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Schematic illustration of the luminescence quenching mechanism in: (a) The core NaYF4: 30%Yb3+, 0.5%Tm3+ nanoparticles, as well as the luminescence enhancement mechanism in (b) The active core/inert shell (NaYF4: 30%Yb3+, 0.5%Tm3+)/NaYF4 nanoparticles; (c) The active core/active shell (NaYF4: 30%Yb3+, 0.5%Tm3+)/NaYbF4 nanoparticles; and (d) The active core/active shell/inert shell (NaYF4: 30%Yb3+, 0.5%Tm3+)/NaYbF4/NaYF4 nanoparticles.
