1. Introduction

Self-assembled monolayer (SAM) has been attracting a wide range of attentions over the last decades [1,2]. The SAM-mediated functionalization of solid surfaces provides great opportunities for the development of various nano- and bio-technologies. Especially, silane based SAMs are extensively studied for multitudinous applications ranging from a conventional interfacial energy modifier to multifarious surface conjugation of functional materials [1,2].

Both surface SAMs and polymer brushes can play a crucial role for the surface mediated alignment control of liquid crystal (LC) molecules [3–6]. Anchoring state of LC molecules critically depends on the length of hydrocarbon chain. The anchoring transition from a planar to homeotropic state is observed at C₅ (n-pentyl) by increasing a chain length [3]. The anchoring has been also known to be affected by wettability, monolayer density, topographical corrugation, and layer thickness [3].

In the field of liquid crystal research and applications, the alignment control of LC molecules is an important issue. Various treatments of solid surfaces are practically employed to regulate alignment of liquid crystals. In addition to mechanical rubbing, the photo-alignment techniques have been reported based on the photo-dimerizable cinnamoyl group as a surface monolayer or polymer side chain [7,8]. The photo-polymerizable acrylate monomers have also been used to induce LC alignment [9,10]. Both cases, the preformed alignment layers combined by the irradiation of linearly polarized UV-light (LPUV) prior to cell assembly and LC loading provide a control of homogeneous planar alignment in the plane of substrates.

On the other hand, sculpting predetermined tilt-orientation of LC director is an essential component for device implementation, which can enormously enhance electro-optical (E.O.) switching characteristics such as wide viewing angle, faster switching time, and enhanced brightness [11,12]. In the current technology for mass production, this has been achieved by photopolymerizable additive doped LC mixture. Conventional technology, currently used for LCD-TV applications, adopts polymer stabilization of LC pretilt (so called, polymer stabilized VA, PSVA) [11–16]. The initial homeotropic alignment is achieved by conventional polymer alignment layers. Photopolymerization-induced phase separation followed by surface deposition of polymer aggregates, by using small amount of mesogenic monomers doped in LC host, stabilizes predetermined tilt orientation of LCs. The effective stabilization is very sensitive to polymerization conditions. The unreacted monomers remained in a host LC can deteriorate physical parameters of LC mixture and cause long term reliability issues.

In this report, we demonstrate a very beneficial use of the photo-reactive SAMs (PR-SAMs) for liquid crystal display applications. The surface monolayer is bifunctional in that it is sufficient not only to instigate LC alignment but also stabilize the tilt orientation of LC director. The photoreactive SAMs formed by silane coupling reaction to oxide surfaces efficiently induce a homeotropic alignment. Subsequent photo-polymerization of methacrylate groups of the PR-SAM by using unpolarized UV-light under predetermined director orientation of nematic LCs facilitates retaining permanent surface pretilt. In addition to the optical and electro-optical characterizations, atomic force microscopy (AFM) is employed for the study. We propose a model for the photopolymerization-induced surface stabilization of the LC pretilt obtained from photoreactive SAMs as alignment layers.

2. Experimental

2.1 Materials

For silane coupling agents, 3-(Trimethoxysilyl)propyl methacrylate, 3-(Trichlorosilyl)propyl methacrylate, trimethoxyoctadecyl silane are used as received from the Sigma-Aldrich. Triethyl amine as received and fresh distilled dry toluene are used for reaction additive and solvent, respectively. Nematic liquid crystal with a negative dielectric anisotropy is used as a host liquid crystal (Δε = - 4.0, Δn = 0.077 and T_NI = 75.0 °C).

2.2 Sample preparations

Unpatterned indium-tin-oxide (ITO) glass and fishbone-patterned indium-zinc-oxide (IZO) glass are used as substrates for two different types of E.O. cell. The fishbone-patterned IZO-substrate 100 nm thick SiO_x is vertically deposited on top of electrode layer at 350 °C by using plasma enhanced chemical vapor deposition (PECVD) for all substrates used in the experiments. The SAMs are covalently coupled to the oxide surface through a solution process [1–4]. For the self-assembled monolayers with and without photoreactive methacrylate group, the corresponding silane agents are coupled to the hydroxy group on the oxide surface by a solution process. The solution is prepared by adding a silane coupling agent and triethylamine to dry toluene as a solvent at 0.03 and 0.1 wt% concentrations, respectively. The substrates are immersed into the solution at room temperature. Reaction time varies from 5 minutes to 5 hours. After a completion of the coupling reaction, the substrates are thoroughly rinsed with a dry toluene for a few times. The substrates dried under demoistrized air are assembled into an electro-optic cells with 4.1 μm and 10.0 μm gaps for fishbone patterned IZO and unpatterned ITO cells, respectively.

The methacrylate groups in a self-assembled monolayer are polymerized by using unpolarized 365 nm UV-light under applied electric field at V₈₀ (voltage for 80% transmittance of a E.O. cell under crossed polarizers). The Spot Cure Model SP-9 (Ushio Inc.) was used for UV-source and no photoinitiator was used for polymerization.

2.3 Characterizations

Polarizing optical microscopy and conoscopic images were taken using a Nikon Eclipse LV 100 POL polarizing optical microscope equipped with Nikon DS-Ri1 CCD camera and Instec STC 200 temperature controller with Instec HCS 402 hot stage. Atomic force microscopy was performed on SPM Nano Focus n-Tracer. Electro-optical switching behaviors are characterized by LCMS-200 (Sesim Photonics Technology, Korea). Optical and electro-optical characteristics of both unpatterned and fishibone-patterned E.O. cells before and after UV-stabilization are then examined by using a polarizing optical microscope (POM) and LCMS-200, respectively. The dismantled cells were thoroughly washed by using excess amount of dry hexane for a few times for a selective removal of LC host. The surfaces before and after UV-irradiation were examimed by the AFM.

3. Results and discussion

The SAM treated substrates, illustrated in Fig. 1(a), are assembled into electro-optic (E.O.) cells. The SAM treated E.O. cells with a LC mixture initially exhibit a homeotropic alignment as seen in Fig. 1 while the surface of SiO_x layer without SAM anchors LC molecules random planar as shown in Fig. 1(b). The macroscopic image of the cell in Fig. 1(c) displays a uniform dark state under crossed polarizers. Polarizing optical microscopy (POM) and conoscopy images in Fig. 1(d) confirm a vertical orientation of a uniaxial optic axis of the LC layer. Although essentially the same homeotropic alignment is achieved for different reaction times of silanization, ranging five minutes to five hours, the degree of defect density varies as a function of reaction time. Approximately ten to thirty minute reaction time results in defect free homeotropic alignment. Five minute inundation results in tiny dot defects and over half an hour immersion induces bright loop defects. The loops grow in their size as reaction time increases.

 

Fig. 1 Conceptual illustration of the PR-SAM treated oxide surface and depolarized optical images of the LC cells: (a) Photo-reactive silylpropyl methacrylate monolayer on the SiOx surface, macroscopic images of the LC cells (b) without and (c) with PR-SAM treatment, (d) polarized optical microscopic and conoscopic images of the vertically aligned LC cell, (e) populated transient defects formed by applying electric field, (f) uniform light state after annihilation of temporary bulk defects under electric field at 2.5 V, and (g) conoscopic figures after stabilization of the director tilt with different sample orientations. Blue arrow in (f) represents the optic axis of LC in the plane of substrate.

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Figures 1(d)–1(f) correspond to the POM images without and with applied electric field. Upon applying electric field to a completely dark state in Fig. 1(d), the cell exhibits a chaotic defects formed due to the absence of surface pretilt as seen in Fig. 1(e), which is a transient state slowly disappearing and turning to a uniform bright state as in Fig. 1(f) [13–16]. It should be noted that director orientation in the plane of a substrate for a uniform state in Fig. 1(f) is not exerted by external force and not uniform through a whole cell. The uniform area is large enough to examine conoscopic figures. It can be observed after complete disappearance of transient defect if the surface is uniform with no permanent defect anchored at the surface. However, the fall-over direction of director is completely random unless certain in-plane anisotropy is induced by external forces such as LC flow, rubbing, LPUV, in-plane magnetic and electric fields. Since surfaces are not treated by such anisotropic external forces, a direct cause of observed in-plane director orientation is uncertain in this case. The photoreactive methacrylate groups in the surface monolayer are then polymerized at the condition under electric field in Fig. 1(f) by irradiating unpolarized 365 nm UV-light with 20 mWcm⁻² for 30 minutes.

Since defects in Fig. 1(e) deteriorate uniformity of the bright state and tardily disappear, they significantly hamper a brightness of transmitted light and response time. After surface stabilization by photopolymerizing SAMs at both surfaces, however, no defect appears during switching from a dark to bright state. Figure 1(d) immediately switches to Fig. 1(f) without showing any evidence of defect formation. The direction of director reorientation is permanently fixed and now always along the blue arrow in Fig. 1(f). This is attributed to the stabilized pretilt of LC molecules at the surface coined by photopolymerization of PR-SAMs under applied electric field. Stabilized LC pretilt is further confirmed by conoscopic images in Fig. 1(g) obtained after photopolymerization with different azimuthal orientations of the cell. In Fig. 1(g), N and SW denote “north” and “southwest” for tilt directions of the optic axis, respectively. Metalope of the conoscopic figures slightly shifts toward the north or southwest from the center in each case, confirming the corresponding tilt of uniaxial optic axis of LCs. As a result of the stabilized pretilt of LC director, E.O. properties can be significantly improved after UV-stabilization [11–16].

Temperature durability of the stabilized LC alignment has been examined after complete sealing of the cell periphery using silicone adhesive. Homeotropic alignment and director tilt are stable up to the isotropic transition temperature. No anchoring transition is observed. The alignment retains its initial state even after heating the stabilized cell at 100 °C for 3 days.

The fishbone-patterned VA-cell, practically used for high end LCD-TV applications, also shows essentially the same behaviors upon applying electric field to the cell before and after UV-stabilization. In the fishbone patterned VA-cell with a fishbone patterned pixel electrode and unpatterned common electrode, each subpixel carries four orthogonal domains with different directions of micro-slit axis. In this case, the initial homeotropic LCs tilt over perpendicular to the micro-slits upon applying electric field due to the concentrated fringe field at edges of patterned electrode and subsequently rotate to be aligned parallel to the slits [13,14,16]. In the absence of LC pretilt, however, the incipient fall-over occurs in random directions toward azimuthal plane of the substrates so that chaotic defects are highly populated. These gradually diminish and turn into uniform light states [13,14,16].

Figures 2(a) and 2(b), obtained from the fishbone patterned VA-cell with PR-SAMs prior to polymerization, exhibit precisely matching characteristics of the VA-cell without LC pretilt at the surface [16]. Many deranged defects are observed from the POM images shown at the center of Figs. 2(a) and 2(b), recorded under electric field corresponding to 50% and 90% transmittance, respectively. These slowly transform to uniform states as shown on the right hand side. Since the optic axes of four different domains in each rectangular subpixel longitudinally arranged in Figs. 2(a)–2(d) are oriented 45° with respect to the polarizer and analyzer and lie in the plane of substrates, the transmittance increases as a function of applied electric field and the viewing angle characteristics are significantly enhanced [13,14,16].

 

Fig. 2 E.O. switching behaviors of the fishbone-patterned LC cells before and after photopolymerization assisted stabilization of the director pretilt: Time resolved POM images upon applying electric field corresponding to 50% of transmittance (a and c), 90% of transmittance (b and d), prior to pretilt stabilization (a and b), and after PR-SAM assisted pretilt stabilization (c and d).

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The cell has been exposed with unpolarized 365 nm UV-light with 20 mWcm⁻² for 30 minutes under applied electric field corresponding to 80% transmittance. Figures 2(c) and 2(d) present time resolved POM images during switching of the cell after polymerization of photo-reactive SAMs. Dramatic differences are observed. In this case, no single disclination is observed. Initial dark state instantaneously switches to a uniform bright state without showing disorganized intermediate defects. As a result, E.O. properties are considerably improved after photopolymerization of the PR-SAMs.

The substantial enhancement of E.O. performance has been achieved by the stabilized tilt orientation of LC molecules. The patterned electrode and applied electric field induce a predetermined tilt of LC director along four orthogonal directions in each subpixel and subsequent photopolymerization of the PR-SAMs permanently restrains and stabilizes the tilt orientation of LC director at the surfaces. Once the applied electric field is removed after polymerization, the LC director (i.e., optic axis) relaxes back to a nearly homeotropic state. In this case, however, the slight tilt of a LC director within a few degrees is memorized by the surface monolayer along four different directions in each subpixel. As a consequence, the initial response of LC molecules to electric field proceeds in harmonized fashion toward destined directions in the presence of LC pretilt, subsequently improving E.O. switching characteristics. This clearly implies that the photo-polymerizable surface monolayer is sufficient to instigate a homeotropic alignment and stabilize predetermined tilt orientation of LC molecules. No such stabilization effect is observed for the cell treated with a trimethoxyoctadecyl silane, which has no phtoreactive group in the SAM.

Figure 3 further demonstrates enhanced E.O. properties effectuated by the PR-SAMs. The voltage-transmittance (V-T) curves in Fig. 3(a) show an enhanced overall transmittance and considerable shift to the left after stabilization. It is obvious that both characteristics of brightness and threshold voltage are substantially improved. This can also be noticed from POM images in Fig. 2. More distinguished progression is observed in grey-to-grey response times as seen in Fig. 3(b). Electric field driven rising responses of the PR-SAM treated VA-cell, prior to photopolymerization, are characterized by a sluggish response in low gray levels and strong grey level dependency of rising response time as shown in Fig. 3(b), presented by red squares, which are intrinsic drawbacks in conventional VA-cells [11,12]. However, the rising times are drastically decreased after a surface monolayer-induced stabilization as presented by red circles in Fig. 3(b). Especially for 10% gray level, rising time decreased from 175.8 ms to 46.8 ms and also for medium to high grays rising times have been changed approximately from above 70 ms to below 30 ms after stabilization. On the other hand, the decaying time ranges about 9 to 13 ms and exhibits no significant difference before and after stabilization as shown by blue squares and circles in Fig. 3(b). All these distinctions in response behavior, before and after stabilization, are corroborative to the existence of predetermined director tilt created by photopolymerization of the PR-SAMs.

 

Fig. 3 E.O. switching characteristics of the fishbone-patterned LC cells before and after PR-SAM assisted stabilization of the director pretilt: (a) Voltage-Transmittance curves for before (black squares) and after (red circles) stabilization, and (b) grey-to-grey response time for both rising (red squares and circles) and decaying (blue squares and circles) times before and after stabilization, respectively.

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Figure 4 illustrates PR-SAM assisted surface stabilization of the tilt orientation of LC director. The photoreactive surface molecule is symbolized as seen in Fig. 4(a), where the head group in red represents a photoreactive methacrylate moiety. Figure 4(b) designates anoxide surface with hydroxyl reaction sites. After completion of a silane coupling reaction and thorough removal of unreacted reagents, the molecular monolayer is formed through covalent bonds to the oxide surface as illustrated in Fig. 4(c). The surface coverage and molecular density depend on the reaction conditions and nature of the oxide surface [1,2]. After assembly of the cell, loaded LC molecules with a negative dielectric anisotropy are uniformly aligned normal to the surface, instigated by the PR-SAMs on both substrates as shown in Fig. 4(d). The LC molecules are not depicted for clarity and, instead, the LC director is represented by a green arrow. If electric field is applied to the homeotropic state in Fig. 4(d), transient defects populate immediately and are slowly annihilated. As a result, uniformly tilted state is obtained along the predetermined direction due to a patterned pixel electrode. In this equilibrium state, the molecules in a surface monolayer are presumably tilted uniformly toward the direction of LC director, denoted by the green arrow, as shown in Fig. 4(e). If the PR-SAMs are polymerized at this condition, the photoreactive methacrylate groups are linked together by polymerization and congealed with a surface tilt as depicted in Fig. 4(f). After removal of applied electric field, the LC director relaxes back to a near homeotropic state but polymerized monolayer retains its orientation tilted and restrains tilt orientation of LC director at the surface. As a result, the LC director slightly inclined from the perfect vertical direction as denoted by a green arrow in Fig. 4(f).

 

Fig. 4 Graphical illustration of the PR-SAM assisted stabilization of a director pretilt: (a) Symbol with a corresponding molecular structure for the photoreactive surface molecule, (b) SiO_x surface with hydroxyl group reactive for silanization reaction, (c) photoreactive surface monolayer after silanization, (d) LC cell with the PR-SAM as a homeotropic alignment layer, (e) predetermined uniform tilted state under applied electric field, and (f) stabilized director pretilt coined by the PR-SAM after photopolymerization and removal of electric field (Pink bundles represent polymerization-induced congealed networked state of the SAM). The green arrows denote orientation of LC director.

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The surface morphologies have been studied by employing AFM. Although 3D images of the surfaces with a SiO_x deposited patterned IZO layer, after the PR-SAM coupling reaction, and after polymerization of the PR-SAM and removal of LCs have been examined, no meaningful difference is observed. The multiple numbers of samples for each condition exhibit no significant difference. Although no morphological distinction is evident in the 3D images of AFM, the statistical data for roughness analysis exhibit some degrees of difference in roughness. The root mean square (RMS) roughness is increased by approximately 45% and 43% after silanization and photostabilization reactions, respectively, from the bare surface of SiO_x deposited patterned electrode. It is still obscure whether the roughness change is responsible for a homeotropic alignment and director stabilization of LCs at the surfaces.

As mentioned earlier, conventional PSVA technology is very sensitive to polymerization conditions. The unreacted monomers remained in a host LC can deteriorate physical parameters of LC mixture and cause long term reliability issues. On the contrary, the proposed approach based on the photoreactive surface monolayer is relatively insensitive to polymerization conditions. Since the PR-SAM is ready-made polymerizable monolayer bound to an oxide surface, polymerization-induced surface localization of polymer networks and unreacted monomers left in a LC host are no longer concerns and thus polymerization time can be substantially abridged. Therefore, the approach based on a photoreactive surface monolayer has multifarious advantages over the conventional PSVA. Since no high temperature process is required for the PR-SAM treatment, it also provides advantage for flexible displays, where low temperature process is essential requirement.

Although we have demonstrated the surface monolayer assisted stabilization of LC pretilt by employing the PR-SAM, our results strongly indicate that other kinds of photoreactive surface layer can achieve the same effect. For instance, other bifunctional polymeric materials with a vertically aligning photoreactive group in the side chain can perform essentially the same effect of surface stabilized vertically aligned LCs.

4. Conclusions

The PR-SAMs are used as a photopolymerizable surface monolayer for the induction of a homeotropic alignment and stabilization of a tilt orientation of LC director. Bifunctional PR-SAMs are efficient to instigate a vertical alignment and stabilize a pretilt of LCs, simultaneously. It is explicitly demonstrated that the photoreactive molecular monolayers is sufficient to achieve a stabilization of a specific configuration of LC director. This indicates that not only silanized monolayer but also many other kinds of surfaces with a photoreactive functionality can accommodate a similar bifunctional implement.