Monolayers and Liquid Crystals

Research in liquid crystal has substantially grown in recent years. Ordering in these mesophases results in light transmission under crossed polarizers. In NPL, we are focusing on the tuning of electro-optical properties of ferroelectric liquid crystals by doping with various types of nanomaterials like metal and metal oxide nanoparticles, quantum dots, carbon nanotubes and graphene etc.

Electrically modulated photoluminescence in ferroelectric liquid crystal

We have studied and reported the effect of electric field on photoluminescence (PL) of a deformed helix ferroelectric liquid crystal (DHFLC) material, namely, FLC 6304 (Rolic, Switzerland). For the first time we are successful in modulating the PL characteristics (both the intensity and wavelength) by applying an electric field over the DHFLC material without any doping. In addition to this, switching of the PL intensity has also been achieved. The probable mechanism has been discussed on the basis of field-induced helix unwinding model in the DHFLC material.

The LC sample cell for the present study was prepared using highly conducting (∼30 Ω/□) indium tin oxide (ITO) coated glass plates. The desired electrode patterns on the ITO substrates were achieved using a photolithographic technique. The active electrode area was 45 mm × 45 mm. The thickness of the cell was maintained uniformly ~ 5 μm using Mylar spacers. The homogeneous alignment was obtained using rubbed polyimide technique. The FLC 6304 material was filled in isotropic phase by means of capillary action and then cooled gradually to room temperature. The phase sequence of this DHFLC material is as follows:

The room temperature PL excitation and emission spectra of the filled LC sample cell was recorded in the fluorescence mode using luminescence spectrometer (Edinburgh, F900, UK) equipped with a xenon lamp. A dc regulated power supply was used for applying external electric field across the LC cell. Dielectric permittivity of the sample was measured using an impedance analyzer (Wayne Kerr, 6540 A, UK).

The PL excitation spectrum of the filled LC sample cell was recorded over the range 200-360 nm using the luminescence spectrometer. Initial parameters like slit width, excitation step, and dwell time were kept constant at 5 nm, 1 nm, and 0.1 s respectively, for the sample. It was found that the FLC 6304 material has a clear absorption peak at 333 nm while that of ITO coated glass plates is at 258 nm. The PL emission of this DHFLC material was then recorded by registering the excitation wavelength at 333 nm.

Fig. 1. PL emission spectra of pure FLC 6304 material excited with 333 nm at various voltages.

Figure 1 shows the PL emission spectra ranging from 350-410 nm at various applied voltages (0-30 V). The emission spectrum of the DHFLC material is found to be voltage dependent. Both PL peak position and intensity get modified by changing the applied voltage. It is observed that there is almost no change in peak position when the applied voltage is less than 3 V. At around 3 V, it gets slightly shifted to lower wavelength and this shift gets pronounced as the voltage is further increased to 4 V. This shifting continues up to 10 V and remains constant thereafter. It is a well-known fact that in DHFLC material the helix is easily distorted by electric field which leads to a change in the refractive index of the material. This further result in shifting of the PL peak position. Hence, the observed shifting in the peak position with voltage is attributed to the change in refractive index due to helix distortion in the DHFLC material. For better understanding, this voltage induced helix distortion is shown schematically in Fig. 2.

Fig. 2. Schematic showing helix unwinding process in DHFLC material: (a) helix deformation at low voltages (0 – 3 V), (b) co-existence of unwound and helical parts (4 – 10 V), and (c) complete helix unwinding above 10 V.

At low voltages (< 3 V), only deformation of helix occurs [Fig. 2 (a)] where the pitch is almost same which results in no change in the PL peak position. The pitch of the present DHFLC material is 0.35 µm which is almost linearly dependent on the electric field above a threshold voltage which is defined as value of the voltage required to switch the DHFLC molecules. Above this threshold voltage (3 V), the twist walls become unstable and the helix starts unwinding which leads to the shift in the peak position towards lower wavelength. Due to the surface inhomogeneity there is a voltage range (4 < V < 10) where unwound and helical parts coexist [Fig. 2 (b)], which is manifested as the observed shifting in the PL emission peaks in this range. In this intermediate voltage range, in some areas the helical structure remains stable while in other areas switching occurs between two unwound states which lead to non-uniform switching. At and above saturation voltage (≥10 V), which is defined as the voltage where complete switching of DHFLC molecules takes place, the helix is unwound everywhere [Fig. 2 (c)] and the switching becomes uniform. This complete unwinding of the helix causes no peak shifting above 10 V.

Fig. 3. Variation of PL intensity excited at 333 nm and response time (inset) with the applied voltage of FLC 6304 material.

Figure 3 shows the variation of PL intensity with the applied voltage. It is clear from the figure that the PL intensity increases above a threshold voltage (3 V) and continues up to a saturation voltage (10 V). Figure 3 (inset) shows the response time of the material as a function of applied voltage which further confirms these values of voltages. The mechanism behind these observations can similarly be explained on the basis of voltage-stimulated unwinding of the helical structure in the DHFLC material. It is known that upon application of voltage, the helix gets destabilized resulting in a highly light scattering state. Hence, the excitation photons undergo multiple scattering by this highly scattering state before emission out of the LC cell to give the enhanced PL brightness. As discussed before, the helix unwinding process in FLC 6304 material starts from around 3 V and retains up to 10 V, which explains the observed increase in the PL intensity in this voltage range. Another possible reason for this variation of the PL intensity could be due to supra-helical structure. However, such structures have not been observed in the present FLC material which is in concurrence to earlier reported work.

[Fig. 4 (a)]. The increase in the light intensity provides evidence of electrically switchable PL in the DHFLC material.

Fig. 4. Electrical switching of PL intensity of FLC 6304 material: (a) photos showing light emission from LC sample cells under excitation (b) Time-dependent switching of PL intensity in response to several cycles of field-off (0 Vµm-1) and field-on (6 Vµm-1), inset shows the corresponding emission spectra.

Time-dependent switching of PL intensity in response to several cycles of field-off (0 Vµm-1) and field-on (6 Vµm-1) is shown in Fig. 4 (b) whereas the corresponding PL emission spectra (λex= 333 nm) are compared in the inset of Fig. 4 (b). The time interval (2 min) between two cycles is accounted to the total time required for each PL scan and stabilization of the applied field. It can be seen that the PL intensity quickly reaches to maximum during the field-on state while it goes to minimum in field-off state. This fast switching can be understood as FLC 6304 material is reported to have fast response time (~ 1-5 ms) to the external applied field. This electrically switchable and repeatable PL intensity reveal the possibility of using DHFLC material in optical switches.

संपर्क जानकारी