Intrinsic Time-Tunable Emissions in Core-Shell Upconverting Nanoparticle Systems.

Color-tunable luminescence has been extensively investigated in upconverting nanoparticles for diverse applications, each exploiting emissions in different spectral regions. Manipulation of the emission wavelength is accomplished by varying the composition of the luminescent material or the characteristics of the excitation source. Herein, we propose core-shell β-NaGdF4 : Tm3+ , Yb3+ /β-NaGdF4 : Tb3+ nanoparticles as intrinsic time-tunable luminescent materials. The time dependency of the emission wavelength only depends on the different decay time of the two emitters, without additional variation of the dopant concentration or pumping source. The time-tunable emission was recorded with a commercially available camera. The dynamics of the emissions is thoroughly investigated, and we established that the energy transfer from the 1 D2 excited state of Tm3+ ions to the higher energy excited states of Tb3+ ions to be the principal mechanism to the population of the 5 D4 level for the Tb3+ ions.