Atomistic Simulations of Mechanics of Nanostructures
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At an interface, the periodicity of atomic arrangement terminates. This termination means the loss of atomic neighbors at a free surface, and therefore the change of electron distribution. As an example, Figure 2 shows electron redistribution near a ZnO ¯ surface.2 Due to the loss of atomic (1120) neighbors, each of the surface bonds (S1, S2, and S3) is strengthened, as indicated by the overlap of electron distribution (as shown by the red contours in Figure 2). This tendency of electron redistribution moves the surface atoms and exerts strain on atoms under the surface. At a metallic surface, similar electron redistribution occurs, leading to surface stress and strain. The surface stress may lead to or facilitate surface reconstruction, change of elastic moduli, phase transformation, dislocation nucleation and motion, and self-organization of nanostructures.3–5 When surface stress itself is insufficient, additional mechanical stress helps facilitate the phase transformation.5,6,7 Local electronic structure plays a key role in determining the nature of defects in crystals. For instance, the general planar fault energy curves, which represent the energy dependency of rigidly shearing a crystal, influence the nature of slip activity in nanocrystalline systems.8 Figure 3a displays the valence electron density in face-centered-cubic (fcc) Al that is rigidly ¯ slip sheared at a {111} plane along the 〈 1¯12〉 direction. The inter {111}-plane bonding is substantially altered. This alteration results in the maximum energy configuration in the generalized stacking fault energy (GSFE) curve displayed in Figure
of Mechanics of Nanostructures
Hanchen Huang and Helena Van Swygenhoven, Guest Editors Abstract Nanostructures can be in the form of nanoparticles or nanograins, nanowires or nanotubes, and nanoplates or multilayers. These nanostructures may be used individually or embedded in a bulk material. In both cases, they share two common features. First, the small dimensions minimize or even eliminate the presence of defects. Second, nanostructures entail large surface or interface areas. The absence of defects makes nanostructure materials stronger than their bulk counterparts, leading to the eventual realization of ideal strength. The presence of surfaces and interfaces may either reduce or increase the strength. Atomistic simulations can provide insight into the deformation mechanism at the atomic and electronic level, something that is very difficult to obtain from experiments. This article describes generic features of nanostructures and summarizes the five areas presented in the articles in this issue.
Introduction
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structure can increase or decrease with dimension, as shown in Figure 1b. These aspects form the focus of this theme issue. a
b 1.3
0D
1.2 1.1
1D
E/E Bulk
A structure becomes nanoscale when at least one of its three dimensions is below 100 nm. Gleiter1 classified nanostructures according to their dimensionality, as schematically shown in Figure 1a. In nanoparticles or nanograins, all three dimens
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