Photonic band-gap effects on photoluminescence of silicon nanocrystals embedded in artificial opals

Si nanocrystals were formed in synthetic opals by Si-ion implantation and their optical properties studied using microphotoluminescence and reflection techniques. The properties of areas with high crystalline quality are compared with those of disordered regions of samples. The photoluminescencespectrum from Si nanocrystals embedded in silica spheres is narrowed by the inhibition of emission at wavelengths corresponding to the opalphotonic pseudoband gap (∼690 nm). Measurements of photoluminescencespectra from individual implanted silica spheres is also demonstrated and the number of emitting Si nanocrystals in single brightly emitting spheres is estimated to be of the order of one thousand.This work was supported by GACR (202/03/0789), NATO (PST.CLG.978100), and by the Royal Swedish Academy of Sciences. One of the authors ~J.V.! appreciates financial support from the French government (program Echange)

Mössbauer and magnetic study of Co x Fe3−x O4 nanoparticles

Magnetic nanoparticles of cobalt ferrites Co x Fe3−x O4 (x = 1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH–CO3. Mechanical milling of the 1-LDH–CO3 leads to the large-scale preparation of nearly spherical nanoparticles of CoFe2O4, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC–FC) in the magnetic properties. Annealing treatment of the 2-LDH–CO3 below 673 K results in the formation of nearly spherical pure Co2FeO4 nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co2FeO4 and CoFe2O4, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co2FeO4

A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3

ɛ-Fe2O3 was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y3Fe5O12) in a silica matrix at 300-C followed by sintering in air at 1,000-C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mossbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K

Mössbauer study of nanodimensional nickel ferrite-mechanochemical synthesis and catalytic properties

Iron-nickel spinel oxide NiFe2O4 nanoparticles have been prepared by the combination of chemical precipitation and subsequent mechanical milling. For comparison, their analogue obtained by thermal synthesis is also studied. Phase composition and structural properties of iron-nickel oxides are investigated by X-ray diffraction and Mössbauer spectroscopy. Their catalytic behavior in methanol decomposition to CO and methane is tested. An influence of the preparation method on the reduction and catalytic properties of iron-nickel samples is established

Sum-frequency generation in sol-gel material doped with binaphthol

A sum-frequency imaging system is used to investigate optical activity in a sol-gel material doped with the chiral molecule binaphthol. We show that this material can be optically structured to embed information that can be retrieved by sum-frequency

Functionalization of a polymer encapsulant with photon conversion

Équipe 104 : NanomatériauxInternational audienceCurrent silicon solar cell modules use a polymer encapsulant to protect the solar cells. This polymer is essential but it has a passive action. In this work, we propose to functionalize the polymer by inserting inorganic nanoparticles in it so that it converts ultraviolet photons into infrared photons. The final goal is to increase the overall efficiency of the silicon solar modules. The polymer proposed is ethylene vinyl acetate which is a commonly used encapsulant, to which Nd, or Yb, or Tb-Yb doped CeO2 nanoparticles are added for the photon conversion. Although no improvement in efficiency was observed in solar cells including the nanoparticles in the encapsulant, the present work paves the way for the integration of photon downconversion into modules without the use of vacuum deposition technologies

Photon conversion in Tb,Yb:Ca x Sr 1-x Al 2 O 4 nanocrystals

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Temperature behaviour of optical properties of Si

Silicon nanocrystals were prepared by Si+-ion implantation and subsequent annealing of SiO2 films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses $(7\times10^{15}$- $2\times10^{17}$ cm$^{-2})$ were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic% . The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature. The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si+-implanted SiO2 films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials

Investigation of KBiFe2O5 as a Photovoltaic Absorber

International audienceKBiFe 2 O 5 (KBFO) was grown by pulsed laser deposition (PLD) on SrTiO 3 (001) (STO), 1 at% Nb-SrTiO 3 (001) (Nb-STO) and MgAl 2 O 4 (001) (MAO). In the case of MAO substrate, epitaxial growth is obtained. As its bandgap is relatively low (1.6 eV in the bulk), KBFO is a promising candidate for oxide photovoltaics. In this work we examine the growth of KBFO by PLD by looking at its structure and composition and we investigate the optical properties of the films obtained. A photovoltaic architecture based on KBFO films is proposed and a solar cell behaviour based on KBFO absorber is obtained