abstract
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The upper limit of Tm3+ doping concentration increases from the well accepted 0.5% to 3% in NaGdF4@Yb/Tm:NaGdF4 NCs with an inert-core/active-shell structure, and the UC intensity increases about 6 times, benefiting from the enhanced energy transfer efficiency from the Yb3+ to Tm3+ ions by confining them in a two-dimensional space.