Characterization of step-edge barrier crossing of para-sexiphenyl on the ZnO (101̄0) surface

Abstract

Mass transport processes of conjugated organic molecules (COMs) on inorganic surfaces are essential elements in thin film deposition for hybrid optoelectronic devices. Defects and in particular surface step-edges dictate the molecular nucleation and growth morphology, which itself determine many physical properties of the resulting hybrid interface. Here, we explore the detailed molecular kinetics and transport rates of a single physisorbed para-sexiphenyl (p-6P) molecule crossing a step-edge (a "hetero-Ehrlich-Schwoebel barrier") on the inorganic ZnO (101[combining macron]0) surface by a combination of all-atom molecular dynamics simulations and passage time theory. We determine temperature- and charge-dependent (free) energy landscapes, position-dependent diffusion coefficients, and ultimately the mean first passage time over the step-edges. We find two completely different step-edge crossing mechanisms, the occurrence and rates of which simultaneously depend on both electrostatic and thermal molecule-surface coupling. In weakly coupled systems, the molecule crosses the step relatively quickly (in nanoseconds) by log-roll mechanisms while for strongly coupled systems, it crosses relatively slowly (in microseconds) in a strictly perpendicular fashion. In the latter process, "internal friction" from intramolecular bending and torsional degrees of freedom contribute a significant corrugation to the overall crossing barrier. Furthermore, we show that crossing pathways can also change qualitatively with step-edge height. The great complexity in hetero-barrier crossing of COMs (in contrast to simple atoms) revealed in this study has implications on the interpretation and possible control of nucleation and growth mechanisms at surface defects in hybrid systems.

Step Bunches, Nanowires and Other Vicinal “Creatures”—Ehrlich–Schwoebel Effect by Cellular Automata

“…It is clear that the ES effect is a common feature in the energy surface landscape. The evidence for its existence is found experimentally [15][16][17][18][19][20] or by ab initio calculations [23][24][25]. Moreover, it was shown that the type of ES can be controlled by appropriate step passivation.…”

“…A kinetic Monte Carlo modeling study by Leal et al [22] was performed to test this idea, and it showed that mounded morphologies can be obtained even with a small barrier while self-affine growth is obtained in the absence of an explicit step barrier. The step height-dependent ES and step-edge crossing mechanisms of the organic molecule on a ZnO surface were studied [23], employing atomistic MD simulations and mean first passage time theory. Additionally, Xiang and Huang [24] performed the density functional theory-based ab initio method to obtain ES values for various step heights on different Cu terminations.…”

Impact of Polarity on Anisotropic Diffusion of Conjugated Organic Molecules on the (101̅0) Zinc Oxide Surface

“…The increase in configurational entropy associated with inter-ring torsion helps the p-6P to reduce the free energy barriers for surface diffusion, similarly as observed for the crossing of an atomic surface step-edge barrier. 39,40 In Fig. 6 a), b) and c) we plot the two-dimensional probability distributions P (x, y)…”

“…The coupling between the different degrees of freedom of adsorbed molecules is a phenomenon already observed and characterized for other molecular adsorbates. 39,41…”

Impact of anisotropic interactions on nonequilibrium cluster growth at surfaces

“…This is even more the case for systems of conjugated organic molecules (COM). In contrast to most atomic systems or systems of nearly spherical organic molecules like fullerene C 60 , elongated organic molecules like diindenoperylene, p-sexyphenyl (6P), the perylene derivative PTCDI-C 8 or pentacene are known to generally in-teract anisotropically with each other when adsorbed on both, organic and inorganic substrates [34,37,39,40,[66][67][68][69][70][71]. Therefore one expects rather complex cluster shapes and corresponding changes in the cluster density, cluster size distribution and the coalescence behavior in the sub-monolayer growth regime as compared to atomic systems.…”