First-principles calculations of the structural and electronic properties of monolayer 1T−MoO2 and WO2

Two-dimensional transition metal dichalcogenides (TMDCs), such as MoS₂ and WTe₂, received extensive attention owing to their diverse crystal structures and intriguing physical properties. In contrast, there has been relatively limited research on transition-metal dioxides (TMDOs) despite sharing the same crystal structures with TMDCs. Here, we investigate the structural and electronic properties of monolayer T-phase MoO₂ and WO₂ using first-principles calculations. Our analysis reveals that their phonon dispersions have prominent imaginary modes, leading to the emergence of two charge density wave phases: 1T^' and √3×√3. Interestingly, we find that 1T^'-MoO₂ and WO₂ exhibit Dirac semimetal behavior, while their √3×√3 phases are ferroelectric semiconductors, with out-of-plane spontaneous polarization of 3.99 and 3.94 pC/m, respectively. This work sheds light on structural and electronic properties of T-phase MoO₂ and WO₂, offering valuable insights for further experimental investigations into similar TMDOs materials.