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In this work we address a novel feature of spreading dynamics of tiny liquid droplets on solid surfaces, namely the case where the ends of the molecules feel different interactions to the surface. We consider a simple model of dimers and short chain--like molecules which cannot form chemical bonds with the surface. We study the spreading dynamics by Molecular Dynamics techniques. In particular, we examine the microscopic structure of the time--dependent precursor film and find that in some cases it can exhibit a high degree of local order. This order persists even for flexible chains. Our results suggest the possibility of extracting information about molecular interactions from the structure of the precursor film., Comment: 4 pages, revtex, no figures, complete file available from ftp://rock.helsinki.fi/pub/preprints/tft/ or at http://www.physics.helsinki.fi/tft/tft_preprints.html (to appear in Phys. Rev. E Rapid Comm.)

We compare computer simulations to experimental scanning tunneling microscopy (STM) images of chloronitrobenzene molecules on a Cu(111) surface. The experiments show that adsorption induced isomerization of the molecules takes place on the surface. Furthermore, not only the submolecular features can be seen in the STM images, but different isomers can also be recognized. The Todorov-Pendry approach to tunneling produces simulated STM images which are in good accordance with the experiments. Alongside with STM simulations in a tight-binding basis, ab initio calculations are performed in order to analyze the symmetry of relevant molecular orbitals and to consider the nature of tunneling channels. Our calculations show that while the orbitals delocalized to the phenyl ring create a relatively transparent tunneling channel, they also almost isolate the orbitals of the substitute groups at energies which are relevant in STM experiments. These features of the electronic structure are the key ingredients of the accurate submolecular observations.

The kinetics of ordering in the Q-state Potts model on a square lattice is studied for quenches from a high-temperature, disordered state to low temperatures below the phase-transition point. Growth laws for the average size of domains are obtained by Monte Carlo studies with Glauber dynamics for Q=3, 4, 6, and 8 for several low temperatures. In addition, the nonequilibrium structure factor is calculated for Q=8 and is shown to satisfy scaling. This study may be of some relevance to island growth in surface science, since the Potts model has been used to describe the adsorption of atoms on substrates.

The possibilities to separate different growth modes of thin films are discussed using results of Monte Carlo simulation. The growth mode can be recognized from the time evolution of the coverage, the mean square deviation of the coverage, the estimated number of clusters on the substrate and snapshots at different stages of growth. In addition to defining and verifying the criteria for layer-by-layer and island modes, we can identify the intermediate simultaneous multilayer growth.

Etude de la cinetique de la germination et de la croissance de couches minces, au moyen des equations de vitesse phenomenologiques modifiees a partir des equations de Venables. Cette approche sert de base pour un modele d'equation de vitesse pour decrire la croissance a plusieurs couches. On discute egalement les effets des impuretes

Multilayer growth using phenomenological rate equations is studied. These equations are developed from those for one-monolayer growth. The basic phenomena include the flux of atoms, desorption, and adsorption of a single adatom and incorporation of adatoms with other adatoms or with a stable cluster. In addition, we investigate the effect of migration of adatoms between layers. The same measures for growth modes are used as in our earlier Monte Carlo studies. These two approaches give consistent results. We conclude that the static and kinetic approaches to determine growth modes differ substantially.

Scanning tunneling microscopy at low temperature reveals that upon dissociative adsorption of oxygen on Ag(001) “hot” adatoms have separated in a far-ranged transient motion to two different intrapair distances around 2 and 4 nm, corresponding to 7 and 14 surface lattice constants, respectively. Manipulation experiments on oxygen atoms displaying different contrasts in the images and model calculations suggest that the transient motion ends up not only in the stable fourfold hollow site but also in two metastable sites.

Atom resolved scanning tunneling microscopy images are known to depend on the nature of the tip and its interaction with the surface. We present a new method for determining barrier height and tip-surface interactions. For Cu(100) images, two distinct tip types are seen. At larger tunnel currents there is evidence for forces acting between tip and surface. We show that a molecular dynamics simulation of tip and surface is able to quantitatively explain the results, and gives a good estimate of both absolute tip-surface separation and the site dependent forces on individual atoms.

A change in bias voltage can reverse the contrast of a scanning-tunneling microscopy (STM) image of an adsorbate molecule on a solid surface. This contrast change may take place not only in a case of tunneling in the neighborhood of a resonance level of the adsorbate, but it can also be seen in the case of a wide energy gap between highest occupied molecular-orbital and lowest unoccupied molecular-orbital levels. In order to find a unified description of contrast changes in STM images, we make comparisons between different approaches to tunneling. We especially consider the role of phase differences between possible tunneling channels. These phase differences can be related to the imaginary and real parts of the Green's function of the system which reveal whether the tunneling takes place through a resonant state or a forbidden gap. Finally, we compare experimental and calculated images of oxygen molecules on a silver surface and analyze the mechanisms of the contrast change in terms of phase shifts in different tunneling channels.

In this work we study the spreading dynamics of tiny liquid droplets on solid surfaces in the case where the ends of the molecules feel different interactions with respect to the surface. We consider a simple model of dimers and short chainlike molecules that cannot form chemical bonds with the surface. We use constant temperature Molecular Dynamics techniques to examine in detail the microscopic structure of the time dependent precursor film. We find that in some cases it can exhibit a high degree of local order that can persist even for flexible chains. Our model also reproduces the experimentally observed early and late-time spreading regimes where the radius of the film grows proportional to the square root of time. The ratios of the associated transport coefficients are in good overall agreement with experiments. Our density profiles are also in good agreement with measurements on the spreading of molecules on hydrophobic surfaces., Comment: 12 pages, LaTeX with APS macros, 21 figures available by contacting mphaataj@rock.helsinki.fi, to appear in Phys. Rev. E

We perform a molecular dynamics (MD) study of non-dissociative adsorption of O 2 molecule on Pt{111} surface at low temperatures, and analyse the STM image of the obtained geometry. The interatomic forces for the simulation are calculated using tight-binding method in density matrix formalism with the constraint of local charge neutrality. The STM image is constructed by calculating tunneling current between a sharp metal tip and the sample. The MD simulation gives an adsorption site with a very plausible bonding behaviour. Furthermore, the corresponding STM image is similar to recent experimental images. The present simulation suggests formation of a larger complex of the oxygen molecule and two Pt atoms. It turns out that the STM image does not directly show the orientation of the O 2 molecule, and has such a complicated origin in interference of atomic orbitals of the formed complex, that an optimization of the surface structure using MD simulation, instead of intuitive considerations, is necessary.