Phonon density of states in lanthanide-based nanocrystals

Abstract

We report a combined inelastic neutron and X-ray scattering study of the phonon density of states of the nano-and microcrystalline lanthanide-based materials NaY0.8Yb0.18Er0.02F4 and NaGd0.8Yb0.18Er0.02F4. While large (20 nm) nanocrystals display the same vibrational spectra as their microcrystalline counterparts, we find an enhanced phonon density of states at low energies, E ≤ 15 meV, in ultra-small (5 nm) NaGd0.8Yb0.18Er0.02F4 nanocrystals which we assign to an increased relative spectral weight of surface phonon modes. Based on our observations for ultra-small nanocrystals, we rationalize that an increase of the phonon density of states in large nanocrystals due to surface phonons is too small to be observed in the current measurements. The experimental approach described in this report constitutes the first step toward the rationalization of size effects on the modification of the absolute upconversion quantum yield of upconverting nanocrystals.

Phonon-Mediated Nonradiative Relaxation in Ln3+-Doped Luminescent Nanocrystals

“…Due to the high SVR of nanocrystals, a lattice expansion is generally observed. For instance, a 1.3% increase in the unit cell volume was observed 64 for the 5-nm-sized NaGdF 4 nanocrystals in comparison to the microcrystalline counterparts. This expansion 3+ demonstrates the direct binding between Eu 3+ and oxygenated ligands on the particle surface, which validates the surface site occupation of Eu 3+ .…”

“…Due to the high SVR of nanocrystals, a lattice expansion is generally observed. For instance, a 1.3% increase in the unit cell volume was observed for the 5-nm-sized NaGdF 4 nanocrystals in comparison to the microcrystalline counterparts. This expansion inevitably causes the elongation of interatomic bond distance in the structure.…”

“…63,213,214 The nature of these dipoles is not yet fully understood. 64 They likely originate from surface defects such as disordered domains and dangling bonds on the surface of nanocrystals. 215,216 Attention needs to be paid on other aspects.…”

“…The influences of additional factors need to be considered. It is reported that there are other forms of vibrations occurring on the surface of Ln 3+ -doped nanocrystals, which can be treated as electric dipoles (Figure c), and these surface-related electric dipoles can trigger the nonradiative relaxation in Ln 3+ -doped nanocrystals as well. ,, The nature of these dipoles is not yet fully understood . They likely originate from surface defects such as disordered domains and dangling bonds on the surface of nanocrystals. , …”

“…None of these findings can be described by the empirical “energy gap law”, raising the question of whether the current knowledge of the nonradiative relaxation of Ln 3+ is adequate. The dilemma is clearly compounded by the decrease in material size, and the emergence of high-frequency vibration modes originating from the surface of nanocrystals makes the analysis challenging. − …”