Mechanistic Investigations on the Dramatic Thermally Induced Luminescence Enhancement in Upconversion Nanocrystals

Luminescent bulk materials generally suffer from thermal quenching, while upconversion nanocrystals (UCNCs) have recently been found to show increase of dramatic emission at elevated temperatures. A deep understanding on this quite different light–heat interaction at the nanoscale is important both scientifically and technologically. Herein, temperature-dependent upconversion luminescence (UCL) is investigated for UCNCs with various sizes, activators (Ho3+, Tm3+, Er3+), and core/shell structures. An anomalous UCL enhancement with increasing temperature is found for UCNCs with larger surface/volume ratios (SVRs). Moreover, this UCL increase shows a pronounced dependence on the SVRs, activators, emitting levels, and measuring environments. Substantial evidence confirms that the thermally induced UCL increase is primarily due to the temperature-dependent quenching effect of surface-adsorbed H2O molecules, instead of the previously proposed surface phonon-assisted mechanism. Temperature-dependent spectral investigations also show that the energy-loss process of Yb3+-sensitized UCNCs is largely due to the deactivation of Yb3+ ions caused by surface quenchers, rather than the direct quenching to activators. UCNCs with an active shell (doped with Yb3+) exhibit similar thermally induced UCL increase, due to energy migration to the surface over the Yb–Yb internet. It implies that active-core/active-shell UCNCs are susceptible to surface quenchers and would be unsuitable for applications in aqueous environments.