Oxygen Vacancies Allow Tuning the Work Function of Vanadium Dioxide

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The evolution of electron valence bands, core levels, and work function of vanadium dioxide (VO 2 ) thin films upon argon ion sputtering and annealing, as commonly done to obtain atomically clean surfaces, with and without low‐pressure oxygen atmosphere during annealing is investigated by ultraviolet and X‐ray photoemission spectroscopy. Both sputtering and annealing in vacuum introduce lower oxidation state V species, leading to an increased intensity of V 3d derived bands close to the Fermi level. Such oxygen deficient surfaces exhibit low work function values as low as 4.40 eV. Annealing the sample under low‐pressure oxygen atmosphere (few times 10 −4 mbar partial O 2 pressure) results in stoichiometric VO 2 surfaces with a high work function of up to 6.70 eV. Moreover, the work function of the VO 2 can be continuously tuned between the high and low limits by adjusting the atomic ratio of oxygen and vanadium at the surface. Appropriately adjusted VO 2 can thus be employed as moderate electron as well as superior hole injecting electrode material in electronic and optoelectronic devices.