Confinement on energy transfer between luminescent centers in nanocrystals

Abstract

The luminescence dynamics of optical centers in nanocrystals depends critically on the phonon density of states (PDOS), which is quite distinct from that of bulk materials. It is shown that energy transfer (ET) in nanocrystals is confined by discrete PDOS as well as direct size restriction. Temperature-, concentration-, and size-dependence of the fluorescence decay from the S3/24 state of Er3+ in Y2O2S nanocrystals have been investigated using laser spectroscopic experiments and computational simulations. A set of microscopic rate equations that govern the evolution of the excitation probability Pi(t) are solved iteratively using a Monte Carlo method. The simulations of ET based on a theoretical model with five parameters are in good agreement with the experimental results. It is shown that phonon-assisted ET processes in Er3+:Y2O2S nanocrystals contribute partly to the fluorescence decay at 295 K, and is negligible at 5 K. For applications, the nanoconfinement effects on ET may significantly reduce the efficiency of sensitized or upconversion luminescence due to the lack of low-frequency phonon modes and restricted excitation migration in nanophosphors.

Synthesis of Hexagonal‐Phase NaYF4:Yb,Er and NaYF4:Yb,Tm Nanocrystals with Efficient Up‐Conversion Fluorescence

“…The decrease in the up-conversion fluorescence has been reported and studied previously for Y 2 O 2 S:Yb,Er up-conversion phosphors. [24] When the size of the Y 2 O 2 S:Yb,Er phosphors decreased to 30 nm, the up-conversion efficiency was reduced to about 22 % of that of the bulk counterpart. When the size decreased from 30 to 2 nm, the efficiency rapidly decreased.…”

“…The decrease in fluorescence efficiency is attributed to the lack of low-frequency phonon modes and restricted excitation migration in nanophosphors. [24][25][26] The decrease in up-conversion fluorescence is also attributed to the organic ligand on the surfaces, which quenched the ions for fluorescence. [18,21] The as-synthesized hexagonal nanocrystals in this work showed a high sensitivity of up-conversion fluorescence.…”

Energy transfer efficiency in YF3 nanocrystals: Quantifying the Yb3+ to Tm3+ infrared dynamics

“…For this reason, different criteria to quantify the lifetimes of nanoparticles can be found in the literature. 24,34,[37][38][39][40] Here, in order to facilitate the comparison to the data previously published for YF 3 crystals, we shall adopt the criteria used by Ostermayer et al 27 and quantify Yb 3þ lifetimes using the time to decay to I/e, where I is Figure 4 as function of Tm 3þ concentration. According to the figure, the results follow the expected linear dependence predicted by Eq.…”

“…[19][20][21][22][23][24] These studies have improved the knowledge of the phenomenology involved in the UC emissions, but very little attention has been devoted to the luminescence dynamics and to the quantification of the physical magnitudes that describe energy transfer in the nanocrystals. In particular, energy transfer parameters, which represent the probability of the processes, have not been determined yet, although they would provide a deeper comprehension of the phenomena and would establish whether there are important spectroscopic differences in comparison to the bulk material.…”

Nanophosphor Coatings: Technology and Applications, Opportunities and Challenges

“…On the other hand, due to the inherent limitation of all combustion synthesis methods, FAD/FSP is best suited for the production of oxides. Nevertheless, certain important non-oxide phosphors can still be obtained by appropriate post-treatment (Y 2 O 2 S:Er, Chen et al, 2003;Liu et al, 2002), flame pyrolysis in controlled inert atmosphere (ZnS:Mn, Athanassiou et al, 2010), or by simply using an appropriate halide source (NaYF:RE, Stepuk et al, 2013). The size of produced nanoparticles is controlled by parameters of the flame and, if needed, can be further decreased by charging them during the synthesis with a corona discharge (Vemury and Pratsinis, 1995).…”