Full shell coating or cation exchange enhances luminescence

Abstract

Core-shell structure is routinely used for enhancing luminescence of optical nanoparticles, where the luminescent core is passivated by an inert shell. It has been intuitively accepted that the luminescence would gradually enhance with the coverage of inert shell. Here we report an "off-on" effect at the interface of core-shell upconversion nanoparticles, i.e., regardless of the shell coverage, the luminescence is not much enhanced unless the core is completely encapsulated. This effect indicates that full shell coating on the luminescent core is critical to significantly enhance luminescence, which is usually neglected. Inspired by this observation, a cation exchange approach is used to block the energy transfer between core nanoparticle and surface quenchers. We find that the luminescent core exhibits enhanced luminescence after cation exchange creates an effective shell region. These findings are believed to provide a better understanding of the interfacial energy dynamics and subsequent luminescence changes.

Fig. 1. Synthesis and spectroscopic characterization of core-shell structured NaErF₄/SiO₂@NaYF₄ Janus nanoparticles with controllable coverage ratio of NaYF₄ inert shell.

a Schematic showing the synthesis process of core-shell structured NaErF₄/SiO₂@NaYF₄ Janus nanoparticles. The surface of NaErF₄ UCNP (blue) was anisotropically coated by a hydrophilic SiO₂ shell (gray) via hydrolysis of TEOS (tetraethyl orthosilicate). After etching treatment by BOE (buffered oxide etchant), part of the SiO₂ shell was removed and the exposed NaErF₄ surface was used for the growth of hydrophobic NaYF₄ shell (yellow). b–e TEM image of NaErF₄ (0% protection), core-shell structured NaErF₄/SiO₂@25%NaYF₄ (25% protection), NaErF₄/SiO₂@75%NaYF₄ (75% protection), and NaErF₄@NaYF₄ (100%protection). Scale bar: 20 nm. f The upconversion emission spectra core-shell structured NaErF₄/SiO₂@NaYF₄ Janus nanoparticles with 0% protection, 25% protection, 75% protection and 100% protection upon excited by 980 nm laser light (3 W cm⁻²). g Comparison of upconversion emission spectra core-shell structured NaErF₄/SiO₂@NaYF₄ Janus nanoparticles with 0% protection, 25% protection, 75% protection and 100% protection in the spectral range from 500 nm to 700 nm. h, i Decay curve at 542 nm, and 658 nm of core-shell structured NaErF₄/SiO₂@NaYF₄ Janus nanoparticles with 0% protection, 25% protection, 75% protection and 100% protection. j Schematic showing the change at composition, luminescence and energy quenching pathways of core-shell structured NaErF₄/SiO₂@NaYF₄ Janus nanoparticles with an increased NaYF₄ shell protection.

Fig. 2. Time-course study on the growth of core-shell NaErF₄@NaYF₄ UCNPs.

a Schematic of the growing process of full NaYF₄shell (yellow) on the NaErF₄ core (blue). During the full shell coating process, the NaYF₄ shell would grow along both sides (red arrow) of the luminescent NaErF₄ core. b The changes of upconversion emission spectra under 980 nm laser light (3 W cm⁻²) over time. Inset is the variations of green to red ratio over time. c The lifetime of red and green emissions over time. d The mechanism of the surface protection of NaErF₄ core by NaYF₄ shell with different thickness. Upon excitation by 980 nm light, the excitation energy is rapidly transferred to the surface quenchers for bare NaErF₄ core. When a thin NaYF₄ shell (shaded region) is coated, the surface defects are partly restored that preferentially promotes the green emission. As the thickness of NaYF₄ shell goes up, the energy loss from Er³⁺ ions to surface defects is further blocked and the possibility of cross relaxation between Er³⁺ ions is greatly raised, which dramatically enhances the red emission.

Fig. 3. Structural characterization of NaErF₄ nanoparticles before and after ion exchange.

a Schematic of the NaErF4 core after ion exchange. The Y³⁺ ions (yellow) would replace some Er³⁺ ions (blue) on the surface and block the energy transfer to the quenching site. b X-ray diffraction (XRD) pattern of the NaErF₄ nanoparticles, obtained before and after exchange with Y³⁺ ions (abbreviated as NaErF₄@Y). c, d TEM image of the NaErF₄ nanoparticles and NaErF₄@Y nanoparticles, respectively. Scale bar: 50 nm. e, f The EDS line-scan profile of NaErF₄ and NaErF4@Y nanoparticles, respectively. g HAADF-STEM (high angle annular dark field scanning TEM) of single NaErF₄@Y nanoparticle and its corresponding elemental mapping images, Scale bar: 10 nm. h, i Full survey XPS spectrum and detailed spectra (Na 1 s, Er 4d, F 1 s, and Y 3d) of NaErF₄@Y nanoparticles.

Fig. 4. Spectroscopic study of NaErF₄ before and after ion exchange.

a Schematic diagram of luminescence quenching caused by energy transfer to the surface. The excitation energy would migrate among Er ions (blue) and travel to the quenching site (dark) on the surface. b Proposed mechanism of luminescence enhancement after ion exchange. The Y³⁺ ions replaced some Er³⁺ ions on the surface and created an effective shell region (shaded area) such that the energy migration to the surface quenching site was blocked. c, d Upconversion emission spectra and corresponding luminescence photographs of the NaErF₄ and NaErF₄@Y nanoparticles upon 980 nm laser excitation (3 W cm⁻²). e, f Lifetime decay curve of Er³⁺ emission at 542 nm and 658 nm from the NaErF₄ and NaErF₄@Y nanoparticles upon 980 nm laser excitation (3 W cm⁻²).