Improving the stability of organosiloxane smectic A liquid crystal random lasers using redox dopants

This report is focus on the development of liquid crystal (LC) vis ible - 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 t he 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 ind uce 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 optimize d 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 s tabilize 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.This is the author accepted manuscript. The final published version is available at http://www.sciencedirect.com/science/article/pii/S0925346715000816

Graphene and chiral nematic liquid crystals: A focus on lasing

© The Royal Society of Chemistry 2015. This work presents the interaction of self-assembled liquid crystalline (LC) unidimensional photonic structures on the surface of polycrystalline graphene. Further, this surface effect is studied through different substrate geometries in the test devices. Primarily, these devices are characterised through polarizing optical microscopy (POM) and their laser emission features in the dye-doped chiral systems. Then the conductive nature of graphene is utilized to apply external electric fields to the photonic medium and its effect is envisaged. These graphene-based devices demonstrate a unique result in polarizing optical micrographs and electro-optic responses which indicates the presence of multidirectional domain formations. Additionally, the LC band-edge lasing from graphene cells is found to be anisotropic and depends on the directionality of the optical pump. This work attempts to lay the foundation for the implementation of a new class of defused chiral nematic liquid crystal based devices e.g. optical filters, notch filters, colour reflectors, and light shutters and may add toward the knowledge base necessary in the substitution of Indium Tin Oxide (ITO) with graphene in traditional LC based display devices.This work was carried out under the GRAPHTED project which is funded by the Engineering and Physical Sciences Research Council UK (project ref. EP/K016636/1, Grant No. EP/H047565/ 1). A. A. K. would like to thank the Higher Education of Pakistan (HEC), P. R. K. acknowledges Lindemann Trust Fellowship. A. A. K. and P. R. K. acknowledge the Cambridge Commonwealth, European and International Trust and for the nancial suppo

Identity, reputation and social interaction with an application to sequential voting

We analyze binary choices in a random utility model assuming that the agent's preferences are affected by conformism (with respect to the behavior of the society) and coherence (with respect to his identity). We apply the analysis to sequential voting when voters like to win

Formation of physical-gel redox electrolytes through self-assembly of discotic liquid crystals: Applications in dye sensitized solar cells

The self-assembly of small molecules into ordered structures is of significant interest in electronic applications due to simpler device fabrication and better performance. Here we present work on the development of self-assembled fibrous networks of thermotropic triphenylene discotic liquid crystals, where 2,3,6,7,10,11-Hexakishexyloxytriphenylene (HAT6) is studied. The formation of interconnected molecular fibres in acetonitrile-based solvents facilitates thermally-reversible physical-gel (non-covalent) preparation, with the HAT6 network providing mechanical support and containment of the solvent. Furthermore, gel formation is also achieved using an acetonitrile-based iodide/tri-iodide redox liquid electrolyte, and the resulting gel mixture is utilised as an electrolyte in dye-sensitized solar cells (DSSCs). Our results show that it is indeed possible to achieve in situ gel formation in DSSCs, allowing for easy cell fabrication and electrolyte filling. In addition, the gel phase is found to increase device lifetime by limiting solvent evaporation. Differential scanning calorimetry (DSC) and polarising optical microscopy (POM) are used to study gel formation, and it is identified that the thermally reversible gels are stable up to working temperatures of 40 °C. It is found that DSSCs filled with gel electrolyte exhibit longer electron lifetime in the TiO 2 photo-anode (≈8.4 ms in the liquid electrolyte to ≈11.4 ms in the gel electrolytes), most likely due to electron screening from the electrolyte by HAT6. Current-Voltage (I–V) and electrochemical impedance spectroscopy (EIS) are used to study the effect of gel formation on conductivity and electrochemical properties, and it is found that confinement of the liquid electrolyte into a gel phase does not significantly reduce ionic conductivity, a problem common with solid-state polymer electrolytes. A 3.8 mM HAT6 gel electrolyte DSSC exhibited a PCE of 6.19% vs. a 5.86% liquid electrolyte reference. Extended device lifetimes studies showed that the gels increase stability of the DSSCs by reducing the rate of solvent evaporation.Cambridge Commonwealth European and International Trust (CCEIT), Higher Education Commission of Pakistan (HEC

Self-assembled liquid crystalline nanotemplates and their incorporation in dye-sensitised solar cells

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$^{-2}$) compared to that of the liquid electrolyte (V$_{OC}$ = 0.694 V and JSC = 10.46 mA cm$^{-2}$). 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.A.A.K. and G.R. would like to thank the Cambridge Commonwealth Trust for financial support. A.A.K. would also like to thank the HEC (Pakistan) for financial support. C.W would like to thank EPSRC Integrated Photonics and Electronics Systems funding

Hybrid graphene nematic liquid crystal light scattering device.

A hybrid graphene nematic liquid crystal (LC) light scattering device is presented. This device exploits the inherent poly-crystallinity of chemical vapour deposited (CVD) graphene films to induce directional anchoring and formation of LC multi-domains. This thereby enables efficient light scattering without the need for crossed polarisers or separate alignment layers/additives. The hybrid LC device exhibits switching thresholds at very low electric fields (< 1 V μm(-1)) and repeatable, hysteresis free characteristics. This exploitation of LC alignment effects on CVD graphene films enables a new generation of highly efficient nematic LC scattering displays as well as many other possible applications.Funding from the EPSRC (Grant No. EP/K016636/1, GRAPHTED) is acknowledged. P.R.K. acknowledges funding from Cambridge Commonwealth Trust (CCT) and the Lindemann Trust Fellowship. A.A.K would like to thank the Higher Education of Pakistan (HEC) and the CCT for financial support. A.C.V. acknowledges funding from the Cambridge Conacyt Scholarship and the Roberto Rocca Fellowship. A.K. would like to thank the Luys Educational Foundation and Hovnanian Foundation for scholarships.This is the author accepted manuscript. The final version is available from the Royal Society of Chemistry via http://dx.doi.org/10.1039/c5nr04094

Stretchable liquid-crystal blue-phase gels

Liquid crystalline polymers are materials of considerable scientific interest and technological value to society [1-3]. An important subset of such materials exhibit rubber-like elasticity; these can combine the remarkable optical properties of liquid crystals with the favourable mechanical properties of rubber and, further, exhibit behaviour not seen in either type of material independently [2]. Many of their properties depend crucially on the particular mesophase employed. Stretchable liquid crystalline polymers have previously been demonstrated in the nematic, chiral nematic, and smectic mesophases [2,4]. Here were report the fabrication of a stretchable gel of blue phase I, which forms a self-assembled, three-dimensional photonic crystal that may have its optical properties manipulated by an applied strain and, further, remains electro-optically switchable under a moderate applied voltage. We find that, unlike its undistorted counterpart, a mechanically deformed blue phase exhibits a Pockels electro-optic effect, which sets out new theoretical challenges and new possibilities for low-voltage electro-optic devices.Comment: 15 pages, 6 figures, additional data and discussion included. Supplementary videos available from F. Castles on reques

Electrically Tunable Scattering from Devitrite–Liquid Crystal Hybrid Devices

Devitrite is normally an unwanted crystalline impurity in the soda-lime-silica glass making process. Thin needles formed by heterogeneous nucleation of devitrite on the glass surface provide unique birefringence properties for potential applications in tunable optical devices. Here, devitrite and a liquid crystal are combined to create an electrically variable optical diffuser. The magnitude and scattering angle of the transmitted light propagating through the diffuser are tuned by varying the voltage between the graphene and indium tin oxide electrodes on either side of the liquid crystal. The threshold voltage to switch the transmitted light from a predominantly horizontal diffusion to a random order is 3.5 V. Angle-resolved measurements show broad diffusion angles of transmitted light with a maximum deflection of ±60°. The dynamically tunable devitrite-liquid crystal hybrid devices may advance the development of currently less viable technologies including beam shaping and automatic light transmission control.Leverhulme Trust, Royal Society, Engineering and Physical Sciences Research Council (IAA Follow on Fund

Electrically Tunable Scattering from Devitrite–Liquid Crystal Hybrid Devices

Devitrite is normally an unwanted crystalline impurity in the soda-lime-silica glass making process. Thin needles formed by heterogeneous nucleation of devitrite on the glass surface provide unique birefringence properties for potential applications in tunable optical devices. Here, devitrite and a liquid crystal are combined to create an electrically variable optical diffuser. The magnitude and scattering angle of the transmitted light propagating through the diffuser are tuned by varying the voltage between the graphene and indium tin oxide electrodes on either side of the liquid crystal. The threshold voltage to switch the transmitted light from a predominantly horizontal diffusion to a random order is 3.5 V. Angle-resolved measurements show broad diffusion angles of transmitted light with a maximum deflection of ±60°. The dynamically tunable devitrite-liquid crystal hybrid devices may advance the development of currently less viable technologies including beam shaping and automatic light transmission control.Leverhulme Trust, Royal Society, Engineering and Physical Sciences Research Council (IAA Follow on Fund

Numerical study of radiative Maxwell viscoelastic magnetized flow from a stretching permeable sheet with the Cattaneo–Christov heat flux model

In this article, the Cattaneo-Christov heat flux model is implemented to study non-Fourier heat and mass transfer in the magnetohydrodynamic (MHD) flow of an upper convected Maxwell (UCM) fluid over a permeable stretching sheet under a transverse constant magnetic field. Thermal radiation and chemical reaction effects are also considered. The nonlinear partial differential conservation equations for mass, momentum, energy and species conservation are transformed with appropriate similarity variables into a system of coupled, highly nonlinear ordinary differential equations with appropriate boundary conditions. Numerical solutions have been presented for the influence of elasticity parameter (), magnetic parameter (M2), suction/injection parameter (λ), Prandtl number (Pr), conduction-radiation parameter (Rd), sheet stretching parameter (A), Schmidt number (Sc), chemical reaction parameter (γ_c), modified Deborah number with respect to relaxation time of heat flux (i.e. non-Fourier Deborah number) on velocity components, temperature and concentration profiles using the successive Taylor series linearization method (STSLM) utilizing Chebyshev interpolating polynomials and Gauss-Lobatto collocation. The effects of selected parameters on skin friction coefficient, Nusselt number and Sherwood number are also presented with the help of tables. Verification of the STSLM solutions is achieved with existing published results demonstrating close agreement. Further validation of skin friction coefficient, Nusselt number and Sherwood number values computed with STSLM is included using Mathematica software shooting quadrature