Photophysical Color Tuning for Photon Upconverting Nanoparticles.

Generating a single higher-energy photon from several photons with lower energy, or photon upconversion, can have useful applications in renewable energy and biological imaging. However, efficient utilization of sub-bandgap infrared radiation in these optoelectronic devices requires high upconversion efficiency and precise tunability of emitted visible light. Although several studies have utilized chemical doping to tune the color of upconverted light, low upconversion efficiency can limit their applicability. In this study, color tuning of upconversion photoluminescence is successfully realized by modulating the photophysics using surface plasmon polaritons. Using absorption of near-infrared light in Yb3+ ions, the occupation of different energy states and the relative energy-transfer rate (from Yb3+ to Er3+ and Tm3+ dopants here) can be simultaneously tuned, with a complete shift in color emission using a chromaticity diagram. Furthermore, the efficacy of color tuning by transforming upconverted light (well matched to their enhanced bandedge absorption) into photocurrent is also demonstrated by using ultrathin two-dimensional semiconductor nanosheets. Therefore, photophysical color tuning and integration of these precisely tuned upconverting nanoparticles with ultrathin semiconductors can pave the way for designed metal nanostructures for highly efficient utilization of low-intensity sub-bandgap infrared radiation in optoelectronic devices.