pubs.acs.org/Langmuir © 2009 American Chemical Society
Vertical Alignment of Liquid Crystals with Negative Dielectric Anisotropy on an Inorganic Thin Film with a Hydrophilic Surface Byoung Har Hwang,† Han Jin Ahn,‡ Soon Joon Rho,§ Soo Sang Chae,† and Hong Koo Baik*,† †
Department of Materials Science and Engineering, Yonsei University, Shinchon-Dong 134, Seodaemun-Gu, Seoul 120-749, Korea, ‡R&D 6 Team, LGDISPLAY Co., Ltd., 1007 Deogeun-Ri, Wollong-Myeon, Paju-Si, Gyeonggi-Do 413-811, Korea, and §LCD R&D Center, LCD Business, Samsung Electronics Co., Ltd., Yongin-City, Gyeonggi-Do 449-711, Korea Received February 12, 2009. Revised Manuscript Received April 10, 2009 The vertical alignment of liquid crystals having negative dielectric anisotropy on an amorphous silicon oxide (a-SiOx) thin film is the consequence of the anisotropic interaction between liquid crystals and a-SiOx thin films. To investigate the mechanism of the vertical alignment, we changed the physicochemical characteristics of alignment layers by controlling the composition, since the anisotropic interaction depends on the nature of both liquid crystals and an alignment layer. The variation of composition gives rise to a change in the polarizability, which is a simple measure of induced-dipole strength at the surface of the alignment layer. There is a critical transition point from planar to vertical alignment of liquid crystals, and it is the long-range van der Waals interaction that is responsible for the vertical alignment. The competition between long-range van der Waals interaction and short-range dipolar interaction were investigated and analyzed in terms of the interfacial energy between liquid crystals and an alignment layer.
Introduction The alignment of liquid crystals has been a topic of interest at both fundamental and technological levels for a long period of time. Several studies on the behavior of nematic liquid crystals (LCs) have been carried out by many researchers in recent years.1,2 In particular, the vertical alignment of LCs has been studied, since it provides an unprecedented contrast ratio and wide viewing angle characteristics. Because of this, it is widely used for liquid crystal displays such as information display devices, large-area LCD TVs, and digital displays in medical devices. One of the most widely used techniques for vertical alignment is surface treatment using silanes or long alkyl side chain alcohols as homeotropic coupling agents.3,4 It is the steric hindrance from these protruding molecules that gives rise to the vertical alignment. Another technique is the oblique evaporation of silicon monoxides.5 The columnar structure obtained by the shadowing effect gives rise to a tilted vertical alignment of LCs. A further example is the vertical alignment layer that has a hydrophobic surface, such as fluorinated diamond-like carbon (FDLC) thin films.6 LCs on a FDLC thin film are aligned vertically, since the interaction between LCs is stronger than the interaction between LCs and the FDLC thin film. Recently, the inorganic silicon oxide (a-SiOx) layer has been intensively studied as a vertical alignment layer, due to its robust *Corresponding author. Mailing address: Department of Material Science and Engineering, Yonsei University, Shinchon-dong 134, Seodaemun-gu, Seoul 120-749, Korea. E-mail:
[email protected]. Telephone: +82-22123-2838. Fax: +82-2-312-5375. (1) Takatoh, K.; Hasegawa, M.; Koden, M.; Itoh, N.; Hasegawa, R.; Sakamoto, M. Alignment Technologies and Applications of Liquid Crystal Devices; Taylor & Francis Inc.: New York, 2005; Chapter 2. (2) Rasing, Th.; Musevic, I. Surfaces and Interfaces of Liquid Crystals; Springer: Berlin, Heidelberg, New York, 2004; Chapter 6. (3) Janning, J. L. Appl. Phys. Lett. 1972, 21, 173. (4) Miller, L. J.; Grinberg, J.; Myer, G. D.; Smythe, D.; Smith, W. Liquid Crystal and Ordered Fluids; Johnson, J. F., Porter, R. S., Eds.; Plenum Press: New York/ London, 1978; Vol. 3. (5) Hiroshima, K. Jpn. J. Appl. Phys. 1982, 21, L761. (6) Ahn, H. J.; Rho, S. J.; Kim, K. C.; Kim, J. B.; Hwang, B. H.; Park, C. J.; Baik, H. K. Jpn. J. Appl. Phys. 2005, 44, 4092.
8306 DOI: 10.1021/la9005339
properties under harsh conditions, such as light and heat, which can cause deterioration of organic materials. However, many issues related to the mechanism of alignment of LCs have not yet been established, especially in the case of the vertical alignment of the LCs on the inorganic alignment layer. Although the vertical alignment is the consequence of complex interactions between the LCs and the alignment layer, only a few studies have focused on the nature of the LC.7-10 For example, Lu7 has shown that the sign of dielectric anisotropy (Δε) of the LC, rather than the columnar structure of a-SiOx thin film, plays an important role in the final alignment of the LCs. Vertical alignment is usually obtained for a LC with a negative Δε, while planar alignment is obtained with a positive Δε. On the other hand, Chen and coworkers8-10 showed that the addition of a material having a strong longitudinal dipole moment to a LC with large negative Δε brings about an alignment transition from the planar to the vertical alignment. These studies clearly showed and logically explained the unique phenomena involved in the alignment of LCs on a-SiOx thin film; however, they were still insufficient to allow full comprehension of how the vertical alignment of LCs on a-SiOx thin film arises. Unresolved questions, for example, were whether the alignment depends on the magnitude of the dielectric anisotropy of LCs or on the deposition method and condition for fabricating a-SiOx thin film. Furthermore, vertical alignment on a flat surface that has a high surface energy is not simple to explain, since it should lead to a planar alignment when basic mechanisms are considered.11 In general, it is very difficult to achieve a vertical alignment on other inorganic oxide layers that are chemically hydrophilic and form physically flat surfaces, for example, cerium oxide, aluminum oxide, titanium oxide, and so on (see the Supporting Information). The fact that (7) Lu, M. Jpn. J. Appl. Phy. 2004, 43, 8156. (8) Chen, C.; Anderson, J. E.; Bos, P. J. Jpn. J. Appl. Phys. 2005, 44, L112. (9) Chen, C.; Bos, P. J.; Kim, J.; Li, Q. J. Appl. Phys. 2006, 99, 123523. (10) Chen, C.; Bos, P. J.; Anderson, J. E. Liq. Cryst. 2008, 35, 465. (11) Creagh, L. T.; Kmetz, A. R. Mol. Cryst. Liq. Cryst. 1973, 24, 59.
Published on Web 05/05/2009
Langmuir 2009, 25(14), 8306–8312
Hwang et al.
Article Table 1. Deposition Conditions for Fabrication of a-SiOx Thin Films
sample S1 S2 S3 S4
base pressure (torr)
working pressure (torr)
-6
1.0 10 3.0 10-6 3.0 10-6 3.0 10-6
-7
1.0 10-4
oxide thin films cannot vertically align LCs although they have similar physical morphology and surface energy to a-SiOx thin film is very interesting, but we cannot explain this phenomenon accurately by currently accepted general mechanisms. Therefore, it is necessary to understand, in a more rigorous way, the interaction between LCs and a-SiOx thin film, looking at it from a more concrete and microscopic point of view, because this interaction is the key that will explain vertical alignment of LCs on inorganic thin films. In this paper, we report that it is the anisotropic interaction between LCs and a-SiOx thin films that determines the alignment of LCs. This interaction is strongly dependent on the physical and chemical nature of a-SiOx. Since LCs with negative Δε have a strong dipole moment perpendicular to the molecular axis, there are two distinguishable interactions that will depend on the interacting direction. These are the dipole/induceddipole interaction and the induced-dipole/induced-dipole interaction between LCs and a-SiOx thin film, in particular near the interface between these.7,10,12 We believe that the induceddipole/induced-dipole interaction brings about the vertical alignment itself, while the vertical alignment can be easily disturbed by dipole/induced-dipole interaction. Experimentally, we investigated the existence and the strength of the induced-dipole/induced-dipole interaction by introducing a screening layer between the LC and the a-SiOx thin film. In order to determine the relationship between the alignment of the LC and the anisotropic interaction, we changed the refractive index of the alignment layer, which led to a change in the polarizability that determines both London dispersion (LD) and Debye forces.13
Experimental Section Preparation of the Amorphous Silicon Oxide Thin Films. To control the refractive index, thin films of amorphous silicon suboxide (a-SiOx) were deposited normal to the substrate by reactive evaporation of silicon monoxide (SiO) in ambient oxygen onto unheated substrates. The chamber was evacuated up to 1 10-6 torr. Table 1 shows the experimental conditions in detail. Indium tin oxide (ITO) coated glass was used as the substrate. Before thin films were deposited, all substrates were cleaned ultrasonically and sequentially for 10 min each with trichloroethylene, acetone, and isopropyl alcohol. SiO powder (99.99% purity) was used as the evaporation source. The film growth rate was maintained using a quartz crystal monitor, and film thickness was fixed at 100 nm. To ensure the sample thickness for each composition, tooling factors were calibrated with an R-step surface profiler. Deposition of the Screening Layer. LD interaction was investigated as the main factor of vertical alignment of LCs having negative Δε on the a-SiOx thin film. FDLC thin films as the screening layer were deposited on the stoichiometric a-SiOx (x ∼ 2) by cosputtering with carbon and polytetrafluoroethylene (PTFE) target to give a changing thickness. Since the hydrophobicity of the FDLC surface is dependent on the ratio of F/C, we controlled the target power ratio, as a single FDLC layer gives rise (12) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985; Chapters 5 and 6. (13) French, R. H. J. Am. Ceram. Soc. 2000, 83, 2117.
Langmuir 2009, 25(14), 8306–8312
deposition rate (A˚/s)
oxygen gas addition
substrate temp.
∼1.5 1 1 1
no no yes yes
RT RT RT RT
to planar alignment of LCs.14 The thickness of the FDLC thin film was changed from 5 to 40 nm. Characterization Techniques. Films were characterized by several different measurements. To confirm the change of the composition of a-SiOx thin films, X-ray photoelectron spectroscopy (XPS) spectra were obtained with a monochromatic XPS (VG microtech, ESCA 2000) system, using the Al KR X-ray line for photoelectron excitation. To investigate the various binding states of the silicon atoms in the films, the Si 2p spectra were decomposed into various Gaussian peaks.15-17 In order to avoid any shift in spectra due to charging and to bring energies to the same reference point, all energies are stated with reference to the C 1s line at a binding energy of 284.6 eV. The change of the composition of a-SiOx thin films was confirmed, and the magnitude of van der Waals (vdW) interaction was estimated by measuring the refractive index of the thin films deposited on the Si(100) substrates at the wavelength of 633 nm with a L117 ellipsometer (Gaertner Scientific Corporation). Contact angles were measured by the sessile drop technique using DI-water and diiodomethane, after which the dispersion and the polar surface energy were calculated using the Owens-Wendt geometric model.18 Finally, optical textures and conoscopic interference images were observed with a polarized optical microscope (POM, Olympus, BX-41) in order to confirm the alignment of LCs. Liquid Crystal Cell Fabrication. To evaluate the alignment of LCs, liquid crystal cells were fabricated. The spacers for a desired cell gap (4.75 μm) were mixed with a UV-curable adhesive, and then, using this adhesive, the cells were assembled with a-SiOx coated substrates. A typical size of an assembled cell was 20 mm 25 mm 4.75 μm. The cell was filled with three types of LCs with negative dielectric anisotropy (mixtures, Δε = -2.0, -3.8, and -4.4 from Merck Co.) at room temperature. Finally, the cells were sealed with the same UV-curable adhesive used for assembly.
Results and Discussion Macroscopic Approach to the Vertical Alignment on the a-SiOx. Generally, vertical alignment of LCs can be easily obtained on a hydrophobic surface or by use of specific physical structures such as columnar structures, ridges, and holes. In contrast, a-SiOx thin films prepared by our method requires none of these features.10 a-SiOx (x ∼ 2) has a hydrophilic surface that usually leads to planar alignment. Even so, LCs having negative Δε can also be aligned vertically on the silicon oxide thin film as shown in Figure 1a. The vertical alignment of LCs is usually explained by the competition between LC-surface interaction and LC-LC interaction. Creagh and Kmetz11 showed that LC orientation is determined by the surface energy of the liquid crystal (γL) and the alignment layer (γS). Their theory sometimes agrees well with experimental observations. For example, an alignment layer having low surface energy, such as FDLC, usually gives rise to a vertical alignment of LCs.6,14 However, this theory cannot (14) Ahn, H. J.; Kim, J. B.; Kim, K, C.; Hwang, B. H.; Kim, J. T.; Baik, H. K. Appl. Phys. Lett. 2007, 90, 253505. (15) Durrani, S. M. A.; Al-Kuhaili, M. F.; Khawaja, E. E. J. Phys.: Condens. Matter 2003, 15, 8123. (16) Nesheva, D.; Bineva, I.; Levi, Z.; Aneva, Z.; Merdzhanova, T.; Pivin, J. C. Vacuum 2003, 68, 1. (17) Shallenberger, J. R. J. Vac. Sci. Technol., A 1996, 14, 693. (18) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
DOI: 10.1021/la9005339
8307
Article
Hwang et al.
Figure 1. DI-water contact angles on the a-SiOx (x ∼ 2) thin film: (a) as-prepared a-SiOx and (b) carbonaceous contaminated a-SiOx, which was exposed to the atmosphere for 7 days. All samples give rise to vertical alignment, but there are some defects in the sample fabricated by contaminated a-SiOx thin films.
precisely explain the vertical alignment of LCs on the a-SiOx thin film. The original silicon oxide surface is hydrophilic, but its surface becomes easily contaminated by carbonaceous material.19-21 Thus, the water contact angle of an a-SiOx thin film exposed for prolonged times to the atmosphere becomes relatively high, compared to an as-prepared thin film as shown in Figure 1. A different contact angle means a different surface energy and a different interaction between LCs and the a-SiOx surface. We fabricated test cells using as-prepared and aged a-SiOx (x ∼ 2) thin films and found that vertical alignment was achieved in all cells, as determined from the orthoscopic and conoscopic observations. Based on this result, we may conclude that the surface energy term is inappropriate for explaining the vertical alignment phenomenon, because there is no large difference in the degree of the vertical alignment even if the surface energy difference is large. Furthermore, in Table 2, we describe the surface energies of various thin films and the alignment state of LCs in the cells fabricated using these films. The result also shows that it is difficult to accurately determine the relationship between surface energy and alignment of LCs on SiOx. For example, sample no. 2 shows the lowest surface energy but it leads to planar alignment of LCs. Consequently, we investigated the vertical alignment of LCs from a more microscopic point of view. vdW Interaction between LC and a-SiOx Thin Film. Several types of interaction including π-π electron coupling, steric hindrance, dipole-dipole interaction, and van der Waals (vdW) interaction are related to the alignment of nematic LCs.22 Among these key factors, vdW interaction has been referred to as the major force that determines the anisotropic anchoring of LCs on the a-SiOx surface.7,10 The vdW forces between molecules in materials result from the interactions of dipoles.13 The summation of the dipolar attractions over the atoms or molecules in a bulk material produces a macroscopic attraction. The vdW forces can be considered as resulting from three additive terms, the Keesom force, the Debye force, and the London dispersion forces, as shown in eq 1. FvdW ≈ FKeesom þ FDebye þ FLD
ð1Þ
The Keesom force is the force between two permanent dipoles, while the Debye force is related to the induction effect by the permanent dipole moment. On the other hand, many molecules that have no permanent dipole moments also exhibit long-range vdW forces, since interatomic bonds in atoms and molecules can (19) (20) (21) (22)
Rana, N. B.; Shadman, F. IEEE Trans. Semicond. Manuf. 2003, 16, 76. Saga, K.; Hattori, T. J. Electrochem. Soc. 1996, 143, 3279. Vig, J. R. J. Vac. Sci. Technol., A 1985, 3, 1027. Okano, K.; Matsuura, N.; Kobayashi, S. Jpn. J. Appl. Phys. 1982, 21, L109.
8308 DOI: 10.1021/la9005339
Table 2. Surface Energy Data for Various Thin Films from Contact Angle Measurementsa contact angle (deg)
surface energy (mJ/m)
sample DI-water diiodomethane S2
47.016
38.667
γs
γPs
γds
alignment
57.143 21.535 35.608
random planar S3 26.308 27.8398 69.942 30.722 39.17 vertical S4 18.45 32.764 71.799 35.973 35.826 vertical FDLC 107.3 72.8022 21.496 0.329 21.167 vertical a The surface energy (mJ/m) depends on the deposition condition; however, not all the SiOx thin films show hydrophobic properties. The sample S2 has the lowest surface energy, but it gives rise to a planar alignment. The data for a FDLC thin film having a hydrophobic surface are also shown as an example.
themselves induce dipole moments in nearby interatomic bonds. These induced-dipole/induced-dipole interactions, the so-called London dispersion (LD) force, result in an attractive force. There are two interactions to consider between LCs and an a-SiOx thin film depending on the interacting direction, namely, the short-range dipolar interaction and the long-range van der Waals interaction.7-10 Short-range dipolar interaction may include charge-charge interaction, charge-dipole interaction, Keesom force, Debye force, hydrogen bonding, or even chemical bonding.23-28 In fact, there can be local fields caused by ions or permanent dipoles at the surface of a-SiOx thin film. These fields are sufficient to affect LC alignment near the alignment surface and lead to an anchoring transition. If the LCs have negative Δε, the LCs near the local field tend to align perpendicular to the field direction. This interaction is the Keesom interaction and will give rise to the planar alignment of LCs.24,25,27 However, we exclude this effect from our discussion because this is apparently responsible for planar alignment but is not effective on the vertical alignment of LCs on a-SiOx surface, as interpreted from our experimental results and references.7-10 On the other hand, LCs with negative Δε have a strong dipole moment perpendicular to the molecular axis of the liquid crystal and this induces the dipole moment at the a-SiOx surface. There is no permanent dipole to the longitudinal direction; consequently, the LD interaction known as long-range vdW interaction occurs. The directions of these interactions are different from each other (23) Alexe-Ionescu, A. L.; Barberi, R.; Bonvent, J. J.; Giocondo, M. Phys. Rev. E 1996, 54, 529. (24) Uchida, T.; Watanabe, H.; Wada, M. Jpn. J. Appl. Phys. 1972, 11, 1559. (25) Barbero, G.; Evangelista, L. R.; Madhusudana, N. V. Eur. Phys. J. B 1998, 1, 327. (26) Osipov, M. A.; Sluckin, T. J.; Cox, S. J. Phys. Rev. E. 1997, 55, 464. (27) Pereira, H. A.; Batalito, F.; Evangelista, L. R. Eur. Phys. J. E 2005, 16, 267. (28) Yaroshchuk, O.; Koval’Chuk, O.; Kravchuk, R. Mol. Cryst. Liq. Cryst. 2005, 438, 195.
Langmuir 2009, 25(14), 8306–8312
Hwang et al.
Article
due to the anisotropy of the LCs. Therefore, these two interactions play an important role in the vertical alignment of LCs on the a-SiOx thin film, and the competition between them may determine the ultimate alignment of the LCs. Considering the direction of interaction and previous work related to the role of dipoles of LC molecules, the LD interaction appears to bring about vertical alignment of LCs while the vertical alignment can be easily disturbed by the Debye interaction.7-10 LD Interaction for Vertical Alignment. The mechanism of alignment of LCs on the a-SiOx thin film was examined from two perspectives. The first was to confirm of the existence of the LD interaction and its role in the alignment of LCs. The second was to examine the way in which the physicochemical characteristics of an inorganic alignment layer relate with the alignment of the LCs. In order to verify the existence of the LD interaction and to confirm that this interaction is responsible for the vertical alignment of LCs on a-SiOx thin films, we inserted a screening layer between the LCs and the stoichiometric a-SiOx (x ∼ 2) thin film.10,29,30 The Debye interaction needs at least one permanent dipole, and its effect sharply decreases as the interaction distance increases.12 Thus, it has little effect on the alignment of the LCs over the screen layer. In contrast, the LD force can influence the alignment of the LCs over the screen layer although its strength is weakened.30-33 If the screen layer is sufficiently thin, both shortrange and long-range interactions can occur between the LC and a screen layer. On the other hand, only the long-range interaction between the LCs and an alignment layer exists, since the shortrange interaction between LCs and an alignment layer is screened by the screen layer. However, if the thickness of the screen layer increases, the strength of the LD interaction between the LCs and an alignment layer will decrease, while interactions between the LC and the screen layer remain. Above a critical screen layer thickness, LC alignment will be governed only by interactions between the LCs and the screen layer. A screening layer of FDLC was deposited as shown in Figure 2a. Since the vertical alignment of LCs on the hydrophobic FDLC surface depends on the fluorine concentration, we controlled the fluorine concentration to be conducive to planar alignment.14 We also changed the thickness of the FDLC thin film to investigate the power of the LD interaction. In order to exclude any other effects, we did not perform any other alignment treatment, such as rubbing, UV irradiation, ion beam irradiation, and so on. The results shown in Figure 2b clearly indicate the existence of LD interaction and its role. Although the FDLC fabricated by our deposition condition should lead to a planar alignment, surprisingly, the vertical alignment was maintained for up to a 30 nm screen layer thickness, based on our orthoscopic and conoscopic observations. The transition of the LC alignment depended on the thickness of the FDLC layer. Up to a 20 nm thickness, the vertical alignment was perfectly maintained. At 30 nm, some defects and degradation of vertical alignment were observed, and the alignment eventually turned into a random planar alignment above 30 nm. These results can be interpreted by the following theoretical approach.28 The total free energy for the unit surface of the junction is f ¼ f13 þ f12
ð2Þ
(29) Gwag, J. S.; Kim, J. C.; Yoon, T. H.; Cho, S. J. J. Appl. Phys. 2006, 100, 093502. (30) Alexe-lonescu, A. L.; Barberi, R.; Giocondo, M.; Cnossen, G.; van der Donk, T. H. Appl. Phys. Lett. 1995, 66, 27. (31) de Gennes, P. G.; Duvois-Violette, E. J. Colloid Interface Sci. 1976, 57, 403. (32) Duvois-Violette, E.; de Gennes, P. G. J. Phys. Lett. 1975, 36, L-255. (33) Blinov, L. M.; Sonin, A. A. Langmuir 1987, 3, 660.
Langmuir 2009, 25(14), 8306–8312
Figure 2. (a) Stoichiometric a-SiOx (x ∼ 2) covered by the FDLC thin film of thickness l. The system is considered as a junction of three different media. The total energy per unit surface is due to the anisotropic vdW interactions between the three media. (b) Polarized optical microscope (POM) images depending on the thickness of FDLC. The conoscopic images are shown in the corners of the pictures. The cross arrow indicates the optical direction of the polarization sheet.
where f12 refers to the LC/FDLC interaction and f13 refers to the LC/a-SiOx interaction. The term f23 is neglected, as it is expected to be independent of the LC orientation. On the other hand, the term f12 includes overall interactions such as LD and dipolar interaction and this is assumed to be independent of the thickness of the screening layer. Here, the term f13 is mainly due to the LD interaction that seems to be responsible for the vertical alignment of LCs. Considering the screening effect of the FDLC layer and characteristics of vdW interaction, the term f13 depends on 1/l3.12,30,31 As the thickness l of the screen layer increases, the LD interaction between LCs and a-SiOx weakens until, eventually, the anchoring transition occurs from the vertical to the planar alignment. Over 30 nm, the f12 overwhelms the LD interaction and leads to the planar alignment of the LCs. Therefore, from experimental results and theoretical analysis, the LD interaction apparently exists between the LC and an alignment layer and it plays an important role in the vertical alignment of the LCs. Competition between Interactions. The LD interaction depends strongly on the nature of the interacting materials. Indeed, the vertical alignment of LCs on a-SiOx thin films is the DOI: 10.1021/la9005339
8309
Article
Hwang et al.
Figure 3. Si 2p spectra of SiOx films with different oxygen concentrations. The Si 2p spectra were fit into five peaks corresponding to Si0 (99.4
eV), Si1+ (100.3 eV), Si2+ (101.2 eV), Si3+ (102.0 eV), and Si4+ (103.0 eV).15 The five valence states of silicon correspond to tetrahedrally coordinated silicon bonded to 0, 1, 2, 3, and 4 oxygen atoms. The relative areas of the peaks were combined with the known oxygen coordination numbers to yield stoichiometry. The refractive index at 633 nm is also shown at the corner of each panel.
consequence of complex interactions; therefore, it should depend on the Δε value of LCs as well as on the physical and chemical nature of the alignment layer, in particular its polarizability. Generally, the strength of induced-dipole moment depends on the polarizability of the interacting material. Polarizability is proportional to the refractive index as follows:12 ! 4πRm NA n2 -1 ¼ 2 Vm n þ2 3
ð3Þ
where n is the refractive index of the medium, Vm is molar volume, and NA is Avogadro’s number. Therefore the refractive index, n, is regarded as a simple measure of the strength of the vdW interaction.13 Experimentally, in order to investigate the relationship between the vertical alignment and vdW interactions, we controlled the refractive index of a-SiOx thin films by controlling their oxygen concentration.15-17 Results of controlled oxygen experiments on Si 2p core-level spectra from XPS and refractive indices from ellipsometry are shown in Figure 3. The refractive index of thin films tends to decrease as the oxygen concentration increases.15 On the other hand, LC textures were observed to confirm the alignment of LCs (Δε = -3.8) on the a-SiOx thin films by conoscopic and orthoscopic POM, as shown in Figure 4. LCs were aligned vertically on the relative oxygen-rich thin films but were not on the oxygen-deficient thin films.34,35 This result is not limited to a-SiOx thin films deposited by thermal evaporation. The a-SiOx thin films deposited by sputtering also show same tendency (see the Supporting Information). Consequently, the (34) Hwang, B. H.; Kim, K. C.; Ahn, H. J.; Kim, J. B.; Hyun, D. C.; Baik, H. K. In Proceedings of the 21st International Liquid Crystal Conference; University of Colorado: Boulder, CD, 2006; p 983. (35) Kim, J. B.; Choi, C. J.; Park, J. S.; Jo, S. J.; Hwang, B. H.; Jo, M. K.; Kang, D. S.; Lee, S, J.; Kim, Y. S.; Baik, H. K. Adv. Mater. (Weinheim, Ger.) 2008, 20, 3073.
8310 DOI: 10.1021/la9005339
alignment of LCs depends on the oxygen concentration of the a-SiOx. In other words, it depends on the refractive index of the a-SiOx thin film. As mentioned above, the strength of vdW interaction can be estimated by the index of refraction. A higher refractive index means a stronger attractive force between interacting materials due to the larger value of induced-dipole moments, which is a function of polarizability. Therefore, LCs near an alignment layer can induce a strong dipole moment at the surface of the alignment layer if the alignment layer has high refractive index or high polarizability. However, the strength of both LD interaction and Debye interaction depends on the polarizability of the alignment layer. In addition, the strength of Debye interaction is larger than that of LD interaction at a short-range interacting distance.12 Consequently, Debye interaction is dominant at a surface layer composed of a monolayer of molecules in direct interaction with the alignment layer. Accordingly, LC alignment can be often different from bulk LC alignment.36 On the other hand, as one moves away from the surface, short-range interactions are gradually weakened, while the long-range interaction is relatively strengthened. Therefore, the competition between two contrary interactions is intense under a certain critical distance; this region is termed the interfacial layer.37 The LC alignment evolves in this region, and this evolution results in a definite anchoring of bulk LCs. Interestingly, LD interaction is a long-range interaction, while Debye interaction is a short-range interaction. Hence, the LD interaction is more effective than the Debye interaction in the interfacial region. However, if the Debye interaction is very strong at the surface region, it strongly influences the LC alignment for a considerable distance. Therefore, the strong dipole/induceddipole interaction disturbs the vertical alignment of the LCs. (36) Feller, M. B.; Chen, W.; Shen, Y. R. Phys. Rev. A 1991, 43, 6778. (37) Schuddeboom, P. C.; Jerome, B. Phys. Rev. E 1997, 56, 4294.
Langmuir 2009, 25(14), 8306–8312
Hwang et al.
Article
Figure 5. Liquid crystal cells with various a-SiOx alignment layers (x ∼ 1.4 and ∼2) viewed between two crossed polarizers. Alignment changes from vertical to planar alignment when Δε of LCs increase on the a-SiOx thin films (x ∼ 1.4). However, there is no alignment transition in liquid crystal cells fabricated by a-SiOx thin films (x ∼ 2). The cell on the right end, filled with LCs having the highest dielectric anisotropy, shows poor vertical alignment. This result shows that the alignment of LCs is strongly dependent on the nature of the interacting materials.
alignment is favored for one with a high refractive index. However, in the current study, we consider LCs with negative Δε, not positive. For the analysis, we considered our system to be two semi-infinite planar media (1) and (3), separated by a planar slab of vacuum (2), as shown in Figure 6a. The dispersive energy and its anisotropy can be obtained from the dielectric functions ε(ω) of the liquid crystal and the solid. If liquid crystal (L) is in contact with an isotropic solid (S), the interface energy γ0 of this system is then given by
Since the competition of various interactions depends on the nature of the interacting materials, the physicochemical characteristics of the LCs also play an important role in their vertical alignment on a-SiOx thin films.9,10 In Figure 5, we show the alignment of LCs with different negative Δε on the various a-SiOx thin films with different oxygen concentrations. Generally, it is well-known that the vertical alignment of LCs that have large negative Δε is very difficult.8 Our experimental results also show the same tendency, and this can be understood on the basis of our previous analysis. More accurately, LCs with large negative Δε induce a stronger dipole moment and this brings about the planar alignment of the LC. On the other hand, even though the LCs have relatively large negative Δε, they can be aligned vertically if the polarizability of the alignment layer is sufficiently small, as shown in Figure 5. vdW Contribution to the Interfacial Energy. On the other hand, we examined the vdW contribution to the interfacial energies corresponding to the anchoring direction of LCs. We applied the analytical expressions of Bernasconi et al.38 to our analysis. They found that a planar alignment is favored for an alignment layer with low refractive index while a vertical (38) Bernasconi, J.; Straddler, S.; Zeller, H. R. Phys. Rev. A 1980, 22, 276.
Langmuir 2009, 25(14), 8306–8312
1 γ0 ðθÞ ¼ - F L, L ðθÞ þ F L, S ðθÞ 2
ð4Þ
where θ is the polar angle of liquid crystal molecules. In the case of comparing planar alignment and vertical alignment, the interfacial energy becomes γ0^ -γ0 ΔR Δd ¼A þB Rav d γ0
ð5Þ
)
Figure 4. POM images of LC texture on a-SiOx thin films with different oxygen concentrations: (a) S1 (x = 1.13), (b) S2 (x = 1.397), (c) S3 (x = 1.69), and (d) S4 (x = 1.904). The cell was filled with LCs (Δε = -3.8). There is an abrupt change of nematic LC alignment from planar to vertical. The azimuthal angle of the LCs is random.
where 3 3Rav ωL0 2 4RS ωS0 2 RS ωS0 2 1A ¼ 2 2 Rav ξS ðξs þ ξL Þ ðξs þ ξL Þ2 4ξL
!
! 4RS ωS0 2 ξL 2 B ¼2 -1 Rav ωL0 2 ξS ðξS þ ξL Þ Rav ¼
1 jj R þ 2R^ , 3
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ξS ¼ ωS0 RS þ 2,
ð6Þ
ð7Þ
1 jj R -R^ 3
ð8Þ
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ξL ¼ ωL0 Rav þ 2
ð9Þ
ΔR ¼
γ0 is the interface energy for a free LC surface, ωx0 is the characteristic absorption frequency of medium (k), d is the intermolecular distance between the LCs and alignment layer, DOI: 10.1021/la9005339
8311
Article
Hwang et al.
Figure 7. Interfacial energy between LCs and a-SiOx thin films depends on both the refractive index of LCs and the alignment of LCs. The vertical alignment of nematic LCs is favored on alignment materials with a lower refractive index.
Figure 6. (a) Geometry for the calculation of the free energy of interaction: d is the intermolecular distance between nematic LCs and the alignment layer. The value of d might depend on the alignment of nematic LCs, but we assumed that it is constant and is about 1 nm. (b) van der Waals contribution to the anchoring versus the refractive index ns of the isotropic solid: the model parameters are ωL0 = 7.54 1015 s-1 and ωS0 = 1016 s-1. Each Δγ value for samples is represented in the figure by arrows.
and the anisotropic intermolecular distance Δd is assumed zero. All parameters and equations are from a previous publication;38 however, the polarizability of LCs, R, is simulated. Bernasconi et al.38 studied the vdW contribution to the surface energy of LCs (free surface) and then considered LCs in contact with an isotropic solid. The surface energy γ is a function of the free energy of interaction F between two media, which is related to dielectric tensor anisotropy. The surface energy of LCs (-1/2FL,L 0 ) was obtained by considering vdW interactions between two semiinfinite LC layers, separated by a planar slab of vacuum. On the other hand, interface energy γ0 (eq 4) reflects the increased energy of the system by contacting LCs with an isotropic solid. Since LC is the uniaxial medium, the increased energy (FL,S(θ)) depends on both contacted isotropic solid and the alignment of LCs. This means that we can expect the alignment of LCs on the specific isotropic solid by comparing the interface energies at each system. We finally present the dependence of the LC alignment on the vdW interaction as a function of the refractive index ns of a-SiOx thin films in Figure 6b. The vdW interaction contribution to the interfacial energy follows a simple trend. It demonstrates that vertical alignment of LCs is favored on alignment layers with lower refractive index. It should be noted here that the anisotropic polarizability of LCs, ΔR, is the slope of eq 5. The result is opposite to the case in which LCs have positive dielectric
8312 DOI: 10.1021/la9005339
anisotropy. The interfacial energy increases as the refractive index increases. From this analysis, we can find the critical point where the sign of the interfacial energy changes from minus to plus, that is, the alignment transition point. In our simulation, this point, n, was 1.8 and this result coincided well with our experimental results. The schematic view of the relationship between interfacial energy and alignment of LCs, depending on the refractive index of a-SiOx thin film, is shown in Figure 7. The interfacial energy between LCs and a-SiOx thin films depends on both the refractive index of a-SiOx thin films and the alignment of LCs, since this is also a consequence of the competition between long-range interaction and short-range interaction. The Debye force is considerably stronger in the case of a-SiOx thin films with higher polarizability. Therefore, the planar alignment is energetically stable and favorable compared to the vertical alignment. Finally, LCs align in a planar arrangement on oxygen deficient a-SiOx thin films.
Conclusion In conclusion, the vertical alignment of LCs with negative Δε is strongly dependent on the physicochemical properties of the a-SiOx thin film. The LC alignment is a consequence of the competition between various interactions including LC-LC interaction, Debye interaction, and LD interaction, among others. LD interaction and Debye interaction appear to play important roles in the alignment of LCs on the a-SiOx thin film. The existence and role of the LD interaction were investigated by inserting a screening layer. The polarizability of a-SiOx thin film is controlled by changes in the refractive index, which is a simple measure of vdW interactions. We observed that LCs tend to align vertically on the a-SiOx thin films with small polarizability. In summary, the LD interaction brings about the vertical alignment itself, while the vertical alignment can be easily disturbed by Debye interaction. Acknowledgment. The authors would like to thank Prof. Oleg Yaroshchuk of the National Academy of Sciences of Ukraine for very helpful discussion. This research was financially supported by Samsung Electronics, Co., Ltd. Supporting Information Available: Vertical alignment on various oxide thin films and the anchoring transition on a-SiOx thin films deposited by sputtering. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2009, 25(14), 8306–8312
