Upconversion Luminescence Intensity Research Articles

Gd3+-Doping Effect on Upconversion Emission of NaYF4: Yb3+, Er3+/Tm3+ Microparticles

β-NaYF4 microcrystals co-doped with Yb3+, Er3+/Tm3+, and Gd3+ ions were synthesized via a hydrothermal method using rare-earth chlorides as the precursors. The SEM and XRD data show that the doped β-NaYF4 form uniform hexagonal prisms with an approximate size of 600–800 nm. The partial substitution of Y by Gd results in size reduction of microcrystals. Upconversion luminescence spectra of microcrystals upon 980 nm excitation contain characteristic intra-configurational ff bands of Er3+/Tm3+ ions. An addition of Gd3+ ions leads to a significant enhancement of upconversion luminescence intensity with maxima at 5 mol % of dopant.

Dual-Wavelength Excited Intense Red Upconversion Luminescence from Er3+-Sensitized Y2O3 Nanocrystals Fabricated by Spray Flame Synthesis.

Er3+-sensitized upconversion nanoparticles (UCNPs) have attracted great attention due to their tunable upconversion (UC) emissions, low cytotoxicity, high resistance to photobleaching and especially multiple effective excitation wavelengths. However, detailed energy conversion between Er3+ and Tm3+ ions in Y2O3 UCNPs is still a problem, especially under multi-wavelength and variable pulse width excitation. In this work, we successfully fabricated a series of Er3+-sensitized Y2O3 nanocrystals by a spray flame synthesis method with a production rate of 40.5 g h−1. The as-prepared UCNPs are a pure cubic phase with a mean size of 14 nm. Excited by both 980 and 808 nm lasers, the tunable upconversion luminescence (UCL) from Er3+ ions was achieved by increasing the Er3+ doping concentration, co-doping Tm3+ ions and extending excitation pulse-width. The investigations of the lifetimes and the laser power dependence of UC emissions further support the proposed mechanism, which provides guidance for achieving effective color control in anticounterfeiting and multiplexed labeling applications. In addition, the red UC emission at about 5 mm beneath the tissue surface was observed in an ex vivo imaging experiment under the excitation of 808 nm laser, indicating that the Y2O3:Er3+/Tm3+ UCNPs have great prospects in further biological applications.

Upconversion in Detail: Multicolor Emission of Yb/Er/Tm‐Doped Nanoparticles under 800, 975, 1208, and 1532 nm Excitation Wavelengths

AbstractSpectroscopic properties of YPO4 nanoparticles doped with Yb3+, Tm3+, and Er3+ ions have been studied in detail. These multiemitting materials are promising not only for photonic or electronic use but especially for anticounterfeiting applications. The nanopowders are synthesized by the facile coprecipitation method with annealing in the air atmosphere at 1000 °C. The structural and morphological studies reveal pure tetragonal nanocrystallites with an average size of 20–30 nm. Most interestingly, under NIR excitation, the samples exhibit intense upconversion (UC) luminescence where the color can be tuned by changing the laser source, switching the excitation wavelengths between 800, 975, 1208, and 1532 nm, double‐wavelength excitation, and by changing laser power density. As a result, a very high color shift, being the result of intensity changes in the emission bands of Er3+ (green and red) and Tm3+ (blue and red) is obtained. The luminescence lifetimes, temporal evolution, and the pump power dependences are also measured to propose the mechanisms responsible for the observed UC emission.

Effect of Li+/Mg2+ co-doping and optical temperature sensing behavior in Y2Ti2O7: Er3+/Yb3+ upconverting phosphors

Abstract The Y2Ti2O7: Er3+/Yb3+ phosphors co-doped with Li+ and Li+/Mg2+ ions were prepared via solid state reaction method. The effects of Li+ and Mg2+ co-doping on the structure and morphology of Y2Ti2O7: Er3+/Yb3+ phosphors were investigated by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) spectra and field emission scanning electron microscopy (FESEM) observations. The 980 nm excitation assisted UC luminescence intensity for green and red band was enhanced by 8.7 and 14.3 times respectively after co-doping with Li+/Mg2+ ions in Y2Ti2O7: Er3+/Yb3+, which could be explained based on the variations in crystal structure and particle size. Using the fluorescence intensity ratio (FIR) technique, the impact of 980 nm pump power on the thermometric behaviors was studied based on the thermally coupled levels of 2H11/2 and 4S3/2 of Er3+ ions. Due to the unique thermal properties of the Y2Ti2O7 host, the temperature increment caused by laser thermal effect was minimized, benefiting the consistency of FIR data over a wide range of pump power density. Besides, a high relative sensor sensitivity of 1.17%K−1 was obtained for Y2Ti2O7: Er3+/Yb3+/Li+/Mg2+ phosphors. These characteristics may make the studied system applicable for optical temperature sensor with high accuracy.

Fast coating of hydrophobic upconversion nanoparticles by NaIO4-induced polymerization of dopamine: Positively charged surfaces and in situ deposition of Au nanoparticles

In this work, hydrophobic upconversion nanoparticles (UCNPs) were transformed into hydrophilic NPs via the rapid oxidative polymerization of dopamine (DA) induced by NaIO4 in a water-in-oil microemulsion system. The polydopamine (PDA)-coated UCNPs synthesized using NaIO4 as the oxidant exhibited a positive charge (+26.4 mV) at pH 7.0. Those prepared using ambient O2 as the oxidant had a negative charge (–15.4 mV). The difference in the composition between PDA-coated UCNPs using NaIO4 (UCNPs@PDA-NaIO4) and those using ambient O2 (UCNPs@PDA-O2) as oxidants was supported by the XPS and FTIR results. The difference in the surface charges may be due to variation in the composition, which probably attributed to the different oxidants used. A series of controlled experiments were carried out to infer the origination of the positive charges of UCNPs@PDA-NaIO4. Furthermore, Au nanoparticles (Au NPs) were deposited on UCNPs@PDA-NaIO4 where PDA served as both the reductant and the stabilizer so that additional reagents and thermal treatments were not necessary. The effect of pH on the deposition of Au NPs was also studied. In addition, the load density of Au NPs on the surface of the PDA-coated UCNPs could be adjusted by changing the dosage of HAuCl4, which could effectively enhance the upconversion luminescence (UCL) intensity. This work provides a method to regulate the surface charge of the PDA coating, and the nanocomposites with unique properties synthesized by this method may be employed in biological imaging, diagnostic analysis, and other biological fields.

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.

RETRACTED ARTICLE: Enhanced luminescence of Mo3+-doped β-NaREF4 nanowires prepared via coprecipitation–solvothermal ion-exchange method and their application in upconversion polyurethane composite

A variety of 0–20 mol% of molybdenum ions (Mo3+)-doped sodium rare-earth (RE) fluoride upconversion nanowires (UCNW) were prepared by Coprecipitate–Solvothermal Ion-Exchange (CSIE) method. In this study, the Mo3+-doped rare-earth hydroxide [REMo(OH)3] precursors were synthesized by the coprecipitation–solvothermal method. Following, the hexagonal phase of the Mo3+-doped sodium rare-earth fluoride (β-NaGdF4: Yb3+/Tm3+/Mo3+, abbreviated as NaREMoF4) nanowires were successfully prepared by ion-exchange reaction, and checked by XRD data analysis. As shown in the EDS results, Mo3+ was uniformly distributed in the β-NaREMoF4. The controlled diameter (20–50 nm) and aspect ratio (20–500) values of β-NaGdF4: Yb3+/Tm3+/Mo3+ nanowires strongly depend on the amount of Mo doping according to TEM images. The upconversion luminescence intensity (UC) of 10 mol% Mo3+-doped β-NaREMoF4 nanowires was increased by one order of magnitude under the 980 nm near-infrared (NIR) excitation in regard to the undoped sample. One-dimensional (1D) upconversion nanowires (UCNW) with 10 mol% Mo3+ doping gives the upconversion polyurethane (UCPU) excellent luminescent performance and about 99% of enhanced tensile strength.

Optimized Multimetal Sensitized Phosphor for Enhanced Red Up-Conversion Luminescence by Machine Learning.

In this research, machine learning including the genetic algorithm (GA) and support vector machine (SVM) algorithm is used to solve the "low up-conversion luminescence (UCL) intensity" problem in order to find the optimal phosphor with enhanced red UCL emission using multielement K/Li/Mn metal modulation. Compared with the first generation of phosphors, the best phosphors' fluorescence intensity occurs in the third generation optimized by the GA, with a stronger brightness (4.91-fold), a higher relative quantum yield (6.40-fold), and an enhanced tissue penetration depth (by 5 mm). The single and multiple dopants effect on the upconversion intensity of K+Li+Mn sensitizers is also studied: the intensity increases first and then decreases with the increase of Yb/Er/K+Li+Mn content, and the optimized K+Li+Mn concentration is 6.03%. In order to confirm the stability of the brightness optimization by the GA, a batch of phosphors was synthesized with the same element proportion, and the similarity of fluorescence intensity of two batches of phosphors was evaluated by the SVM algorithm with the classification accuracy index. Finally, the optimized phosphor was used for bioimaging and phosphor-LED.

Enhanced red and near-infrared upconversion luminescence properties in CaSc2O4 microcrystals

CaSc2O4 co-doped with Yb/Er, Yb/Tm was successfully prepared through a high-temperature solid-state method. The enhanced red and near-infrared upconversion luminescence intensities in CaSc2O4 co-doped with Yb/Er, Yb/Tm were observed. As the increase of Ca2+ ion doping concentration, the crystal phase evolved from cubic Sc2O3 to orthorhombic CaSc2O4, then to cubic CaO. When Ca2+ ion doping concentration is about 40 mol%, CaSc2O4 co-doped with Yb/Er has the maximum red emission intensity (666 nm) which is increased by 39.5 times than that of Sc2O3 microcrystals (MCs). Similarly, the maximum near-infrared emission intensity (807 nm) in CaSc2O4 co-doped with Yb/Tm is enhanced by 9 times than that of Sc2O3 MCs when Ca2+ ion doping concentration is about 40 mol%. They are in accordance with that Er3+ ion and Tm3+ ion exhibit the longest luminescence lifetime in CaSc2O4 MCs than that in Sc2O3 and CaO MCs.

Laser-induced thermal effect and the role of Nd3+ in Tm3+/Yb3+/Nd3+ triply doped LaNbO4 up-conversion phosphors under 808 nm excitation

In this work, intense blue up-conversion (UC) luminescence in LaNbO4: Tm3+/Yb3+/Nd3+ phosphors were obtained under the excitation of 808 nm. The laser-induced thermal effect and the role of Nd3+ in this type of phosphors were discussed. Compared with Tm3+ singly doped and Tm3+/Yb3+ doubly doped samples, thousands-fold and 30-fold overall enhancement in the blue emission intensity of Tm3+ in Tm3+/Yb3+/Nd3+ triply doped LaNbO4 were achieved with the help of Nd3+ sensitizing and the energy transfer bridging Yb3+ ion. The luminescence-enhancement effect exhibited a strong dependence on Nd3+ doping concentration, and the optimal concentration of Nd3+ was found to be 1.0 mol%. Once the doping concentration of Nd3+ exceeded this value, concentration dependent luminescence quenching occurred. Moreover, the laser-induced thermal effect on Tm3+-doped samples was investigated by using Er3+-doped sample as the thermal probe. It was found that the elevation of sample temperature induced by laser irradiation depended on both the doping concentrations of Nd3+ and the excitation power density of 808 nm laser.

Controlled synthesis and upconversion luminescence properties of Yb3+/Er3+ co-doped Bi2O3 nanospheres for optical and X-ray computed tomography imaging

Bi2O3 nanospheres co-doped with Yb3+/Er3+ upconverting ions pair have been successfully synthesized via a facile hydrothermal method followed by a heat treatment. The resulting Bi2O3:Yb3+,Er3+ nanospheres show a monodisperse spherical morphology with narrow size distribution (~180 nm) and display intense upconversion luminescence under 980 nm laser excitation. It is found that the crystal structure, morphology and upconversion luminescence properties of nanospheres have a direct relationship with the doping concentration of Yb3+ and the subsequent calcination temperature. These Bi2O3:Yb3+,Er3+ nanospheres can be prepared in large quantities and can be easily modified with a layer of biocompatible polyethyleneimine (PEI) molecules. The yielded nanospheres could not only maintain uniform size and morphological characteristics, but also show good water dispersibility and biocompatibility. Remarkably, these Bi2O3:Yb3+,Er3+ upconversion nanophosphors could also act as an effective contrast agent for X-ray computed tomography (CT) imaging, which show higher contrast efficacy than commercial iodine-based contrast agent. The proposed facile synthetic route and inexpensive matrix materials pave the way for broad use of these Bi2O3:Yb3+,Er3+ nanospheres as ideal dual-mode bioimaging probes in biomedical field.

A Multi‐Functional Highly Efficient Upconversion Luminescent Film with an Array of Dielectric Microbeads Decorated with Metal Nanoparticles

AbstractUpconversion nanoparticles (UCNPs) have been integrated with photonic platforms to overcome the intrinsically low quantum efficiency limit of upconversion luminescence (UCL). However, platforms based on thin films lack transferability and flexibility, which hinders their broader and more practical application. A plasmonic structure is developed that works as a multi‐functional platform for flexible, transparent, and washable near‐infrared (NIR)‐to‐visible UCL films with ultra‐strong UCL intensity. The platform consists of dielectric microbeads decorated with plasmonic metal nanoparticles on an insulator/metal substrate. Distinct improvements in NIR confinement, visible light extraction, and boosted plasmonic effects for upconversion are observed. With weak NIR excitation, the UCL intensity is higher by three orders of magnitude relative to the reference platform. When the microbeads are organized in a square lattice array, the functionality of the platform can be expanded to wearable and washable UCL films. The platform can be transferred to transparent, flexible, and foldable films and still emit strong UCL with a wide viewing angle.

Effect of F/O ratio on up-conversion and down-conversion luminescence properties of Er3+/Yb3+ co-doped SiO2–Al2O3–AlF3-Gd2O3–Na2O glass

Er3+/Yb3+ co-doped SiO2–Al2O3–AlF3-Gd2O3–Na2O glass with different F/O ratios were prepared. The effect of F/O ratio on the structure and optical properties of the glasses was investigated. The amorphous nature of the glasses was determined by XRD, and the glass transition temperature (Tg) and the crystallization peak temperature (Tc) were obtained from the DSC curves. The change in the structure of the glass network was analyzed by infrared spectroscopy. The density and absorption spectra of the bulk glass was also investigated. The excitation spectrum, emission spectrum and 4S3/2 level fluorescence lifetime of Er3+ ions in the glasses were tested by a Xenon lamp with adjustable wavelength. The up-conversion luminescence spectrum of Er3+ ions was measured under excitation of a 980 nm laser. The results show that the green (2H11/2, 4S3/2) and red (4F9/2) up-conversion luminescence intensity is the highest when the F/O ratio is 0.214. According to the variation law of up-conversion luminescence intensity and pump power, it is confirmed that the up-conversion green (2H11/2 → 4I15/2, 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) light of Er3+ ions are the three-photon absorption process based on excited state absorption (ESA) and energy transfer up-conversion (ETU) mechanism. Finally, energy transfer transitions between Er3+ and Yb3+ are also analyzed. The CIE chromaticity coordinates were calculated from the emission spectrum obtained at two different excitation wavelengths of 378 nm and 980 nm.

Effect of hydrothermal synthesis conditions on up-conversion luminescence intensity of β-NaYF4 : Er3+, Yb3+ submicron particles

The differences in the luminescence intensities of up-conversion β-NaYF4 : Er3+, Yb3+ particles synthesised by the hydrothermal method under various synthesis conditions are studied. The results of the study lead to the conclusion that in order to achieve the maximum luminescence intensity in such particles, it is necessary to use ammonium fluoride and a medium with pH = 3. In this case, the length of the particles increases, up to the formation of rod-shaped particles. Based on the data on the size of the coherent scattering region and on microstresses, we can assume that the particles are polycrystals. At the same time, limiting the size of the coherent scattering region is possible due to the defective structure. When the nanoparticles are synthesised in a medium with pH = 3, hydrolysed regions containing OH groups are formed on the crystallite surface. The presence of these groups does not affect the intensity of up-conversion luminescence of submicron-size particles.

Effects of Ca2+ doping on upconversion luminescence intensity and crystal field asymmetry of β-NaYF4:Yb3+/Er3+ microcrystals

The green and red upconversion (UC) emissions of 35 mol% Ca2+ doped β-NaYF4:Yb3+/Er3+ microcrystals were improved to 102.8 and 43.1 times higher than those of Ca2+-free counterparts. X-ray powder diffraction spectra, scanning electron microscopy, Fourier transform infrared spectroscopy, upconversion luminescence (UCL) spectroscopy and Judd-Ofelt analysis were performed to investigate the mechanism of UCL intensity enhancement caused by Ca2+ doping. The hexagonal-to-cubic phase transition was observed when Ca2+ doping concentration exceeded 35 mol%. The incorporation of Ca2+ into hexagonal microcrystals lattice induced unit cell volume enhancement and microcrystals surface to volume ratio variation, which affected the UCL intensity by hindering the non-radiative energy loss process via Yb3+/Er3+ ions and changing the amount of surface quenchers adsorbed on the microcrystals surface. The hypersensitive transition related local crystal field asymmetry around Er3+ in the grown hexagonal microcrystals was also investigated by Judd-Ofelt analysis but unexpectedly found to be the less dominant factor for the UCL intensity variation. This study helps to realize the mechanism of impurity ion doping resulted UCL intensity enhancement, which is instructive for high-performance UC microcrystals synthesis.

Growth and enhanced upconversion luminescence intensity of Mg2+ and Cr3+ co-doped β-NaYF4:Yb3+/Er3+ microcrystals

In this study, Cr3+ and Mg2+ co-doped β-NaYF4:Yb3+/Er3+ (NYFYE) microcrystals were synthesized for the first time by the facile hydrothermal method. Cr3+ and Mg2+ co-doping was confirmed to influence the structure, morphology and UCL property of NYFYE microcrystals. The growth mechanism of NYFYE microcrystals doped with different Cr3+ and Mg2+ was discussed in detail. With 16 ​mol% Cr3+ and 4 ​mol% Mg2+ co-doping into NYFYE, the UC emissions were enhanced to 7 times (green) and 4 times (red) higher than those of the undoped counterpart, and the mechanism was explored by considering the different roles of Cr3+ and Mg2+, the surface quenching and concentration quenching effects. This finding suggests the co-doping of two different impurity ions may be a better approach than corresponding single impurity ion doping for morphology control and UCL intensity enhancement, which is instructive for high performance UC crystals design.

Optimal spectral phase control of femtosecond laser-induced up-conversion luminescence in Sm3+:NaYF4 glass

The spectral phase of the femtosecond laser field is an important parameter that affects the upconversion (UC) luminescence efficiency of dopant lanthanide ions. In this work, we report an experimental study on controlling the UC luminescence efficiency in Sm3+:NaYF4 glass by 800-nm femtosecond laser pulse shaping using spectral phase modulation. The optimal phase control strategy efficiently enhances or suppresses the UC luminescence intensity. Based on the laser-power dependence of the UC luminescence intensity and its comparison with the luminescence spectrum under direct 266-nm femtosecond laser irradiation, we propose herein an excitation model combining non-resonant two-photon absorption with resonance-mediated three-photon absorption to explain the experimental observations.

Highly Sensitive Detection of Amaranth Realized with Upconversion Nanoparticles-Based Solid Sensor

High performance of synthetic colorants detection system has great importance for food safety, yet its quantitative analysis with high sensitivity remains challenging. Herein, core–shell NaGdF4: Yb3+, Er3+@NaYF4 upconversion nanoparticles based solid sensor (UCNPs-SS) is designed and explored in this work, where the detection limit of 0.01 μg mL−1 can be achieved thanks to the effective energy transfer between amaranth and this fluorescence-based sensor. Besides, ratiometric detection technique is employed in this system to ensure the accuracy of the detection, hence, insignificant change of the upconversion luminescence intensity of this UCNPs-SS is observed when the potentially interfering substances are present. Moreover, this UCNPs-SS exhibits good stability for recycled detections. Hence, the precise amaranth detection of three commercial soft drinks and excellent recoveries are achieved via our core–shell UCNPs-SS in this work. These positive results indicate that this proposed fluorescence detect approach not only exhibits excellent sensitivity, but also provides feasibility for practical applications.

Neoteric Conjugative Electrospinning towards Alloplastic Nanofiber Yarns Affording Enhanced Upconversion Luminescence and Tailored Magnetism

AbstractUpconversion luminescent‐magnetic [Y2O3:Yb3+, Er3+/polyacrylonitrile (PAN)]//[CoFe2O4/PAN] alloplastic nanofiber yarns (ANYs) are successfully prepared by a novel conjugative electrospinning. The ANYs consist of two different kinds of nanofibers, respectively called as [CoFe2O4/PAN] magnetic nanofibers and [Y2O3:Yb3+, Er3+/PAN] upconversion luminescent nanofibers, and the two kinds of nanofibers are combined together and twisted to form ANYs. Pumped by 980‐nm laser, the ANYs exhibit excellent upconversion luminescence (UCL) and orange‐red fluorescence. ANYs demonstrate tailored saturation magnetization by modulating CoFe2O4 nanoparticles (NPs) content. The influence mechanism of magnetic CoFe2O4 NPs on the UCL of ANYs is detailedly explored. The relationship between mechanical properties and twist for ANYs is explored through the tensile test. Compared with the counterpart coessential nanofiber yarns (CNYs), ANYs exhibit about 10 times stronger UCL intensity owing to the complete separation of the upconversion luminescent substance from the magnetic substance and thereby great reduction of impact of the magnetic substance on the upconversion luminescent substance. Thus, concurrent enhancive upconversion luminescence (UCL) and tunable magnetism of the ANYs are successfully achieved. The detailed formation mechanism of ANYs is advanced and a novel fabrication technique of yarns is established and can be popularized to prepare other bi‐ and multi‐functional nanofibrous yarns.

Влияние температуры апконверсионных частиц NaYF4:Er,Yb на формирование люминесценции

The intensity of upconversion luminescence depends nonlinearly on the excitation intensity. The aim of this work is to study the effect of the temperature of NaYF4:Er,Yb upconversion particles on the dependence of the luminescence intensity on the excitation intensity. The synthesized particles were observed to have the shape of a hexagonal prism with a width of about 440 nm and a height of 445 nm. The upconversion luminescence spectra were obtained in the temperature range of 22–55° C with the excitation intensity in the range of 1.5–9.4 W/cm2. The obtained results show the green-band luminescence photons can be generated by means of two-step and three-step mechanisms: the contribution of these mechanisms depends on the temperature of the particles. As the temperature increases, the contribution of the three-stage mechanism of green luminescence excitation is enhanced.