How to Prepare the Brightest Luminescent Coatings?

We address here the question of studying the parameters affecting the brightness of luminescent nanoparticulate coatings, among which are the absorption rate, the internal quantum yield of the phosphor nanoparticles, and the extraction factor of the emitted light in a solid angle perpendicular to the substrate. Experimental investigations are achieved on spray-deposited YVO₄:Eu particles, a system whose synthesis and properties are well documented so that particles of different sizes and microstructure can be considered. This allows a quantitative evaluation of the factors affecting film brightness. Considering a film made from raw colloidal particles, this work shows that its brightness is limited by a factor of 5 due to altered quantum yield of nanoparticles, a factor of 1.75 by dielectric effects and a factor of 2.4 by light extraction issues. This investigation, through providing quantitative evaluations of these different parameters, opens the way toward a possible rational design of inorganic luminescent coatings, with a possible improvement of brightness that could reach a factor of 30 as compared to simple films made directly from colloidal suspensions

Influence of Protected Annealing on the Magnetic Properties of γ‑Fe₂O₃ Nanoparticles

It is usually considered that nanoparticles synthesized by low-temperature routes present structural disorder, from extended defects to local rearrangements (e.g., vacancy ordering or inversion in spinel ferrites), that may severely impact their magnetic properties. In the present work, we have investigated the influence of postsynthesis thermal treatments on 7-nm-sized γ-Fe₂O₃ nanoparticles prepared by room temperature coprecipitation of ferric and ferrous salts in alkaline medium, followed by the dispersion of the preformed particles in a sol–gel silica binder. Such protected annealing in a refractory matrix prevents coalescence and growth, thus preserving the mean size and size distribution of the pristine particles. Structural characterizations show that heat treatments up to 1000 °C turned the raw grains into well-crystallized particles without transformation into hematite. This strategy thus allows accounting for the influence of structural rearrangements on magnetic properties at fixed particle size. For such 7 nm particles, postsynthesis heat treatments were found to mainly influence the shell of misaligned spins at the surface

Synthesis and Luminescent Properties of REVO₄–REPO₄ (RE = Y, Eu, Gd, Er, Tm, or Yb) Heteronanostructures: A Promising Class of Phosphors for Excitation from NIR to VUV

Despite presenting very similar structural properties, rare earth (RE) phosphate and vanadate solids display different spectroscopic behaviors, which makes the elaboration of REVO₄–REPO₄ mixed structures an interesting prospect for the design of luminescent materials with improved activity. This work describes the application of a two-step colloidal precipitation approach for the formation of REVO₄–REPO₄ heteronanostructures and an investigation of their luminescent properties. The growth of the phosphate phase over REVO₄ particles was kinetically evaluated through spectroscopic methodologies comprising the observation of the VO₄^3– → Eu³⁺ energy transfer and the absorption of vanadium­(V) peroxocomplexes in solution. This confirmed an effective coating of the precursor nanoparticles. In order to obtain materials with enhanced properties, a protected annealing methodology in SiO₂ was applied, leading to highly crystalline nanorods with low degree of aggregation. The final materials display efficient emissions in the red, green, and blue (RGB) regions under VUV, UV, or NIR excitation according to their composition. The described structures are promising for light generation in several different systems and can be tuned to provide RGB emissions as VUV-excited phosphors and as NIR or UV colloidal luminescent biomarkers

Siloxanol-Functionalized Copper Iodide Cluster as a Thermochromic Luminescent Building Block

A copper iodide cluster bearing reactive silanol groups exhibits thermochromic luminescence properties sensitive to its chemical environment and is thus a suitable building block for the synthesis of optically active materials

Mechanochromic and Thermochromic Luminescence of a Copper Iodide Cluster

The mechanochromic and thermochromic luminescence properties of a molecular copper(I) iodide cluster formulated [Cu₄I₄(PPh₂(CH₂CHCH₂))₄] are reported. Upon mechanical grinding in a mortar, its solid-state emission properties are drastically modified as well as its thermochromic behavior. This reversible phenomenon has been attributed to distortions in the crystal packing leading to modifications of the intermolecular interactions and thus of the [Cu₄I₄] cluster core geometry. Notably, modification of the Cu−Cu interactions seems to be involved in this phenomenon directly affecting the emissive properties of the cluster

Siloxanol-Functionalized Copper Iodide Cluster as a Thermochromic Luminescent Building Block

A copper iodide cluster bearing reactive silanol groups exhibits thermochromic luminescence properties sensitive to its chemical environment and is thus a suitable building block for the synthesis of optically active materials

Mechanochromic and Thermochromic Luminescence of a Copper Iodide Cluster

The mechanochromic and thermochromic luminescence properties of a molecular copper(I) iodide cluster formulated [Cu₄I₄(PPh₂(CH₂CHCH₂))₄] are reported. Upon mechanical grinding in a mortar, its solid-state emission properties are drastically modified as well as its thermochromic behavior. This reversible phenomenon has been attributed to distortions in the crystal packing leading to modifications of the intermolecular interactions and thus of the [Cu₄I₄] cluster core geometry. Notably, modification of the Cu−Cu interactions seems to be involved in this phenomenon directly affecting the emissive properties of the cluster

Multifunctional Rare-Earth Vanadate Nanoparticles: Luminescent Labels, Oxidant Sensors, and MRI Contrast Agents

Collecting information on multiple pathophysiological parameters is essential for understanding complex pathologies, especially given the large interindividual variability. We report here multifunctional nanoparticles which are luminescent probes, oxidant sensors, and contrast agents in magnetic resonance imaging (MRI). Eu³⁺ ions in an yttrium vanadate matrix have been demonstrated to emit strong, nonblinking, and stable luminescence. Time- and space-resolved optical oxidant detection is feasible after reversible photoreduction of Eu³⁺ to Eu²⁺ and reoxidation by oxidants, such as H₂O₂, leading to a modulation of the luminescence emission. The incorporation of paramagnetic Gd³⁺ confers in addition proton relaxation enhancing properties to the system. We synthesized and characterized nanoparticles of either 5 or 30 nm diameter with compositions of GdVO₄ and Gd_0.6Eu_0.4VO₄. These particles retain the luminescence and oxidant detection properties of YVO₄:Eu. Moreover, the proton relaxivity of GdVO₄ and Gd_0.6Eu_0.4VO₄ nanoparticles of 5 nm diameter is higher than that of the commercial Gd³⁺ chelate compound Dotarem at 20 MHz. Nuclear magnetic resonance dispersion spectroscopy showed a relaxivity increase above 10 MHz. Complexometric titration indicated that rare-earth leaching is negligible. The 5 nm nanoparticles injected in mice were observed with MRI to concentrate in the liver and the bladder after 30 min. Thus, these multifunctional rare-earth vanadate nanoparticles pave the way for simultaneous optical and magnetic resonance detection, in particular, for <i>in vivo</i> localization evolution and reactive oxygen species detection in a broad range of physiological and pathophysiological conditions

Polymorphic Copper Iodide Clusters: Insights into the Mechanochromic Luminescence Properties

An in-depth study of mechanochromic and thermochromic luminescent copper iodide clusters exhibiting structural polymorphism is reported and gives new insights into the origin of the mechanochromic luminescence properties. The two different crystalline polymorphs exhibit distinct luminescence properties with one being green emissive and the other one being yellow emissive. Upon mechanical grinding, only one of the polymorphs exhibits great modification of its emission from green to yellow. Interestingly, the photophysical properties of the resulting partially amorphous crushed compound are closed to those of the other yellow polymorph. Comparative structural and optical analyses of the different phases including a solution of clusters permit us to establish a correlation between the Cu–Cu bond distances and the luminescence properties. In addition, the local structure of the [Cu₄I₄P₄] cluster cores has been probed by ³¹P and ⁶⁵Cu solid-state NMR analysis, which readily indicates that the grinding process modifies the phosphorus and copper atoms environments. The mechanochromic phenomenon is thus explained by the disruption of the crystal packing within intermolecular interactions inducing shortening of the Cu–Cu bond distances in the [Cu₄I₄] cluster core and eventually modification of the emissive state. These results definitely establish the role of cuprophilic interactions in the mechanochromism of copper iodide clusters. More generally, this study constitutes a step further into the understanding of the mechanism involved in the mechanochromic luminescent properties of metal-based compounds

Photostrictive/Piezomagnetic Core–Shell Particles Based on Prussian Blue Analogues: Evidence for Confinement Effects?

High-quality core–shell particles, which associate a photostrictive core (Rb_0.5Co­[Fe­(CN)₆]_0.8·<i>z</i>H₂O, <b>RbCoFe</b>) and a ferromagnetic shell (Rb_0.2Ni­[Cr­(CN)₆]_0.7·<i>z</i>′H₂O, <b>RbNiCr</b>), were successfully grown by a multistep protocol based on coprecipitation in water. High-resolution transmission electron microscopy shows that well-defined heterostructures are formed and that the core–shell interface is abrupt with the epitaxial relationship [001](001)<b>RbCoFe</b>//[001]­(001)<b>RbNiCr</b>, confirmed by simulations of the X-ray diffraction line widths. The core particles are monocrystalline, with 50 nm sides, and the shell consists of large platelet-like crystallites, with a height that corresponds to the shell thickness and lateral dimensions comparable to the size of the core particles. Analysis of the diffracted intensities as a function of shell thickness (9–26 nm) shows that the epitaxial shell growth does not lead to a thick pseudomorphic layer at the interface. In contrast, Williamson–Hall plots suggest that a structural relaxation takes place to adapt the mismatched lattices, with the formation of misfit dislocations distributed over the entire shell thickness. This later finding is indicative of an effective mechanical coupling within the heterostructures. However, a magnetization increase by only a few percent was observed under light irradiation for these <b>RbCoFe</b>@<b>RbNiCr</b> particles. We showed from in situ synchrotron X-ray diffraction measurements that these small changes most likely reflect confinement effects as photoswitching of the core phase is partly or completely blocked depending on the shell thickness