Surface-induced crystallization in supercooled tetrahedral liquids

Abstract

Surfaces have long been known to have an intricate role in solid-liquid phase transformations. Whereas melting is often observed to originate at surfaces, freezing usually starts in the bulk, and only a few systems have been reported to exhibit signatures of surface-induced crystallization. These include assembly of chain-like molecules, some liquid metals and alloys and silicate glasses. Here, we report direct computational evidence of surface-induced nucleation in supercooled liquid silicon and germanium, and we illustrate the crucial role of free surfaces in the freezing process of tetrahedral liquids exhibiting a negative slope of their melting lines (dT/dP|coexist<0). Our molecular dynamics simulations show that the presence of free surfaces may enhance the nucleation rates by several orders of magnitude with respect to those found in the bulk. Our findings provide insight, at the atomistic level, into the nucleation mechanism of widely used semiconductors, and support the hypothesis of surface-induced crystallization in other tetrahedrally coordinated systems, in particular water in the atmosphere.

Ice nucleation at the nanoscale probes no man’s land of water

“…Our molecular dynamics (MD) simulations were carried out using a coarse-grained water model (mW) 13 , which is both computationally efficient and accurate in describing many of the thermodynamic properties of water and ice. The ice nucleation rates were computed by employing the forward flux sampling method [14][15][16] , which allowed us to collect a large number of nucleation trajectories (B200) at several conditions. We computed ice nucleation rates of droplets, with radii between 2.4 and 6.1 nm, over a wide temperature range from 205 to 240 K, well into 'no man's land'.…”

“…In fact, although melting initiated at the surface has been often observed in experiments, surface freezing is much less common 23,24 . (2) For tetrahedral liquids exhibiting a negative slope of their melting lines (dT/dP| coexist o0), for instance silicon and water, we proposed that the presence of a liquid-vapour surface enhances the crystallization rate: indeed in these systems the density decrease upon crystallization may be better accommodated in the vicinity of the liquid surface 16,25 . (3) In a liquid nano-droplet, due to the curvature of the surface, there exists a non-negligible Laplace pressure, which can be expressed as p L ¼ 2g lv /r, where g lv and r are the liquid-vapour surface tension and the radius of the droplet, respectively.…”

“…Using the molecular volumes and the surface tension of the liquid-vacuum interface 16 for silicon modelled with the Tersoff potential 26 , we obtained an enhancement of the nucleation rate in the subsurface liquid layer with respect to that of the bulk by a factor of B10 2 at the temperature of 0.95 T m (see Supplementary Note 2). This estimate is in agreement with previous numerical results that showed a thousand fold increase in the calculated nucleation rate in the silicon liquid slabs 16 .…”

“…Using the molecular volumes and the surface tension of the liquid-vacuum interface 16 for silicon modelled with the Tersoff potential 26 , we obtained an enhancement of the nucleation rate in the subsurface liquid layer with respect to that of the bulk by a factor of B10 2 at the temperature of 0.95 T m (see Supplementary Note 2). This estimate is in agreement with previous numerical results that showed a thousand fold increase in the calculated nucleation rate in the silicon liquid slabs 16 . On the other hand, from the thermodynamic quantities of the mW water model 13,17 , we find that our CNT model predicts only a marginal increase of ice nucleation rate in the vicinity of flat water surfaces, for example, R p /R b B3 at 230 K. To confirm the prediction of the mW water model, we computed ice nucleation rates in supercooled water slabs using 4,096 mW water molecules, and indeed found no significant difference in the computed nucleation rates from those in bulk water (see Supplementary Fig.…”

Solidification in a Supercomputer: From Crystal Nuclei to Dendrite Assemblages

“…Therefore, multiple nucleation in a large-scale system is usually achieved with the aid of inducing factors such as a high pressure 53 and surface fluctuation. 67 We successfully achieved spontaneous nucleation from an undercooled iron melt without any inducing factor in a million-atom molecular dynamics simulation on a GPU supercomputer using the code described in the previous subsection. The simulation methodology basically followed the simulation in the previous subsection.…”