In Situ Tracking of Low-Temperature VO2 Crystallization via Photocombustion and Characterization of Phase-Transition Reliability on Large-Area Flexible Substrates

Low-temperature processing methods targetting for sol–gel metal oxides have been limited to amorphous metal oxide films because a much higher energy budget is required for crystalline functional metal oxide formation. Herein, we report the photocombustion developed to dramatically reduce the crystallization temperature of a sol–gel VO2 film from 600 to 250 °C by combining deep ultraviolet irradiation with a carbon-free oxidizer for reactive radical generation. The morphological transition and crystal evolution of the sol–gel films annealed via various conditions including combustion, photoactivation, and photocombustion were thoroughly investigated using in situ and ex situ TEM techniques, allowing us to separate the regimes of amorphous and crystal solid formation. TGA-DTA, UV–vis spectroscopy, and EDX analyses reveal that, at [AN]/[VO­(acac)2] of 1.5, impurity removal and V–O–V polycondensation are most effective and suggest that oxygen vacancies generated during reduction play a key role in accelerating crystallization. Functional VO2 films were successfully deposited on polyimide substrates for large-area flexible phase-transition device arrays, while their excellent uniformity and phase-transition reliability were confirmed under various bending conditions. We believe that the low-temperature crystallization via DUV-assisted photocombustion is useful for strain-sensitive low temperature devices of crystalline metal oxide films fabricated via sol–gel routes.