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Segregation at surfaces of metal-covalent binary liquids is oftennon-classical and in extreme cases such as AuSi, the surface crystallizes abovethe melting point. In this study, we employ atomic-scale computationalframeworks to study the surface crystallization of AuSi films and droplets as afunction of composition, temperature and size. For temperatures in the range$T_s^\ast=765-780$K above the melting point $(T_s^\ast\approx1.3\,T_m)$, boththin film and droplet surfaces undergo a first order transition, from a 2DAu$_2$Si crystalline phase to a laterally disordered yet stratified layer. Thethin film surfaces exhibit an effective surface tension that increases withtemperature and decreases with Si concentration. On the other hand, fordroplets in the size range $10-30$ nm, the bulk Laplace pressure alters thesurface segregation as it occurs with respect to a strained bulk. Above$T_s^\ast$ the size effect on the surface tension is small, while for$T<T_s^\ast$ the surface layer is strained and composed of 2D crystallitesseparated by extended grain boundary scars that lead to large fluctuations inits energetics. As a specific application, all-atom simulations of AuSidroplets on Si(111) substrate subject to Si surface flux show that thesupersaturation dependent surface tension destabilizes the contact line viaformation of a precursor wetting film on the solid-vapor interface, and hasramifications for size selection during VLS-based routes for nanowire growth.Our study sheds light on the interplay between stability and energetics ofsurfaces in these unique class of binary alloys and offers pathways forexploiting their surface structure for varied applications such as catalyticnanocrystal growth, dealloying, and polymer crystallization.

Stability and energetics of 2D surface crystals in liquid AuSi thin films and nanoscale droplets