  Low-dimensional boundaries between phases and domains in organic thin films
are important in charge transport and recombination. Here, fluctuations of
interfacial boundaries in an organic thin film, acridine-9-carboxylic acid
(ACA) on Ag(111), have been visualized in real time, and measured
quantitatively, using Scanning Tunneling Microscopy. The boundaries fluctuate
via molecular exchange with exchange time constants of 10-30 ms at room
temperature, yielding length mode fluctuations that should yield characteristic
f-1/2 signatures for frequencies less than ~100 Hz. Although ACA has highly
anisotropic intermolecular interactions, it forms islands that are compact in
shape with crystallographically distinct boundaries that have essentially
identical thermodynamic and kinetic properties . The physical basis of the
modified symmetry is shown to arise from significantly different substrate
interactions induced by alternating orientations of successive molecules in the
condensed phase. Incorporating this additional set of interactions in a lattice
gas model leads to effective multi-component behavior, as in the
Blume-Emery-Griffiths (BEG) model, and can straightforwardly reproduce the
experimentally observed isotropic behavior. The general multi-component
description allows the domain shapes and boundary fluctuations to be tuned from
isotropic to highly anisotropic in terms of the balance between intermolecular
interactions and molecule-substrate interactions. Key words: Organic thin film,
fluctuations, STM, molecular interactions, diffusion kinetics, phase
coexistence
