Optical control of exotic properties in strongly correlated electronic materials is very attractive.

What is it about?

Herein, we demonstrate a vertical heterojunction made of a strongly correlated electronic oxide thin film VO2 and a conductive 0.05 wt% Nb-doped TiO2 (Nb:TiO2) single crystal, whose metal–insulator transition (MIT) behaviors across the heterointerface can be efficiently modulated by visible light irradiation. It is observed that the magnitude of the MIT change is strongly suppressed by light illumination. The amplitude of the MIT decreases from ~350 in the dark state to ~7 in the illuminated state, obeying a logarithmic law with respect to the light power density. The optical tunability of the junction resistance is logarithmically proportional to the light power density, and a 320-fold on/off ratio is achieved with an irradiance of 65.6-mW/cm2 below the MIT temperature. The junction resistance is switched in a reversible and synchronous manner by turning the light on and off. While the VO2 thin film is metallic above the MIT temperature, the optical tunability is remarkably weakened, with a one-fold change remaining under light illumination. These results are co-attributed to a net reduction in the apparent barrier height and the photocarrier-injection-induced metallization of the VO2 heterointerface through a photovoltaic effect in the n-n-type junction at low temperature. Additionally, the optical tunability is minimal, resulting from the quite weak modulation of the already metallic band structure in the Schottky-type junction above the MIT temperature.

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Photo by Pascal Mauerhofer on Unsplash

Photo by Pascal Mauerhofer on Unsplash

Why is it important?

(1)The metal-insulator transition in a strongly correlated electronic archetype VO2 is modulated by light irradiance through constructing a vertical heterojunction device. (2) We broaden the spectral response of the VO2/Nb:TiO2 heterojunction from the UV to visible region. The junction resistance switching in a fast and reversible manner by visible light illumination is experimentally demonstrated at room temperature, indicating a potential application in photoelectronic devices. (3) A new mechanism of the optical excitation is observed in the VO2/Nb:TiO2 heterojunction, deep defect-level transition rather than conventional direct band gap transition under visible light illumination. This mechanism indicates that optical control of MIT in the VO2-based heterojunction can be realized by defect engineering, which has not been reported. This concept can be extended to the other strongly correlated electronic heterostructures for the desired functionalities.

Perspectives