Atomic-Scale Structural Analysis of Homoepitaxial LaF₃:Yb,Tm Core–Shell Upconversion Nanoparticles Synthesized through a Microwave Route

Abstract

Core−shell upconversion nanoparticles (UCNPs) have been intensely studied and are anticipated to affect fields including solar cells and imaging, sensing, and biomedical applications where both optical enhancement and tuning are of great importance. In the case of homoepitaxial core−shell designs, a lack of understanding of the shell's dopant concentration effect on crystal growth is due to difficulties in distinguishing atomistically between the core and shell using experimental approaches.Here we demonstrate the critical role of the homoepitaxial shell's dopant composition on structural and morphological properties which are crucial for upconversion activity. Yb 3+ ion doped (active) and undoped (inert) LaF 3 shell designs are grown on a 20% Yb, 2% Tm codoped hexagonal phase LaF 3 core through a microwaveassisted synthesis route. We further analyzed the crystal ordering through X-ray diffraction analysis and atomic-scale imaging at the core−shell interface with high-angle annular dark field and annular bright field modes of Cs-corrected STEM. The inert shell showed greater optical enhancement than the active shell at all major emission peaks, with the greatest enhancement factors of 13.5 and 7.0 at blue and red emissions, respectively. The tensile strain (+0.23%) of the core is eliminated by the inert shell, which resulted in a −0.05% compressive strain as well as enlargement of the unit cell, whereas the active shell resulted in a tensile strain of +0.53% with a slight increase in unit cell parameters. FFT images are analyzed for further effects, including lattice distortion and directionality of shell growth. The inert shell yields a multifaceted encapsulation resulting in a round morphology with notable distortions, while the active shell with minor distortions has an elongated morphology with sharper edges and preferential growth in the [001] direction. Our work provides direct evidence from the core−shell interface of homoepitaxial upconversion nanoparticles of doped and undoped shell composition and related structural properties.

Nanocomposites based on lanthanide-doped upconversion nanoparticles: diverse designs and applications

“…Growing homogeneous shell on UCNPs through epitaxial growth is generally considered to be an effective approach to reduce the original nanocrystal surface defect density, thereby improving the UCL efficiency. Various core–shell UCNPs have been constructed, such as inert core–shell structures (NaYF 4 :Yb/Ln@NaYF 4 (Ln = Er, Tm) 50 – 52 , NaGdF 4 :Yb/Tm@NaGdF 4 53 , 54 , LaF 3 :Yb/Tm@LaF 3 55 ) or active core–shell structures (NaYF 4 :Yb/Tm@NaYF 4 :Yb/Er 56 , NaGdF 4 :Yb/Er@NaGdF 4 :Yb 57 , NaGdF 4 :Yb/Tm@NaGdF 4 :Eu 58 , and BaGdF 5 :Yb/Er@BaGdF 5 :Yb 59 ). The heterogeneous core–shell structure refers to the different host lattices between the core and shell, which has been proven to be an effective strategy to form hybrid nanostructures 60 – 69 .…”

Doping of Semiconductors at Nanoscale with Microwave Heating (Overview)

“…Lanthanum trifluoride (LaF 3 ) is an ionic compound that is utilized as core-shellup conversion nanoparticles (UCNPs) for different filed of applications like sensing, biomedical, and solar cells. Tek et al reported Yb 3+ ion-doped (active) and undoped (inert) LaF 3 shell coatings on a 20% Yb, 2% Tm codoped hexagonal phase LaF 3 core with the help of microwave -assisted synthesis route [58]. They observed higher optical enhancement of inert shell compared with the active shell at all prominent emission peaks, which is explained with the energy band diagram indicating the energy transfer pathways for the Yb 3+ and Tm 3+ − co-doping (Figure 8).…”

“…(a) UCPL data for core, active shell, and inert shell nanoparticles under 980 nm continuous-wave excitation. (b) Energy diagram showing the corresponding transition of UCPL of (a) where the energy transfer pathways for the Yb3+ and Tm 3+ -codoped up conversion nanoparticles are depicted[58].…”

Photon upconversion of Nd3+–Yb3+–Tb3+ doped core–shell–shell–shell nanoparticles

“…Researchers have tried to utilize different strategies such as changing the host lattice, energy transfer, surface passivation, surface plasmon coupling, and photonic crystals to modulate rare-earth luminescence. 17–27 For example, Yan et al built a NaGdF 4 :Yb/Er@NaGdF 4 :Nd/Yb core–shell structure, in which Er 3+ and Nd 3+ were doped separately in the core and shell, and the energy back transfer process can be inhibited through spatial separation. 28 Furthermore, Yao et al synthesized sandwich-structured nanoparticles (NPs) including a NaYF 4 :Yb quenching-shield layer between the activator core and Nd 3+ doped shell.…”