Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Interpretation of europium(III) spectra

Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Energy of the first 4f7→4f65d transition of Eu2+ in inorganic compounds

On BaMgAl10O17:Eu2+ phosphor degradation mechanism: thermal treatment effects

The Tamazeght alkaline complex (High Atlas Mountains of Morocco) is a middle Eocene volcanic to plutonic mass, ranging from silica-saturated to silica-undersaturated rocks and carbonatites. Some of the most evolved units are nepheline syenites and pegmatites, which show evidence of hydrothermal alteration. These rocks also contain the highest concentrations of Zr in the complex, although these concentrations are not sufficient to form exploitable reserves. High field strength element (HFSE) minerals (predominantly the zirconosilicates) commonly occur as secondary phases that are richer in Ca and F (cancrinite (Na6Ca(CO3)) (AlSiO4)6•2H2O), calcic catapleiite ((Ca,Na)ZrSi3O9•2H2O), rinkite ((Ca,Ce)4Na(Na,Ca)2Ti(Si2O7)2F2(0,F)2). Furthermore, they partly or completely replace primary HFSE phases (e.g., zircon, eudialyte (Na4(Ca,Ce)2(Fe2+,Mn,Y)ZrSi8O22(OH,Cl)2)). Altered nepheline crystals adjacent to these replaced minerals host fluid inclusions that, based on textural evidence, are interpreted to have formed during alteration. The inclusions have high salinity (25 wt % NaCl equiv), are Ca rich, and were trapped at ~300°C. They contain several solids in addition to a liquid phase and a vapor bubble. One of the most common daughter minerals is a Ca-La-Ce phase (parisite?). Other daughter or trapped minerals include zircon, a Ti (±Nb) silicate or oxide (rutile?), an Mn hydroxide(?), and less commonly, sphalerite and galena. The occurrence of zircon, parisite, and rutile are evidence for mobilization of Zr and other HFSE during hydrothermal alteration.

Based on data available for the solubility of ZrO2 in H2O-HF solutions and estimates for the concentration of Zr in the fluids at the Tamazeght complex (possibly as high as ~0.05 m), we suggest that HFSE transport occurred by mixed metal OH-F complexing. This transport requires a fluid with an elevated F concentration and a temperature of ±300°C. In addition, it can only take place in the near absence of Ca. We propose that a residual F-rich brine evolving from crystallization of pegmatites at <500°C leached and transported HFSE. Upon interaction with surrounding limestone, the fluid cooled rapidly and gained Ca. This caused deposition of fluorite (fluorite is very sparingly soluble), which buffered aF to low levels in the fluid causing precipitation of the HFSE in the form of secondary minerals. Because the hydrothermal event was short-lived and relatively restricted in space, HFSE were not concentrated to ore grades.

/pdf/hydrothermal-mobilization-of-high-field-strength-elements-in-3561e3ma7r.pdf

Hydrothermal Mobilization of High Field Strength Elements in Alkaline Igneous Systems: Evidence from the Tamazeght Complex (Morocco)

Photoionization and luminescences in BaF2:Eu2+

Absorption, excitation and emission spectra and lifetime measurements in cerium-doped lanthanum fluoride single crystals were obtained using both one- and two-photon spectroscopy techniques. It is shown that two kinds of fluorescent centres are present. In weakly doped systems, a rapid fluorescence around 300 nm dominates, due to Ce3+ ions at normal sites. In heavily doped systems, an additional longer fluorescence appears at higher wavelengths due to Ce3+ ions occupying perturbed sites and acting as traps. In CeF3, the energy migrates among Ce3+ ions and is finally trapped so that only the trap emission is detected at room temperature. Significant energy losses in the fluorescence of Ce3+ at normal sites are explained by up-conversion and ionization processes and by non-radiative surface recombination phenomena for the case of strong absorption. Two-photon excitation spectra in the region of trap absorption confirm the role of traps in emission mechanisms and show narrow lines which could be vibronic structures of various kinds of traps existing in these materials.

https://iopscience.iop.org/article/10.1088/0953-8984/4/24/016/pdf

One- and two-photon spectroscopy of Ce3+ ions in LaF3-CeF3 mixed crystals

In this paper, we analyze the fluorescence and the photoconductivity properties of Yb 2+ in CaF 2 , SrF 2 and BaF 2 . We present a new approach to the interpretation of the «anomalous» emission usually observed in such systems by an impurity trapped exciton which is the lowest excited state of the system. We show that the excited «d» electron of Yb 2+ is delocalized in the «s» orbitals of next neighbor Ca 2+ ions from where it can either autoionize or recombine with Yb 3+ giving rise to the red shifted luminescence. The photoconductivity spectra show that when the electron is excited into the higher energy bands it autoionizes without any prior relaxation. The strong temperature dependence of lifetime shows that the exciton level is split into two close levels and the study of the risetime of the decays allow us to calculate the energy of E u , T 2u level and the probability of tunneling from this level into the exciton level Dans cet article, on analyse les proprietes de fluorescence et de photoconductivite de Yb 2+ dans CaF 2 , SrF 2 et BaF 2 . On presente une nouvelle interpretation de l'emission «anormale» habituelement observee dans de tels systemes par un exciton piege sur l'impurete. On montre que l'electron «d» de Yb 2+ excite se delocalise dans les orbitales «s» des ions Ca 2+ voisins a partir d'ou il peut soit s'auto-ioniser soit se recombiner avec l'ion Yb 3+ en donnant la luminescence decalee vers le rouge. Les spectres de photoconductivite montrent que lorsque l'electron est excite dans les bandes de haute energie il s'auto-ionise sans relaxation prealable. La forte variation thermique des durees de vie montre que le niveau excitonique est scinde en deux sous-niveaux tres proches et l'etude du temps de montee permet de calculer l'energie du niveau E u , T 2u ainsi que la probabilite de l'effet tunnel entre ce niveau et l'exciton

Bernard Moine

Papers

On BaMgAl10O17:Eu2+ phosphor degradation mechanism: thermal treatment effects

Hydrothermal Mobilization of High Field Strength Elements in Alkaline Igneous Systems: Evidence from the Tamazeght Complex (Morocco)

Photoionization and luminescences in BaF2:Eu2+

One- and two-photon spectroscopy of Ce3+ ions in LaF3-CeF3 mixed crystals

Luminescence and photoionization processes of Yb2+ in CaF 2, SrF2 and BaF2