Evidence of shallow band gap in ultrathin 1T′-MoTe2 via infrared spectroscopy

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 MoS2 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 three-layer (or 2-nm)-thick 1T'-MoS2 is clearly observed at a low-temperature region below 50 K. No discernible optical band gap is observed in samples thicker than similar to 4 nm. The observed thickness-dependent band-gap 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.