Characterizations of a graphene-polyimide hybrid electro-optical liquid crystal device

A planar liquid crystal (LC) cell is designed by placing together a monolayer graphene coated glass slide and a planar-aligning polyimide layer on an indium tin oxide (ITO) coated glass slide. It is shown that the monolayer graphene film on one side of the cell can serve as the planar-alignment agent. At the same time, the monolayer graphene film also functions as the transparent electrode. The successful optical and electro-optical operations of this hybrid LC cell with monolayer graphene on one side and with ITO and associated planaraligning polyimide on the other side are demonstrated. The measured optical transmission of the graphene electrode is found to be much better than that of the combined ITO-PI layers. The electro-optical effect and the dynamic electro-optic response of the LC in this graphenepolyimide-based hybrid cell reveal the typical director reorientation of the LC on the application of an electric field.

In another direction, optical anisotropic properties of liquid crystals (LCs) make them an ideal optical material for the electro-optical display technology.Semi-transparent capacitive LC cells are used in various LC devices.Such LC cells have two major components: the indium tin oxide (ITO) electrodes and the polyimide (PI) alignment layers.Since MGP has a few advantages over ITO electrodes, such as better mechanical strength, higher chemical resistance, and higher mechanical flexibility, several reports in the literature have shown that MGP can serve as an ITO replacement in an LC device [11][12][13][14][15][16].
The alignment of the LC molecules on a substrate dramatically influences the electrooptical characteristics of the LC [17].In a conventional LC cell, the PI layers (at both sides) are employed to align the LC unidirectionally in the cell.The LC molecules align with alkyl side chains on the unidirectionally rubbed PI layers, creating a uniform and planar nematic director (  n ) profile (also called the easy axis) in the cell.Then, an external electric field generated between two ITO layers reorients  n from its initial planar state.Here we report the fabrication and electro-optical characterizations of an MGP-PI hybrid LC cell, where the twodimensional MGP sheet on one side of the cell serves as the planar-alignment agent.Additionally, the MGP film serves as the transparent conductive electrode as well.

Experiments, results, and discussion
In this section, we present (a) the fabrication of the MGP-PI hybrid LC cell with optical transmission properties of MGP and PI/ITO slides, (b) the electro-optical effect of the LC in the cell, and finally, (c) the dynamic response of the LC in the cell.

Fabrication and optical characterization of MGP-PI hybrid cell
The LC molecules coherently align on the two-dimensional MGP [18][19][20][21] and other graphene-like surfaces [22][23][24][25] due to epitaxial interactions between the benzene rings of the LC molecules and the hexagonal lattice of MGP, employing the π−π electron stacking interaction.This interaction results in a planar alignment of the LC on the hexagonal lattice.This planar anchoring process of the LC is reinforced with a binding energy, ranging from 0.7 to 2.0 eV/molecule [21][22][23]26], due to a considerable amount of charge exchange between the LC molecule and the hexagonal lattice.The π−π stacking interaction is illustrated in Figs.1(a), 1(b), 1(c) by overlaying the LC's benzene rings on the hexagonal MGP structure.Thus, the LC can gain a planar-aligned state over a large-scale dimension on MGP [27][28][29][30] due to this strong π−π stacking interaction [31][32][33][34].Therefore, we employed the MGP film to serve as the planar-alignment agent in addition to the conductive electrode on one side in an LC cell.The other side of the cell had a regular rubbed PI alignment layer, and the reason for using this MGP-PI hybrid cell (and not an MGP-MGP cell) is explained below.The hexagonal symmetry on the MGP surface causes the LC director,  n to adopt three different orientations separated by 60° [35,36].This three-fold alignment degeneracy of  n is schematically shown in Figs.1(a), 1(b), 1(c).First, the standard polymethyl-methacrylate (PMMA) assisted wet transfer method [37] was employed to transfer the commercially obtained (Graphene Supermarket, Inc.) large-scale MGP film [38] from a copper foil onto a glass substrate.The sheet resistance of the graphene film on the glass was found to be ~660 Ω/ .To realize the alignment degeneracy, a thin LC layer was created by placing a small droplet of E7 liquid crystal (T NI = 60.5 °C, EMD Millipore Corporation) on the MGP sample and then gently blowing the droplet away by a dust blower.The thin LC layer on the MGP sample was heated up (to the isotropic phase) and cooled down (to the nematic phase) multiple times to remove any remaining order resulted from the coating process.A crossedpolarized optical microscope was used to study the alignment of the LC on the MGP sample.First, two MGP coated glass slides were placed together (with the MGP sides facing each other) to make an MGP-MGP cell (with an average cell-gap of 14 μm), where the MGP films serve as the planar-alignment agent on both sides of the cell.To investigate the director alignment, E7 filled MGP-MGP cell was examined under the crossed-polarized microscope, and Fig. 1(f) shows the micrograph of the LC E7 in the MGP-MGP cell where the average LC director is at 45° with respect to the crossed polarizers.The LC, inside the MGP-MGP cell, generates local defect-like texture (see Fig. 1(f)), resulting from the three-fold alignment degeneracy of LC on the hexagonal structure of MGP.To achieve a better unidirectional planar alignment on a large-scale using MGP, we fabricated a hybrid cell with a rubbed PI layer on one side and the two-dimensional MGP sheet on the other side of the cell.A unidirectionally rubbed planar-aligning PI (KPI-300B, Kelead Photoelectric Materials Co., Ltd.) substrate on an ITO coated glass slide (from Instec, Inc.) and the MGP coated glass slide were placed together to make an MGP-PI hybrid cell with an average cell-gap of 17 μm.Inside the cell, the MGP film and the PI substrate faced each other.On the glass slide, the ITO layer was 230 nm thick, and the PI layer was 60 nm thick.Before fabricating this MGP-PI hybrid cell, the optical transmission spectra of the ITO/PI sample and the MGP sample were taken separately using FLAME-S-XR1-ES (Ocean Optics, Inc.) spectrometer and DH-2000-BAL UV-VIS-NIR (Ocean Optics, Inc.) light source.The transmission spectra for these two samples in the wavelength (λ) range from 300 nm (UV) to 1000 nm (near IR) are shown in Fig. 2. The MGP sample clearly indicates more optical transparency than the ITO/PI sample, except for a small range, as shown in Fig. 2. Therefore, replacing one of the two ITO/PI slides in a conventional cell by the MGP slide is expected to reduce the transmissive losses over a broad spectral range in the LC device.The MGP-PI hybrid LC cell is schematically illustrated in Fig. 3(a).Figure 3(b) shows the picture of this hybrid cell.The alignment of the LC in the hybrid cell was studied by rotating the cell under the crossed-polarized microscope and examining the transmitted intensity.The micrographs in Figs.3(c) and 3(d) show the orientation of  n at 45° (uniformly bright) and 0° (uniformly dark), respectively, with respect to the analyzer.This is the same optical behavior of a standard PI-PI LC cell [29].The normalized transmitted intensity through the cell as a function of the rotation angle under the crossed-polarized microscope is shown in Fig. 3(e).Note that the intensity goes from a maximum (bright) to a minimum (dark) when the cell rotated by 45°.Thus, these micrographs confirm the unidirectional planar-alignment of the LC in the MGP-PI hybrid cell.Unlike Fig. 1(f), the LC texture in Fig. 3(c) depicts a uniform director field without any local defects.The presence of the unidirectionally rubbed PI alignment layer at one side of the hybrid cell enforces the LC to choose the alignment on the MGP surface parallel to the PI's rubbing direction.See Fig. 3(a).This mechanism minimizes the elastic distortion of the director field in the cell and reduces the three-fold degeneracy to the uniaxial alignment of the LC on MGP.Consequently, the LC gains a uniform planar alignment (i.e., an easy axis) over a large-scale dimension on MGP.Thus, it is experimentally verified now that the two-dimensional MGP film can function as a planar-alignment agent on one side in the hybrid cell which has a rubbed PI layer on the other side.
We fabricated another cell with a rubbed PI layer on one side and a plain glass slide (with no MGP) on the other side.This PI-Glass cell gives us a sense of the alignment of the LC in the absence of the MGP film.Figure 3(f) shows the micrograph of LC E7 in the PI-Glass cell where the PI's rubbing direction is at 45° with the crossed polarizers.The LC texture clearly indicates multiple domains without having a uniform planar-alignment.As the cell was rotated by 45°, the rubbing direction of the PI became parallel to the polarizer, and the micrograph in Fig. 3(g) shows a non-uniform dark texture.This proves that the absence of the MGP on one side of the cell disrupts the unidirectional planar-alignment.

Electro-optical effect of LC in MGP-PI hybrid cell
The electro-optical effect [17] of LC E7 in the MGP-PI hybrid cell was studied to realize the field induced LC director rotation in the cell.The electro-optical effect is essential for LCD technology and observed when the applied electric field across the cell exceeds its threshold value.As  n rotates from the planar state to the homeotropic state with increasing electric field, the effective birefringence, <Δn> changes as a function of applied voltage, causing a change in the phase difference, where d is the cell-gap and λ is the wavelength of the monochromatic light.According to Eq. ( 1), a change in the phase difference results in an oscillatory optical signal at the exit of the analyzer.The electro-optical effect of the LC in the MGP-PI hybrid cell was studied using an optical setup comprising a He-Ne laser beam (5-mW, λ = 633 nm) traveling through a polarizer, the MGP-PI hybrid LC cell (where  n was oriented at 45° with respect to the polarizer), a crossed analyzer, and into a photodetector.The output of the photodetector was fed into a dc voltmeter to measure the transmitted intensity, I as a function of the applied ac voltage (f = 1000 Hz) across the cell.The cell was also mounted under the crossed-polarized microscope (with  n at 45° with respect to the crossed polarizers) to take several micrographs of the MGP-PI cell at different applied voltages (f = 1000 Hz).Note that when the voltage was applied across the hybrid cell, the electric field was generated between the MGP electrode and the ITO electrode.
Figure 4 shows the electro-optical effect of LC E7 in the MGP-PI hybrid cell.Figures 4(a 1), as shown in Fig. 4(f).The intensities of the micrographs do not directly correspond to the transmittance curve, as the micrographs were taken under a white light, and the transmittance curve was obtained using a red laser (i.e., a monochromatic source).Note that Figs.4(b), 4(c), and 4(d) show parallel-lines like patterned texture (along the diagonal) when the voltage is applied.We believe that they appear because of the small asymmetry of the electric field due to the mismatch of the conductivity between the ITO electrode and the graphene electrode.4(f) presents the same transmittance curve in a smaller voltage range.This inset depicts six maxima, confirming a complete director rotation from planar state to homeotropic state in the hybrid cell.The inset also shows a typical Fréedericksz transition upon the application of an electric field across the cell.Thus, the micrographs and the transmittance curve reveal that the MGP-PI hybrid cell exhibits the required electro-optical effect for an LC device-where the two-dimensional MGP film at one side of the cell simultaneously serves as the planar-alignment agent and the transparent conducting electrode.

Dynamic response of LC in MGP-PI hybrid cell
The field-induced dynamic response of the LC is given by two characteristic times, τ rise and τ decay [17].When the voltage is turned on, the time taken by  n to rotate from planar to homeotropic configuration is defined as τ rise .On the other hand, the time taken by  n to relax back to planar from homeotropic configuration after the voltage is turned off is defined as τ decay .These two characteristic times are described as where d is the cell-gap, 1 is the rotational viscosity, Δε is the dielectric anisotropy, V is the applied voltage, ε 0 the is free space permittivity, and K 11 is the splay elastic constant.These switching times were studied using the optical setup similar to the electro-optical effect setup described in section 2.2.In this case, the output of the detector was fed into a digital storage oscilloscope which detected the change in transmitted intensity when a squarewave voltage of 30 Hz was applied across the hybrid cell.Transmittance responses were studied for several applied voltages much higher than the threshold switching voltage.In Fig. 5(a), the left y-axis represents the normalized transmitted intensity of LC E7 in the MGP-PI hybrid cell as a function of time when a voltage (30 V) was turned on and off.The right y-axis shows the applied square-wave voltage across the cell.At t = 0, as the voltage is turned off, the transmitted intensity increases as a function of time.The optical switching off, τ off is defined by the time the transmitted intensity takes to increase from 10% to 90% of its maximum value.At t = 16.6 ms, the applied voltage is turned on, and the transmitted intensity falls as a function of time.The optical switching on, τ on is the time the transmitted intensity takes to decrease from 90% to 10% of its maximum value.Note that τ rise and τ decay (Eq.( 2) are not directly equal to the electro-optic responses -τ on and τ off , respectively.However, after the field is turned on or off, the optical response shown in Fig. 5(a) is mainly due to the director's rotation.Therefore, if the backflow of the cell is neglected, one can write τ rise ∝ τ on and τ decay ∝ τ off .
Figure 5(b) shows τ on (top panel) and τ off (bottom) as a function of peak-to-peak voltage, V pp across the hybrid cell.τ on was fitted according to Eq. ( 2) for τ rise , using the room temperature E7 LC parameters [28]: γ 1 = 160 mP.s, Δε = 14, and K 11 = 8 pN, with the cellgap, d = 17 μm.The dotted curve shows the fitting in the top panel in Fig. 5(b).The fitted curve is consistent with the experimental data-which confirms that the MGP at one side of the cell can successfully replace the ITO layer as well as the PI layer, exhibiting the typical dynamic electro-optic response.
As shown in Eq. ( 2), τ off does not depend on the applied voltage when the applied voltage, V>>V th .Note that this dynamic electro-optic experiment was carried out in the high voltage (V>>V th ) regime-which is known as the transient nematic relaxation mode [39,40].In this mode, the decay time can be as fast as ~3 ms even for a cell-gap larger than 15 μm [14,39,40].
In our experiment, V th = 0.8 V (see Fig. 2(f)) for this MGP-PI hybrid cell.Our applied voltage range is from 12 V to 45 V (i.e., V>>V th ).The value of τ off , shown in Fig. 5(b), for this hybrid cell (d = 17 μm) is in the same range of the fast transient nematic relaxation mode [14,39,40].

Conclusion
To summarize, it is demonstrated that the two-dimensional MGP sheet can serve both as the planar-alignment agent and the transparent electrode, simultaneously, at one side of the cell.The appearance of the uniform planar-alignment of MGP-PI hybrid cell and its absence in the PI-Glass cell is a clear indication that MGP can serve as the planar-alignment agent.This MGP-PI hybrid cell shows (a) a uniaxial planar LC alignment, (b) a typical Fréedericksz transition through electro-optical effect, and (c) a dynamic electro-optic response of the LC upon the application of an electric field.Additionally, it is also shown that the MGP on a glass slide is significantly more transparent than the ITO/PI combination on a glass slide for a wide spectral range.Note that the combined thickness of an ITO electrode and a PI alignment layer is about 300 nm, while the two-dimensional MGP sheet is about 0.37 nm thick [31].Replacing an ITO/PI slide by an MGP slide reduces this effective thickness significantly and offers the potential to decrease the transmissive losses over a wide range of spectral bands.Presented results are expected to advance the methodology toward nanoscale manipulation of LCs using their interactions with MGP.

Fig. 1 .
Fig. 1.A schematic representation of the alignment of nematic LC molecules (ellipsoids) on MGP (honeycomb structure) due to π−π electron stacking.The MGP crystal is oriented along  a .The three-fold alignment degeneracy is illustrated as the nematic director,  n is orientated at (a) −60°, (b) 0°, (c) + 60°, with respect to  a .(d) Microphotograph of a thin layer of nematic LC E7 on the MGP film on a glass substrate under the crossed-polarized microscope.(e) Micrograph of the LC sample when rotated by 45° under the crossed-polarized microscope.(f) Micrograph of the MGP-MGP cell filled with LC E7 under the crossed-polarized microscope where the average director field,  n is at 45° with respect to the crossed polarizers (bright state).The black bar in micrograph (d) presents 50 μm.

Figure 1 (
d) shows the micrograph of E7 LC-coated MGP on the glass substrate.An abrupt directional change of  n occurs at the degenerate domain boundaries.Several light and dark domains in the micrograph indicate the degenerate planar alignment of the LC on the MGP sample.When the sample was rotated through 45° under the crossed-polarized microscope, the degenerate domains changed their intensities, as shown in Fig. 1(e).It is, therefore, difficult to obtain a unidirectional planar alignment of the LC on MGP on a large scale.

Fig. 2 .
Fig. 2. Optical transmission intensity as a function wavelength for ITO/PI on glass and MGP on glass, listed in the legend.The visible wavelength range is shown by colors in the x-axis.The inset picture shows the two samples.Except for the shaded region (520 nm ≥ λ ≥ 570 nm), the MGP sample shows more optical transparency than the ITO/PI sample.

Fig. 3 .
Fig. 3. (a) A schematic representation of the MGP-PI hybrid cell which contains an MGP on one slide and ITO/PI on the other slide.(b) The picture of the MGP-PI hybrid cell.(c), (d) Micrographs of the bright and dark states, respectively, of the MGP-PI hybrid cell filled with LC E7 under the crossed-polarized microscope.(e) Normalized transmitted intensity of the MGP-PI hybrid cell as a function of the angle of rotation under the crossed-polarized microscope.Micrographs of the PI-Glass cell filled with LC E7 under the crossed-polarized microscope where the PI's rubbing direction is at (f) 45° and (g) 0° with respect to the polarizer.The black bar in micrograph (c) and (f) presents 50 μm.
n is initially oriented at 45° with the crossed polarizers, and I o is the initial intensity of the plane polarized light incident on the LC cell, then the transmitted intensity, I at the exit of the analyzer varies as[17] ), 4(b), 4(c), 4(d), and 4(e) are the micrographs of the LC texture in the MGP-PI cell at different applied ac voltages.The transmittance, o I I of LC E7 in the MGP-PI hybrid cell as a function of the applied ac voltage (f = 1000 Hz) clearly exhibits the oscillatory behavior according to Eq. (

= 6 .
When  n undergoes a complete rotation from planar state to homeotropic state, the number of maxima in the transmittance curve in Fig. 4(f) is approximately given by Δ d n λ [17].Using our experimental parameters (i.e.Δn = 0.225 for LC E7, λ = 633 nm and, MGP-PI hybrid cell-gap, d = 17 μm), we obtain Δ d n λ The inset in Fig.

Fig. 5 .
Fig. 5. (a) Left y-axis: Normalized transmitted intensity as a function of time when a peak-topeak voltage (V pp = 30 V) is turned off at t = 0, and then turned on at t = 16.6 ms, across the cell (at T = 25° C).Right y-axis: The applied voltage profile across the cell as a function of time.(b) τ on (top panel) and τ off (bottom panel) as a function of V pp for LC E7 in the MGP-PI hybrid cell.