General Strategy for Doping Impurities (Ge, Si, Mn, Sn, Ti) in Hematite Nanocrystals

The doping of foreign atoms is critical in tailoring the properties and potential applications of semiconductor nanocrystals. A general strategy for successfully incorporating various impurities (e.g., Ge, Si, Mn, Sn, Ti) inside the regular crystal lattice of hematite (alpha-Fe2O3), a promising candidate for water splitting and environmental protection, is developed. Liquid-phase laser ablation-derived colloidal clusters are used as doping precursors for the metastable growth of doped hematite nanocrystals, thereby avoiding surfactants and hazardous liquid byproducts. The doping percentage, morphology, and structure of the hematite nanocrystals are greatly affected by the type and amount of the colloidal precursors used. High-resolution transmission electron microscopy and the corresponding component analysis reveal that the dopant atoms either form superlattice structures (Ge and Si) or distribute as disordered solid solutions (Mn, Sn, Ti) inside the crystal lattice of hematite. The optical absorption spectra and the resulting band gaps of the doped-hematite nanocrystals are investigated. Typical electronic transitions consisting of ligand to metal charge transitions, Fe3+ d-d transitions, and pair excitations distinctly occur in the optical spectra. The simultaneous incorporation of impurities and preferential growth mechanism of hematite nanocrystals are also further elaborated.