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Abstract
Liquid crystals (LCs) have been a vital component of modern communication and photonic technologies. However, traditional LC alignment on polyimide (PI) requires mechanically rubbing treatment to control LC orientation, suffering from dust particles, surface damage, and electrostatic charges. In this paper, LC alignment on organic single-crystal rubrene (SCR) has been studied and used to fabricate rubbing-free LC devices. A rubrene/toluene solution is spin-coated on the indium–tin–oxide (ITO) substrate and transformed thereafter to the orthorhombic SCR after annealing. Experimental result reveals that SCR-based LC cell has a homogeneous alignment geometry, the pretilt angle of LCs is low and the orientation of LCs is determined with capillary filling action of LCs. LC alignment on SCR performs a wider thermal tolerance than that on PI by virtue of the strong anchoring nature of LCs on SCR due to van der Waals and π–π electron stacking interactions between the rubrene and LCs. SCR-based LC cell performs a lower operation voltage, faster response time, and higher voltage holding ratio than the traditional PI-based LC cell. Organic SCR enables to play a role as weakly conductive alignment layer without rubbing treatment and offers versatile function to develop novel LC devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Display technology has steadily but fundamentally influenced human lives, and has been generally acknowledged as an indispensable aspect of the modern world. The use of nematic liquid crystals (LCs) in a wide range of high-tech devices, such as photonics [1], sensors [2], light shutters [3], modulators [4], and LC displays (LCDs) [5], has been intimately connected to the necessity to macroscopically align LC molecules along a desired direction (dynamically favorable direction or easy axis) [6]. LC orientation could be determined by some physical or chemical interactions, such as an electric field [7], surface modifications [7,8], and pressure gradients [9]. Numerous materials and strategies have been developed over the past decades. The most common technique is using organic polyimide (PI) as an alignment layer accompanied with rubbing treatment to align LC molecules. Although the mechanical rubbing method is robust and straightforward, the evident disadvantage is that mechanical friction causes dust particles, surface damage, or electrostatic charge generation, thereby markedly limiting its application for high-quality LC photonic and optoelectronic devices. New functional materials and contact-free approaches, such as photo-alignment, have been intensively designed to overcome these drawbacks [10–13]. The earliest photo-alignment research was reported in 1988, when an azobenzene monolayer was irradiated with unpolarized light, in which the alignment of LCs near the monolayer changed from perpendicular to parallel to monolayer [14]. Later, an azo dye-doped polymer has been found to induce an in-plane anisotropy with polarized ultraviolet (UV) irradiation, resulting in a preferred in-plane alignment direction of LCs on the polymer [15]. Photo-alignments using other materials immediately followed, such as the illumination of linearly polarized UV light on cinnamoyl side-chain polymers, inducing a planar LC alignment perpendicular to the polarization direction of UV light [16]. Under a linearly polarized UV exposure, PI resulted in anisotropic depolymerization, causing a planar LC alignment perpendicular to the polarization direction of UV light [17]. Thereafter, self-assemble monolayer (SAMs) methods had entered the field of LC alignment, further determining a specific LC orientation. The resulting LC cells possessed remarkable electro-optical properties, such as memory effect, frequency modulation response, fast response, and low driving voltage [18–21]. Unfortunately, some problems remained unsolved, such as weak interactions with LCs, aggregation-induced light leakage, and unreliable LC alignment against heat, light, and moisture, all of which contributed to poor device quality. Recently, it had been reported that the planar LC alignment could be achieved by the coherent overlay of benzene rings of the LC molecules on the 2D hexagonal nanosurface, e.g., single-crystal graphene (SCG) and boron nitride (h-BN). The organic SCG possessed high conductivity and optical transmittance, and therefore, it was used as transparent electrode in the electronic device. By contrast, the inorganic h-BN was an insulator, which exhibited a very high structural, thermal, and chemical stability. Consequently, the h-BN was often used as a dielectric substrate in nano-electronic devices. The epitaxial alignment of LC molecules on SCG [22,23] and h-BN nanosheet [24,25] had been employed in the rubbing-free LC device. However, selectivity among the three symmetric easy axes of LCs on the hexagonal lattice of SCG or h-BN was difficult to control [26], so that they were not appropriate for large-area LC devices. Moreover, the SCG or h-BN deposition on the substrate required complicate procedures. The director configurations of SCG-based and h-BN-based LC devices, e.g., homogeneous or splay geometry, also have yet to be confirmed.
Organic single-crystal rubrene (SCR) had a tremendous potential interest because of its excellent performance in electronics and optoelectronics. SCR had been the global priority owing to its exciting potential in the research and applications of thin films and flexible electronic devices [27–30]. Rubrene, as a typical p-type organic semiconductor material, held an extremely high field-effect mobility (20 cm2/V s) and could be achieved by fabricating field-effect transistors on the surface of high-quality SCR film [31]. The approach of organic epitaxy growth was the choice of the most suitable substrate with defined surface properties in terms of symmetry and corrugation (macroscopic and a microscopic scale), thereby implying that it could grow high-quality SCR film [32–34]. Numerous viable technologies have been introduced recently to achieve highly ordered and uniaxially oriented SCR thin films comprising large single-crystalline grains that could be successfully prepared using an abrupt heating technique [27,35]. Another noteworthy feature of rubrene was its ability to act as an efficient 2D crystalline prototype, which enabled highly ordered, crystalline organic films on van der Waals substrates, thereby exhibiting high carrier mobility comparable with those of single-crystal counterparts [33,36,37]. In general, a 2D crystal lattice of layered materials should enable organic molecules to assemble into well-ordered structures via van-der Waals interactions, while minimizing random heterogeneous nucleation processes [33]. This situation was beneficial in the development of nanodevices and optoelectronics [26,38,39]. However, the application of SCR on LC devices has yet to be reported.
Over the past few years, modal LC lens has become a favorable alternative to traditional adaptive optical elements, since it overcomes the limitation of aperture size and has low power consumption, no moving parts, and electrically tunable focus. The layout of modal LC lens contained the rubbed alignment layer and weakly conductive layer (WCL) which were used to align the LC molecules orderly and spread the fringing electric field toward the aperture center, respectively [40–42]. The WCL ensures that modal LC lens can be addressed with a low voltage by the effect analogous to transmission line. Considering the evolutionary development of modal LC lens, it is important to seek a weakly conductive alignment material without rubbing treatment. As mentioned previously, SCG and h-BN have been used to fabricate rubbing-free LC device. However, they cannot be used as a weakly conductive alignment layer for modal LC lens due to either high conductivity or insulator property and undefined orientation of LCs on the hexagonal nanosurfaces. Up to now, the planar LC alignment on weakly conductive material without rubbing treatment has yet to be investigated.
The current study first reports the LC alignment on organic SCR, and further discusses the thermal and electrical stabilities of SCR-based LC cell. The pretilt angle and alignment configuration of LCs in SCR-based LC cell has been confirmed using crystal rotation method. The orthorhombic structure of SCR created on the indium–tin–oxide (ITO) substrate has been verified using X-ray diffraction (XRD) measurement. LC orientation in SCR-based LC cell can be determined with the flow direction during capillary filling action. For SCR-based LC cell, rubbing-free treatment protects the cell from dust contamination, surface damage, and electrostatic charges. Thermal and electrical stabilities of the SCR-based LC cell have been confirmed in accordance with polarized optical microscope (POM) and voltage-dependent transmissions. SCR-based LC cell has the advantages of wide thermal tolerance, good electrical stability, low operation voltage, and fast response time. The voltage holding characteristic of SCR-based LC cell has been examined by dynamic transmission with varying voltage polarity. Unlike conductor SCG and insulator h-BN, semiconductor SCR can act as a weakly conductive alignment layer without rubbing treatment, particularly for modal LC lens application.
2. Experimental methods
In this study, 5,6,11,12-tetraphenylnaphthacene (rubrene) powder was purchased from Acros Organic Fisher Scientific (US). Rubrene powder was dissolved in toluene at different concentrations of 0.5 1.0, 1.5, and 2.0 wt.% (hereafter defined as rubrene concentration). Rubrene/toluene solution was ultrasonically stirred for 5 min at room temperature. Prior to use, all solutions were placed at room temperature overnight for the complete dissolution of solutes. The rubrene solution was spin-coated on a cleaned ITO glass substrate with a size of 2 cm × 3 cm by 2000 rpm spin speed for 20 s. Amorphous rubrene film was directly transformed into a highly ordered SCR thin film with an abrupt heating process by placing a sample onto a hot plate pre-heated at 170 °C for 1 min under ambient conditions. All procedures were carried out in a dark room [27,35].
Nematic LC E7 (Daily Polymer Corp., Taiwan) was used in the experiment. It had a nematic–isotropic phase transition temperature (TNI) of 64 °C, birefringence (Δn) of 0.22, dielectric anisotropy (Δɛ) of 14.1, and elastic constant K11, K22, and K33 values of 12.0, 5.9, and 17.1 pN, respectively, at 20 °C. A 3-µm-thick empty cell composed of two ITO substrates deposited with SCR (or PI) was prepared, defined as SCR (or PI) cell. Inner surfaces of the substrates in the PI cell were rubbed in antiparallel directions. By contrast, rubbing-free treatment was used in the SCR cell. Thickness of the empty cell was confirmed with the interference method [43]. LC mixture was heated to the isotropic phase and filled in the empty cells thereafter via capillary action. After filling, LC cell was cooled down to the nematic phase.
Optical observations and determination of LC orientation were performed using POM (DM EP, Leica, Wetzlar, Germany). The crystal system and crystalline nature were analyzed using an XRD-6000 diffractometer (Shimadzu cor., Japan) equipped with an Fe–Κα radiation source (λ = 1.93 Å). Thickness and surface morphology of SCR created on the ITO substrate was measured using scanning electron microscopy (SEM). Pretilt angle of LCs on SCR was measured using the crystal rotation method [44]. The voltage-dependent transmissions (V–T) of the LC cells were measured using the following setup. The LC cell was placed between a pair of crossed polarizers which transmission axes had an angle of 45° with respect to the LC orientation direction. A He–Ne laser with a wavelength of 632.8 nm was normally incident on the LC cell, where a square-wave voltage at a frequency of 1 kHz was subjected to the LC cell. The rotational viscosity (γ) and ion density (n) of the LC mixture were obtained by the transient current and dielectric spectrum methods [45,46], respectively.
3. Results and discussion
The rubrene film deposited on the ITO substrate was confirmed with XRD patterns (Fig. 1(a)). Evidently, smooth signal without peaks indicated amorphous rubrene before annealing. By contrast, the highly crystalline nature of rubrene caused the sharp, intense, and well-defined Bragg diffraction peaks after annealing. Moreover, the XRD pattern of orthorhombic SCR was calculated using the Vienna Ab Initio Simulation Package (VASP) and 3D structural models for visualization program by Visualization for Electronic Structural Analysis (VESTA). The peak positions of the measured XRD pattern are consistent with those of the calculated XRD pattern (red line in Fig. 1(a)), indicating that the crystalline rubrene created with annealing was orthorhombic SCR (Fig. 1(b)). The created crystalline rubrene had lattice constants of a = 26.43, b = 7.08, and c = 14.18 Å, which were estimated by substituting the major peaks (h, k, l) (2 0 0), (0 0 2), and (1 1 2) in the XRD pattern into Eq. (1).
Fig. 1. (a) XRD patterns of rubrene deposited on ITO substrate before and after annealing, and calculated XRD pattern of orthorhombic SCR; (b) structure scheme of orthorhombic SCR; (c) calculated alignment geometry between the LC molecule and rubrene; (d) top view of SEM texture of SCR.
LC molecules can be macroscopically oriented in the SCR LC cell through capillary flow. LC orientations with various directions of capillary injections have been confirmed by POM (Figs. 2(a)–2(b)). Dark textures are observed when the direction of capillary injection is parallel or perpendicular to the transmission axes of the polarizers. By contrast, bright textures are obtained if the direction of capillary injection has a 45° angle with respect to the transmission axes of the polarizers. Accordingly, the texture characteristic of the SCR LC cell is similar to that of the traditional planar LC cell with rubbed PI layer, confirming that LC molecules are oriented along the direction of the capillary injection. Uniform colors throughout the cells confirm that LC molecules were aligned homogenously. Figure 2(c) shows that when the angle between the direction of capillary injection and transmission axes of the polarizers increases from 0° to 45°, the intensity increases from minimum to maximum value. Once the angle further increases more than 45°, the intensity starts to decline from maximum to minimum value. This intensity transition from dark to bright (bright to dark) at every 45° confirms that SCR imposes a planar LC alignment. In our investigation, when the cell gap exceeds 20 µm, the alignment quality of LCs on SCR degrades significantly, possibly due to the high flow speed.
Fig. 2. POM textures of the 0.5wt% SCR LC cells with directions of capillary injections at (a) 0° and (b) 45°. (c) Intensity of POM texture as a function of the angle between the direction of capillary injection and the transmission axes of polarizers. P, A, and I indicate the transmission axes of polarizer and analyzer and the direction of capillary injection, respectively.
Transmittance as a function of incident angle (T-A) of LC cell has been obtained to confirm the LC alignment configuration by crystal rotation method. For a homogeneous cell with small tilt angle θ, the measured T-A curve is symmetrical about a non-zero angle ψ with a maximum phase retardation [56], as shown in Fig. 3(a). By contrast, if ψ = 0, the LC tilt angle is zero or the LCs are aligned in symmetrical configurations (e.g., splay or bend geometries). Further, the sign of ψ determines the tilt direction of LCs in the cell. For SCR LC cell, ψ > 0 represents the LC directors are tilted clockwise wherein the capillary flow in the + x direction (Fig. 3(b)). This confirms that the LCs in the SCR LC cell is aligned in the homogeneous configuration with small tilt angle. The numerical calculation also discloses that the homogeneous LC alignment has a significantly lower free energy density than the splay LC alignment. When supplied an electric field, the reverse tilt domain occurs in the nematic LC device using homogeneous alignment with zero tilt angle or splay alignment. The disclination at the boundary of reverse tilt domain deteriorates the cell performance [57]. In SCR LC cell, the homogenous alignment with small tilt angle prevents the formation of reverse tilt domain.
Fig. 3. (a) Transmittance as a function of incident angle of 0.5 wt% SCR LC cell. (b) Schematic representation of LC director distribution in the 0.5 wt% SCR LC cell with capillary flow in the + x direction.
The alignment geometries of LCs on SCG [6] and h-BN [25] were achieved by dragging and blowing the LC droplets, respectively. The overlapping of benzene rings of LC molecules on the hexagonal structures of SCG and h-BN causes that the LCs still align along one of the closest armchair directions even the dragging direction was not parallel to the armchair direction of SCG; the unidirectional blow also created the degenerate domains in the h-BN LC cell. However, in our SCR LC cell, the benzene rings of LC molecule align without overlapping with those of rubrene tetracene owing to competition between the van der Waals and π–π electron stacking interactions. Moreover, the rubrene tetracene is oriented in a periodically orthogonal form. The LCs on SCR are completely oriented along the flow direction of the capillary filling without alignment degeneracy.
To clarify the thermal stability of the LC alignment, PI and SCR LC cells were heated up to a maximum temperature Tmax and cooled down thereafter to room temperature. Tmax was higher than the clearing point TNI (64 °C) of LC mixture. Tmax used were 180 °C, 200 °C, and 220 °C, which corresponds to TNI + 116 °C, 136 °C, and 156 °C, respectively. As shown in Figs. 4(a)–4(b), initial alignment is nearly perfectly recovered when the maximum temperature was TNI + 116°C, confirming that the PI and SCR surfaces remember the initial LC alignment. However, this memory effect is gradually weakened from TNI + 136°C, in which numerous defect domains appear in the textures. After heating to TNI + 156 °C, POM textures of the PI LC cell look similar with various sample rotation angles, indicating that initial alignment is nearly eliminated. By contrast, the POM texture of the SCR LC cell still performs the intensity transition from dark (bright) to bright (dark) when the cell was rotated by 45°, although the defect domains appear. The defect domains may be attributed to the partially dissolve of the coated rubrene at high temperature. The weak but clear presence of the preset LC alignment even at TNI + 156 °C implies the strong anchoring nature of LCs on SCR. The polar anchoring energy of LCs on SCR was measured as 3.3 × 10−4 J/m2, which can be considered as strong anchoring, comparable to that of LCs on rubbed PI layer (1.7 × 10−4 J/m2) and can explain the conservation of the LC alignment in the heating test up to TNI + 156 °C. LC alignment on SCR remains unchanged even after several months, thereby confirming the stability of LC alignment on SCR.
Fig. 4. Temperature tolerance tests of (a) PI and (b) 0.5wt% SCR LC cells after annealing above TNI. P, A, I, and R indicate the transmission axes of polarizer and analyzer, the direction of capillary injection, and rubbing direction, respectively.
Figures 5(a)–5(b) show the POM textures of SCR LC cells with various rubrene concentrations. When the concentration exceeds 1 wt%, the defect in POM texture significantly increases, indicating that LC alignment is disturbed. In Figs. 6(a)–6(b), SEM textures reveal that the thickness and surface roughness of SCR increases with rubrene concentrations. The increased roughness of SCR disturbs the LC alignment on SCR. Furthermore, the resistivities of substrate surface deposited with PI, 0.5 wt% SCR, and 2 wt% SCR were measured approximately 10, 6, and 4 Ωm, respectively. The resistivity of substrate surface deposited with SCR is lower than that of the substrate surface deposited with PI owing to the high electron mobilities of SCR [58]. The resistivity of the substrate surface deposited with SCR decreases with rubrene concentrations. As the rubrene concentration increases, SCR possibly acts as an electrode to replace the ITO electrode for LC devices. However, the increased roughness degrades LC alignment quality, which can be improved with the enhanced crystalline growth quality of SCR by the vapor phase transport or thermally deposition method [27,33].
Fig. 5. POM textures of SCR LC cells with rubrene concentrations of (a) 0.5 and (b) 1 wt%. P, A, and I indicate the transmission axes of polarizer and analyzer and the direction of capillary injection, respectively.
Figure 7 demonstrates the electric-optical characteristics of the PI and SCR LC cells. In Fig. 7(a), the threshold voltages (Vth) of the SCR LC cells almost remain constant at ∼0.9 V which is slightly lower than that (1 V) of the PI LC cell. Meanwhile, the V-T curve of the LC cell shifts toward the low-voltage side with rubrene concentration, indicating SCR deposition decreases the operation voltage of the cell. The results are attributed to the uneven tilt angle of LCs near the rough SCR surface and the decreased resistivity of substrate surface with SCR deposition. To examine the electrical stability of LC alignment on SCR, the V-T curve of the 0.5 wt% SCR LC cell was measured while the applied voltage (V) was gradually increased from 0 to 10 V and thereafter declined from 10 to 0 V. As shown in the inset of Fig. 7(a), the V-T curves in V increase and decline processes are perfectly overlapped, confirming the excellent electrical stability owing to strong anchoring of LCs on SCR. The T-V curve of SCR LC cell remains unchanged after cycles of voltage operations. Figure 7(b) shows the response time of the PI and SCR LC cells. When the cell was turned on (off) from 2 to 10 V (10 to 2 V), the rise (fall) time was defined as the time required for the transmission shifts from 90% to 10% (10% to 90%) of the maximum transmission. The rise time (τon) is significantly less than the fall time (τoff) because of the former’s electric torque-driven reorientation, whereas the latter has a free-relaxation reorientation. The relevance between the τon, τoff, Vth, and γ refers to follows [5]:
Fig. 7. Electro-optical characteristics of the PI and SCR LC cells with various rubrene concentrations. (a) V-T curves of the LC cells. The inset shows the V-T curves of 0.5 wt% SCR LC cell in V increase and decrease processes. (b) Response time of the LC cells. (c) Rotational viscosities and ion densities of the LC mixtures in the cells.
Figure 8(a) shows the dynamic transmissions of the LC cells addressed with a square wave of 2 V at 30 Hz. Within each voltage period, the transmission change of SCR LC cell is significantly less than that of PI LC cell. The voltage holding ratio (VHR) of the LC cell is defined as the ratio between the minimum and maximum transmissions in a voltage period. As shown in Fig. 8(b), the VHRs of the SCR LC cell are significantly higher than those of PI LC cell, because of the trapping of free-ions by SCR substrate.
4. Conclusions
The LC alignment on orthorhombic SCR has been reported. The calculated geometry between the rubrene and LC molecules reveals that the benzene rings of LC molecule tend to align along the rubrene tetracene owing to π–π electron stacking and van der Waals interactions. Notably, the rubrene tetracene in orthorhombic SCR aligns without an intrinsic preferential direction, thereby resulting in that the alignment direction of LCs on SCR can be determined with the capillary flow. The SCR LC cell has a homogeneous rather than splay alignment geometry, and the pretilt angle is low. LC alignment on SCR exhibits excellent thermal and electrical stabilities owing to the strong anchoring nature of LCs on SCR. The SCR LC cell emerges a lower operation voltage than the PI LC cell due to the rough SCR surface and its low resistivity. The fall time of the SCR LC cell is less than that of the PI LC cell because of the decreased viscosity of the LC mixture by free-ion trapping of SCR. The SCR LC cell also provides a relatively high VHRs than the PI LC cell. The conductivity of SCR increases with rubrene concentrations, making SCR is prospective to develop modal LC lens. The possibility of applying SCR on one-drop filling technique for large area LC display is also under evaluation. The interesting and unique features of LCs on SCR offer a novel potential for rubbing-free LC devices.
Funding
Ministry of Science and Technology, Taiwan (107-2112-M-018-003-MY3, 109-2811-M-018-500, 110-2112-M-018-009, 110-2811-M-018-501).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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