Improvement of Electro-Optical Properties of PSLC Devices by Silver Nanowire Doping

Polymer stabilized liquid crystal (PSLC) devices modulating the light that goes through them have broad applications. In this study, to improve the electro-optical properties of a PSLC device, Ag nanowires with diameter about 20 nm were doped into PSLC active layer with different concentrations. The influence of Ag nanowires concentration on the driving voltage, on-state response time and frequency modulation characteristics of a PSLC device were studied. The results indicate that the doping of Ag nanowires can reduce the driving voltage of PSLC cell up to 42%. The response time of the PSLC cell can decrease by about 41%. Meanwhile, frequency modulation does not show significant improvement upon Ag nanowire doping. Overall, Ag nanowire doping can improve the electro-optical properties of PSLC device effectively.


Introduction
Smart windows [1,2] that can modulate light are a type of intelligent building material and can save energy in buildings. At present, most commercially available smart windows are controlled by electric field. Materials with such function are mainly liquid crystal (LC) [3][4][5] and electrochromic oxide. For LC based smart windows, the orientation of LC molecule will change when an electronic field is applied. The incident light will be modulated by the LC molecules, and be reflected, scattered and so on. Electrochromic materials show a change of color as induced either by an electron transfer (redox) process or by a sufficiently high electrochemical potential [6]. Compared with electrochromic smart glass, LC based smart glass has many advantages, such as low cost, good stability, easy regulation and fabrication of large size preparation. As a result, LC based smart windows are expected to occupy the mainstream of smart glass market [7].
Privacy windows that can switch between scatting state and transparent state are one of the most popular types of LC-based smart glass. There are two kinds of privacy windows: polymer dispersed

Materials
The silver nanowires used in this study (purchased from Shanghai Chaowei New Materials Co., Ltd., Shanghai, China) have diameter of 20-35 nm and a length of 10-20 µm. The SEM image of the silver nanowires is shown in Figure 2.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 11 polymer network will disturb the rotation and a multi-domain structure will form. The incident light 95 will be scattered and the device becomes opaque.

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The materials used in the PSLC device include a negative LC mixture, a photo polymerizable LC 106 monomer and a photo initiator.  The materials used in the PSLC device include a negative LC mixture, a photo polymerizable LC monomer and a photo initiator. The negative LC mixture was purchased from Jiangsu Hecheng new materials Co. Ltd., Nanjing, China. The dielectric constant ∆E is −8.3, n e is 1.633, n o is 1.484, melting point is −40 • C, and clear point is 94 • C. The photo polymerizable monomer used in this study is RM82 from Merck KGaA, Darmstadt, Germany. The photo initiator is Irgacure 651 (IR651, BASF SE, Ludwigshafen, Germany). The liquid crystal monomer has acrylate groups at both ends. Under UV irradiation, these acrylate groups will react with each other and form a polymer network. This reaction was started by the photo-initiator. The molecular structure of RM82 and IR651 are shown in Figure 3. The alignment agent used in this study is a vertically oriented polyimide purchased from Shenzhen Dalton electronic materials Co. Ltd., Shenzhen, China.

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The concentration of materials used for PSLC fabrication in this study is shown in Table 1. The amount of negative LC was adjusted slightly when the concentration of silver nanowire changed.

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The PSLC mixtures with different silver nanowire concentration were filled into a cell with 5 µm   The concentration of materials used for PSLC fabrication in this study is shown in Table 1. The amount of negative LC was adjusted slightly when the concentration of silver nanowire changed.

Fabrication Process of PSLC Device
The PSLC mixture with negative LC, RM82 and IR651 were mixed together according to the ratio in Table 1 with magnetic stirrer for 30 min at 60 • C. The silver nanowire solution was diluted to a concentration of 0.5 mg/mL using ethanol, and dispersed for 1 h by ultrasonication. Then 20, 40, 100, or 200 mL of silver nanowire solution were added to 0.1 g PSLC mixed solution, respectively, resulting in 0.01, 0.02, 0.05, 0.1 wt% of silver nanowires concentration in PSLC. After another 30 min of ultrasonication, the mixtures were placed at room temperature for 72 h to remove the ethanol.
The PSLC mixtures with different silver nanowire concentration were filled into a cell with 5 µm gap and ITO electrode on both internal surface by capillary force at 60 • C. The samples were cooled on hot stage to room temperature slowly after the cells were fully filled. Then, the samples were photo polymerized by 320-400 nm UV light with light intensity of 500 mW/cm 2 for 300 s at room temperature.

Electro-Optical Property Characterization
The alternating electric field with a square wave function was provided by a function generator (33220A, Agilent Technologies Inc., Santa Clara, CA, United States). The electric signal from the function generator was amplified through a high-voltage linear amplifier (F20A, FLC Electronics AB, Partille, Sweden). The output voltage was measured by an oscilloscope (DSOX3032T, Keysight Technologies, Santa Rosa, CA, United States). Transmittance of the samples were measured by spectrometer (HR2000+, Ocean Optics Inc., Largo, FL, United States). The wavelength of 550 nm was select to evaluate the transmittance. A halogen lamp (LS-3000, Keyence (CHINA) Co., Ltd., Shanghai, China) was used to provide white light. Response time of the samples were measure by photodiode (FSD1010, Thorlab Inc., Newtown, NJ, United States) with time resolution of 65 ns. The output signal of photodiode was amplified and then transformed by an acquisition card (USB6002, National Instruments, Austin, TX, United States), then collected by computer with a LabVIEW program. A 5 W White LED was used to provide white light. The experiment setup is shown in Figure 4.  the transmittance increased as the Ag nanowire concentration was increased up to 0.1 wt.%. This is 158 because, when the concentration is too high, Ag nanowires cannot be evenly distributed in the LC 159 and tend to aggregate together, resulting in the interconnection between the silver nanowires [34].

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The reduction of the residual DC voltage through metal nanomaterial doping in the nematic LC 177 is discussed in the literature [35,36]. Metal nanomaterials mixed in LC materials can capture ionic 178 impurities and enhance the electro-optic properties of LCs. Due to the small size effect of the silver nanowire, when a voltage is applied, the surface of the silver nanowire will be polarized, and thus 180 the ionic impurities in the liquid crystal mixture will be absorbed by the silver nanowire surface.

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Therefore, the reason that doping a certain concentration of Ag nanowires can reduce the threshold

Effect of Ag Nanowires on the Driving Voltage of PSLC Devices
The voltage-transmittance (V-T) curves, threshold voltages and leakage current of the Ag nanowire doped PSLC according to the Ag nanowire concentration are shown in Figure 5. The V-T curve of PSLC devices under 1 kHz square wave AC driving are shown in Figure 5a. All PSLC devices have transmittance above 95% at off state and the transmittance decreases as the voltage increases. All devices can reach a scattering state with transmittance under 18%. However, the transmittance at both on and off state decreased slightly with small amount of Ag nanowire doping at 0.01 wt.%, and the transmittance increased as the Ag nanowire concentration was increased up to 0.1 wt.%. This is because, when the concentration is too high, Ag nanowires cannot be evenly distributed in the LC and tend to aggregate together, resulting in the interconnection between the silver nanowires [34]. When the voltage continuously increased, aggregation of silver nanowires will form a local short circuit and burn up the device, thus affecting the device performance and increasing the transmittance of the device.
Meanwhile, the V-T curve of the PSLC had a different tendency: the threshold voltages (V 10 and V 90 ) shifted as a function of the Ag nanowire concentration. It shifted to the left side first as the Ag nanowire concentration increased to 0.02 wt.%, and then shifted to the right side as the Ag nanowire concentration continued to increase. The V-T curve of PSLC devices doped with 0.01 wt.%, 0.02 wt.%, and 0.05 wt.% Ag nanowires are on the left side of the undoped PSLC device. However, the V-T curve of PSLC device doped with 0.1 wt.% Ag nanowires is on the right side of the undoped PSLC device, meaning it has worse performance than the undoped PSLC device. The voltages for 10% transmission (V 10 ) and 90% transmission (V 90 ) show the same tendency. As shown in Figure 5b, V 10 and V 90 greatly decreased with a small amount of doping at 0.01 wt.% Ag nanowires, and the values decreased as the Ag nanowire concentration was increased up to 0.02 wt.%. However, the V 10 and V 90 values started increasing when the Ag nanowire concentration continued increasing up to 0.1 wt.%. When the doping concentration is 0.02 wt.%, the threshold voltage is lowest, reduced by 35-42% compared with undoped PSLC device.
The reduction of the residual DC voltage through metal nanomaterial doping in the nematic LC is discussed in the literature [35,36]. Metal nanomaterials mixed in LC materials can capture ionic impurities and enhance the electro-optic properties of LCs. Due to the small size effect of the silver nanowire, when a voltage is applied, the surface of the silver nanowire will be polarized, and thus the ionic impurities in the liquid crystal mixture will be absorbed by the silver nanowire surface. Therefore, the reason that doping a certain concentration of Ag nanowires can reduce the threshold voltage of the PSLC device is Ag nanowire doping can effectively eliminate the ionic impurities in the LC and reduce the conductivity in the PSLC device. Figure 5c shows the current-voltage curves of PSLC devices doped with different concentrations of Ag nanowires. At a certain voltage, the current of PSLC devices doped with 0.01 wt.% and 0.02 wt.% Ag nanowires decreasing when the concertation of Ag nanowire increases. When the Ag nanowire doping concentration is 0.02 wt.%, the current is the lowest, which represents the lowest conductivity. This is because the ionic impurities in the PSLC mixture is captured by the Ag nanowires. When the conductivity of LC material is the lowest, the electric field and force applied on the liquid crystal molecule is the strongest. Therefore, the threshold voltage is the lowest and the response is the fastest. As a result, small amount of Ag nanowire doping can reduce the amount of ionic impurities and device conductivity effectively, making the device have a better electro-optical response. This result is consistent with the V-T curves and threshold voltage variation tendency in Figure 5a,b. Since the length of the silver nanowires used in this study is about 20 µm, the density of Ag nanowire was too high when the doping concentration of the Ag nanowires increased to 0.05 wt.% and 0.1 wt.%. and the Ag nanowires became short-circuited with each other and increased the conductivity, causing the actual electric field to decrease and the driving voltage to increase.

Effect of Ag Nanowires on Response Time of PSLC Devices
The response time of PSLC with different Ag nanowire concentration with 30 V, 1 kHz square wave driving is shown in Figure 6a. Here, both the turn on or rising time from clear state to scattering state (an interval from 90% transmittance to 10% transmittance) and the turn off or falling time shows the same variation tendency with threshold voltage as the Ag nanowire concentration increases. The turn on time of PSLC first decreased to about a half and then increased to about double of undoped PSLC as Ag nanowire concentration increased. However, the change of turn off time was very limited, with all around 5 ms.
PSLC as Ag nanowire concentration increased. However, the change of turn off time was very 209 limited, with all around 5 ms.
210 Figure 6b shows the comparison of temporal response of a pure PSLC and a PSLC with 0.02 211 wt.% Ag nanowire doping. There was no noticeable difference in the turn off time of the samples, but 212 the turn on time of PSLC with 0.02 wt.% Ag nanowire doping was only 12.9 ms, i.e. 41% lower than 213 the undoped PSLC.
214 Figure 6c shows the input signal and output signal of a PSLC with 0.02 wt.% Ag nanowire 215 doping. It shows the Ag nanowire doped PSLC has very fast response to electric field. The turn off 216 process of a PSLC device is elastic response, during which the negative LC molecule reorientation is 217 driven by the vertical alignment layer and vertical aligned LC network. This process is almost linear 218 and the Ag nanowire doping had very little influence on it. The turn on process is dielectric response.

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Ag nanowire doping can lower the ionic impurities in LC, which would enhance the electric field and 220 thus increase the speed of dielectric response. On the other hand, Ag nanowire doping can increase 221 the polar anchoring energy in the PSLC [37], which would also shorten the turn on time.   Figure 6b shows the comparison of temporal response of a pure PSLC and a PSLC with 0.02 wt.% Ag nanowire doping. There was no noticeable difference in the turn off time of the samples, but the turn on time of PSLC with 0.02 wt.% Ag nanowire doping was only 12.9 ms, i.e. 41% lower than the undoped PSLC. Figure 6c shows the input signal and output signal of a PSLC with 0.02 wt.% Ag nanowire doping. It shows the Ag nanowire doped PSLC has very fast response to electric field. The turn off process of a PSLC device is elastic response, during which the negative LC molecule reorientation is driven by the vertical alignment layer and vertical aligned LC network. This process is almost linear and the Ag nanowire doping had very little influence on it. The turn on process is dielectric response. Ag nanowire doping can lower the ionic impurities in LC, which would enhance the electric field and thus increase the speed of dielectric response. On the other hand, Ag nanowire doping can increase the polar anchoring energy in the PSLC [37], which would also shorten the turn on time.

Effect of Ag Nanowires on Frequency Response of PSLC Devices
Figure 7a-c shows the transmittance versus driving frequency curve of PSLC device with different Ag nanowire doping concentration at 10 V, 20 V and 30 V respectively. At 10 V, the transmittance of all PSLC devices decreases first and then increase as the frequency increases. Lowest transmittance can be obtained at 10 kHz. At 20 V and 30 V, the change of transmittance from 100 Hz to 100 kHz is negligible. At 1 MHz, the PSLC devices stop responding to electric field, which may be because this frequency is higher than the crossover frequency (f c ) of the dielectric anisotropy of the LC used in this experiment. Under high driving voltage, the difference between PSLC devices with different Ag nanowire doping concentration is very small. The difference of F-T curves under low driving voltage is mainly caused by the variation of threshold voltage. As shown in Figure 7d, from 100 Hz to 10 kHz, the threshold voltage of all PSLC devices first decreases and then increases as the frequency increases. This trend is the same for all PSLC devices. This is because the dielectric constants of LC used in this study increase first and then decrease as a function of the frequency regardless of Ag nanowire doping concentration. The influence of driving frequency on the response time of PSLC at 30 V with different Ag nanowire doping concentration is shown in Figure 7e. It shows that, with the gradual increase in frequency, from 100 Hz to 10 kHz, the device response time has a slight downward trend, but not much. When the frequency increases from 10 kHz to 100 kHz, the response time gradually increases as the frequency increases. This trend is also the same for all PSLC devices. Therefore, the PSLC devices in this study have a good frequency modulation property, but the Ag nanowire doping has very little influence on the frequency response.

Conclusions
Electro-optical properties of PSLC device were successfully improved by Ag nanowires doping into PSLC active layer. When doping concentration is only 0.02 wt.%, the threshold voltage V 10 and V 90 values can be reduced by 35-42% compared to undoped PSLC device. The main reason is Ag nanowire doping can effectively eliminate the ionic impurities in LC and thus reduce the conductivity of PSLC active layer. The turn on response time of PSLC device with 0.02 wt.% Ag nanowire doping is only 12.9 ms, which is 41% lower than that of undoped PSLC device. However, the change of turn off response time is negligible. This is because Ag nanowire doping can increase the polar anchoring energy and lower the ionic impurities in LC mixture, thus enhancing the dielectric response of LC molecules. Since the turn off process is an elastic response, doping has limited influence on this.
The Ag nanowire doped PSLC shows similar frequency modulation performance with undoped PSLC device, with no significant improvement. All the present results imply that Ag nanowire doping can improve the performance of electro-optical devices such as adaptive optics devices, e.g. PSLC smart windows, which in general require a fast response and low threshold voltage.