Size-dependent structural phase transition of face-centered-cubic metal nanowires
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ng Au as an example, we have investigated the epitaxial bain paths of 〈001〉 oriented face-centered-cubic metal nanowires. It demonstrates that there are one stable and one metastable phase, having the lattice constant ratio c/a of about 0.6 and 1.0, respectively. Even without any external stimuli, the surface-tension-induced intrinsic stress in the interior may drive the nanowires to phase transform spontaneously for surface-energy minimization. However, this structural transition depends on the feature sizes of the nanowires. Specifically, only when the cross-section areas are reduced to 4.147 nm2 or so can the surface energy and the intrinsic stress satisfy the thermodynamic and kinetic conditions simultaneously.
I. INTRODUCTION
In nanometer scale, materials usually present fantastic behaviors due to their distinct structural properties, such as larger surface-to-volume ratio, lower atomic coordination number, and considerably increased surface free energy and surface tension. More and more research has been focused on this field, from materials design and fabrication to performance evaluations, and has tried to apply these new types of materials to further miniaturize electronic, optical, and sensor devices.1–6 On the whole, the fabrication methods for nanometer-scale elements can be grouped into two classes: “bottom-up”7–13; and “top-down.”14–18 In the former case, nanoelements are usually deposited by utilizing certain physical or chemical processes with the aid of a template, while the latter case involves the fabrication of nanocomponents from bulk materials by cutting through optical-beam, electronbeam, ion-beam, or scanning-probe lithography. Compared with the bottom-up approaches, the top-down approaches possess great flexibility and can be used to prepare nanoelements of nearly any shape. During the top-down processes, when at least one of the dimensions of the materials shrinks to nanometer scale, a large proportion of atoms can “feel” the existence of surfaces. In such a case, numerous dangling bonds certainly will increase the surface free energy,19,20 and the asymmetrical electron redistribution may induce great surface tension.21 In the equilibrium state, a certain magnitude of
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0153 J. Mater. Res., Vol. 22, No. 5, May 2007
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intrinsic compressive stress should be produced in the interior of materials to balance the surface tension, moreover, the smaller the feature sizes of the materials, the larger the surface effect and the larger the intrinsic stress. Just because of this, the top-down fabricated nanomaterials, for instance, nanoparticles, nanowires, and nanofilms, should be driven by the intrinsic compressive stress to contract for the surface to lower its energy, and thus the structural phase transition occurs. Kondo and Takayanagi22 observed through high-resolution transmission electron microscopy that a face-centered-cubic (fcc) {100} orien
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