Evidence of shallow bandgap in ultra-thin 1T'-MoTe2 via infrared spectroscopy

TL;DR

This study reveals a measurable shallow bandgap in ultra-thin 1T'-MoTe2 using infrared spectroscopy, showing a transition from metallic to semiconducting behavior as thickness decreases below 4 nm.

Contribution

It provides the first direct optical evidence of a shallow bandgap in ultra-thin 1T'-MoTe2 and correlates this with thickness-dependent electronic properties.

Findings

Optical gap of 28 +/- 2 meV observed in 3-layer 1T'-MoTe2 at low temperature.

No discernible bandgap in samples thicker than ~4 nm.

Thickness-dependent metal-semiconductor transition below 2 nm.

Abstract

Although van der Waals (vdW) layered MoS2 shows the phase transformation from the semiconducting 2H-phase to the metallic 1T-phase through chemical lithium intercalation, vdW MoTe2 is thermodynamically reversible between the 2H- and 1T'-phases, and can be further transformed by energetics, laser irradiation, strain or pressure, and electrical doping. Here, thickness- and temperature-dependent optical properties of 1T'-MoTe2 thin films grown by chemical vapor depsition are investigated via Fourier-transformed infrared spectroscopy. An optical gap of 28 +/- 2 meV in a 3-layer (or 2 nm) thick 1T'-MoTe2 is clearly observed at a low temperature region below 50K. No discernible optical bandgap is observed in samples thicker than ~4 nm. The observed thickness-dependent bandgap results agree with the measured dc resistivity data; the thickness-dependent 1T'-MoTe2 clearly demonstrates the metal-semiconductor transition at a crossover below the 2 nm-thick sample.