New One-Dimensional Material Nb₂Se₉: Theoretical Prediction of Indirect to Direct Band Gap Transition due to Dimensional Reduction

Layered two-dimensional (2D) transition metal dichalcogenides (TMDCs) often form 2D sheets and some of these show an indirect to direct band gap transition as the number of layers decreases from that of the bulk structure. Recently, a new one-dimensional (1D) material of Nb₂Se₉ is successfully prepared by solid state reaction. This material is semiconducting and composed of periodically stacked single-chain atomic crystals (SCAC) where the SCACs form inorganic bulk crystals due to strong bonds within the chain but with weak inter-chain interactions. To determine the potential applications of our newly developed 1D nanowire, theoretical prediction of its material properties is performed. As a first step, the band structures of bundles of Nb₂Se₉ SCACs, which are composed of 1–7 single chains, are calculated by using density functional theory. Unlike the bulk structure of Nb₂Se₉, the chain bundles composed of up to 21 single SCACs would have a direct band gap. Accordingly, it is expected that an Nb₂Se₉ bundle SCAC with a diameter of 3.6 nm can cause the electronic transition without being disturbed by the phononic environment due to the direct band gap, and can therefore be used in photoluminescence applications.