Brown University School of Engineering

Joint Materials/Solid Mechanics Seminar Series: “Characterizing and tuning the intrinsic properties of organic thin films and 2D materials via strain”

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Monday, November 25, 2013 4:00pm - 5:00pm

Ashwin Ramasubramaniam, University of Massachusetts, Amhert will present two examples from recent work on utilizing mechanical deformations to characterize and/or tune the intrinsic properties of organic semiconductor thin films and 2D materials. The first example employs the wrinkling instability of thin films as a metrology tool for characterizing the elastic properties of rubrene, a conjugated organic semiconductor. This technique alleviates handling difficulties associated with the small dimensions and fragility of rubrene crystals. We perform complementary Density Functional Theory (DFT) calculations, employing state-of-the-art van der Waals exchange correlation functionals, to extract the complete set of nine independent elastic constants of orthorhombic rubrene. Proper accounting of van der Waals interactions within DFT is shown to accurately capture the orientation dependence of buckling wavelengths in experiments. Thereafter, the effects of mechanical strain on charge transport in rubrene are examined and quantified using DFT-parameterized semi-classical models and Boltzmann transport studies. The second example demonstrates the use of curvature to dynamically and reversibly tune the electronic properties of layered transition-metal dichalcogenides (TMDCs). Monolayer TMDCs display valley-selective circular dichroism due to time-reversal symmetry and inversion asymmetry, making them promising candidates for valleytronics. In contrast, few-layer TMDCs possess both time-reversal and inversion symmetry and hence, lose these desirable valley-selective properties. By using density-functional tight-binding electronic structure simulations, we show that bending of multilayer MoS2 sheets can break band degeneracies and localize states on distinct layers due to bending-induced strain-gradients across the sheets. We propose a strategy for employing bending deformations as a simple yet effective means of tuning band gaps while simultaneously tuning coupling between spin, valley, and layer pseudospin of charge carriers in few-layer TMDCs.