Infoscience

Thesis

Déposition d'atomes et d'agrégats d'argent triés en taille sur le palladium (100): une étude par diffusion d'hélium

The aim of this research project is to study the deposition of thermal atoms and small ionised clusters on well defined surfaces. For this, an installation has been developed at the Institut de Physique Expérimentale to produce mass selected ionic metal clusters using a sputtering source and to deposit them on well defined metallic surfaces. The chosen investigation method is Thermal Energy Atom Scattering (TEAS), a specularly reflected helium beam probes the surface and offers a very high sensitivity to the surface point defects; this is an absolutely non perturbative technique and an in-situ and in-time monitoring of the deposition. The principle of the method consists of using the difference in size between a point defect (like for example an adatom) and its cross section for the specular beam (which is about ten times larger). When two adatoms meet to form a dimer for example, their cross section overlap, which results in an increase in the reflected helium beam. As a first step we studied the deposition of atoms evaporated from a Knudsen cell. The small incident kinetic energy related to thermal evaporation insures that they do not create defects in the substrate (vacancies, implanted or interstitial atoms). So we studied the deposition and the very first steps of the nucleation and growth of Ag on Pd(100). For this we developped a new method that studies the form of the specularly reflected helium signal as a function of coverage: the signal is compared to a theoretical signal which is computed by resolving a system of rate equations. We found that even at a temperature much lower than the temperature at which the Ag adatoms become thermally mobile on the Pd(100) surface (typically 160 K), there is a neighbor driven mobility of the atoms on the substrate. Silver adatoms are not stable if they are too close to each other or to close to a cluster, they move towards already existing particles [VFM+94b]. The dynamical behavior of the adparticle has also been studied with the same method, by solving the nucleation and growth rate equations for small particles. The diffusion parameters (activation barrier Ed = 0.37 ± 0.03 eV and pre-exponential factor ν0 = 8 109 s-1) have been extracted for the system Ag on Pd(100) [FVH+95]. The structure of the particles and the damage creation due to the impact of the ionised Ag particles (Edep= 20 and 95 eV) have also been studied by TEAS. It is the first experimental work that studied specifically the structural problems due to the deposition of small clusters. We deposited Ag1+, Kr+, Ag7+ and Ag19+ in well defined conditions (low and controlled contamination of the surface, precise calibration of the cluster current density in the probed area). The main results are the following [VFM+94a, VFM+95, VFG+95]: Ag1+ Many defects are created by the deposition of these particles (implanted atoms, surface vacancies, substrate adatoms ejected from the first surface layers). Most of these defects are annealed at 350 K except the Ag atoms implanted in the first surface layer that are still stable at this temperature.   Ag7+ There are still a great number of surface defects produced by the impact of these particles even if the impact energy per particle is reduced. A mean number of four atoms seem to be implanted for an impact energy of Edep = 95 eV whereas only one particle is implanted at Edep = 20 eV. The rest of the cluster atoms are severely fragmented over large distances.   Ag19+ Although the impinging clusters are lightly fragmented they seem to recombine as soon as the mobility of the adatoms is active. Between Ts = 200 and 300 K, one observes a slow dynamical change of the surface that we attribute to a compactification of the clusters. At higher surface temperature, one observes the destruction of these structures by a 2-dimensional evaporation. Molecular Dynamics simulations of the impact of atoms and clusters of our system are in good agreement with the experimental results, except for the fragmentation of incident Ag particles and the low temperature mobility.

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