Step Velocity Growth Models for Molecular Crystals: Two Molecules in the Unit Cell

Kink sites are a fundamental characteristic of steps found on crystals growing in layered regimes. For asymmetric organic molecules, kink densities are usually modeled at equilibrium, owing to a lack of understanding of nonequilibrium kink density behavior. Hence, crystal growth models approximate kink density at its equilibrium value despite observations showing its strong dependence on supersaturation. In this work, we develop a simplified steady-state framework for obtaining nonequilibrium kink densities and propose a pathway for predicting step velocity as a function of kink density for molecular crystals with Z = 2 growth units in the unit cell. The kink density and step velocity predictions are validated with kinetic Monte Carlo (kMC) simulation data from the literature. The overarching goal is translation of the framework to provide rapid predictions of crystal properties such as crystal shape, morphology, and size distribution.