Orbital electronic occupation effect on metal-insulator transition in Ti _x V_1-x O₂

A series of Ti _x V_1-x O₂ (0%  ⩽  x  ⩽  4.48%) thin films on c-plane sapphire substrates have been fabricated by co-sputtering oxidation solutions, and the metal-insulator transition temperature (T _MIT) of Ti _x V_1-x O₂ films rises monotonically at the rate of 1.64 K/at.% Ti. The x-ray diffraction measurement results show that, after Ti⁴⁺ ion doping, the rutile structure expands along the c _r axis while shrinking along the a _r and b _r axis simultaneously. It makes the V-O bond length shorter, which is believed to upshift the π ^* orbitals. The rising of π ^* orbitals in Ti-doped VO₂ has been illustrated by ultraviolet-infrared spectroscopy and first-principles calculation. With the Ti⁴⁺ ion doping concentration increasing, the energy levels of π ^* orbitals are elevated and the electronic occupation of π ^* orbitals decreases, which weakens the shielding for the strong electron-electron correlations in the d_|| orbital and result in the T _MIT rising. The research reveals that the T _MIT of VO₂ can be effected by the electronic occupancy of π ^* orbitals in a rutile state, which is helpful for developing VO₂-based thermal devices.