Nature of step-edge barriers for small organic molecules

JE Goose and EL First and P Clancy, PHYSICAL REVIEW B, 81, 205310 (2010).

DOI: 10.1103/PhysRevB.81.205310

A detailed examination of the Ehrlich-Schwoebel barrier that governs transport of molecules over step edges for small organic molecules shows that the nature of this barrier for molecular systems is far richer than has been previously understood. While such barriers for atomic systems can be represented by one numerical value, we show that step-edge energy barriers for molecular systems depend sensitively on the angle of approach to the step edge which, in turn, depends on the ability of the molecule to explore rotational degrees of freedom. Thus a multiplex of barriers can be obtained for edge descent depending on the angle of approach. Studies of seven aromatic molecules (small acenes, C-60, rubrene, diindenoperylene, and sexiphenyl) that cover a range of size, shape, and rotational freedom explore how the degree of molecular twisting and bending affects the value of the Ehrlich-Schwoebel barrier on a surface composed of itself ("self"-Schwoebel barriers) and of other organic molecules ("hetero"-Schwoebel barriers). Nonspherical small organic molecules exhibit a strong tendency to "log roll" over step edges at angles around 20 from parallel to the step edge as the lowest- energy descent mechanism. Rigid models of small organic molecules fail to capture conformational flexibility during edge descent, resulting in the production of considerably higher barriers. Intriguingly, while bending and twisting have only a small direct effect on the magnitude of the barrier, they play a crucial role on the angle of approach to the step edge which turns out to have a large impact on the barrier height. In addition, we quantify the importance of selecting a sufficiently representative potential model and of employing an appropriately conducted search mechanism. Finally, the broad scope of this study allows us to correlate Schwoebel barriers with the binding energy of the adsorbed molecule to the surface, obviating the need to undertake molecular simulations for molecules not studied here and making possible accurate predictions of self- and hetero-Ehrlich-Schwoebel barriers.

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