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(a) Energy level representation of Yb/Tm doped UCNPs-4 under different excitation wavelength; upconversion emission of the (b) 8% Tm-doped and (c) 1% Tm-doped UCNPs-4; (d) energy level diagram of Yb/Tm doped UCNPs-4; (e) schematic illustration of the photon avalanche process; (f) transient response of the upconverted emission from Tm-doped UCNPs-4 [30].
