Thickness-driven emergence of a highly insulating bulk state in SnTe nanoplates.

Abstract

A primary objective for ideal topological electronic devices is the precise modulation of topological surface conduction, fully decoupled from bulk transport interference. To date, this objective remains unfulfilled in both topological insulators and topological crystalline insulators because of severe charge doping induced by intrinsic defects. Thin nanoplates of SnTe, a prototypical topological crystalline insulator, are promising candidates for topological transistors, in which an electrostatic gate is predicted to open or close a gap in the topological surface states rather than shift the bulk carrier density. However, realizing such devices has been fundamentally hindered by the pronounced metallic conduction of SnTe crystals, which arises from tin-vacancy-induced self-doping that pins the Fermi level deep within the valence band. In this work, we report the thickness-dependent resistance of SnTe nanoplates synthesized under ambient conditions of the chemical vapor deposition experiment. A substantial increase in resistance is observed as the nanoplate thickness is reduced below a critical threshold of 6.5 ± 0.2 nm. In parallel, systematic Raman spectroscopy reveals thickness-dependent phonon mode shifts in these nanoplates. Integrating structural, optical, and electrical analyses, we propose a thickness-dependent band-structure evolution model. This model attributes the pronounced insulating state in ultrathin nanoplates to compressive strain and a reduced density of Sn vacancies. Achieving a highly insulating bulk state in a topological crystalline insulator represents a critical milestone toward decoupling topological surface transport from bulk conduction, paving the way for purely gate-controlled topological transistors.