Insight into the Luminescence Alternation of Sub‐30 nm Upconversion Nanoparticles with a Small NaHoF₄ Core and Multi‐Gd³⁺/Yb³⁺ Coexisting Shells

Abstract

With narrow luminescence peak width, reduced photobleaching, high physical and chemical stability, low toxicity, etc., [4-8] RE-doped UCNPs have been widely utilized in all sorts of fields, [9-17] especially in photoinduced therapies. [18-24] Among these UCNPs, activator (usually Er 3+ , Tm 3+ , Ho 3+) or sensitizer (usually Yb 3+) highly doped ones, provided with unique optical properties like exceptional brightness, single-band emission, lifetime tuning, and so on, [25] which are of great bioapplication potentials, have become a hot research focus recently. [25-33] However, progress has been severely limited as once the activator or sensitizer concentration goes beyond certain values, upconversion luminescence (UCL) intensities would be substantially weakened. [34-38] Take Tm 3+-activated UCL for example. As reported, the UCL intensity of blue emission fell straightly downward to less than 5% when Tm 3+ concentration rose from 2 to merely 10 mol%. [39] Energy loss pathways could easily occur under highly doped circumstances, mainly including acute cross-relaxation (CR) processes within activators, energy-back-transfer (EBT) processes between activators and sensitizers, as well as resonant energy transfer to lattice and surface defects. [40-44] Therefore, probing into these highly doped nanoparticles for the sake of effective methods to improve and finetune their luminescence performances would be urgently required for deeper theory study and wider application use. Holmium (Ho) is a commonly adopted RE element emitting both red and green characteristic upconversion peaks in visible waveband under 980 nm laser excitation, with crest values approximately located at 540 and 645 nm, respectively. [45-47] Similar to Er 3+ , an overall bright green UCL could be obtained in low activator and sensitizer doping concentration. [48-50] To reduce the green/red (G/R) ratio for purer red emission, Ce 3+ is normally employed for its CR processes with Ho 3+ , which could increase the population of excited states 5 I 7 and 5 F 5 of Ho 3+ to promote the red emission. [51-57] In addition to this, other techniques involving high excitation power [58,59] or unwonted types of laser with specific parameters [60,61] could also implement high purity of red UCL, which actually gets unsuitable or too complicated for biouse. In this sense, exploring more targeted strategies would bring out more flexibility for Hobased UCL materials to distinguish themselves. It is absolutely imperative for development of material science to adjust upconversion luminescence (UCL) properties of highly doped upconversion nanoparticles (UCNPs) with special optical properties and prominent application prospects. In this work, featuring NaHoF 4 @NaYbF 4 (Ho@Yb) structures, sub-30 nm core-multishell UCNPs are synthesized with a small NaHoF 4 core and varied Gd 3+ /Yb 3+ coexisting shells. X-ray diffraction, transmission electron microscopy, UCL spectrum, UCL lifetime, and pump power dependence are adhibited for characterization. Compared with the former work, exc...

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