Tetragonal Phase Transformation in Gold Nanowires

Abstract

First principle, tight binding, and semi-empirical embedded atom calculations are used to investigate a tetragonal phase transformation in gold nanowires. As wire diameter is decreased, tight binding and modified embedded atom simulations predict a surface-stress-induced phase transformation from a face-centered-cubic (fcc) ⟨100⟩ nanowire into a body-centered-tetragonal (bct) nanowire. In bulk gold, all theoretical approaches predict a local energy minimum at the bct phase, but tight binding and first principle calculations predict elastic instability of the bulk bct phase. The predicted existence of the stable bct phase in the nanowires is thus attributed to constraint from surface stresses. The results demonstrate that surface stresses are theoretically capable of inducing phase transformation and subsequent phase stability in nanometer scale metallic wires under appropriate conditions.

A surface Cauchy–Born model for nanoscale materials

“…Therefore, the issue at hand is to develop an expression for the energy density along the surfaces of a body. If the effects of surfaces are not considered, a standard bulk Cauchy-Born relationship can be utilized to derive the stress [27] as described above in (9). Once the surface and bulk energy densities are determined, the stresses arising from bulk and surface energetics can be defined using the relationships (5) and (9) defined in this section.…”

“…For example, gold nanowires have been seen in molecular dynamics (MD) simulations to undergo phase transformations from an initially face-centred cubic (FCC) 1 0 0 orientation to body-centred tetragonal (BCT) that are entirely surface stress driven [8,9] once the cross sectional length decreases below about 2 nm. In addition, shape memory [10,11] and pseudoelastic [10,12,13] behaviour has also been recently shown in MD simulations of FCC metal nanowires.…”

Cross-sectional geometry dependence of spontaneous phase transformation of copper nanowires

“…In this situation, however, the bcc structure is still unstable and will further transform into hcp [4]. Finally, the hcp structure extends from the ends of the nanowire to its center.…”

“…In general, the properties of nanowires are strongly dependent on their structures and exhibit distinct size effects [4]. Much attention has been attracted to investigations on the mechanical behavior of nanowires made of different materials [5,6].…”

Nanoprobing Fracture Length Scales

“…These tests have had moderate success determining elastic modulus and yield strengths with freestanding thin films, but require substantial experimental expertise to operate. Atomistic simulations by Gall et al (2005) of nanowire tensile tests have also given significant insight into length scale behavior.…”