Microstructure-Dependent Optical Properties of Doped Spinel Oxide Nanosystems*

Doped spinel crystals show long-lasting emissions (LLP) in the visible and near-IR regions that can be activated by X-ray or UV-visible light. Such properties make this material a promising candidate for background-free deep-tissue bioimaging, photodynamic or photon-induced therapy (PIT), optical sensing, and many other applications. The optical properties, however, are highly dependent on the dopant environment and the nature of the trap states responsible for the LLP. Annealing changes the defect structures of Cr and the trap states and the strength of the energy transfer between the different Cr defects and significantly affects the optical properties. We found crystal field decrease at low temperatures caused by quasi-melted pseudocrystalline surface, making them temperature sensitive, an order of magnitude increase in the photon yield upon X-ray or UV illumination, and temperature-dependent LLP. The outcome of the fundamental research on the defect structure is that at optimum annealing conditions, activating the IR700 photoactive dye used in PIT, with X-ray excitation, or by the LLP that promises PIT for deep-seated or embedded tumors or other diseases is possible.