Amplifying Upconversion by Engineering Interfacial Density of State in Sub-10 nm Colloidal Core/Shell Fluoride Nanoparticles

Spectral Analysis for Photon Emission of Rare-Earth Ions in Single Plasmonic Hot Spot

Rare earth ion luminescent materials have attracted extensive attention due to their wide applications in biochemical sensing and bioimaging. However, the low quantum yield and weak luminescence intensity have restricted their further development. Realizing the modulation of rare-earth ions' emission behavior has become a hot topic in the interdisciplinary fields of materials and chemometrics. Herein, the regulation of the electron decay process and emission of photon signals of rare earth ion (Eu3+) has been achieved in a well-defined plasmonic nanocavity. This nanocavity consists of Ag shell-isolated nanoparticles (SHINs) and an ultraflat Au film, which are separated by a polymer dielectric spacer and CaF2:Eu3+ nanoparticles. Contrary to the intrinsic photoluminescence of CaF2:Eu3+, a factor of 408 increase in the spontaneous emission rate and simultaneously an 800-fold enhancement in the emission intensity have been realized in nanocavities via comprehensive spectroscopic analysis. Additionally, the evolution law between the plasmon resonances and the luminescent enhancement as well as the emission spectrum of Eu3+ indicates a highly effective modulation of emission behavior by plasmons. This presents a novel strategy for enhancing the performance of optical micro- and nanodevices based on rare-earth ion materials, demonstrating significant potential in applications such as bioimaging and surface detection analysis.

Flexible X-ray Imaging and Stable Information Storage of SrF2:Eu Based on Radio-Photoluminescence

X-ray imaging has garnered widespread interest in biomedical diagnosis and nondestructive detection. The exploration of radio-photoluminescence has hastened the advancement of X-ray information storage. However, significant challenges persist in achieving the prolonged imaging of curved objects without attenuation. Here, europium-doped strontium fluoride (SrF2:Eu) is meticulously created to exhibit a linear response to an extensive range of X-ray doses (maximum dose > 5000 Gy), showcasing excellent X-ray information reading/erasing reusability properties (10 cycles). This is accompanied by a red-to-blue emission transition under UV excitation, sustaining for 150 days without attenuation. To elucidate the phenomena of irradiated photoluminescent discoloration and the reversible X-ray storage of SrF2:Eu, we propose an electron-vacancy trap (valence conversion) mechanism, information stably retained by the SrF2:Eu-based device under ambient conditions due to high energy barriers. The time-lapse readout capability is further demonstrated for three-dimensional imaging of curved objects (10 lp mm-1) based on SrF2:Eu embedded within a polydimethylsiloxane (SrF2:Eu@PDMS). The SrF2:Eu demonstrates time-lapse imaging, reversible radio-photoluminescence, and recoverable X-ray storage, offering a promising avenue for optical information encryption and anticounterfeiting applications.

Understanding of Lanthanide-Doped Core-Shell Structure at the Nanoscale Level

The groundbreaking development of lanthanide-doped core-shell nanostructures have successfully achieved precise optical tuning of rare-earth nanocrystals, leading to significant improvements in energy transfer efficiency and facilitating multifunctional integration. Exploring the atomic-level structural, physical, and optical properties of rare-earth core-shell nanocrystals is essential for advancing our understanding of their fundamental principles and driving the development of emerging applications. However, our knowledge of the atomic-level structural details of rare-earth nanocrystal core-shell structures remains limited. This review provides a comprehensive discussion of synthesis strategies, characterization techniques, interfacial ion-mixing phenomena, strain effects, and spectral modulation in core-shell structures of rare-earth-doped nanocrystals. Additionally, we prospectively discuss the challenges encountered in studying the fine structures of rare-earth-doped core-shell nanocrystals, particularly the increasing demand for researchers to integrate interdisciplinary knowledge and utilize high-end precision instruments.

Excitation power-dependent multicolor upconversion in NaLnF4:Er3+ under 1532 nm irradiation for anti-counterfeiting application

Upconversion (UC) materials are renowned for their ability to convert low-energy photons into high-energy ones. The manipulation of parameters allows for the observation of multicolored UC luminescence (UCL) within a single material system. While modulation of multicolored UCL commonly relies on excitation at approximately 980 nm, investigation into multicolored UC materials activated by a 1532 nm excitation source remains comparatively scarce. In this work, we introduce NaLnF4:Er3+ as a novel class of smart luminescent materials. When the power density of a 1532 nm laser increases from 0.5 to 20.0 W/cm2, the emission peak positions remain unchanged, but the red-to-green (R/G) ratio decreases significantly from 18.82 to 1.48, inducing a color shift from red to yellow and ultimately to green. In contrast, no color variation is observed when NaLnF4:Er3+ is excited with a 980 nm laser at different power densities. This power-dependent multicolored UCL of NaLnF4:Er3+ excited at 1532 nm can be attributed to the competitive processes of upward pumping and downward relaxation of electrons on the 4I9/2 level of Er3+. By utilizing the unique UC characteristics of NaLnF4:Er3+, its potential utility in anti-counterfeiting applications is demonstrated. Our research highlights the distinctive optical properties of NaLnF4:Er3+ and provides novel insights into the use of luminescent materials in optical anti-counterfeiting technologies.

Amplifying Photon Upconversion in Alloyed Nanoparticles for a Near-Infrared Photodetector

Photon upconverison has attracted a substantial amount of interest in diverse fields due to its characteristic anti-Stokes emissions. However, obtaining intense emission under low-power laser irradiation has remained a challenge. Here we report a mechanistic design of activator-sensitizer alloyed nanoparticles to achieve bright upconversion under weak infrared irradiation. This design allows a nearest sensitizer-activator separation to facilitate efficient energy transfer that results in remarkably enhanced upconversion (>2 orders of magnitude) under 0.26 W cm-2 irradiation compared to that of the Er sublattice, and the upconversion quantum yield also shows a 20-fold increase. Interestingly, the alloyed nanoparticles exhibit a gradual change in emission color with an increase in Yb3+ content, and moreover, their emission colors can be dynamically controlled by simply modulating the excitation laser power and pulse widths. Such alloyed nanoparticles show great promise for application in a near-infrared photodetector.

Er3+/Tm3+ co-activated core@shell nanoarchitectures: tunable upconversion luminescence and high-security anti-counterfeiting

Currently, developing advanced optoelectronic materials is of great importance to solving serious problem of fake and shoddy products. Lanthanide-doped nanomaterials are particularly suitable for addressing this issue, but limited by the realization of multiple upconverison (UC) emissions upon a single-wavelength laser excitation. Herein, it is proven that the co-dopant of blue/near-infrared (NIR)-emitting activators (Tm3+) and green/red-emitting centers (Er3+) in a particular designed core-shell nanoarchitecture allows the achievement of multiple luminescence over wide spectral region for optical security. In our study, cubic-phased NaYbF4:Tm/Er@CaF2 nanocrystals have been successfully synthesized through a layer-by-layer coprecipitation strategy, which presents visible multicolor UC luminescence and invisible NIR UC emission upon 980 nm laser excitation by just regulating the laser power and temperature. Significantly, the unique luminescent characteristics enables the designed UC nanoparticles a promising candidate for advanced anti-counterfeiting. This works offers a reference to develop advanced optoelectronic materials for practical application in optical security.

Luminescence Temperature Sensing and First Principles Calculation of Photoelectric Properties in C12A7 Co-Doped Eu3+ Ions

Developing high-resolution, high-accuracy fluorescent thermometers is challenging. In this study, the optical properties and thermal sensing of Yb-, Tm-, and Eu-co-doped C12A7 (C12A7:Yb/Eu/Tm), with flower-like structure upconversion microparticles, were studied. Eu³⁺ doping induced an approximately 6-fold change in the upconversion luminescence (UCL) output in comparison with C12A7:Yb/Tm microparticles. The maximum relative temperature sensitivity (S) of C12A7:Yb/Eu/Tm reached 3.0% K^-1, representing an approximately 5-fold difference compared with the value of C12A7:Yb/Tm. In particular, the multicolor upconversion emission of C12A7:Yb/Eu/Tm can easily change from blue to white UCL with increasing temperature. Moreover, the band structure, total density, and optical coefficient of C12A7:Yb/Eu/Tm were investigated via density functional theory. The total density of O atoms increased in comparison with the total density of pure C12A7, indicating that substitution of Ca²⁺ by Yb/Eu/Tm produced positive vacancies on the cage structure. The optical coefficient of C12A7 was improved by the Yb/Eu/Tm dopant. The thermally regulated multicolor characteristics and thermally coupled energy levels of Tm³⁺ provide "dual adjustment temperature sensing", which is a promising strategy for realizing accurate and effective temperature sensors.

Manipulation of time-dependent multicolour evolution of X-ray excited afterglow in lanthanide-doped fluoride nanoparticles

External manipulation of emission colour is of significance for scientific research and applications, however, the general stimulus-responsive colour modulation method requires both stringent control of microstructures and continously adjustment of particular stimuli conditions. Here, we introduce pathways to manipulate the kinetics of time evolution of both intensity and spectral characteristics of X-ray excited afterglow (XEA) by regioselective doping of lanthanide activators in core-shell nanostructures. Our work reported here reveals the following phenomena: 1. The XEA intensities of multiple lanthanide activators are significantly enhanced via incorporating interstitial Na⁺ ions inside the nanocrystal structure. 2. The XEA intensities of activators exhibit diverse decay rates in the core and the shell and can largely be tuned separately, which enables us to realize a series of core@shell NPs featuring distinct time-dependent afterglow colour evolution. 3. A core/multi-shell NP structure can be designed to simultaneously generate afterglow, upconversion and downshifting to realize multimode time-dependent multicolour evolutions. These findings can promote the development of superior XEA and plentiful spectral manipulation, opening up a broad range of applications ranging from multiplexed biosensing, to high-capacity information encryption, to multidimensional displays and to multifunctional optoelectronic devices.

X-ray activated afterglow nanomaterials are desirable components for advanced optoelectronic applications. Here, the authors present pathways to modulate the stimulus-responsive color emissions in lanthanide-doped fluoride core-shell nanoparticles.

Improved relative temperature sensitivity of over 10% K-1 in fluoride nanocrystals via engineering the interfacial layer

Real-time in situ temperature sensing is of significance in the bio-medical field; however, the low relative temperature sensitivity S_r is one of the major obstacles in the development of nanothermometers. Herein, we provide an effective route that engineers the interfacial layer in a core/shell/shell nanostructure to enlarge the temperature-dependent luminescence intensity ratio (LIR) variations followed by an improved S_r. The CaF₂ interlayer is employed to inhibit the interaction between the core and outer shell, and increase the interfacial phonon energy to enhance the negative thermal quenching effect (TQE) of Nd³⁺ ions in the outer shell and positive TQE of Er³⁺ ions in the core layer. Based on the temperature-dependent LIR variations of Er (650 nm) to Nd (800 nm), the maximum S_r of 10.01% K^-1 and minimum S_r of % 2.56% K^-1 are achieved.

NIR-I-Responsive Single-Band Upconversion Emission through Energy Migration in Core-Shell-Shell Nanostructures

Here we report a new strategy to tune both excitation and emission peaks of upconversion nanoparticles (UCNPs) into the first infrared biowindow (NIR-I, 650-900 nm) with high NIR-I-to-NIR-I upconversion efficiency. By introducing the sensitizer Nd³⁺ , activator Er³⁺ , energy migrator Yb³⁺ and energy manipulator Mn²⁺ into specific region to construct proposed energy migration processes in the designed core-shell-shell nanoarchitecture, back energy transfer (BET) from activator to sensitizer or migrator can be greatly blocked and the NIR-to-red upconversion emission can be efficiently promoted. Consequently, BET-induced photon quenching and the undesired green-emitting radiative transition are entirely eliminated, leading to high-efficiency single-band red upconversion emission upon 808 nm NIR-I laser excitation. Our findings provide insights into fundamental lanthanide interactions and advance the development of UCNPs for bioapplications with techniques that overturn traditional limitations.