Comprehensive Optical Band-Edge Characterization for Multilayered MoTe2 and Its Application in van der Waals-Stacked Heterojunction Devices.

Abstract

MoTe2 is considered a promising 2D material for solar energy and optoelectronic applications owing to its suitable bandgap value and specific excitonic behaviors. However, its band-edge and excitonic transitions have not been fully elucidated. In this study, micro-thermoreflectance (µTR) spectroscopy results show that multilayered 2H-MoTe2 exhibits multiple excitonic features, including A1s, B1s, A', C, D, E, F, and G excitons, as well as one indirect-gap related feature, observed in a 500 nm-thick nanoflake at 300 K. Thickness-dependent micro-photoluminescence (µPL) measurement reveals that the PL emission is undetectable at a thickness of ≈40 nm (56 layers), but it is initially detected at 0.944 eV for a thinner thickness of ≈20 nm (28 layers), and finally, it shifts to 1.042 eV and presents the highest PL intensity when the thickness decreases to 5 nm (7 layers). Density functional theory (DFT) band structure calculations reveal that monolayer MoTe2 is a direct semiconductor with the highest bandgap, which diminishes and finally converts to an indirect band with at ≈45 layers, nearly consistent with the thickness-dependent µPL results. From the DFT calculations, the A1s, B1s, A', C, D, E, F, and G band-edge exciton features in the µTR spectra of multilayered MoTe2 are verified and assigned. Additionally, a prototype p-SnS/n-MoTe2 stacking heterojunction device is fabricated. The built-in potential of the heterojunction diode is ≈0.62 V, matching well with the measured work function difference between the two heterojunction materials.