Dielectric, electro-optical, and photoluminescence characteristics of ferroelectric liquid crystals on a graphene-coated indium tin oxide substrate

Multilayer graphene was deposited on indium tin oxide (ITO) -coated glass plates and characterized by suitable techniques. A liquid crystal sample cell was designed using graphene deposited ITO glass plates without any additional treatment for alignment. Ferroelectric liquid crystal (FLC) material was filled in the sample cell. The effect of multilayer graphene on the characteristics of FLC material was investigated. The extremely high relative permittivity of pristine graphene and charge transfer between graphene and FLC material were consequences of the enormous increase in relative permittivity for the graphene-FLC (GFLC) system as compared to pure FLC. The presence of multilayer graphene suppresses the ionic impurities, comprised in the FLC material at lower frequencies. The ionic charge annihilation mechanism might be responsible for the reduction of ionic impurities. The presence of graphene reduces the net ferroelectricity and results in a change in the spontaneous polarization of pure FLC. Rotational viscosity of the GFLC system also decreases due to the strong π-π interaction between the FLC molecule and multilayer graphene. The photoluminescence of the GFLC system is blueshifted as compared to pure FLC, which is due to the coupling of energy released in the process of charge annihilation and photon emission.

Figure 1

The diagram for the fabrication of a liquid crystal sample cell with the help of multilayer graphene-coated ITO glass plates.

Figure 2

The characterization of deposited multilayer graphene by (a) Raman spectrum, (b) x-ray diffraction (XRD) spectrum, (c) scanning electron microscopic (SEM) image, and (d) UV-visible absorbance spectrum. The inset of (d) shows the absorption profile estimated from the SEM image at the dark-bright region.

Figure 3

The polarizing optical micrographs of (a) pure FLC and (b) GFLC system under crossed polarizer conditions at 35 °C.

Figure 4

The variation of relative permittivity of (a) pure FLC and (b) graphene-FLC on the frequency scale for different temperatures (40 °–60 °C), whereas (c) represents the variation of tanδ (ε″/ε′) at 40 °C on the frequency scale and (d) shows the variation of ac conductivity with the change in temperature for pure FLC and graphene-FLC at 200 and 500 Hz. The insets of (a) and (b) represent the variation of relative permittivity of pure FLC and GFLC systems with the change in temperature at 200 and 500 Hz, respectively.

Figure 5

(a) Variation of polarization with the change in bias voltage for FLC material with and without the presence of multilayer graphene on the ITO surface at 35 °C. The saturation value of polarization on the bias voltage scale is the spontaneous polarization of the material. (b) represents the change in rotational viscosity as a function of bias voltage at 35 °C.

Figure 6

The behavior of response time on the bias voltage scale for the FLC material with and without the presence of multilayer graphene on the ITO surface at 35 °C.

Figure 7

The variation of photoluminescence (PL) intensity on the wavelength scale for (a) graphene-FLC at room temperature (35 °C) and (b) the Gaussian fit of PL intensity curve to disclose the existence of the multiple peaks. The inset of (a) represents the PL intensity curve of the pure FLC whereas the inset of (b) shows the PL of pristine multilayer graphene. The excitation wavelength was 310 nm in the present investigation.