  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.
