Effect of Azo Dye on Electro-Optic and Optical Band Gap Characteristics in ZnO Nanoparticles Induced Vertically Aligned Liquid Crystal Cells

Alignment of liquid crystal (LC) molecules in vertical direction is the most promising and interesting aspect for display devices in view of scientific and technological growth. Moreover, in accordance with the photovoltaic device applications, liquid crystals (LCs) show numerous applications. In the present work, the impact of dye as dopant with appropriate amount, over the electro-optic (E-O) and band gap properties as well as phase behaviour of vertically aligned liquid crystals (VALCs) has been studied and discussed. Initially, zinc oxide (ZnO) nanoparticles (NPs) were mixed to induce the vertical alignment (VA) in confined cell. Then, to prepare the dye doped LC sample, 0.125% of azo dye as dopant was uniformly mixed in the host sample. The results showed enhanced E-O characteristics with reduction in optical band gap as calculated using UV Visible study in 0.125% dye doped cell as compared to host sample cell.


INTRODUCTION
Liquid crystals (LCs) are a fascinating state of matter, as it possesses soft crystalline nature and have some orientational order also.In addition to these characteristics, LC also encloses large optical anisotropy that proves themselves a powerful component for fabrication of efficient display devices.Furthermore, as the alignment control of LC molecules is most basic requirement for the production of LC display devices so a number of researches were carried out to find the feasible ways [1][2][3].The anchoring transition of LC molecules from homogeneous to vertical state was well explained by Berreman's theory [4].Recently, many authors reported the use of different nanoparticles (NPs) for vertical alignment (VA) of LC [5][6][7][8][9].But spherical zinc oxide (ZnO) NPs proved much useful to achieve the VA in nematic LC even without using any surface treatment [10].In addition, many material characteristics were studied using different techniques [11][12][13].Moreover, wide view angle, outstanding hues and high brightness levels of coloured LC displays prepared via doping of controlled amount of dye in LC host have become the great interest of the researchers.Consequently, different types of research works were carried out in order to achieve bright and pure colors in LC displays using variety of dyes [14][15][16][17][18][19].Out of the used dyes, azo dye proves most commonly used dye for fabricating this kind of LC displays due to their properties compatible to the LCs.The azo dye molecules have extended and rod-like molecular shape and are more suitable to get aligned with the LC host [20][21][22][23][24][25].Thus, in the present work, a dye doped vertically aligned LC (VALC) cell was formed by mixing 0.125% of azo dye in host LC-ZnO NPs mixture.The voltage-transmittance (V-T) characteristics, band gap properties and phase behaviour were studied and compared with host NPs induced VALC cell.

EXPERIMENTAL SECTION
In this work, nematic liquid crystalline host sample MJ98468 (Merck, UK) having transition temperature equal to 75°C was used.The no = 1.4742, ne = 1.5512where no and ne are ordinary and extra-ordinary refractive indices possessed by the LC molecule respectively, and the negative dielectric anisotropy (∆ε) of the LC is = -4.The spherical ZnO NPs with standard particle size less than 50 nm was purchased from SIGMA-ALDRICH and used as collected from the company for doping in host LC [10].In addition, azo dye with its chemical structure as shown in figure 1 was used as another dopant material and uniform mixture of LC + 0.125% (wt/wt) of azo dye was prepared via continue stirring for a night using hot plate and magnetic stirrer.In this study, test cells (NPs induced VALC cell and dye + NPs Induced VALC cell) were prepared with indium tin oxide (ITO) coated glass substrates with 10 μm tape spacers.Here, the mylar spacer was used to control the cell gap.The surface morphology and V-T characteristics of both sample cells were studied using Polarized optical microscope (POM) (Nikon LV100POL, Japan) by fixing cells position sandwiched between perpendicular polarizers at 45 o direction with laser light of wavelength 632 nm and also a square wave with frequency 60 Hz.In addition, the prepared cell assemblies were passed through many heating and cooling cycles via placing over the hot stage further linked with a temperature controller (THMS600E, Linkam, UK) at a constant rate of 0.5°C/ min for the uniform dissemination of LC molecules.Furthermore, absorption characteristics of sample cells were studied using UV-Vis spectrophotometer (Thermo Scientific, Evolution 300) by placing the cells one by one into the chamber of spectrophotometer further connected with a computer.The system was set for base line firstly by placing the empty cells inside the chamber and then system was ordered to run for test cells.Subsequently, optical band gap of cells was measured.

Morphological and V-T Characteristics
The POM images of surface morphology of NPs induced VALC and 0.125% azo dye doped NPs induced VALC cells in ON and OFF states at room temperature under crossed polarizers at a magnification of 10X are shown in figure 2. The morphological view of these cells is associated with the alignment of LC molecules in vertical and homogeneous (planar) direction in OFF state and ON state respectively.Initially, in OFF state (at 0 applied voltage), the morphological view as shown in  The maximum (Tmax), minimum (Tmin) transmittance and contrast ratio (CR) values in accordance with the applied voltage are shown in figure 3 (b-d) respectively.Here, contrast ratio was measured by the transmittance values of transparent and opaque state and given by ratio of Tmax (maximum transmittance in ON state) and Tmin (minimum transmittance in OFF state).As a result, higher contrast was reported for (NPs + 0.125% dye) doped VALC sample in comparison to NPs induced VALC sample, because of the possible absorption of some light by the dye molecules [26].

Optical Band Gap Study
In order to study the optical properties of NPs induced VALC and 0.125% dye doped NPs induced VALC cells, UV-Visible study was carried out to examine the optical band gap (Eg).The Tauc plot [27] between ℎ () and (αhν) Thus, using equations 1 and 2, plot of figure 4a between ℎ ()and (ℎ) 2 was made in which the author extrapolate the linear region at  = 0 to find value of optical band gap for investigated cells.The comparison of band gap values for both cells is revealed in figure 4b where it has been found that dye doping in host sample reported the decreased value of band gap (2.394 eV < 2.538 eV) because electrons promotion in π orbital from lower energy state to higher energy state known as molecular orbital occurs via light absorption by azo dye in the visible regions [28][29].Moreover, the declination of band gap in (0.125% dye + NPs) VALC cell leads to augmented conductivity resulting in more suitability for the fabrication of photovoltaic devices.

Temperature Dependent Phase Behaviour
The phase behavior of NPs induced VALC and 0.125% dye doped NPs induced VALC cells was studied using temperature controller by placing both the cells on hot plate linked to temperature controller and the sample cells were made to pass through consecutive heating and cooling cycles at a constant rate of 0.5 0 C per minute.The microscopic textures were recorded during both the cycles using Linksys software connected to microscope fixed to the CCD camera and the results showed the lowering of transition (isotropic) temperature with addition of 0.125% dye into the NPs induced sample due to disturbance in intrinsic LC order caused by dye molecules as molecular additives or impurity atoms.

CONCLUSION
NPs induced VALC sample was prepared and doped with 0.125% of azo dichroic dye.The effect of dye as dopant was investigated for transmission and optical band gap properties of cells under examination.POM study for 0.125% azo dye doped VALC gives the signature of vertical alignment under crossed polarizer in OFF state as confirmed from dark state.Further, addition of 0.125% dye improved the characteristics of display devices in terms of reduced threshold and operating voltages (1.72V, 1.88V) in comparison to NPs induced VALC cell (1.80V, 1.98V).Moreover, CR for 0.125% dye doped NPs induced VALC cell was also reported to be improved significantly 195 as compared to 139 of NPs induced VALC sample cell.More interestingly, the optical band gap value of 0.125% dye doped NPs induced VALC cell found to be much lowered 2.394eV compared with 2.538eV of NPs induced VALC cell leading to improved conductivity.In addition, the reduction in isotropic temperature has been reported with the addition of 0.125% azo dye in NPs induced VALC sample.Thus, the performance of 0.125% dye doped NPs induced VALC cell with augmented properties like better CR, low applying voltages and lesser optical band gap proves to be of large significance for the energy efficient LC displays and moreover it leads to broaden the areas of dye doped NPs induced VALCs in view of its applications in different display devices.

Figure 1 .
Figure 1.Chemical structure of orange azo dye used in experiment as one of the dopant materials.

figure 2 (Figure 2 .
figure2 (a and c), the dark textures of both cells is a direct signature of VA state whereas on applying voltage, the switching of LC molecules occur from vertical state towards planar state as reflected in figure2 (b and d) with white states.This change in textural view is due to shifting of alignment properties of molecules from vertical to planar with respect to the voltage applied over them.Basically, these states of surface morphological textures are further used to study the transmission and band gap properties of cells.Moreover, these textures clearly investigated the substantial lowering in threshold (VT) and operating (VO) voltages for 0.125% dye doped NPs induced VALC cell compared to NPs induced VALC cell as reflected in figure3a.It seems that the addition of dye as dopant in host sample weakens the strength of (LC + dye) bond in comparison to the (LC+NP) bond strength in case of NPs doping.Thus, the anchoring strength significantly declined which resulted in quick switching of molecules in case of dye doped cell.

Figure 3 .
Figure 3. a) Threshold and operating voltages b) Maximum transmission (Tmax) c) Minimum transmission (Tmin) and d) Contrast ratio comparison of NPs induced VALC and 0.125% azo dye doped NPs induced VALC cells.

2 8 (
has been used to find out the value of Eg as shown in figure 4a.Here, h stands for Plank's constant,  depicts corresponding frequency (ν = hc λ where c denotes speed of light,  stands for wavelength of light used) and α is absorption coefficient.The important equations involved to find Eg are hν (Joule) = 19.10 17 ) (λ)  hν (eV) = hν(Joule) 1.6 ×10 −19

Figure 4 .
Figure 4. a) Determination of optical band gap (Eg) values b) band gap comparison for NPs induced VALC and 0.125% azo dye doped NPs induced VALC cells.