Figure 5.

Log-scale I-V curves recorded after being annealed at 425 K under large voltage excursions. I-V curves recorded at 425 K (a) and at 300 K (b) for an individual WO₃ nanowire with asymmetric contacts before (square) and after (circle, triangle) being annealed under large positive (+4 V) (triangle) and negative (−2 V) (cirlce) bias voltages at 425 K in vacuum. Insets at the lower left and right corner are schematic diagrams showing the distributions of positively charged oxygen vacancies.