Reduction of surface coverage of finite systems due to geometrical steps

Abstract

The coverage of vicinal, stepped surfaces with molecules is simulated with the help of a twodimensional Ising model including local distortions and an Ehrlich-Schwoebel barrier term at the steps. An effective two-spin model is capable to describe the main properties of this distorted Ising model. It is employed to analyze the behavior of the system close to the critical points. Within a well-defined regime of bonding strengths and Ehrlich-Schwoebel barriers we find a reduction of coverage (magnetization) at low temperatures due to the presence of the surface step. This results in a second, low-temperature transition besides the standard Ising order-disorder transition. The additional transition is characterized by a divergence of the susceptibility as a finite-size effect. Due to the surface step the mean-field specific heat diverges with a power law.

Adsorption of PTCDA on a partially KBr covered Ag(111) substrate

“…Here we present for the first time ordered growth of single monolayers (ML) of 3,4,9,10-perylene-tetracarboxylicdianhydride (PTCDA) on a Ag(111) substrate partially covered by an ultrathin potassium bromide (KBr) film. Different adsorption patterns are observed on (I) pure Ag(111), (II) on 1 ML KBr|Ag(111), and (III) on 2 ML KBr|Ag(111) and rationalized by simulations with a extended Ising-type model [27].…”

“…Without an external magnetic field H the transition is of second order, otherwise it is of first order [27,[40][41][42]. In our new model the influence factors for the adsorption of molecules on surfaces are mapped onto this Ising model in the following way: the spin s i denotes the site occupancy with s i = 1 for occupied and s i = −1 for unoccupied sites.…”