R. Monot

Controlled Deposition of Size-Selected Silver Nanoclusters

Variable-temperature scanning tunneling microscopy was used to study the effect of kinetic cluster energy and rare-gas buffer layers on the deposition process of size-selected silver nanoclusters on a platinum(111) surface. Clusters with impact energies of ≤1 electron volt per atom could be landed nondestructively on the bare substrate, whereas at higher kinetic energies fragmentation and substrate damage were observed. Clusters with elevated impact energy could be soft-landed via an argon buffer layer on the platinum substrate, which efficiently dissipated the kinetic energy. Nondestructive cluster deposition represents a promising method to produce monodispersed nanostructures at surfaces.

Hard and soft landing of mass selected Ag clusters on Pt(111)

Mass selected Ag/sub n/ clusters (n=1,7,19) from a secondary ion source have been deposited onto a Pt(111) substrate at low temperature. The surface and resulting cluster morphology have subsequently been characterized within the same UHV chamber by variable temperature STM as a function of cluster size, kinetic impact energy, and substrate temperature. The kinetic energy per cluster atom was found to be the decisive parameter for a controlled deposition. Noble gas buffer layers ( approximately=10 ML Ar), which were pre-adsorbed onto the surface at low temperatures, were found to efficiently dissipate the impact energy opening up the possibility of soft landing clusters with elevated kinetic energy.

Solvent caging by diatomic molecules in the photodissociation of i2m clusters

Abstract The reaction I 2 M+ hv →I 2 (B)+M is studied following excitation of the I 2 M cluster above its B state dissociation limit with M being the diatomics H 2 , N 2 , O 2 and I 2 . Dispersion of the iodine fluorescence allows for the determination of the relative populations of a number of the vibrational levels v of the B state. The relative importance of the hindered dissociation pathway appears to increase with decreasing photon energy as compared to the alternative (unobserved) path in which the I-I bond is broken, and the photofragment recoil energy tends to increase with increasing mass of M. We also find an exceptionally large wavelength dependence of the I 2 (B) yield for the dissociation of i 2 O 2 .

Measurement of macroscopic diffusion of CO on Pt(111) by thermal helium scattering

A new method to measure the diffusion of adsorbed molecules over macroscopic distance on single crystal surfaces is presented and demonstrated with the system CO on Pt(111). A controlled coverage of molecules, typically of the order of 0.01 monolayer, is deposited on a well-defined small spot of the crystal surface. The specular intensity of a helium beam reflected on the same surface spot is recorded as a function of time in order to follow the decrease of the average coverage due to the diffusion of the molecules. The data extracted from measurements made between 273 and 373 K are in agreement with an Arrhenius law corresponding to an activation energy E/sub D/=6+or-0.3 kcal/mol and a pre-exponential factor D/sub 0/=(5.0+or-1)*10/sup -7/ m/sup 2//s.

Energetic cluster deposition on a dislocation network: Ag7 on 2 ML Ag/Pt(111)

Abstract Mass selected Ag 7 clusters have been deposited at two different impact energies (20 and 95 eV) on a dislocation network formed by evaporating 2 ML of Ag on Pt(1xa01xa01). The surface has been investigated by low temperature STM as a function of annealing temperature. The clusters adsorption site, stability and ordering depend on impact energies and annealing temperatures. We find that the hcp small triangle and dislocation region of the reconstructed unit cell form preferred pinning centers at low temperature, at higher temperature most clusters are situated in the fcc hexagon of the unit cell. The results are compared to experiments with thermal evaporated Ag atoms.

Rare gases physisorbed on Pt(1 1 1): a low-temperature STM investigation

Abstract The structure and the phase transitions of Xe, Kr and Ar physisorbed on Pt(1xa01xa01) in the submonolayer-coverage range has been explored by low-temperature scanning tunneling microscopy. The hexagonal incommensurate rotated (HIR) → hexagonal incommensurate (HI) transition with decreasing Xe coverage has been observed unambiguously. The onset of adlayer rotation occurs for Xe islands larger than 700 A 2 . Xe dimers physisorbed on Pt(1xa01xa01) are commensurately ordered on the Pt(1xa01xa01) surface. The experimental results are compared to molecular dynamic simulations. The orientation of the high order of commensurability (HOC) Kr structure with respect to the surface strongly depends on the terrace width and on the step morphology. Kr monolayer misorientation ranging from 0° to 30° has been observed depending on the surface morphology. The Ar monolayer adopts a HOC structure aligned with respect to the Pt(1xa01xa01) surface while for low coverage, the most energitically favorable position of the Ar islands corresponds to a R (1.875× n )° ( n an integer) HI structure.

Infrared vibrational predissociation spectroscopy of (SF6)mArn clusters: Implications for effective laser isotope separation

Selective infrared vibrational predissociation (IRVP) of Van der Waals clusters in a free jet is shown to be an effective way to separate isotopes. Absolute separation factors α in excess of 1.1 are observed upon irradiation of a beam of SF6 diluted in argon with a 20 W cw CO2 laser. Selection of the wavelength permits enrichment or depletion of any one of the sulfur isotopes in the mixture. In order to understand the observed wavelength dependence of the isotope enrichment we have measured the effective IRVP spectra of the individual Van der Waals clusters (SF6)mArn with 1 ⩽ m ⩽ 3 and n ⩽ 9. A systematic study of these spectra reveals a significant red-shift which occurs upon attaching Ar atoms to (SF6)m. Such shifts can attain as much as 10 cm−1, as in the case between SF6 and SF6Ar9.

Laser Probing of Cluster Formation and Dissociation in Molecular Beams

An SF6 molecular beam is irradiated with a cw CO2 laser. The multiple effects of the infrared radiation are analyzed with several mass spectrometric techniques. Detailed information is obtained on the extent of the collisional region of the adiabatic gas expansion. Infrared vibrational predissociation spectra of the van der Waals clusters are obtained.

Surface Atomic Scale Engineering by Deposition of Mass Selected Clusters: STM and Helium Scattering Analysis

Wide effort is actually carried to use metallic clusters for technological applications like nanosized structured new materials, thin films, surface coatings, etc. Prepared in the gas phase, the clusters are generally deposited on surfaces, either as building blocks in a growth process [1], or to function as a tool to modify the characteristics of the surface [2, 3]. No size selection has been applied so far to the clusters in these applications. However, clusters formed and mass selected in the gas phase and subsequently deposited in a controlled way, is a promising alternative in nanostructure formation on surfaces. Clusters are systems containing typically from 2 to 2000 atoms or molecules. They have been studied for their specific properties, which are size dependent and different from both the atoms (or molecules) and the bulk material [4], mostly due to their large surface to volume ratio. Cluster deposition is of fundamental interest as it differs radically from conventional thermal atom deposition, a field which is now well established [5]. The difference arises mainly from the fact that new parameters, such as the cluster size and the deposition energy, may be used to tailor the collision outcome [6]. These input parameters open new perspectives in the controlled growth of such structures, with new phenomena not accessible within conventional atom deposition or atomic manipulation by scanning probe methods. For instance, the size or equivalently the number of atoms constituting the clusters to be deposited can be tuned for a specific inherent property. In that sense, one could prepare the systems as free clusters before deposition and then deposit them. An important issue for future technological applications of cluster deposition is the relation between the size of the incident cluster’s and the size of the islands obtained on the substrate. Experimental evidence for the importance of the precise definition of the cluster size can be found in the catalytic activity of clusters on oxide substrates [7] and the minimum size of a silver cluster to form an image speck in the photographic process [8].