Key Role of Metal-to-Metal Charge Transfer Transition between Mo⁶⁺ and Bi³⁺ for Enhancement in NIR Luminescence of Gd₂MoO₆:Bi,Yb Nanophosphor

Near-infrared (NIR) luminescent materials play a significant role in various application fields, including food analysis, medical diagnosis, and bioimaging. Consequently, there is a growing demand for the development of efficient NIR phosphors. In this study, we successfully synthesized Bi3+-sensitized NIR luminescent nanophosphors, specifically Yb3+-doped Gd2MoO6 (GMO), utilizing a solvothermal reaction technique. The luminescent properties were comprehensively assessed with photoluminescence (PL), PL excitation (PLE) spectra, and time-resolved PL spectra at different temperatures. Additionally, electronic structures were estimated to gain insights into the PL mechanism, employing density functional theory calculations. It was found that GMO:Bi,Yb exhibited NIR luminescence due to the 4f-4f transitions of Yb3+ under ultraviolet light excitation, and the luminescence was sensitized by introducing Bi3+ ions. While the PL spectra in the visible region showed an increased intensity following Bi3+ doping, the absence of an additional peak suggested that the enhancement mechanism differs from the typical processes. The time-resolved PL spectra, wavelength-dependent PLE spectra, and theoretical calculation of GMO:Bi confirmed that the MMCT transitions between Mo6+ and Bi3+ ions contributed to its enhancement. The insights from this study should provide a valuable concept for developing efficient Yb3+-doped NIR luminescence phosphors.