Structural, Optoelectronic and Thermodynamical Properties of 1T Phase of Transition Metal Oxides TMO₂ (TM = Zr and Hf): A first-principles Study

Monolayer transition metal oxides have emerged as an exciting area of study of two-dimensional materials. It is crucial to comprehend the structural stability and physical characteristics of various dielectric materials at the ultrathin level due to the diminishing size of microelectronic equipment. In this regard, we have studied the structural, optoelectronic and thermodynamical characteristics of the 1T phase of transition metal oxides TMO₂ (TM = Zr and Hf) using first-principles computations. The structural stability of both monolayers has been assessed by formation enthalpy computations, which suggest that both monolayers are thermodynamically stable and can be synthesized experimentally. The optimized lattice parameters for 1T-ZrO₂ and 1T-HfO₂ are found to be 3.15 and 3.17 Å, respectively. The phonon calculations assure the dynamical stability of both monolayers as there are no imaginary frequencies present in the phonon dispersion plots. In order to assess the thermodynamical stabilities of TMO₂ monolayers, various thermodynamical characteristics have been determined for 0 to 1000 K. Notably, the TMO₂ monolayers depict their thermal stability as both monolayers possess negative free energies up to 1000 K. The maximum calculated formation enthalpy value for 1T-ZrO₂ and 1T-HfO₂ is − 0.86 and − 0.85 eV, respectively, at 1000 K. Furthermore, the 1T-ZrO₂ show the high thermal stability than 1T-HfO₂ because it has a lower Helmholtz free energy value. Electronic properties have been analyzed through density of states and band structures calculations. The calculated band gaps for 1T-ZrO₂ and 1T-HfO₂ monolayer are 4.30 and 5.41 eV, respectively. The difference in the band gaps develops due to the varying energy levels by the valence orbitals of Zr (5s², 4d²) and Hf (6s², 5d²). The indicated difference acts as the main distinguishing feature among the studied metal oxides. Moreover, the valence bands in both metal oxide monolayers are mostly obtained from the p-states of Oxygen, and d-states of Zr and Hf. Furthermore, numerous optical characteristics for both under-study monolayers have been computed and compared. The first critical edge for 1T-ZrO₂ and 1T-HfO₂ are found to be 2.13 and 1.95, respectively. The highest absorption peaks for 1T-ZrO₂ and 1T-HfO₂ are noted at 5.43 and 6.24 eV. It is found that the absorptivity of 1T-HfO₂ is 2.91% higher as compared to 1T-ZrO₂. The high value of dielectric constants, efficient thermodynamical stability and wide band gaps of under-investigation TMO₂ (TM = Zr and Hf) make them the potential contenders for 2D FETs. In short, our findings suggested that monolayers TMO₂ are ideal choices for a range of applications and could help in developing efficient optoelectronic devices.