10 research outputs found
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 YVO<sub>4</sub>: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 Ī³āFe<sub>2</sub>O<sub>3</sub> 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 Ī³-Fe<sub>2</sub>O<sub>3</sub> nanoparticles
prepared by room temperature coprecipitation of ferric and ferrous
salts in alkaline medium, followed by the dispersion of the preformed
particles in a solāgel 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 Ā°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 REVO<sub>4</sub>āREPO<sub>4</sub> (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 REVO<sub>4</sub>āREPO<sub>4</sub> 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 REVO<sub>4</sub>āREPO<sub>4</sub> heteronanostructures
and an investigation of their luminescent properties. The growth of
the phosphate phase over REVO<sub>4</sub> particles was kinetically
evaluated through spectroscopic methodologies comprising the observation
of the VO<sub>4</sub><sup>3ā</sup> ā Eu<sup>3+</sup> energy transfer and the absorption of vanadiumĀ(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 SiO<sub>2</sub> 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 [Cu<sub>4</sub>I<sub>4</sub>(PPh<sub>2</sub>(CH<sub>2</sub>CHī»CH<sub>2</sub>))<sub>4</sub>] 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 [Cu<sub>4</sub>I<sub>4</sub>] cluster core geometry. Notably, modification of the CuāCu
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 [Cu<sub>4</sub>I<sub>4</sub>(PPh<sub>2</sub>(CH<sub>2</sub>CHī»CH<sub>2</sub>))<sub>4</sub>] 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 [Cu<sub>4</sub>I<sub>4</sub>] cluster core geometry. Notably, modification of the CuāCu
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). Eu<sup>3+</sup> 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 Eu<sup>3+</sup> to Eu<sup>2+</sup> and reoxidation by oxidants, such as H<sub>2</sub>O<sub>2</sub>, leading to a modulation of the luminescence emission. The incorporation of paramagnetic Gd<sup>3+</sup> 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 GdVO<sub>4</sub> and Gd<sub>0.6</sub>Eu<sub>0.4</sub>VO<sub>4</sub>. These particles retain the luminescence and oxidant detection properties of YVO<sub>4</sub>:Eu. Moreover, the proton relaxivity of GdVO<sub>4</sub> and Gd<sub>0.6</sub>Eu<sub>0.4</sub>VO<sub>4</sub> nanoparticles of 5 nm diameter is higher than that of the commercial Gd<sup>3+</sup> 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 CuāCu bond distances and the luminescence
properties. In addition, the local structure of the [Cu<sub>4</sub>I<sub>4</sub>P<sub>4</sub>] cluster cores has been probed by <sup>31</sup>P and <sup>65</sup>Cu 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 CuāCu bond distances in the [Cu<sub>4</sub>I<sub>4</sub>] 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 CoreāShell Particles Based on Prussian Blue Analogues: Evidence for Confinement Effects?
High-quality coreāshell particles,
which associate a photostrictive core (Rb<sub>0.5</sub>CoĀ[FeĀ(CN)<sub>6</sub>]<sub>0.8</sub>Ā·<i>z</i>H<sub>2</sub>O, <b>RbCoFe</b>) and a ferromagnetic shell (Rb<sub>0.2</sub>NiĀ[CrĀ(CN)<sub>6</sub>]<sub>0.7</sub>Ā·<i>z</i>ā²H<sub>2</sub>O, <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 coreāshell interface is abrupt with the epitaxial
relationship [001](001)<b>RbCoFe</b>//[001]Ā(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 (9ā26 nm) shows that the epitaxial
shell growth does not lead to a thick pseudomorphic layer at the interface.
In contrast, WilliamsonāHall 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