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

Abstract

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 Bi 3+ -sensitized NIR luminescent nanophosphors, specifically Yb 3+ -doped Gd 2 MoO 6 (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 Yb 3+ under ultraviolet light excitation, and the luminescence was sensitized by introducing Bi 3+ ions. While the PL spectra in the visible region showed an increased intensity following Bi 3+ 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 Mo 6+ and Bi 3+ ions contributed to its enhancement. The insights from this study should provide a valuable concept for developing efficient Yb 3+ -doped NIR luminescence phosphors.