T. Hjelt

Diffusive spreading of chainlike molecules on surfaces

We study the diffusion and submonolayer spreading of chainlike molecules on surfaces. Using the fluctuating bond model, we extract the collective and tracer diffusion coefficients Dc and Dt with a variety of methods. We show that Dcsud has unusual behavior as a function of the coverage u. It first increases, but after a maximum goes to zero as u ! 1. We show that the increase is due to entropic repulsion that leads to steep density profiles for spreading droplets seen in experiments. We also develop an analytic model for Dcsud which agrees well with the simulations. [S0031-9007(96)00253-0]

Velocity correlations and memory functions in surface diffusion

Abstract We introduce a generalized velocity correlation function and the corresponding memory function for the study of tracer diffusion of interacting particles on surfaces. These functions can be obtained by using discrete particle displacement variables either from experimental or simulation data. We study the behavior of these functions for diverse systems and find that in most cases, for both functions, there is an intermediate power-law decay ∝ t − x which spans about two orders of magnitude in time. The exponent x depends on the range and the strength of interactions. For the Langmuir gas with on-site exclusions only, x ≈2. For other interacting systems we find that, when strong attractive interactions are present, x tends to decrease from two to unity, while strong repulsion may cause x to be larger than two. These results demonstrate that the velocity correlation function can be used to gage the importance of interaction effects for surface diffusion.

Nonexponential decay of velocity correlations in surface diffusion: The role of interactions and ordering

We study the diffusive dynamics of adparticles in two model systems with strong interactions by considering the decay of the single-particle velocity correlation function φ(t). In accordance with previous studies, we find φ(t) to decay nonexponentially and follow a power-law φ(t)∼t−x at intermediate times t, while at long times there is a crossover to an exponential decay. We characterize the behavior of the decay exponent x in detail in various ordered phases and in the vicinity of phase boundaries. We find that within the disordered phase, the behavior of x can be rationalized in terms of interaction effects. Namely, x is typically larger than two in cases where repulsive adparticle–adparticle interactions dominate, while attractive interactions lead to x<2. In ordered phases, our results suggest that the behavior of x is mainly governed by ordering effects that determine the local structure in which adatoms diffuse. Then the decay is characterized by 1

How to measure velocity correlations from surface diffusion experiments by STM

Abstract We introduce a generalized velocity-correlation function φ g ( t ) for the study of tracer diffusion of interacting particles on surfaces. This function is defined in terms of particle displacement variables that can be obtained from both scanning tunneling microscopy experiments and simulations. We demonstrate this approach by considering a simple model adsorption system in which we find φ g ( t ) to decay in a non-exponential fashion that can be well characterized by a power-law φ g ( t )∼ t − x in time t . The decay exponent x provides information of predominant adatom–adatom interactions and ordering effects as is shown in this work.

Diffusion of end-grafted chain-like molecules on surfaces

We have studied the diffusion of short end-grafted chain-like molecules on smooth surfaces using the fluctuating bond model and Monte Carlo simulations. In our simulations, one end of each chain is restricted to lie on the surface, where it can move in a diffusive manner. The other segments of the chains can fluctuate freely above the surface. We study the behavior of the collective diffusion coefficient, D C (θ), as a function of the coverage, θ. Through simulations, we demonstrate that with relatively short end-grafted chains, D C (θ) first increases as a function of 0 but then eventually approaches zero for θ→1. This is caused by an entropy-induced repulsive interaction in analogy to the case of purely 2D diffusion of polymers.

Memory Effects and Memory Functions in Surface Diffusion

We consider the diffusive dynamics of particles in various model systems with strong interactions. We study the temporal dependence of the single-particle velocity autocorrelation function o(t), and its corresponding memory function. We find o(t) to decay non-exponentially and in most cases follow a power-law o(t) t- x at intermediate times i, while at long times there is a crossover to an exponential decay. We characterize the possible values of the decay exponent x, and show that x correlates with interaction and ordering effects. In many cases, the memory function follows behavior similar to that of o(t). These results suggest that o(t) can be used to obtain information about the ordering of the system and about the nature of predominant interactions between adparticles using experimental techniques such as scanning tunneling microscopy, in which o(t) can be measured in terms of discrete adparticle displacements. Finally, our studies suggest that the decay of velocity correlations in collective diffusion is qualitatively similar to the case of tracer diffusion.