Pascal Andreazza

Nucleation and initial growth of platinum islands by plasma sputter deposition

The nucleation and islands growth of platinum have been followed from an early stage through to complete substrate coverage. Thus, this study of initial stages of thin film growth has allowed to improve the understanding of growth mechanisms and to control the final nanomaterials structure. In the presented deposition method, the metal atom source is a negatively biased metal wire submitted to the bombardment of ions created in HF plasma. The originality of this plasma sputtering technique is that in addition to the metal atom flux, the substrate surface is submitted to a high flux of low energy ions, inducing a high atomic surface mobility and modifying the growth mode with respect to conventional deposition techniques (magnetron sputtering or evaporation). Platinum deposits have been investigated by ex situ characterisation techniques: grazing incidence-small angle X-ray scattering (GISAXS) and X-ray diffraction (GIXD) in conjunction with transmission electron microscopy (TEM). Using morphological parameters as coverage rate, islands number density and islands size, different mechanisms (scale laws, islands mobility, coalescence, etc.) governing the growth are described vs. Ar ion and Pt atom energies and fluxes.

Color-switchable, emission-enhanced fluorescence realized by engineering C-dot@C-dot nanoparticles

This paper reports the preparation and properties of color-switchable fluorescent carbon nanodots (C-dots). C-dots that emit dark turquoise and green-yellow fluorescence under 365 nm UV illumination were obtained from the hydrothermal decomposition of citric acid. Dark green fluorescent C-dots were obtained by conjugating prepared C-dots to form C-dot@C-dot nanoparticles. After successful conjugation of the C-dots, the fluorescence emission undergoes a blue-shift of nearly 20 nm (∼0.15 eV) under UV excitation at 370 nm. The C-dots emit goldenrod, green-yellow, and gold light under excitation at 455 nm, which shows that the prepared C-dots are color-switchable. Furthermore, conjugation of the C-dots results in enhanced, red-shifted absorption of the π-π* transition of the aromatic sp(2) domains due to the conjugated π-electron system. N incorporation in the carbon structure leads to a degree of dipoles for all the aromatic sp(2) bonds. The enhanced absorption in a wide range from 226 to 601 nm indicates extended conjugation in the C-dot@C-dot structure. The time-resolved average lifetimes for the three different types of C-dots prepared in this study are 7.10, 7.65, and 4.07 ns. The radiative rate (reduced decay lifetime) increases when the C-dots are conjugated in the C-dot@C-dot nanoparticles, leading to the enhanced fluorescence emission. The fluorescence emission of the C-dot@C-dot nanoparticles can be used in applications such as flow cytometry and cell imaging.

Vacuum ultraviolet annealing of hydroxyapatite films grown by pulsed laser deposition

The effect of a post-deposition vacuum ultraviolet (VUV) radiation-assisted annealing treatment performed under 1 bar of oxygen at moderate temperatures (450 °C) upon thin hydroxyapatite (HA) films grown by the pulsed laser deposition technique was investigated. The HA layers were deposited at 650 °C under different partial oxidizing pressures without any water vapor and exhibited, besides the HA crystalline phase, tetracalcium phosphate and calcium oxide phases, more so for the films grown at lower oxidizing pressures. After the VUV-assisted anneal the layers were transformed into high quality crystalline HA films, exhibiting Ca/P ratio values closer to 1.67, the value for stoichiometric HA. The content of the other crystalline phases initially present was reduced significantly. Infrared spectroscopy also showed that the amount of OH− in the films increased after the treatment. The combination of these two low temperature techniques opens the possibility of growing high quality HA layers without signific...

Excimer laser synthesis of thin AlN coatings

Abstract A new technique for the production of thin AlN films on Al samples by direct laser synthesis is reported. This has the advantages of good adhesion, low temperature deposition and localisation. A metallic, pure Al sample is irradiated by an excimer laser under a nitrogen containing atmosphere at high intensity levels such that a plasma is formed above the sample. The active nitrogen atoms and ions contained in this plasma react with the hot melted Al surface resulting in the synthesis of an AlN layer. The influence of the incident laser fluence, nature and pressure of the ambient atmosphere and of the number of laser pulses on the chemical composition and crystalline structure of the synthesised layers is reported. Under optimised conditions a few μm thick polycrystalline AlN layer, containing only a few percent oxygen and exhibiting high hardness and very good adhesion, has been prepared on Al.

Structural and chemical surface modifications of a stainless steel with implanted molybdenum and carbon ions

Abstract The surface transformation of stainless steel was investigated in the case of single and multi-element ion implantation in order to improve the localized corrosion resistance. Molybdenum and carbon ions (Mo alone and Mo-C) were implanted into austenitic stainless steels (316L type). Several surface techniques (X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXD)) were used to determine the surface modifications, such as phase transformation (γ→α), amorphization, precipitation and stable or metastable phase formation, e.g. substitution (for Mo) or insertion (for C) solid solution.

Surface carburization of aluminum alloys by excimer laser

Abstract The excimer laser cementation process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. It would be interesting to use aluminum alloys in the automotive industry widely because of their low density, corrosion resistance and good workability. The motor weight can be reduced by replacing usual materials such as iron–steel by light alloys treated to increase their wear resistance. Ceramic materials generally exhibit great strength, resistance to wear and oxidation. The use of laser beams allows surface treatment to be located at the parts strongly exposed to wear and friction. The surface undergoes a transformation leading to an increase in hardness without changing the dimensions of the piece, thus avoiding post machining after treatment. The laser process is especially suitable for environment protection as there is no pollution by chemical solvent or emanation. An excimer laser beam is focused onto the alloy surface in a cell containing 1 bar methane or propylene gas. A vapor plasma expands from the surface and a shockwave dissociates and ionizes the ambient gas. It is assumed that nitrogen or/and carbon from the plasma in contact with the surface penetrates into the depth. Thus, it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The carbon concentration profiles are determined from Rutherford backscattering spectroscopy (RBS) and scanning electron microscopy (SEM). Crystalline quality is evidenced by grazing incidence X-ray diffraction (GIXD) technique. Transmission electron microscopy (TEM) gives the in-depth microstructure. The polycrystalline cemented layer obtained is several micrometers thick and composed of a pure composition (columnar microstructure) top layer (200–500-nm thick) standing on a diffusion layer (grains). Fretting test measurements exhibit an improvement of the surface mechanical behavior for some experimental conditions.

Solvothermal Vapor Annealing of Lamellar Poly(styrene)-block-poly(d,l-lactide) Block Copolymer Thin Films for Directed Self-Assembly Application

Solvothermal vapor annealing (STVA) was employed to induce microphase separation in a lamellar forming block copolymer (BCP) thin film containing a readily degradable block. Directed self-assembly of poly(styrene)-block-poly(d,l-lactide) (PS-b-PLA) BCP films using topographically patterned silicon nitride was demonstrated with alignment over macroscopic areas. Interestingly, we observed lamellar patterns aligned parallel as well as perpendicular (perpendicular microdomains to substrate in both cases) to the topography of the graphoepitaxial guiding patterns. PS-b-PLA BCP microphase separated with a high degree of order in an atmosphere of tetrahydrofuran (THF) at an elevated vapor pressure (at approximately 40-60 °C). Grazing incidence small-angle X-ray scattering (GISAXS) measurements of PS-b-PLA films reveal the through-film uniformity of perpendicular microdomains after STVA. Perpendicular lamellar orientation was observed on both hydrophilic and relatively hydrophobic surfaces with a domain spacing (L0) of ∼32.5 nm. The rapid removal of the PLA microdomains is demonstrated using a mild basic solution for the development of a well-defined PS mask template. GISAXS data reveal the through-film uniformity is retained following wet etching. The experimental results in this article demonstrate highly oriented PS-b-PLA microdomains after a short annealing period and facile PLA removal to form porous on-chip etch masks for nanolithography application.

PdPt catalyst synthesized using a gas aggregation source and magnetron sputtering for fuel cell electrodes

, +33 (0)2 3849 4352 KEYWORDS. gas aggregation source, magnetron sputtering, platinum, nanoclusters, catalyst. Abstract. PdPt catalysts with different morphologies and atomic ratios have been synthesized on native SiO2/Si and on proton exchange membrane. The combination of the gas-aggregation source and of the magnetron sputtering techniques allows the formation of quasi core-shell Pd0.97Pt0.03@Pt nanoclusters. Transmission electron microscopy and grazing incidence wide angle X-ray scattering measurements on Pd-rich core reveal a mean diameter of 4 nm and a fcc structure. The Pt shell around the half of the Pd-rich core is formed by magnetron sputtering which leads to the increase of nanocluster diameter (up to 10 nm) and of the overall Pt content (up to 85%). The membranes coated by PdPt core catalyst and PdPt@Pt catalyst (resulting in the formation of catalyst coated membrane) are incorporated into fuel cells and their electrical characteristics are measured. The association of the two deposition techniques resulting in the formation of quasi core-shell PdPt@Pt nanoclusters improves the startup step of the fuel cell.

Cluster organization in co-sputtered platinum-carbon films as revealed by grazing incidence X-ray scattering

Nanostructured platinum-carbon thin films were prepared by magnetron co-sputtering method for designing efficient catalytic thin films, like fuel cells electrodes. The in-depth morphology of composite films was studied using surface sensitive X-ray techniques (grazing incidence small-angle scattering and reflectivity), consolidated by electron microscopy investigations. This study elucidates the growth mode of co-sputtered platinum-carbon thin film: 2-nm-sized platinum clusters are growing in surrounding simultaneously growing carbon columns (20-nm diameter range). In particular, the platinum cluster growth and distribution in the plane of the substrate surface are driven by surface diffusion and coalescence phenomena. Finally, this anisotropic distribution of platinum clusters correlated to the textured morphology of carbon matrix leads to a catalytic thin film morphology very suitable for electrochemical processes in fuel cell electrodes.

The use of plasmas in catalysis: catalyst preparation and hydrogen production

Abstract Plasma technologies are introduced in the field of hydrogen production related to fuel cells. Two ways are described: Plasma synthesis of catalysts and membranes for the production and purification of hydrogen and, direct production of hydrogen based on atmospheric plasma-assisted methane steam reforming.