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Orbital change manipulation metal-insulator transition temperature in W-doped VO2.

A series of epitaxial V1-xWxO2 (0 ≤ x ≤ 0.76%) nanocrystalline films on c-plane sapphire substrates have been successfully synthesized. Orbital structures of V1-xWxO2 films with monoclinic and rutile states have been investigated by ultraviolet-infrared spectroscopy combined with first principles calculations. Experimental and calculated results show that the overlap of π* and d∥ orbitals increases with increasing W doping content for the rutile state. Meanwhile, in the monoclinic state, the optical band gap decreases from 0.65 to 0.54 eV with increasing W doping concentration. Clear evidence is found that the V1-xWxO2 thin film phase transition temperature change comes from orbital structure variations. This shows that, with increasing W doping concentration, the decrease of rutile d∥ orbital occupancy can reduce the strength of V-V interactions, which finally results in phase transition temperature decrease. The experimental results reveal that the d∥ orbital is very important for the VO2 phase transition process. Our findings open a possibility to tune VO2 phase transition temperature through orbital engineering.