Evidence of shallow band gap in ultrathin <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>1</mml:mn><mml:msup><mml:mi>T</mml:mi><mml:mo>′</mml:mo></mml:msup><mml:mtext>−</mml:mtext><mml:mi>MoT</mml:mi><mml:msub><mml:mi mathvariant="normal">e</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> via infrared spectroscopy
Jin Cheol Park, Eilho Jung, Sangyun Lee, Jungseek Hwang, Young Hee Lee
Abstract
Although van der Waals (vdW) layered $\mathrm{Mo}{\mathrm{S}}_{2}$ shows the phase transformation from the semiconducting $2H$ phase to the metallic $1T$ phase through chemical lithium intercalation, vdW $\mathrm{MoT}{\mathrm{e}}_{2}$ is thermodynamically reversible between the $2H$- and $1{T}^{\ensuremath{'}}$ phases, and can be further transformed by energetics, laser irradiation, strain or pressure, and electrical doping. Here, thickness- and temperature-dependent optical properties of $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ thin films grown by chemical vapor depsition are investigated via Fourier-transformed infrared spectroscopy. An optical gap of $28\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}2$ meV in a three-layer (or 2-nm)-thick $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ is clearly observed at a low-temperature region below 50 K. No discernible optical band gap is observed in samples thicker than \ensuremath{\sim}4 nm. The observed thickness-dependent band-gap results agree with the measured dc resistivity data; the thickness-dependent $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ clearly demonstrates the metal-semiconductor transition at a crossover below the 2-nm-thick sample.