Efficient Tailoring of Upconversion Selectivity by Engineering Local Structure of Lanthanides in Na(x)REF(3+x) Nanocrystals

Multi-Wavelength Excitable Multicolor Upconversion and Ratiometric Luminescence Thermometry of Yb3+/Er3+ Co-Doped NaYGeO4 Microcrystals

Excitation wavelength controllable lanthanide upconversion allows for real-time manipulation of luminescent color in a composition-fixed material, which has been proven to be conducive to a variety of applications, such as optical anti-counterfeiting and information security. However, current available materials highly rely on the elaborate core-shell structure in order to ensure efficient excitation-dependent energy transfer routes. Herein, multicolor upconversion luminescence in response to both near-infrared I and near-infrared II (NIR-I and NIR-II) excitations is realized in a novel but simple NaYGeO4:Yb3+/Er3+ phosphor. The remarkably enhanced red emission ratio under 1532 nm excitation, compared with that under 980 nm excitation, could be attributed to the Yb3+-mediated cross-relaxation energy transfers. Moreover, multi-wavelength excitable temperature-dependent (295-823 K) upconversion luminescence realizes a ratiometric thermometry relying on the thermally coupled levels (TCLs) of Er3+. Detailed investigations demonstrate that changing excitation wavelength makes little difference for the performances of TCL-based ratiometric thermometry of NaYGeO4:Yb3+/Er3+. These findings gain more insights to manipulate cross-relaxations for excitation controllable upconversion in single activator doped materials and benefit the cognition of the effect of excitation wavelength on ratiometric luminescence thermometry.

Dramatic Impact of Materials Combinations on the Chemical Organization of Core-Shell Nanocrystals: Boosting the Tm3+ Emission above 1600 nm

This article represents the first foray into investigating the consequences of various material combinations on the short-wave infrared (SWIR, 1000-2000 nm) performance of Tm-based core-shell nanocrystals (NCs) above 1600 nm. In total, six different material combinations involving two different types of SWIR-emitting core NCs (α-NaTmF4 and LiTmF4) combined with three different protecting shell materials (α-NaYF4, CaF2, and LiYF4) have been synthesized. All corresponding homo- and heterostructured NCs have been meticulously characterized by powder X-ray diffraction and electron microscopy techniques. The latter revealed that out of the six investigated combinations, only one led to the formation of a true core-shell structure with well-segregated core and shell domains. The direct correlation between the downshifting performance and the spatial localization of Tm3+ ions within the final homo- and heterostructured NCs is established. Interestingly, to achieve the best SWIR performance, the formation of an abrupt interface is not a prerequisite, while the existence of a pure (even thin) protective shell is vital. Remarkably, although all homo- and heterostructured NCs have been synthesized under the exact same experimental conditions, Tm3+ SWIR emission is either fully quenched or highly efficient depending on the type of material combination. The most efficient combination (LiTmF4/LiYF4) achieved a high photoluminescence quantum yield of 39% for SWIR emission above 1600 nm (excitation power density in the range 0.5-3 W/cm2) despite significant intermixing. From now on, highly efficient SWIR-emitting probes with an emission above 1600 nm are within reach to unlock the full potential of in vivo SWIR imaging.

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Excitation-Controlled Host-Guest Multicolor Luminescence in Lanthanide-Doped Calcium Zirconate for Information Encryption

Efficient control over lanthanide luminescence by regulating excitations offers a real-time and reversible luminescence-managing strategy, which is of great importance and highly desirable for various applications, including multicolor display and information encryption. Herein, we studied the crystal structure, luminescence properties, and mechanisms of undoped and Tb3+/Eu3+-doped CaZrO3 in detail. The intrinsic purple-blue luminescence from host CaZrO3 and the introduced green/red luminescence from guest dopants Tb3+/Eu3+ were found to have different excitation mechanisms and, therefore, different excitation wavelength ranges. This enables the regulation of luminescent color through controlling the excitation wavelengths of Tb3+/Eu3+-doped CaZrO3. Furthermore, preliminary applications for information encryption with these materials were demonstrated using portable UV lamps of 254 and 302 nm. This study not only promotes the development of multicolor luminescence regulation in fixed-composition materials, but also advances the practical applications of lanthanide luminescent materials in visually readable, high-level anti-counterfeiting and information encryption.

Ultrapure single-band red upconversion luminescence in Er3+ doped sensitizer-rich ytterbium oxide transparent ceramics for solid-state lighting and temperature sensing

Achieving single-band upconversion (UC) is a challenging but rewarding approach to attain optimal performance in diverse applications. In this paper, we successfully achieved single-band red UC luminescence in Yb2O3: Er transparent ceramics (TCs) through the utilization of a sensitizer-rich design. The Yb2O3 host, which has a maximum host lattice occupancy by Yb3+ sensitizers, facilitates the utilization of excitation light and enhances energy transfer to activators, resulting in improved UC luminescence. Specifically, by shortening the ionic spacing between sensitizer and activator, the energy back transfer and the cross-relaxation process are promoted, resulting in weakening of green energy level 4S3/2 and 2H11/2 emission and enhancement of red energy level 4F9/2 emission. The prepared Yb2O3: Er TCs exhibited superior optical properties with in-line transmittance over 80% at 600 nm. Notably, in the 980nm-excited UC spectrum, green emission does not appear, thus Yb2O3: Er TCs exhibit ultra-pure single band red emission, with CIE coordinates of (0.72, 0.28) and color purity exceeding 99.9%. To the best of our knowledge, this is the first demonstration of pure red UC luminescence in TCs. Furthermore, the luminescent intensity ratio (LIR) technique was utilized to apply this pure red-emitting TCs for temperature sensing. The absolute sensitivity of Yb2O3: Er TCs was calculated to be 0.319% K-1 at 304 K, which is the highest level of optical thermometry based on 4F9/2 levels splitting of Er3+ known so far. The integration between pure red UC luminescence and temperature sensing performance opens up new possibilities for the development of multi-functional smart windows.

Tuning the Red-to-Green-Upconversion Luminescence Intensity Ratio of Na3ScF6: 20% Yb3+, 2% Er3+ Particles by Changes in Size

Na₃ScF₆: 20% Yb³⁺, 2% Er³⁺ samples were synthesized with different reaction times and reaction temperatures using the solvothermal method. We carried out a series of tests on Na3ScF6 crystals. The XRD patterns showed that the monoclinic phases of the Na₃ScF₆ samples could be synthesized under different reaction conditions, and doping with Yb³⁺ ions and Er³⁺ ions did not change the crystal structures. The SEM images showed that the sizes of the samples gradually increased with reaction time and reaction temperature. The fluorescence spectra showed that the emission peaks of the prepared samples under 980 nm near-infrared (NIR) excitation were centered at 520 nm/543 nm and 654 nm, corresponding to the ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively. With the increasing size of the samples, the emission intensities at 654 nm increased and the luminescence colors changed from green to red; at the same time, the red-to-green luminescence intensity ratios (IR/IG ratios) increased from 0.435 to 15.106-by as much as ~34.7 times. Therefore, this paper provides a scheme for tuning the IR/IG ratios of Na₃ScF₆: 20% Yb³⁺, 2% Er³⁺ samples by changing their sizes, making it possible to enhance the intensity of red upconversion, which has great potential for the study of color displays and lighting.

Enhancing Photoluminescence of CsPb(ClxBr1-x)3 Perovskite Nanocrystals by Fe2+ Doping

The doping of impurity ions into perovskite lattices has been scrupulously developed as a promising method to stabilize the crystallographic structure and modulate the optoelectronic properties. However, the photoluminescence (PL) of Fe²⁺-doped mixed halide perovskite NCs is still relatively unexplored. In this work, the Fe²⁺-doped CsPb(Cl_xBr_1-x)₃ nanocrystals (NCs) are prepared by a hot injection method. In addition, their optical absorption, photoluminescence (PL), PL lifetimes, and photostabilities are compared with those of undoped CsPb(Br_1-xCl_x)₃ NCs. We find the Fe²⁺ doping results in the redshift of the absorption edge and PL. Moreover, the full width at half maximums (FWHMs) are decreased, PL quantum yields (QYs) are improved, and PL lifetimes are extended, suggesting the defect density is reduced by the Fe²⁺ doping. Moreover, the photostability is significantly improved after the Fe²⁺ doping. Therefore, this work reveals that Fe²⁺ doping is a very promising approach to modulate the optical properties of mixed halide perovskite NCs.

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.

Tunable and Enhanced NIR-II Luminescence from Heavily Doped Rare-Earth Nanoparticles for In Vivo Bioimaging

The last decade has witnessed the booming development of optical imaging in the second near-infrared (NIR-II, 1000-1700 nm) window for disease screening and image-guided surgical interventions, due to the merits of multi-color observations and high spatio-temporal resolution in deep tissue. Therefore, bright and multispectral NIR-II probes are required and play a key role. Here, we report the synthesis of a set of bright rare-earth based NIR-II downshifting nanoparticles (DSNPs) with hexagonal phase (β phase). As compared with the widely reported DSNPs (β-NaYF4@NaYF₄:20Yb/(0.5-2)A@NaYF₄; A = Ho, Pr, Tm or Er) previously, we reveal that the concentrations of both sensitizers and activators can be further highly doped, not limited by the concentration quenching effect. Our results demonstrate that the optimized formula in the heavily doped DSNPs (β-NaYF₄@NaYbF₄:A@NaYF₄, A = 20Ho, 3Pr, 4Tm or 10Er) leads to 1.2- to 4.2-folds NIR-II luminescence enhancement. Especially for the heavily Er-doped DSNPs with long-wavelength photons extending to the NIR-IIb window (1500-1700 nm), we can further boost their luminescence through introducing a beneficial cross-relaxation and host matrix with higher phonon energy (cubic phase NaYF₄@NaYbF₄:10Er/5Ce@NaYF₄), leading to a total of ∼11.4-fold enhancement. The resulting biocompatible, bright NIR-II emitting DSNPs enable us to in vivo monitor the cerebral vessels through the intact scalp and skull, as well as two-color dynamic tumor imaging with high spatial resolution. This work suggests the potential of the heavily doped DSNPs for multiplexed imaging in cerebrovascular abnormalities toward the diagnosis and therapy of the cerebral diseases.

Local Structure Engineering in Lanthanide-Doped Nanocrystals for Tunable Upconversion Emissions

Upconversion emissions from lanthanide-doped nanocrystals have sparked extensive research interests in nanophotonics, biomedicine, photovoltaics, photocatalysis, etc. Rational modulation of upconversion emissions is highly desirable to meet the requirements of specific applications. Among the diverse developed methods, local structure engineering is fundamentally feasible, through which the upconversion emission intensity, selectivity, wavelength shift, and lifetime can be tuned effectively. The underlying mechanism of the local-structure-dependent upconversion emissions lies in the degree of parity hybridization and energy level splitting of lanthanide ions as well as the interionic energy transfer efficiency. Over the past few years, there has been significant progress in local-structure-engineered upconversion emissions. In this Perspective, we first introduce the principles of upconversion emissions and typical characterization methods for local structure. Subsequently, we summarize recent achievements in tuning of upconversion emissions through local structure engineering, including host composition adjustment, external field regulation, and interfacial strain management. Finally, we propose a few perspectives that should tackle the current bottlenecks. This Perspective is expected to deepen the understanding of local-structure-dependent upconversion emissions and arouse adequate attention to the engineering of local structure for desired properties of inorganic nanocrystals.

Modulating electron population pathways for time-dependent dynamic multicolor displays

Multicolor luminescent nanoparticles (NPs) show several potential emerging applications. In this work, we provide a new route that integrates the afterglow and upconversion (UC) that originate in a single activator to achieve color variations without the modulation of any other parameters. The Er³⁺ ions in Na₃HfF₇:Yb/Er NPs exhibit bright green afterglow upon X-ray irradiation and single-band red UC under 980 nm laser excitation, which are attributed to the significantly different electron population pathways. The UC intensity is stable and the afterglow decreases gradually over time, thus the output color is clearly changed from green to red naturally via illuminating the pre-X-ray-irradiated NPs with a 980 nm laser. Furthermore, the fine emission profiles of Er³⁺, Ho³⁺ and Tm³⁺ are achieved upon X-ray irradiation. Our results develop a new approach for time-dependent dynamic color displays and a simple route to revealing the electronic fine structures of lanthanide activators at room temperature.

Full shell coating or cation exchange enhances luminescence

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.

Core-shell designs enhance the luminescence of lanthanide-doped upconversion nanoparticles (UCNPs), but the effect of shell coverage was insufficiently characterized. Here the authors demonstrate, on a series of core-shell UCNPs with various shell coverage ratios, an on-off effect by which luminescence is enhanced only when a full coverage is achieved.

Label-Free Ratiometric Upconversion Nanoprobe for Spatiotemporal pH Mapping in Living Cells

Sensing and imaging pH inside living cells are of paramount importance for better penetrating cellular functions and disease diagnostics. Herein, we engineered an original pH sensor by a simple one-step self-assembly of poly(ethylene glycol) (PEG)ylated phospholipid (DSPE-PEG) and a phenol red small molecule on the surface of upconversion nanoparticles (UCNPs) to form a phospholipid monolayer for sensing and imaging the change of intracellular pH. The sensor showed excellent reversibility and rapid response to the pH variations. Furthermore, this pH sensing system could measure spatial and temporal pH changes during endocytosis and interrogate the pH fluctuations inside cells under external stimuli. Our experimental results revealed that the pH sensor was able to map spatial and temporal pH fluctuations inside living cells, showing its potential application in diagnostics and pH-related study of cell biology.

Exploring Heterostructured Upconversion Nanoparticles: From Rational Engineering to Diverse Applications

Upconversion nanoparticles (UCNPs) represent a class of optical nanomaterials that can convert low-energy excitation photons to high-energy fluorescence emissions. On the basis of UCNPs, heterostructured UCNPs, consisting of UCNPs and other functional counterparts (metals, semiconductors, polymers, etc.), present an intriguing system in which the physicochemical properties are largely influenced by the entire assembled particle and also by the morphology, dimension, and composition of each individual component. As multicomponent nanomaterials, heterostructured UCNPs can overcome challenges associated with a single component and exhibit bifunctional or multifunctional properties, which can further expand their applications in bioimaging, biodetection, and phototherapy. In this review, we provide a summary of recent achievements in the field of heterostructured UCNPs in the aspects of construction strategies, synthetic approaches, and types of heterostructured UCNPs. This review also summarizes the trends in biomedical applications of heterostructured UCNPs and discusses the challenges and potential solutions in this field.

Low-Temperature-Induced Controllable Transversal Shell Growth of NaLnF4 Nanocrystals

Highly controllable anisotropic shell growth is essential for further engineering the function and properties of lanthanide-doped luminescence nanocrystals, especially in some of the advanced applications such as multi-mode bioimaging, security coding and three-dimensional (3D) display. However, the understanding of the transversal shell growth mechanism is still limited today, because the shell growth direction is impacted by multiple complex factors, such as the anisotropy of surface ligand-binding energy, anisotropic core–shell lattice mismatch, the size of cores and varied shell crystalline stability. Herein, we report a highly controlled transversal shell growth method for hexagonal sodium rare-earth tetrafluoride (β-NaLnF₄) nanocrystals. Exploiting the relationship between reaction temperature and shell growth direction, we found that the shell growth direction could be tuned from longitudinal to transversal by decreasing the reaction temperature from 310 °C to 280 °C. In addition to the reaction temperature, we also discussed the roles of other factors in the transversal shell growth of nanocrystals. A suitable core size and a relative lower shell precursor concentration could promote transversal shell growth, although different shell hosts played a minor role in changing the shell growth direction.

Design of a bi-functional NaScF4: Yb3+/Er3+ nanoparticles for deep-tissue bioimaging and optical thermometry through Mn2+ doping

An approximately monochromatic red upconversion (UC) emission is successfully realized in NaScF₄: Yb³⁺/Er³⁺ nanoparticles (NPs) through Mn²⁺ ions doping without phase transition. The Mn²⁺ ions play a role of bridge during the energy transfer process from green emission state ²H_11/2/⁴S_3/2 of Er³⁺ to red emission state ⁴F_9/2 of Er³⁺, which significantly accelerates the red UC enhancement. The strongest red luminescence is observed in the sample containing 10% Mn²⁺ ions (Mn-10) with an enhancement factor of 7.5 times. Meanwhile, an ultrasensitive optical thermometry in the physiological temperature region can be realized by utilizing the fluorescence intensity ratio (FIR) between two thermally coupled Stark transitions of Er³⁺: ⁴I_13/2 → ⁴I_15/2, locating in the near-infrared (NIR) long wavelength region of the second biological window. Its relative sensitivity S_R can be expressed by 340/T², which is much higher than most optical thermometers based on thermally coupled Stark sublevels reported by the previous papers. Beyond that, an ex vivo experiment is designed to evaluate the penetration depth of the red and NIR emission of Mn-10 in the biological tissues, revealing that they can reach depth of at least 3 mm and 5 mm respectively. More importantly, the increasing tissue thickness has almost no effect on the FIR values. All the results show that the present sample is a promising bi-functional nano probe which can be used for bioimaging and temperature sensing in the deep tissues through the strong red UC emission and ultrasensitive NIR optical thermometer, respectively.

Local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for biological applications

This feature article overviews the recent advances in the local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for various biological applications.

Luminescent europium(iii) complexes based on tridentate isoquinoline ligands with extremely high quantum yield

Synthesis and characterization of two new lanthanide Eu(iii) complexes based on tridentate isoquinoline ligands with high luminescence efficiency, extended excitation wavelength and good thermal stability.

Multimodal Luminescent Yb3+ /Er3+ /Bi3+ -Doped Perovskite Single Crystals for X-ray Detection and Anti-Counterfeiting

Anti-counterfeiting techniques have become a global topic since they is correlated to the information and data safety, in which multimodal luminescence is one of the most desirable candidates for practical applications. However, it is a long-standing challenge to actualize robust multimodal luminescence with high thermal stability and humid resistance. Conventionally, the multimodal luminescence is usually achieved by the combination of upconversion and downshifting luminescence, which only responds to the electromagnetic waves in a limited range. Herein, the Yb³⁺ /Er³⁺ /Bi³⁺ co-doped Cs₂ Ag_0.6 Na_0.4 InCl₆ perovskite material is reported as an efficient multimodal luminescence material. Beyond the excitation of ultraviolet light and near-infrared laser (980 nm), this work extends multimodal luminescence to the excitation of X-ray. Besides the flexible excitation sources, this material also shows the exceptional luminescence performance, in which the X-ray detection limit reaches the level of nGy s^-1 , indicating a great potential for further application as a colorless pigment in the anti-counterfeiting field. More importantly, the obtained double perovskite features high stability against both humidity and temperature up to 400 °C. This integrated multifunctional luminescent material provides a new directional solution for the development of multifunctional optical materials and devices.

Tailoring local coordination structure of the Er3+ ions for tuning the up-conversion multicolor luminescence

The regulation of the local structure around Er³⁺ ions is an important channel for adjusting the characteristic of up-conversion luminescence. In this paper, the cubic-phased Er³⁺:CaF₂ crystals with different Er³⁺ doping concentrations were fabricated with temperature gradient technique (TGT) method and the effect of the local coordination structure of the Er³⁺ ions on its luminescence performance was investigated. The local coordination structure of Er³⁺ ions was simulated by density functional theory. The computational results show that clusters evolve from low order to high order with the increase of Er³⁺ ion doping concentration. In this evolution process, the local structure transforms from cubic structure to the co-existence of cubic and lower symmetric square anti-prism structures. Meanwhile, the distance between Er³⁺ ions in the cluster decreased first and then increased slightly, and in dimers and trimers this distance reached the minimum. Under 980 nm excitation, with the increase of Er³⁺ ion concentration, the intensity ratios of the red and green emissions of Er³⁺:CaF₂ first increased from 0.61 to 42.03 and then decreased to 12.11. The corresponding up-conversion luminescence gamut was adjusted from monochrome green to red to red-yellow. This work provides a new thread for realizing upconversion multicolor luminescence by regulating the clusters of rare earth ions.

Experimental and Simulation Insights into Local Structure and Luminescence Evolution in Eu3+-Doped Nanocrystals under High Pressure

Tremendous effort has been devoted to tailoring structure-correlated properties, especially for the luminescence of lanthanide nanocrystals (NCs). High pressure has been demonstrated as a decent way to tune the performance of lanthanide NCs; however, little attention has been paid to the local structure evolution accompanied by extreme compression and its effect on luminescence. Here, we tailor the local structure around lanthanide ions with pressure in β-NaGdF₄ NCs, in which Eu³⁺ ions were doped as optical probes for local structure for the sensitive electric dipole transition. As the pressure increases, the intensity ratio of the ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transition decreases monotonically from 2.04 to 0.81, implying a higher local symmetry around Eu³⁺ ions from compression. In situ X-ray diffraction demonstrates that the sample maintains the hexagonal structure up to 33.5 GPa, and density functional theory calculations reveal the tendency of the local structure to vary under high pressure.

Hydrothermal synthesis and site symmetry tuning of polycrystalline YVO4:Eu nanoparticles via a continuous-flow microreactor

Yttrium orthovanadate (YVO₄) has been widely used as a host material for low- and medium-power diode-pumped solid-state lasers due to its excellent thermal, mechanical, and optical properties. This work demonstrates the synthesis and site symmetry tunning of polycrystalline YVO₄:Eu nanoparticles with uniform size and shape using a continuous-flow microreactor at high pressures. High-quality YVO₄:Eu nanoparticles were created using a residence time of fewer than 20 s. Carefully controlling the heat flux and flow rate can produce the YVO₄:Eu nanoparticles showing different crystallinity, crystal morphologies, site symmetry around Eu³⁺, and therefore optical emission. The site symmetry of YVO₄:Eu is adjusted without any stoichiometric modification of the precursors by simply varying the flow rate and heat flux of the microreactor. The site symmetries of the as-synthesized YVO₄:Eu nanoparticles are studied by investigating their photoluminescent emission spectra and computational model of first-principle density functional theory (DFT). The DFT model indicates that the oxygen vacancy influenced the V-O association and the overlap between Eu 4f and V 3d states which can contribute to different optical transitions and, therefore, distinct emission spectrum. The use of a continuous flow microreactor at high pressure provides better understandings of the hydrothermal syntheses of functional nanoparticles and enables scalable manufacturing concurrently.

Study of near-infrared light-induced excitation of upconversion nanoparticles as a vector for non-viral DNA delivery

Clinical requirements have necessitated the development of biomedical nanomaterials that can be implanted into tissues or bodies.

Efficient modulation of upconversion luminescence in NaErF4-based core–shell nanocrystals

Efficient modulation of upconversion luminescence in heavily-doped core–shell nanocrystals by the tuning of [F]/[RE] ratio during synthesis.

Photon management properties of Yb-doped SnO2 nanoparticles synthesized by the sol-gel technique

SnO2 is a transparent large band gap semiconductor, particularly interesting for optoelectronic and photovoltaic devices, mainly because its conduction can be easily tuned by doping or by modulating the amount of oxygen vacancies. Besides, rare earth doping was successfully exploited for up conversion properties. Here we report on the functionalization of SnO2 nanoparticles with optically active Yb3+ ions using the sol-gel method, which allows UV to NIR spectral (down) conversion. As starting solutions we used stable non-alkoxide metal-organic compounds, which is rather uncommon. Transmission electron microscopy analysis demonstrated the formation of small well-crystallized nanoparticles while X-ray photoelectron spectroscopy measurements have revealed that the Yb is well inserted in the host matrix and has a 3+ valence state. All nanoparticles present large absorption in the UV-visible range (250 to 550 nm) and a band gap that decreases down to 2.72 eV upon doping. The UV energy converted into NIR on the basis of efficient energy transfer from SnO2 to the Yb3+ ions ranges between 250 and 400 nm. Reference undoped SnO2 nanoparticles with a mean size of 20 nm allow converting UV light into broad visible emission centered at 650 nm. The incorporation of up to 3.5 at% of Yb3+ ions into the SnO2 host matrix results in a spectacular decrease of the nanoparticle size down to 6.6 nm. This allowed also the shift of the photoluminescence to NIR in the 970-1050 nm range. The energy level structure of Yb3+ in SnO2 was successfully determined from the deconvolution of the Yb emission. This emission is significantly enhanced by increasing the doping level. All optical measurements suggest that these nanoparticles can be efficiently used as down-shifting converters.

Imaging dopant distribution across complete phase transformation by TEM and upconversion emission

Correlating dopant distribution to its optical response represents a complex challenge for nanomaterials science. Differentiating the "true" clustering nature from dopant pairs formed in statistical distribution complicates even more the elucidation of doping-functionality relationship. The present study associates lanthanide dopant distribution, including all significant events (enrichment, depletion and surface segregation), to its optical response in upconversion (UPC) at the ensemble and single-nanoparticle level. A small deviation from the Er nominal concentration of a few percent is able to induce clear differences in Er UPC emission color, intensity, excited-state dynamics and ultimately, UPC mechanisms, across tetragonal to monoclinic phase transformation in rationally designed Er doped ZrO2 nanoparticles. Rare evidence of a heterogeneous dopant distribution leading to the coexistence of two polymorphs in a single nanoparticle is revealed by Z- and phase contrast transmission electron microscopy (TEM). Despite their spatial proximity, Er in the two polymorphs are spectroscopically isolated, i.e. they do not communicate by energy transfer. Segregated Er, which is well imaged in TEM, is absent in UPC, while the minor phase content overlooked by X-ray diffraction and TEM is revealed by UPC. The outstanding sensitivity of combined TEM and UPC emission to subtle deviations from uniform doping in the diluted concentration regime renders such an approach relevant for various functional oxides supporting lanthanide dopants as emitters.

Lanthanide-Doped Photoluminescence Hollow Structures: Recent Advances and Applications

Lanthanide-doped nanomaterials have attracted significant attention for their preeminent properties and widespread applications. Due to the unique characteristic, the lanthanide-doped photoluminescence materials with hollow structures may provide advantages including enhanced light harvesting, intensified electric field density, improved luminescent property, and larger drug loading capacity. Herein, the synthesis, properties, and applications of lanthanide-doped photoluminescence hollow structures (LPHSs) are comprehensively reviewed. First, different strategies for the engineered synthesis of LPHSs are described in detail, which contain hard, soft, self-templating methods and other techniques. Thereafter, the relationship between their structure features and photoluminescence properties is discussed. Then, niche applications including biomedicines, bioimaging, therapy, and energy storage/conversion are focused on and superiorities of LPHSs for these applications are particularly highlighted. Finally, keen insights into the challenges and personal prospects for the future development of the LPHSs are provided.

Structure-Property Relationships in Lanthanide-Doped Upconverting Nanocrystals: Recent Advances in Understanding Core-Shell Structures

The production of upconverting nanostructures with tailored optical properties is of major technological interest, and rapid progress toward the realization of such production has been made in recent years. Ultimately, accurate understanding of nanostructure organization will lead to design rules for accurately tailoring optical properties. Here, the context of open questions still of general importance to the upconversion and nanocrystal communities is presented, with a particular emphasis on the structure-property relationships of core-shell upconverting nanocrystals. Although the optical properties of the latter have been thoroughly investigated, little is known regarding their atomic-scale organization. Indeed, solving the atomic-scale structure of such nanomaterials is challenging because of their intrinsic nonperiodic nature. Familiar concepts of crystallography are no longer appropriate; chemical and structural modulation waves must be introduced. To reveal the exact core-shell structures, innovative characterization techniques need to be applied and developed, as discussed herein. The continued development and application of structural characterization techniques will be vital to consolidate the currently incomplete link between atomic-scale structure and upconversion properties. This will ultimately provide a valuable contribution to the emerging detailed guidelines on how to better design upconverting nanostructures to achieve given optical properties in terms of efficiency, absorption, spectral emission, and dynamics.

Mass production of poly(ethylene glycol) monooleate-modified core-shell structured upconversion nanoparticles for bio-imaging and photodynamic therapy

Developing robust and high-efficient synthesis approaches has significant importance for the expanded applications of upconversion nanoparticles (UCNPs). Here, we report a high-throughput synthesis strategy to fabricate water-dispersible core-shell structured UCNPs. Firstly, we successfully obtain more than 10 grams core UCNPs with high quality from one-pot reaction using liquid rare-earth precursors. Afterwards, different core-shell structured UCNPs are fabricated by successive layer-by-layer strategy to get enhanced fluorescence property. Finally, the hydrophobic UCNPs are modified with poly(ethylene glycol) monooleate (PEG-OA) though a novel physical grinding method. On the basis of mass-production, we use the as-prepared PEG-UCNPs to construct an 808-nm stimuli photodynamic therapy agent, and apply them in cancer therapy and bio-imaging.

Thermally Stable Copper(II)-Doped Cesium Lead Halide Perovskite Quantum Dots with Strong Blue Emission

All-inorganic perovskite quantum dots (QDs) have emerged as potentially promising materials for lighting and displays, but their poor thermal stability restricts their practical application. In addition, optical characteristics of the blue-emitting CsPbX₃ QDs lag behind their red- and green-emitting counterparts. Herein, we addressed these two issues by doping divalent Cu²⁺ ions into the perovskite lattice to form CsPb_{1- x}Cu _xX₃ QDs. Extended X-ray absorption fine structure (EXAFS) measurements reveal that doping smaller Cu²⁺ guest ions induces a lattice contraction and eliminates halide vacancies, which leads to an increased lattice formation energy and improved short-range order of the doped perovskite QDs. This results in the improvement of both the thermal stability and the optical performance of CsPb_{1- x}Cu _x(Br/Cl)₃ QDs, which exhibit bright blue photoluminescence at 450-460 nm, with a high quantum yield of over 80%. The CsPb_{1- x}Cu _xX₃ QD films maintain stable luminescence performance even when annealed at temperatures of over 250 °C.

Rare Earth Hydroxide as a Precursor for Controlled Fabrication of Uniform β-NaYF₄ Nanoparticles: A Novel, Low Cost, and Facile Method

In recent years, rare earth doped upconversion nanocrystals have been widely used in different fields owing to their unique merits. Although rare earth chlorides and trifluoroacetates are commonly used precursors for the synthesis of nanocrystals, they have certain disadvantages. For example, rare earth chlorides are expensive and rare earth trifluoroacetates produce toxic gases during the reaction. To overcome these drawbacks, we use the less expensive rare earth hydroxide as a precursor to synthesize β-NaYF₄ nanoparticles with multiform shapes and sizes. Small-sized nanocrystals (15 nm) can be obtained by precisely controlling the synthesis conditions. Compared with the previous methods, the current method is more facile and has lower cost. In addition, the defects of the nanocrystal surface are reduced through constructing core–shell structures, resulting in enhanced upconversion luminescence intensity.

Enhanced Red Emission in Er3+-Sensitized NaLuF4 Upconversion Crystals via Energy Trapping

Luminescence efficiency of trivalent lanthanide-doped upconversion (UC) materials is significantly limited by luminescence concentration quenching. In this work, red UC emission is dramatically enhanced in Er³⁺-sensitized NaLuF₄ UC crystals through energy trapping under multiple excitation wavelengths. Cross-relaxation quenching and the energy migration to internal lattice defects are simultaneously suppressed by confining the excitation energy in the Er³⁺ activator after introducing the Tm³⁺ or Ho³⁺ energy trapping center. The enhanced red UC emission (Er³⁺: 660 nm) mainly comes from the effective excitation energy confinement by Tm³⁺ and Ho³⁺ trapping centers through an easy energy transfer between Er³⁺ and Tm³⁺/Ho³⁺: ⁴I_11/2 (Er³⁺) → ³H₅ (Tm³⁺) → ⁴I_13/2 (Er³⁺) and ⁴I_11/2 (Er³⁺) → ⁵I₆ (Ho³⁺) → ⁴I_13/2 (Er³⁺). It is found that the confining efficiency of excitation energy in Er³⁺-sensitized NaLuF₄ crystals is higher than that in Yb³⁺/Er³⁺ cosensitized NaLuF₄ crystals, and the luminescence efficiency of Er³⁺-sensitized NaLuF₄ crystals is much higher than that of Er³⁺-based host sensitization UC crystals (NaErF₄). Moreover, Er³⁺-sensitized UC particles can be efficiently excited by three different wavelengths (808, 980, and 1532 nm), indicating huge advantages for applications in bioimaging, anticounterfeiting, and solar cells.

Energy transfer mechanism dominated by the doping location of activators in rare-earth upconversion nanoparticles

Research on the energy transfer mechanism of rare-earth-doped upconversion nanoparticles (UCNPs) has been an important area due to the increasing demand for tuning multicolor emission and enhancing the upconversion efficiency; however, because of large energy mismatch, many lanthanide activators, such as Eu3+, cannot realize highly efficient near infrared-to-visible upconversion by simple codoping of Yb3+. Therefore, introduction of other ions to assist the energy transfer process is required. Herein, we prepared core-shell nanoparticles with different doping locations to investigate the upconversion energy transfer mechanism. The upconversion luminescence (UCL) of core-shell nanoparticles was investigated by steady-state luminescence and time-resolved luminescence spectra. The UCL behaviors in these different multi-activator core-shell nanoparticles were observed. The results revealed different energy transfer channels influenced by the doping location of activators. This study may open up new avenues of structure design for fine-tuning of multicolor UCL for specific applications.

Three primary color emissions from single multilayered nanocrystals

The achievement of three-primary-color luminescence in a single material will lead to revolutionary developments of many advanced applications such as dynamic display with ultra-high resolution, and complex anti-counterfeiting. Here we report the realization of steady-state three-primary-color emission in single multilayered NaYF4 upconversion (UC) nanoparticles. In this core-shell structure, a novel design of a tri-sensitizer, i.e., Nd3+, Yb3+ and Er3+ ions, is utilized, which effectively absorbs the excitation photons of 808, 980 and 1550 nm, and then exhibits blue, red and green emissions, respectively. By simply combining the three primary color emissions, tunable full-color luminescence was achieved in this single material. These nanoparticles have demonstrated promising potential applications in dynamic display and multiple anti-counterfeiting.

A general strategy for tailoring upconversion luminescence in lanthanide-doped inorganic nanocrystals through local structure engineering

A local structure around lanthanide (Ln3+) emitters in Ln3+-doped upconversion nanocrystals (UCNCs) is of fundamental importance in tailoring their upconversion luminescence (UCL) features. However, a general strategy responsible for the local-structure-dependent UCL in Ln3+-doped UCNCs has not been conclusively established to date. Herein, we report a new class of alkaline zirconium fluoride-based Yb3+/Er3+ co-doped UCNCs featuring a diversity of crystallographic structures for Ln3+ ion doping, which thereby allow us to thoroughly understand the origin underlying the local-structure-dependent UCL of the Er3+ ion for the first time. We reveal that the high-symmetry crystal lattice of Yb3+/Er3+ co-doped UCNCs may incur the large UCL red-to-green intensity ratio of Er3+ regardless of their identical elemental compositions. In combination with the first-principles calculations, we show that such local-structure-dependent UCL of Er3+ is primarily due to the varied electronic band structures induced by the Yb3+/Er3+ doping in different crystallographic structures of alkaline zirconium fluorides. These findings may open up a new avenue for constructing high-quality UCNCs with a tailored UCL profile and lifetime for diverse applications.

Facile synthesis of upconversion nanoparticles with high purity using lanthanide oleate compounds

A novel strategy for preparing highly pure NaYF₄-based upconversion nanoparticles (UCNPs) was developed using lanthanide oleate compounds [Ln(OA)₃] as the precursor, denoted as the Ln-OA preparation method. Compared to the conventional solvothermal method for synthesizing UCNPs using lanthanide chloride compounds (LnCl₃) as the precursor (denoted as the Ln-Cl method), the Ln-OA strategy exhibited the merits of high purity, reduced purification process and a uniform size in preparing core and core-shell UCNPs excited by a 980 or 808 nm near infrared (NIR) laser. This work sheds new insight on the preparation of UCNPs and promotes their application in biomedical fields.

Near-infrared optical and X-ray computed tomography dual-modal imaging probe based on novel lanthanide-doped K0.3Bi0.7F2.4 upconversion nanoparticles

A novel K_0.3Bi_0.7F_2.4 upconversion (UC) matrix has been prepared successfully by a solvothermal method. K_0.3Bi_0.7F_2.4:Yb³⁺/Ln³⁺ (Ln = Er, Ho, Tm) upconversion nanoparticles (UCNPs) show a corresponding excellent upconversion luminescence (UCL) under 980 nm laser irradiation. Especially, the strong near-infrared (NIR) UCL of K_0.3Bi_0.7F_2.4:20% Yb³⁺/0.5% Tm³⁺ (abbreviated as BYT) UCNPs is suitable for deep tissue optical imaging. Moreover, the high X-ray absorption coefficient of Bi makes the as-prepared UCNPs favorable for computed tomography (CT) imaging. The citrate-coated BYT UCNPs show good biocompatibility through the MTT assay towards HeLa cells and low hemolytic properties by hemolysis assay, which could be applied for in vivo optical and CT imaging. After intravenous injection of citrate-coated BYT UCNPs for one month, blood biochemistry and histology analysis of mice suggest the UCNPs have a negligible toxicity in vivo, implying citrate-coated BYT could be employed as a safe bioprobe for NIR optical and CT dual-modal imaging.

Phase segregation enabled scandium fluoride–lanthanide fluoride Janus nanoparticles

A phase-segregation based protocol enables the fabrication of a series of scandium fluoride–lanthanide fluoride Janus particles.

Simultaneous size adjustment and upconversion luminescence enhancement of β-NaLuF4:Yb3+/Er3+,Er3+/Tm3+ microcrystals by introducing Ca2+ for temperature sensing

β-NaLuF₄:Yb³⁺/Er³⁺ microcrystals have been obtained through a facile hydrothermal method at a relatively low temperature (180 °C) within only two hours.

Calcium-based biomaterials for diagnosis, treatment, and theranostics

Calcium-based biomaterials with good biosafety and bio-absorbability are promising for biomedical applications such as diagnosis, treatment, and theranostics.

Soft X-ray activated NaYF4:Gd/Tb scintillating nanorods for in vivo dual-modal X-ray/X-ray-induced optical bioimaging

Lanthanide (Ln) nanocrystals using soft X-ray as an excitation source have received significant research interest due to the advantages of unlimited penetration depth of X-ray light. In this study, we demonstrated an efficient scintillator based on NaYF₄:Gd nanorods (denoted as NRs) doped with different contents of terbium (Tb) ions for optical bioimaging under X-ray irradiation. The experimental results showed that the emission intensity was correlated to the doping contents of Tb³⁺, and the largest emission intensity was achieved by doping 15% Tb under excitation by soft X-ray light. In addition, the emission intensity of the as-prepared NRs can be significantly improved by increasing the excitation power and irradiation times of the X-ray. Owing to the efficient X-ray-induced emission, these NRs were successfully used as probes for X-ray-induced optical bioimaging with high sensitivity. In addition, the dual-modal X-ray imaging and X-ray induced optical bioimaging were performed on a mouse, which indicated that the NRs were promising dual-modal bioprobes. Therefore, the X-ray activation nature of the designed NRs makes them promising probes for biomedicine and X-ray-induced photodynamic therapy (PDT) applications owing to the unlimited penetration depth of X-ray excitation source and absence of autofluorescence.

Controlled Synthesis of BaYF5:Er3+, Yb3+ with Different Morphology for the Enhancement of Upconversion Luminescence

In this work, Er³⁺/Yb³⁺-codoped BaYF₅ with different sizes and shapes have been synthesized by a simple solvothermal method. By changing the fluoride source, pH value, solvent, surfactants, Yb³⁺ concentration, temperature, and reaction time, the optimum synthetic conditions of BaYF₅:Er³⁺, Yb³⁺ were found to improve the upconversion luminescent properties. It is found that the emission intensity of green and red light is enhanced for several times by the way of using NaBF₄ as a fluoride source with the comparison of NH₄F and NaF. Moreover, the effects of different surfactants are not the same. Adding 5% polyetherimide (PEI) as surfactant can also improve the upconversion emission. On the contrary, when sodium citrate (CIT) as another surfactant was used to add, the sizes of the nanocrystals were gradually increased and the luminous properties also declined.

Enhancement of Eu3+ Red Upconversion in Lu2O3: Yb3+/Eu3+ Powders under the Assistance of Bridging Function Originated from Ho3+ Tridoping

The red upconversion (UC) emission of Eu³⁺ ions in Lu₂O₃: Yb³⁺/Eu³⁺ powders was successfully enhanced by tridoping Ho³⁺ ions in the matrix, which is due to the bridging function of Ho³⁺ ions. The experiment data manifest that, in Yb³⁺/Eu³⁺/Ho³⁺ tridoped system, the Ho³⁺ ions are first populated to the green emitting level ⁵F₄/⁵S₂ through the energy transfer (ET) processes from the excited Yb³⁺ ions. Subsequently, the Ho³⁺ ions at ⁵F₄/⁵S₂ level can transfer their energy to the Eu³⁺ ions at the ground state, resulting in the population of Eu³⁺⁵D₀ level. With the assistance of the bridging function of Ho³⁺ ion, this ET process is more efficient than the cooperative sensitization process between Yb³⁺ ion and Eu³⁺ ion. Compared with Lu₂O₃: 5 mol % Yb³⁺/1 mol % Eu³⁺, the UC intensity of Eu³⁺⁵D₀→⁷F₂ transition in Lu₂O₃: 5 mol % Yb³⁺/1 mol % Eu³⁺/0.5 mol % Ho³⁺ is increased by a factor of 8.

Insights into the growth mechanism of REF3 (RE = La-Lu, Y) nanocrystals: hexagonal and/or orthorhombic

The synthesis of REF₃ (RE = La-Lu, Y) nanocrystals with controlled phase structures has so far remained a challenge. Herein we have developed a one-for-all synthetic procedure that allows the successful synthesis of REF₃ nanocrystals in a controlled manner. Experimental results showed that the radius of RE ions determines the phase structure: pure hexagonal REF₃ (RE = La-Eu), a mixture of hexagonal and orthorhombic REF₃ (RE = Gd), and pure orthorhombic REF₃ (RE = Tb-Lu, Y) nanocrystals are obtained along with the decrease of the ionic radius. As Gd is positioned exactly in the middle of the lanthanides row, GdF₃ nanocrystals were used as a model to further investigate how the molar ratio of F^- : Gd³⁺, the doping of RE ions with different ionic radii, and the doping concentration of certain RE ions affects the crystal structure of the final product.