Thickness-dependent energy gap of van der Waals epitaxial Cr1+δTe2 films: An insulator-metal transition

Abstract

Recent research on ferromagnetic 2D materials renewed the interest in Cr-Te-system films [Cr1+δTe2 (0 ≤ δ ≤ 1)], a kind of pseudo-layer compounds which were found to be an alternative to pure van der Waals materials. Here, the phase structure and an insulator–metal transition (IMT) induced by interface effect of Cr-Te-system films grown epitaxially on mica substrates were investigated. In situ reflection high energy electron diffraction, scanning tunneling microscopy, x-ray diffraction, scanning transmission electron microscopy, and scanning tunneling microscope measurements showed the phase structure of the high-quality as-grown films was Cr3Te4 (δ = 0.5). By combining scanning tunneling spectrum and ultraviolet–visible absorption spectroscopy, we found that the energy gap of the film is remarkably thickness-dependent and an IMT phenomenon was observed at a critical film thickness of about 4 nm (the bandgap decreased from 2.5 eV to 0 eV as the thickness increased from 2 nm to 24 nm). These findings will promote the development of practical applications of Cr1+δTe2 in photoelectronics and electronics devices.