Efficient Broadband Near-Infrared CaMgGe2O6:Cr3+ Phosphor for pc-LED.

Broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are essential to integrate near-infrared spectrometers into mobile devices for the rapid and noninvasive detection of biological components. However, efficient broadband NIR phosphors with a peak emission wavelength longer than 800 nm are deficient. In this study, CaMgGe2O6:Cr3+ phosphor was prepared by a high-temperature solid-state reaction. The phosphor doped with 0.02Cr3+ showed an emission band at 845 nm with a broad bandwidth of 160 nm and a high quantum yield of 84% under 450 nm excitation. The broadband NIR pc-LED was fabricated using CaMgGe2O6:0.02Cr3+ phosphor based on a blue light-emitting diode (LED) chip. A photoelectric efficiency of 27.2% @ 10 mA and an NIR output power of 57.98 mW @ 100 mA were achieved, which are the highest values reported yet for broadband NIR pc-LEDs with a peak wavelength longer than 800 nm. Using the fabricated NIR pc-LED as the light source, the characteristic absorption spectra of some substances were obtained. All of the results indicated that the CaMgGe2O6:Cr3+ phosphor has considerable potential in near-infrared spectroscopic applications.

Two-dimensional materials towards separator functionalization in advanced Li-S batteries.

Functional separators have played important roles in improving the electrochemical performance of lithium-sulfur (Li-S) batteries by addressing the key issues of both the sulfur cathode and lithium anode. Compared with other materials that are used for separator functionalization, two-dimensional (2D) materials with atomic layer thickness and infinite lateral dimensions feature several advantages of ultra-thin laminate structure, remarkable physical properties and tunable surface chemistry, which show potential applications in separator functionalization towards addressing the issues of both the shuttle effect and formation of Li dendrites in Li-S batteries. In this review, the unique advantages of 2D materials for separator functionalization in Li-S batteries and their common construction methods are introduced. Then, recent progress and advances in the construction of 2D materials functional separators are summarized in detail towards inhibiting the shuttle effect of polysulfides and suppressing Li dendrite growth in Li-S batteries. Finally, some opportunities and challenges of 2D materials for constructing high-performance functional separators are proposed. We anticipate that this review will provide new insights into separator functionalization for developing advanced Li-S batteries.

Functional separators towards the suppression of lithium dendrites for rechargeable high-energy batteries.

Lithium metal battery (LMB) is considered to be one of the most promising electrochemical energy storage devices due to the high theoretical specific capacity and the lowest redox potential of metallic lithium; however, some key issues caused by lithium dendrites on the lithium metal anode seriously hinder its real-world applications. As an indispensable part of LMBs, the separator could serve as a physical barrier to prevent direct contact of the two electrodes and control ionic transport in batteries; it is an ideal platform for the suppression of lithium dendrites. In this review, the mechanism of lithium dendrite nucleation and growth are firstly discussed and then some advanced techniques are introduced for the precise characterization of lithium dendrites. On the basis of dendritic nucleation and growth principle, several feasible strategies are summarized for suppressing lithium dendrites by utilizing functional separators, including providing a mechanical barrier, promoting homogeneous lithium deposition, and regulating ionic transport. Finally, some challenges and prospects are proposed to clear the future development of functional separators. We anticipate that this paper will provide a new insight into the design and construction of functional separators for addressing the issues of lithium dendrites in high-energy batteries.

Cation-Selective Separators for Addressing the Lithium-Sulfur Battery Challenges.

Lithium-sulfur batteries (LSBs) have become one of the most promising candidates for next-generation energy storage systems owing to their high theoretical energy density, environmental friendliness, and cost effectiveness. However, real-word applications are seriously restricted by an undesirable shuttle effect and Li dendrite formation. In essence, uncontrollable anion transport is a key factor that causes both polysulfide shuttling and dendrite formation, which creates the possibility of simultaneously addressing the two critical issues in LSBs. An effective strategy to control anion transport is the construction of cation-selective separators. Significant progress has been achieved in the inhibition of the shuttle effect, whereas addressing the problem of Li dendrite formation by utilizing a cation-selective separator is still under way. From this viewpoint, this Review analyzes critical issues with regard to the shuttle effect and Li dendrite formation caused by uncontrollable anion transport, based on which roles and advantages of cation-selective separators toward high-performance LSBs are presented. According to the separator-construction principle, the latest advances and progress in cation-selective separators in inhibiting the shuttle effect and Li dendrite formation are reviewed in detail. Finally, some challenges and prospects are proposed for the future development of cation-selective separators. This Review is anticipated to provide a new perspective for simultaneously addressing the two critical issues in LSBs.

Multi-peaked broad-band red phosphor Y3Si6N11:Pr3+ for white LEDs and temperature sensing.

The indispensable broad-band red phosphors for LED lighting generally show a long emission tail for wavelengths longer than 650 nm, which consumes excitation energy but contributes little luminance. Here, we report, for the first time, a broad red emission band with a steep falling edge at 652 nm, formed of widely distributed 1D2 → 3H4 emission lines of Pr3+ in Y3Si6N11 due to a large Stark splitting of the 3H4 (930 cm-1) and 1D2 (725 cm-1) levels. The red emission exhibits a 43 nm bandwidth, which is the widest in Pr3+-doped phosphors reported so far. The red Y3Si6N11:Pr3+ phosphor was applied for the fabrication of 310 nm UV chip-based white LEDs, and a high color rendering index of 96 at a low correlated color temperature of 4188 K was achieved. Furthermore, a temperature-sensing scheme was proposed based on the temperature-dependent intensity ratios of the emission lines from the thermally coupled and large Stark splitting levels of the 1D2 state. Relative sensitivities as a function of temperature were studied in the range of 93-473 K. The findings of this study indicate that Y3Si6N11:Pr3+ is an attractive broad-band red phosphor for both high color rendering white LEDs and temperature sensing applications.

Electrochromic Nanochannels for Visual Nanofluidic Manipulation in Integrated Ionic Circuits.

Nanochannel system provides a promising platform to create nanofluidic components in large-scale integrated circuits for "lab-on-a-chip" applications. However, it is a big challenge to achieve in situ monitoring on microscopic nanofluidic manipulation of single nanofluidic components in the integrated ionic circuit. Herein, we present a simple approach to realize visual nanofluidic manipulation in asymmetric nanochannels by the functionalization of an electrochromic polyaniline coating, which demonstrates redox-tunable surface charge accompanied by a visible color variation. The electrochromic nanochannels present a green color when behaving as ionic diodes, while the color turns to light yellow in a manner of ionic resistor. Moreover, both ionic transport behavior and color transition could respond well with alternating switch between redox states, contributing to a reversible and stable visual nanofluidic manipulation of electrochromic nanochannels. This work will create new avenues on in situ characterizing nanofluidic manipulation of nanofluidic components in integrated ionic circuits for intelligent sensing and detection applications.

Nanochannels regulating ionic transport for boosting electrochemical energy storage and conversion: a review.

Electrochemical power sources, as one of the most promising energy storage and conversion technologies, provide great opportunities for developing high energy density electrochemical devices and portable electronics. However, uncontrolled ionic transport in electrochemical energy conversion, typically undesired anion transfer, usually causes some issues degrading the performance of energy storage devices. Nanochannels offer an effective strategy to solve the ionic transport problems for boosting electrochemical energy storage and conversion. In this review, the advantages of nanochannels for electrochemical energy storage and conversion and the construction principle of nanochannels are introduced, including ion selectivity and ultrafast ion transmission of nanochannels, which are considered as two critical factors to achieve highly efficient energy conversion. Recent advances in applications of nanochannels in lithium secondary batteries (LSBs), electrokinetic energy conversion systems and concentration cells are summarized in detail. Nanochannels exist in the above systems in two typical forms: functional separator and electrode protective layer. Current research on nanochannel-based LSBs is still at the early stage, and deeper and broader applications are expected in the future. Finally, the remaining challenges of nanochannel fabrication, performance improvement, and intelligent construction are presented. It is envisioned that this paper will provide new insights for developing high-performance and versatile energy storage electronics based on nanochannels.

Observation of a red Ce3+ center in SrLu2O4:Ce3+ phosphor and its potential application in temperature sensing.

In Ce3+ activated SrLn2O4 type phosphors (Ln = Y, Lu, Sc, etc.) only one Ce3+ center was previously reported to show a blue emission band. In this paper, we report the observation of a second Ce3+ center in SrLu2O4:Ce3+. The new center shows a red emission band peaking at 600 nm with an excitation band at 485 nm. We attributed the new center (Ce(ii)) to the substitution of the Lu3+ site and the original blue center (Ce(i)) to the substitution of the Sr2+ site. Spectroscopy studies indicate that Ce(i) centers are preferentially formed at a low doping concentration and the number ratio of Ce(i)/Ce(ii) decreases with increasing Ce3+ concentration until beyond 0.002. The fluorescence lifetimes of the two centers were measured for various doping concentrations. Energy transfer from Ce(i) to Ce(ii) was observed. It was found that the emission intensity of Ce(ii) centers reduces much faster than that of Ce(i) with increasing temperature from 83 K up to 350 K, implying their potential application in temperature sensing based on their temperature dependent intensity ratios. A relative sensing sensitivity as high as 2.28% K-1 at 283 K was achieved.

Efficient Visible-to-NIR Spectral Conversion for Polycrystalline Si Solar Cells and Revisiting the Energy Transfer Mechanism from Ce3+ to Yb3+ in Lu3Al5O12 Host.

The so-called Shockley-Queisser converting efficiency limit of Si solar cells is believed to be surpassed by using the spectral converter. However, searching for efficient spectral converting materials is still a challenging task. In this paper, efficient visible-to-NIR spectral conversion for polycrystalline Si solar cells has been demonstrated in Ce3+ and Yb3+ codoped Lu3Al5O12. Moreover, the underlying energy transfermechanism from Ce3+ to Yb3+ is systematically re-investigated by the detailed excitation and emission spectra as well as fluorescent decay curves, and our results demonstrate that fast metal-to-metal charge transfer from Ce3+ to nearby Yb3+ is the dominant energy transfermechanism. Finally, we provide new evidence that Ce4+-Yb2+ charge-transfer state is responsible for the relatively low quantum efficiency of NIR emission in Ce3+ and Yb3+ codoped system.

Correction: Dye-embedded YAG:Ce3+@SiO2 composite phosphors toward warm wLEDs through radiative energy transfer: preparation, characterization and luminescence properties.

Correction for 'Dye-embedded YAG:Ce3+@SiO2 composite phosphors toward warm wLEDs through radiative energy transfer: preparation, characterization and luminescence properties' by Guo-Hui Pan, Jiahua Zhang et al., Nanoscale, 2018, DOI: 10.1039/c8nr07360k.

The Surface Coating of Commercial LiFePO4 by Utilizing ZIF-8 for High Electrochemical Performance Lithium Ion Battery.

The requirement of energy-storage equipment needs to develop the lithium ion battery (LIB) with high electrochemical performance. The surface modification of commercial LiFePO4 (LFP) by utilizing zeolitic imidazolate frameworks-8 (ZIF-8) offers new possibilities for commercial LFP with high electrochemical performances. In this work, the carbonized ZIF-8 (CZIF-8) was coated on the surface of LFP particles by the in situ growth and carbonization of ZIF-8. Transmission electron microscopy indicates that there is an approximate 10 nm coating layer with metal zinc and graphite-like carbon on the surface of LFP/CZIF-8 sample. The N2 adsorption and desorption isotherm suggests that the coating layer has uniform and simple connecting mesopores. As cathode material, LFP/CZIF-8 cathode-active material delivers a discharge specific capacity of 159.3 mAh g-1 at 0.1C and a discharge specific energy of 141.7 mWh g-1 after 200 cycles at 5.0C (the retention rate is approximate 99%). These results are attributed to the synergy improvement of the conductivity, the lithium ion diffusion coefficient, and the degree of freedom for volume change of LFP/CZIF-8 cathode. This work will contribute to the improvement of the cathode materials of commercial LIB.

Two Ce3+ centers induced broadband emission in Y3Si6N11:Ce3+ yellow phosphor.

The Y3Si6N11:Ce3+ yellow phosphor shows a well-known ∼150 nm broad emission band, exhibiting a potential application in UV or blue based white LEDs. We report the observation of two Ce3+ emitting centers, the superposition of which forms the broad emission band. One center has a green emission band peaked at 539 nm (Ce1) with the first excitation band at 420 nm. The other has a red emission band peaked at 600 nm (Ce2) with the first excitation band at 485 nm. The two Ce3+ centers are assigned to the substitution for two Y sites in Y3Si6N11. It was found that the Ce2 emission intensity is continuously enhanced relative to that of Ce1 with an increasing Ce3+ concentration, thus leading to a redshift of the broadband. Meanwhile, a more notable fluorescence lifetime shortening of Ce1 compared to Ce2 with an increasing Ce3+ concentration was observed. These results suggest the occurrence of energy transfer from Ce1 to Ce2. The temperature-dependent luminescence intensity of Y3Si6N11:Ce3+ was also studied in the range of 93 to 473 K.

Dye-embedded YAG:Ce3+@SiO2 composite phosphors toward warm wLEDs through radiative energy transfer: preparation, characterization and luminescence properties.

The most common yellow phosphor for wLEDs, Y3Al5O12:Ce3+ (YAG:Ce3+), suffers from a deficiency of red in its spectral content of light. In this paper, a new strategy is provided to tailor the Ce3+ spectral profile through surface-located dye molecules of ATTO-Rho101, which feature intense, broad absorption in the green-yellow spectral region of Ce3+ emission as well as bright red emission. Sphere-shaped and highly dispersed micrometer and nanometer-sized YAG:Ce3+ (micro/nano-YAG:Ce3+) was synthesized through a modified solvothermal method. Surface SiO2 coating and simultaneous dye embedding were performed on the solvothermally derived YAG:Ce3+, heat-treated micro-YAG:Ce3+ and commercial phosphors. Efficient radiative transfer/reabsorption from Ce3+ in the inner core of YAG to the dye molecules in the SiO2 outer shell, irrespective of the size of the phosphors, was demonstrated in the accumulated YAG:Ce3+@SiO2 + dye powder upon blue light excitation; this enhanced its red emission. Fluorescence microscopy was demonstrated to be a powerful tool to identify the reabsorption phenomenon of the powdered materials. Packaging the heat-treated micro-YAG:Ce3+@SiO2 + dye phosphors on blue LED chips yielded a warm wLED (Ra∼ 93), but an Ra of only ∼79 was obtained for the wLED with commercial YAG:Ce3+@(SiO2 + dye)5 due to the low concentration of phosphors dispersed in the epoxy resin and the resulting decreased reabsorption by dye molecules. Surface-protonated amine species were found to induce Ce3+→ Ce4+ oxidation upon activation by heating or photoirradiation and then quench the photoluminescence (PL) of micro-YAG:Ce3+ even after surface modification by SiO2, YAG or being embedded in an epoxy resin matrix. High calcination temperatures greatly improved the PL stability of micro-YAG:Ce3+ through the removal of surface-capped species. The dye in the silica matrix showed high stability against heating and irradiation due to the so-called "caging effects"; however, decreased photo-stability was found in commercial YAG:Ce3+@(SiO2 + dye)5 due to the incomplete and/or loose SiO2 layer grown during multiple surface modifications.

Efficient Blue-emitting Phosphor SrLu2O4:Ce3+ with High Thermal Stability for Near Ultraviolet (~400 nm) LED-Chip based White LEDs.

Blue-emitting phosphors for near ultraviolet (NUV) based tri-color RGB phosphor blend converted white light emitting diodes (LEDs) have been extensively investigated in the past few years. LED chip peaked near 400 nm is the most efficient among the NUV chips currently. However, most of blue phosphors show inefficient excitation around 400 nm. Herein, a novel blue phosphor SrLu2O4:Ce3+ matching well with near 400 nm chip and showing high thermal stability has been developed. The photoluminescence spectrum presents a broad emission band peaking at 460 nm with a bandwidth of nearly 90 nm. By optimizing the Ce3+ concentration, an internal quantum efficiency (IQE) as high as 76% was achieved. Furthermore, 86% of the room-temperature emission intensity is still maintained at 150 °C, indicating a good thermal stability and practicality. A series of white LEDs were fabricated based on 405 nm chips coated with a blend of the new blue phosphor with the commercial yellow and red phosphors. High color rendering indexes (≥90) were achieved while the correlated color temperature was tuneable in the range of 3094 to 8990 K. These results suggest that SrLu2O4:Ce3+ can be utilized as a blue-emitting phosphor in NUV based white LEDs.

Highly efficient upconversion emission of Er3+ in δ-Sc4Zr3O12 and broad-range temperature sensing.

Developing optical temperature sensors with a wider range, higher sensitivity and repeatability based on Er3+/Yb3+ doped upconverting phosphors has always been at the forefront of temperature measurement technologies. Here, we report the intense green upconversion luminescence in Er3+/Yb3+ doped δ-Sc4Zr3O12 for the first time and its temperature sensing performance is investigated. The structure of δ-Sc4Zr3O12 is given by Rietveld refinement of XRD data and the site occupancy of Er3+ ions has been determined. Compared with cubic Sc2O3 and ZrO2, under 972 nm excitation, the green emission from Er3+ centers in Sc4Zr3O12 is increased by 59-fold and 264-fold, respectively. By experimental analysis, this enhancement of upconversion luminescence is attributed to the low-symmetrical environment of Er3+, generation of Yb3+ clusters and high internal efficiency of Yb3+ emission in Sc4Zr3O12. In addition, the fluorescence intensity ratio of two green emission bands (2H11/2/4S3/2 → 4I15/2) is studied as a function of temperature ranging from 303 to 793 K in Sc4Zr3O12. The maximum sensitivity observed via calculation is 0.00634 K-1 at 573 K, and the sensitivity is still as high as 0.00534 K-1 at 793 K. The stability of a Sc4Zr3O12 thermometer is also examined via a recycling test. These findings suggest that δ-Sc4Zr3O12 is a promising upconversion host and could achieve high-sensitivity optical temperature sensing with a wide measuring range.

Enhanced ∼2 µm Emission of Tm3+ in Lu2O3 by Addition of a Trace Amount of Er3.

Er3+-induced intensity enhancement of ∼2 µm emission is observed in 2 atom % Tm3+ doped Lu2O3 under 782 nm excitation. The maximum enhancement reaches 41.9% with only 0.05 atom % Er3+. Er3+ introduces a new quantum cutting process which is proved to be a Tm3+ → Er3+ → Tm3+ forward-backward energy transfer (FBET) system. The FBET system is observed to work efficiently even at very low Er3+ concentration. Thus, energy loss due to energy migration among Tm3+ ions is suggested to be suppressed by the FBET process. The Tm3+ → Er3+ → Tm3+ FBET system may be a new route to improve the performance of Tm3+ lasers.

Inhomogeneous-Broadening-Induced Intense Upconversion Luminescence in Tm3+ and Yb3+ Codoped Lu2O3-ZrO2 Disordered Crystals.

Near-infrared (980 nm) to near-infrared (800 nm) and blue (490 nm) upconversion has been studied in 0.2% Tm3+ and 10% Yb3+ codoped Lu2O3-ZrO2 solid solutions as a function of the ZrO2 content in the range of 0-50%, prepared by a high-temperature solid-state reaction. The continuous enhancement of upconversion luminescence is observed with increasing ZrO2 content up to 30%. Analyses of the Yb3+ emission intensity and lifetime indicate enlarged absorption of a 980 nm excitation laser and enhanced energy transfer from Yb3+ to Tm3+ with the addition of ZrO2. The spectrally inhomogeneous broadening of the dopants in this disordered solid solution is considered to play the main role in the enhancement by providing better matches with the excitation laser line and increasing the spectral overlap for efficient energy transfer from Yb3+ to Tm3+. In addition, the inhomogeneous broadening is also validated to improve energy migration among Yb3+ ions and energy back transfer from Tm3+ to Yb3+. Hence, it is understandable that a drop in the upconversion luminescence intensity occurs as the concentration of ZrO2 is further increased from 30% to 50%. This work indicates the possibility of disordered crystals to achieve intense upconversion luminescence for biological and optoelectronic applications.

The Inductive Effect of Neighboring Cations in Tuning Luminescence Properties of the Solid Solution Phosphors.

Forming solid solutions through cation substitution is an efficient way to improve the luminescence properties of Ce3+ or Eu2+ activated phosphors and even to develop new ones, which is badly needed for phosphor-converted white LEDs. Here, we report new color tunable solid solution phosphors based on Eu2+ activated K2Al2B2O7 as a typical case to demonstrate that, besides crystal field splitting of 5d levels, centroid shift and Stokes shift can be dominant in tuning excitation and emission spectra as well as thermal stability of solid solution phosphors, both of which were previously considered to be negligible. Moreover, a general model involving the inductive effect of neighboring cations is proposed to explain the obvious variations in centroid shift and Stokes shift with cation substitution. Our work is propitious for the construction of more reasonable structure-property relations and thus offers theoretical guidance for designing solid solution phosphors.

Highly Efficient Green-Emitting Phosphors Ba2Y5B5O17 with Low Thermal Quenching Due to Fast Energy Transfer from Ce3+ to Tb3.

This paper demonstrates a highly thermally stable and efficient green-emitting Ba2Y5B5O17:Ce3+, Tb3+ phosphor prepared by high-temperature solid-state reaction. The phosphor exhibits a blue emission band of Ce3+ and green emission lines of Tb3+ upon Ce3+ excitation in the near-UV spectral region. The effect of Ce3+ to Tb3+ energy transfer on blue to green emission color tuning and on luminescence thermal stability is studied in the samples codoped with 1% Ce3+ and various concentrations (0-40%) of Tb3+. The green emission of Tb3+ upon Ce3+ excitation at 150 °C can keep, on average, 92% of its intensity at room temperature, with the best one showing no intensity decreasing up to 210 °C for 30% Tb3+. Meanwhile, Ce3+ emission intensity only keeps 42% on average at 150 °C. The high thermal stability of the green emission is attributed to suppression of Ce3+ thermal de-excitation through fast energy transfer to Tb3+, which in the green-emitting excited states is highly thermally stable such that no lifetime shortening is observed with raising temperature to 210 °C. The predominant green emission is observed for Tb3+ concentration of at least 10% due to efficient energy transfer with the transfer efficiency approaching 100% for 40% Tb3+. The internal and external quantum yield of the sample with Tb3+ concentration of 20% can be as high as 76% and 55%, respectively. The green phosphor, thus, shows attractive performance for near-UV-based white-light-emitting diodes applications.

Investigation of the Energy-Transfer Mechanism in Ho3+- and Yb3+-Codoped Lu2O3 Phosphor with Efficient Near-Infrared Downconversion.

A high-temperature solid-state method was used to synthesize the Ho3+- and Yb3+-codoped cubic Lu2O3 powders. The crystal structures of the as-prepared powders were characterized by X-ray diffraction. The energy-transfer (ET) phenomenon between Ho3+ ions and Yb3+ ions was verified by the steady-state spectra including visible and near-infrared (NIR) regions. Beyond that, the decay curves were also measured to certify the existence of the ET process. The downconversion phenomena appeared when the samples were excited by 446 nm wavelength corresponding to the transition of Ho3+: 5I8→5G6/5F1. On the basis of the analysis of the relationship between the initial transfer rate of Ho3+: 5F3 level and the Yb3+ doping concentration, it indicates that the ET from 5F3 state of Ho3+ ions to 2F5/2 state of Yb3+ ions is mainly through a two-step ET process, not the long-accepted cooperative ET process. In addition, a 62% ET efficiency can be achieved in Lu2O3: 1% Ho3+/30% Yb3+. Unlike the common situations in which the NIR photons are all emitted by the acceptors Yb3+, the sensitizers Ho3+ also make contributions to the NIR emission upon 446 nm wavelength excitation. Meanwhile, the 5I5→5I8 transition and 5F4/5S2→5I6 transition of Ho3+ as well as the 2F5/2→2F7/2 transition of Yb3+ match well with the optimal spectral response of crystalline silicon solar cells. The current research indicates that Lu2O3: Ho3+/Yb3+ is a promising material to improve conversion efficiency of crystalline silicon solar cell.