Synthesis, characterization, and room-temperature ferromagnetism in cobalt-doped zinc oxide (ZnO:Co²⁺) nanocrystals encapsulated in carbon

Carbon-encapsulated, cobalt-doped ZnO nanocrystals were synthesized by the one-step RAPET technique with Co:Zn atomic ratios of 0, 0.011, 0.037, 0.082, and 0.183. The substitution of Zn²⁺ by Co²⁺ decreased the XRD peak intensities of the Co²⁺-occupied lattice planes due to the lower atomic scattering factor of Co²⁺ with respect to Zn ²⁺. The size of the nanocrystals varied with the variation in cobalt concentration. X-ray photoelectron spectroscopy studies revealed the presence of Co2+. The electron transfer was evident from the Zn 4s level to the unfilled higher charge density Co 3d level. In the photoluminescence spectra, band edge exciton transitions and the presence of oxygen vacancies were observed. The observed ferromagnetism was intrinsic in nature and not due to any metallic Co segregation or any cobalt oxide phase formation. The spin-split impurity band (due to hybridization between the charge carriers in Co 3d and Zn 4s) and the defect-bound carriers (from the interstitial Zn_ivacancies) account for the room-temperature ferromagnetism for lower and higher cobalt concentrations, respectively. The 9-15 nm graphitic carbon shell protects the semiconductor core from oxidation of the Co²⁺ dopant. The carbon atoms at the proximity of the ferromagnetic core get spin-polarized to contribute a weak magnetic order.