Defect-Induced Room-Temperature Ferromagnetism in SnO2 Nanowires Controlled by UV Light Irradiation

Nonferromagnetic SnO2 nanowires show ferromagnetism at room temperature, which is significantly enhanced upon irradiation by ultraviolet (UV) light of wavelength 365 nm. The structural characterizations on as-grown, UV-irradiated, and O-2-annealed samples show no measurable difference in X-ray diffraction profiles and field-emission scanning and transmission electron microscopy images. But optical absorption, photoluminescence, and X-ray photoelectron spectroscopy reveal a significant effect of UV irradiation and establish a remarkable enhancement of O vacancies in SnO2 nanowires upon UV light irradiation, which diminishes after annealing in the presence of O-2 gas. The as-grown and O-2-annealed nanowires show weak ferromagnetism at room temperature, but the UV-irradiated nanowires show strong ferromagnetism. The origin of such ferromagnetism in pure (undoped) SnO2 is attributed to the presence of oxygen vacancies, which is tunable externally using UV irradiation.