Article pubs.acs.org/JPCC
Alignment of a Columnar Hexagonal Discotic Liquid Crystal on SelfAssembled Monolayers Zuhoor H. Al-Lawati,† Richard J. Bushby,‡ and Stephen D. Evans*,† †
Molecular and Nanoscale Physics, School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, U.K. School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
‡
ABSTRACT: The alignment of the columnar hexagonal (Colh) phase of the discotic liquid crystal 2,3,6,7,10,11hexakis(undecyloxy)triphenylene on well-characterized surfaces comprised of thiol-on-gold self-assembled monolayers (SAMs) has been studied using polarizing optical microscopy in combination with evanescent wave ellipsometry. Homeotropic alignment is preferred for both COOH- and CH3functionalized SAMs (both high and low energy surfaces). Even when the SAM layer is comprised of edge-on oriented triphenylene units, evanescent wave ellipsometry shows that the liquid crystal aligns predominantly in a homeotropic (face-on) manner. These results confirm the view that the surface acts primarily as a mechanical barrier and although the chemical nature of the surface can enhance the ease or quality of the alignment of the Colh phase, it is of secondary importance.
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INTRODUCTION Molecular species that give rise to “discotic” liquid crystals (DLCs) are usually made up of a flat disk-shaped aromatic core surrounded by flexible alkyl, alkoxy, or acyloxy side chains. In the discotic nematic phase (ND) the short axes of these disks are all more-or-less aligned with the director, but there is no long-range positional order. In the much more common columnar phases (Colh, Colr, etc.), not only is there this orientational order but also there is positional order of the disks in two dimensions. The disks are stacked center-on-center to form columns which are ordered on a regular, most often hexagonal, lattice. However, there is no correlation of the positions of the disks column-to-column: no three-dimensional order. At a solid surface the DLC can adopt a homeotropic (disks face-on to the surface), planar (disks edge-on to the surface), or tilted-planar orientation as shown schematically in Figure 1. Whereas the ND phase aligns rapidly and spontaneously, columnar phases only align when thin films are cooled very slowly (at 0.1 °C/min or less) from the isotropic phase or when thin films are annealed ∼1 °C (or less) below the columnar-toisotropic transition temperature. This can be a very slow process, and not only does the aligning effect of the surface propagate very slowly out into the sample but often it only propagates over a few tens of micometers. However, although it can be very difficult to control the orientation of columnar phases, control over their orientation is the key to developing new applications.1−11 When a dilute solution of a discogen in an organic solvent is brought into contact with a solid surface an adsorbed monolayer often forms at the interface in which the disks lie face-on to the surface.12−16 Face-on orientation (homeotropic alignment) also dominates the behavior of both ND2,17,18 and © 2013 American Chemical Society
Figure 1. Schematic of the alignment of DLCs at solid surfaces.
Colh phases of DLCs. Homeotropic alignment has been reported for many columnar liquid crystal phases in experiments where thin films have been sandwiched between two identical solid surfaces including various triphenylene-based DLCs sandwiched between glass surfaces,19−22 between glass coated with materials such as mellitic acid and hexahydroxytriphenylene,18 between silicon,23,24 between ITO25−27 and aluminum-coated surfaces,28−33 and on various inorganic “single crystals”.18 Homeotropic alignment has also been reported for gel-stabilized triphenylene-based LCs sandwiched between ITO surfaces,25 CPI triphenylene/hexaphenyltripheReceived: November 9, 2012 Revised: January 25, 2013 Published: March 18, 2013 7533
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nylene DLC mixtures between glass34−36 and ITO surfaces,37,38 truxene-based DLCs between silicon surfaces,23,24 tricycloquinazoline-based DLCs between aluminum surfaces,39 pyrenebased DLCs between ITO surfaces,40 carbazole-based DLCs between glass and ITO surfaces,40 porphyrin-based DLCs between ITO surfaces,41,42 phthalocyanine-based DLCs between ITO surfaces,43,44 and hexabenzocoronene-based DLCs between glass,45,46 ITO,47 and aluminum-coated surfaces.46 In a more systematic study of surface alignment effects, it has been shown that for one particular tetraalkoxyphthalocyanine DLC the Colh phase aligns in a homeotropic manner on hydrophilic, hydrophobic, lypophilic, fluorophilic, and metallic surfaces.48 Furthermore, the way in which homeotropic Colh domains grow from the surface has been investigated in considerable detail for HAT8 [hexakis(octyloxy)triphenylene] on glass49 and for other DLCs on silica.50 Most experimental studies of DLC alignment have relied heavily on polarizing optical microscopy (POM), but this method has significant limitations. In particular, although it is very easy to unambiguously identify homeotropic aligned columnar hexagonal systems using POM (the samples appear black under crossed polarizers, remain black when rotated, but become highly birefringent when sheared or when cooled below the Colh−crystal transition temperature), it is not usually possible to distinguish between the mosaic texture associated with an unorientated sample and that for a planar sample in which there is a random in-plane distribution of the director. As a result, POM can be used to prove that there is homeotropic anchoring, but it cannot be used to prove that it is planar. Furthermore, POM cannot distinguish between a thin unoriented sample and one in which the orientation at the surface is homeotropic but that in the bulk is still random: probably quite a common situation in thin films. In this paper we report studies the alignment of the Colh phase of HAT11, hexakis(undecyloxy)triphenylene (1), on thiol-on-gold self-assembled monolayers (SAMs) which provide well-defined well-characterized surfaces. We have used a mixture of both POM and evanescent wave ellipsometry (EWE). EWE gives an unambiguous indication of the alignment within ∼4000 Å of the surface. The sign for the change in the Brewster angle is opposite for homeotropic and for planar alignment, and so, unlike POM, EWE can distinguish between planar and homeotropic alignment.51 The SAMs which we have used are shown in Figure 2. They include a CO2H-terminated high-energy surface SAM-2, a CH3-terminated low-energy surface SAM-3, and a SAM composed of DLC-like moieties 4 in which the triphenylenes are known to lie edge-on to the surface.52 In experiments on calamitic liquid crystals it has been shown that a SAM comprised of liquidcrystal look-alike moieties can be use to dictate the alignment of bulk liquid crystal phases,53,54 and so it was expected that the SAM derived from the thiol 4 would promote planar anchoring of the Colh phase of HAT11. However, this was not the case!
Figure 2. Molecular formula of HAT11 and of the SAM-forming materials used in this study.
cm was used throughout. High-purity (99.99%) temperannealed gold wire (0.75 mm diameter) was supplied by Goodfellow. Glass equilateral prisms with high refractive index of 1.85 (Tih53) were purchased from Galvoptics, UK. 2,3,6,7,10,11-Hexakis(undecyloxy)triphenylene (HAT11, 1) was synthesized as previously described55 and was purified by column chromatography and recrystallization twice from ethanol. DSC (heating/cooling rate 10 °C min−1, second heating/cooling cycle): K 53 °C (71 J g−1) Colh 66 °C (3.6 J g−1) I 56 °C (−2.9 J g−1) Colh 33 °C (−72 J g−1) K. lit. K 53 or 54 °C Colh 65 or 66 °C I.56,57 2-(5′-Thiopentyloxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (4) was synthesized and purified as previously described.52 Both of these compounds gave 1H NMR spectra in agreement with those reported. Substrate Preparation. Glass microscope slides and high refractive index prisms (Tih53) were cleaned by ultrasonication for 15 min in a 10% solution of Decon 90 in Milli-Q water. Each slide and prism was rinsed 10 times in Milli-Q water and then dried under a stream of oxygen free nitrogen. The slides were ultrasonicated in dichloromethane, and the prisms were ultrasonicated in methanol for 15 min; both were removed and dried in a stream of zero grade nitrogen, rinsed under Milli-Q water, and dried under nitrogen. The slides were immersed in piranha solution (70:30, v/v, H2SO4/H2O2) at ∼85 °C for 10 min. The substrates were then rinsed in Milli-Q water and dried under nitrogen. The slides and prisms were placed separately in an Edwards Auto 306 thermal evaporator. A 30 nm gold layer was deposited onto a chromium adhesion layer (3 nm thick) on slides, and in the case of the prisms a 20 nm gold layer was evaporated directly onto the base of the prisms. The deposition rate was maintained at 0.08 nm s−1 and at a base pressure of 6 × 10−7 mbar. The samples were rotated during the evaporation
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EXPERIMENTAL SECTION Materials. Dichloromethane (>99.9%), hydrogen peroxide (27.5 wt %), methanol (>99.9%), 16-mercaptohexadecanoic acid, and hexadecanethiol were used as received from SigmaAldrich. Sulfuric acid (98%) was supplied by Fisher Scientific. Glass microscope slides (thickness 0.8 mm) purchased from Agarand were cut to approximately half of their original length. Millipore Milli-Q water with a resistivity better than 18.2 MΩ 7534
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Differential Scanning Calorimetry (DSC). A PerkinElmer DSC7 with a thermal analysis controller TAC7/DX and Pyris series software-DSC7A was used to determine the transition temperatures of HAT11.
process of the gold in order to produce a uniformly deposited gold film which will reduce the likelihood of uniform azimuthal alignment.58 The gold-coated slides were cleaned immediately prior to use by placing them in freshly prepared piranha solution for 2 min, followed by a rinse with Milli-Q water. The gold-coated prisms were rinsed with methanol, dried with nitrogen, rinsed with Milli-Q water, and dried with nitrogen before use. SAM Formation. The clean gold-coated slides and prisms were immersed in a 1 mM SAM solution in methanol or dichloromethane for 18 h at 23 °C. The samples were then removed from the solution, rinsed with methanol or dichloromethane, dried with a nitrogen stream, rinsed with Milli-Q water, and again dried. The quality of the SAMs was checked using water contact angle measurements and X-ray photoelectron spectroscopy.51,59 Cell Arrangement. Two structures of HAT11 LC cell were produced. The one used for EWE used a SAM-coated slides as the bottom surface and a SAM-coated prism as the top surface. The one used for POM used SAM-coated slides as both the bottom and the top surfaces. HAT11 was placed on a SAMcoated slide, and 23 μm thick poly(ethylene terephthalate) (PET) spacers were placed on either side. The slide was heated to melt the liquid crystal, and the SAM-coated prism (or SAMcoated slide) was then added. Evanescent Wave Ellipsometry (EWE). A Jobin Yvon UVISEL spectroscopic ellipsometer was used to measure the pseudo Brewster angle of the incident light (the incident angle when rp is minimum, Δ = 270°, and ρ = 0) on the cell through the phase transition from isotropic to nematic or columnar and vice versa. The temperature of the cell was maintained with accuracy of 0.05 °C using a PID controlled Peltier heater. The temperature was allowed to reach equilibrium for 6 min before each reading was taken. The angular resolution was 0.01°, and the wavelength was set to 633 nm. The phase-modulated ellipsometer was operated in mode II (A = ±45°, M = ±0°, and P-M = ±45°).60 The change in the Brewster angle as a function of temperature through the isotropic−columnar transition of the LCs was traced through the cooling and heating runs in order to check for hysteresis. The prism/LC/slide cell was set up horizontally so that the incident light beam passing through the prism was totally internally reflected from the glass/gold/ SAM/LC interface.61 The LC surface area under investigation (∼1 mm2, the diameter of the light beam) was much smaller than the surface area of the cell and therefore is unaffected by the edge effects. In addition, the LC film is much thicker (23 μm) than the penetration depth of the evanescent field (∼150 nm). However, the director-fluctuation-mediated interactions between the two opposing boundaries of the cell cannot be ignored. Consequently, identical opposing boundaries of known properties were used so the alignment is enforced at the surface.51 For the data shown in Figure 4, the samples were first slowly cooled from the isotropic into the columnar phase and then reheated back into the isotropic. The “stray” points (at −0.2 on the reduced temperature scale in Figure 4a and at +0.3 in Figure 4b) are probably artifactual and most probably reflect ambiguity in the solution of the optical model. Polarizing Optical Microscopy (POM). A Leica microscope with polarizers below and above a sample stage was used. The temperature of the LC samples was controlled during the experiments using a Linkam heating stage. Images were taken using a standard digital camera (Olympus SP350) attached to the microscope eyepiece
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RESULTS Polarizing Optical Microscopy. Figure 3a shows the optical texture obtained from HAT11 confined between two
Figure 3. (a) Optical texture observed for a 23 μm thick cell of HAT11 between COOH SAMs of alkanethiol (SAM-2) immediately after cooling from isotropic into columnar phase by ∼0.7 °C. (b) After annealing for 5 h at this temperature. (c) The optical texture observed for a 23 μm thick cell of HAT11 between CH3-functionalized SAMs (SAM-3), after cooling from isotropic into the columnar phase by ∼0.5 °C. (d) After annealing at this temperature for 5 h. (e) The optical texture of a 23 μm thick cell of HAT11 on the thiopentyloxytriphenylene SAM-4 after cooling from the isotropic to columnar phase by ∼0.5 °C. (f) After 2 h annealing at this temperature. In each case the cooling rate was 0.1 °C/min.
COOH SAM-functionalized gold surfaces and cooled at 0.1 °C/min from the isotropic (just) into the Colh phase. The mosaic texture shown is typical of that for a randomly oriented sample. When the sample was annealed at ∼0.7 °C below the Colh/I transition temperature, it slowly became homeotropic/ black. As shown in Figure 3b, after 5 h, homeotropic regions had developed. These regions remained black upon rotation of the sample to 45° and 90° relative to the polarizers. Figure 3c shows the optical texture obtained from HAT11 confined between two CH3 SAM functionalized gold surfaces 7535
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and cooled at 0.1 °C/min from the isotropic (just) into the Colh phase. Once again, when the sample was annealed at ∼0.5 °C below the Colh/I transition temperature for ∼5 h, homeotropic regions had developed as shown in Figure 3d. Figure 3e shows the optical texture obtained when the liquid crystal was confined between two SAMs obtained using the thiol 4 when the cell was cooled to about ∼0.5 °C below the Colh/I transition temperature. Figure 3f shows that there was no significant change when the sample was annealed for ∼2 h at this temperature Evanescent Wave Ellipsometry. Figure 4 shows the EWE results for all three SAM systems. In all three systems there is a decrease in the reduced Brewster angle on passing from the
isotropic to the columnar phase, indicating (at least predominantly) homeotropic alignment. In the case of the CO2H-terminated SAM the decrease in the reduced Brewster angle was about −0.09° on (Figure 4a) for the CH3-terminated SAM about −0.10° (Figure 4b) and for the triphenylene-based SAM-4 about −0.055° (Figure 4c). In each case the estimated error is ∼±0.005.51 Hysteresis was observed in all three systems. This suggests that there is some process, in these systems, which is slow relative to the time scale of the experiment (which itself was very slow: 0.025 °C/min). The nature of this process is unclear.
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DISCUSSION The thermal stability of thiol-on-gold SAMs is very variable. It depends on the quality of the gold surface, the quality of the SAM, the chain length of the SAM-forming material, the degree of order within the SAM, and whether the SAM is heated in the air or in contact with a liquid.62 HAT11 was chosen as the liquid crystal for the present study because it has one of the lowest known Col/I transition temperatures. Nevertheless, at 70 °C and over the rather long time periods needed to align the columnar phase (up to 5 h for the OPM studies, 1−3 h for the initial, cooling, stage in the ellipsometry experiments), there is likely to be some loss of the SAM material. This “thinning” of the SAM is most likely to be a problem for SAM-3 which is the most disordered of the SAMs we have used. When a homologue of SAM-3 was heated at 70 °C, ellipsometry showed that the thickness of the SAM layer decreased by 12% after 3 h and by 20% after 12 h.59 However, although there was evidently some “thinning” of the SAM, there was no significant change in the water contact angle. Furthermore, experiments with patterned SAMs show that small amounts of “thinning” do not affect liquid crystal alignment. Hence, SAMs patterned with SAM-2 and a fluorocarbon analogue of SAM-3 showed that the aligning effect of the pattern persisted for at least 6 h at ∼50 °C when heated in contact with a cyanobiphenyl liquid crystal. It only began to degrade rapidly above ∼90 °C. In a further control experiment, we have shown that, when a 23 μm thick cell of HAT11 sandwiched between plain gold-coated slides is annealed for 14 h and monitored by POM, the alignment remained random. Hence, the degree of homeotropic alignment evident in Figure 3b,d must be dictated by the presence of the SAM layer.59 Taken overall, these results all suggest that, although there is probably a little “thinning” of the SAM layers under the experimental conditions we have used for HAT11, a little loss of SAM material from the surface should not be a problem. The many previous studies of alignment of Colh phases of DLCs described in the Introduction show that they usually align in a homeotropic manner on high-energy surfaces. Hence, the observation of homeotropic alignment for HAT11 for the CO2H-terminated SAM-2 is entirely as expected. In the POM experiment it is clear that, even after annealing for 5 h, the sample is not homeotropically aligned throughout the whole film, but compared to many of the studies reported in the literature, a film of 23 μm is quite thick, and in these thicker films it is always much more difficult to achieve alignment throughout the entire thickness of the sample. The EWE experiment shows a change in the reduced Brewster angle of −0.09 + 0.005° on cooling from the isotopic to the Colh phase. This unambiguously confirms that the alignment of the surface layer (first ∼4000 Å) is homeotropic.51 Since the parallel and perpendicular components of the refractive index for
Figure 4. EWE experimental data for HAT11 (a) on a COOHterminated SAM (SAM-2) The Brewster angle in the isotropic phase at T ≫ TIC was θB* = 62.19°. (b) EWE experimental data for HAT11 on a CH3-terminated SAM (SAM-3). The Brewster angle in the isotropic phase at T ≫ TIC was θB* = 62.15°. (c) EWE experimental data for HAT11 on a SAM made from the triphenylene derivative 4. The Brewster angle in the isotropic phase at T ≫ TIC was θB* = 61.82°. In each case the heating/cooling rate was 0.025 °C/min. 7536
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This is quite different to the situation which pertains in the case of calamitic liquid crystals where control of the surface chemistry, for example, control over the nature of a SAM surface or over the orientation within the “seed layer” provides a very simple way in which the alignment of the liquid crystal can be controlled.53,54,61,64,65 Established commercial applications of DLCs such as the manufacture of optical compensating films 1,2 and the production of carbon fibres3−5 rely on the ease with which ND phases can be aligned. Development of future applications of DLCs such as organic field effect transistors (OFETs)2,6−8 and photovoltaic devices2,9−11 will rely on solving the much more difficult problem of how to control the alignment of columnar phases. The fact that Col phases show homeotropic alignment at solid surfaces means that they align in exactly the manner required for most photovoltaic applications, but it creates a problem for the fabrication of bottom-gated OFET devices. A normal bottom-gated OFET device requires planar alignment of the Colh phase with control over the in-plane angle of the director such that the favored conduction pathway runs from source to drain electrode. Probably the best solution to this problem will lie in exploiting thin films of DLCs which are open to the air where the planar anchoring at the liquid crystal-to-air interface usually proves to be the dominant interaction overriding homeotropic anchoring at the liquid crystal-to-solid surface.6−8,50,66
HAT11are not known, the magnitude of the change in the Brewster angle cannot be calculated. However, these refractive indices values are known for the closely related hexakis(heptyloxy)triphenylene HAT7, and for this the calculated changes (for a layer of ∼4000 Å or greater thickness) are −0.11° for perfect homeotropic alignment and +0.05° for perfect planar alignment.51 An unaligned, random layer would show no change. The negative sign of the observed change confirms that the alignment is homeotropic, and its magnitude is consistent with good homeotropic alignment given that, for slightly longer side chains in HAT11, the anisotropy in the refractive index will be slightly lower as will the change in the reduced Brewster angle. There is much less information in the literature concerning the alignment of Colh phases of DLCs on low-energy surfaces.48 On the CH3-terminated SAM-3 the POM experiment shows that, after annealing for 5 h, HAT11 is partly homeotropically aligned, and the negative sign in the change of the reduced Brewster angle (−0.10 ± 0.005°) unambiguously confirms that the alignment of the surface layer is homeotropic. The fact that the magnitude of this change is almost the same on both SAM-2 and SAM-3 shows that the alignment is equally good on both SAM surfaces and further suggests a wholly or almost wholly homeotropic sample in both cases. The SAM based on the triphenylene derivative SAM-4 has previously been characterized in some detail.52 STM studies showed the presence of short (∼10 nm) stacks of alkoxytriphenylene molecules which are oriented edge-on and separated by about 2.2−2.5 nm. It was hoped that the presence of such an underlying “seed layer” would promote planar alignment of a Colh overlayer. The POM experiment showed no evidence for homeotropic alignment after annealing for 2 h. However, the sign of the change of the reduced Brewster angle of −0.055 ± 0.005° strongly suggests that the alignment of the surface layer is predominantly homeotropic although the fact that it is rather smaller than that observed for the other two SAM surfaces probably suggests incomplete homeotropic alignment and probably weaker anchoring. The fact that this particular SAM does not produce a planar aligned Colh layer may be related to the incommensurate nature of the two lattices. The average column-to-column separation in SAM-4 is 2.2−2.5 nm,52 and that in the Colh phase of HAT11 is 2.7 nm.62
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AUTHOR INFORMATION
Corresponding Author
*Phone 0113 343 3852; Fax 0113 34 33900; e-mail s.d.evans@ leeds.ac.uk. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Ministry of Education in Sultanate of Oman for the provision of a PhD scholarship to Z.H.A. and Dr. Mark Bonner for the DSC measurements.
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REFERENCES
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CONCLUSIONS All previous studies of the alignment of Colh phase of DLCs suggest that, when thin films are sandwiched between identical solid surfaces, homeotropic alignment is preferred, and it is known that HAT11 itself adsorbs at the tetradecane/Au(111) interface in a face-on manner.63 However, although a wide range of DLCs has been investigated, most of the literature reports are limited to alignment of Colh phases on high-energy surfaces. In the present study we have confirmed that the Colh phase of HAT11 aligns in a homeotropic manner on both highenergy and low-energy surfaces and even on surfaces that are comprised of a “seed layer” of edge-on alkoxytriphenylene moieties. This strengthens the view that “alignment of Colh phases of DLCs is intrinsically homeotropic regardless of the nature of the DLC or the nature of the surface48”: that the surface acts purely as a mechanical barrier, and although it is known that a specific interaction between the surface and the discogen can be used to enhance the ease or quality of the alignment, this factor seems to be of secondary importance. 7537
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