Energy level diagram and kinetics of luminescence of Ag nanoclusters dispersed in a glass host.

A site-selective spectroscopy study of Ag nanoclusters dispersed in oxyfluoride glass hosts has been carried out. The nano- to millisecond, essentially non-exponential, luminescence kinetics of Ag nanoclusters has been detected in the spectral range from 450 to 1000 nm, when excited at discrete wavelengths in the range 250 to 450 nm. Based on these experimental observations, the energy level configuration coordinate diagram for the involved ground and excited singlet/triplet states of the Ag nanoclusters has been proposed and confirmed by the density functional theory (DFT). The sites for the Ag nanoclusters are argued to be multiple. The structure/geometry of the involved Ag nanoclusters has been suggested to involve spin-paired dimers Ag²âº, or tetramers Ag4²âº, with a varying elongation/distortion along the tetramer diagonals.

Preparation and luminescence of bulk oxyfluoride glasses doped with Ag nanoclusters.

Bulk oxyfluoride glasses doped with Ag nanoclusters have been prepared using the melt quenching technique. When pumped in the absorption band of Ag nanoclusters between 300 to 500 nm, these glasses emit a very broad luminescence band covering all the visible range with a weak tail extending into the near infrared. The maximum of the luminescence band and its color shifts to the blue with a shortening of the excitation wavelength and an increasing ratio of oxide to fluoride components, resulting in white color luminescence at a particular ratio of oxide to fluoride; with a quantum yield above 20%.

Cathodo- and photoluminescence in Yb(3+)-Er(3+) co-doped PbF(2) nanoparticles.

We have prepared and studied the PbF(2):(Yb(3+),Er(3+)) co-doped nanoparticles, with chemical formula (Yb-Er)(x)Pb(1-x)F(2+x), where x = 0.29, Yb(3+)/Er(3+) = 6, and estimated the energy efficiency for their cathodoluminescence, mostly of Yb(3+), and up-conversion photoluminescence of Er(3+) to reach more than 0.5% and 20%, respectively, which may be the highest to date for rare-earth doped nanoparticles. Electron beam induced temperature rise in the nanoparticles has been estimated by measuring the ratio of green emission bands of Er(3+). These high efficiencies are due to high doping level of nanoparticles and due to low phonon energy of the PbF(2) host.

Colour tuneability in sol-gel nano-glass-ceramics comprising Yb(3+)-Er(3+)-Tm3+ Co-doped NaYF4 nanocrystals.

White light can be obtained by up-conversion processes in a cheap and efficient way, being of great interest for lighting, displays, and photonic integrated devices. Here we report a generation of white light under 980 nm laser infrared excitation in Y(b3+)-Er(3+)-Tm3+ triply doped sol-gel derived nano-glass-ceramics containing NaYF4 nanocrystals where co-dopants ions are partitioned. Efficient simultaneous blue, green, and red up-conversion emissions give rise to a balanced white overall emission and corresponding up-conversion mechanisms have been analyzed. Changes in pump power intensity or heat treatment temperature widely vary the ratio of emission bands, resulting in a colour tuneability which has been analyzed and quantified in terms of CIE standard chromaticity diagram.

Undoped and Eu3+ doped In2O3 quantum-dots in transparent glass-ceramics.

Nano-structured glass-ceramics comprising In2O3 nanocrystals (quantum dots) in a silica glassy matrix have been synthesized for the first time to our knowledge, by thermal treatment of sol-gel precursor glasses with different concentrations of In2O3. Undoped and Eu(3+)-doped samples have been obtained and characterized. By means of X-ray Diffraction and Transmission Electron Microscopy analysis, the precipitation of In2O3 cubic crystalline nanoparticles was confirmed. The mean radii of these nanocrystals, from 1 to 4 nm, are comparable to the exciton Bohr radius, corresponding to wide band-gap semiconductor quantum-dots. Under interband UV excitation of the nanocrystals, a broad visible emission is observed in the undoped samples. Meanwhile, the Eu(3+)-doped samples only show emission from these ions which are efficiently excited by energy transfer from the In2O3, nanocrystals. Selective excitation of the Eu3+ ions allow us to discern those remaining in the silica glassy matrix from the ones located in the interface SiO2-In2O3.

Optical nanoheater based on the Yb3+-Er3+ co-doped nanoparticles.

Yb(3+)-Er(3+) co-doped fluoride nanoparticles have been prepared. When pumped by 975 nm laser diode into absorption band of Yb(3+), the laser-induced temperature rise up to 800 degrees C has been detected in the nanoparticles by measuring the ratio of the intensities of the thermalised up-conversion luminescence bands (2)H(11/2)-->(4)I(15/2) and (4)S(3/2)-->(4)I(15/2) of Er(3+). These results show that a controlled optical heating of the nanoparticles and their surrounding nano-volumes can be realised, while the location and temperature rise of the nanoparticles and heated nano-volumes can be detected distantly by means of luminescence.

Measurement of quantum yield of up-conversion Luminescence in Er(3+)-doped nano-glass-ceramics.

A measurement of quantum yield of up-conversion luminescence has been done for the Er(3+)-doped transparent oxyfluoride glass-ceramics 32(SiO,)9(AlO1.5)31.5(CdF2)18.5(PbF2)5.5(ZnF2): 3.5(ErF3) mol%, where most of Er3+ dopants partition in 8 nm diameter nano-crystals Er10Pb25F65. The yield was found by newly proposed method using the pump power dependence of the resonant luminescence. The result of the measurement points out that a theoretical maximum of 50% may be reached for the up-conversion luminescence yield in this material. This high yield is shown to be due to low phonon energy and short inter-dopant distances in the nano-crystals.

Luminescence of nanostructured SnO2-SiO2 glass-ceramics prepared by sol-gel method.

Nanostructured silica based glass-ceramics samples of composition (100 - x)SiO2-xSnO2, with x from 1 to 10, have been synthesized by thermal treatment of precursor sol-gel glasses. The average size of the obtained SnO2 nanocrystals, calculated by using the X-ray diffraction, can be predetermined by using well-controlled concentration of tin precursor. The mean radius ranging from 1.6 to 5.5 nm, is comparable to the exciton Bohr radius, corresponding to wide band-gap semiconductor quantum-dots in an insulator SiO2 glass. A spectroscopy study in terms of optical absorption and photoluminescence spectra has been carried out as a function of SnO2 concentration. Size-dependent red-shifts of excitation and emission bands, with increasing of tin precursor concentration, point to the quantum confinement effect. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations. The band gap increase is in agreement with results, based on the effective mass model. The recombination of conduction band electron with oxygen vacancies is proposed to explain the luminescence red-shift.

Site-selective spectroscopy in Sm(3+)-doped sol-gel-derived nano-glass-ceramics containing SnO(2) quantum dots.

Nano-glass-ceramics of composition 95SiO(2)-5SnO(2) doped with 0.4 Sm(3+) (mol%) were synthesized by the thermal treatment of precursor sol-gel glasses. Structural and luminescence measurements were carried out. The precipitated SnO(2) nanocrystals in the glass matrix constitute a wide bandgap quantum-dot system with size comparable to the bulk exciton Bohr radius. A site-selective excitation, by energy transfer from the semiconductor host, reveals that a fraction of the Sm(3+) ions are incorporated in the SnO(2) nanocrystals, whereas the rest remains in the silica glassy phase. An evolution in the Sm(3+) emission spectra has been observed when the SnO(2) nanocrystals are excited with different UV wavelengths, which has been ascribed to selective excitation of nanocrystal sets with predetermined size.

Three-body Coulomb problem probed by mapping the Bethe surface in ionizing ion-atom collisions.

The three-body Coulomb problem has been explored in kinematically complete experiments on single ionization of helium by 100 MeV/u C(6+) and 3.6 MeV/u Au(53+) impact. Low-energy electron emission ( E(e)<150 eV) as a function of the projectile deflection theta(p) (momentum transfer), i.e., the Bethe surface [15], has been mapped with Delta theta(p)+/-25 nanoradian resolution at extremely large perturbations ( 3.6 MeV/u Au(53+)) where single ionization occurs at impact parameters of typically 10 times the He K-shell radius. The experimental data are not in agreement with state-of-the-art continuum distorted wave-eikonal initial state theory.

Abrupt rise of the longitudinal recoil ion momentum distribution for ionizing collisions.

We report on the experimental observation of an abrupt rise in the longitudinal momentum distribution of recoil ions created in proton helium collision. The details of this structure can be related to electrons traveling with the velocity of the projectile [electron capture to the continuum (ECC)]. The longitudinal as well as the transverse distribution of the recoil ions can be explained as a continuation of the momentum distribution from ions resulting from electron capture illustrating the smooth transition from the capture to bound states of the projectile to the ECC.