CVD GROWTH AND RAMAN SPECTROSCOPY STUDY OF VAN DER WAALS IN2SE3
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Using micro-Raman spectroscopy and finite-element simulations, we determine the in-plane thermal conductivity of suspended two-dimensional single-crystal In2Se3 grown by chemical vapor deposition. The thermal conductivity shows a strong dependence on the layer thickness: it reaches ~ 60 W/m·K at the thickness of 35 nm, and it reduces to ~ 4 W/m·K for the 5 nm-thick layer. This dependence demonstrates the significance of phonon surface scattering, and also indicates changes to the phonon dispersion relations as the layer thickness decreases. The determination of the thickness-dependent thermal conductivity provides an important practical basis for advancing 2D In2Se3-based device technologies, and, more generally, also enables fundamental insight into the limiting mechanisms for 2D thermal transport. We demonstrate a van der Waals Schottky junction defined by crystal phase multilayer In2Se3. Interestingly, these Schottky junctions exhibit enhanced thermoelectric properties. It has large Seebeck coefficients, on the order of 104 – 105 µV/K, with the thermoelectric figure-of-merit (ZT) approaching ~ 1. These greatly enhanced thermoelectric properties are attributed to the Schottky energy barriers at the junction interface, which lead to hot carrier transport and shift the balance between thermally and field-driven currents. We compare the electrical and thermal characteristics of - and -In2Se3, through variable-temperature Hall measurements, micro-Raman spectroscopy and related thermometry, as well as finite-element simulations. Our results show that, while the electron density in the metallic -In2Se3 is about four orders of magnitude higher than that in -In2Se3, the thermal conductivity of -In2Se3 is lower than that of -In2Se3 by one order of magnitude, which can be attributed to the enhanced anharmonic phonon coupling in -In2Se3. This combination of a very low thermal conductivity (~ 1 W/mK) and a high electrical conductivity makes -In2Se3 a potentially efficient thermoelectric material.