B J Inkson

Nanoscale tomography in materials science

In materials science, various techniques for three-dimensional reconstruction of microstructures have been applied successfully for decades, such as X-ray tomography and mechanical sectioning. However, in the last decade the family tree of methods has grown significantly. This is partly through advances in instrumentation. The introduction of the focused ion beam microscope and the transformation of transmission electron microscopy into a multipurpose analytical and structural tool have made major impacts. The main driving force for progress is perhaps the advent of nanotechnology with the need to achieve nanometer-scale resolution and the desire to get a real three-dimensional view of the nanoscale world.

Bottom-up nanoconstruction by the welding of individual metallic nanoobjects using nanoscale solder.

We report that individual metallic nanowires and nanoobjects can be assembled and welded together into complex nanostructures and conductive circuits by a new nanoscale electrical welding technique using nanovolumes of metal solder. At the weld sites, nanoscale volumes of a chosen metal are deposited using a sacrificial nanowire, which ensures that the nanoobjects to be bonded retain their structural integrity. We demonstrate by welding both similar and dissimilar materials that the use of nanoscale solder is clean, controllable, and reliable and ensures both mechanically strong and electrically conductive contacts. Nanoscale weld resistances of just 20Omega are achieved by using Sn solder. Precise engineering of nanowelds by this technique, including the chemical flexibility of the nanowire solder, and high spatial resolution of the nanowelding method, should result in research applications including fabrication of nanosensors and nanoelectronics constructed from a small number of nanoobjects, and repair of interconnects and failed nanoscale electronics.

Growth and microstructural stability of epitaxial Al films on (0001) α-Al2O3 substrates

Abstract Al films were grown epitaxially on single-crystal α-Al2O3 substrates by magnetron sputtering and molecular beam epitaxy, respectively. The microstructure and thermal stability of these films were analysed in detail using X-ray diffraction methods and electron microscopy techniques. The films consist of two twin-related growth variants, related by a 180° rotation around the film normal resulting in a {111} Al || (0001) α-Al2O3, and ± 1 10 > Al || 10 1 0 > α-Al2O3 orientation relationship. The Al variants are separated by Σ3 { 2 1 1 } Al twin boundaries possessing a rigid body translation of the {111} Al planes across the boundary plane in order to reduce their energy. Motion of the twin boundaries was observed by annealing plan-view samples in situ in a transmission electron microscope. The twin boundaries advance in jerky motion at velocities of several μm/s at temperatures of ~400 °C, resulting in grain coarsening. In all cases, heat treatments resulted in increased area fraction of one twin variant, which finally will result in single-crystal films upon further annealing.

A miniaturized TEM nanoindenter for studying material deformation in situ

A miniaturized nanoindenter system has been designed and fabricated to carry out localized in situ deformation studies in a high resolution transmission electron microscope (TEM). The coarse positioning is carried out with the help of small inertial drives so that the whole system could fit into the specimen holder of the JEOL 2010 microscope. The fine positioning is achieved with a piezoelectric tube and the force is measured with the help of a four bar flexible hinge spring element. The ability of the system to correlate the force–distance data with the events observed in TEM is demonstrated.

Nanoscale characterization of CoPt/Pt multilayer nanowires

CoPt/Pt multilayer nanowires have been successfully fabricated by a dc electrodeposition technique into anodic aluminium oxide (AAO) templates, and their chemistry and crystal structure characterized at the nanoscale. It was found that each individual multilayered nanowire had a regular periodic structure like bamboo. However, the periodicity of diverse nanowires from the same specimen varied in magnitude. The Pt layers grew to the full width of the AAO template. However, the cobalt-rich layers in each nanowire did not fully fill the template pores during growth, resulting in a repeatably oscillating nanowire diameter. The chemical composition of multilayer nanowires was measured to be (Co72Pt28/Pt)n. Both the Co72Pt28 and Pt layers were polycrystalline, consisting of fcc nanocrystals <5 nm in size. The variable nanowire periodicity and wire diameter will result in variable mechanical strength, resistance and GMR effect.

Accurate electrical testing of individual gold nanowires by in situ scanning electron microscope nanomanipulators

This work presents an accurate measurement of electrical properties of individual gold nanowires, directly measured by nanomanipulators in situ in a scanning electron microscope. The electrical testing of 55nm width gold nanowires, with a bamboo-type polycrystalline micorstructure, shows that individual gold nanowires have an ideal resistivity of about 2.26μΩcm and remarkably high failure current density of 4.94×108Acm−2. The measurement of resistance (R) versus nanowire length of individual nanowires shows that the intrinsic conductivity of the gold nanowire is 4.45×107Ω−1m−1. There is no evidence that the polycrystalline grain structure, 55nm width and 500–2800nm length, generates any size-induced electrical effects. The accurate electrical testing of gold nanowires should be significant for nanodevices and nanoelectronics using them as building blocks or interconnects.

Indentation mechanics of Cu-Be quantified by an in situ transmission electron microscopy mechanical probe

In situ transmission electron microscopy was used to study, in real time, the sub-surface deformation taking place in Cu-Be alloy during nanoindentation. A twinned region of the material was indented with a sharp tungsten tip in a specially developed transmission electron microscopy (TEM) holder. A flexible hinge-based force sensor was used to measure the force on the indenter, and the force-displacement curve for the tip was obtained by tracking the tip in the sequential images of a TEM video of the indentation process. Step-like structures similar to 50 nm in size resulting from the tip surface roughness were observed to generate clusters of dislocations in the sample when they come in contact with the softer Cu-Be. With this setup, the forces and the mean pressure associated with such an individual deformation event in a nanostructured TEM sample were measured.

Three-dimensional chemical analysis of tungsten probes by energy dispersive x-ray nanotomography

The chemical distribution of oxide layers around functional tungsten nanotips is studied using electron tomography. Three-dimensional element distribution functions are derived for such tips, giving insight into the subsurface chemistry. Energy dispersive x-ray (EDX) spectroscopy is coupled to computed tomography to reconstruct slices across the tip. It is finally shown how the surface reconstruction by geometric tomography from annular dark field scanning transmission electron microscopy images can be combined with EDX tomography reconstructions to reduce backprojection artefacts and improve the sharpness of the surface contours.

Evolution of residual stress and crack morphologies during 3D FIB tomographic analysis of alumina

Three‐dimensional focused ion beam (FIB) tomography is increasingly being used for 3D characterization of microstructures in the 50 nm–20 μm range. FIB tomography is a destructive, invasive process, and microstructural changes may potentially occur during the analysis process. Here residual stress and crack morphologies in single‐crystal sapphire samples have been concurrently analyzed using Cr3+ fluorescence spectroscopy and FIB tomography. Specifically, maps of surface residual stress have been obtained from optically polished single‐crystal alumina [surface orientation (1 ī 0 2)], from FIB milled surface trenches, from Vickers micro‐indentation sites (loads 50 g–300 g), and from Vickers micro‐indentation sites during FIB serial sectioning. The residual stress maps clearly show that FIB sputtering generates residual stress changes. For the case of the Vickers micro‐indentations, FIB sputtering causes significant changes in residual stress during the FIB tomographic serial sectioning. 3D reconstruction of the crack distribution around micro‐indentation sites shows that the cracks observed are influenced by the location of the FIB milled surface trenches due to localized stress changes.

A comprehensive Monte Carlo calculation of dopant contrast in secondary-electron imaging

Two-dimensional dopant mapping using secondary electrons in a scanning electron microscope is a useful and rapid technique for studying dopant distributions with high spatial resolution in semiconductor materials and devices. However, it has not yet found widespread application because the quantification of dopant concentrations currently lacks a firm theoretical model. This paper addresses the issue by means of Monte Carlo modelling. We demonstrate that by taking account of the electron affinity in Monte Carlo simulations to calculate the secondary-electron emission from doped silicon the dopant contrast can be explained. This paper also provides a firm theoretical model about the existence of surface effects in dopant contrast imaging.