Growth of films with anisotropic particles: Simulations and rate equations

Abstract

By means of computer simulations and kinetic rate equations, we study the formation of a film of rod-like particles which are deposited on a substrate. The rod-rod interactions are hard with a short-range attraction of variable strength and width, and the rod-substrate interactions favor lying rods with a variable strength. For a rod aspect ratio of 5 and deposition of up to an equivalent of one monolayer of standing rods, we demonstrate a rich variety of growth modes upon variation of the three interaction parameters. We formulate rate equations for the time evolution of densities of islands composed of standing, lying, and mixed rods. Input parameters such as diffusion constants, island capture numbers, and rod reorientation free energies are extracted from simulations, while rod reorientation attempt frequencies remain as free parameters. Numerical solutions of the rate equations in a simple truncation show rough qualitative agreement with the simulations for the early stage of film growth but an extension to later stages requires to go significantly beyond this simple truncation.

Pentacene Crystal Growth on Silica and Layer-Dependent Step-Edge Barrier from Atomistic Simulations

“…It is likely that with a more realistic (lower) deposition flux, the growth of both types of nuclei would be diffusion-limited and hence that each amorphous nucleus would generate an island, thus reconciling the experimental finding of very low critical sizes with the two-step growth process illustrated here. The two-step nucleation of crystalline islands appears to be a quite general mechanism for elongated π-conjugated molecules, as it was already observed in MD simulations of pentacene and sexithiophene on C 60 , and of small pentacene clusters on SiO 2 . , A reorientation process, following the initial formation of a small nucleus of lying molecules, was also suggested for the vapor growth of para-sexiphenyl on the basis of simulated annealing force-field simulations and also by Monte Carlo simulations of model rodlike particles . Moreover, an increasing number of experimental reports is appearing concerning two-step nucleation processes for organic crystals in solution. ,, Our findings suggest that the two-step model could possibly be extended to crystallization on surfaces by vapor deposition and adopted in the corresponding theories.…”

“…34,35 A reorientation process, following the initial formation of a small nucleus of lying molecules, was also suggested for the vapor growth of para-sexiphenyl on the basis of simulated annealing force-field simulations 36 and also by Monte Carlo simulations of model rodlike particles. 37 Moreover, an increasing number of experimental reports is appearing concerning two-step nucleation processes for organic crystals in solution. 3,38,39 Our findings suggest that the two-step model could possibly be extended to crystallization on surfaces by vapor deposition and adopted in the corresponding theories.…”

“…The intermediate steps of the deposition protocol provide realistic morphologies of growing crystalline islands, which are employed in the following for the calculation of the step-edge or Ehrlich–Schwoebel barrier. ESB has been rarely calculated so far for organic molecules, and most often through energy-minimization techniques, whose results are limited by the arbitrariness of the molecular path chosen, ,, do not include entropic effects, and therefore are sometimes contradictory or at variance with experiments. A paradigmatic case is that of sexiphenyl, for which the barrier is extremely sensitive to the tilt angle of the monolayer, going from 6 to 15 kcal/mol for tilts of 43° and 17°, respectively, and for which a wide range of values is obtained by varying the calculation method. , The drawbacks of energy-minimization schemes is overcome here by performing free-energy driven MD simulations at room temperature, in which a pentacene probe molecule is pushed by a biasing force to move across the steps without any further restriction on its pathway.…”

Power functional theory for Newtonian many-body dynamics

“…Amati et al (2019) for recent work) and to the use of rate equations (see e.g. Dixit et al (2018) for recent work) would be valuable. It would be interesting to see how power functional theory can be useful in going beyond classical nucleation theory (Lutsko, 2018), investigating the "dynamical barrier" in many-body correlations in hard spheres (Robinson et al, 2019), and making connections to stochastic thermodynamics (Leonard et al, 2013;Seifert and Speck, 2010;Seifert, 2012), and in particular to the concept of the entropy production rate, see e.g.…”