1. Introduction

Photonic crystalline arrangements of the constituent particles of colloidal systems have previously been reported. Such a periodic system is commonly accompanied by structural color reflection, depending on the periodic pitch [1, 2]. Certain colloids composed of disk-like particles with a large aspect ratio can have a swollen, periodic lamellar structure that exhibits structural color [3]. It has been reported that a periodic lamellar structure can be formed to some degree even in colloids of polydispersed disk-like particles [4].

Graphene oxide (GO) is a typical disk-like two-dimensional material, and its aspect ratio defined as the lateral diameter divided by the thickness is usually in the range of several thousands [5]. Aqueous GO dispersions exhibit the nematic liquid crystal phase [6, 7], and it was recently reported that a well prepared aqueous GO dispersion can be composed of a periodic lamellar structure in the visible light range, hence exhibiting structural color reflection [8]. Considering the large polydispersity of GO particles, this photonic crystalline property of GO dispersions is somewhat unexpected.

Aqueous GO dispersions show high electrical susceptibility and a large Kerr coefficient [5, 9–11]. Although several methods are available for use in aligning the GO flakes in dispersions [12], the application of electric fields is the simplest and most effective of these. The combination of the photonic crystalline property of GO dispersions and their control through electrical alignment may lead to a new application of GO dispersions in reflective color display devices.

In this study, we fabricated electro-optical cells containing aqueous GO dispersions with varying structural colors, and demonstrated that structural color reflection can be electrically controlled. In particular, it was observed that the design of the electrodes and driving scheme employed significantly influence the electrical response of structural color reflection.

2. Experimental

Aqueous GO dispersions were prepared by oxidizing graphite powder (200 mesh, purity 98%) and exfoliating the graphite-oxide particles, via the standard Hummers method [5, 13]. The mean diameter and the standard deviation of the GO particles were found to be 0.5 μm and 0.04 μm, respectively, via dynamic light scattering measurement (Zetasizer Nano by Malvern Instrument Ltd, UK) [14]. A series of structural colors emerged from red to blue when white light was incident on the cells containing the samples with the GO concentrations from 0.2 to 0.55 wt%, which was the similar as the range found by Peng et al. [8].

For electro-optical analyses, the samples were sandwiched by two substrates, and different electrode geometries were employed depending on the purpose of experiments. The cell thickness was approximately 1 mm for all cases. Spectral analyses were performed using an integrating sphere-type reflective spectrophotometer (i1 pro, X-Rite Inc., USA), and specular component excluded (SCE) reflectance, which is typically used for the analysis of display devices, was measured.

3. Results and discussion

We fabricated two cells with different electrode geometries so as to produce a horizontal electric field in one and vertical electric field in the other, as shown in Figs. 1(a-i) and 1(b-i), respectively. A 0.3 wt% GO dispersion with a green structural color was injected into the cells. For the cell that was subjected to horizontal electric fields [Fig. 1(a-i)], wall-type electrodes covered with silver were placed. The spacing between two wall electrodes was 5 mm. When we applied a square wave electric voltage at 10 kHz and 10 V to the cells, the structural color reflection became more uniform, although it was slightly weakened in some areas, as shown in Fig. 1(a-ii).This result indicates that the horizontal field leads GO particles to align parallel to the substrate. However, when the cell was observed under a transmission polarized microscope, an optical birefringent pattern was induced [Fig. 1(a-iii)]. It is therefore possible for a GO dispersion to simultaneously exhibit both color reflection and optical birefringence. Although GO particles align parallel to the substrate, the order parameter along the electric field direction will be higher than that along the other axis within the substrate plane, giving rise to birefringence. Moreover, the structural color reflection was mostly caused by the periodic structure near the surface. As long as the alignment near the surface is not distorted, structural color reflection can be sustained under the application of horizontal fields.

 

Fig. 1 (a) Electro-optical response in the cell subject to horizontal electric fields: (a-i) cell configuration, (a-ii) reflective microscopic observation without and with an electric field, and (a-iii) transmission polarized microscopic (POM) observation without and with an electric field. Arrows denote the directions of the polarizer and analyzer. (b) Electro-optical response in the cell subject to vertical electric fields: (b-i) cell configuration, and (b-ii) reflective microscopic observation without and with an electric field. (b-iii) Reflective microscopic observation at 10 min and 16 hr after removal of the electric field.

Download Full Size | PDF

When a vertical electric field was applied, however, as shown in Fig. 1(b-i), the structural color soon disappeared [Fig. 1(b-ii)]. The vertical electric field caused the GO basal planes to align vertically on the surface, where no periodicity exists along the direction perpendicular to the surface, so the structural color disappeared. Through the application of the vertical electric field, the structural color could be easily erased, although its recovery process was very slow [Fig. 1(b-iii)]. The field was removed to observe the recovery process of structural color in the cell. After 10 min, only the edges of the cell exhibited weak structural color reflection. After 16 h, the structural color was significantly recovered, but was different to that of the initial state. This difference may be due to the presence of a sediment of GO particles, which is commonly observed when GO dispersions are stored over long timescales [6]. A slow response time is usually not a desirable feature in a switching device. However, it indicates that the vertically aligned dark state can be regarded as a meta-stable state and can be used to develop a bi-stable device, as long as a method for restoring the bright state is provided.

In the next experiment, we prepared another set of cells with varying GO concentrations and different structural colors [Fig. 2]. The initial colors of the cells at 0 V varied from blue to reddish yellow as the GO concentration decreased from 0.54wt% to 0.27wt%. When vertical electric fields were applied to the cells, the reflected colors gradually faded away, becoming dark. However, the voltage required to erase the structural color increased dramatically as the GO concentration increased. The application of a 2 V signal was sufficient to erase the structural color in the 0.27 wt% cell, but in the case of the 0.54 wt% cell (deep blue), even a 10 V signal was not sufficient to erase the structural color. This can be explained by the increasing inter-flake friction obtained with increasing GO concentration. GO particles in a higher concentration GO dispersion have less freedom to rotate and exhibit an increased inter-particle friction, and thus become electrically desensitized, which has been confirmed in electro-optical birefringent switching of GO dispersions [5, 11].

 

Fig. 2 Change in color reflection depending on the GO concentration and voltage. Inset: Nematic volume ratio as a function of concentration, after phase separation in the bottle had been completed.

Download Full Size | PDF

We stored the bottles containing GO dispersions of varying concentrations for two weeks in order to obtain a separation into the dense nematic and dilute isotropic phases [the inset of Fig. 2]. Structural color was observed only in the nematic phase (bottom) and no color was observed in the isotropic phase (top). The volume fraction of nematic phase increased with increasing GO concentration. Thus, GO dispersions with the structural color were in the biphasic state, and as the nematic volume fraction decreased, the structural color exhibited a red shift. For 0.54 wt% sample, the nematic fraction reached more than 0.8, implying strong inter-particle interaction that is responsible for the electrical desensitization.

Figure 3 (a) shows the spectral reflectance for the cells with various GO concentration, and the spectral reflectance for the 0.39 wt% GO cell as a function of the applied voltage. The spectrum of the 0.39 wt% cell exhibits a clear peak at 500 nm at a voltage of 0 V, and the corresponding interlayer distance was approximately 188 nm. The peak intensity decreased as the applied voltage increased, and at voltages above 10 V, the color reflection completely disappeared. The peak intensities as a function of applied voltage are given for all cells in Fig. 3(b). The peak reflectance decreased with an increased applied voltage, but the required voltage significantly increased with increasing GO concentration. Figure 3(c) shows the peak reflectance as a function of the cell thickness for 0.34 wt% GO sample. The reflectance almost linearly increased when the cell thickness increased up to 0.5 mm, but beyond 0.5 mm, the slope became much gentle. This indicates that as the cell thickness is thicker than 0.5 mm, the quality of the periodic lamellar structure in the middle of the cell is less influenced by the surface and is deteriorated. Overall, all the spectra are broad and the reflectance is very low, implying that the quality of periodicity in our GO samples is not good.

 

Fig. 3 (a) The spectra of GO cells at various GO concentrations (solid lines) and at various voltages for 0.39 wt% GO sample (2V, 4V, 6V, 8V, and 10V for square, diamond, triangle, circle and star pointed lines, respectively). (b) Normalized reflectance as a function of voltage for different GO concentrations. (c) Peak reflectance as a function of cell thickness for 0.34 wt% GO sample.

Download Full Size | PDF

We introduced a new design for the electrodes [Fig. 4(a)] in order to restore the initial structural reflection in the cell, which is essential for use in real applications. The position of the electrodes on the bottom substrate overlapped that on the top substrate. We applied an electric voltage in three different ways as shown in Fig. 4, in which the red electrodes denote the application of a 10 V and 10 kHz signal, and the black electrodes are electrically grounded. Initially the cell filled with a 0.39 wt% GO dispersion reflected a blue color [Fig. 4(a)]. By applying the signal denoted on-signal in Fig. 4(b) that is practically identical to the vertical electric field, the structural color disappeared within 3 min, indicating a vertical alignment of the GO flakes. Two different off-signals were tested to restore the initial state. Off-signal1 is a real voltage-off state [Fig. 4(c)], and spontaneous rotational diffusion is involved in the restoring process. In this case, a weak structural color was recovered after 15 min. Off-signal 2 however employs a horizontal electric field [Fig. 4(d)], in which the electric field directly induces the rotation of GO particles, aligning them parallel to the surface. On applying off-signal 2, the structural color was recovered within 3 min between neighboring electrodes, although in the area near electrodes, the color was not recovered. The erasing and recovering of the structural color could be repeatedly performed by applying the on-signal and off-signal 2, respectively. The response time, 3 min, is not sufficient for applications, but it may be improved further by optimizing materials, which remains as a future work.

 

Fig. 4 (a) Initial color reflection in the cell containing new electrode design. Dimensions are given. (b) The cell became dark on applying the on-signal. The red electrodes denote the application of 10 kHz and 10 V signal, and the black electrodes are grounded. (c) Structural color was partially recovered 15 min after switching the signal off, and (d) exhibited a quicker recovery through the application of horizontal electric field (off-signal 2); (e) and (f) birefringence patterns in the cell subject to the on-signal and off-signal 2, respectively.

Download Full Size | PDF

Birefringence patterns were observed for the cell when subject to the on-signal and off-signal 2, respectively, as shown in Figs. 4(e) and 4(f). In Fig. 4(e), the birefringence pattern and overall luminance are almost invariant under the rotation of crossed polarizers, implying that GO particles were vertically aligned and the normal axes of GO basal planes were randomly distributed within a plane parallel to the substrate as illustrated in Fig. 5(a). This birefringence indicates the presence of local nematic order, but without uniform direction. Bright lines appeared near the electrode, due to the fringe field. The overall GO alignment of the cell when subject to the on-signal setting is shown in Fig. 5(a). The birefringence pattern of the cell for off-signal 2, shown in Fig. 4(f), was clearly different when the crossed polarizers were rotated. The birefringence patterns indicate that GO particles preferentially align themselves along the field direction between two neighboring electrodes, that is, the normal axes of GO basal planes mostly aligned vertically to the substrate, as shown in Fig. 5(b). One should note that the perfect alignment of GO particles parallel to a surface cannot induce birefringence. The presence of birefringence indicates that the overall ordering of the normal vectors [the red arrows shown in Fig. 5(b)] of GO particles is biaxial. The mean direction of the normal vectors was along the z-axis, but the variation of the normal vectors along the x-axis was stronger than that along the y-axis. In particular, such a biased variation is stronger in the bulk regions of the cells due to the absence of the surface field. Near the surface, the GO particles mostly align parallel to the surface, producing a clear structural color. Near the electrode, GO particles mostly aligned vertically, as shown in Fig. 5(b).

 

Fig. 5 The schematic alignment of GO particles (a) in the cell under on-signal, and (b) in the cell under off-signal 2. Red arrows indicate the normal vectors for each GO basal plane, and the Cartesian coordinate system is indicated in order to explain the GO alignment between two electrodes.

Download Full Size | PDF

4. Conclusion

Well-prepared aqueous GO dispersions can exhibit Bragg reflection with structural color. We demonstrated that the structural color in a cell containing an aqueous GO dispersion can be electrically erased and rewritten through the introduction of a appropriate electrode structure and driving mechanism. Since periodic GO alignment parallel to the substrate is responsible for color reflection, the application of a vertical field can destroy the structural color by rearranging the GO alignment along the vertical direction. In the presence of a vertical field, the reflection color becomes black. The operation voltage of the cell increases with increasing GO concentration due to an increase in inter-particle friction. When the field is removed, the initial structural color can be partially recovered via spontaneous diffusive particle motion, although this process is extremely slow. This process can be accelerated by applying a horizontal field to cause the GO particles to align parallel to the surface. By applying vertical and horizontal fields repeatedly, the structural color can be written and erased accordingly.