Anne Pillonnet

Luminescence and Scintillation Properties at the Nanoscale

This contribution is a review of the luminescence and scintillation properties of nanoparticles (NP), particularly doped insulators. Luminescence spectroscopy is an appropriate tool to probe matter at the nanoscale. Luminescence is also the last stage of the scintillation process. Specific surface and structural effects occurring in NP are reported. Their consequences on the NP luminescence properties are discussed. Parts of the effects are related to the preparation method. On the other hand some intrinsic properties of the nanostructures which can modify the optical properties are described: quantum confinement and dielectric confinement. The response under high-energy excitation is also discussed. It appears that their size can be used as a tool to describe the spatial distribution of electronic excitations induced by the relaxation process after the high-energy excitation. Finally, potentiality to grow transparent bulk materials based on small Nps agglomeration via soft chemistry route is presented. It is a promising approach toward the development of scintillating materials.

Bragg surface wave device based on porous silicon and its application for sensing

Results concerning a Bragg surface wave device based on porous silicon and intended for sensing application are reported. Existence of optical surface waves on Bragg structures is experimentally shown. Such device is expected to be very sensitive to the grafting of biological molecules. The authors demonstrate this sensing effect by grafting of amine chemical groups. The optical characterization using m-line spectroscopy shows that the increase of the coupling angle is about 20° after the amine grafting. The authors show that porosity is essential for reaching this high sensitivity.Results concerning a Bragg surface wave device based on porous silicon and intended for sensing application are reported. Existence of optical surface waves on Bragg structures is experimentally shown. Such device is expected to be very sensitive to the grafting of biological molecules. The authors demonstrate this sensing effect by grafting of amine chemical groups. The optical characterization using m-line spectroscopy shows that the increase of the coupling angle is about 20° after the amine grafting. The authors show that porosity is essential for reaching this high sensitivity.

Coupling distance between Eu3+ emitters and Ag nanoparticles

Emission by rare earth emitters has been experimentally investigated based on their position relative to metallic nanoparticles by using Ag/Y2O3/Eu:Y2O3/Y2O3/Ag multilayer samples on- or off-plasmon resonance. Comparison with simple models revealed two different coupling regimes at short and long distances. The optimal coupling distance was determined.

Metal enhanced fluorescence in rare earth doped plasmonic core-shell nanoparticles

We theoretically and numerically investigate metal enhanced fluorescence of plasmonic core-shell nanoparticles doped with rare earth (RE) ions. Particle shape and size are engineered to maximize the average enhancement factor (AEF) of the overall doped shell. We show that the highest enhancement (11 in the visible and 7 in the near-infrared) is achieved by tuning either the dipolar or the quadrupolar particle resonance to the rare earth ions excitation wavelength. Additionally, the calculated AEFs are compared to experimental data reported in the literature, obtained in similar conditions (plasmon mediated enhancement) or when a metal-RE energy transfer mechanism is involved.

HfO2:X (X = Eu3+, Ce3+, Y3+) Sol Gel Powders for Ultradense Scintillating Materials

Hafnium dioxide (HfO 2) presents a high crystalline density which makes it attractive for host lattice activated by rare earths for applications as scintillating materials. HfO 2 powders doped with Eu (3+) or Ce (3+) luminescent ions are prepared by sol gel process. The annealing temperature and the concentration of doping ions are optimized to provide the powder presenting the best scintillation yield. The powders are crystallized in monoclinic phase whatever annealing temperature above 800 degrees C. The emission spectra are characterized by a white broadband between 400 and 600 nm. After optimization, the most efficient composition, namely HfO 2:2.5% Eu 1% Y (molar percent) exhibits a scintillation yield about 31,000 photons/MeV, which is about 3.8 times that of the standard Bi 3Ge 5O 12 (BGO) commercial powder.

Fluorescence of Cr3+ doped alumina optical waveguides prepared by pulsed laser deposition and sol–gel method

Abstract The fluorescence properties of Cr 3+ in optical waveguides of doped alumina prepared by sol–gel or pulsed laser deposition (PLD) are compared with those of Cr-doped alumina powders obtained by sol–gel method and having γ, θ and α crystalline phases. Cr 3+ is located in γ crystallites in sol–gel films but the three phases are present in PLD films.

Local refractive index probed via the fluorescence decay of semiconductor quantum dots.

We present a novel approach for convenient tuning of the local refractive index around nanostructures. We apply this technique to study the influence of the local refractive index on the radiative decay time of CdSe/ZnS quantum dots with three distinct emission wavelengths. The dependence of the luminescence decay time on the environment is well described by an effective medium approach. A critical distance of about 80 nm is found for the determination of the effective local index of refraction. An estimation for the emitting-state quantum efficiency can be extracted.

Valence state of europium doping ions during pulsed-laser deposition

The evolution of europium as a doping ion during the pulsed-laser deposition process of Eu : Al2O3 films has been studied. A decrease in oxygen pressure in the deposition chamber generated the growth of γ-Al2O3 crystallites and a conversion of the 3+ to a 2+ valence state of europium ions. Excitation-selective emission of Eu2+ and fluorescence line narrowing of Eu3+ revealed that two kinds of europium site families were created in the alumina matrix. Time-of-flight emission spectroscopy shows that oxygen came preferentially from the target for the studied range of pressure.

Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement

Pulsed laser deposition was used to deposit aluminum thin films of various thicknesses (tAl) ranging from 5 to 40 nm and to investigate their growth process when they are deposited onto SiO2 and Y2O3. Atomic force microscopy and x-ray reflectivity measurements show that the structure of the Al films are related to the wettability properties of the underlaying layer. Onto SiO2, ultra-smooth layers of aluminum are obtained, due to a perfect wetting of SiO2 by Al. In contrast when deposited onto Y2O3, percolated Al layers are observed with apparent pore size decreasing from 200 to 82 nm as t(Al) is increased from 5 to 40 nm, respectively. This particular morphology is related to partial dewetting of Al on Y2O3. These two different growth mechanisms of aluminum depend therefore on the surface properties of SiO2 and Y2O3. The plasmon resonance of such Al nanostructures in the UV region was then analyzed by studying the coupling between Eu(3+) rare earth emitters and Al.

Plasmonic enhancement of Eu:Y2O3 luminescence by Al percolated layer

The coupling between Eu(3+) rare earth emitters and Al has been investigated in multilayer structures, which consist of an Eu:Y2O3 phosphor film deposited between percolated and continuous Al films. Passive buffer Y2O3 layers were deposited between phosphor and Al films with different thicknesses to analyze the role of the Eu-Al distance on the nanostructuration and emission of the Eu:Y2O3 film. By using Eu(3+) emitters as local structural probes completed by transmission electron microscopy analyses, we show that the deposition on Al promotes the growth of the cubic crystallites. A fluorescence analysis allows us to evaluate the presence of a perturbed structural shell around the cubic core of the crystallites. Moreover, the enhancement observed at short distances is attributed to the localized plasmon resonance of the percolated upper Al film.