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Diffusion and aggregation of large antimony and gold clusters deposited on graphite

Département de Physique des Matériaux, UniversitéClaude Bernard- Lyon I, 43 Boulevard du 11 November 1918, 69622 Villeurbanne Cedex, France; Institut de Recherches sur la Catalyse, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France
Surface Science (Impact Factor: 1.87). 01/1996; DOI: 10.1016/S0039-6028(96)00875-8

ABSTRACT We present a study of the first stages of growth of thin films produced by low-energy cluster beam deposition (LECBD) on graphite. Our experiments are analyzed in the framework of new models including three physical ingredients, which are the deposition, the diffusion and the aggregation of the clusters. The comparison between computer simulations of the model and the experimental structures reveals that only the incident clusters diffuse on the graphite, the clusters stick irreversibly upon contact, and allow us to quantify the diffusion of clusters on graphite. Two kinds of metallic cluster films are studied: thin films produced by deposition of antimony clusters (containing 2300 and 250 atoms) and others by deposition of gold clusters (containing about 250 atoms). In both cases, we find that the clusters, in spite of their large size, diffuse very rapidly on the surface. The different microscopic diffusion mechanisms proposed in the literature are investigated, but none is compatible with our experimental results. Finally, we suggest a collective mechanism where the cluster rotates on the surface as a rigid entity to explain our results.

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    ABSTRACT: Thin film applications have become increasingly important in our search for multifunctional and economically viable technological solutions of the future. Thin film coatings can be used for a multitude of purposes, ranging from a basic enhancement of aesthetic attributes to the addition of a complex surface functionality. Anything from electronic or optical properties, to an increased catalytic or biological activity, can be added or enhanced by the deposition of a thin film, with a thickness of only a few atomic layers at the best, on an already existing surface. Thin films offer both a means of saving in materials and the possibility for improving properties without a critical enlargement of devices. Nanocluster deposition is a promising new method for the growth of structured thin films. Nanoclusters are small aggregates of atoms or molecules, ranging in sizes from only a few nanometers up to several hundreds of nanometers in diameter. Due to their large surface to volume ratio, and the confinement of atoms and electrons in all three dimensions, nanoclusters exhibit a wide variety of exotic properties that differ notably from those of both single atoms and bulk materials. Nanoclusters are a completely new type of building block for thin film deposition. As preformed entities, clusters provide a new means of tailoring the properties of thin films before their growth, simply by changing the size or composition of the clusters that are to be deposited. Contrary to contemporary methods of thin film growth, which mainly rely on the deposition of single atoms, cluster deposition also allows for a more precise assembly of thin films, as the configuration of single atoms with respect to each other is already predetermined in clusters. Nanocluster deposition offers a possibility for the coating of virtually any material with a nanostructured thin film, and therein the enhancement of already existing physical or chemical properties, or the addition of some exciting new feature. A clearer understanding of cluster-surface interactions, and the growth of thin films by cluster deposition, must, however, be achieved, if clusters are to be successfully used in thin film technologies. Using a combination of experimental techniques and molecular dynamics simulations, both the deposition of nanoclusters, and the growth and modification of cluster-assembled thin films, are studied in this thesis. Emphasis is laid on an understanding of the interaction between metal clusters and surfaces, and therein the behaviour of these clusters during deposition and thin film growth. The behaviour of single metal clusters, as they impact on clean metal surfaces, is analysed in detail, from which it is shown that there exists a cluster size and deposition energy dependent limit, below which epitaxial alignment occurs. If larger clusters are deposited at low energies, or cluster-surface interactions are weaker, non-epitaxial deposition will take place, resulting in the formation of nanocrystalline structures. The effect of cluster size and deposition energy on the morphology of cluster-assembled thin films is also determined, from which it is shown that nanocrystalline cluster-assembled films will be porous. Modification of these thin films, with the purpose of enhancing their mechanical properties and durability, without destroying their nanostructure, is presented. Irradiation with heavy ions is introduced as a feasible method for increasing the density, and therein the mechanical stability, of cluster-assembled thin films, without critically destroying their nanocrystalline properties. The results of this thesis demonstrate that nanocluster deposition is a suitable technique for the growth of nanostructured thin films. The interactions between nanoclusters and their supporting surfaces must, however, be carefully considered, if a controlled growth of cluster-assembled thin films, with precisely tailored properties, is to be achieved. Tunna filmer, som bäst några atomlager smala skikt av material, uppfyller en central roll inom nanovetenskapen, en ny vetenskapsgren som fokuserar på egenskaperna och tillämpningen av material på en atomär eller molekylär skala - en längdskala på omkring 1-100 nanometer. Med hjälp av tunna filmer kan de fysikaliska och kemiska egenskaperna av en yta fullständigt förändras, utan att nämnvärt öka på storleken av det föremål som täckts med filmen. Nanoklusterdeponering är en lovande metod för tillväxten av tunna filmer. Nanokluster, nanometerstora sammanhopningar av atomer, har exotiska egenskaper vilka kan skilja sig markant från egenskaperna av både de enskilda atomerna och större stycken av materialet i fråga. På grund av den stora ytan, i förhållandet till volymen av ett kluster, samt den begränsade volymen i vilken atomerna och deras elektroner kan röra sig, kan ett kluster bete sig som ett helt nytt grundämne, med egenskaper vilka starkt beror på det specifika antalet atomer i just det klustret. Hoppet om att överföra dessa spännande egenskaper på tunna filmer, genom tillväxt av så kallade nanostrukturerade tunna filmer, är en drivkraft för utvecklingen av nanoklusterdeponering. I denna avhandling studerades tillväxten av tunna filmer genom deponering av metallnanokluster med hjälp av både experiment och datorsimuleringar. Både den grundläggande växelverkan mellan enskilda kluster och ytor samt mekanismerna som styr klustrens växelverkningar vid deponering av ett flertal kluster undersöktes i detalj. Hela processen av filmdeponering, från de första klustren till ett tjockare lager av kluster i en film, kartlades noggrannt. Strukturen av filmer tillverkade med nanoklusterdeponering är starkt beroende av både storleken på klustren samt den energi med vilken de kolliderar med ytan. Hur bland annat dessa faktorer påverkar de slutliga filmerna bestämdes i avhandlingen. Samtidigt fastställdes även ett gränsvärde för dessa parametrar, vid vilket tillväxten av nanostrukturerade filmer överhuvudtaget kan ske. Ytterligare modifiering av de deponerade filmerna, med målet att förbättra deras mekaniska egenskaper, utan att negativt påverka deras nanostruktur, presenteras även. Bombardemang med jonstrålar introduceras som en möjlig metod för detta. Slutligen undersöktes även själv-arrangemang av kluster på olika ytor; hur naturliga variationer i växelverkningen mellan kluster och ytor kan leda till de mest varierande slutresultat. Resultaten av denna avhandling visar hur nanokluster effektivt kan användas för tillväxten av nanostrukturerade tunna filmer. Avhandlingen visar att växelverkningen mellan kluster och ytor noggrannt måste beaktas, ifall en kontrollerbar använding av nanoklusterdeponering skall vara möjlig.
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