Influence of the annealing treatments on the luminescence properties of SiO∕SiO2 multilayers

International audienceThe formation of silicon nanocrystals ͑Si-ncs͒ and their room temperature photoluminescence ͑PL͒ properties were investigated in samples elaborated by the evaporation method. Silicon oxide ͑SiO͒ single layer and SiO / SiO 2 multilayers with different SiO layer thicknesses from 1 to 5 nm were prepared and annealed at different temperatures up to 1050°C. The structure and the formation of Si-nc were studied by transmission electron microscopy ͑TEM͒ and by Fourier transform infrared ͑FTIR͒ absorption spectroscopy. It is demonstrated that the Si-ncs appear by the phase separation process from SiO due to the annealing treatments. Contrary to the SiO single layer, the multilayers are a powerful system to obtain highly luminescent Si-nc and to control the Si-nc size for SiO layer thicknesses lower than or equal to 4 nm. It is clearly shown that, in agreement with the quantum confinement theory, the PL energy is a decreasing function of the Si-nc size. However, thanks to the correlation between FTIR, TEM, and PL results, it is demonstrated that the PL energy is also strongly dependent on the quality of the matrix in which the Si-nc are embedded. A model based on the existence of a SiO x shell surrounding the Si-nc is proposed to explain the PL results

Lattice QCD calculation of ππ\pi\pi scattering length

We study s-wave pion-pion ($\pi\pi$) scattering length in lattice QCD for pion masses ranging from 330 MeV to 466 MeV. In the "Asqtad" improved staggered fermion formulation, we calculate the $\pi\pi$ four-point functions for isospin I=0 and 2 channels, and use chiral perturbation theory at next-to-leading order to extrapolate our simulation results. Extrapolating to the physical pion mass gives the scattering lengths as $m_\pi a_0^{I=2} = -0.0416(2)$ and $m_\pi a_0^{I=0} = 0.186(2)$ for isospin I=2 and 0 channels, respectively. Our lattice simulation for $\pi\pi$ scattering length in the I=0 channel is an exploratory study, where we include the disconnected contribution, and our preliminary result is near to its experimental value. These simulations are performed with MILC 2+1 flavor gauge configurations at lattice spacing $a \approx 0.15$ fm.Comment: Remove some typo

Indirect excitation of Er3+ ions in silicon nitride films prepared by reactive evaporation

International audienceEr-doped silicon nitride films were obtained by reactive evaporation of silicon under a flow of nitrogen ions and were annealed at temperatures up to 1300°C. Samples were studied by infrared absorption and Raman spectrometries and by transmission electron microscopy. The 1.54 m Er-related photoluminescence ͑PL͒ was studied in relation with the structure with pump excitation at 488 and 325 nm. Steady-state PL, PL excitation spectroscopy, and time-resolved PL were performed. The results demonstrate that Er 3+ ions are indirectly excited both via silicon nanocrystals and via localized states in the silicon nitride matrix. Er-doped silicon-based materials have attracted much attention in the scientific community because of their potential use for optoelectronics. 1 Indeed, Er 3+ ions can emit sharp luminescence at 1.54 m, which is the commonly used wavelength for optical communications. The Er sensitization has been widely studied in Si rich SiO 2 layers. In silica containing silicon nanocrystals ͑Si-nc͒, the Er-related photolu-minescence is strongly improved due to a strong energy transfer from Si-nc to Er 3+ ions. 2-4 The Er 3+ ions can then be indirectly excited by Si-nc which have an absorption cross section several orders of magnitude higher than that of direct Er excitation. While SiN x is a particularly interesting host matrix for electrically pumped light-emitting devices, the Er excitation mechanism in silicon nitride films is still not clear. Similarly to the SiO x based samples, the sensitization of Er 3+ ions by Si nanoparticules has been reported in SiN x samples prepared by plasma enhanced chemical vapour deposition ͑PECVD͒ 5 or by magnetron sputtering. 6 However, some works have also demonstrated that indirect excitation of Er 3+ ions could occur via electronic states localized in the SiN x band tail states. 7,8 In this letter, we study the Er-related PL at 1.54 m in Er-doped silicon nitride thin films prepared by an ion-beam-assisted evaporation technique. The evolutions of the structure and of the PL properties with the annealing treatments are studied. It is demonstrated that the Er excitation is indirect and that Si-nc is able to improve the PL intensity. It is also shown that another indirect excitation path presumably exists in the amorphous SiN x matrix. Silicon was evaporated from an electron beam gun with a deposition rate equal to 0.1 nm/s. The 200 nm thick films were deposited on silicon substrates maintained at 100°C. The nitrogen ions were provided by an electron cyclotron resonance microwave plasma source. The nitrogen flow was regulated by maintaining the total pressure in the evaporation chamber at 2 ϫ 10 −5 Torr. The Er doping was performed from an effusion cell. Rutherford backscattering spectrom-etry was used to analyze the chemical content of the film. The Si, N, O, and Er atomic concentrations are equal to 47%, 48%, 5%, and 0.3%, respectively. The oxygen content is due to the low density of the layer and to exposure to the air. This concentration corresponds to a 12 at. % Si excess compared to the Si 3 N 4 equilibrium stoichiometry. The Fourier transform infrared ͑FTIR͒ experiments were carried out with a spectrometer with a resolution of 2 cm −1. Raman measurements were carried out with a mutichannel spectrometer equipped with a 1800 grooves mm −1 grating. The samples were excited by the 514 nm line from an argon laser. Transmission electron microscopy was performed with a 200 keV microscope. For the steady-state PL experiments, the samples were excited by a 30 mW He-Cd laser using the 325 nm line or by a 60 mW laser diode emitting at 488 nm. For the PL excitation ͑PLE͒ experiments, the samples were excited by an optical parametric oscillator laser. The PL signal was measured by a photomultiplier tube cooled at 190 K. For the time-resolved PL experiments, the samples were pumped by the 355 nm line of a frequency-tripled YAG:Nd laser. The laser pulse frequency, energy, and duration were typically equal to 10 Hz, 50 J, and 20 ns, respectively. The time response of the detection system was better than 1 s. Figure 1͑a͒ shows the FTIR spectra of the films for as-deposited sample and samples annealed at 1000 and 1100°C. The spectrum shows a very intense band at around 850 cm −1 , characteristic of the asymmetric stretching vibration of the SiN bonds. 9 The spectra are not significantly modified for annealing temperatures lower than 1000°C since only a 6 cm −1 shift occurs to higher wavenumbers. For higher annealing temperature, the peaks shift again a few cm −1 and a shoulder appears at high wavenumbers, demonstrating a modification of the SiN bonds, which could be correlated to the precipitation of Si-nc. 1

Ecological speciation in sympatric palms: 1. Gene expression, selection and pleiotropy

Ecological speciation requires divergent selection, reproductive isolation and a genetic mechanism to link the two. We examined the role of gene expression and coding sequence evolution in this process using two species of Howea palms that have diverged sympatrically on Lord Howe Island, Australia. These palms are associated with distinct soil types and have displaced flowering times, representing an ideal candidate for ecological speciation. We generated large amounts of RNA-Seq data from multiple individuals and tissue types collected on the island from each of the two species. We found that differentially expressed loci as well as those with divergent coding sequences between Howea species were associated with known ecological and phenotypic differences, including response to salinity, drought, pH and flowering time. From these loci, we identified potential 'ecological speciation genes' and further validate their effect on flowering time by knocking out orthologous loci in a model plant species. Finally, we put forward six plausible ecological speciation loci, providing support for the hypothesis that pleiotropy could help to overcome the antagonism between selection and recombination during speciation with gene flow

A Study of the Formation of Single- and Double-Walled Carbon Nanotubes by a CVD Method

The reduction in H2/CH4 atmosphere of aluminum-iron oxides produces metal particles small enough to catalyze the formation of single-walled carbon nanotubes. Several experiments have been made using the same temperature profile and changing only the maximum temperature (800-1070 °C). Characterizations of the catalyst materials are performed using notably 57Fe Mo¨ssbauer spectroscopy. Electron microscopy and a macroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe3+, R-Fe, ç-Fe-C, Fe3C) is correlated to their location in the material. The nature of the particles responsible for the high-temperature formation of the nanotubes is probably an Fe-C alloy which is, however, found as Fe3C by postreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors the formation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes are mostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formation mechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the first one but that subsequent ones are formed outside. It is also possible that under given experimental conditions, the smallest (<2 nm) catalyst particles preferentially produce double-walled rather than single-walled carbon nanotubes

Mössbauer Spectroscopy Involved in the Study of the Catalytic Growth of Carbon Nanotubes

Single-walled and thin multiwalled carbon nanotubes are prepared by a catalytic-chemical-vapor-deposition method involving the simultaneous formation of Fe or Co nanometric particles from oxide solid solutions based on Al2O3, MgAl2O4 or MgO. This paper is an overview of the authors’ work on the characterization by Mössbauer spectroscopy used in complement to electron microscopy and specific-surface-area measurements. It is notably attempted to correlate the nature of the different iron phases in the carbon nanotube-metal-oxide powders with the formation mechanisms of the nanotubes. Massive composites and hydrogen storage are proposed as possible applications

Large and robust electrical spin injection into GaAs at zero magnetic field using an ultrathin CoFeB/MgO injector

We demonstrate a large electrical spin injection into GaAs at zero magnetic field thanks to an ultrathin perpendicularly magnetized CoFeB contact of a few atomic planes (1.2 nm). The spin-polarization of electrons injected into GaAs was examined by the circular polarization of electroluminescence from a Spin Light Emitting Diode with embedded InGaAs/GaAs quantum wells. The electroluminescence polarization as a function of the magnetic field closely traces the out-of-plane magnetization of the CoFeB/MgO injector. A circular polarization degree of the emitted light as large as 20% at 25 K is achieved at zero magnetic field. Moreover the electroluminescence circular polarization is still about 8% at room temperature.Comment: *Corresponding author: [email protected]

HTS Dipole Insert Developments

n/

J/psi azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158 GeV per nucleon

The J/$\psi$ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/$\psi$ mesons at SPS energies. Hence, the measurement of J/$\psi$ elliptic anisotropy, quantified by the Fourier coefficient v$_2$ of the J/$\psi$ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/$\psi$ suppression observed in Pb-Pb collisions. We present the measured J/$\psi$ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v$_{2}$ as a function of the collision centrality and of the J/$\psi$ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p$_{\rm T}$ sub-samples. The extracted v$_{2}$ values are significantly larger than zero for non-central collisions and are seen to increase with p$_{\rm T}$.Comment: proceedings of HP08 conference corrected a typo in one equatio