Properties of a Thermotropic Nematic Liquid Crystal Doped with Graphene Oxide

Graphene oxide (GO) nanoparticles of two different sizes are dispersed in the nematic liquid crystal (LC) 5CB covering a wide concentration range. The dielectric properties, as well as the electrooptic behavior, including threshold voltage, elastic constant, and response times are investigated as a function of GO concentration. It is found that small graphene oxide flakes of mean size of 560 nm are better and easier dispersible than larger flakes of 2.8 μm mean size. The nematic–isotropic transition only increases slightly for the (GO+LC) hybrid systems. For increasing GO concentrations the threshold voltage and splay elastic constant dramatically increase, until saturation for a concentration of ≈1% by weight GO. The field driven switching‐on time is practically independent of concentration, while the switching‐off time, which is purely elastically driven, exhibits a strongly decreasing behavior. Dielectric spectroscopy reveals a noncollective relaxation which is absent in the neat liquid crystal. This may be attributed to a drastically slowed down molecular relaxation related to the rotation around the short axis of the liquid crystal molecules. When heating the thermotropic liquid crystal into its isotropic phase, the latter acts as a solvent for the GO particles, and a lyotropic nematic phase with largely reduced birefringence is formed.


Introduction
Nematic liquid crystals (NLCs) are essential materials for the scientifi c and industrial progress of the electrooptic display technology. The key characteristic that makes NLCs practical It also exhibits an extremely large Kerr effect, [ 15 ] which could enhance the electrooptic properties of novel display types, such as blue phase LCDs. In electronics, reduced graphene oxide could act as transparent electrode materials for LEDs or solar cells, but especially also as a replacement for electrode material indium tin oxide (ITO) used for liquid crystal electrooptic cells and displays, orienting the GO at the substrate surfaces through a water based lyotropic liquid crystal. Further applications could be envisioned in biomedical applications, such as drug delivery systems, as cell membranes exhibit liquid crystalline order; or in the area of tunable sensor materials.
In this paper, we investigate the dispersion of GO fl akes into a standard thermotropic nematic material, 5CB. GO is made of monolayer graphene platelets that are surface and edge-oxygenated in the form of carboxyl, hydroxyl, or epoxy groups, [ 16,17 ] and have strong mechanical properties, chemical functionalization capability, and a large specifi c surface area. These oxygen functional groups allow high solubility of GO in water and other polar solvents. However, due to the anisotropic property of the medium, and the disturbance in the free energy and entropy, the dispersion of GO in an NLC is a major challenge. [ 18 ] GO fl ake additive differs from graphene fl ake additives in many signifi cant ways. Liquid-phase exfoliated graphene fl akes, like the ones that have been previously added to NLCs, are comprised of small fl akes of few-layer graphene; typically 500 nm laterally and 1-5 layers thick. More importantly, fl ake thickness and lateral size correlate, bigger fl akes are also the thicker fl akes. GO on the other hand provides us with 100% monolayer fl akes, with better control over fl ake size. Also, whereas graphene is a hydrophobic material, GO is a hydrophilic material, and this will result in a different interaction with liquid and liquid crystal solvents.
In this paper, the evolution of aggregates, along with the nematic properties of phase transition temperature and electrooptic parameters will be presented and discussed as a function of GO concentration. In addition, we study differently sized GO fl akes, and in particular also the high concentration regimes, which was motivated by investigations of lyotropic systems. [ 16,17,[19][20][21]

Characterization of the GO Flakes
The average size and polydispersity of GO fl akes were characterized using scanning electron microscopy (SEM), with typical images for the small and the large fl akes shown in Figure 1 a,b. The GO fl akes exhibit irregular polygonal shapes with relatively broad size distributions. The small fl akes (GO-A) and the large fl akes (GO-B) had a mean equivalent diameter of 0.56 ± 0.32 µm and 2.8 ± 1.6 µm, respectively. As shown in Figure 1 c, the size distribution also contains smaller fragments, which are thought to be produced as a result of breaking down fl akes during the exfoliation process. [ 16 ] Nevertheless, the two distributions are clearly separated, so that one can speak of two different GO sheet sizes.

Optical Polarizing Microscope Textures
The observation of the mixture (GO+LC) textures in sandwich cells by polarizing microscopy indicates the quality of alignment. As indicated in Figure 2 , the orange texture inside the square is homogeneously aligned where GO fl akes are well dispersed. The dark brown area inside the circle illustrates a GO aggregate, which disturbs the liquid crystal alignment. In addition, the aggregate sizes of the small fl akes (Figure 2 a) evolve slowly when compared to those of large fl akes, Figure 2 b. This implies that the small fl akes are better dispersible than the large ones, as qualitatively expected. Interestingly, the aggregate sizes saturate at approximately 0.6 wt% of GO. Beyond this concentration, all of the director fi eld appears strongly dominated by the dopant. Thus, the nematic liquid crystal loses its long range orientational order by doping GO fl akes in excess of approximately 0.6 wt%. This will be discussed in further detail below when the electrooptic behavior is reported.
At very large concentrations of GO in 5CB, samples become extremely viscous, so that capillary fi lling of cells would not be possible anymore. Squeezing the sample to thin cell gaps appears to induce phase separation, which is more pronounced for the larger than the smaller fl akes. This indicates that there is a stronger interaction between large GO fl akes, leading to a stronger tendency for aggregation.
For various concentrations, the dispersions exhibited a change of state at the clearing temperature of 5CB. The

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dispersion of GO in the now isotropic phase of the thermotropic liquid crystal, exhibits a possible lyotropic nematic phase with a clearly detectable, but small birefringence. The isotropic phase of 5CB could thus behave as a solvent for the dispersed graphene oxide ( Figure 3 a,b). This implies that the material exhibits orientational order, which can be attributed to one of two mechanisms, or a combination thereof. First, GO experiences the medium as an isotropic liquid to form a lyotropic nematic phase, which have recently been studied intensively with water as a solvent [ 16,17,20,21 ] (Figure 3 c). Or second, due to the surface anchoring of 5CB molecules on the GO fl akes, a large amount of mesogens are ordered at the aggregates, leading to very large and pronounced pretransitional effects, simulating a nematic phase.
One of the drawbacks in these investigations of the GO fl akes is opacity; with increasing the concentration the cell needed more backlight to capture the texture, as can be seen from the transmitted light intensity as a function of concentration (Figure 3 d).

Fractal Analysis of the GO Aggregates
The aggregates of graphene oxide in 5CB exhibit a complicated, irregular structure, which suggests a further investigation by fractal methods. The images with a resolution of 2048 × 1088 pixels were analyzed as a function of GO concentration, with the box dimension method, using the fractal analysis software (Benoit 1.3). The textures were carefully thresholded and converted to a binary image, as exemplary demonstrated in Figure 4 a,b. The fractal dimension D b is calculated from the proportionality [ 22 ] Adv where N is the number of boxes of length d being occupied by the aggregate. Figure 4 c depicts the fractal box dimension as a function of graphene oxide concentration. It is found that this saturates at a value of approximately D b = 1.9 at 0.2 wt% of large GO fl akes, and 0.4 wt% for the smaller GO fl akes. It should be noted that D b = 1.9 is exactly the dimension one would expect for 2D percolation clusters at the percolation threshold, p c . And indeed, at approximately these concentrations, the GO aggregates form a cluster that spans the whole sample, as expected.

Nematic-Isotropic Transition ( T N -I )
The nematic to isotropic transition of 5CB, being a single-component room-temperature nematic, occurs within ±0.1 °C at 35.4 °C. The suspension of GO fl akes into 5CB is not strictly a molecular doping process, so one would not expect a resulting behavior described by the thermodynamics of mixtures. Nevertheless, it is a colloidal suspension process where GO fl akes disperse in the LC matrix. Thus, an effect on the system is to be expected, even if different than that from molecular mixtures. Even more so, if the nanoparticles possess a certain functionality, such as ferroelectric or ferromagnetic properties. For instance, Kurochkin et al. have shown that ferroelectric nanoparticles behave as a dopant, changing the order parameter of the nematic phase and enhancing the clearing temperature, [ 23 ] whereas multiwall carbon nanotubes resulted in a decrease. [ 8 ] By determining the change of transmitted light intensity between crossed polarizers and simultaneously measuring the cell capacitance, the nematic to isotropic transition can be determined accurately. The two different GO fl ake sizes exhibited a similar qualitative trend on the phase diagram, with the larger fl akes leading to slightly higher transition temperatures than those observed for the smaller GO fl akes ( Figure 5 ). This may be caused by a difference in mesogen anchoring on the fl akes.

Dielectric Properties
The resulting parameters of the dielectric measurements, ε ′ ⊥ , ε′ ԽԽ , and Δε′, as a function of GO concentration in 5CB are shown in Figure 6 a,b. The dielectric anisotropy decreases for both the small and the large GO fl akes until a constant value is reached at ≈0.6%-1% by weight. It is interesting to note that the dielectric constants parallel and perpendicular Adv. Optical Mater. 2016, 4, 1541-1548 www.MaterialsViews.com www.advopticalmat.de  to the director exhibit a different behavior for the small and the large fl akes above this concentration, with the former one increasing, while the values for latter stay constant. The overall dielectric behavior may be attributed to liquid crystal molecules being anchored at the surface of the graphene oxide sheets. As the concentration increases, more liquid crystal is infl uenced by the GO and the dielectric response decreases. ε ′ ԽԽ decreases, as more molecules are anchored in planar conditions, while ε ′ ⊥ increases. This would indicate, that the graphene oxide sheets remain parallel to the cell boundaries (Figure 6 c), as can be expected.

Fréedericksz Transition
The re-orientation of the director under an electric fi eld E is referred to as a Fréedericksz transition. For a nematic liquid crystal, the director experience a torque proportional to Δ ε ′ E 2 , [ 24 ] where Δ ε ′ is the dielectric anisotropy, Δ′ = ε ′ ԽԽ − ε ′ ⊥ . Due to anchoring conditions, a threshold voltage V th is observed, which is of importance for the application of liquid crystals in display devices where K 11 is the splay elastic constant.
For both the small and the large GO fl akes, the threshold voltage and the determined splay elastic constant are depicted in Figure 7 a,b, respectively. For increasing GO concentration an increase of threshold and elastic constant are observed, until saturation is reached for both at a concentration of about 1% by weight. The effect is much more pronounced for the small GO fl akes than the larger ones, and leads to threshold voltages which are approximately one order of magnitude larger than those of the neat liquid crystal. Also the elastic splay constant drastically increases (note the logarithmic scale in Figure 7 b by about two orders of magnitude).
It is again likely that both the drastic increase in threshold voltage, as also that of the splay elastic constant, are due to a strong planar anchoring of the liquid crystal molecules on the surface of the GO sheets, which remain parallel to the substrate plane as an electric fi eld is applied. Increasing fl ake concentration leads to an increase in threshold voltage and elastic constant, until all of the liquid crystal is dominated by the dispersed graphene oxide. At this concentration, saturation behavior is observed. Note, that the GO saturation concentration of ≈1% by weight is observed for dielectric, threshold and elastic parameters, and corresponds quite well to the concentration where the fractal dimension of aggregates saturated and a cluster of GO spans the whole sample.

Electrooptical Response Time
The two typical response times of the director for the fi eld-off and fi eld-on state are [ 25,26 ] τ γ π τ γ ε ε π = = Δ ′ − and  voltage. Figure 8 depicts both times as a function of GO concentration for both the small and the large fl ake sizes. The on-time represents the electric fi eld driven switching process, and is found to be rather independent of dopant concentration, within the limits of error. This is the case for both fl ake sizes, which display the same switching-on times (Figure 8 a). These results imply, that with increasing concentration the GO-liquid crystal dispersions becomes drastically more viscous, a behavior which is indeed qualitatively observed simply by shearing or steering the samples. In Figure 8 b the off-time is depicted as a function of GO concentration. This process is purely elastically driven and shows a decreasing switching time for increasing GO concentration. The behavior is more pronounced for the large fl akes than for the small ones.

Conclusion
Graphene oxide-thermotropic nematic dispersions were investigated for two different sizes of GO fl akes and as a function of concentration. It is found that the transition from the nematic to the isotropic state of the thermotropic liquid crystal only slightly increases with graphene oxide concentration, and that smaller graphene fl akes are easier to disperse than larger ones, which was to be expected. Electrooptic properties which are relevant for applications of such dispersions in devices, such as threshold voltages, response times, and elastic constants, were investigated for increasing concentration of two differently sized GO fl akes. It is demonstrated that   threshold voltage and elastic splay constant strongly increase with increasing concentration, until saturation at a GO concentration of ≈0.6%-1% by weight. While the switching-on times are practically unaffected, the off-times strongly decrease for increasing GO concentration, more so for the large fl akes than the smaller ones. The results imply that the viscosity of the dispersions strongly increases for increasing GO concentration, a behavior which is also observed qualitatively, and which can be expected. Dielectric spectroscopy reveals a relaxation, which is absent in the neat liquid crystal material. This is attributed to strongly anchored liquid crystal molecules on the surface of the GO fl akes, thus drastically hindering the molecular rotation around the short axis, and decreasing the relaxation frequency dramatically. The latter scenario also accounts for the saturation in threshold voltage, dielectric anisotropy, and elastic constant at ≈1% GO in 5CB, as discussed above. It is interesting to note that when the thermotropic liquid crystal is heated above its clearing temperature, it acts like a solvent, and a lyotropic graphene oxide nematic liquid crystal with a strongly reduced birefringence, as compared to 5CB, is observed.

Experimental Section
First, stable dispersions of GO-water were prepared by a modifi ed Hummers method, [ 27 ] and then transferred by solvent exchange into isopropanol making Solution A of a known concentration. Mixtures of different weight percentage (wt%) of GO fl akes were prepared by adding the defi nite amount of Solution A to (70 mg) of 5CB (SYNTHON Chemicals GmbH & Co. KG, Germany), making Solution B. To achieve a stable suspension; the solutions were sonicated for 1 h. To prevent the GO from chemical reduction, the solvents were evaporated at 45 °C for 24 h. After cooling below the clearing temperature, the remaining GO in 5CB dispersion was sheared and mixed with a spatula for better dispersion. For consistency, the pure 5CB was treated the same way by dissolving in isopropanol followed by the evaporation to provide same preparation conditions needed for later comparisons. ITO coated glasses from VisionTek Systems Ltd. of thickness 1.1 mm, and ITO resistance 10 Ω ٗ -1 , were cut into 1.5 cm × 2 cm sized substrates. The glasses were washed in different solvents and sonicated for 30 min. The ITO was then etched to leave the central parts of the glass with 0.5 cm width of ITO. Etching was carried out by using polyimide Kapton tape as a protective layer and immersion in hydrochloric acid (30%) for 7 min. The glasses were again washed for the fi nal time, dried and plasma cleaned for 2.5 min to remove any organic residues. At last, the substrates were spin coated with a solution of polyvinyl alcohol in water (0.5 mg mL -1 ) and unidirectionally and antiparallel rubbed with a velvet cloth. Instead of fi lling the premade cells by the usual capillary action method, the mixture of GO+5CB was sandwiched between the ITO glasses and then sealed with UV glue (Norland N68). The cell thickness was controlled by a Mylar spacer (13 µm). This process was found to produce more reliable results, because size exclusion due to capillary fi lling is avoided. The cell assembly process is summarized schematically in Figure 9 . Finally, the quality of the cells was tested by polarizing microscopy.
Optical textures of different concentrations of GO dispersions in 5CB were investigated using a Leica DMLP polarizing microscope and a digital image acquisition system (uEye CP). A precision temperature controller (Linkam TMS 94) with a relative accuracy of ±0.1 °C controlled the sample temperature in the hotstage chamber.
Dielectric spectroscopy investigations were performed using an Agilent Precision LCR Meter E4980A, which was operated in the parallel equivalent circuit mode in a frequency range of 20 Hz to 2 MHz with an applied voltage, V ac , of 0.05-20 V. The perpendicular component of the dielectric permittivity ε′ ⊥ was measured below the Fréedericksz threshold V th while the parallel component of the dielectric permittivity ε′ ԽԽ that can be measured as the electric fi eld was increased to well above the Fréedericksz transition.
All of the electrooptic experiments were made using a microscope based system with an optical bandpass fi lter (Thorlab) of 632 nm wavelength for monochromatic light. For the threshold measurements, the voltage sweep of the LCR meter was synchronized with the transmitted light intensity recorded by a photodiode through a digital multimeter (Agilent 34401A). The electrooptical response time was measured with a carrier signal of 5 kHz and 20 V amplitude, modulated with a 50 mHz square wave, using a signal generator (Agilent 33220A). The transmitted light intensity was recorded on a digital oscilloscope (Tektronix TDS 2024C) and averaged to reduce statistical noise. Apparatus operation and data acquisitions were carried out using LabView.