A material exhibits polymorphism properties when it can adopt different crystal structures while keeping the exact same chemical composition [1]. Since numerous material properties depend on the crystal structure, being able to control polymorphism effects is crucial for a large variety of technological applications. At the nanometric scale, crystal structures that would be unstable otherwise can become preponderant because surface effects play an increasing role when decreasing the material size [2]. Among the different ways to synthesize nanomaterials, a hot liquid nanodroplet can be deposited on a solid substrate thus generating the templated-freezing as in physical vapor deposition technique [3]. While numerous works have been dedicated to bulk heterogeneous nucleation and to nanoscale effects in liquid droplets, the interplay between the two phenomena and especially how it can lead to different polymorphisms have rarely been investigated.
We use Lennard-Jones (LJ) interactions for both the liquid phase and the substrate represented by an atomistic FCC crystal. While the stable bulk phase for LJ is FCC, with HCP being a metastable alternative, we are able to stabilize nanocrystals with a predominantly BCC structure by varying the lattice mismatch between the substrate and the freezing species. We present phase diagrams showing the variation between these three phases as a function of lattice mismatch, temperature and droplet size. Furthermore, we show strong evidence that when not forming FCC crystal, the system can begin to freeze into BCC which then, under certain circumstances, transforms to HCP thus evidencing a multi-step nucleation pathway.
References
[1] D. Braga, F. Grepioni, L. Maini, and M. Polito, Crystal Polymorphism and Multiple Crystal Forms, Springer (2009).
[2] J. McHale, A. Auroux, A. Perrotta, and A. Navrotsky, Science 277, 788 (1997).
[3] D. M. Mattox, Handbook of physical vapor deposition (PVD) processing, William Andrew (2010).
