Monitoring the electronic, thermal and optical properties of two-dimensional MoO2 under strain via vibrational spectroscopies: a first-principles investigation

Abstract

This study presents the electronic, mechanical, thermal, vibrational and optical properties of the MoO₂ monolayer under the effect of biaxial and uniaxial compressive/tensile strain, using first-principles calculations based on density functional theory. It has been found that the mechanical strength of MoO₂ is higher than other MoX₂ (X = S, Se, Te) monolayers. Dynamical stability analysis shows that MoO₂ is stable up to 6% compressive and at least 8% tensile strain. Strain dependent Raman modes are investigated along the biaxial directions. It was obtained that phonon softening and hardening occurred under tensile and compressive strain owing to the increase and decrease of the bond lengths of the MoO₂ structure. Our results also imply that the electronic band structure of the MoO₂ monolayer can be tuned with strain and the energy bandgap decreases with increasing biaxial tensile strain up to 4%. For larger values of strain, a semiconductor to semimetal transition is observed; however, this kind of transition is not observed for uniaxial tensile strain. Besides, we report that MoO₂ has a negative thermal expansion coefficient (TEC) in the range of extremely low-temperatures (0 K to 33 K) similar to other 2D MoX₂ monolayers. For temperatures above 600 K, it possesses a positive TEC with an approximate maximum value of 12 × 10⁻⁶ K⁻¹. We carried out optical property calculations by solving the Bethe–Salpeter equation and found that the MoO₂ monolayer has two strongly bound excitons below the quasiparticle absorption edge. Overall, our results shed light on experimental studies and suggest that the MoO₂ monolayer should be an excellent candidate for new design layered semiconductors, electronics, and optoelectronic devices.