Stability and Structural Transition of Gold Nanowires under Their Own Surface Stresses
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Stability and Structural Transition of Gold Nanowires under Their Own Surface Stresses Ken Gall1 , Michael Haftel2 , Jiankuai Diao1∗ , Martin L. Dunn1 , Noam Bernstein2 , and Michael J. Mehl2 1 Department of Mechanical Engineering, University of Colorado at Boulder Boulder, CO 80309. 2 Center for Computational Materials Science, Naval Research Laboratory, Washington, D.C. 20375 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) 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. INTRODUCTION Gold (Au) nanowires have potential application in nanotechnology due to their relative ease of fabrication1,2, stability at small scales3-13 , capacity for biomolecule functionalization14-17 , and high conductivity. Recent studies have demonstrated that low-dimension Au materials can exist in non- face centered cubic (fcc) structures. When thinned below a critical size, Au nanowires have been observed to transform into a helical multi-shell structure10 . Small Au nanowires are predicted to undergo an fcc to body centered cubic (bct) phase transformation18 . Atoms in single chain Au nanowires have different spacing than atoms along close-packed directions in bulk Au12 . In order to fully exploit Au nanowires in emerging nanosystems, it is critical to understand their unique structures from a fundamental perspective. In addition, the study of phase stability and transformation in nanometer scale solids has broad implications. From a basic science point of view, the study of phase changes in nanometer scale materials provides fundamental information on solid-state transformations not easily ascertained in bulk solids19-20 . From an application standpoint, control of metastable phases in nanometer scale materials may provide a means for small-scale actuation, analogous to martensitic transformations observed in nanoscale biological systems21,22. Nanometer scale solids possess unique properties due in part to their large ratio of surface area to volume. Free surfaces in solids give rise to surface energy and surface stress. Surface stresses, which are typically tensile in metals, cause contraction of surface atoms relative to bulk atoms, resulting in “intrinsic” compressive stresses within materials. Intrinsic stresses are defined as stres
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