Understanding the Chemical Nature of the Buried Nanostructures in Low Thermal Conductive Sb-Doped SnTe by Variable-Energy Photoelectron Spectroscopy

Abstract

Nanoprecipitates embedded in a matrix of thermoelectric materials decrease the lattice thermal conductivity significantly by extensive heat carrying phonon scattering. Recently, two-dimensional layered intergrowth nanostructures of SnmSb2nTe3n+m embedded in SnTe matrix have provided record low lattice thermal conductivity in SnTe, but an understanding of the chemical nature of these layered nanostructures is still not clear. Herein, we studied the chemical nature of the intergrowth nanostructures of a series Sb-doped SnTe by variable-energy X-ray photoelectron spectroscopy at synchrotron, which is well known to probe buried interfaces and embedded nanostructures. The primary oxidation states of Sb, Sn, and Te in these intergrowth structures are found to be in +3, +2, and −2, respectively, which is expected from the composition. However, both the Sn and Sb are found to be slightly oxidized in the surface. From the intensity variation with photon energy, we have found a thin layer of SnO2 (∼4.5 nm) on the sample surfaces and the thickness decreases with Sb doping. Te is also found in 0 oxidation states, which corroborates with the variation of Sn vacancies with Sb doping. The valence band features near the edge do not change significantly with Sb doping. This understanding of the chemical nature of low lattice thermal conductive Sb-doped SnTe will help further to design the thermoelectric materials with their surface phenomenon.