Nanoscale gold pillars strengthened through dislocation starvation

It has been known for more than half a century that crystals can be made stronger by introducing defects into them, i.e., by strain-hardening. As the number of defects increases, their movement and multiplication is impeded, thus strengthening the material. In the present work we show hardening by dislocation starvation, a fundamentally different strengthening mechanism based on the elimination of defects from the crystal. We demonstrate that submicrometer sized gold crystals can be 50 times stronger than their bulk counterparts due to the elimination of defects from the crystal in the course of deformation

A Laboratory Experiment Using Nanoindentation to Demonstrate the Indentation Size Effect

A laboratory experiment using nanoindentation to demonstrate the indentation size effect is described. This laboratory introduces students to sophisticated instrumentation at low cost and low risk and utilizes recent research in the materials community as its foundation. The motivation, learning objectives, experimental details, data, and data analysis are presented. This experiment is intended for use in an upper-division materials science elective at the university level and has been successfully used in laboratory courses for senior undergraduates and first-year graduate students at Stanford University and Santa Clara University

Electromigration failure by shape change of voids in bamboo lines

The behavior of electromigration-induced voids in narrow, unpassivated aluminum interconnects is examined, using scanning electron microscopy. Some electromigration tests were interrupted several times in order to observe void nucleation, void growth, and finally the failure of the conductor line. It is found that voids which opened the line have a specific asymmetric shape with respect to the electron flow direction. Besides void nucleation and void growth, void shape changes can consume a major part of the lifetime of the conductor line. A first attempt to model these processes on the basis of diffusion along the void surface shows that voids with a noncircular initial shape tend to produce the fatal asymmetry due to electron wind effects, with the anisotropy of surface energy possibly playing only a minor role

On void nucleation and growth in metal interconnect lines under electromigration conditions

Electromigration failure in rigidly passivated metal interconnect lines is studied with particular reference to the vacancy supersaturations and hydrostatic stresses that can be developed at blocking grain boundaries under electromigration conditions. It is shown that the high stresses needed for homogeneous void nucleation to occur are probably too high to be developed by electromigration and that failure is more likely to involve the growth of pre-existing voids. We also show that the amount of void growth that can occur at a blocking grain boundary by electromigration of vacancies down the adjoining grain boundaries is small relative to the dimensions of the line unless the adjoining grain boundaries are continuous in a very long section of the line. This suggests that other mechanisms of void growth are responsible for electromigration failure. An analysis of the electromigration of small pre-existing voids shows that above a critical size, large voids migrate faster than smaller ones. This leads to a catastrophic process in which large voids can catch up with and coalesce with smaller ones, growing in size and migrating more rapidly as they do so. We conclude that the migration and coalescence of preexisting voids is a more likely mechanism of electromigration failure

High‐Throughput Growth of Microscale Gold Bicrystals for Single‐Grain‐Boundary Studies

The study of grain boundaries is the foundation to understanding many of the intrinsic physical properties of bulk metals. Here, the preparation of microscale thin‐film gold bicrystals, using rapid melt growth, is presented as a model system for studies of single grain boundaries. This material platform utilizes standard fabrication tools and supports the high‐yield growth of thousands of bicrystals per wafer, each containing a grain boundary with a unique tilt character. The crystal growth dynamics of the gold grains in each bicrystal are mediated by platinum gradients, which originate from the gold–platinum seeds responsible for gold crystal nucleation. This crystallization mechanism leads to a decoupling between crystal nucleation and crystal growth, and it ensures that the grain boundaries form at the middle of the gold microstructures and possess a uniform distribution of misorientation angles. It is envisioned that these bicrystals will enable the systematic study of the electrical, optical, chemical, thermal, and mechanical properties of individual grain boundary types.Studies of single grain boundaries are enabled through the preparation of microscale gold bicrystals via rapid melt growth. This material platform supports the high‐throughput and high‐yield growth of gold bicrystals on amorphous oxide. Crystallization is mediated by platinum doping, which decouples crystal nucleation from growth. These bicrystals offer model systems for the systematic study of individual defect properties.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151275/1/adma201902189-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151275/2/adma201902189.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151275/3/adma201902189_am.pd

Dislocation junctions and jogs in a free-standing FCC thin film

Abstract Dislocation junctions and jogs in a free-standing FCC thin film have been studied using 3-dimensional dislocation dynamics simulations. Due to the unconstrained motion of surface nodes and dislocation annihilation at the free surface, junctions and jogs are unstable except for some uncommon conditions. If the film thickness is thin enough for a significant portion of dislocation network to be terminated at the free surface, junctions and jogs can exist for only a finite time during deformation. Thus, the creation of junction/jog-related dislocation sources and their performance are more limited as the film thickness decreases. This effect could lead to insufficient dislocation multiplication to balance dislocation annihilation at the free surface

Modelling dislocations in a free-standing thin film

Abstract We present a set of efficient numerical algorithms to accurately compute the forces on dislocations in free-standing thin films. We first present a spectral method for computing the image stress field of dislocations in an isotropic elastic half space and a free-standing thin film. The traction force on the free surface is decomposed into Fourier modes by a discrete Fourier transform and the resulting image stress field is obtained by superimposing analytic solutions in the Fourier space. Dislocations intersecting free surfaces are discussed, including the use of virtual segments and the associated uniqueness of their solutions. The efficiency of the algorithm is enhanced by incorporating the analytical solutions for straight dislocations intersecting free surfaces. A comprehensive algorithm, including a flow diagram, is formulated and the numerical convergence of these algorithms discussed. As a benchmark, we compute the equilibrium orientation of a threading dislocation in a free-standing thin film. Good agreement is observed between the predictions from the dislocation dynamics model and those from molecular static simulations and the line tension model

Transcription Factors Bind Thousands of Active and Inactive Regions in the Drosophila Blastoderm

Identifying the genomic regions bound by sequence-specific regulatory factors is central both to deciphering the complex DNA cis-regulatory code that controls transcription in metazoans and to determining the range of genes that shape animal morphogenesis. We used whole-genome tiling arrays to map sequences bound in Drosophila melanogaster embryos by the six maternal and gap transcription factors that initiate anterior–posterior patterning. We find that these sequence-specific DNA binding proteins bind with quantitatively different specificities to highly overlapping sets of several thousand genomic regions in blastoderm embryos. Specific high- and moderate-affinity in vitro recognition sequences for each factor are enriched in bound regions. This enrichment, however, is not sufficient to explain the pattern of binding in vivo and varies in a context-dependent manner, demonstrating that higher-order rules must govern targeting of transcription factors. The more highly bound regions include all of the over 40 well-characterized enhancers known to respond to these factors as well as several hundred putative new cis-regulatory modules clustered near developmental regulators and other genes with patterned expression at this stage of embryogenesis. The new targets include most of the microRNAs (miRNAs) transcribed in the blastoderm, as well as all major zygotically transcribed dorsal–ventral patterning genes, whose expression we show to be quantitatively modulated by anterior–posterior factors. In addition to these highly bound regions, there are several thousand regions that are reproducibly bound at lower levels. However, these poorly bound regions are, collectively, far more distant from genes transcribed in the blastoderm than highly bound regions; are preferentially found in protein-coding sequences; and are less conserved than highly bound regions. Together these observations suggest that many of these poorly bound regions are not involved in early-embryonic transcriptional regulation, and a significant proportion may be nonfunctional. Surprisingly, for five of the six factors, their recognition sites are not unambiguously more constrained evolutionarily than the immediate flanking DNA, even in more highly bound and presumably functional regions, indicating that comparative DNA sequence analysis is limited in its ability to identify functional transcription factor targets