Atomic-scale observation of Te-terminated surface reconstruction of sapphire substrate for the epitaxial growth of a 2D platinum ditelluride thin film

Abstract

Nowadays, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant interest in a variety of applications, including electronics, optoelectronics, biosensors, and electrocatalytic hydrogen generation because of their distinctive physical and chemical properties. Among various TMDCs, 2D platinum ditelluride (PtTe2), as a member of noble transition metal dichalcogenides (nTMDCs) has been widely investigated as a promising candidate in electronic and optoelectronic devices due to novel electronic feature of semi metal-to-semiconductor transition with decreasing layer thickness. Despite its highly attractive electronic properties, there are still critical issues in controlling the layer numbers and growing high-quality 2D films with a single crystal domain. In particular, synthesizing large-scale, uniform PtTe2 films on substrates presents a significant challenge. In this work, we successfully synthesized large-area, epitaxially-grown PtTe2 thin films on sapphire substrates. As a critical layer for growing epitaxial PtTe2 films, we directly visualize the existence of a pseudomorphic Te atomic adlayer at the interface between PtTe2 thin film and sapphire substrate using cross-sectional scanning transmission electron microscopy (STEM). The large-area uniform PtTe2 thin films were grown by Pt tellurization using chemical vapor deposition (CVD). Firstly, the 10 nm Pt film was deposited on c-sapphire (α-Al2O3) substrate via a sputtering method. In the CVD chamber, Te powders were evaporated and flowed toward the Pt film for the tellurization reaction. The cross-sectional PtTe2 specimen for TEM analysis was prepared by a focused ion beam (FIB) technique. In the high-resolution TEM (HRTEM) images of cross-sectional specimen, it was confirmed that PtTe2 thin films grow with a 30 degrees rotation about c-axis of sapphire and the epitaxial relation between the PtTe2 layer and sapphire substrate can be described as PtTe2(0001)//Al2O3(0001) and PtTe2[11-20]//Al2O3[1-100]. At the interface of the heterojunction, the lattice mismatch between the two materials is overcome due to the van der Waals (vdW) force. To understand the epitaxial relationship between the PtTe2 thin film and the sapphire substrate, we performed atomic-scale STEM imaging at the interface. Interestingly, additional atomic Te layer was inserted at the interface between PtTe2 and sapphire. These results demonstrate that the vdW gap can be stabilized and a high-quality epitaxial film can be obtained by the surface reconstruction of sapphire substrate.