Pushan Ayyub
TIFR, India
Title: The microscopic origin of size-dependent lattice contraction and expansion
Biography
Biography: Pushan Ayyub
Abstract
With a decrease in the particle size, the lattice parameters in a large class of metallic nanoparticles (Ag, Al, Au, Cu, Ni, Pd, Pt, Bi, Sn, etc.) show a contraction as compared to their corresponding bulk values. Interestingly, among the metal nanoparticles listed above that exhibit a lattice contraction, all except Bi and Sn have a face centered cubic (fcc) crystal structure. The size dependence of the lattice parameter in fcc metals can be generally fitted to a Laplace-Young type equation, which suggests that they can be represented by a simple liquid-droplet model in which surface-tension-like forces are the most dominant. On the other hand, the few metals known to exhibit a systematic lattice expansion in the nanoparticle form include Cr, Fe, Nb, V and Ta, each of which happens to have a body centered cubic (bcc) structure. To understand the physical basis for this striking empirical correlation, we have carried out a detailed microscopic study based on ab-initio density functional theory (DFT). Our simulations on representative bcc (Nb) and fcc (Cu) nanoclusters elucidate the importance of a capping layer on the metal nanoparticles and succeed in provide a consistent understanding of this apparently puzzling observation. It is important to appreciate that size-driven changes in the lattice parameters is a non-trivial effect with significant consequences, in some cases dominating over quantum size effects and other types of surface effects. Thus, size-induced lattice expansion has been invoked to understand the (a) persistence of superconductivity down to unexpectedly small sizes, (b) appearance of a magnetic moment in isolated Fe atoms embedded in a nanocrystalline metals, and (c) destruction of ferroelectricity in nanocrystalline oxides.