Controlling energy transfer between multiple dopants within a single nanoparticle

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Core-shell rare-earth-doped nanostructures in biomedicine

The current status of the use of core-shell rare-earth-doped nanoparticles in biomedical applications is reviewed in detail. The different core-shell rare-earth-doped nanoparticles developed so far are described and the most relevant examples of their application in imaging, sensing, and therapy are summarized. In addition, the advantages and disadvantages they present are discussed. Finally, a critical opinion of their potential application in real life biomedicine is given.

Depleted upconversion luminescence in NaYF4:Yb3+,Tm3+ nanoparticles via simultaneous two-wavelength excitation

Depleted UCL in NaYF₄:Yb³⁺,Tm³⁺ UCNPs upon simultaneous excitation at 980 nm and 1550 nm is attributed to the STED process.

Assessing inter lanthanide photophysical interactions in co-doped titanium dioxide nanoparticles for multiplex assays

This work assesses inter lanthanide photophysical interactions in titanium dioxide nanoparticles towards the development of multiplex assays.

Mechanisms of the blue emission of NaYF4:Tm3+ nanoparticles excited by an 800 nm continuous wave laser

Simultaneous multiwavelength excitation and the quantum transition principle are utilized to study the blue emission mechanisms of NaYF₄:Tm³⁺ UCNPs under 800 nm CW laser excitation.

Enhancement of near-infrared to near-infrared upconversion luminescence in sub-10-nm ultra-small LaF(3):Yb(3+)/Tm(3+) nanoparticles through lanthanide doping

In this Letter, I present a versatile strategy to enhance the near-infrared to near-infrared (NIR-to-NIR) upconversion luminescence from sub-10-nm ultra-small LaF(3):Yb(3+)/Tm(3+) colloidal nanoparticles through lanthanide doping under 980 nm laser excitation. It is interesting that the NIR-to-NIR upconversion emission at 801 nm of LaF(3):Yb(3+)Tm(3+) nanoparticles can be improved by increasing the Tm(3+) doping concentration or by introducing another lanthanide activator (Er(3+) or Ho(3+)) as a sensitizer. The luminescence enhancement effect showed a strong dependence on the doping concentrations of activator ions (Tm(3+), Er(3+), or Ho(3+)). Particularly, adding 1 mol. % Ho(3+) ions into LaF(3):Yb(3+)Tm(3+) nanoparticles induced a 2.85-fold enhancement in NIR 801 nm emission of Tm(3+) ions. The related upconversion emission mechanisms were investigated and discussed.

Giant enhancement of upconversion emission in (NaYF₄:Nd³⁺/Yb³⁺/Ho³⁺)/(NaYF₄:Nd³⁺/Yb³⁺) core/shell nanoparticles excited at 808 nm

In this work, colloidal hexagonal-phase (NaYF4:Nd3+/Yb3+/Ho3+)/(NaYF4:Nd3+/Yb3+) core/shell nanoparticles with intense visible upconversion emissions under 808-nm laser excitation were prepared. Compared with the core-only nanoparticles, a maximum 990-fold overall enhancement in the emission intensity of Ho3+ ions was achieved with the help of active-shell coating design, due to the significant increase in the near-infrared absorption and efficient energy transfer from Nd3+ primary-sensitizers to Ho3+ activators via Yb3+ bridging sensitizers. The luminescence-enhancement effect exhibited a strong dependence on the doping concentrations of NaYF4:Nd3+/Yb3+ active-shell. The optimal concentrations of Nd3+ and Yb3+ ions in the active-shell layer were found to be 30 and 5 mol. %, respectively. Moreover, the upconversion emission intensity of NaYF4:Nd3+/Yb3+-coated nanoparticles was about 2.5 times higher than the one coated with a NaYF4:Nd3+ active-shell.

A Strategy to enhance Eu3+ emission from LiYF4:Eu nanophosphors and green-to-orange multicolor tunable, transparent nanophosphor-polymer composites

LiYF₄:Eu nanophosphors with a single tetragonal phase are synthesized, and various strategies to enhance the Eu³⁺ emission from the nanophosphors are investigated. The optimized Eu³⁺ concentration is 35 mol%, and the red emission peaks due to the ⁵D₀ →⁷F_J (J = 1 and 2) transitions of Eu³⁺ ions are further enhanced by energy transfer from a sensitizer pair of Ce³⁺ and Tb³⁺. The triple doping of Ce, Tb, and Eu into the LiYF₄ host more effectively enhances the Eu³⁺ emission than the core/shell strategies of LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) and LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) architectures. Efficient energy transfer from Ce³⁺ to Eu³⁺ through Tb³⁺ results in three times higher Eu³⁺ emission intensity from LiYF₄:Ce(15%), Tb(15%), Eu(1%) nanophosphors compared with LiYF₄:Eu(35%), which contains the optimized Eu³⁺ concentration. Owing to the energy transfer of Ce³⁺ → Tb³⁺ and Ce³⁺ → Tb³⁺ → Eu³⁺, intense green and red emission peaks are observed from LiYF₄:Ce(13%), Tb(14%), Eu(1-5%) (LiYF₄:Ce, Tb, Eu) nanophosphors, and the intensity ratio of green to red emission is controlled by adjusting the Eu³⁺ concentration. With increasing Eu³⁺ concentration, the LiYF₄:Ce, Tb, Eu nanophosphors exhibit multicolor emission from green to orange. In addition, the successful incorporation of LiYF₄:Ce, Tb, Eu nanophosphors into polydimethylsiloxane (PDMS) facilitates the preparation of highly transparent nanophosphor-PDMS composites that present excellent multicolor tunability.

Recent advances in energy transfer in bulk and nanoscale luminescent materials: from spectroscopy to applications

We discuss optical energy transfer involving ions, QDs, molecules etc., together with the relevant applications in different areas.

Combinatorial approaches for developing upconverting nanomaterials: high-throughput screening, modeling, and applications

This review surveys the use of combinatorial and high-throughput techniques for the rapid discovery, optimization, and application of upconverting nanomaterials.

Time-resolved luminescent biosensing based on inorganic lanthanide-doped nanoprobes

In this feature article, we review the latest advancements in lanthanide-doped luminescent nanocrystals as time-resolved luminescent nano-bioprobes, from their fundamental optical properties to their potential applications for ultrasensitive biodetection and high-resolution bioimaging.

Light upconverting core–shell nanostructures: nanophotonic control for emerging applications

Nanophotonic control of light upconversion in the hierarchical core–shell nanostructures, their biomedical, solar energy and security encoding applications were reviewed.

Ultrasensitive polarized up-conversion of Tm(3+)-Yb3+ doped β-NaYF4 single nanorod

Up-conversion luminescence in rare earth ions (REs) doped nanoparticles has attracted considerable research attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, biological imaging, and so forth. However, there have been no reports on REs doped nanoparticles to investigate their polarized energy transfer up-conversion, especially for single particle. Herein, the polarized energy transfer up-conversion from REs doped fluoride nanorods is demonstrated in a single particle spectroscopy mode for the first time. Unique luminescent phenomena, for example, sharp energy level split and singlet-to-triplet transitions at room temperature, multiple discrete luminescence intensity periodic variation with polarization direction, are observed upon excitation with 980 nm linearly polarized laser. Furthermore, nanorods with the controllable aspect ratio and symmetry are fabricated for analysis of the mechanism of polarization anisotropy. The comparative experiments suggest that intraions transition properties and crystal local symmetry dominate the polarization anisotropy, which is also confirmed by density functional theory calculations. Taking advantage of the REs based up-conversion, potential application in polarized microscopic multi-information transportation is suggested for the polarization anisotropy from REs doped fluoride single nanorod or nanorod array.

Efficient Dual-Modal NIR-to-NIR Emission of Rare Earth Ions Co-doped Nanocrystals for Biological Fluorescence Imaging

A novel approach has been developed for the realization of efficient near-infrared to near-infrared (NIR-to-NIR) upconversion and down-shifting emission in nanophosphors. The efficient dual-modal NIR-to-NIR emission is realized in a β-NaGdF4/Nd(3+)@NaGdF4/Tm(3+)-Yb(3+) core-shell nanocrystal by careful control of the identity and concentration of the doped rare earth (RE) ion species and by manipulation of the spatial distributions of these RE ions. The photoluminescence results reveal that the emission efficiency increases at least 2-fold when comparing the materials synthesized in this study with those synthesized through traditional approaches. Hence, these core-shell structured nanocrystals with novel excitation and emission behaviors enable us to obtain tissue fluorescence imaging by detecting the upconverted and down-shifted photoluminescence from Tm(3+) and Nd(3+) ions, respectively. The reported approach thus provides a new route for the realization of high-yield emission from RE ion doped nanocrystals, which could prove to be useful for the design of optical materials containing other optically active centers.

Combinatorial discovery of lanthanide-doped nanocrystals with spectrally pure upconverted emission

Nanoparticles doped with lanthanide ions exhibit stable and visible luminescence under near-infrared excitation via a process known as upconversion, enabling long-duration, low-background biological imaging. However, the complex, overlapping emission spectra of lanthanide ions can hinder the quantitative imaging of samples labeled with multiple upconverting probes. Here, we use combinatorial screening of multiply doped NaYF(4) nanocrystals to identify a series of doubly and triply doped upconverting nanoparticles that exhibit narrow, spectrally pure emission spectra at various visible wavelengths. We then developed a comprehensive kinetic model validated by our extensive experimental data set. Applying this model, we elucidated the energy transfer mechanisms giving rise to spectrally pure emission. These mechanisms suggest design rules for electronic level structures that yield robust color tuning in lanthanide-doped upconverting nanoparticles. The resulting materials will be useful for background-free multicolor imaging and tracking of biological processes.

Impacts of core-shell structures on properties of lanthanide-based nanocrystals: crystal phase, lattice strain, downconversion, upconversion and energy transfer

This feature article highlights the new development and current status of rare-earth (RE) based core-shell nanocrystals, which is one of the new classes of hybrid nanostructures. Attractive properties of rare-earth based nanomaterials include extremely narrow emission bands, long lifetimes, large Stoke's shifts, photostability and absence of blinking that can be exploited for biophotonic and photonic applications. Core-shell nanostructures have been attracting a great deal of interest to improve the luminescence efficiency by the elimination of deleterious cross-relaxation. The main focus of this feature article is to address the impacts of core-shell structures on the properties of lanthanide based nanocrystals including crystal phase, lattice strain, downconversion emission, upconversion emission and energy transfer. We describe general synthetic methodologies to design core-shell nanostructure materials. An interesting finding reported is that the local environment of an ion in the core-shell structure significantly affects the modifications of radiative and nonradiative relaxation mechanisms. Finally, a tentative outlook on future developments of this research field is given. Here, we attempt to identify the critical parameters governing the design of luminescent lanthanide based core-shell nanostructures.

Bandwidth broadening of near-infrared emission through nanocrystallization in Bi/Ni co-doped glass

We demonstrated an effective way to broaden the bandwidth of near-infrared (NIR) emission from Bi/Ni codoped 58SiO₂₋21ZnO-13Al₂₋O₃₋5TiO₂₋3Ga₂O₃ glass through nanocrystallization. The nanocrystallized glass shows ultra-wide NIR luminescence with a full width at half maximum (FWHM) of 350 nm and long lifetime up to 476 µs. The observed broadband NIR emission, attributed to energy transfer suppression between Ni and Bi active centers, was realized by a separation process with Ni²⁺ ions selectively incorporated into nanocrystals. This bandwidth engineering through nanocrystallization inside glass suggests a promising approach for enhancement of glass functionality and construction of broadband light sources.

Tuning upconversion through energy migration in core-shell nanoparticles

Photon upconversion is promising for applications such as biological imaging, data storage or solar cells. Here, we have investigated upconversion processes in a broad range of gadolinium-based nanoparticles of varying composition. We show that by rational design of a core-shell structure with a set of lanthanide ions incorporated into separated layers at precisely defined concentrations, efficient upconversion emission can be realized through gadolinium sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. Furthermore, the use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. Indeed, the findings described here suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.

Preparation and Characterization of Rare Earth Doped Fluoride Nanoparticles

This paper reviews the synthesis, structure and applications of metal fluoride nanoparticles, with particular focus on rare earth (RE) doped fluoride nanoparticles obtained by our research group. Nanoparticles were produced by precipitation methods using the ligand ammonium di-n-octadecyldithiophosphate (ADDP) that allows the growth of shells around a core particle while simultaneously avoiding particle aggregation. Nanoparticles were characterized on their structure, morphology, and luminescent properties. We discuss the synthesis, properties, and application of heavy metal fluorides; specifically LaF₃:RE and PbF₂, and group IIA fluorides. Particular attention is given to the synthesis of core/shell nanoparticles, including selectively RE-doped LaF₃/LaF₃, and CaF₂/CaF₂ core/(multi-)shell nanoparticles, and the CaF₂-LaF₃ system.

Designer emission spectra through tailored energy transfer in nanoparticle-doped silica preforms

This Letter provides a qualitative proof of concept for purposefully tailoring the emission spectrum of glass by spatially localizing dissimilar dopants to control the degree of energy transfer. More specifically, modified-chemical-vapor-deposition-derived silica preforms were solution doped with either a solution of individually Eu(3+)- or Tb(3+)-doped nanoparticles or a solution of Eu(3+)/Tb(3+)-codoped nanoparticles. The preform prepared using the codoped nanoparticles exhibited energy transfer from the Tb(3+) to the Eu(3+) ions, whereas the preform containing individually doped nanoparticles yielded only discretely Tb(3+) or Eu(3+) emissions. The extension of this work to broadband amplifiers and lasers is discussed.

Single-molecule fluorescence imaging of TiO(2) photocatalytic reactions

Heterogeneous photocatalysts have both potential and demonstrated applications for use in the water-splitting reaction that produces hydrogen, the degradation of organic pollutants, the surface wettability conversion, etc. In this feature article, we have focused on the in-site observation of various reactive oxygen species (ROS), such as singlet oxygen ((1)O(2)) and the hydroxyl radical ((*)OH), generated by the photoexcitation of TiO(2) nanomaterials using single-molecule fluorescence spectroscopy. The spatially resolved photoluminescence (PL) imaging techniques enable us to determine the location of the (photo)catalytically active sites that are related to the heterogeneously distributed defects on the surface. We also present the results that revealed the formation and reaction dynamics of the photogenerated charge carriers in individual TiO(2) nanoparticles. Furthermore, we introduce the single-molecule single-mismatch detection of the nucleotide sequence upon the photoexcitation of a novel nanoconjugate consisting of TiO(2) and DNA on the basis of the mechanistic aspects. Notably, the present conjugates can recognize the difference in a single nucleotide. Consequently, this article provides a significant opportunity to understand the temporal and spatial distributions of ROS generated during the photoirradiation of TiO(2) nanomaterials and directly explore the microscopic world in many fields ranging from fundamental physics and chemistry to practical applications.

Photoluminescent characterization of atomic diffusion in core-shell nanoparticles

Eu3+ doped LaF3 nanoparticles with core/shell morphologies were synthesized and selected spectroscopic properties were measured as a function of heat treatment times and temperatures. More specifically, the relative intensity of photoluminescence spectra, both through direct excitation of the lanthanide as well as phonon sideband spectra were evaluated with increasing amounts of time held at specific temperatures. A one dimensional approximation was used to compute an effective diffusion coefficient for the rare earth dopants in LaF3. Despite the simplicity of the model employed, the calculated diffusion coefficients based on the spectroscopic results are accurate within an order of magnitude in comparison to other fluoride crystals yielding a simplified approach to estimating kinetic and diffusion effects in optical materials.