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

The vibrational spectrum of CaCO3 aragonite: A combined experimental and quantum-mechanical investigation

The vibrational properties of CaCO3 aragonite have been investigated both theoretically, by using a quantum mechanical approach (all electron Gaussian type basis set and B3LYP HF-DFT hybrid functional, as implemented in the CRYSTAL code) and experimentally, by collecting polarized infrared (IR) reflectance and Raman spectra. The combined use of theory and experiment permits on the one hand to analyze the many subtle features of the measured spectra, on the other hand to evidentiate limits and deficiencies of both approaches. The full set of TO and LO IR active modes, their intensities, the dielectric tensor (in its static and high frequency components), and the optical indices have been determined, as well as the Raman frequencies. Tools such as isotopic substitution and graphical animation of the modes are available, that complement the analysis of the spectrum

Phenomenological quantum confinement models for excitons and phonons applied to photoluminescence and Raman spectra of silicon nanocrystals

International audienceThe photoluminescence and vibrational properties of silicon nanocrystals are studied in a multilayered system elaborated by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The multilayer systems are deposited on a glass substrate (Herasil). The photoluminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. Contrary to the same study realized with multilayer systems deposited on silicon substrate, the two confinement models (phononic and excitonic) do not lead to the same size distribution. An amorphous silicon phase was also detected in Raman spectroscopy that prevented a good fitting accuracy by the model. Contribution of the substrate to the thermal crystallization process is thus discussed, as well as the origin of the photoluminescence and vibrational properties in terms of quantum confinement or interfacial defects

The use of microwave plasma-assisted CVD on nanostructured iron catalysts to grow isolated bundles of carbon nanotubes

International audienceCatalysts play a key role in the growth of carbon nanotubes. The microwave plasma-assisted chemical vapor deposition (MPACVD) method is now commonly used for directional and conformal growth of carbon nanotubes (CNTs) on substrates. In this work, we report on the effect of H2 plasma pre-treatment on the diameter and density of iron catalyst nanoparticles for different iron layer thicknesses in order to grow isolated bundles of CNTs. Atomic force microscopy shows first that as plasma power density increases, iron nanoparticle diameters decrease, which is due to the increasing of gas dissociation giving more ion bombardment energy, and second that the diameter of nanoparticles decreases with the catalyst thickness. The?growth of CNT was carried out under different CH4 concentrations for different iron film thicknesses. Transmission electron microscopy and Raman spectroscopy show that the synthesized CNT were of good quality and had an outer diameter between 5 and 10?nm

Highly chlorinated Escherichia coli cannot be stained by propidium iodide

International audienceSeveral studies have shown that the staining by fluorochromes (DAPI, SYBR Green II, and TOTO-1) of bacteria is altered by chlorination. To evaluate the effect of chlorine (bleach solution) on propidium iodide (PI) staining, we studied Escherichia coli in suspension and biomolecules in solution (DNA, RNA, BSA, palmitic acid, and dextran) first subjected to chlorine and then neutralized by sodium thiosulphate. The suspensions and solutions were subsequently stained with PI. The fluorescence intensity of the PI-stained DNA and RNA in solution dramatically decreased with an increase in the chlorine concentration applied. These results explain the fact that for chlorine concentrations higher than 3 micromol/L Cl2, the E. coli cells were too damaged to be properly stained by PI. In the case of highly chlorinated bacteria, it was impossible to distinguish healthy cells (with a PI-impermeable membrane and undamaged nucleic acids), which were nonfluorescent after PI staining, from cells severely injured by chlorine (with a PI-permeable membrane and damaged nucleic acids) that were also nonfluorescent, as PI penetrated but did not stain chlorinated nucleic acids. Our results suggest that it would be prudent to be cautious in interpreting the results of PI staining, as PI false-negative cells (cells with compromised membranes but not stained by PI because of nucleic acid damage caused by chlorine) are obtained as a result of nucleic acid damage, leading to an underestimation of truly dead bacteria

Local heterogeneity for a Eu3+-doped glass evidenced by time-resolved fluorescence spectroscopy coupled to scanning near-field optical microscopy

International audienceTime-resolved fluorescence spectroscopy (TRFS) was applied to an aluminate glass sample doped with Eu3+ cation as a fluorescent probe of chemical environment and local symmetry. Conventional far field experiments revealed the presence of two different phases an amorphous phase featured by a highly disordered environment surrounding the Eu3+ cation and a more ordered polycrystalline phase that shows a significant increase of the Eu3+ fluorescence decay time compared to the amorphous phase. Near-field fluorescence spectra and decay kinetics were performed in the frontier region between the two phases using a home-built scanning near-field optical microscope (SNOM). SNOM-TRFS experiments confirmed the presence of local heterogeneities in this part of the glass at a sub-micrometric spatial scale. Polycrystalline sites were featured by an important shear-force interaction with the probing fiber optic tip, a longer fluorescence decay time and a higher Stark splitting of the 5D07FJ (J = 1 to 4) electronic transitions of the Eu3+ cations

Embedded Silicon Nanocrystals Studied by Photoluminescence and Raman Spectroscopies: Exciton and Phonon Confinement Effects

International audienceThe optical and vibrational properties of silicon nanocrystals are studied in two systems elaborated by evaporation. The first one is constituted by a thick SiO layers. The second one is a multilayered sample made by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The luminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. The coherence between the two models and experimental data is demonstrated and gives support to the notion of exciton and phonon confinement effect in silicon nanocrystals embedded within silica matrix

Electrochemically controlled cocrystallisation of caffeine:1-hydroxy-2-naphthoic acid

Controlled formation of cocrystals is an important objective in drug development. Here, cocrystallisation of caffeine and 1-hydroxy-2-naphthoic acid was investigated at the interface between two immiscible electrolyte solutions under chemical polarisation. In this way, selective cocrystallisation was achieved, as verified by X-ray diffraction and Raman spectroscopy. Positive interfacial potentials favoured the formation of one polymorphic form of caffeine:1-hydroxy-2-naphthoic acid cocrystal. This approach to electrochemically controlled cocrystallisation opens up new possibilities for drug development