6,692 research outputs found

    Quantification of Ion Migration in CH3NH3PbI3 Perovskite Solar Cells by Transient Capacitance Measurements

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    Solar cells based on organic-inorganic metal halide perovskites show efficiencies close to highly-optimized silicon solar cells. However, ion migration in the perovskite films leads to device degradation and impedes large scale commercial applications. We use transient ion-drift measurements to quantify activation energy, diffusion coefficient, and concentration of mobile ions in methylammonium lead triiodide (MAPbI3) perovskite solar cells, and find that their properties change close to the tetragonal-to-orthorhombic phase transition temperature. We identify three migrating ion species which we attribute to the migration of iodide (I-) and methylammonium (MA+). We find that the concentration of mobile MA+ ions is one order of magnitude higher than the one of mobile I- ions, and that the diffusion coefficient of mobile MA+ ions is three orders of magnitude lower than the one for mobile I- ions. We furthermore observe that the activation energy of mobile I- ions (0.29 eV) is highly reproducible for different devices, while the activation energy of mobile MA+ depends strongly on device fabrication. This quantification of mobile ions in MAPbI3 will lead to a better understanding of ion migration and its role in operation and degradation of perovskite solar cells

    Shape Memory Polymers as 2D Substrates and 3D Scaffolds for the Study of Cell Mechanobiology and Tissue Engineering

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    Tissue engineering is a promising, fast-growing field that combines cells, signals, and scaffolds to regenerate damaged tissues. To develop new, functional, engineered tissues, it is becoming increasingly important to understand how cell-material interactions affect the cell mechanobiological response. As a result, recent efforts have focused on developing complex synthetic materials that can mimic the dynamic in vivo cell environment. In this work, shape memory polymers (SMPs) were employed to develop dynamic 2D substrates and 3D scaffolds that undergo programmed changes in shape under cell compatible conditions. These substrates and scaffolds were applied in vitro and in vivo to demonstrate their potential as platforms to study cell mechanobiology and as functional tissue engineered constructs. The first part of this dissertation describes the fabrication and application of an SMP bilayer system capable of forming nano-scale wrinkles under cytocompatible conditions. Wrinkled substrates with easily tunable characteristics were employed to control the degree of cell alignment, with increased wrinkle amplitude and wrinkle orientation resulting in increased cell alignment until reaching a point of saturation. Active wrinkling with attached and viable cells was found to enable cell alignment to be “turned-on” on command. Additionally, cell migration on wrinkled substrates was assessed using quantitative, statistical-physics-based metrics which revealed cell motility atop anisotropic wrinkled substrates and which was more oriented and persistent than cell motility atop flat isotropic controls The second part of this dissertation describes the fabrication and application of porous 3D SMPs capable of expanding under physiological temperatures. A modified porogen-leaching approached was employed to fabricate highly porous, interconnected SMP scaffolds with tunable properties. The potential of SMP foams for use as synthetic bone substitutes was demonstrated in a mouse segmental defect model, where expanding foams were deployed intraoperatively to fill and conform to a critical size defect. Stiff SMP foams were able to maintain defect stability in a load-bearing application and integrated with the native bone after 12 weeks. Furthermore, deployable SMP foams showed potential for use as deployable cell-based therapies to facilitate bone repair, as expanding foams were able to support osteogenic differentiation of attached stem cells. This work demonstrates the potential of SMPs to be employed as dynamic materials to study cell-material interactions in dynamic environments and to aid in the development of functional tissue engineered constructs

    Self-supporting graphene films and their applications

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    The self-supporting monolayer material which is graphene has excited enormous interest over the ten years since its discovery due to its remarkable electrical, mechanical thermal and chemical properties. In this paper we describe our work to develop chemical vapour deposition methods to grow monolayer graphene on copper foil substrates and the subsequent transfer process. Raman microscopy, scanning electron microscopy and atomic force microscopy (AFM) are used to examine the quality of the transferred material. To demonstrate the process we describe transfer onto patterned SiO2/Si substrates which forms freely suspended graphene with focus on circular wells forming graphene drums. These show interesting mechanical properties which are being explored as nanomechanical resonators.UK NMS Programme, the EU EMRP (European Metrology Research Programme) projects MetNEMS and GraphOh

    Photoelectrochemical synthesis of DNA microarrays

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    Optical addressing of semiconductor electrodes represents a powerful technology that enables the independent and parallel control of a very large number of electrical phenomena at the solid-electrolyte interface. To date, it has been used in a wide range of applications including electrophoretic manipulation, biomolecule sensing, and stimulating networks of neurons. Here, we have adapted this approach for the parallel addressing of redox reactions, and report the construction of a DNA microarray synthesis platform based on semiconductor photoelectrochemistry (PEC). An amorphous silicon photoconductor is activated by an optical projection system to create virtual electrodes capable of electrochemically generating protons; these PEC-generated protons then cleave the acid-labile dimethoxytrityl protecting groups of DNA phosphoramidite synthesis reagents with the requisite spatial selectivity to generate DNA microarrays. Furthermore, a thin-film porous glass dramatically increases the amount of DNA synthesized per chip by over an order of magnitude versus uncoated glass. This platform demonstrates that PEC can be used toward combinatorial bio-polymer and small molecule synthesis.Defense Advanced Research Projects Agency (N66001-05-X6030)National Science Foundation (Grant CCR0122419

    Silicon and glass micromachining for μTAS

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