J.-P. Mosnier

Control of ZnO nanorod array density by Zn supersaturation variation and effects on field emission

We demonstrate control of ZnO nanorod density for self-organized growth on ZnO buffer layers on Si by varying Zn supersaturation during the initial growth phase, thereby altering the competition between 2D and 1D growth modes. Higher initial supersaturation favours nanorods of diameter 1000, attributed to sharp facet edges, and indicate that lower density arrays have more uniform emission due to a reduction in screening effects.

Time-integrated laser-induced plasma spectroscopy in the vacuum ultraviolet for the quantitative elemental characterization of steel alloys

This paper demonstrates that time-integrated space-resolved laser-induced plasma spectroscopy (TISR-LIPS) is a useful technique in the vacuum ultraviolet (VUV) for the quantitative determination of the carbon content in steels. The standard reference samples used were carbon-iron alloys containing a relatively wide concentration range of carbon (0.041-1.32%). In the experiments the output of a Q-switched Nd:YAG (1064?nm) laser, with approximately a 1?J maximum output pulse energy and approximately a 12?ns temporal pulse width, was focused onto the surface of each sample (under vacuum) in order to produce the emitting plasma. A fore-slit mounted in the target chamber allowed spatially-resolved spectral measurements in the axial direction of the plasma and provided emission lines that were almost free of the background continuum. A 1?m normal incidence vacuum spectrometer, equipped with a 1200 grooves mm-1 concave grating and a micro-channel plate/photodiode array detector combination, was used as the detection system. A particularly interesting feature of this work is the demonstration that VUV spectroscopy allows ionic lines to be used and linear calibration curves were obtained for the five carbon spectral lines (from C+ and C2+) under investigation. The limits of detection for all lines were determined; the lowest detection limit (87?10?ppm) was obtained from the C2+ 97.70?nm line, which compares favourably with the only available value in the literature of 100?ppm.

Characterization and antimicrobial efficacy against E. coli of a helium/air plasma at atmospheric pressure created in a plastic package

A plasma source, sustained by the application of a floating high voltage (?15?kV) to parallel-plate electrodes at 50?Hz, has been achieved in a helium/air mixture at atmospheric pressure (P?=?105?Pa) contained in a zip-locked plastic package placed in the electrode gap. Some of the physical and antimicrobial properties of this apparatus were established with a view to ascertain its performance as a prototype for the disinfection of fresh produce. The current?voltage (I?V) and charge?voltage (Q?V) characteristics of the system were measured as a function of gap distance d, in the range (3???103???Pd???1.0???104?Pa?m). The electrical measurements showed this plasma source to exhibit the characteristic behaviour of a dielectric barrier discharge in the filamentary mode and its properties could be accurately interpreted by the two-capacitance in series model. The power consumed by the discharge and the reduced field strength were found to decrease quadratically from 12.0?W to 4.5?W and linearly from 140?Td to 50?Td, respectively, in the range studied. Emission spectra of the discharge were recorded on a relative intensity scale and the dominant spectral features could be assigned to strong vibrational bands in the 2+ and 1? systems of N2 and , respectively, with other weak signatures from the NO and OH radicals and the N+, He and O atomic species. Absolute spectral intensities were also recorded and interpreted by comparison with the non-equilibrium synthetic spectra generated by the computer code SPECAIR. At an inter-electrode gap of 0.04?m, this comparison yielded typical values for the electron, vibrational and translational (gas) temperatures of (4980???100)?K, (2700???200)?K and (300???100)?K, respectively and an electron density of 1.0???1017?m?3. A Boltzmann plot also provided a value of (3200???200?K) for the vibrational temperature. The antimicrobial efficacy was assessed by studying the resistance of both Escherichia coli K12 its isogenic mutants in soxR, soxS, oxyR, rpoS and dnaK selected to identify possible cellular responses and targets related with 5?min exposure to the active gas in proximity of, but not directly in, the path of the discharge filaments. Both the parent strain and mutants populations were significantly reduced by more than 1.5 log cycles in these conditions, showing the potential of the system. Post-treatment storage studies showed that some transcription regulators and specific genes related to oxidative stress play an important role in the E. coli repair mechanism and that plasma exposure affects specific cell regulator systems.

Growth of ZnO nanostructures on Au-coated Si: Influence of growth temperature on growth mechanism and morphology

ZnO nanostructures were grown on Au-catalyzed Si silicon substrates using vapor phase transport at growth temperatures from 800 to 1150 °C. The sample location ensured a low Zn vapor supersaturation during growth. Nanostructures grown at 800 and 850 °C showed a faceted rodlike morphology with mainly one-dimensional (1D) growth along the nanorod axis. Samples grown at intermediate temperatures (900, 950, and 1050 °C) in all cases showed significant three dimensional (3D) growth at the base of 1D nanostructures. At higher growth temperatures (1100 and 1150 °C) 3D growth tended to dominate resulting in the formation of a porous, nanostructured morphology. In all cases growth was seen only on the Au-coated region. Our results show that the majority of the nanostructures grow via a vapor-solid mechanism at low growth temperatures with no evidence of Au nanoparticles at their tip, in sharp contrast to the morphology expected for the vapor-liquid-solid (VLS) process often reported as the growth mechanism on Au-c...

(20−23) ZnO thin films grown by pulsed laser deposition on CeO2-buffered r-sapphire substrate

Composite ZnO∕CeO2 thin films were grown epitaxially on r-sapphire substrates using the pulsed laser deposition technique. Their crystalline properties were established using x-ray diffraction and showed the ZnO (wurtzite structure) and CeO2 (fluorite structure) layers to be highly textured with the (20−23) and (100) orientations, respectively. ϕ-scan measurements were also carried out and the (20−23)ZnO‖(100CeO2), [1−210]ZnO‖⟨011⟩ CeO2 epitaxial relations established. The rocking curve profiles indicated that the ZnO films grew as four crystallographically equivalent domains. Series of rocking curve and χ−scan measurements at varying ϕ angles, respectively, were used to investigate the domain structure. These showed that the normal to the (20−23) plane in each domain is tilted away from the substrate normal towards one of the four equivalent CeO2 ⟨111⟩ directions by ∼1.60. Atomic force microscopy measurements showed that the ZnO∕CeO2 composite film has a granular microstructure with a rough surface (typi...

Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma

Recently, the spectrum of singly ionized chromium has come under theoretical scrutiny by Dolmatov (1996 J. Phys. B: At. Mol. Opt. Phys. 29 L673) and also by Donnelly et al (1997 J. Phys. B: At. Mol. Opt. Phys. 30 L285). Both of these theoretical investigations, employing the spin-polarized RPAE (SPRPAE) and R-matrix techniques, respectively, concerned the 3p photoabsorption cross section from the 3d5 6S5/2 ground state of Cr+. The two calculations are consistent with each other and agree with the previously reported experimental data for the 3p4s resonances and the onset of the 3p3d giant resonance (Costello 1991 Phys. Rev. A 43 1441). However, beyond the peak of the giant resonance, both calculations result in cross sections which differ considerably from the experimental data. From further dual-laser plasma experiments carried out under different spatio-temporal conditions and atomic structure calculations on metastable states this discrepancy can be ascribed to the presence of metastable excited singly ionized chromium within the plasma.

Electron and ion stagnation at the collision front between two laser produced plasmas

We report results from a combined optical interferometric and spectrally resolved imaging study on colliding laser produced aluminium plasmas. A Nomarski interferometer was used to probe the spatio-temporal distribution of electron densities at the collision front. Analysis of the resulting interferograms reveals the formation and evolution of a localized electron density feature with a well-defined profile reminiscent of a stagnation layer. Electron stagnation begins at a time delay of 10 ns after the peak of the plasma generating laser pulse. The peak electron density was found to exceed 1019 cm−3 and the layer remained well defined up to a time delay of ca 100 ns. Temporally and spectrally resolved optical imaging was also undertaken, to compare the Al+ ion distribution with that of the 2D electron density profile. This revealed nascent stagnation of singly charged ions at a delay time of 20 ns. We attribute these results to the effects of space charge separation in the seed plasma plumes.

Structural characterization of ZnO thin films grown on various substrates by pulsed laser deposition

ZnO thin films were grown by pulsed laser deposition on three different substrates: sapphire (0?0?0?1), MgO (1?0?0) and fused silica (FS). The structure and morphology of the films were characterized by x-ray diffraction and scanning electron microscopy and defect studies were carried out using slow positron implantation spectroscopy (SPIS). Films deposited on all substrates studied in this work exhibit the wurtzite ZnO structure and are characterized by an average crystallite size of 20?100?nm. However, strong differences in the microstructure of films deposited on various substrates were found. The ZnO films deposited on MgO and sapphire single-crystalline substrates exhibit local epitaxy, i.e. a well-defined relation between film crystallites and the substrate. Domains with different orientation relationships with the substrate were found in both films. On the other hand, the film deposited on the FS substrate exhibits fibre texture with random lateral orientation of crystallites. Extremely high compressive in-plane stress of ????14?GPa was determined in the film deposited on the MgO substrate, while the film deposited on sapphire is virtually stress-free, and the film deposited on the FS substrate exhibits a tensile in-plane stress of ????0.9?GPa. SPIS investigations revealed that the concentration of open-volume defects in the ZnO films is substantially higher than that in a bulk ZnO single crystal. Moreover, the ZnO films deposited on MgO and sapphire single-crystalline substrates exhibit a significantly higher density of defects than the film deposited on the amorphous FS substrate.

Vacuum-UV absorption spectrum of a laser-produced chromium plasma: 3p-subshell photoabsorption by Cr2+ ions

The dual laser plasma photoabsorption technique has been used to measure the time-resolved vacuum-UV photoabsorption spectrum of a chromium plasma. Resonant photoabsorption cross sections, constructed with the aid of Hartree-Fock calculations, and weighted in accordance with the plasma temperature, have been used to produce the synthetic Cr2+ spectra. The relevant plasma temperature and ionization balance are obtained from simple analytical models for various times during the expansion phase of the plasma plume. The experimental spectra taken at delays of 32, 62 and 90 ns compare well with Cr2+ spectra computed for corresponding predicted temperatures. It is found that in order to produce synthetic spectra that match experiment well, it is necessary to take into account absorption from many states belonging to the Cr2+ ground state configuration 3p63d4, while states from the nearest metastable configuration 3p63p34s make a negligible contribution.

4d photoabsorption spectra of Indium (In II?In IV)

The 4d photoabsorption spectra of singly to triply ionized indium have been recorded using the dual laser plasma technique in the 24.5–55 eV photon energy range. The data substantiate and add to previously classified lines in In II and In IV while many new lines are tentatively identified in In III. Transitions were assigned with the aid of Hartree–Fock calculations.