Phase-Dependent Band Gap Engineering in Alloys of Metal-Semiconductor Transition Metal Dichalcogenides

Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)-electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, a novel form of bandgap engineering involving alloying non-isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe(2)with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the1Tand2Hphases, are produced each with emergent electronic structure. At room temperature, it is observed that the1Tand2Hphases are semiconducting and metallic, respectively. For the1Tstructure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps in the range of 0.42-0.58 at 77 K. Electron diffraction patterns of the1Tstructure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions that result in an uncommon 4 x 4 supercell, rotated approximately 4 degrees from the lattice. Density-functional-theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in1T-Mo0.5Nb0.5Se2, but not in the2Hphase. This work expands the boundaries of alloy-based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying.