Malik Qasim - Academia.edu (original) (raw)

Papers by Malik Qasim

Communications in Nonlinear Science and Numerical Simulation, 2010

A homotopy analysis method (HAM) is employed to investigate the unsteady magnetohydrodynamic (MHD... more A homotopy analysis method (HAM) is employed to investigate the unsteady magnetohydrodynamic (MHD) flow induced by a stretching surface. An incompressible viscous fluid fills the porous space. The heat and mass transfer analyses are also studied. Series solutions have been constructed. Comparative study between the series and exact solutions is also given. The effects of embedded parameters in the considered problems are examined in detail.

Optical Materials, 2015

This report is focus on the development of liquid crystal (LC) visible-light scattering devices f... more This report is focus on the development of liquid crystal (LC) visible-light scattering devices for random lasers. These light-scattering devices are based upon binary mixtures that consist of an organosiloxane smectic A LC and a wide temperature range nematogen LC. Both the temperature range of the smectic A phase and the dielectric anisotropy of the binary mixture are increased compared with that of the neat organosiloxane compound. In the latter case, the increase in the dielectric anisotropy results in a reduction of the magnitude of the electric field required to induce a clear state. Furthermore, it is found that the electric field threshold continues to decrease with increasing concentration of the nematic compound. For the random laser devices, the pyrromethene 597 laser dye was added to a mixture that was optimized for scattering and it was found that the absorption properties of the dye becomes unstable in the presence of the electro-hydrodynamic instabilities that are required to generate scattering in the LC cells. This is believed to be due to electro-chemical reactions that occur at the electrodes. To avoid dye degradation and ensure repeatable electro-optic behaviour, a reduction-oxidation (redox) couple is dispersed within the dye-doped binary mixture. It is shown that the addition of redox dopants helps to stabilize the dye in the scattering mixtures, and also increases the long-term repeatability of the scattering behaviour. Finally, we conclude by characterizing the random laser emission of the dye-doped binary mixture and demonstrate improved stability.

Organic Electronics, 2016

Discotic liquid crystals (DLCs) are considered promising materials for organo-electronic applicat... more Discotic liquid crystals (DLCs) are considered promising materials for organo-electronic applications. Columnar alignment of DLCs leads to anisotropic charge transport with high charge carrier mobility. However, pure DLCs exhibit low intrinsic charge carrier density which limits bulk conductivity. This research studies the alignment and conductivity properties of small molecule triphenylene-based DLCs to develop hole transport layers for potential applications in organic semiconductor devices. Binary mixtures of homologous DLCs of the hexakis(n-alkyloxy)triphenylene series (HAT6 and HAT10) are formulated. Mesophase characteristics and columnar alignment of these mixtures are characterized using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Alignment, orientation and order of columnar packing in the mixtures is studied using X-ray diffraction (XRD) and grazing incidence wide angle X-ray scattering (GIWAXS) measurements. It is identified that binary mixture formation strongly effects the columnar alignment in solution processed films. Furthermore, to increase charge carrier density in the DLC films a strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ) is added as a p-type dopant, followed by an extensive characterization of its doping effect. POM, DSC thermal scans, UVevisible spectroscopy, photo-luminescence spectroscopy (PL) and I-V measurements are utilized to characterize and establish the improvement of hole conduction in the doped films. It is observed that F4TCNQ-doped triphenylene DLC films exhibit twofold increase in hole conductivity, making the materials highly relevant for charge transport applications.

Physical Review E, 2012

It is known that bimesogenic liquid crystals exhibit a marked "odd-even" effect in the flexoelast... more It is known that bimesogenic liquid crystals exhibit a marked "odd-even" effect in the flexoelastic ratio (the effective flexoelectric coefficient to the average elastic coefficient), with the ratio being higher for the "odd-spaced" bimesogens (those with an odd number of alkyl groups in the spacer chain) than their neighboring even-spaced counterparts. To determine the contribution of each property to the flexoelastic ratio, we present experimental results on the flexoelectric and elastic coefficients of two homologous nonsymmetric bimesogens which possess odd and even alkyl spacers. Our results show that, although there are differences in the flexoelectric coefficients, there are substantially larger differences in the effective elastic coefficient. Specifically, the odd bimesogen is found to have both a low splay elastic coefficient and a very low bend elastic coefficient which, when combined, results in a significantly lower effective elastic coefficient and consequently a higher flexoelastic ratio.

Electrochimica Acta, 2016

Liquid junction dye-sensitised solar cells (DSSCs) suffer from solvent evaporation and leakage wh... more Liquid junction dye-sensitised solar cells (DSSCs) suffer from solvent evaporation and leakage which limit their large-scale production. Here, we have prepared DSSC using a simple and cheap fabrication process with improved photovoltaic parameters and stability. A binary mixture of Smectic A (SmA) and Nematic Liquid Crystal (NLC) was used to provide a self-assembled template for a polymerisable reactive mesogen LC. The layered structure of SmA combined with a low viscosity NLC forms a polygonal structure that provides an ordered and continuous template for reactive mesogens. Once the reactive mesogen is polymerised under UV light, the SmA:NLC mixture is washed away, resulting in a polymer network template containing nanochannels. We demonstrate the incorporation of these templates into DSSCs and find that DSSCs containing these nanochannels show improved open-circuit voltage (V OC) (0.705 V) and short-circuit current (J SC) (13.25 mA cm À2) compared to that of the liquid electrolyte (V OC = 0.694 V and J SC = 10.46 mA cm À2). The highest obtained power conversion efficiency with Sm-PE was 5.94% which is higher than that of the reference solar cell (5.51%). These can be attributed to the improved ionic conductivity and ionic diffusion of Sm-PE where the presence of the nanochannels aided the ionic conduction in the polymer electrolyte. In addition, it is hypothesized that the light scattering effect of the polymerised reactive mesogen also contributed to the improved performance of the photovoltaic devices. This finding is important because it is known fact that when a polymer is added to liquid electrolyte, the ionic conductivity will decrease although the stability is improved.

ACS Applied Materials & Interfaces, 2016

Since 1980’s a lot of interest has been drawn to devise optical filters based on cholesteric phas... more Since 1980’s a lot of interest has been drawn to devise optical filters based on cholesteric phase for instance, notch filter,1–3 colour reflector,4,5 and light shutter 6–8. All of these devices utilised the unique feature of chiral nematic liquid crystals which is the well-known selective reflection band. One of the benefits of using chiral nematic LCs is that the reflection band can be “tuned” by subjecting the LC to a range of external stimuli. An active area of research is to use light to control the reflection band by using molecular structures that contain a photo-active element. This is advantageous as it offers a means of controlling the structure remotely and is particularly attractive as an optical shutter component for sensor devices. One limitation, however, is that the reflection band only exists for one handedness of circularly polarized light. Here we present a polariser free optically activated shutter based upon a short-pitch chiral nematic liquid crystal device. Th...

Sensors, 2015

This proof-of-concept study proposes a novel sensing mechanism for selective and label-free detec... more This proof-of-concept study proposes a novel sensing mechanism for selective and label-free detection of 2,4,6-trinitrotoluene (TNT). It is realized by surface chemistry functionalization of silica nanoparticles (NPs) with 3-aminopropyl-triethoxysilane (APTES). The primary amine anchored to the surface of the silica nanoparticles (SiO2-NH2) acts as a capturing probe for TNT target binding to form Meisenheimer amine-TNT complexes. A colorimetric change of the self-assembled (SAM) NP samples from the initial green of a SiO2-NH2 nanoparticle film towards red was observed after successful attachment of TNT, which was confirmed as a result of the increased separation between the nanoparticles. The shift in the peak wavelength of the reflected light normal to the film surface () and the associated change of the peak width were measured, and a merit function taking into account their combined effect was proposed for the detection of TNT concentrations from 10 −12 to 10 −4 molar. The selectivity of our sensing approach is confirmed

Analytical Chemistry, 2015

The real-time sensing of metal ions at point-of-care requires integrated sensors with low energy ... more The real-time sensing of metal ions at point-of-care requires integrated sensors with low energy and sample consumption, reversibility, and rapid recovery. Here, we report a photonic nanosensor that reversibly and quantitatively reports on variation in the concentrations of Pb 2+ and Cu 2+ ions in aqueous solutions (<500 μL) in the visible region of the spectrum (λ max ≈400-700 nm). A single 6 ns laser pulse (λ=532 nm) was used to pattern a ~10 µm thick photosensitive recording medium. This formed periodic AgBr nanocrystal (ø ~5-20 nm) concentrated regions, which produced Bragg diffraction upon illumination with a white light source. The sensor functionalized with 8-hydroxyquinoline allowed sensing through inducing Donnan osmotic pressure and tuning its lattice spacing. The sensor quantitatively measured Pb 2+ and Cu 2+ ion concentrations within the dynamic range of 0.1-10.0 mM with limits of detection of 11.4 and 18.6 µM in under 10 min. The sensor could be reset in 3 min and was reused at least 100 times without compromising its accuracy. The plasmonic nanosensor represents a simple and label-free analytical platform with potential scalability for applications in in vitro diagnostics and environmental monitoring.

Advanced Energy Materials, 2014

Nanoscale, 2014

We demonstrate a rare combination of polymer wrapped single walled carbon nanotubes with liquid c... more We demonstrate a rare combination of polymer wrapped single walled carbon nanotubes with liquid crystals to produce switchable optoelectronic devices.

Journal of Materials Chemistry C, 2014

Tuneable optical sensors have been developed to sense chemical stimuli for a range of application... more Tuneable optical sensors have been developed to sense chemical stimuli for a range of applications from bioprocess and environmental monitoring to medical diagnostics. Here, we present a porphyrinfunctionalised optical sensor based on a holographic grating. The holographic sensor fulfils two key sensing functions simultaneously: it responds to external stimuli and serves as an optical transducer in the visible region of the spectrum. The sensor was fabricated via a 6 nanosecond-pulsed laser (350 mJ, l ¼ 532 nm) photochemical patterning process that enabled a facile fabrication. A novel porphyrin derivative was synthesised to function as the crosslinker of a polymer matrix, the light-absorbing material, the component of a diffraction grating, as well as the cation chelating agent in the sensor. The use of this multifunctional porphyrin permitted two-step fabrication of a narrow-band light diffracting photonic sensing structure. The resulting structure can be tuned finely to diffract narrow-band light based on the changes in the fringe spacing within the polymer and the system's overall index of refraction. We show the utility of the sensor by demonstrating its reversible colorimetric tuneability in response to variation in concentrations of organic solvents and metal cations (Cu 2+ and Fe 2+) in the visible region of the spectrum (l max z 520-680 nm) with a response time within 50 s. Porphyrinfunctionalised optical sensors offer great promise in fields varying from environmental monitoring to biochemical sensing to printable optical devices.

RSC Advances, 2014

Photonic crystals are materials that are used to control or manipulate the propagation of light t... more Photonic crystals are materials that are used to control or manipulate the propagation of light through a medium for a desired application.