Undercooling Skin Pre-Melting

There is an extensive database describing surface and nanosolid Tm suppression [21-26]. For instance, a photoelectron emission study [27] confirmed that lithium (110) surface melting occurs 50 K below the bulk (454 K). A temperature-resolved XRD analysis revealed that the of nanometer-sized drugs (polymer) also drops (by 33 and 30 K for 7.5-nm-sized griseofulvin and 11.0-nm-sized nifedipine, respectively) in a 1/R fashion [28]. STM measurements of a reversible, temperature-driven structural surface phase transition of Pb/Si(lll) nanoislands indicates that the transition temperature decreases with inverse of domain size and the phase transition is independent of the processes of cooling or heating [29]. 

The I m of a Pd nanowire is lower than the bulk value but higher than that of the cluster with countable number of atoms proceeding in a surface pre-melting manner. A quasi-liquid skin grows from the surface radially inward for both cluster and wire, followed by a breakdown of order in the remaining solid core at the transition temperature [30]. 

The size effect on the nanosolid melting has been modeled in terms of classical thermodynamics and atomistic MD simulations [31-46]. In general, the size-dependent TinCK) follows the empirical scaling relationship  

Classical thermodynamics based on the surface Laplace and the Gibbs-Duhem equations [43] have derived that Kq obeys the following relations [41, 44]  

The liquid shell nucleation (LSN) model [34] assumes that a liquid layer of thickness Kq is in equilibrium at the surface, which indicates that the surface melts before the core of the solid. 

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