Surface transport and band gap structure of exfoliated 2H-MoTe$_2$ crystals

TL;DR

This study investigates the surface transport properties and band gap structure of exfoliated 2H-MoTe$_2$ crystals, revealing ambipolar transport and quantifying the indirect and direct band gaps using multiple techniques.

Contribution

It provides the first detailed analysis of surface transport and band gap structure in MoTe$_2$, expanding understanding of Te-based 2D semiconducting TMDs.

Findings

Ambipolar transport achieved with hole and electron mobilities of 10-30 cm$^2$/Vs.

Indirect gap $E_{ind}$ = 0.88 eV and direct gap $E_{dir}$ = 1.02 eV at room temperature.

Potential transition to a direct gap in thin MoTe$_2$ layers.

Abstract

Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe$_2$ crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm$^2$/Vs at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, that allow the measurement of the indirect gap ($E_{ind}$), and optical transmission spectroscopy on crystals of different thickness, that enables the determination of both the direct ($E_{dir}$) and the indirect gap. We find that at room temperature $E_{ind}$ = 0.88 eV and $E_{dir}$ = 1.02 eV. Our results suggest that thin MoTe$_2$ layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs.