Kaushlendra Agrahari

Thermal and optical study of semiconducting CNTs-doped nematic liquid crystalline material

We report the thermal and spectroscopic analysis of the carbon nanotubes (CNTs)-doped nematic liquid crystal (NLC) material. The CNTs have been oriented in the p-ethoxybenzylidene p-butylaniline NLC. The thermal study of the CNTs doped nematic mixtures shows a significant decrease in the isotropic to nematic phase transition temperature. However higher doping concentration of CNTs has led to the further increase in transition temperature. The UV-Visible spectroscopy has been attempted on the CNTs/NLC mixtures at room temperature. The investigated NLC present one absorption band corresponding to π–π* electronic transition. A red shift of λmax with the increasing concentration of CNTs in the mixture has been observed. The band gap of NLC has been found to decrease after the doping of CNTs. The absorbance was measured for the UV light, polarized parallel and perpendicular to the LC director in the planar aligned cell.

Manifestation of strong magneto-electric dipolar coupling in ferromagnetic nanoparticles−FLC composite: evaluation of time-dependent memory effect

ABSTRACT Rod-shaped 5 wt.% copper-doped ZnO (ZnO:Cu2+) ferromagnetic nanoparticles (NPs), prepared by hydrothermal method, were dispersed in ferroelectric liquid crystal (FLC) named Felix 17/100. The effect of ferromagnetic NPs on the physical properties of FLC material (Felix 17/100) has been investigated by dielectric, electro-optical and polarising optical microscopic methods. A noteworthy time-dependent memory has been observed in the NPs-dispersed FLC composite attributed to the coupling of magnetic field associated to NPs with the director orientation of FLC. Improvement in spontaneous polarisation and dielectric susceptibility of FLC material has been ensued with the addition of ferromagnetic NPs. Faster electro-optic response, at lower applied voltage, has also been observed in NPs-dispersed FLC composite. These changes are accredited to the magneto-electric dipolar coupling existing due to the interactions between magnetic-dipole and electric-dipole moments of magnetic NPs and FLC material, respectively. The formation of periodic domains capable to show memory effect has been observed in composite. The observed time-dependent memory was confirmed by dielectric and electro-optical methods. FLC material enriched with the properties of ferromagnetic NPs can be utilised in advanced multifunctional optical devices, time-dependent memory-based security devices and computational purposes. Graphical Abstract

Effect of Cd1−xZnxS/ZnS core/shell quantum dot on the optical response and relaxation behaviour of ferroelectric liquid crystal

ABSTRACT Dielectrics, polarizing optical microscopic and electro-optical measurements have been carried out on a core/shell quantum dot Cd1−xZnxS/ZnS dispersed ferroelectric liquid crystal (FLC). In the present study, quantum dots were dispersed into two different concentrations of 0.1 and 0.25 wt./wt.% in pure FLC. The electro-optical parameters of pure and QDs dispersed FLC were carried out as a function of applied voltage. A significant improvement in optical response time of QDs dispersed FLC system is one of the major finding of the present study which may be useful for fabrication of faster liquid crystal system.

Dual photoluminescence and charge transport in an alkoxy biphenyl benzoate ferroelectric liquid crystalline–graphene oxide composite

An optimized concentration of graphene oxide (GO) has been dispersed in a ferroelectric liquid crystalline (FLC) material namely 4′-(octyloxy)-[1,1′-biphenyl]-4-yl 4-(heptan-2-yloxy)benzoate, to prepare a FLC–GO composite. Temperature dependent photoluminescence (PL) measurements for the FLC–GO composite were conducted between 30–100 °C. We observed a superlinear increase in the PL with increasing temperature. The time resolved luminescence study exhibits a bi-exponential decay time with a shorter life time for the FLC–GO composite and confirms the surface energy transfer from GO to FLC. Charge transport and current–voltage (I–V) characteristics for the FLC–GO composite have been investigated at ambient conditions by using current sensing atomic force microscopy. For the FLC–GO composite, critical diode like nonlinear I–V curves have been obtained in which the charge transport is assigned to the thermally active intermolecular hopping at room temperature. The FLC material yields ionic charge mobilities of 1.45 × 10−5, 1.26 × 10−5 and 9.83 × 10−6 cm2 V−1 s−1 in isotropic, chiral nematic (N*) and chiral smectic C (SmC*) phases. The dispersion of GO significantly enhances the ionic mobility in the composite which was observed to be 2.71 × 10−4, 2.69 × 10−4 and 2.65 × 10−4 cm2 V−1 s−1 for the aforementioned phase sequence. Physical interactions between GO and FLC molecules were confirmed by FTIR and polarized optical microscopy. In-plane coupling between the orientation of GO and the long molecular axis of the FLC molecules remarkably enhances the band intensity of CO, C–H, COO, C–O and C–H vibrations. The size of multi-domain fan texture in the SmC* phase has been enhanced after the dispersion of GO. The cobweb like networking in the oily streaks texture of the N* phase confirms the interesting molecular architecture via planar anchoring between FLC molecules and GO. This work opens new avenues towards applications in pico-ampere current-regulated electronic devices and opto-electronics.

Study of an interesting physical mechanism of memory effect in nematic liquid crystal dispersed with quantum dots

ABSTRACT The present study is based on effect of dispersing Cd1−xZnxS/ZnS core/shell quantum dots (QDs) on the memory behaviour of nematic liquid crystal 2020 with the variation of dopant concentration and applied voltage. Around 26% and 45% memory storage in QDs dispersed nematic matrix (MIX 1 and MIX 2) has been the core finding. The presence of ionic charges at low-frequency regime along with their reduction in QDs dispersed nematic matrix has been confirmed from tan δ curve. Pure nematic LC as well as nematic/QD mixtures depict volatile memory effect that depends upon concentration of QDs. The existence of memory due to storage of charge on QDs has been further confirmed from the dielectric, polarising optical micrographs and electro optical study under the influence of bias voltage. The observation of memory effect is attributed to the ion capturing and ion releasing phenomenon. The dispersion of QDs in nematic material plays an important role to enhance memory parameter by capturing and releasing the ionic charges under the application of bias voltage which has been confirmed from capacitance-voltage curve. Graphical Abstract

Analysis of optical properties and mechanism of photoluminescence enhancement of quantum dot - ferroelectric liquid crystal composite

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP