Light Conversion upon Photoexcitation of NaBiF4:Yb3+/Ho3+/Ce3+ Nanocrystalline Particles

NaBiF₄ nanocrystalline particles were synthesized by means of a facile precipitation synthesis route to explore upconversion emission properties when doped with lanthanide ions. In particular, the incorporation of the Yb³⁺-Ho³⁺-Ce³⁺ triad with controlled ion concentration facilitates near-IR pumping conversion into visible light, with the possibility of color emission tuning depending on Ce³⁺ doping amount. We observed that introducing a Ce³⁺ content up to 20 at.% in NaBiF₄:Yb³⁺/Ho³⁺, the chromaticity progressively turns from green for the Ce³⁺ undoped system to red. This is due to cross-relaxation mechanisms between Ho³⁺ and Ce³⁺ ions that influence the relative efficiency of the overall upconversion pathways, as discussed on the basis of a theoretical rate equation model. Furthermore, experimental results suggest that the photoexcitation of intra-4f Ho³⁺ transitions with light near the UV-visible edge can promote downconverted Yb³⁺ near-IR emission through quantum cutting triggered by Ho³⁺-Yb³⁺ energy transfer mechanisms. The present study evidences the potentiality of the developed NaBiF₄ particles for applications that exploit lanthanide-based light frequency conversion and multicolor emission tuning.

Time-Gated Imaging of Latent Fingerprints with Level 3 Details Achieved by Persistent Luminescent Fluoride Nanoparticles

The discovery of X-ray-charged persistent luminescence (PersL) in fluoride nanoparticles enables these materials to emit photons without real-time excitation, which provides a great possibility for the development of new luminescent nanotechnologies. In this work, we developed NaLuF₄:Mn nanoparticles with intense green PersL and functionalized surfaces and accordingly achieved time-gated imaging of latent fingerprints (LFPs) with Level 3 details. These surface-modified NaLuF₄:Mn nanoparticles exhibited near-spherical morphology, long-lasting emission for several hours, appropriate trap depth distribution, and tight chemical bonding with amino acids from fingerprints, thus greatly improving the accuracy of LFP imaging in a variety of environments. The developed NaLuF₄:Mn PersL nanoparticles are expected to find broad applications in the fields of LFP imaging and in vivo biological imaging.

Revisiting the Enhanced Red Upconversion Emission from a Single β-NaYF4:Yb/Er Microcrystal By Doping with Mn2+ Ions

The presence of manganese ions (Mn2+) in Yb/Er-co-doped nanomaterials results in suppressing green (545 nm) and enhancing red (650 nm) upconversion (UC) emission, which can achieve single-red-band emission to enable applications in bioimaging and drug delivery. Here, we revisit the tunable multicolor UC emission in a single Mn2+-doped β-NaYF4:Yb/Er microcrystal which is synthesized by a simple one-pot hydrothermal method. Excited by a 980 nm continuous wave (CW) laser, the color of the single β-NaYF4:Yb/Er/Mn microrod can be tuned from green to red as the doping Mn2+ ions increase from 0 to 30 mol%. Notably, under a relatively high excitation intensity, a newly emerged emission band at 560 nm (2H9/2 → 4I13/2) becomes significant and further exceeds the traditional green (545 nm) emission. Therefore, the red-to-green (R/G) emission intensity ratio is subdivided into traditional (650 to 545 nm) and new (650 to 560 nm) R/G ones. As the doped Mn2+ ions increase, these two R/G ratios are in lockstep with the same tunable trends at low excitation intensity, but the tunable regions become different at high excitation intensity. Moreover, we demonstrate that the energy transfer (ET) between Mn2+ and Er3+ contributes to the adjustment of R/G ratio and leads to tunable multicolor of the single microrod. The spectroscopic properties and tunable color from the single microrod can be potentially utilized in color display and micro-optoelectronic devices.

Upconversion Composite Nanoparticles for Tumor Hypoxia Modulation and Enhanced Near-Infrared-Triggered Photodynamic Therapy

The efficacy of the conventional photodynamic therapy (PDT) is markedly suppressed by limited penetration depth of light in biological tissues and oxygen depletion in the hypoxic tumor microenvironment. Herein, mesoporous silica nanospheres with fine CaF₂:Yb,Er upconversion nanocrystals entrapped in their porous structure are synthesized via a thermal decomposition method. After subsequently coating with a thin MnO₂ layer and loading with a photosensitizer, Chlorin e6 (Ce6), a new type of nanoscale PDT platform is obtained. Within such composite nanoparticles, Mn²⁺ ions doped into the lattice of CaF₂ crystals effectively enhance the near-infrared (NIR)-triggered red-light upconversion photoluminescence for exciting the adsorbed Ce6 via resonance energy transfer, enabling the improved photodynamic phenomenon. Meanwhile, the MnO₂ coating modulates the hypoxic tumor microenvironment by in situ generating O₂ through the reaction with tumor endogenous H₂O₂. Both mechanisms acting synchronously lead to the superior therapeutic outcome in NIR-triggered photodynamic tumor therapy.

Theranostic system based on NaY(Mn)F4:Yb/Er upconversion nanoparticles with multi-drug resistance reversing ability

An innovative theranostic system (D-UNT) for MDR tumors diagnosis and therapy based on the red emitter NaY(Mn)F₄:Yb/Er with optimized luminescence was developed.

A facile methodology for regulating the size of hexagonal NaYF4:Yb3+,Er3+ upconversion nanocrystals

The novelty of the work: size adjustment of hexagonal NaYF₄:Yb³⁺,Er³⁺ upconversion nanocrystals by varying only the ratio of oleic acid to rare-earth acetates.

Facile synthesis of ultrasmall hexagonal NaYF4:Yb3+,Er3+ upconversion nanocrystals through temperature oscillation

A simple and efficient temperature oscillation methodology has been successfully developed to synthesize ultrasmall (10.8 nm) and uniform hexagonal NaYF₄:Yb³⁺,Er³⁺ upconversion nanocrystals.

Transition Metal-Involved Photon Upconversion

Upconversion (UC) luminescence of lanthanide ions (Ln³⁺) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln³⁺, transition metal (TM) ions, e.g., Mn²⁺, usually possess a single broadband emission due to its 3d⁵ electronic configuration. Wavelength-tuneable single UC emission can be achieved in some TM ion-activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln³⁺ can be modulated by TM ions (specifically d-block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln³⁺ owing to the well-shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d⁰ ion-containing hosts (d⁰ normally viewed as charged anion groups, such as MoO₆^6- and TiO₄^4-) may also have a strong influence on the electric dipole transition of Ln³⁺, resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide-ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln³⁺ tuned by TM or d⁰ ions, and the UC of d⁰ ion-centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.

Understanding the effect of Mn2+ on Yb3+/Er3+ upconversion and obtaining a maximum upconversion fluorescence enhancement in inert-core/active-shell/inert-shell structures

NaYF₄@NaYF₄:Er³⁺/Yb³⁺/Mn²⁺@NaYF₄ (C/S_d/S) nanoparticles were synthesized which show an obvious efficiency enhancement of red upconversion emission.

Enhancing Solar Cell Efficiency Using Photon Upconversion Materials

Photovoltaic cells are able to convert sunlight into electricity, providing enough of the most abundant and cleanest energy to cover our energy needs. However, the efficiency of current photovoltaics is significantly impeded by the transmission loss of sub-band-gap photons. Photon upconversion is a promising route to circumvent this problem by converting these transmitted sub-band-gap photons into above-band-gap light, where solar cells typically have high quantum efficiency. Here, we summarize recent progress on varying types of efficient upconversion materials as well as their outstanding uses in a series of solar cells, including silicon solar cells (crystalline and amorphous), gallium arsenide (GaAs) solar cells, dye-sensitized solar cells, and other types of solar cells. The challenge and prospect of upconversion materials for photovoltaic applications are also discussed.

Synthesis of a novel bifunctional nanocomposite with tunable upconversion emission and magnetic properties

A method of Co²⁺ ions codoping for significantly enhancing upconversion emission intensity and simultaneous controlling sparamagnetic properties in β-NaYF₄:Yb/Er nanoparticles, with well maintaining their morphology and highly disperse.