The extremely elongated growth spirals observed using atomic force microscopy on the largest {10 (1) over bar} faces of the needle crystals of vapor-grown metal-free beta-phthalocyanine are analyzed. An analytical model for the growth of anisotropic rectangular spirals is proposed to relate step distances and the step energies to the driving force for crystalization. The model is confronted with the experimental results, with kinetic Monte Carlo simulations, and with a growth step analysis. The model is well able to describe the observed and simulated spiral shapes. In addition, the experimental driving force for crystallization Could be estimated and turns Out to be reduced considerably as a result Of diffusion limitation when compared to the value obtained for ideal gas behavior.
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