Ultrathin layers of beta-tellurene grown on highly oriented pyrolytic graphite by molecular-beam epitaxy

Monolayer Tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) substrate by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted beta-tellurene, whereas for thicker films, the cell size is more consistent with that of the (10-10) surface of bulk Te crystal. Scanning tunneling spectroscopy measurements show the films are semiconductors with the energy bandgaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valance-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.