Highly Efficient LiYF4:Yb(3+), Er(3+) Upconversion Single Crystal under Solar Cell Spectrum Excitation and Photovoltaic Application

Enhanced photoluminescence and photocatalytic properties in Dy-doped sodium zinc molybdate synthesized via a green microwave-assisted method

This study focuses on the structural, photoluminescence and photocatalytic properties of a Dy3+-doped sodium zinc molybdate phosphor. The samples were synthesized using a green microwave-assisted method owing to its efficiency in time and energy consumption. The powder X-ray diffraction (P-XRD) analysis confirmed the monoclinic structure of as-synthesized Dy3+-doped sodium zinc molybdate with space group C2/m. The most prominent excitation band appeared at 348 nm under 590 nm, and the emission spectra for all samples exhibited a sharp peak at around 590 nm, which is attributed to the 4F9/2 → 6H13/2 electric dipole transition of Dy3+ ions. The emission intensity increases with higher Dy3+ doping levels, reaching maximum intensity at a concentration of 6 mol% of Dy3+ in sodium zinc molybdate. Additionally, the Commission Internationale de l'Éclairage (CIE) chromaticity coordinates of the Dy3+ sodium zinc molybdate phosphor places it within the orangish region. Dy3+ is also found to play a great role in enhancing the photocatalytic activity of sodium zinc molybdate under UV light. These findings indicate that the as-synthesized phosphor holds promise for white LED and water purification applications.

Upconversion circularly polarized luminescence of cholesteric liquid crystal polymer networks with NaYF4:Yb,Tm UCNPs

Upconversion circularly polarized luminescence (UC-CPL) exhibits promising potential for application for anti-counterfeiting and displays. Upconversion nanoparticles (UCNPs), NaYF4:Yb,Tm, with uniform morphology and high crystallinity, were prepared via a simple solvothermal method. These UCNPs were embedded into cholesteric liquid crystal polymer network (CLCN) films. The UC-CPL performance of these films was investigated using left- and right-handed circular polarizers. After calibration, the |gcallum| values (up to 0.33) were obtained for the free-standing CLCN-UCNPs films, while a |gcallum| value of 0.43 was achieved for the CLCN-UCNPs-coated PET film. Moreover, a combined system comprising a PMMA-UCNPs layer and a CLCN layer yielded an ultra-large |gcallum| value of up to 1.73. Flexible and colourful patterned CLCN films were fabricated using photomasks, offering potential applications in anti-counterfeiting. This study not only successfully prepared UC-CPL-active materials based on CLCNs and UCNPs, but also demonstrated the chiral filtering effect of CLCN films in upconversion luminescent materials.

The marriage of perovskite nanocrystals with lanthanide‐doped upconversion nanoparticles for advanced optoelectronic applications

The exceptional optoelectronic properties of lead halide perovskite nanocrystals (PeNCs) in the ultraviolet and visible spectral regions have positioned them as a promising class of semiconductor materials for diverse optoelectronic and photovoltaic applications. However, their limited response to near‐infrared (NIR) light due to the intrinsic bandgap (>1.5 eV) has hindered their applications in many advanced technologies. To circumvent this limitation, it is of fundamental significance to integrate PeNCs with lanthanide‐doped upconversion nanoparticles (UCNPs) that are capable of efficiently converting low‐energy NIR photons into high‐energy ultraviolet and visible photons. By leveraging the energy transfer from UCNPs to PeNCs, this synergistic combination can not only expand the NIR responsivity range of PeNCs but also introduce novel emission profiles to upconversion luminescence with multi‐dimensional tunability (e.g., wavelength, lifetime, and polarization) under low‐to‐medium power NIR irradiation, which breaks through the inherent restrictions of individual PeNCs and UCNPs and thereby opens up new opportunities for materials and device engineering. In this review, we focus on the latest advancements in the development of PeNCs‐UCNPs nanocomposites, with an emphasis on the controlled synthesis and optical properties design for advanced optoelectronic applications such as full‐spectrum solar cells, NIR photodetectors, and multilevel anticounterfeiting. Some future efforts and prospects toward this active research field are also envisioned.

Preparation of NaYF4:Tm, Yb, and Gd Luminescent Nanorods/SiO2 Nanospheres Composite Thin Film and Its Application in Perovskite Solar Cells

In this study, we aim to minimize light loss and achieve high power conversion efficiencies (PCE) in perovskite solar cells (PSCs) by employing a spectral conversion film component with antireflection properties. In our scheme, NaYF4:Tm, Yb, and Gd luminescent nanorod/silica nanosphere-based thin films are applied on CH3NH3PbI3 PSCs to improve the device efficiency. The film was fabricated by spin coating an aged silica sol containing NaYF4:Tm, Yb, and Gd luminescent nanorods. The size and the spectral conversion properties of the NaYF4:Tm, Yb, and Gd luminescent nanorods were controlled by tuning the Gd3+ ion concentration. The microstructure and the transmittance properties of the thin film were controlled by changing the concentration of NaYF4:Tm, Yb, and Gd luminescent nanorod in silica sol. The thin films have excellent spectral conversion properties while exhibiting a maximum transmittance. The photovoltaic performance of PSCs with NaYF4:Tm, Yb, and Gd luminescent nanorod/silica nanosphere-based thin films was systematically investigated. The light transmittance was optimized to 95.1% on a cleaned glass substrate, which resulted in an average increase of about 3.0% across the broadband range of 400-800 nm. The optimized films widen the spectrum of light absorbed by conventional PSC cells and reduce reflections across a broad range, enhancing the photovoltaic performance of PSCs. As a result, the PCE of the PSC increased from 14.51% for the reference device without a thin film to 15.67% for the PSC device with an optimized thin film. This study presents a comprehensive solution to the problem of Fresnel reflection and spectral response mismatch of the PSCs, which provides new ideas for the light management of PSCs.

High-Performance and Stable Perovskite Solar Cells Using Carbon Quantum Dots and Upconversion Nanoparticles

Upconversion nanoparticles (UCNPs) and carbon quantum dots (CQDs) have recently received a lot of attention as promising materials to improve the stability and efficiency of perovskite solar cells (PSCs). This is because they can passivate the surfaces of perovskite-sensitive materials and act as a spectrum converter for sunlight. In this study, we mixed and added both promising nanomaterials to PSC layers at the ideal mixing ratios. When compared to the pristine PSCs, the fabricated PSCs showed improved power conversion efficiency (PCE), from 16.57% to 20.44%, a higher photocurrent, and a superior fill factor (FF), which increased from 70% to 75%. Furthermore, the incorporation of CQDs into the manufactured PSCs shielded the perovskite layer from water contact, producing a device that was more stable than the original.

Review for Rare-Earth-Modified Perovskite Materials and Optoelectronic Applications

In recent years, rare-earth metals with triply oxidized state, lanthanide ions (Ln³⁺), have been demonstrated as dopants, which can efficiently improve the optical and electronic properties of metal halide perovskite materials. On the one hand, doping Ln³⁺ ions can convert near-infrared/ultraviolet light into visible light through the process of up-/down-conversion and then the absorption efficiency of solar spectrum by perovskite solar cells can be significantly increased, leading to high device power conversion efficiency. On the other hand, multi-color light emissions and white light emissions originated from perovskite nanocrystals can be realized via inserting Ln³⁺ ions into the perovskite crystal lattice, which functioned as quantum cutting. In addition, doping or co-doping Ln³⁺ ions in perovskite films or devices can effectively facilitate perovskite film growth, tailor the energy band alignment and passivate the defect states, resulting in improved charge carrier transport efficiency or reduced nonradiative recombination. Finally, Ln³⁺ ions have also been used in the fields of photodetectors and luminescent solar concentrators. These indicate the huge potential of rare-earth metals in improving the perovskite optoelectronic device performances.

Enhancement of Upconversion Luminescence by the Construction of a 3Yb-Er-Hf Sublattice Energy Cluster and Surface Defect Elimination

Nanotetragonal LiYF₄:RE (Tm,Er,Ho) is a kind of excellent upconversion luminescence (UCL) material potentially used in many fields, while the enhancement of UC emission and regulation of luminescence lifetime are still a challenge. Herein, a strategy was reported to enhance UCL performance with the aid of the construction of a 3Yb-Er-Hf sublattice energy cluster with the introduction of Hf⁴⁺ and the interception of surface defect fluorescence quenching. UCL was obviously decreased by Hf⁴⁺ doping without surface defect elimination, but after the interception of surface defect quenching, UCL was dramatically enhanced more than 300-fold with an Er³⁺/Hf⁴⁺ mole ratio of 1:1. The contribution of UCL enhancement by the construction of a 3Yb-Er-Hf sublattice energy cluster is about 1.5 times of the sample without energy cluster construction. Interestingly, the lifetime of UCL can also be regulated by this strategy. According to the results of systematical microstructure analyses and UCL performance behaviors examined by X-ray powder diffraction (XRD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and fluorescence spectrophotometry (FS) methods, the possible mechanism of UCL enhancement was proposed. This work may be an inspiration for researchers to design and develop high-performance UCL nanomaterials.

Safe and Scalable Polyethylene Glycol-Assisted Hydrothermal Synthesis and Laser Cooling of 10%Yb3+:LiLuF4 Crystals

Rare earth doped lithium fluorides are a class of materials with a wide variety of optical applications, but the hazardous reagents used in their synthesis often restrict the amount of product that can be created at one time. In this work, 10%Yb3+:LiLuF4 (Yb:LLF) crystals have been synthesized through a safe and scalable polyethylene glycol (PEG)-assisted hydrothermal method. A combination of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and photoluminescence (PL) measurements were used to characterize the obtained materials. The influence of reaction temperature, time, fluoride source, and precursor amount on the shape and size of the Yb:LLF crystals are also discussed. Calibrated PL spectra of Yb3+ ions show laser cooling to more than 15 K below room temperature in air and 5 K in deionized water under 1020 nm diode laser excitation measured at a laser power of 50 mW.

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.

An overview of boosting lanthanide upconversion luminescence through chemical methods and physical strategies

Lanthanide-doped upconversion nanoparticles have attracted extensive research interest due to their promising applications in various fields.

Harvesting Sub-bandgap Photons via Upconversion for Perovskite Solar Cells

Lanthanide-based upconversion (UC) allows harvesting sub-bandgap near-infrared photons in photovoltaics. In this work, we investigate UC in perovskite solar cells by implementing UC single crystal BaF₂:Yb³⁺, Er³⁺ at the rear of the solar cell. Upon illumination with high-intensity sub-bandgap photons at 980 nm, the BaF₂:Yb³⁺, Er³⁺ crystal emits upconverted photons in the spectral range between 520 and 700 nm. When tested under terrestrial sunlight representing one sun above the perovskite's bandgap and sub-bandgap illumination at 980 nm, upconverted photons contribute a 0.38 mA/cm² enhancement in the short-circuit current density at lower intensity. The current enhancement scales non-linearly with the incident intensity of sub-bandgap illumination, and at higher intensity, 2.09 mA/cm² enhancement in current was observed. Hence, our study shows that using a fluoride single crystal like BaF₂:Yb³⁺, Er³⁺ for UC is a suitable method to extend the response of perovskite solar cells to near-infrared illumination at 980 nm with a subsequent enhancement in current for very high incident intensity.

Plasmon-induced double-field-enhanced upconversion nanoprobes with near-infrared resonances for high-sensitivity optical bio-imaging

High-sensitivity optical imaging can be achieved through improving upconversion photoluminescence (UCPL) efficiency of localized surface plasmon resonance (LSPR)-enhanced excitation and emission. Herein, we report a type of UCPL nanoprobe, Au nanospheres assemblage@Gd₂O₃:Yb³⁺/Ln³⁺(Ln = Er, Ho, Tm), which exhibits emission enhancements from 46- to 96-fold as compared with its Au-free counterparts. The aggregation and interaction among Au nanospheres embedded inside the nanoprobe brings about three characteristic LSPR peaks in visible and near-infrared regions according to simulated and experimental absorption spectra, resulting in both excitation and emission fields simultaneously intensified all through the entire nanoprobe. We addressed a characteristic wavelength dependence on emission amplifications, which could be elucidated by a LSPR-enhanced UCPL mechanism and relevant rate equations that we addressed. The nanoprobe was verified to have a superior capability for optical bio-imaging with a negligible toxicityin vitroandin vivo. This study realizes a synchronous double-field-enhanced upconversion of optical nanoprobein situ, and may gain an insight into its mechanism underlying for LSPR-induced UCPL enhancement.

Photon Upconversion for Photovoltaics and Photocatalysis: A Critical Review

Opportunities for enhancing solar energy harvesting using photon upconversion are reviewed. The increasing prominence of bifacial solar cells is an enabling factor for the implementation of upconversion, however, when the realistic constraints of current best-performing silicon devices are considered, many challenges remain before silicon photovoltaics operating under nonconcentrated sunlight can be enhanced via lanthanide-based upconversion. A photophysical model reveals that >1-2 orders of magnitude increase in the intermediate state lifetime, energy transfer rate, or generation rate would be needed before such solar upconversion could start to become efficient. Methods to increase the generation rate such as the use of cosensitizers to expand the absorption range and the use of plasmonics or photonic structures are reviewed. The opportunities and challenges for these approaches (or combinations thereof) to achieve efficient solar upconversion are discussed. The opportunity for enhancing the performance of technologies such as luminescent solar concentrators by combining upconversion together with micro-optics is also reviewed. Triplet-triplet annihilation-based upconversion is progressing steadily toward being relevant to lower-bandgap solar cells. Looking toward photocatalysis, photophysical modeling indicates that current blue-to-ultraviolet lanthanide upconversion systems are very inefficient. However, hope remains in this direction for organic upconversion enhancing the performance of visible-light-active photocatalysts.

Modulated Luminescence of Lanthanide Materials by Local Surface Plasmon Resonance Effect

Lanthanide materials have great applications in optical communication, biological fluorescence imaging, laser, and so on, due to their narrow emission bandwidths, large Stokes' shifts, long emission lifetimes, and excellent photo-stability. However, the photon absorption cross-section of lanthanide ions is generally small, and the luminescence efficiency is relatively low. The effective improvement of the lanthanide-doped materials has been a challenge in the implementation of many applications. The local surface plasmon resonance (LSPR) effect of plasmonic nanoparticles (NPs) can improve the luminescence in different aspects: excitation enhancement induced by enhanced local field, emission enhancement induced by increased radiative decay, and quenching induced by increased non-radiative decay. In addition, plasmonic NPs can also regulate the energy transfer between two close lanthanide ions. In this review, the properties of the nanocomposite systems of lanthanide material and plasmonic NPs are presented, respectively. The mechanism of lanthanide materials regulated by plasmonic NPs and the scientific and technological discoveries of the luminescence technology are elaborated. Due to the large gap between the reported enhancement and the theoretical enhancement, some new strategies applied in lanthanide materials and related development in the plasmonic enhancing luminescence are presented.

Room-Temperature Upconversion in a Nanosized {Ln15} Molecular Cluster-Aggregate

The successive absorption of low-energy photons to the accumulation of the intermediate excited states leading to higher energy emission is still a challenge in molecular architectures. Contrary to low-phonon solids and nanoparticles, the rational construction of molecular systems containing an excess of donor atoms in relation to acceptor ones is far from trivial. Moreover, the vibrations caused by high-energy oscillators commonly present on coordination compounds result in serious drawbacks on molecular upconversion. To overcome these limitations, we demonstrate that upconversion can be achieved even at room temperatures through the use of molecular cluster-aggregates (MCAs). To achieve the upconverted emission, we synthesized a MCA containing 15 lanthanide ions, {Er₂Yb₁₃}, ensuring an excess of donor atoms. With the excitation on the ytterbium ion, the characteristic green and red emissions from erbium were obtained at room temperature. To prove the mechanism behind the upconversion process, four other compositions were synthesized and studied, namely, {Y₁₃Er₂}, {Y₁₀Er₅}, {Er₁₀Yb₅}, and {Y₁₀Er₁Yb₄}. Upconversion quantum yield values on the order of 10^-3% were obtained, values 100000 times higher than for previously reported lanthanide-based molecular upconverting systems. The presented methodology is an interesting approach to address a fine composition control and harness the upconversion properties of nanoscale molecular materials.

LiYF4-nanocrystal-embedded glass ceramics for upconversion: glass crystallization, optical thermometry and spectral conversion

Glass ceramics (GCs) can perfectly integrate nanocrystals (NCs) into bulk materials. Herein, GCs containing LiYF4 NCs were fabricated via a traditional melt-quenching method and subsequent glass crystallization. Structural characterization was carried out via X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) analysis, suggesting the precipitation of LiYF4 NCs from a glass matrix. Taking Eu3+ as a structural probe, the spectrographic features provide compelling evidence for the partition of dopants. In particular, intense upconversion (UC) emission was achieved when co-doped with Yb3+ and Er3+. Temperature-dependent UC emission behaviour was also established based on the fluorescence intensity ratio (FIR) of Er3+, to study its properties for optical thermometry. Furthermore, spectral conversion was attained through cross relaxation (CR) between Ce3+ and Ho3+, tuning from green to red with various Ce3+ doping concentrations. There is evidence that LiYF4 NC-embedded GCs were favorable for UC, which may be extremely promising for optical thermometry and spectral conversion applications. This work may open up new avenues for the exploration of GC materials for expansive applications.

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.

Temperature dependent quantum cutting in cubic BaGdF5:Eu3+ nanophosphors

A task-specific ionic liquid (IL) is employed as a structure directing agent for the synthesis of quantum cutting BaGdF₅:Eu³⁺ nanophosphors.

Photon management to reduce energy loss in perovskite solar cells

Despite the rapid development of perovskite solar cells (PSCs) over the past few years, the conversion of solar energy into electricity is not efficient enough or cost-competitive yet. The principal energy loss in the conversion of solar energy to electricity fundamentally originates from the non-absorption of low-energy photons ascribed to Shockley-Queisser limits and thermalization losses of high-energy photons. Enhancing the light-harvesting efficiency of the perovskite photoactive layer by developing efficient photo management strategies with functional materials and arrays remains a long-standing challenge. Here, we briefly review the historical research trials and future research trends to overcome the fundamental loss mechanisms in PSCs, including upconversion, downconversion, scattering, tandem/graded structures, texturing, anti-reflection, and luminescent solar concentrators. We will deeply emphasize the availability and analyze the importance of a fine device structure, fluorescence efficiency, material proportion, and integration position for performance improvement. The unique energy level structure arising from the 4fn inner shell configuration of the trivalent rare-earth ions gives multifarious options for efficient light-harvesting by upconversion and downconversion. Tandem or graded PSCs by combining a series of subcells with varying bandgaps seek to rectify the spectral mismatch. Plasmonic nanostructures function as a secondary light source to augment the light-trapping within the perovskite layer and carrier transporting layer, enabling enhanced carrier generation. Texturing the interior using controllable micro/nanoarrays can realize light-matter interactions. Anti-reflective coatings on the top glass cover of the PSCs bring about better transmission and glare reduction. Photon concentration through perovskite-based luminescent solar concentrators offers a path to increase efficiency at reduced cost and plays a role in building-integrated photovoltaics. Distinct from other published reviews, we here systematically and hierarchically present all of the photon management strategies in PSCs by presenting the theoretical possibilities and summarizing the experimental results, expecting to inspire future research in the field of photovoltaics, phototransistors, photoelectrochemical sensors, photocatalysis, and especially light-emitting diodes. We further assess the overall possibilities of the strategies based on ultimate efficiency prospects, material requirements, and developmental outlook.

Advances in inorganic-based colloidal nanovehicles functionalized for nitric oxide delivery

Nitric oxide (NO) is an important pharmaceutical agent of considerable therapeutic interest ascribed to its vasodilative, tumoricidal and antibacterial effects. Rapid development of functional nanomaterials has provided opportunities for us to achieve controllable exogenous delivery of NO. In the current review, a variety of functionalized colloidal nanovehicles that have been developed to date for nitric oxide delivery are reported. Specifically, we focus on inorganic nanomaterials such as semiconductor quantum dots, silica nanoparticles, upconversion nanomaterials, carbon/graphene nanodots, gold nanoparticles, iron oxide nanoparticles as the functional or/and supporting materials to carry NO donors. N-diazeniumdiolates, S-nitrosothiols, nitrosyl metal complexes and organic nitrates as main types of NO donors have their own unique properties and molecular structures. Conjugating the NO donors of different forms with appropriate nanomaterials results in NO delivery nanovehicles capable of releasing NO in a dose-controllable or/and on-demand manner. We also consider the therapeutic applications of those NO delivery nanovehicles, especially their applications for cancer therapy. In the end, we discuss possible future directions for developing exogenous NO delivery systems with more desired structure and improved performance. This review aims to offer the readers an overall view of the advances in functionalized colloidal nanovehicles for NO delivery. It will be attractive to scientists and researchers in the areas of material science, nanotechnology, biomedical engineering, chemical biology, etc.

Dye Sensitization and Local Surface Plasmon Resonance-Enhanced Upconversion Luminescence for Efficient Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved rapid progress in this decade. However, the limited solar spectral utilization has restricted the further improvement of performance of the PSCs. One promising approach to solving this problem is utilizing IR to visible upconversion nanoparticles (UCNPs) in the PSC devices. Despite being confined by the lower quantum yield (QY) and smaller absorption cross section of the traditional UCNPs, their application is still a great challenge. In this work, the IR-783 dye-sensitized core/shell NaYF₄:Yb³⁺, Er³⁺@NaYF₄:Yb³⁺, and Nd³⁺ UCNPs were synthesized and coupled with plasmonic Au nanorods films. Thereby, the upconversion luminescence (UCL) intensity was enhanced by about 120-fold, whereas the luminescent QY was improved from 0.2 to 1.2%. Then, the composite UCNPs were assembled on the SnO₂ layer of the PSCs, which resulted in the power conversion efficiency (PCE) increasing from 19.4 to 20.5% under simulated 100 mw/cm² AM 1.5G irradiation. Up to now, it is the highest PCE for the PSCs based on various upconversion devices. Under the irradiation of a sun concentrator (1 W/cm²), the PCE of the device can be further improved to 21.1%. In-depth studies indicate that under standard sunlight irradiation, the improvement of PCE is due to both the IR to visible UCL and the scattering effect of the UCNPs. Under irradiation of a sun concentrator, the UCL contributes dominantly to the further improvement of PCE. This work provides an effective method for increasing the luminescent QY utilized in the PSCs and is of great significance for future PSCs that use sunlight concentrator.

Study on the effect of sodium-nanoparticle in rice root development

Micro-nanoparticles can enter the root tissue of plant cells along with the multiple lanes, and then accumulate in the tissue. But the plant physiological effect is still less studied. In this study, rice seedlings at germination stage were treated with 100 µM NaBiF4 and BiF3. We found that exogenous application of NaBiF4 treatment inhibited the elongation of rice roots and promoted the generation of adventitious roots, but treated BiF3 did not mediate obvious phenotype. Further analysis of the peroxidase activity in related tissues showed that NaBiF4 induced the activity of SOD and CAT decreased, and POD increased, while BiF3 only induced the activity of SOD to reduced, but the activity of CAT and POD were no changed. Further analysis of the sodium element and potassium element concentration in tissues showed that only the NaBiF4 treatment reduced content of potassium, but not sodium. Finally, stress-related genes OsMT1, OsMT2, OsOVP1, OsNIP2;1, and OsMT2b were analyzed by Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR). These results showed that NaBiF4 induced the expression of OsMT2, OsOVP1, OsNIP2;1 decreased, and OsMT2b increased. However, BiF3 only induced expression of OsMT1 increased. These results provide a physiological basis for further analysis of the effects of sodium salt-nanoparticles in crop plants.

Interface-Dependent Radiative Lifetimes of Yb3+, Er3+ Co-doped Single NaYF4 Upconversion Nanowires

The development of upconversion nanomaterials for many photonic applications requires a detailed understanding of their radiative lifetimes that in turn depend critically on local environmental conditions. In this work, hexagonal (β-phase) sodium-yttrium-fluoride (NaYF₄) nanowires (NWs) were synthesized and substitutionally co-doped with a luminescent solid solution of trivalent erbium and ytterbium ions. A single-beam laser trapping instrument was used in tandem with a piezo-controlled, variable-temperature stage to precisely vary the nanowire's distance from the substrate. The spontaneous photoluminescence lifetime of the ⁴S_3/2 → ⁴I_15/2 transition from Er³⁺ ions was observed to change by >60% depending on the ions' separation distance from a planar (water/glass) dielectric interface. The ⁴S_3/2 state lifetime is observed to increase by a factor of 1.62 ± 0.01 as the distance from the quartz coverslip increases from ∼0 nm to ∼40 μm. Less significant changes in the luminescence lifetime (≤10%) were observed over a temperature range between 25 and 50 °C. The distance dependence of the lifetime is interpreted quantitatively in the context of classical electromagnetic coupling between Er³⁺ ions within the nanowire and the adjacent dielectric interface. We also demonstrate potential applications of the NaYF₄ NWs for both controlling and probing temperatures at nanometer scales by integrating them within a poly(dimethylsiloxane) composite matrix.

Synthesis and Rational design of Europium and Lithium Doped Sodium Zinc Molybdate with Red Emission for Optical Imaging

Highly efficient fluorescent and biocompatible europium doped sodium zinc molybdate (NZMOE) nanoprobes were successfully synthesized via Polyol method. Non-radiative defect centres get reduced with Li+ co-doping in NZMOE nanoprobes. XRD spectra and Rietveld refinement confirmed successful incorporation of lithium ion and crystallinity was also improved with Li+ co-doping. The shape of phosphor is rod shaped, as determined by TEM. Significant enhancement in photoluminescence intensity was observed with 266, 395 and 465 nm excitations. Profound red emission was recorded for 5 at% Li+ co-doped NZMOE nanoprobes with 266 nm excitation. It shows high asymmetry ratio (~15), color purity (94.90%) and good quantum efficiency (~70%). Judd Ofelt parameters have been calculated to measure intensity parameters and radiative transition rates. In order to measure biocompatibility of the nanoprobes, cytotoxicity assays were performed with HePG2 cells. The fluorescence emitted from phosphor material treated HePG2 cells was also measured by Laser Scanning Confocal Microscopy. The bright red fluorescence in HePG2 cells treated with very low concentration (20 μg/ml) of phosphor material indicates that it could be a promising phosphor for biological detection or bio-imaging.

Resonant Plasmon-Enhanced Upconversion in Monolayers of Core-Shell Nanocrystals: Role of Shell Thickness

The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er³⁺-doped NaYF₄ core as a function of the thickness of a NaLuF₄ shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er³⁺ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCNC monolayers is further enhanced by electron-beam-lithographic-designed Au nanodiscs deposited either on top of or buried within the monolayer. It is observed that the improvement by the nanocrystal shells is greater than that of the Au nanodiscs.

SC, Chidangil S, George SD. Post annealing induced manipulation of phase and upconversion luminescence of Cr3+ doped NaYF4:Yb,Er crystals

The role of post synthesis annealing at different temperatures (200–600 °C) on the structural as well as luminescence properties of NaY_80%F₄:Yb_17%,Er_3% prepared via a coprecipitation method was found to change the structure from a cubic to hexagonal phase with a concomitant increase in upconversion luminescence by 12 times for the green region and 17 times for the red region. Addition of the Cr³⁺ ions (5–20 mol%) into the host followed by post annealing at 200–600 °C causes that the samples to exhibit phase dependent and upconversion luminescence behavior that depend upon the doping concentration as well as the annealing temperature. The inductively coupled optical emission spectroscopy reveals that only 1/600 times of the desired volume of the co-dopant goes to the lattice and it can manifest visible spectral changes in the diffuse reflectance spectra of the samples. The samples co-doped with Cr³⁺ ion concentrations of 10–15% and post-annealed at 600 °C were found to have maximum emission with an enhancement factor of 24 for the green region and 33 for the red region. In addition, the laser power dependent studies reveal that even for the power density levels 3.69 W cm⁻² to 32.14 W cm⁻², the samples are in the saturation regime and most of the samples investigated here follow a single photon process, and a few samples show a slope value less than 1 for laser power dependent intensity plots. The results show the remarkable promise of controlled tailoring of the properties of upconversion crystals via post annealing and co-doping.

Co-dopant (Cr³⁺ ion) concentration as well as post annealing found to change the structural as well as luminescence properties of Cr³⁺ ion doped NaY_80%F₄:Yb_17%,Er_3% prepared via a co-precipitation method.

Photovoltaic Effect of a Ferroelectric-Luminescent Heterostructure under Infrared Light Illumination

In this report, a ferroelectric-luminescent heterostructure is designed to convert infrared light into electric power. We use BiFeO₃ (BFO) as the ferroelectric layer and Y₂O₃:Yb,Tm (YOT) as the upconversion layer. Different from conventional ferroelectric materials, this heterostructure exhibits switchable and stable photovoltaic effects under 980 nm illumination, whose energy is much lower than the band gap of BFO. The energy transfer mechanism in this heterostructure is therefore studied carefully. It is found that a highly efficient nonradiative energy transfer process from YOT to BFO plays a critical role in achieving the below-band-gap photon-excited photovoltaic effects in this heterostructure. Our results also indicate that by introducing asymmetric electrodes, both the photovoltage and photocurrent are further enhanced when the built-in field and the depolarization field are aligned. The construction of ferroelectric-luminescent heterostructure is consequently proposed as a promising route to enhance the photovoltaic effects of ferroelectric materials by extending the absorption of the solar spectrum.

Recent Advances of Rare-Earth Ion Doped Luminescent Nanomaterials in Perovskite Solar Cells

Organic-inorganic lead halide based perovskite solar cells have received broad interest due to their merits of low fabrication cost, a low temperature solution process, and high energy conversion efficiencies. Rare-earth (RE) ion doped nanomaterials can be used in perovskite solar cells to expand the range of absorption spectra and improve the stability due to its upconversion and downconversion effect. This article reviews recent progress in using RE-ion-doped nanomaterials in mesoporous electrodes, perovskite active layers, and as an external function layer of perovskite solar cells. Finally, we discuss the challenges facing the effective use of RE-ion-doped nanomaterials in perovskite solar cells and present some prospects for future research.

Using lead chalcogenide nanocrystals as spin mixers: a perspective on near-infrared-to-visible upconversion

This perspective highlights recent advances in the field of PbS NC-sensitized near-infrared-to-visible upconversion based on triplet–triplet annihilation in rubrene.

High-efficiency near-infrared enabled planar perovskite solar cells by embedding upconversion nanocrystals

Integration of the upconversion effect in perovskite solar cells (PSCs) is a facile approach towards extending the spectral absorption from the visible to the near infrared (NIR) range and reducing the non-absorption loss of solar photons. However, the big challenge for practical application of UCNCs in planar PSCs is the poor compatibility between UCNCs and the perovskite precursor. Herein, we have subtly overcome the tough compatibility issue using a ligand-exchange strategy. For the first time, β-NaYF₄:Yb,Er UCNCs have been embedded in situ into a CH₃NH₃PbI₃ layer to fabricate NIR-enabled planar PSCs. The CH₃NH₃I-capped UCNCs generated from the ligand-exchange were mixed with the perovskite precursor and served as nucleation sites for the UCNC-mediated heteroepitaxial growth of perovskite; moreover, the in situ embedding of UCNCs into the perovskite layer was realized during a spin-coating process. The resulting UCNC-embedded perovskite layer attained a uniform pinhole-free morphology with enlarged crystal grains and enabled NIR absorption. It also contributed to the energy transfer from the UCNCs to the perovskite and electron transport to the collecting electrode surface. The device fabricated using the UCNC-embedded perovskite film achieved an average power-conversion efficiency of 18.60% (19.70% for the best) under AM 1.5G and 0.37% under 980 nm laser, corresponding to 54% and 740-fold increase as compared to that of its counterpart without UCNCs.

Highly efficient up-conversion luminescence in Er3+/Yb3+ co-doped Na5Lu9F32 single crystals by vertical Bridgman method

Er³⁺/Yb³⁺ co-doped Na₅Lu₉F₃₂ single crystals with different concentrations of Yb³⁺ ions were prepared to investigate their phase structure, up-conversion (UC) properties and mechanism of UC luminescence by Bridgman method. Under 980 nm near-infrared (NIR) excitation, three sharp UC emission bands topping at green ~525 nm, ~548 nm and red ~669 nm were obtained in Er³⁺/Yb³⁺ doped Na₅Lu₉F₃₂ single crystals which are attributing to the transitions of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively. The quadratic dependence of pump power on UC emission indicated that two-photon process is in charge of the transition from excited state of Yb³⁺ ions to lower state of Er³⁺ ion in Na₅Lu₉F₃₂ single crystals. The long-accepted mechanism for the production of red and green emissions through up-conversion (UC) under 980 nm excitation in Er³⁺/Yb³⁺ co-doped materials apply in the Na₅Lu₉F₃₂ host was displayed. The enhancement of the red emission was observed due to a cross-relaxation (CR) process of the form: ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2. Furthermore, an ideal yellowish green light performance could be achieved with 1.0 mol% Er³⁺ doped certain Yb³⁺ concentrations samples, and its external quantum efficiency approached to 6.80% under 5.5 Wcm⁻² 980 nm excitation which can be applied in developing UC displays for electro-optical devices.

Towards Efficient Spectral Converters through Materials Design for Luminescent Solar Devices

Single-junction photovoltaic devices exhibit a bottleneck in their efficiency due to incomplete or inefficient harvesting of photons in the low- or high-energy regions of the solar spectrum. Spectral converters can be used to convert solar photons into energies that are more effectively captured by the photovoltaic device through a photoluminescence process. Here, recent advances in the fields of luminescent solar concentration, luminescent downshifting, and upconversion are discussed. The focus is specifically on the role that materials science has to play in overcoming barriers in the optical performance in all spectral converters and on their successful integration with both established (e.g., c-Si, GaAs) and emerging (perovskite, organic, dye-sensitized) cell types. Current challenges and emerging research directions, which need to be addressed for the development of next-generation luminescent solar devices, are also discussed.

Manipulating the emission intensity and lifetime of NaYF4:Yb3+,Er3+ simultaneously by embedding it into CdS photonic crystals

Photonic crystals (PCs) have long been considered effective for tuning upconversion luminescence due to their photonic band gap (PBG) and the redistribution of density of optical states (DOS). Although the emission intensity can be changed obviously by the PC effect, rarely an obvious lifetime change consistent with theory is observed due to the low refractive index of PS or SiO₂ spheres in the commonly used PCs. Herein, CdS/NaYF₄:Yb³⁺,Er³⁺ composite PCs with a high refractive index contrast are fabricated in one step with upconversion nanoparticles filled inside CdS PCs. When the upconversion emission peak lies at the edge of the PBGs of the composite PCs, a dramatic decrease in lifetime by 28% and 41% is observed for the green and red emissions, respectively. At the same time, obvious emission intensity enhancements are also observed. In contrast, PS PCs with a low refractive index contrast show a slight effect on the lifetime of upconversion luminescence with their emission peak at the edge of the PBGs. Our results agree well with theory and prove that a sufficiently large refractive index contrast is necessary for PCs to dramatically tune the luminescence lifetime and intensity simultaneously.