated with a transparent conductor. Finally, we test the alignment and electro-optical response of the a liquid cell that is made from the treated glass. On the technology side, homeotropic alignment has been identified as a potentially useful arrangement for displays (see for example, [8 –10]). This is true for the case of so-called vertical cells and mixed alignment displays. The interest in homeotropic displays stems from the potential advantages in terms of speed of response and viewing angle. For N4, our main interest stems from the role liquid crystals have played in the study of pattern formation in anisotropic systems [11,12]. The general problem of pattern formation refers to the study of transitions in system subjected to external driving [13]. Traditionally, a system is prepared in a spatially uniform state. Above a critical value of the driving, transitions to states with periodic structures (patterns) are observed. Among the advantages of using liquid crystals is the ability to control the alignment to select varying degrees of anisotropy. One common pattern forming system is electroconvection in nematic liquid crystals [11,14,15]. This utilizes materials with a negative dielectric anisotropy in a planar alignment between two parallel glass plates. (The dielectric anisotropy is the difference between the dielectric constant when the material is aligned parallel to an electric field and when it is aligned perpendicular to an electric field). When driven with an ac electric voltage, there is a transition from the uniform conducting state to a periodic convecting state. An interesting variation on electroconvection is to start with a homeotropic sample. In this case, a material with a negative dielectric anisotropy typically undergoes the Free´dericksz transition at some initial critical voltage, as it prefers to align perpendicular to the electric field. This is a transition from homeotropic alignment to planar alignment. Then, at a higher critical voltage, there is a transition to electroconvection [16 – 18]. Therefore, the ability to produce homeotropic samples of nematic liquid crystals with negative dielectric anisotropy for studies of electroconvection is useful. Though a detailed study of electroconvection in homeotropic N4 is outside the scope of this paper, it is this example of electro-optical response on which we report in this paper as a test of the alignment of the liquid crystal samples. (There have also been studies of homeotropic samples in which there is a direct transition to electroconvection, e.g., see [19], but these cases involve a positive dielectric anisotropy).
2. Materials and characterization techniques We used arachidic acid obtained from Sigma-Aldrich with a quoted purity of 99%. It was used without further purification. The arachidic acid was dissolved in spectral grade chloroform obtained from EM Science at a concentration of 1 mg/ml. The liquid crystal N4 is a eutectic mixture of the two isomers of 4-methoxy-4V-n-butylazoxybenzene (CH3O-C6H4NON-C6H4-C4H9 and CH3OC6H4-NNO-C6H4-C4H9). It was obtained from EM Industries (a Merck company), now EMD Chemicals Inc. [20]. The N4 was used without further purification. The monolayers were formed on an ionic subphase composed of 1 mM CaCl2 solution in purified water. The water was deionized water with a resistivity of 18.2 MVcm. Two different methods were used to include the N4 in the arachidic acid monolayers. First, an arachidic acid monolayer was formed by placing drops of the arachidic acid/chloroform solution on the subphase. Subsequently, a solution of 3.95 10 3 mol/l of N4 in chloroform was added to the surface. The second method involved placing different mixtures of arachidic acid and N4 (described later) directly on the surface. The most basic characterization of Langmuir monolayers is the measurement of the surface pressure versus area isotherms (or isotherms, for short) [21,22]. For monolayers, the surface pressure (P) is defined as the surface tension of pure water (c w ) minus the surface tension of the water– monolayer system (c), i.e., P = c w c. We made isotherm measurements using a KSV 5000 LB trough of maximum area 150 475 mm2. The monolayer is compressed by moving a single barrier at a rate of 10 mm/min (or 15 cm2/min). The surface pressure is monitored during the compression. Phase transitions within the monolayer are identified by the presence of plateaus (first order transitions) or kinks (second order transitions) in the isotherm. The isotherms were measured at a temperature of 22 -C. In addition to surface-pressure area isotherms, we also characterized the composition of the LB films using UV-visible absorption spectroscopy recorded on a Hewlett-Packard 8453 diode array spectrophotometer. The LB films are deposited on a CaF2 substrate for this characterization. The LB films were made by passing the substrate through the Langmuir monolayer in a vertical orientation at a constant rate of 10 mm/min. The number of layers transferred to the substrate was set by the number of times the substrate was passed through the monolayer, with one layer transferred during each lowering and raising of the substrate through the monolayer. 3. Results and discussion Fig. 1 shows the isotherms for the pure arachidic acid system in which 50 Al of the 1 mg/ml arachidic acid solution was placed on the trough. Fig. 1 also shows the arachidic acid systems subjected to the addition of N4 in 10 Al increments of the 3.95 10 3 mol/l N4 solution each time. The pure arachidic acid isotherms are consistent with the behavior expected from previous measurements of this system. Because
J. Collins et al. / Thin Solid Films 496 (2006) 601 – 605
Fig. 1. Plot of isotherms for arachidic acid (solid black curve), and arachidic acid with the addition of N4 (dashed curves). Reading from left to right, each dashed curve represents an addition of 10 Al of the N4 solution, as described in the text. All curves are plotted as a function of the area per molecule for the arachidic acid molecules. Each isotherm shows a single compression and expansion. For the pure arachidic acid case, the compression and expansion are essentially identical. For the cases with N4, the direction of each curve is indicated by the arrows.
we remain below the collapse pressure, the measured isotherm is reproducible in compression/expansion, as can be seen by the overlap of the solid and dashed line. We are able to identify the three phases of arachidic acid that are expected at room ˚ 2/molecules, temperature. Up to an area per molecule of 20.5 A the phase is gaseous. At this point (the ‘‘liftoff point’’), we observed the initial rise in the isotherm that corresponds to the L2 phase of the monolayer. This exists until the observed kink at a surface pressure of 22.5 mN/m. This corresponds to the transition to the LV2 phase. As expected, there are no observed plateaus in the isotherm. As the N4 is added to the monolayer with the arachidic acid in the gas phase, it is expected that the N4 inserts into the monolayer. This expectation is confirmed by the changes in the liftoff point and the existence of a plateau in the isotherm at 12 mN/m. The isotherms are still being plotted as a function of the area/molecule for the number of arachidic acid molecules present in the monolayer. Therefore, the fact that the liftoff point occurs at a higher apparent area per molecule is due to the space occupied by the N4 molecules. Using the isotherms in Fig. 1, one can compute the effective liftoff area as a function of the amount of N4 added to the system. Using the fact that the areas are in terms of the initial amount of arachidic acid, one can estimate the area per molecule occupied by N4 at liftoff based on the shift. For example, for the 10 Al addition, there are 2.4 1016 N4 molecules. The additional area at liftoff is 50.3 cm3. This corresponds to each N4 molecule occupying an area of ˚ 2. Similar calculations for the 20 approximately 21 T .0.1 A ˚ 2. This Al and 30 Al additions also yield an area of 21 T .0.1 A suggests that the N4 is occupying a similar area as the arachidic acid molecules at liftoff, and one can expect a vertical orientation for the N4 molecules in the L2 phase of the monolayer. The next issue is the nature of the plateau. It is known that other liquid crystal systems are able to form multilayers at the
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Fig. 2. Isotherms for three examples of the arachidic acid/N4 mixtures. The mixtures are (by mole percent) 43% (solid curve), 60% (dashed curve), and 69% (dotted curve) N4.
air –water interface under compression. The plateau region is indicative of the formation of a multilayer of N4. The fact that the isotherms ultimately converge on the pure arachidic acid isotherm suggests that the N4 lays on top of a dense arachidic acid monolayer. The subsequent expansion illustrates that most of the N4 is recovered, as the isotherms returns to the compression isotherm at high areas. Though not directly related to the focus of this paper, this collapse mechanisms is interesting and requires further study. An alternate method of formation for the monolayer is to make a mixture of arachidic acid and N4 and directly form a Langmuir monolayer from the mixture. The results for isotherms from this solution are given in Fig. 2. Mixtures by mole percent of 43 – 57%, 60 –40% and 69– 31% of N4 and arachidic acid, respectively are premixed before introduction on the trough. A plateau region at approximately 13 mN/m is observed for these systems. In this case, the shift in liftoff area is also consistent with the amount of N4 present. However, it is interesting that in this case, the expansion curves do not ever recover the compression curves. This suggests a loss of
Fig. 3. Ultraviolet-visible absorption spectra for bulk N4 (dashed line), blank substrate (dotted line), arachidic acid/N4 film transferred at 10 mN/m (lower solid line) and arachidic acid/N4 film transferred at 20 mN/m (upper solid curve). The optical density is given in arbitrary units, as the relative intensity is the important factor. The bulk film is relative to the left axis, with all other cases relative to the right axis. The key feature is the N4 peak at 350 nm that is visible in both LB films.
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Fig. 4. Critical voltage (V c ) for the onset of the Free´dericksz transition (squares) and electroconvection (circles) as a function of applied frequency. As expected, the Free´dericksz transition is independent of frequency, and the transition to electroconvection increases with increasing frequency.
material from this system. It will be interesting to explore the differences between the two film formation mechanisms in future work. However, for the purposes of obtaining LB films with N4 molecules orientated perpendicular to the surface, both types of film suggest that the best conditions for transfer are just before the plateau region. In this region, the N4 is still fully incorporated into the arachidic acid monolayer, and it is oriented perpendicular to the surface. To test that the N4 remains incorporated during the formation of the LB film, the monolayer is transferred at 10 mN/m on to a CaF2 substrate. A UV-visible absorption spectrum of LB films with different numbers of layers is carried out. A typical result is presented in Fig. 3. For comparison, we show the spectra for the plain substrate and bulk N4. The peak for the N4 molecules is clearly observable at 350 nm in all of the LB examples, with the expected increase in amplitude of the peak as the number of layers is increased. To test for homeotropic alignment, a liquid crystal cell was made using the following procedure. An indium – tin oxide (ITO) coated glass slide was washed sequentially in water, in NH3 solution, and in an ultrasonic bath with a solution of 350 ml distilled water, 50 ml sodium hydroxide, and 40 ml liquinox. We selected the mixture of 43% N4 and 57% arachidic acid diluted in chloroform and formed a monolayer
(a)
(b)
that was compressed to 10 mN/m. The pressure was held constant while the ITO glass is coated with 10 layers of the Langmuir monolayers. After the coating of the surface, the glass is baked in an oven at a temperature of 50 -C. A 25 Am mylar spacer is placed between the two ITO glasses. Two opposite sides of the cell are sealed with epoxy. The cell is filled with N4 by capillary action. The two remaining sides of the cell are sealed with 5 minute epoxy. The alignment was checked by observing the samples between crossed polarizers. This confirmed the general homeotropic alignment. As the ultimate goal is to produce samples useful for the study of electroconvection in homeotropic samples, we further tested the alignment by measuring the response of the N4 samples to an applied ac electric field as a function of both applied voltage and frequency. As discussed, N4 has a negative dielectric anisotropy. Therefore, at each frequency, we expect to observe the Free´dericksz transition followed by electroconvection. To measure the response, the applied ac voltage was increased in steps of 0.05 V. A waiting time of 120 s was used at each step. The temperature was maintained at 30 -C. The system was placed between cross polarizers and imaged from above. In the undistorted state, the image is uniformly black. Above the Free´dericksz transition, as the director tilts, the total intensity of the image increased continuously as a function of the applied voltage. This increase in intensity can be used to determine the onset voltage for the Free´dericksz transition. Finally, at the critical value for electroconvection, one observes periodic patterns in the image. For the purposes of this paper, we made a quantitative measure of the Free´dericksz transition as a test of the alignment characterization and report on some qualitatively features of the subsequent electroconvection. A more detailed study of the electroconvection will be the subject of future work. The response to the applied electric field is summarized in Fig. 4. As expected, the Free´dericksz transition is independent of the applied frequency, but the onset to convection is frequency dependent. For strong alignment, one expects the Free´dericksz transition to occur at V cF = (K 33 / (e a e o ))1 / 2, where e a is the difference between the dielectric constant parallel and perpendicular to the electric field (e a = e || e –), K 33 is the bend elastic constant, and e o = 8.85 10 12 C2 / Nm2. For our
(c)
Fig. 5. Three images of the sample covering an area of 3.05 3.05 mm2. The scale bar represents 1 mm. The images were taken with an applied frequency of 5000 Hz and a voltage of (a) 9.05 V (b) 9.55 V and (c) 14.5 V.
J. Collins et al. / Thin Solid Films 496 (2006) 601 – 605
(a)
(b)
(c)
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quantitative characterization of these transitions is beyond the scope of this paper and will be the subject of future work. In summary, we have demonstrated that one can form monolayers of the liquid crystal N4 within an arachidic acid monolayer, and we have characterized the phase behavior of this mixed system at a single temperature. This system can be transferred to ITO coated substrates using standard LB techniques. The resulting surface provides homeotropic alignment for N4 in a standard electroconvection cell when other standard alignment techniques failed. The cells were tested both by imaging the initial state with cross polarizers and by measuring the response to an applied electric field. Acknowledgments This work was supported by NSF grant DMR-9975479 and PRF 39070-AC9. The authors thank Wytze Van Der Veer for his help in using the UCI Laser Facility.
Fig. 6. Four images of the sample covering an area of 3.05 3.05 mm2. The scale bar represents 1 mm. The images were taken with an applied frequency of 5000 Hz and a voltage of (a) 14.55 V (b) 14.60 V (c) 14.79 V and (d) 14.95 V.
measured value of V cF = 9.3 V and the reported value of e a = 0.2 [20], we get K 33 = 15.5 10 12 N. It is worth briefly commenting on the nature of the patterns that are observed above the transition to electroconvection. Typical images are summarized in Figs. 5 and 6. All of the images in Figs. 5 and 6 cover 3.05 3.05 mm2. It should be noted that both figures focus on the case for higher frequencies. For frequencies lower than 1500 Hz, the initial electroconvection state is chaotic, and single images are not particularly useful. Fig. 5 illustrates the two main transitions. Fig. 5a is below the Free´dericksz transition; hence, it is uniformly dark. Fig. 5b illustrates a state above the Free´dericksz, but it is below the transition to electroconvection. Finally, Fig. 5c shows the initial state above the transition to electronconvection for high values of the applied frequency. In this regime, the orientation of the rolls is essentially uniform throughout the cell, and independent of the underlying Free´dericksz domain. (The Free´dericksz domains are distinguished by different degrees of overall intensity due to the crossed polarizers). Fig. 6 illustrates the secondary transition exhibited by the convecting state as the voltage is increased. In Fig. 6a, the the orientation of the rolls has begun to align with the Free´dericksz domains. As the voltage is increased, even more ‘‘solid looking’’ boundaries develop between the roll orientations and the domains of electroconvection shrink in size (Fig. 6b– d). It is interesting to note that this series of patterns is different from that reported in Ref. [16] for a homeotropic cell containing the liquid crystal 4-methoxybenzylidene-4V-butylaniline. The
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