Chantal Boulmer-Leborgne

Nanoparticle formation by femtosecond laser ablation

Ultra short femtosecond (fs) pulses for the laser ablation of materials lead to deposited films which are very different from those obtained by the well-known classical nanosecond (ns) pulsed laser deposition (PLD). In very specific cases, epitaxial thin films can be obtained, whereas in the majority of materials, the films formed by fs PLD are constituted by the random stacking of nanoparticles (Nps) in the 10–100 nm size range. As a result, fs PLD has been rapidly considered as a viable and efficient method for the synthesis of Nps of a wide range of materials presenting interesting physical properties and potential applications. The Np synthesis by fs laser ablation has been studied, and theoretical investigations have been reported to establish their formation mechanisms. Two possibilities can be assumed to explain the Np synthesis: direct cluster ejection from the target or collisional sticking and aggregation in the ablated plume flow.

Growth of apatite films by laser ablation: Reduction of the droplet areal density

The pulsed laser deposition of calcium phosphate (apatite) thin films using targets with different optical absorptivities has been studied. It was found that the surface morphology of the grown films greatly depends on the target optical properties, the higher the optical absorption coefficient a, the lower the droplet density on the surface of the deposited films. The temperature profiles inside the different targets during the action of a laser pulse were obtained by numerically solving the heat diffusion equation. These simulations indicated for the low a value targets a large volume heating with the formation of a thick layer of melted material, and the occurrence of a sub‐surface superheating effect once the evaporation began. For higher a value targets, the temperature profiles corresponded to the surface heating case, where the evaporation process began at earlier times and was restricted to the outermost surface region. Time‐ and space‐resolved optical emission spectroscopy investigations showed t...

Light emission from germanium nanoparticles formed by ultraviolet assisted oxidation of silicon-germanium

Nanocrystalline Ge particles embedded in a SiO2 layer, directly formed by an excision process from a SiGe strained layer during low temperature ultraviolet‐assisted oxidation are shown to exhibit visible photoluminescence. The emission maxima of the photoluminescence spectra are situated at around 2.18 eV, a value that corresponds, according to recent data to an average particle size of 5 nm, in excellent agreement with our previous Raman and transmission electron microscopy measurements of particle size. It is proposed that stress effects associated with the oxidation of small spherical particles allow the formed nanocrystalline Ge to survive during prolonged ultraviolet oxidation of the SiGe layer.

GaN thin films deposition by laser ablation of liquid Ga target in nitrogen reactive atmosphere

Abstract GaN thin films were deposited by laser ablation of liquid Ga target in nitrogen reactive atmosphere. An Nd-YAG laser (λ=1.06 μm, τFWHM=10 ns) of 0.35 J energy/pulse was used as laser source. The nitrogen pressure was varied in the range of 10−2 to 10−1 mbar. As substrates, we used (0001) sapphire plates and (100) Si wafers, coated or uncoated with ZnO as buffer layers, and heated below 300°C. Different analysis techniques evidenced the characteristics of the deposited films. SIMS profiles corresponding to N and Ga in-depth distribution carried out the presence of layers of the order of 130–150 nm, with uniform distribution of Ga and N inside the layer. XPS studies evidenced the Ga–N bonding. The N1s signal contains as main peak the one centered at 397.3 eV and corresponding to Ga–N bond. From the distance between the photoelectron Ga 3d peak and the Auger Ga LMM peak, the calculated Auger parameter of 1083.9 eV corresponds to the one reported in literature for GaN compound (1084.05 eV). Both techniques evidenced an oxygen contamination below 5%. XRD recorded spectra show the presence of a peak assigned to (002) GaN crystalline orientation. Optical absorption spectroscopy studies in the UV–visible range evidenced a high transparency (over 80% transmission) for the deposited films. The energy band gap obtained from the absorption spectra was found to be larger than 3.6 eV.

Excimer laser induced surface nitriding of aluminium alloy

Abstract This study describes a technique for the growth of thin nitride layer on aluminium alloy samples by direct laser synthesis with the advantages of good adhesion and localisation. The laser irradiation process is performed using an XeCl excimer laser (λ=308 nm, 50 Hz) under a nitrogen atmosphere. The laser induced plasma interacts with the melted sample surface resulting in nitrogen atom diffusion and reaction into the depth of the sample. Plasma spots were overlapped by two dimensional laser beam displacement to ensure the complete coating of the surface with a specified laser fluence (1.6 J/cm2) and number of pulses (500), while not removing of the nitride layer already synthesised. Under these conditions, the diffusion layer penetrates a few μm deep, but its crystalline quality is preserved. Interesting information on the layer formation and composition are drawn from nuclear analysis (RBS and NRA) to determine the nitrogen and oxygen (contaminant) concentration profiles.

Influence of irradiation conditions on plasma evolution in laser-surface interaction

The plasma plume induced by pulsed CO2 laser irradiation of a Ti target at power densities up to 4×108 W cm−2 was studied by emission spectroscopy. Time‐ and space‐resolved measurements were performed by varying laser intensity, laser temporal pulse shape, ambient gas pressure, and the nature of the ambient gas. Experimental results are discussed by comparison with usual models. We show that shock wave and plasma propagation depend critically on the ratio Ivap/Ii, Ivap being the intensity threshold for surface vaporization and Ii the plasma ignition threshold of the ambient gas. Spectroscopic diagnostics of the helium breakdown plasma show maximum values of electron temperature and electron density in the order of kTe∼10 eV and ne=1018 cm−3, respectively. The plasma cannot be described by local thermodynamic equilibrium modeling. Nevertheless, excited metal atoms appear to be in equilibrium with electrons, hence, they can be used like a probe to measure the electron temperature. In order to get informatio...

Plasma formation resulting from the interaction of a laser beam with a solid metal target in an ambient gas

The formation and role of a plasma resulting from the interaction between a laser beam and a metal target in an ambient gas are reviewed in this paper. The plasma is initiated after the production of primary electrons by multiphoton absorption and thermionic photoemission mechanisms. Vaporization of the surface then occurs at a lower threshold than theoretically deduced, due to surface defects and impurities. Vapour ionization is first a thermal process, primary electrons gaining energy by inverse Bremsstrahlung than electron cascade growth occurs. The plasma propagates with absorption waves; laser-supported combustion or detonation waves depending on the laser irradiance regime. The ablation process is favoured with the shortest wavelengths, whereas ambient gas breakdown occurs with the largest wavelengths. The nature of the ambient gas must be adapted for the chosen application. Plasma development can be an inconvenience with target shielding effect when the ionization degree is too high and with laser-supported detonation wave generation occurring in this case. However, transparent plasmas provide optimum laser machining, as good surface coupling with plasma formation occurs which is useful for drilling or welding.

A contribution to the understanding of the plasma ignition mechanism above a metal target under UV laser irradiation

In this paper, the plasma ignition process above a metallic surface submitted to UV laser irradiation is studied. An easy model based on the hypothesis of thermal equilibrium between ejected vapour and heated surface, and of a local thermodynamic equilibrium state of the vapour, is used to characterize the metallic vapour at the end of the laser pulse. Then the efficiency of the different elementary mechanisms liable to sustain or to prevent the ionization process in this medium is discussed depending on the laser power density. In this work, the calculations are applied to the case of the interaction between an excimer XeCl laser beam ( nm, ns) and titanium target. It is shown that the thermal heated metallic vapour is partially ionized and contains excited and singly ionized species at high densities ( atoms ). The electron temperature in this medium is found to be around 1 eV. The study of the ionization rise in the vapour evidences the important role played by the single-photon ionization process and the electron/ion inverse bremsstrahlung effect. For laser power densities above 100 MW (laser fluence of 2 J cm), the ionization level is found to increase before the laser pulse end, and a thermal evaporation regime is reached. As the laser power density exceeds 500 MW (fluence of 10 J ), an avalanche breakdown is liable to occur in the vapour before the pulse end and the plasma governs the evaporation mode. The results presented here are in good agreement with experimental observations and with results from more complex models reported in the literature.

Direct carbidation of titanium as a result of multipulse UV-laser irradiation of titanium samples in an ambient methane gas

Abstract We report the efficient synthesis of titanium carbide layers as an effect of multipulse excimer laser irradiation of titanium samples in a methane atmosphere.

ZnO sublimation using a polyenergetic pulsed electron beam source: numerical simulation and validation

This paper details the electro-thermal study of the sublimation phase on a zinc oxide surface. This thermodynamic process occurs when a ZnO target is bombarded by a pulsed electron beam source composed of polyenergetic electrons. The source delivers short pulses of 180 ns of electrons with energies up to 16 keV. The beam total current reaches 800 A and is focused onto a spot area 2 mm in diameter. The Monte Carlo CASINO program is used to study the first stage of the interaction and to define the heat source space distribution inside the ZnO target. Simulation of the second stage of interaction is developed in a COMSOL multiphysics project. The simulated thermal field induced by space and time heat conduction is presented. Typically for a pulsed electron beam 2 mm in diameter of electrons having energies up to 16 keV, the surface temperature reaches a maximum of 7000 K. The calculations are supported by SEM pictures of the target irradiated by various beam energies and numbers of pulses.