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Manipulating the morphology of inorganic nanostructures, such as their chirality and branching structure, has been actively pursued as a means of controlling their electrical, optical and mechanical properties. Notable examples of chiral inorganic nanostructures include carbon nanotubes 1 , 2 , gold multishell nanowires 3 , mesoporous nanowires 4 , 5 and helical nanowires 6 , 7 , 8 . Branched nanostructures 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 have also been studied and been shown to have interesting properties for energy harvesting 17 and nanoelectronics 18 . Combining both chiral and branching motifs into nanostructures might provide new materials properties. Here we show a chiral branched PbSe nanowire structure, which is formed by a vapour–liquid–solid branching from a central nanowire with an axial screw dislocation. The chirality is caused by the elastic strain of the axial screw dislocation, which produces a corresponding Eshelby Twist 19 , 20 in the nanowires. In addition to opening up new opportunities for tailoring the properties of nanomaterials, these chiral branched nanowires also provide a direct visualization of the Eshelby Twist. The electrical, optical and mechanical properties of nanowires depend on their morphology. Nanowires that possess both chirality and a branched structure may therefore possess new material properties. Such nanowires can be formed by vapour–liquid–solid branching from a central PbSe nanowire with an axial screw dislocation.

We examined the stress associated with crystallite coalescence during the initial stages of growth in thin polycrystalline films with island growth morphology. As growing crystallites contacted each other at their bases, the side-walls zipped together until a balance was reached between the energy associated with eliminating surface area, creating a grain boundary and straining the film. Our estimate for the resulting strain depends only on interfacial free energies, elastic properties, and grain size and predicts large tensile stresses in agreement with experimental results. We also discuss possible stress relaxation mechanisms that can occur during film growth subsequent to the coalescence event.

The phenomena of pile-up and sink-in associated with nanoindentation have been found to have large effects on the measurements of the indentation modulus and hardness of copper. Pile-up (or sink-in) leads to contact areas that are greater than (or less than) the cross-sectional area of the indenter at a given depth. These effects lead to errors in the absolute measurement of mechanical properties by nanoindentation. To account for these effects, a new method of indenter tip shape calibration has been developed; it is based on measurements of contact compliance as well as direct SEM observations and measurements of the areas of large indentations. Application of this calibration technique to strain-hardened (pile-up) and annealed (sink-in) copper leads to a unique tip shape calibration for the diamond indenter itself, as well as to a material parameter, a, which characterizes the extent of pile-up or sink-in. Thus the shape of the indenter tip and nature of the material response are separated in this calibration method. Using this approach, it is possible to make accurate absolute measurements of hardness and indentation modulus by nanoindentation.

Nix and Gao established an important relation between microindentation hardnessand indentation depth. Such a relation has been verified by many microindentation experiments (indentation depths in the micrometer range), but it does not always hold in nanoindentation experiments (indentation depths approaching the nanometer range). We have developed a unified computational model for both micro- and nanoindentation in an effort to understand the breakdown of the Nix–Gao relation at indentation depths approaching the nanometer scale. The unified computational model for indentation accounts for various indenter shapes, including a sharp, conical indenter, a spherical indenter, and a conical indenter with a spherical tip. It is based on the conventional theory of mechanism-based strain gradient plasticity established from the Taylor dislocation model to account for the effect of geometrically necessary dislocations. The unified computational model for indentation indeed shows that the Nix–Gao relation holds in microindentation with a sharp indenter, but it does not hold in nanoindentation due to the indenter tip radius effect.

While Black’s equation for electromigration (EM) in interconnects with n  = 1 is rigorously based on the principles of electrotransport, n  > 1 is more commonly observed empirically. This deviation is usually attributed to Joule heating. An alternative explanation is suggested by the recent discovery of EM plasticity. To examine this possibility, we have retested samples that had been previously subjected to a predamaging phase of high temperature and current densities to determine whether the loss of median time to failure (MTF) is retained. We find that the predamaged samples exhibit MTFs that are permanently reduced, which is a characteristic of EM plasticity.

We recently proposed a theory of mechanism-based strain gradient (MSG) plasticity to account for the size dependence of plastic deformation at micron- and submicronlength scales. The MSG plasticity theory connects micron-scale plasticity to dislocation theories via a multiscale, hierarchical framework linking Taylor's dislocation hardening model to strain gradient plasticity. Here we show that the theory of MSG plasticity, when used to study micro-indentation, indeed reproduces the linear dependence observed in experiments, thus providing an important self-consistent check of the theory. The effects of pileup, sink-in, and the radius of indenter tip have been taken into account in the indentation model. In accomplishing this objective, we have generalized the MSG plasticity theory to include the elastic deformation in the hierarchical framework.

Plastic behavior has previously been observed in metallic interconnects undergoing high-current-density electromigration (EM) loading. In this study of Cu interconnects, using the synchrotron technique of white-beam x-ray microdiffraction, we have further found preliminary evidence of a texture correlation. In lines with strong (111) textures, the extent of plastic deformation is found to be relatively large compared with that of weaker textures. We suggest that this strong (111) texture may lead to an extra path of mass transport in addition to the dominant interface diffusion in Cu EM. When this extra mass transport begins to affect the overall transport process, the effective diffusivity, D^sub eff^, of the EM process is expected to deviate from that of interface diffusion only. This would have fundamental implications. We have some preliminary observations that this might be the case, and report its implications for EM lifetime assessment herein. [PUBLICATION ABSTRACT]

The indentation experiment is a popular method for the investigation of mechanical properties of thin films. By application of conventional methods, the hardness and the stiffness of the film material can be determined by limiting the indentation depth to well below the film thickness so that the substrate effects can be eliminated. In this work a new method is proposed, which allows for a determination of the reduced modulus as well as the nonlinear hardening behaviour of both the film and substrate materials. To this end, comparable deep indentations are made on the film substrate composite to obtain sufficient information on the mechanical properties of both materials. The inverse problem is solved by training neural networks on the basis of finite-element simulations using only the easily measurable hardness and stiffness behaviour as input data. It is shown that the neural networks are very robust against noise in the load and depth. The identification of the material parameters of aluminium films on different substrates results in a significant increase in yield stress and initial work-hardening rate for a reduction of the film thickness from 1.5 to 0.5 µm, while the elastic modulus and the extent of work hardening remain constant.

Using transmission electron microscopy we observe the dissociation of 60° misfit dislocations at the interface of SiGe/Si multilayers, extending into the substrate for distances of 5.0–7.5 nm. Analysis using elasticity theory shows that this dissociationis the equilibrium configuration for individual 60° misfit dislocations, as it is for 60° mixed dislocations in bulk Si, and that the compressively strained multilayer film serves mainly to position the partial dislocations and stacking fault with respect to the free surface. We observe both undissociated 60° and Lomer edge dislocations after annealing, and conclude that these result from dislocation climb in the interface. Since the dislocations move off their slip plane during climb, they cannot remain dissociated. Significant climb and Lomer dislocation formation for these low misfit layers is observed at temperatures above 850 °C and for samples with a high initial dislocation density, such as found in thicker as-grown samples. The dislocation configuration formed during annealing is distinct from that reported to form during growth of higher mismatch films: the Lomer dislocations tend to be segmented, with the segments connected by perfect 60° dislocations.

The storage and loss stiffnesses for the composite response of the sample, indenter, and load frame during dynamic nanoindentation are derived. In the first part of the analysis, no physical model is assigned to the composite system. It is shown that this case is equivalent to the conventional nanoindentation analysis. In the second part of the analysis, the sample is modeled as a standard linear solid in series with the indenter and load frame. The results for the storage and loss stiffnesses as computed by the two methods differ by at most ∼3% for the elastomeric system under consideration. Results for the storage and loss moduli are also similar. The relative merits and weaknesses of each analysis are discussed.