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

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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 (Ho 3+, Tm 3+, Er 3+ ), 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 H 2 O molecules, instead of the previously proposed surface phonon-assisted mechanism. Temperature-dependent spectral investigations also show that the energy-loss process of Yb 3+ -sensitized UCNCs is largely due to the deactivation of Yb 3+ ions caused by surface quenchers, rather than the direct quenching to activators. UCNCs with an active shell (doped with Yb 3+ ) 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.