We present the formation of unique and striking dendritic "seahorse" patterns in the growth of fullerene-tetracyanoquinodimethane (C 60 -TCNQ) or pure TCNQ thin films. The films were fabricated by an ionized-cluster-beam deposition method. Energetic neutral and charged clusters were deposited on amorphous carbon substrates. Transmission electron microscopy reveals that the elemental pattern is a "seahorse" -that is, an S-shaped form, with "fins" on the outer edges of the curved arms forming the S. Such forms possess an approximate symmetry under rotations by π, but strongly break two-dimensional inversion symmetry. A novel formation mechanism is proposed, involving the aggregation of neutral and charged clusters, such that some electrostatic charge is trapped on each growing island. This charge gives rise to a longrange field which biases the growth in a nontrivial way. The broken symmetry arises from the strong amplification of noise in the diffusive aggregation process by the effects of the electrostatic field -that is, the symmetry breaking is spontaneous. This picture is tested by applying a transverse electric field during growth: for sufficiently strong fields, the S-shaped "seahorses" lose their curvature, while retaining the feature of having two main arms. These results demonstrate the importance of electrostatic effects in the growth process, and are consistent with the growth mechanism described here. 337 Fractals 1998.06:337-350. Downloaded from www.worldscientific.com by YALE UNIVERSITY on 05/15/15. For personal use only. Fractals 1998.06:337-350. Downloaded from www.worldscientific.com by YALE UNIVERSITY on 05/15/15. For personal use only.

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.

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