Evolutional carrier mobility and power factor of two-dimensional tin telluride due to quantum size effects

Abstract

There has been emerging research of novel two-dimensional (2D) layered materials recently, due to their striking geometric, electronic and thermoelectric properties caused by the quantum confinement effect. However, the current reported thermoelectric performance of thin films is usually size-sensitive and hence may not be superior to that of their bulk counterparts. It is thus important to determine the size effect for low-dimensional thermoelectric materials, based on the theoretical tools of quantum mechanics. To achieve this goal, we studied 2D SnTe single crystals with a varied layer thickness as the characteristic length of materials, to eliminate the other coupled effects of interface engineering or universal defects in polycrystalline thin films. In this work, we demonstrate that the strategy of the quantum confinement effect is highly sensitive to the layer thickness of 2D SnTe materials, and a critical size of 3 layers exists, above which an abrupt degradation of the mobility and thermoelectric parameters occurs. The thermoelectric performance is optimal in monolayer SnTe and then gradually decays, until 6 layers, which gets close to the bulk feature. Correspondingly, the power factor (PF) and the ZT values exhibit evident layer-tunability as a combined effect of layer-dependent relaxation time, effective mass, electrical conductivity and Seebeck coefficient. Our study provides a profound physical understanding of the low-dimensionality strategy for high-performance thermoelectric materials. The intrinsic thermoelectric properties of ultra-thin 2D materials can be favourable as compared to those of the bulk single crystals.