Orbital electronic occupation effect on metal-insulator transition in TixV1-xO2

A series of T(i)xV(1-x)O(2) (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 TixV1-xO2 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 pi* orbitals. The rising of pi* 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 pi* orbitals are elevated and the electronic occupation of pi* orbitals decreases, which weakens the shielding for the strong electron-electron correlations in the d(II) 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 pi* orbitals in a rutile state, which is helpful for developing VO2-based thermal devices.