Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 122, Issue 45, Pages 26142-26152Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.8b07899
Keywords
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Funding
- Natural Science Foundation of Jiangsu Province of China [BK20160073]
- Jiangsu Key RD Program [BE2015102]
- Fundamental Research Funds for the Central Universities [2242018K40107]
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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 (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.
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