Chemical vapor transport synthesis of a selenium-based two-dimensional material

Abstract

Selenium-based layered materials, and in particular transition-metal diselenides (TMDSs), have intriguingproperties in the monolayer limit. Materials such as $MoSe_2$ , $WSe_2$ , and $NbSe_2$ display striking features such as spinvalley coupling at the valence-band edges and offer great potential for optoelectronics applications. Although a dozenof other TMDSs have been realized or proposed, whether two-dimensional chalcogens are possible or not is still an openchallenge. In this work, we show the chemical vapor transport synthesis of a novel, atomically thin selenium-basedmaterial on oxidized silicon substrates. This new member of the two-dimensional materials family has a unique Ramanspectrum similar to that of bulk selenium and has an optical gap of ∼1.57 eV at room temperature determined bythe photoluminescence. No transition metals are found in the stoichiometry of the crystals. Analysis of high-resolutiontransmission electron micrographs of the monolayers reveals a distinctive set of hexagonal spots indicating a sixfoldsymmetry of the lattice. Atomic force microscopy measurements show the monolayer thickness to be ∼0.75 nm.