Ultrathin layers of β-tellurium grown on highly oriented pyrolytic graphite by molecular-beam epitaxy

Two-dimensional (2D) materials have been the subject of intensive research in recent years. New 2D materials are continuously put forward by theory and realized by experiments. A recent theory has predicted some layered structures of tellurium (Te), the monolayers of which represent new two-dimensional (2D) systems with attractive electronic and optoelectronic properties. In this work, we employ molecular-beam epitaxy to fabricate ultrathin Te films on highly oriented pyrolytic graphite (HOPG). Using scanning tunneling microscopy, we find that the grown Te ultrathin layers have rectangular surface cells with the cell size being consistent with the predicted β-tellurene. For thicker films, the cell size is found more consistent with that of the [10-10] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements suggest the β-tellurium 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 can be explained by the strong coupling of states at the VBM but a weak coupling at conduction band minimum as revealed by density functional theory (DFT) calculations.