Energy Migration Research Articles

Ultrahighly Efficient Narrowband Red Luminescence of Uniquely Distorted Mn4+ Octahedron in the Feldspar‐Type LED Phosphor

AbstractThe mineral structure‐inspired discovery of red phosphors has become a research hotspot that can help compensate for the missing red components in cold white light‐emitting diodes (WLEDs). Herein, first the feldspar‐type structure is utilized to develop narrowband red‐emitting CaAl2Si2O8:Mn4+/Mn4+, Mg2+ phosphors. Rietveld refinement results confirm the Mn4+ occupation of the distorted [Ca1O6] octahedron, resulting in a strong nephelauxetic effect, and thus exhibits bright red luminescence with a narrow full width at half maximum (43 nm). Compared with CaAl2Si2O8:Mn4+, the fluorescence intensity of CaAl2Si2O8:Mn4+, Mg2+ is further enhanced by up to 210% owing to the decreasing nonradiative decay rate from 2Eg excited state, which weakens the concentration quenching effect resulting from energy migration along the adjacently aggregated Mn4+. CaAl2Si2O8:Mn4+, Mg2+ shows preferable thermal stability with an ultrahigh quantum efficiency (90.3%), surpassing other current Mn4+‐doped oxide phosphors and UCr4C4‐type nitride phosphors. A fabricated prototype WLED obtains ideal warm white light with a low correlated color temperature (3081 K) and a wide color gamut covering 112% of the National Television System Committee standard. Guided by natural mineral‐type structural prototypes, this study demonstrates the possibility of designing a uniquely distorted local structure to achieve a superior luminescence performance of Mn4+ for warm WLEDs.

Spark plasma sintering and optical properties of Tm3+ and Tm3+ /Yb3+ doped NaLaF4 transparent glass-ceramics

Tm3+ doped oxyfluoride glass-ceramics (GCs) containing NaLaF4 nanocrystals (NCs) have been obtained by spark plasma sintering (SPS). First, the precursor glasses were melted and then milled and sieved to a suitable particle size. Glass powder pellets were sintered by spark plasma sintering under vacuum conditions. The SPS processing parameters (temperature, pressure, and holding time) were optimized to obtain transparent glass-ceramics. The times of SPS processing are considerably shorter compared with those for the preparation of these GCs by conventional thermal treatment. All glass-ceramics contain nanocrystals of the β- NaLaF4 phase with an average crystal size of 20 nm, but the more highly doped samples (2Tm3+ and 0.5Tm3+/2Yb3+) show evidence of the presence of another phase corresponding with α-NaLaF4. The luminescence properties of the near infrared (NIR) emissions of Tm3+ for different concentrations reveal the presence of concentration quenching of the 3H4 and 3F4 levels. The analysis of the decay from the 3H4 level with increasing concentration is consistent with a dipole-dipole quenching process assisted by energy migration, whereas the self-quenching of the 3F4→3H6 emission can be attributed to fast diffusion. Energy transfer between Yb3+ and Tm3+ ions is confirmed by the NIR and upconverted (UC) emissions after Yb3+ excitation at 975 nm. No UC emission is observed under 791 nm excitation of Tm3+ ions.

Orthogonal Trichromatic Upconversion with High Color Purity in Core‐Shell Nanoparticles for a Full‐Color Display

AbstractMaterials with tunable emission colors has attracted increasing interest in both fundamental research and applications. As a key member of light‐emitting materials family, lanthanide doped upconversion nanoparticles (UCNPs) have been intensively demonstrated to emit light in any color upon near‐infrared excitation. However, realizing the trichromatic emission in UCNPs with a fixed composition remains a great challenge. Here, without excitation pulsed modulation and three different near‐infrared pumping, we report an experimental design to fine‐control emission in the full color gamut from core–shell‐structured UCNPs by manipulating the energy migration through dual‐channel pump scheme. We also demonstrate their potential application in full‐color display. These findings may benefit the future development of convenient and versatile optical methos for multicolor tuning and open up the possibility of constructing full‐color volumetric display systems with high spatiotemporal resolution.

Orthogonal Trichromatic Upconversion with High Color Purity in Core-Shell Nanoparticles for a Full-Color Display.

Materials with tunable emission colors has attracted increasing interest in both fundamental research and applications. As a key member of light-emitting materials family, lanthanide doped upconversion nanoparticles (UCNPs) have been intensively demonstrated to emit light in any color upon near-infrared excitation. However, realizing the trichromatic emission in UCNPs with a fixed composition remains a great challenge. Here, without excitation pulsed modulation and three different near-infrared pumping, we report an experimental design to fine-control emission in the full color gamut from core-shell-structured UCNPs by manipulating the energy migration through dual-channel pump scheme. We also demonstrate their potential application in full-color display. These findings may benefit the future development of convenient and versatile optical methos for multicolor tuning and open up the possibility of constructing full-color volumetric display systems with high spatiotemporal resolution.

Aging Effects on the Exciton Relaxation and Diffusion Processes in CsPbBr3 Nanocrystals

AbstractFully inorganic perovskite nanocrystals (NCs) have been widely investigated due to their potential as very interesting active materials for several types of photonic and optoelectronic devices. Despite several experiments designed to investigate the basic emission properties of these NCs, a clear and complete understanding of their photophysics is still missing. In this work, temperature‐dependent steady state and time‐resolved photoluminescence (PL) measurements are used to investigate the nature of the emitting states, the origin of the excitation relaxation dynamics and the effects of aging upon long exposure to wet air for thin films of CsPbBr3 NCs prepared by coprecipitation. It is demonstrated that both free excitons and localized excitons contribute to the NC emission and that electron traps within the conduction band, ≈14 meV and ≈80 meV above the band edge, determine thermal emission quenching. Moreover, it is shown that the non‐exponential PL relaxation dynamics are due to short‐range energy migration within a disordered distribution of localized states between 10 and 100 K, with activation of a second, long‐range diffusion process at higher temperatures. It is also demonstrated that aging determines the variations in defect levels, exciton‐phonon coupling and exciton relaxation dynamics. The results substantially improve the current understanding of the basic photophysics of CsPbBr3 NC films and of the aging effects and are expected to provide a useful guide for future characterization of other similar materials.

Simulation of bulk and grain boundary diffusion phenomena in a high entropy CoCrFeMnNi alloy by molecular dynamics

A series of calculations on the self-diffusion behavior of high entropy CoCrFeMnNi alloy were carried out using molecular dynamics methods. By computing both vacancy formation energy and atomic migration energy of the constituent elements in the alloy, the diffusional activation energy of each element is obtained, and the self-diffusion coefficients for bulk diffusion were calculated, with the values exhibiting close to of experiments. A model for structures of symmetrically tilted grain boundary is established, with Σ9 and Σ27 grain boundaries studied based on the coincidence site lattice theory. Measured by the full width at half maxima of the radial distribution function, it is found that the grain boundaries with low index are more ordered than those with high plane index, and the atom fluctuation occurred in the low-indexed grain boundaries is less intensively and sensitively to temperature change. Meanwhile, the diffusion coefficients of ordered grain boundaries are generally smaller than those of disordered grain boundaries. Compared with the experimental values of grain boundary diffusion, the diffusion activation energy of configured grain boundaries from coincidence site lattice is smaller than that of normal large-angle grain boundaries.

Protonated organic cations enhance the luminescence performance and thermal stability of fluoride red phosphor K2TiF6:Mn4+

The application of red luminescent phosphors composed of Mn4+-activated fluoride in WLEDs has aroused great interest. However, most of the fluoride phosphors doped with Mn4+ have poor luminescent thermostability. In order to improve their luminescent thermostability, a new type of organic-inorganic hybrid red luminescent phosphor K2TiF6:PIPH22+,Mn4+ (PIPH22+ = protonated piperazine) was synthesized by a simple cation exchange method in this work. Protonated piperazine was incorporated into K2TiF6 lattice, and the strong hydrogen bond between H- and F- enabled the organic group to exist stably in the inorganic lattice. Compared with K2TiF6:Mn4+, the addition of PIPH22+ increases distance between adjacent [TiF6]2- and inhibits the energy migration of Mn4+ ions between [MnF6]2- to reduce nonradiative transitions. Therefore, the hybrid sample K2TiF6:PIPH22+,Mn4+ has higher emission intensity and quantum yield. Under the excitation of 467 nm blue light, K2TiF6:PIPH22+, Mn4+ exhibits sharp red emission near 631 nm, and the luminescence intensity is 2.14 times that of K2TiF6:Mn4+. K2TiF6:PIPH22+,Mn4+ has significant negative thermal quenching and higher luminescence thermostability, which can be attributed to electron-phonon interaction mechanism at high temperature. The prototype WLED prepared with YAG:Ce3+, K2TiF6:PIPH22+,Mn4+ and InGaN blue LED chip can emit warm white light (CCT = 2795 K, CRI = 91.1) under 20 mA current, and the luminescence efficiency was 105.8 Lm/W. The prototype WLED was tested under 20–120 mA current, and the results showed that the changes of CCT and CRI were stable. These results indicate the potential of K2TiF6:PIPH22+,Mn4+ in WLED applications.

Ab-initio DFT calculations and experimental investigations into optoelectronic and structural properties of Ca9Al(PO4)7:Sm3+ orange phosphor

Nanocrystalline phosphors of composition Ca9Al(PO4)7:xSm3+ (x = 0.01–0.16) were successfully synthesized using a urea-assisted highly efficient solution combustion technique. The structural, morphological, and photoluminescence features of the series of white powdered samples were studied using powder X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and photoluminescence characterization techniques. The structural framework and crystal phase purity of Ca9Al0.90Sm0.10(PO4)7 phosphor were investigated by Rietveld refinement analysis. The results showed that the replacement of Al3+ ions with Sm3+ ions in Ca9Al0.90Sm0.10(PO4)7 nanophosphor didn't alter the host lattice crystal structure and a trigonal lattice having R3c(161) space group was established. Meanwhile, the average particle size evaluated from the Scherrer equation was further confirmed by transmission electron microscopic studies revealing a size domain ranging from 35-60 nm. Structural optimization of the pure host via first-principle routing disclosed the density of states and electronic band structure having a band gap of 4.738 eV, which was found to be quite comparable to the experimentally observed value (4.37 eV) from the diffuse reflectance spectra. Many key optical properties, such as optical conductivity, extinction coefficient, loss function, and refractive index were determined by using a calculated dielectric function. Furthermore, photoluminescence studies inferred that 10 mol% of Sm3+ ion in Ca9Al(PO4)7 host matrix gave the best luminescence emission intensity. Detailed photoluminescent analysis of Ca9Al(PO4)7:xSm3+ phosphors demonstrated that on excitation at 401 nm, an intense reddish-orange emission was received at 600 nm (4G5/2 → 6H7/2 transition). The critical distance for energy migration came out to be 22 Å which strongly favors the mechanism behind luminescence quenching as dipole-quadrupole interactions resulting from the over-doping of activator ions. The chromaticity coordinates (0.5216, 0.3630) were obtained using MATLAB computations. These results favor the applicability of Sm3+ doped Ca9Al(PO4)7 phosphors as the reddish-orange emitter in near-ultraviolet excited white light-emitting diodes.

Understanding FRET in Upconversion Nanoparticle Nucleic Acid Biosensors.

Upconversion nanoparticles (UCNPs) have been frequently applied in Förster resonance energy transfer (FRET) bioanalysis. However, the understanding of how surface coatings, bioconjugation, and dye-surface distance influence FRET biosensing performance has not significantly advanced. Here, we investigated UCNP-to-dye FRET DNA-hybridization assays in H2O and D2O using ∼24 nm large NaYF4:Yb3+,Er3+ UCNPs coated with thin layers of silica (SiO2) or poly(acrylic acid) (PAA). FRET resulted in strong distance-dependent PL intensity changes. However, the PL decay times were not significantly altered because of continuous Yb3+-to-Er3+ energy migration during Er3+-to-dye FRET. Direct bioconjugation of DNA to the thin PAA coating combined with the closest possible dye-surface distance resulted in optimal FRET performance with minor influence from competitive quenching by H2O. The better comprehension of UCNP-to-dye FRET was successfully translated into a microRNA (miR-20a) FRET assay with a limit of detection of 100 fmol in a 80 μL sample volume.

Temperature-Dependent Reversible Optical Properties of Mn-Based Organic–Inorganic Hybrid (C8H20N)2MnCl4 Metal Halides

Organic-inorganic metal halides (OIMHs) have abundant optical properties and potential applications, such as light-emitting diodes, displays, solar cells, and photodetectors. Herein, we report zero-dimensional Mn-based OIMH (C8H20N)2MnCl4 single crystals synthesized by a simple slow evaporation method, which exhibit intense green emission at 520 nm originating from 4T1-6A1 transition of Mn2+ ions. Large organic cations in the crystal structure result in the isolated [MnCl4]2- tetrahedrons, and the closest Mn-Mn distance reaches 9.07 Å, which effectively inhibits the migration of excitation energy between adjacent Mn2+ emission centers, thus achieving a high quantum yield (∼87%) and a long photoluminescence (PL) lifetime (3.42 ms). The different optical and structural properties at low and high temperatures are revealed by temperature-dependent PL and X-ray diffraction spectra. The PL spectra and lifetimes under the heating and cooling processes indicate that the optical property transitions are reversible at 220/240 K. Our work provides a promising strategy for building multifunctional optoelectronic materials and insights into the understanding convertible photophysical properties from isomers of metal halides.

Interface Engineering Enables High-Performance Sb Anode for Sodium Storage

Heterointerface engineering has been verified to be an effective approach to enhance the energy density of alkali-ion batteries by resolving inherent shortcomings of single materials. However, the rational construction of heterogeneous composite with abundant heterogeneous interfaces for sodium-ion batteries (SIBs) is still a significant challenge. Herein, inspired by density functional theory calculations, interface engineering can greatly decrease the energy bandgap and migration barrier of Na ions in Sb and Na3Sb phases, as well as enhance the mechanical properties. A porous heterointerface MOFC-Sb is fabricated by utilizing MOF-C as a support and buffer, exhibiting excellent electrochemical performances for sodium storage. The MOF-C-Sb anode with its rich heterointerface presents an improved electrochemical performance of 540.5 mAh g-1 after 100 cycles at 0.1 A g-1, and 515.9 mAh g-1 at 1.6 A g-1 in term of sodium storage, efficiently resolving the serious volume expansion issues of metal Sb. These results indicate the structural superiority of heterointerface-engineered structure and afford valuable information for the rational design and construction of Sb-based anode materials for high-performance electrochemical energy storage.

Limited energy migration and circumscribed multiphonon relaxation produced non-concentration quenching in a novel dazzling red-emitting phosphor.

White LED applications are still constrained by extremely efficient narrow band red emitting phosphors. Meanwhile, the concentration quenching induced by energy migration is the main reason that limits the emission intensity of a red emitting phosphor. Therefore, developing a novel red emitting material with energy migration limitations seems necessary. Here, we proposed and realized the non-concentration quenching doping of Eu3+ ions in a Sr9Y2-2xW4O24:xEu3+ (0 ≤ x ≤ 1.0) phosphor for the first time by means of host preferential selection. By clearly investigating the crystal structure and luminescence kinetics, the long-distance between the nearby Eu3+ ions and the low phonon energy are the main reasons that suppress the energy migration and the cross-relaxation among Eu3+ ions. These advantages result in a high internal (90.47%) and external quantum efficiency (42.1%) of Sr9Eu2W4O24. With the help of the Judd-Ofelt theory and the large value of oscillator strength Ω2, Eu3+ ions are verified to occupy the non-symmetric lattice site with high color purity (94.4%). In addition, only 5.2% emission intensity loss at 140 °C can guarantee its application in LED devices. Moreover, the SYWO:Eu3+ phosphor has high thermal tolerance, high color stability, excellent moisture resistance and superior physical/chemical stability, and thus has broad practical spectral application prospects. The prepared WLED shows superior performance, and the calculated NTSC values are as high as 101.8% and 104.7%, respectively. For comparison, the optical performances of the Sr9Y2W4O24:Eu3+ phosphor outperform those of the standard commercial red phosphors, Y2O3:Eu3+ and Y2O2S:Eu3+, and almost match that of K2MnF6. These results may pave the way for fresh approaches to the study of high-performance Eu3+-activated phosphors.

High-intensity first near-infrared emission through energy migration in multilayered upconversion nanoparticles.

The development of Tm3+ 807 nm first near-infrared (NIR-I, 700-1000 nm) emission with second near-infrared (NIR-II, 1000-1700 nm) excitation is urgently needed, due to its potential application in biomedicine. In this work, a range of NaErF4:Yb@NaYF4:Yb@NaYF4:Yb,Tm@NaYF4 multilayer core-shell structure upconversion nanoparticles (UCNPs) were successfully prepared by a co-precipitation method. The strongest UC emissions can be obtained by changing the concentration of Yb3+ in the core and the first shell, and the proposed UC process was discussed in detail. The analysis shows that high-intensity NIR-I emission (807 nm) from Tm3+ and visible light from Er3+ were achieved through the energy migration among Yb3+ and the energy back transfer from Yb3+ to Er3+ under 1532 nm excitation. Besides, compared to bilayer UCNPs, multilayer core-shell UCNPs display superior optical performance. The high-intensity NIR-I emission at 807 nm (Tm3+:3H4 → 3H6) under 1532 nm NIR-II excitation demonstrates huge advantages in bioimaging.

Optimization of Data and Energy Migrations in Mini Data Centers for Carbon-Neutral Computing

Due to large-scale applications and services, cloud computing infrastructures are experiencing an ever-increasing demand for computing resources. At the same time, the overall power consumption of data centers has been rising beyond 1&#x0025; of worldwide electricity consumption. The usage of renewable energy in data centers contributes to decreasing their carbon footprint and overall electricity costs. Several green-energy-aware resource allocation approaches have been studied recently. None of them takes advantage of the joint migration of <i>jobs</i> and <i>energy</i> in green data centers to increase energy efficiency. This paper presents an optimization approach for energy-efficient resource allocation in mini data centers. The observed momentum around edge computing makes the design of geographically distributed mini data centers highly desirable. Our solution exploits both virtual machines (VMs) and energy migrations between green compute nodes in mini data centers. These nodes have energy harvesting, storage, and transport capabilities. They enable the migration of VMs and energy across different nodes. Compared to VM allocation alone, joint-optimization of VM and energy allocation reduces utility electricity consumption by up to 22&#x0025;. This reduction can reach up to 28.5&#x0025; for the same system when integrating less energy-efficient servers. The gains are demonstrated using simulation and a Mixed Integer Linear Programming formulation for the resource allocation problem. Furthermore, we show how our solution contributes to sustaining the energy consumption of old-generation and less efficient servers in mini data centers.

A Novel Energy Efficient Multi-Dimensional Virtual Machines Allocation and Migration at the Cloud Data Center

A Novel Energy Efficient Multi-Dimensional Virtual Machines Allocation and Migration at the Cloud Data Center

Understanding enormous redshifts in highly concentrated Mn2+ phosphors

The enormous redshift observed for luminescence in highly doped Mn2+ phosphors is shown to be not caused by strongly exchange coupled Mn2+-pairs but by energy migration to near-infrared luminescent trap centers.

BaF2:Eu2+/3+,Tb3+nanofibres achieve enhanced multicolor luminescence and white-light emissionviamulti-channel excitation and energy migration procedure

BaF2:Eu2+/3+,Tb3+nanofibres with multicolor luminescence and white-light emission are constructedviaelectrospinning and di-crucible fluorination technology.

Amorphous TiO&lt;sub&gt;2&lt;/sub&gt; ultrathin nanosheet for stable high-rate lithium storage

The use of intercalation-type metal oxides as anode materials in rechargeable lithium-ion batteries is appealing due to their reduced risk of Li plating at low voltages. However, their implementation for fast-charging applications is limited by their lower energy and power density, as well as cycling instability. Herein, we present an amorphous TiO&lt;sub&gt;2&lt;/sub&gt; nanosheet that exhibits exceptional cycling stability with a high capacity of 231 mA &lt;b&gt;·&lt;/b&gt; h &lt;b&gt;·&lt;/b&gt; g&lt;sup&gt;−1&lt;/sup&gt; after 200 cycles at 500 mA &lt;b&gt;·&lt;/b&gt; g&lt;sup&gt;−1&lt;/sup&gt; and 156.7 mA &lt;b&gt;·&lt;/b&gt; h &lt;b&gt;·&lt;/b&gt; g&lt;sup&gt;−1&lt;/sup&gt; after 1000 cycles at a high current density of 6 A &lt;b&gt;·&lt;/b&gt; g&lt;sup&gt;−1&lt;/sup&gt;. We attribute the enhanced rate performance to the amorphous nature with high isotropy, which facilitates low energy migration paths and ion availability and can accommodate large changes in volume. This work suggests that amorphization represents a promising strategy for developing unconventional metal oxide electrode materials with high-rate performance.

Revisiting molecularly conformation-planarized organic dyes for NIR-II fluorescence imaging.

Fluorescence imaging in the second window (NIR-II, 1000-1700 nm) provides deeper penetration depth and higher resolution, but there is still a dilemma for designing NIR-II dyes for simultaneously enhancing fluorescence efficiency and prolonging excitation wavelength. Herein, a molecular conformation planarization strategy has been revisited to guide the synthesis of two donor-acceptor-donor dyes (named T-BBT and BT-BBT). On the one hand, conformational planarization can extend the absorption peaks of T-BBT and BT-BBT to the NIR region with high molar extinction coefficients of 30.5 × 103 and 16.4 × 103 L (mol-1 cm-1) at 1064 nm, respectively. On the other hand, structural rigidity can weaken electronic vibration coupling-related non-radiative decay pathways, whereby both T-BBT and BT-BBT display rather high fluorescence efficiencies of 3.6% and 13.5% in solution. Furthermore, a molecular doping strategy is adopted to alleviate fluorescence quenching in the aggregated state by suppressing long-distance energy migration, and 2.5 wt% doped BT-BBT nanoparticles show a high fluorescence efficiency of 2.0%, which enables the application of in vivo deep NIR-II fluorescence imaging for vessels and tumors with high resolution under 980 nm excitation. This work demonstrates that organic dyes with structural planarization can bridge the gap between NIR-II absorption and fluorescence efficiency.

Evolution of π-Peptide Self-Assembly: From Understanding to Prediction and Control.

Supramolecular materials derived from the self-assembly of engineered molecules continue to garner tremendous scientific and technological interest. Recent innovations include the realization of nano- and mesoscale particles (0D), rods and fibrils (1D), sheets (2D), and even extended lattices (3D). Our research groups have focused attention over the past 15 years on one particular class of supramolecular materials derived from oligopeptides with embedded π-electron units, where the oligopeptides can be viewed as substituents or side chains to direct the assembly of the central π-electron cores. Upon assembly, the π-systems are driven into close cofacial architectures that facilitate a variety of energy migration processes within the nanomaterial volume, including exciton transport, voltage transmission, and photoinduced electron transfer. Like many practitioners of supramolecular materials science, many of our initial molecular designs were designed with substantial inspiration from biologically occurring self-assembly coupled with input from chemical intuition and molecular modeling and simulation. In this feature article, we summarize our current understanding of the π-peptide self-assembly process as documented through our body of publications in this area. We address fundamental spectroscopic and computational tools used to extract information regarding the internal structures and energetics of the π-peptide assemblies, and we address the current state of the art in terms of recent applications of data science tools in conjunction with high-throughput computational screening and experimental assays to guide the efficient traversal of the π-peptide molecular design space. The abstract image details our integrated program of chemical synthesis, spectroscopic and functional characterization, multiscale simulation, and machine learning which has advanced the understanding and control of the assembly of synthetic π-conjugated peptides into supramolecular nanostructures with energy and biomedical applications.