The Role of a Phonon Bottleneck in Relaxation Processes for Ln-Doped NaYF₄ Nanocrystals

Abstract

The localized inner 4f shell transitions of lanthanide ions are largely independent of the local surroundings. The luminescence properties of Ln3+ ions doped into nanocrystals (NCs) are therefore similar to those in bulk crystals. Quantum size effects, responsible for the unique size-dependent luminescence of semiconductor NCs, are generally assumed not to influence the optical properties of Ln3+-doped insulator NCs. However, phonon confinement effects have been reported to hamper relaxation between closely spaced Stark levels in Ln3+-doped NCs. At cryogenic temperatures emission and excitation from higher Stark levels was observed for Ln3+ ions in NCs only and were explained by a cutoff in the acoustic phonon spectrum. Relaxation would be inhibited as no resonant low energy (long wavelength) acoustic phonon modes can exist in nanometer sized crystals, and this prevents relaxation by direct phonon emission between closely spaced Stark levels. This phenomenon is known as a phonon bottleneck. Here, we investigate the role of phonon confinement in Ln-doped NCs. High resolution emission spectra at temperatures down to 2.2 K are reported for various Ln3+ ions (Er3+, Yb3+, Eu3+) doped into monodisperse 10 nm NaYF4 NCs and compared with spectra for bulk (microcrystalline) material. Contrary to previous reports, we find no evidence for phonon bottleneck effects in the emission spectra. Emission from closely spaced higher Stark levels is observed only at high excitation powers and is explained by laser heating. The present results indicate that previously reported effects in NCs may not be caused by phonon confinement.

Apparent self-heating of individual upconverting nanoparticle thermometers

“…We note that others have attributed related changes in the spectra 63 or luminescence lifetimes 64 of Yb 3+ /Er 3+ co-doped materials to a broad class of effects involving impeded thermalization of a large non-equilibrium phonon distribution, frequently referred to as a “phonon bottleneck”. Yet the interpretation of such results remains far from fully settled, with recent studies 65 disputing some of the earlier works and instead attributing the results to nanoparticle heating. Such effects are also expected to occur primarily at cryogenic temperatures and for nanoparticles dimensions of ~10 nm or smaller, and we thus believe that such effects are unlikely to play a large role in our measurements.…”

Thermal enhancement of upconversion emission in nanocrystals: a comprehensive summary

“…Recently, the role of the phonon confinement effect in controlling the relaxation between close-spaced Ln 3+ Stark levels in ultra-small UCNPs was revisited. 48 The UCL intensities of Tm 3+ were also recorded at elevated temperatures in core-shell Yb 3+ /Tm 3+ codoped NPs, co-doped NPs after annealing at 773 K and codoped μm-sized rods. As temperature increased, the thermal quenching of UCL was observed in all the samples, which foreboded that surface phonon was only created by direct chelating between Yb 3+ and O 2of the surface ligand.…”

Inflection in size-dependence of thermally enhanced up-conversion luminescence of UCNPs

“…4 On the one hand, the spectra profiles of UCNP upconversion luminescence remain the same as that of bulk counterparts since the electronic energy level structure of the localized 4f N configuration of lanthanide ions can hardly be affected by quantum confinement in the insulating nanocrystals. 5 On the other hand, UCNPs suffer from the significant size-dependent luminescence efficiency decrease, 6 which compromises the functionality of UCNPs. Fortunately, the unique anomalous luminescence thermal enhancement recently discovered in UCNPs has been found to be size-dependent as well, 7–9 which initials great opportunities for applications such as optical thermal sensing and anti-counterfeiting.…”