Chiral patterns arising from electrostatic growth models

Abstract

Recently, unusual and strikingly beautiful seahorse-like growth patterns have been observed under conditions of quasitwo-dimensional growth. These 'S'-shaped patterns strongly break two-dimensional inversion symmetry; however such broken symmetry occurs only at the level of overall morphology, as the clusters are formed from achiral molecules with an achiral unit cell. Here we describe a mechanism which gives rise to chiral growth morphologies without invoking microscopic chirality. This mechanism involves trapped electrostatic charge on the growing cluster, and the enhancement of growth in regions of large electric field. We illustrate the mechanism with a tree growth model, with a continuum model for the motion of the one-dimensional boundary, and with microscopic Monte Carlo simulations. Our most dramatic results are found using the continuum model, which strongly exhibits spontaneous chiral symmetry breaking, and in particular finned 'S' shapes like those seen in the experiments.

Chiral pattern formation: Combined transmission electron microscopy and atomic force microscopy study of tetracyanoquinodimethane thin film grown by vacuum evaporation

“…Moreover, TEM examinations of different samples, prepared simultaneously with or without applied electric field (0−10 3 V/cm), showed that there are no obvious difference among the patterns of them. Therefore, contrary to the reported electrostatic growth models, 21 we conclude that charged clusters are unnecessary to the chiral pattern formation.…”

“…16−19 Recently, it was reported the observation of quasi-twodimensional chiral patterns in growth of C 60 -TCNQ by an ionized-cluster-beam (ICB) method. 20 Theoretical investigations have also been carried out on the broken symmetry of these patterns, 21 where, motivated by the presence of electric charged clusters in ICB deposition, the charged clusters are considered to be the crucial role that determines the chiral pattern formation. However, the actual growth mechanism and the role of charged clusters are still under debate.…”

Unique Dendritic Patterns in Organic Thin Films

“…Referring to Ref. 16 for details, we here note simply that, for one set of assumptions which gave promising results, we obtain growth at a point (x, y) in the boundary of a growing island proportional to exp[χ|E(x, y)| 2 /2kT ] (where T is a temperature appropriate to the diffusing clusterswhich may be higher than the substrate temperature T s ).…”

“…A detailed description of this modeling work is given in Ref. 16. The models studied include deterministic and nondeterministic "tree" growth models 31 ; Monte-Carlo (MC) simulations 32 of diffusive aggregation; and a continuum model for the motion of the island boundary.…”

“…Inspired by these novel patterns, we have proposed an equally novel growth mechanism 15,16 which can give rise to similar forms. The mechanism involves as an essential element the presence of some electrostatic charge trapped on each growing island.…”