Effect of surface defects on optical properties of upconverting nanoparticles

Since up-converting nanoparticles (UCNPs) with core-shell structure can significantly enhance photoluminescence efficiency, they have good application prospects in optical imaging to guide bioimaging, therapeutics, anti-counterfeiting and solar cells. It is generally the outer coating that eliminates the quenching point and separates the core from the surrounding deactivator (ligand, solvent), thereby effectively inhibiting surface-related deactivation. Studies have shown that the surface capture of doped ions can suppress the quenching of excitation energy and can be suppressed by the core-shell structure.

[Introduction]

Recently, Professor Qiu Jianbei from Kunming University of Science and Technology in China and Professor Yu Siu Fung from Hong Kong Polytechnic University (co-author) used a wet chemical annealing process to restore lanthanide doping from surface defects (ie, disorder, vacancy and gap defects). KLu ₂ F ₇ bare core UCNPs. The prepared UCNPs have a uniform thickness of only a few atomic layers, and the surface defects are identified on the atomic scale by aberration-corrected high-angle circular dark field scanning transmission electron microscopy (HAADF-STEM). The thermal annealing method is used to recover the surface defects of UCNPs, and the corresponding up-conversion photoluminescence intensity above one order of magnitude is improved, which has a good potential application prospect. The research results were published in the internationally renowned journal ACS Nano under the title "Direct Identification of Surface Defects and their Influence on the Optical Characteristics of Upconn Nanoparticles".

Figure 1. Preparation and TEM characterization of UCNPs

(a) Schematic diagram of the wet chemical annealing process of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs;

(b) TEM images of synthetic UCNPs;

(c) thermal annealing at 240 ° C to synthesize UCNPs;

(d) HRTEM map in (b);

(e) FFT contour;

(f) a size distribution map of UCNPs as they grow;

(g) HRTEM map in (c);

(h) FFT contour;

(i) Size distribution of annealed UCNPs after growth.

Figure 2. SEM characterization of UCNPs

(a) HAADF-STEM image of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs;

(b) HAADF-STEM image of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs after thermal annealing at 240 °C;

(c) the intensity profile recorded in the orange-direction scan in (a);

(d) the intensity profile recorded in the green direction scan in (b);

(e) an enlarged crystal edge structure image in (a);

(f) An enlarged crystal edge structure image in (b).

Figure 3. Conversion attenuation and energy transfer mechanism of UCNPs

(a) Upconversion photoluminescence spectra of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs;

(b) Conversion curves of ⁴ S _3/2 - ⁴ I _15/2 in UCNPs at 543 nm;

(c) Conversion curves of ⁴ F _9/2 - ⁴ I _15/2 in UCNPs at 668 nm;

(d) Conversion curves of ² F _5/2 - ² F _7/2 at 980 nm in UCNPs;

(e) Simplified energy transfer diagram of UCNPs excited by 980 nm continuous wavelength (CW) laser.

Figure 4. Energy transfer and emission spectra of UCNPs

(a) Energy transfer of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs under 980 nm short pulse and long pulse excitation;

(b) Up-conversion emission spectra of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs before thermal annealing;

(c) Upconversion emission spectra of KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs after thermal annealing.

Figure 5. Microcavity laser changes in UCNPs

(a) KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs obtained micro-cavity laser spectroscopy at 980 nm laser excitation at room temperature;

(b) Output power and emission linewidth and excitation power diagrams;

(c) P _th , Δλ and 1/ D are based on the green emission measurements of the microcavity laser.

【summary】

The HAADF-STEM was used to directly observe the aggregation structure of thin uniform thickness KLu ₂ F ₇ : 38% Yb ³⁺ , 2% Er ³⁺ UCNPs. Using the wet chemical reaction method and the arrangement of cerium ions in the structure, the activator found on the crystallization site of Lu ³⁺ is clearly demonstrated without causing undesirable defects. The prepared KLu ₂ F ₇ : 38% Yb3+, 2% Er3+ UCNPs increased the up-conversion photoluminescence intensity by more than one order of magnitude under 980 nm continuous wave laser excitation. Surface thermal annealing process is a feasible method to overcome surface defects when preparing UCNPs.