Kinetic control of competing nuclei in a dimer lattice-gas model

Nucleation is a key step in the synthesis of new material from a solution of its components. Competition of clusters with different structures is often observed in nature. Free energy calculations can show which structure is most stable at each cluster size, favouring the stable bulk structure as the cluster size approaches the thermodynamic limit. Nevertheless, significant populations of large and very long lived metastable post-critical nuclei are often observed in experiments.

Well-established lattice-gas models can be used to gain insight into the basic physics of nucleation pathways. We have studied nucleation in a system of two interacting dimer types, using grand canonical Monte Carlo simulations. Two possible dimer-rich phases can be nucleated from the metastable parent phase. We calculate the free energy barrier to nucleation as a function of cluster size for each phase using the Umbrella Sampling method and tune the relative barrier heights by changing the interaction energies of the particles. In our model, either the stable or metastable structures may nucleate first, leading to a mixed population. We have shown that the dominant phase in this population can be controlled by tuning the relative kinetics of the two dimers. To demonstrate this, we calculate the nucleation rate of both phases using the Forward Flux Sampling method. We also estimate the relative population of stable and metastable crystallites by modelling the nucleation of each as independent processes occurring at constant rates.