Understanding the trends of phase transition in dn (n = 0 to 6) transition metal disulfides during carrier injection: a perspective from d-orbital filling

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have different structural phases including the H-phase, T-phase, and various distorted phases. Structural phase transitions between phases are of significant importance for realizing unique electronic properties and integrating different functionalities in one material. In the current work, we systematically investigate the energy-favorable phases (EFPs) of groups IVB to XB (d⁰ to d⁶) transition metal disulfide (TMDS) monolayers during electron and hole injections and extract a rule that governs the different trends of phase transition (PT) from the perspective of d-orbital filling. Only from crystal and ligand field splitting, one can get hints for the PT trends between undistorted phases only. In order to quantitatively distinguish the phases and include the distorted phases, we adopt a work function (WF) analysis method, namely, for different phases, comparing their WFs or band edges relative to the vacuum level. Specifically, metallic phases use their WFs, while semiconducting phases consider either electron affinity (EA) or ionization energies (IEs) for electron or hole injections, respectively, which can be obtained from density functional theory calculations or from a database, and the effect of d-orbital filling is included naturally in the WF or IE/EA. Moreover, our WF analysis method is applicable also for VSe₂ with spin-polarization and electron correlation. Our work provides a unified understanding for the PT trends of TMDSs with various dⁿ (n = 0 to 6) filling and hence is helpful for guiding phase-engineering and preparation of metastable or single phase 2D materials.