Holger Vach

Evaporation of small fragments during the scattering of argon clusters at thermal kinetic energies from a graphite surface

We present a thermokinetic model together with new experimental results for the scattering of large argon clusters off a graphite surface. Both angular and time‐of‐flight distributions are shown for a large range of surface temperatures, incidence angles, and incident cluster sizes. A quantitative comparison between the proposed thermokinetic model and our measurements allows one to interpret most of the experimental results as due to thermal evaporation of very small fragments from their parent clusters gliding along the surface. The coefficient of tangential velocity conservation cF and the local temperature Tlocal of the evaporating fragments have been determined quantitatively. Although the investigated parameters were varied over a large range, Tlocal remains essentially constant around (140 ± 20) K. The coefficient cF turns out to be approximately (0.80 ± 0.05) independent of surface temperature and incident cluster size for all incidence angles larger than 40°. It increases, however, rapidly to 1.4...

Argon cluster scattering from a graphite surface

Abstract Experimental results from the scattering of argon clusters off a graphite surface are presented. Angular distributions of cluster fragments have been recorded at monomer, dimer and trimer masses. While some of our findings are similar to previous observations and can be interpreted in the framework of existing theories, two distinctly new features have been seen: first, the angular distributions of the scattered fragments are clearly correlated with their masses; and second, a new grazing exit angle component appears for sufficiently large incident angles or backing pressures, which is interpreted as evidence of large cluster fragments surviving the surface collision.

Dynamic zone structure model for the surface scattering of large van der Waals clusters at thermal kinetic energies

A semiempirical dynamic zone structure model is presented to explain the behavior of the grazing exit angle component previously observed in experiment and simulation during the scattering of large van der Waals clusters from surfaces. The proposed model that is based on a simple energy balance and the Leidenfrost phenomenon does not only qualitatively reproduce the measured importance of the grazing exit angle component as a function of incident cluster size, incident velocity, angle of incidence, and surface temperature, but it also gives an order‐of‐magnitude estimate for the size of the large fragments scattered in this component and for the involved picosecond interaction times. Recent results obtained from both trajectory calculations and experiments show very good agreement with the predictions concluded from the proposed model.

Experimental evidence of enhanced diffuse monomer scattering in cluster-surface collisions. Arn on graphite

Abstract We report the first experimental observation of diffuse scattering of monomers exhibiting significant velocity thermalization in large argon clusters collisions with a pyrolytic surface. Angular distributions of the outcoming argon atoms show a diffuse component for which time-of-flight spectra are independent of incidence angle and incoming cluster size, while they vary significantly with surface temperature. The amplitude of the diffuse scattering component is enhanced by up to a factor of three for incident clusters over its value for incident monomers. This enhancement is discussed by using a simple model of atom trapping-desorption on flat surfaces.

Soft landing of silicon nanocrystals in plasma enhanced chemical vapor deposition

Plasma-generated silicon nanocrystals have been selectively trapped on a cooled substrate to yield nanocrystalline films. We here present experimental evidence that the contribution of positively charged nanocrystals largely dominates the film deposition. As a direct application, we illustrate how the use of a simple substrate bias voltage allows us to “toggle switch” between 100% nanocrystalline and 100% amorphous layers. Moreover, we demonstrate that the applied bias voltage can be used to “tune” the photoluminescence of the nanocrystals between 630 and 730nm.

IMPURITY DYNAMICS IN BINARY VAN DER WAALS CLUSTERS CREATED BY PICK-UP

We present results from molecular dynamics simulations concerning the creation of binary van der Waals clusters under a very large range of possible experimental pick-up conditions. Special emphasis is put on the dynamical processes occurring during and after the “pick-up” of Ne, SiF4, Kr, and Xe by Arn clusters with n ranging from 53 to 5000 atoms. Both Ne and SiF4 impurities are shown to normally reside in cluster surface states. For certain experimental conditions, however, both dopants may present transient matrix states. Matrix states are found to be the most probable final locations for Kr and Xe dopants under all considered conditions. We show that the dopant penetration depth crucially depends on cluster size, cluster velocity, nature of the dopant, and buffer gas pressure and that the final result is not always predictable from simple equilibrium considerations.

Average cluster size determination in supersonic beams from angular distribution measurements after scattering by a buffer gas

A method for the determination of average cluster size in supersonic beams is presented. Based on angular distribution broadening of the beams caused by passing through a buffer gas, this method is well suited for in situ determination of the mean cluster size when the apparatus contains a movable detector with sufficient spatial resolution. The shape and width of the beam profile after scattering by a buffer gas are evaluated theoretically as functions of buffer gas pressure and atom-cluster collision cross-section. Experimental results are presented for an argon beam, yielding average cluster sizes between 300 and 7000 atoms depending on the stagnation pressure. Simple criteria to assess the applicability of the method to a given experimental situation are discussed. The average cluster sizes determined in this work agree quite satisfactorily with previously published values for similar beam generation conditions.

Ultrahigh‐vacuum multichamber apparatus for molecule‐surface interaction studies

We present a complete apparatus for molecule‐surface interaction studies. Three UHV chambers are connected together allowing preparation, characterization, transfer, and experimental investigations with well‐defined surface samples under ultrahigh‐vacuum conditions. A chopped, supersonic molecular beam with well‐controlled profile enters the main UHV chamber where it is scattered by the sample under study. Detection systems have been designed to measure angular and time‐of‐flight distributions and rotational populations of beam particles scattered off the surface sample giving access to the energy exchange between the internal degrees of freedom of the scattered molecules and the surface. The characteristics of our apparatus are reported and experimental tests for nitrogen and argon molecular beams scattered off a graphite surface are shown.

Photodissociation of HCl and small (HCl)m complexes in and on large Arn clusters

Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Ar n clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes n=140-1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are m=3 and 6 and n=8-248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for.

Formation of Silicene Nanosheets on Graphite

The extraordinary properties of graphene have spurred huge interest in the experimental realization of a two-dimensional honeycomb lattice of silicon, namely, silicene. However, its synthesis on supporting substrates remains a challenging issue. Recently, strong doubts against the possibility of synthesizing silicene on metallic substrates have been brought forward because of the non-negligible interaction between silicon and metal atoms. To solve the growth problems, we directly deposited silicon on a chemically inert graphite substrate at room temperature. Based on atomic force microscopy, scanning tunneling microscopy, and ab initio molecular dynamics simulations, we reveal the growth of silicon nanosheets where the substrate-silicon interaction is minimized. Scanning tunneling microscopy measurements clearly display the atomically resolved unit cell and the small buckling of the silicene honeycomb structure. Similar to the carbon atoms in graphene, each of the silicon atoms has three nearest and six second nearest neighbors, thus demonstrating its dominant sp2 configuration. Our scanning tunneling spectroscopy investigations confirm the metallic character of the deposited silicene, in excellent agreement with our band structure calculations that also exhibit the presence of a Dirac cone.