Energy Loss Mechanism of Upconversion Core/Shell Nanocrystals

Abstract

Small-sized upconversion nanocrystals (<10 nm) show a quite low luminescence efficiency. Even if these nanocrystals are coated by a 2 nm thick inert shell, the core/ shell nanocrystals still exhibit weak upconversion luminescence. The involved energy loss mechanism is under debate. Here, we have demonstrated that the major contribution to the low upconversion efficiency is ascribed to an overtone vibrational energy transfer from electronic transition of the Yb 3+ excited state to overtone transitions of deactivating group vibrations by dipole−dipole coupling. The maximum coupling distance reaches ∼11 nm. Moreover, we first find that an ultrathick inert shell (>11 nm) is not beneficial for upconversion luminescence due to a strong scattering effect. A novel lifetime model is proposed to precisely describe the decay times of the Yb 3+ 2 F 5/2 state and Er 3+ 4 S 3/2 state as a function of inert-shell thickness. Based on an insight into the luminescence loss, we design β-Na91%YbF 4 :9%Er@NaGdF 4 nanocrystals with a 11.2 nm inert shell to completely block surface quenching and achieve more efficient and stronger upconversion emission.

Thermal enhancement of upconversion emission in nanocrystals: a comprehensive summary

“…Moreover, the characteristic OH stretching vibration band was clearly observed in the FT-IR spectrum of sample even after heating treatment at 570 K. In the same year, Shao's group gave an additional contribution to this discussion investigating the UCNPs luminescence loss mechanisms. 57 A series of core-shell Yb 3+ /Er 3+ doped β-NaGdF4 NPs with an identical core diameter (5.7 nm) and different shell thicknesses (from 1.1 to 17.7 nm) were studied. The thinnest shell coating co-doped UCNPs were used to record the UCL intensity at elevated temperatures in several atmospheres (air, Ar, Ar/H2O, and Ar/D2O).…”

“…15c), the UCL quenching of Yb 3+ /Er 3+ co-doped sample was efficiently suppressed with about 6 nm thick inert-shell coating, while Shao et al believed that a far thicker inert-shell (~11 nm) was required. 57 (Fig. 15d) The validity of these statements is relative depending on specific sample treatments, experimental conditions, and data analysis criteria.…”

Volume and surface effects on two-photonic and three-photonic processes in dry co-doped upconversion nanocrystals

“…To identify differences in the population dynamics we focused on the Yb 3+ emission and the green and red Er 3+ emission. The emission intensity and lifetime of the Yb 3+ energy level 2 F 5/2 , which can be excited at wavelengths from about 900 to 1,200 nm, are known to be sensitive to surface quenching and particularly to quenching by high energy vibrators like -OH or by MPR [28,32]. The green ( 4 S 3/2 ) and the red ( 4 F 9/2 ) Er 3+ emission can be quenched by efficient CR at elevated Er 3+ concentrations (concentration quenching) [26] and the 4 F 9/2 level can be populated via backenergy transfer (BET) from Er 3+ to Yb 3+ [59].…”

“…Despite considerable advances in the synthesis of high quality core/shell nanoparticles (NPs), minimizing the incorporation of UCL-quenching high energy vibrators like OH groups into UCNC cores during particle synthesis and protecting emissive lanthanide ions from surface quenching by sufficiently thick surface shielding shells [26][27][28][29][30], the UC efficiencies of nanocrystals nevertheless remain still lower than those of the respective bulk materials. Such quenching effects, which can also affect the spectral distribution of UCL, are particularly pronounced for very small and ultrasmall UCNPs with sizes ≤ 10 nm or even ≤ 5 nm, in water, and at low P [31].…”

Thermal-responsive multicolor emission of single NaGdF4:Yb/Ce/Ho upconversion nanocrystals for anticounterfeiting application

“…For NaGdF 4 :Yb/Ce/Ho NCs with the size of ∼11 nm, a large number of H 2 O molecules in air can be adsorbed on their surface at room temperature due to a large surface-tovolume ratio. The Yb 3+ excited state energies are directly quenched by an overtone energy transfer from an electric dipole of Yb 3+ ion to an atomic dipole of H 2 O molecule [31]. Only a small amount of Yb 3+ excited state energy is transferred to the nearby Ho 3+ ion, so the UCL is weak at room temperature.…”