Graphene Glass Inducing Multi-Domain Orientations in Cholesteric

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Graphene Glass Inducing Multi-Domain Orientations in Cholesteric Liquid Crystals Devices towards Wide Viewing Angles Huihui Wang, Bingzhi Liu, Ling Wang, Xudong Chen, Zhaolong Chen, Yue Qi, Guang Cui, Huanhuan Xie, Yanfeng Zhang, and Zhongfan Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01773 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Graphene Glass Inducing Multi-Domain Orientations in Cholesteric Liquid Crystals Devices towards Wide Viewing Angles Huihui Wang, 1BingzhiLiu,1 LingWang,4 XudongChen,3 ZhaolongChen,1Yue Qi,1Guang Cui,1Huanhuan Xie,1 Yanfeng Zhang,*,1,2,3and Zhongfan Liu*,1,3

1 Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China

2 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China

3 Beijing Graphene Institute, Beijing 100091, People’s Republic of China

4 Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77840, USA

Corresponding authors: Zhongfan Liu ([email protected]), Yanfeng Zhang ([email protected]).

KEYWORDS: Graphene glass, multi-domain, cholesteric liquid crystals, π-π interaction, wide viewing angle

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ABSTRACT: The photonic reflection of a cholesteric liquid crystals (ChLCs) device depends on the spatial distribution of the orientations of their helical axes, and many orientation techniques have been developed so far. In this study, we select the hybrids of graphene directly grown on quartz glass as platforms to construct ChLCs based devices. This special design makes graphene serve both as an alignment layer and a conductive layer, thus affording a more simplified device fabrication route. We reveal that, multi-domain structures can be evolved for ChLCs on the polycrystalline monolayer graphene on quartz glass, as evidenced by polarized optical microscope (POM) characterizations. The disparate orientations of the helical axes of ChLCs and the formation of multi-domain structures are proposed to be induced by the different domain orientations of graphene, leading to a wide viewing angle of the ChLCs-based devices. Moreover, the pitch of ChLCs is also observed to play a key role in the relative orientations of ChLCs. A wide viewing angle of the ChLCs-based device is also detected especially in the infrared spectrum region. Briefly, this work should provoke the application of graphene glass as a perfect transparent electrode in the fabrication of liquid crystal-based devices showing broad application potentials in intelligent laser protection and energy-saving smart window.

Graphene possesses a number of superior properties, such as good mechanical strength and flexibility, and exceptionally high electrical and thermal conductivities, which makes it a revolutionary material in the fabrication of high performance electronic devices, transparent flexible devices, etc.1,

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Latest

demonstrations in direct chemical vapor deposition (CVD) growth of graphene on glass by our group mark game-changing breakthroughs to bypass the complex transfer steps, as well as to provide more application potentials with the combination of a functional substrate material of glass.3-7 Considering the advanced surface properties of the type hybrid material of graphene glass (e.g., conductivity, hydrophobicity and 2


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biocompatibility), as distinguished from or complementary to those of traditional glass, various applications, including low-cost heating devices, transparent electrodes, photocatalytic plates, biosensors, and biocompatible cell culture media, etc., have been developed recently.

It is well known that ChLCs devices can be used in diverse fields such as intelligent laser protection,

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next-generation ultrafast reflective display9 and energy-saving smart window, etc.10 However, the orientation control of ChLCs molecules is highly required for the fabrication of such high-quality ChLCs-based devices.11, 12 The alignment of LCs is determined by the nature of the substrate surface.13 An alignment layer was usually generated by the surface treatment,14 and the mechanical rubbing of a polymer layer on the targeted surface was usually applied. However, this process has some shortcomings, such as the introduction of dust particles, creation of electrostatic charges, etc. In this regard, a non-rubbing alignment technique is highly desirable for the development of large-scale, wide viewing-angle, high-resolution LCDs. And a number of alternative alignment techniques have been reported,15-19 but none of these have so far been implemented in large-scale manufacturing.

In general, the orientational arrangement of LCs is dependent on their van der Waals interactions with substrates.20,

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The discovery of two dimensional (2D) functional materials, such as graphene, have

provoked the interests of researchers to imply such atomic thin materials as an alignment layers in the related devices.22 For graphene, the excellent combinations of good electrical conductivity and optical transmittance in the visible range, together with good chemical stability make this conception become more realistic.23-25 Recent researches have demonstrated that, LC molecules can be aligned along the crystalline orientation of CVD-derived graphene on PET or glass, as a consequence of relative strong epitaxial interactions between LCs molecules and graphene.26, 3


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Furthermore, the variations in the

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molecular orientations of the nematic LCs were also proposed to be mediated by the multi-domain structure of graphene.28, 29

In this work, we demonstrate the direct atmospheric-pressure CVD (APCVD) growth of high quality graphene on high-temperature resistant solid quartz glass, without using any metal catalysts. Large-area uniform, high quality graphene with tunable optical transparencies and electrical conductivities have been achieved on quartz. Intriguingly, we present that, the as-produced graphene glass can assist the alignment of the over-coated ChLCs molecules. That means, the polycrystalline, multi-domain graphene on glass can alter the helical axes of ChLCs and generate a multi-domain structure, thus providing a wider viewing angle for graphene glass based ChLCs devices. The influence of chirality effect is also discussed in the orientations of ChLCs molecules, which is essential for display technologies and applications. In this context, graphene glass is promising to serve as an excellent transparent electrode to replace indium tin oxide (ITO), for the fabrication of high-performance LC-based devices with a simplified device structure and a much wider viewing angle.

RESULTS AND DISCUSSION

The direct growth of graphene on quartz glass was carried out by a facile atmospheric pressure (AP) CVD route with CH4 as carbon precursor, without the aid of any metallic species or catalysts. The growth method is described in detail in the methods section. The photograph in Figure 1a presents a large-scale quartz sample (10 cm × 6 cm) after the APCVD growth of graphene (growth parameters: 500 sccm Ar, 100 sccm H2, and 5 sccm CH4 at 1080 °C, for 3 h). The CVD-derived glass shows a good transparency to the underneath Peking University diagram. The corresponding scanning electron microscopy (SEM) image of the as-synthesized graphene on quartz glass growing for 3 h is displayed in Figure 1b, showing the 4


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formation of a uniform layer with the coverage of ~ 99%. To characterize the detailed structures at the atomic level, the samples were then transferred onto the Cu grids, and examined using high-resolution transmission electron microscopy (HR-TEM). Sectional views of the film edges indicate that, most of the graphene film is of monolayer thickness, as representatively displayed in Figure 1c. A transparency of ∼ 97.25% at 550 nm was then determined by the ultraviolet-visible (UV-vis) transmittance spectroscopy measurement, providing a more straightforward proof of the large-area uniformity of the graphene film. Furthermore, the sheet resistances at 125 points over a 5 cm × 5 cm graphene/quartz glass plate were also measured by the four-probe method showing a relative narrow sheet resistance distribution, with the specific values varying from 4.1 ∼ 5.4 kΩ sq−1 and an average value ~ 4.8 kΩ sq−1. The relative homogenous sheet resistance in a large scale is also displayed in a two dimensional mapping image shown in Figure 3d, wherein a uniform green color appears in most of the area. The inset in Figure 1e exhibits the typical Raman spectrum of the as-grown graphene film. Three prominent peaks appear at ~ 1340, 1580, and 2683 cm−1, corresponding to the D, G, and 2D bands typical for graphene, respectively. For more direct evidence, Raman mapping of the I2D/IG in Figure 1e again displays a rather uniform color contrast, reconfirming the rather high thickness uniformity of the as-synthesized graphene on glass.

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Figure 1. (a) Photograph of 6 × 10 cm2 graphene glass featuring a good transparency to the underneath “Peking University” diagram (growth parameters: 500 sccm Ar, 100 sccm H2, and 5 sccm CH4 at 1080 °C, for 3 h). (b) SEM image of the CVD-graphene on quartz glass showing a rather uniform contrast. (c) TEM image of transferred graphene on Cu grids showing its monolayer thickness. (d) Distribution of sheet resistances (collected from 125 points) of monolayer graphene on glass. Inset exhibits the sheet resistance mapping on a 5 cm × 5 cm area of the as-grown graphene showing an average value of ~ 4.8 kΩ sq−1, at a transmittance of ~ 97.25% @550 nm. (e) Raman spectrum (inset) and Raman mapping image of I2D/IG ratio on the as-grown graphene. (f) Heating of the ChLCs coated graphene glass above the clearing transition temperature (92 °C). A dark contrast appears for the polarized OM image, confirming the isotropic phase feature of ChLCs. (g) Polarized OM image of the sample after cooling down to RT (30 °C) indicating the cholesteric phase feature of ChLCs. A bulk-like texture with distinguishable boundaries addresses the multi-domain structure of ChLCs. Accordingly, a rather high transmittance (inset of (g)) can be noticed for the ChLCs cell.

Considering the structural property of the graphene coating layer itself as well as the good transparency, reasonable sheet resistance, extra high stability of the hybrid material, the graphene glass was directly 6


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utilized as substrates for fabricating LCs cells with 20 µm thick spacers. This action was attempted for investigating the alignment effect of polycrystalline, multi-domain monolayer graphene on ChCLs molecules, thus for constructing high performance LCs devices. As known that, ChLCs is a variant of the nematic LCs, which spontaneously forms a macroscopic helical structure either when the LCs molecules are inherently chiral or when the chirality is externally introduced.30 Here, HNG 726200-100 (TNI = 373.2K, ∆ε = - 4.0), was used as the nematic LCs material with a chiral dopant of methylcyanobiphenyl (CB-15, 10wt%) to induce chirality. The phase-isotropic phase transition temperature is 92 °C. Hereby, the cell was filled with ChLCs above 92 °C by the capillary action, followed by cooling down to room temperature (RT) to generate the cholesteric phase. The phase change of ChLCs was examined by polarized optical microscopy (POM). The POM image (Figure 1f) of the texture exhibits a dark colour at 92 °C, in line with its isotropic phase. This is mediated by the isotropic refractive index associated with the random orientations of the ChLCs molecules. Interestingly, the isotropic phase can be changed into cholesteric phase with a planar alignment, by cooling down the sample to room temperature (Figure 1g). The planar texture exhibits distinguishable boundaries, corresponding to the multi-domains structures of ChLCs. Moreover, the ChLCs cell shows rather high transmittance under natural lighting, as demonstrated in the inset of Figure 1g, which in return addresses the macroscopic alignment feature of the ChLCs molecules by the graphene layer.

As reported previously, Ryotaro Ozaki and co-workers utilized a rapid thermal process to induce the alignment of ChLCs molecules into a planar texture.31 Note that, only after twenty cycles of annealing treatment, the ChLCs molecules began to adopt planar orientations. In contrast to this reference, the hexagonal structure (e.g. the graphene lattice) of graphene on a glass template was selected to induce a

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highly ordered packing of ChLCs molecules only by one cycle annealing treatment, which was explained to be caused by relative strong π-π interactions. Specifically, the polycrystalline multi-domain monolayer graphene grown on glass was proposed to be able to mediate the regional orientation of the helical axes of ChLCs, producing a similar multi-domain structure of ChLCs.

Generally, a reflective device displays the desired images through interacting with the surrounding light. Therefore, a wide viewing angle is particularly important for such devices that are intended for use in arbitrary ambient light and for viewing by groups of people. In the ChLCs, the rod-like mesogenic molecules self-organize into the helical superstructure which exhibits a selective reflection of circularly polarized light. The photonic reflection of ChLCs is angle-dependent. As a result, for light propagating off-axis, the reflected light wavelength is shifted to shorter wavelengths because the effective pitch length of the helix shortens to pcosθ, where p is the pitch of the helix, and θ is the incidence angle.32 The traditional approach to improve the angular properties of a ChLCs device is to tilt the helix axis within the device, through introducing some complicated microstructures on the glass substrate.33, 34

To understand the alignment effect, the alignment behavior of ChLCs (13 wt% of CB-15 was added to HNG 726200-100) on different substrates were then intensively explored, including bare quartz glass, quartz glass with an alignment layer (PVA) (as introduced by a mechanical rubbing technique), as well as the graphene glass, as shown in Figure 2. The surface characteristics of bare quartz glass was firstly examined with atomic force microscopy (AFM), with the data shown in Figure 2a. The surface is very flat showing a root-mean-square (RMS) value of ~ 0.621 nm, which is attributed to the formation of terraces and steps or defects generated during the preparation of glass. The POM image of the ChLCs confined by two graphene glass plates is displayed in Figure 2b. Some defect structures are commonly observed for 8


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ChLCs on quartz, arising from the formation of isotropic cores. Moreover, many defects that agglomerate within the ChLCs films generate a variety of focal conic textures in the absence of a uniform alignment. As shown in Figure S1 (in the supporting information), a linear polarized broadband white light was used as a light source, and the reflection spectrum and the intensity were measured using a spectrometer, where the angle of incidence was set at various angles -30°, 0° and 30° respectively. The transmittance spectrum in the infrared spectrum range (1200-2400 nm) of the focal conic ChLCs cell is shown in Figure 2c, with the maximum transmittance (70.0%) occurring at angles of -30°, 0° and 30°, respectively. Notably, the Bragg refection peak is not visible in the spectrum. This is because when the texture of the ChLCs is switched to the focal conic texture, the Bragg reflection disappears, and ChLCs scatters the incident light due to their randomly distributed helical axes.

Figure 2. (a) AFM image for the bare quartz glass surface. (b) POM image showing the formation of a focal conic texture 9


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of ChLCs on bare quartz. Herein, 13 wt% of CB-15 was added to HNG 726200-100 for inducing chirality. (c) Transmittance spectra measured within the near infrared region (1200 -2400 nm) of ChLCs on bare quartz showing no any obvious reflection peaks under various viewing angles (-30°, 0°, 30°). (d) AFM image for the quartz glass surface with the rubbed PVA polymer layer. (e) POM image indicating the evolution of a perfect planar texture of ChLCs on quartz with the coated PVA polymer layer as an alignment layer. (f) Transmittance spectra corresponding to sample (e) revealing a reflection peak at 0°. (g) AFM image for the graphene glass surface. (h) POM image addressing the formation of an imperfect planar texture of ChLCs with oily streaks and bulk-like textures. (i) Transmittance spectra of a ChLCs film on CVD-derived graphene on quartz glass presenting obvious peaks under various viewing angles (-30°, 0°, 30°) showing wide viewing angles characteristics.

In comparison, the RMS value of the quartz glass with an alignment layer (PVA) (as introduced by mechanical rubbing) was also measured as ~ 4.18 nm, as shown in the AFM image in Figure 2d. Microgrooves as marked by dashed lines were produced to be aligned parallel to the rubbing direction. This template induced a uniform orientation of the helical axes of ChLCs, and a perfect planar texture was evolved to be free of oily streaks (Figure 2e). Figure 2f manifests the transmittance spectra of the planar texture of the ChLCs cell. Specifically, the reflection intensity reaches a maximum value at approximately 90°, and declines rapidly as the angle deviates further from the Bragg reflection angle. The mirror reflection effect of the planar aligned ChLCs makes it unsuitable for use in reflective devices.

In this study, polycrystalline, multi-domain graphene on glass was also applied to achieve the uniform orientations of ChLCs molecules with irregular helical axes towards the formation of multi-domain structures, so as to widen the viewing angles of related devices. The AFM image of the graphene glass (Figure 2g) addresses a very flat surface which is free of any macroscopic surface structures. The π-π 10


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stacking interactions between the phenyl rings of ChLCs molecules and graphene should determine the relative orientations of the helical axes of ChLCs. From the POM image shown in Figure 2h, It can be noticed that, the oily streaks in the planar texture is increased, and the POM texture of ChLCs-coated graphene show slightly different colors on different locations, confirming disparate orientations of ChLCs molecules. The increase of the oily streaks is likely due to the difference of Fresnel reflection coefficients at the substrate/ChLCs interface which is dependent on the helix phase, since the incident light is linearly polarized. Briefly, the above results indicate that, the aligned orientations of the helical axes of ChLCs are possibly determined by the different domain orientations of the polycrystalline graphene. Unexpectedly, a wide viewing angle of photonic reflection is achieved in such multi-domain ChLCs cell. Figure 2i presents the transmittance spectra of the multi-domain ChLCs cell that is fabricated with graphene glass as an alignment layer. Notably, the peak of the infrared reflection spectrum is redshifted at an angle of -30° and buleshifted at an angle of 30°, respectively. The viewing zone of the graphene glass-based ChLCs device is thus exceeding 60°. In this regard, unlike the planarly aligned ChLCs with a coated PVA polymer layer as the alignment layer (Figure 2d), the multi-domain graphene glass surface contributes to alter the helical axes of ChLCs, producing a similar multi-domain structure. A wider viewing angle in the infrared spectrum region for graphene glass-based ChLCs device (Figure 2i) is thus achieved than the above-mentioned device with a PVA-polymer alignment layer. It should be noted that, the visible light reflection of ChLCs is also possible through the optimization of parameters such as viscosity35, 36 elastic constants,37 as well as the appropriate graphene nanostructures with controlled patterns of lines or dots.38-41

In order to show the multi-domain structural feature of graphene on glass, the initial growth process of graphene on quartz glass is carefully examined. When exposing quartz substrate to methane gas precursor,

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randomly distributed nuclei are formed at first, which increases in size with extending the growth time. After 1 h growth, graphene islands are randomly evolved on the substrate surface (Figure 3a) with a lateral size of 200 nm. With increasing growth time to 2 h, the islands enlarge their sizes and start to merge with each other. Ultimately, a near complete graphene film is evolved on quartz, with only a few open patches remained (Figure 3b). Similar data are also noticeable by the AFM studies in Figure 3c. The formation of nearly round shape domains and the evolution of multi-domain structures are in good agreement with the previous work.42, 43 With further increasing growth time, the secondary and ternary layers can also be achieved as shown in Figure S2 (seeing in the supporting information).

As is already known that, the hexagonal graphene lattice has three arm-chair edge directions with a 120° interval. Accordingly, LCs molecules on a single crystalline graphene domain tend to align along one of the three orientations. 44 The similarly oriented LCs molecules tend to form a specific domain, and the abrupt directional change usually occurs at the domain boundaries. In this work, the first monolayer of ChLCs can be viewed as nematic LCs, which was aligned along the crystalline orientations of the composite graphene domains, as a consequence of relative strong π-π interactions between LCs molecules and graphene. Hereby, the anchored direction of the LCs molecules should vary depending on the lattice direction of each graphene domain. However, the ChLCs contains helical domains, in which the orientation vector successively turns a small torsion angle from one layer to the next one along the helical axis direction. The surface-induced alignment of the LCs bulk is also mediated by this long-range interaction, as shown in Figure 3d illustrating the interaction right in the x-y plane. In this situation, the preferable alignment of the first ChLCs layer was realized by multi-domain graphene through relative strong π-π interactions between ChLCs and graphene. Concurrently, due to a long-range orientational

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interaction, the bulk-like alignment of ChLCs was also induced by a bulk elastic interaction. Specifically, the orientation of the helical axis of ChLCs depends on the alignment direction of the first monolayer, and the order will propagate inside the ChLCs along a common direction, while keeping their centers of mass homogeneously distributed in space. As a result, multi-domain distributions based on the different helix axes of ChLCs in a planar texture are directly created by using the similarly multi-domain structures of graphene on glass, with graphene as an alignment layer. This mechanism can be verified by the presence of different domains with disparate polarization angles for ChLCs on graphene glass, as also shown in Figure 1g, 2h. ChLCs is known to exhibit a circular-polarization selective Bragg reflection band over the near infrared wavelength region, as determined by the reflective index and pitch. In a conventional planar texture where the helical axis was aligned unidirectionally at the substrate surface, ChLCs can act as flat dielectric mirrors, reflecting light at an angle fulfilling the law of reflection.45 Here, it is intriguing to see that, by introducing a multi-domain structure of ChLCs by the similarly multi-domains graphene glass template, the reflected light phase becomes multi-dimension accordingly, leading to a wide-angle reflection as shown in Figure 3d when viewing in the z axis. Moreover, the reflection maintains its circular polarization selectivity, making this material distinctive compared to the structure of a Morpho didius butterfly.46 In the published work, the randomness in the height and spacing of the shelf-structures play a crucial role in mediating a wide-angle, diffuse reflection.

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Figure 3. (a) SEM image for graphene growth on quartz for 1 h with the formation of randomly distributed islands. (b) SEM image after 2 h growth showing the formation of nearly complete graphene film due to the merging of individual domains. (c) AFM height image of graphene growth on quartz for 2 h (corresponding to sample (b)). (d) Schematics of the alignment of ChLCs on the graphene coated glass. A view in the x-y plane (multi-domain planar texture of ChLCs, middle panel): the ChLCs molecules were oriented by the long-range interaction arising from the relative strong π-π interactions between the phenyl rings of ChLCs molecules and graphene. A view in the z axis (corresponding to the wide-angle reflection, right panel): the reflected light phase was multi-dimension due to the multi-domain distributions of the helix axes.

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Figure 4. Alignments of ChLCs with different helix pitches (P) on graphene coated glass surfaces (by adjusting the contents of the chiral dopant). When the sample was heated above the clearing transition temperature (92 °C), the POM image became dark, confirming the evolution of an isotropic phase of ChLCs. The inset shows the helix pitch of the ChLCs measured by a Cano wedges method. After cooled down from 80°C to 30°C, the ChLCs molecules present planar textures, and the transmittance of the samples @550 nm was variable from 95.19%, 88.94%, 78.91% to 72.06%, respectively, for (a) P1 = 5.246 µm, (b) P2 = 4.354 µm, (c) P3 = 2.890 µm, (d) P4 = 1.782 µm. Scale bar: 200 µm.

In order to verify the universality, it is necessary to investigate the alignment effect of graphene for ChLCs with different helix pitch (P). For such a purpose, the content of the chiral dopant was adjusted deliberately to obtain a series of samples with different P. Figure 4 presents the changes from isotropic to cholesteric phases with the formation of planar textures for ChLCs with different helix pitches (P) on graphene coated glass surfaces, by cooling down process from 80 °C to 30°C. Notably, the increase of the oily streaks in the planar textures is accompanied with the decrease of P (P1 = 5.246 µm, P2 = 4.354 µm, P3 = 2.890 µm, P4 = 1.782 µm), and the transmittance of the corresponding cell decreases obviously from 95.19% to 88.94%, 78.91%, 72.06%, respectively. This is because the intermolecular interaction in the 15


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ChLCs with a short-pitch is stronger than that with a long-pitch, and the anchoring property of graphene on the ChLCs molecules is relatively weakened. Correspondingly, the fractured surfaces of cross-linked LC samples with different P were studied by SEM. Notably, planar layers were observed in the samples, confirming the homogeneous alignment of ChLCs on the graphene glass, as shown in Figure S3 (in the supporting information). When the P value is set at 2.890 µm, the planar texture (Figure. 4c) presents less oily streaks, and the maximum transmittance of the cell is enhanced up to ~ 78.91%. And the bulk structure presents various colors, as shown in the POM texture.

According to above experimental results, it can be inferred that, the chirality of ChLCs, as well as the interface interactions of ChLCs with the multi-domain graphene on glass are responsible for reducing the oily streak defects. As known that, there was an energy barrier between the focal conic and planar textures of ChLCs, which was P-dependent. The longer P, the lower energy barrier for the switch from the focal conic texture to the planar one (see Figure S4 in the supporting information), and it was reported that an entropy-driven cholesteric phase strongly depended on the helical pitch P.47 When the P value of ChLCs is short enough, the free energies of the focal conic texture and the planar texture are similarly low (both textures are stable relatively), followed with a hard switch. Corresponding simulation has been conducted basing on strong chiral attractive interactions.48 In this regard, through a careful selection of P on a given surface, multi-domain graphene on glass is promising to align the helical axes of ChLCs, by virtue of the relative strong interface π-π stacking interaction, as well as the intermolecular interaction in the cholesteric phase.

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Figure 5. (a) Schematic of the graphene-glass-based trans-PSLC device at the power off state (left panel) and photograph of the device also at power off state. (b) Corresponding POM image of the graphene-glass-based trans-PSLC at the power off state showing a planar texture. (c) Schematic of the graphene-glass-based trans-PSLC device at power on state (left) and photograph of the device at power on state. (d) Corresponding POM image of the graphene-glass-based trans-PSLC at the power on state showing the formation of a focal conic texture. (e) Response time (inset) and transmittance plots dependent on the applied voltage of the graphene-glass-based trans-PSLC. (f) Transmittance spectra of the graphene-glass-based trans-PSLC at power off state presenting various viewing angles in the infrared spectrum region. Here, 8 wt% of CB-15 was added to HNG 726200-100 for inducing chirality. A different reflection peak position was induced as compared with that of Figure 2 with a difference dopant concentration. 17


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Considering the relatively high conductivity/transparency of graphene glass, the good visible light transparency of ChLCs on graphene glass, it is thus desirable to construct some photoelectric devices by applying an external field. The left panel in Figure 5a presents the sketch of the ChLCs-related device directly fabricated on graphene glass. Herein, the planar alignments of ChLCs with the evolution of multi-domain at the substrate surface are marked with red arrows for emphasis. Specifically, a layer of polymer network stabilized liquid crystals (PSLC, 20 µm thick) is sandwiched between two pieces of graphene glass with an intrinsic transmittance of 90% (ChLCs possessing the similar composition as that of Figure 4b). The corresponding photograph of the ChLCs-based device is displayed in the right panel of Figure 5a, with a sample size of 10 cm ×6 cm. The words of “graphene glass” and the colored figure are placed underneath the device for the transparency demonstration. The corresponding POM image (Figure 5b) presents the evolution of a similar multi-domain structure of ChLCs. Accordingly, the reflection of light on the related device maintains its circular polarization selectivity, leading to semi-transparent feature under specified lighting.

Intriguingly, by applying a suitable electric field between the two electrodes (AC, 120V 50Hz), the distribution of the helical axes of ChLCs becomes disorder, as displayed in the schematic image in Figure 5c. The related ChLCs-device was driven to a strong light-scattering state with a reduced transparency, showing an opaque and ivory white color. This is because the planar texture of ChLCs is turned into a focal conic texture, as characterized by POM image shown in Figure 5d. The electro-optical behavior of ChLCs-device was also investigated by plotting the contrast ratio as a function of the driving voltage. The threshold voltage (Vth) is identified as 18.47 V and the saturation voltage (Vsat) is 91.61V, as shown in Figure 5e. Moreover, it is intriguing to see that, the device exhibits quite short rise (~ 0.5 ms) and decay

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time (~ 25 ms), as shown by the inset of Figure 5e. Note that, the directly synthesized graphene on glass is featured with the traits of high transparency, superior conductivity and relative high stability. Compared with the ChLCs devices using traditional alignment techniques, the resulting devices which is free of any additional alignment layer can exhibit wider viewing angle in the infrared spectrum region due to the formation of multi-domain cholesteric superstructures. Using such graphene glass, polymer-stabilized ChLCs device can be facilely and reversibly switched between high transparent and strong light-scattering states by applying a low electric field, which is expected to offer a pathway for developing advanced multifunctional photonic devices towards energy- and safety-related applications.

Considering the evolution of a similar multi-domain structure of ChLCs (Figure 5b), the infrared spectrum reflections of the ChLCs-device (with a different dopant concentration as that of Figure 3i) were also measured at various viewing angles, as shown in Figure 5f. Unlike in the planar aligned ChLCs, the multi-domain structure of graphene on glass can induce regional alignments of the helical axes of ChLCs. The viewing zone of the ChLCs-device with the use of graphene glass similarly exceeds 60° in the infrared light region.

In summary, the direct CVD fabricated graphene glass has exhibited many traits, e.g., inert surface with a hexagonal lattice, tunable optical transparency, reasonable electrical conductivity, etc. By virtue of the hexagonal lattice and multi-domain structure of CVD-derived graphene on glass, the regional alignments of the helical axes of ChLCs are realized with the evolution of a similar multi-domain structure of ChLCs, leading to much wider viewing angle in the related devices especially in the infrared spectrum region. This utilization of a graphene layer as the alignment layer avoids the traditional route that is usually involved with lengthy nanofabrication procedures. Moreover, due to the random orientations of the helical axes of 19


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ChLCs, as well as the circular polarization selectivity of graphene glass-based ChLCs device, the visible light transmittance can be switched reversibly by an external electric field. Briefly, this work demonstrates that, graphene glass can be applied to high-performance wide viewing angle ChLCs devices with graphene as an alignment layer, as well as a transparent electrode. For this reason, this technique has long found wide technological applications to fabricate various optical devices in the optoelectronic industry.

Experimental Section. Methane precursor based APCVD growth of graphene glass without the aid of any metallic species or catalysts. In a typical CVD process, thoroughly cleaned quartz glass (with deionized water, acetone, and ethanol in sequence) was loaded into a horizontal quartz tube (3-inch diameter) inside of a three-zone high-temperature furnace (Lindberg/Blue). Prior to heating, the chamber was flushed with 500 sccm Ar to remove air. The furnace was then heated to the desired growth temperature and stabilized for about 10 min. Typical growth conditions were 500 sccm Ar, 100 sccm H2, and 5 sccm CH4 at 1000-1120 °C for 1-5 h. In this growth process, the thermally decomposed CHx (x = 0-4) fragments absorb and desorb on the glass surface, and nucleate at some preferred sites (scratches, defects, etc.), when the concentration of the active carbon species exceeds a critical value. And then the as-nucleated graphene domain expands through attaching C adatoms to the domain edge. Intriguingly, the nucleation capacity can be further activated by prior annealing the substrate in air at high temperature, which could promote the adsorption of active carbon species though the C-O or H-O binding. On the other side, the intermediate product of SiC may be formed through the carbothermal reduction of silica to serve as catalyst of graphene growth on quartz glass. Finally, these small, individual islands merge with each other with the formation of a continuous film on the glass substrate.

Graphene-glass-based trans-PSLC. The graphene-glass-based trans-PSLC consists of two pieces of 20


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graphene glass and a sandwiched LCs layer. The ChLCs used in the study were HNG 726200-100 (TNI = 373.2K, ∆ε = - 4.0, Jiangsu Hecheng Display Technology Co. Ltd.), the chiral dopant CB15 (Merck Co., Ltd) were used. The monomer was a difunctional monomer C6M (Merck). The photoinitiator was Irgacure 651 (Ciba-Geigy). All of the above materials were used as received without further purification. The compositions of the samples were prepared and vigorously stirred until a homogeneous mixture formed. Then, the mixture was sandwiched between two pieces of graphene coated glass substrates by capillary filling into the cell. The film thickness was controlled by 20.0 ± 1.0 µm thick polyester spacers. The samples were irradiated for polymerisation by a UV lamp (365 nm 35-W Hg lamp, PS135, UV Flood, Stockholm, Sweden) for 10.0 min at 318.2 K. The performance of the trans-PSLC was measured using the liquid crystal device parameters tester (LCT-5016C).

Characterization. The prepared samples were systematically characterized using SEM (Hitachi S-4800, operating at 1 kV), Raman spectroscopy (Horiba, Lab RAM HR-800, 514 nm laser excitation, 100 × objective lens), UV-Vis spectroscopy (Perkin-Elmer Lambda 950 spectrophotometer), TEM (FET Tecnai F20, 200 kV; JEM-ARM200F equipped with double post-specimen spherical aberration correctors, 80 kV), and four-probe resistance measuring meter (Guangzhou 4-probe Tech Co. Ltd., RTS-4). The optical textures of the samples were studied by polarizing optical microscopic (POM) (ZEISS, Axio Scope.A1) equipped with hot-stage (LINKAM, LTS460E).

ASSOCIATED CONTENT Supporting Information. Detailed SEM micrograph for the graphene sample on quartz growing for 4, 5 h and the fracture of the pitch of ChLCs network graphene are included. Schematic of pitch on the influence

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of the energy barrier between different texture is also presented. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2016YFA0200103),

the

Beijing

Municipal

Science

and

Technology

Commission

(No.

Z161100002116020), the Ministry of Science and Technology of China (2013CB932603), and the National Natural Science Foundation of China (51432002, 51290272).

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Table of content

The hybrids of graphene directly grown on quartz glass were utilized to construct cholesteric liquid crystals (ChLCs) based devices with graphene serving as both alignment and conductive layers, towards the fabrication of structurally simplified, wide viewing angle devices.

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Graphene Glass Inducing Multi-Domain Orientations in Cholesteric

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