Understanding the Phase Transition Evolution Mechanism of Partially M2 Phased VO₂ Film by Hydrogen Incorporation

Abstract

Studies on the hydrogen incorporated M1 phase of VO2 film have been widely reported. However, there are few works on an M2 phase of VO2. Recently, the M2 phase in VO2 has received considerable attention due to the possibility of realizing a Mott transition field-effect transistor. By varying the postannealing environment, systematic variations of the M2 phase in (020)-oriented VO2 films grown on Al2O3(0001) were observed. The M2 phase converted to the metallic M1 phase at first and then to the metallic rutile phase after hydrogen annealing (i.e., for H2/N2 mixture and H2 environments). From the diffraction and spectroscopy measurements, the transition is attributed to suppressed electron interactions, not structural modification caused by hydrogen incorporation. Our results suggest the understanding of the phase transition process of the M2 phase by hydrogen incorporation and the possibility of realization of the M2 phased-based Mott transition field-effect transistor.

Correlation between Symmetry and Phase Transition Temperature of VO2 Films Deposited on Al2O3 Substrates with Various Orientations

“…In contrast, the peak area of ω O‐M1 is larger (smaller) than that of ω O‐M2 for the VO 2 film deposited on c ‐plane Al 2 O 3 ( m ‐plane Al 2 O 3 ). This indicates that the film deposited on m ‐plane Al 2 O 3 ( c ‐plane Al 2 O 3 ) has the highest (lowest) symmetry of VO 6 octahedron because the M2 phase is more symmetrical than the M1 phase, [ 48 ] which is in line with the V K ‐edge XANES result. As the temperature increases, the peak area of ω O‐M1 (ω O‐M2 ) decreases (increases), although the degree of change is different.…”

Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping

“…The obtained lattice constant of VO 2 (M 1 ) (Figure S8a) is calculated as a M1 = 5.39 Å, b M1 = 4.56 Å, c M1 = 5.67 Å, and β M1 = 121.93°and that of M 2 phase is as a M2 = 9.15 Å, b M2 = 5.93 Å, c M2 = 4.58 Å, and β M2 = 90.81°(Figure S8d), respectively, similar to our experimental observations and the phase variation of vanadium dioxide in previous reports. 54,55 Figure S8b,c demonstrates the atomic variation of the M 1 phase after cobalt doping (Co z5 M 1 and Co z10 M 1 , respectively). The bond length between Co and O was calculated as 1.92 Å, 1.95 Å, compared to 1.93 Å in M 1 , indicating that the VO 2 structure is significantly affected by different doping concentrations and Co has two different bonding environments, in line with the XPS measurement above.…”

“…The d || band splitting generates a 0.9 eV bandgap, similar to the previous calculation result on V 3d controlled MIT in VO 2 . 55 The V 3d orbital splits into d || and d || * (Figure S10a) because of the V−V dimer change during the phase transition. 59 In the conduction band, d || * hybridizes with π*.…”

The Role of Local DFT+U Minima in the First-Principles Modeling of the Metal–Insulator Transition in Vanadium Dioxide

“…Since only half of the cations participate in this distortion, the resulting band gap is reduced compared with the fully dimerized M phase but still pronounced in accordance with experiments. 74 Previous DFT+U studies involving the pristine M2 phase have relied on two different values for the U parameter to stabilize the mixed dimerized/nondimerized arrangement of V ions upon geometry optimization 76 or have completely refrained from ionic relaxation, studying the phase properties at the experimental structures only. 77,78 Testing different initial V d occupations for the M2 structure, we found a relaxation toward the metallic R phase for some of the metastable states, losing the initial partial dimerization, while others preserved the M2 geometry.…”