The property variations of ZnO responding to UV irradiations

(1) An enhanced preferred crystalline orientation in ZnO nanorods is achieved by applying ultraviolet (UV) illumination during sample deposition. The crystal field, generated from internal charge transfer between different energy bands by carrier excitations, drives crystalline growth in a preferred orientation. This work applies a novel crystalline engineering model, such that the preferred crystalline orientation in sample growth can be controlled by exciting carriers between assigned bands, which match the direction of crystal momentum between assigned bands via UV illumination.

(2) The saturated room-temperature ferromagnetism observed from nitrogen-doped zinc oxide (ZnO:N) increases significantly after ultraviolet light (UV) irradiation. During the period of irradiation, the resistivity as a function of time decreases dramatically and reaches saturation eventually. The log scale of the resistivity as a function of irradiation time reveals two sequential linear lines with different slops. Both of linear lines correspond to different dominant transports which result from some electrons recombination captured by the impurity band induced from the N doping. Similarly, the reverse properties of resistivity change appear at the samples after turning off the UV irradiation. The experiments convince us of the magnetic mechanism arising from the Coulomb excitation.

(3) The crystalline growth of the preferred orientation in the ZnO nanorods will be profoundly enhanced as we irradiate a specific wave length of ultra-violate (UV) during the sample fabrication. The specific UV photon energy is equivalent to the ZnO band gap. Both samples, with and without UV irradiated undoped nanorods, exhibit the anisotropic ferromagnetism. The electric polarization yields the ferromagnetic anisotropy along the c-axis, which suppresses the ferromagnetism in the same polarized direction and creates a similar multiferroic characteristic. Besides, the specific UV irradiation significantly enhances the ferromagnetic anisotropy.

References
[1] Chang-Feng Yu, Jen-Yu Tsai, Shin-Pon Ju, Hsiung Chou and Shih-Jye Sun*, “UV-assisted deposition of ZnO nanorods”, Physica Scripta, 85, 015604 (2012).

[2] Chang-Feng Yu, Shih-Jye Sun*, Hua-Shu Hsu, Hsiung Chou,” UV irradiations induce variations of ferromagnetism and resistivity in nitrogen doped ZnO films”, Physics Letters A 378, 1965 (2014).

[3] Chang-FengYu, Shih-Jye Sun*, Hua-Shu Hsu, “UV irradiations enhance anisotropy of ZnO nanorods in crystal growth and ferromagnetism”, Physics Letters A 379, 211 (2015).