J. Murray Gibson

We used Fluctuation Electron Microscopy (FEM) to measure the correlation length of bond ordering on the nanoscale in atactic polystyrene samples of molecular weight of 350 kg/mol. After annealing, the bond correlations became more pronounced and moved to a shorter length scale of ∼15Å, within the range of agreement with small-angle neutron scattering estimates of persistence length. We also observe on a very local scale (<1 μm) a degree of liquid crystallinity that has not been reported previously in averaged diffraction measurements, most likely because the material is globally, but not locally, isotropic. The method we used, which relies on the sensitivity of FEM to bond–bond correlations, provides a new measure of bond persistence in a polymer, which can be accomplished on nanoscale volumes, particularly important for understanding the structure and properties of nanostructured polymers and polymer thin films and interfaces.

Anomalously low values of the normalised variance in fluctuation electron microscopy (FEM) have been frequently reported. We present three experimental corrections for quantitative interpretation that significantly modify conventional approaches. FEM relies on measurements of intensity statistics in coherent nanodiffraction patterns. We demonstrate that sampling the nanodiffraction patterns with a pixelated detector removes high‐frequency signals and reduces statistical variance. The most significant impact is on the background normalised variance, which arises from random atomic alignments and is distinct from the normalised variance peaks associated with the correlated alignments of medium‐range order. Indeed, we show that if the peaks are background‐subtracted, their height is much less affected by the detector effect, provided the experimental conditions are optimised. We show that shot noise correction must also be adjusted to account for the camera Modulation Transfer Function (MTF) effects. Additionally, we demonstrate through experiment that the traditional method of thickness correction for a‐Si is inadequate and propose an alternative approach to address thickness variations. We speculate on the origin of the anomalous thickness effect in terms of displacement decoherence due to sample ‘fluttering’ under irradiation.

Self-ion implantation amorphization is an established approach to study the structure and properties of amorphous silicon (a-Si). Fluctuation electron microscopy has consistently observed Medium-Range Order (MRO) in this system that is not consistent with the Continuous Random Network (CRN) model. Using this technique, we find that the degree of MRO first increases on thermal annealing and then decreases before finally recrystallizing. We discuss this new result in the light of previous experimental studies and recent theoretical observations on the favorability of the paracrystalline (PC) model over the CRN in a-Si. At ion doses far above the minimum required to amorphize, a high defect density is found in the PC phase, which anneals out at 500 °C. The PC structure after 500 °C annealing is independent of the initial implantation conditions and appears to represent a metastable and highly ordered structure. Higher-temperature annealing causes a reduction in the degree of MRO and the structure approaches but does not reach a fully CRN before eventually crystallizing above 600 °C. The effect of high-dose implantation is to increase the defect density in the as-implanted state, and the annealing of these defects is likely responsible for the large characteristic heat evolution at low temperature.

This paper presents a kinematical scattering model that describes the variance of the speckle intensity observed in dark-field images of amorphous materials. A simple expression is given for the limiting case of uncorrelated atoms, that takes into account partial spatial coherence of the electron illumination.

Wide bandgap III-nitride semiconductors have emerged as highly promising materials for the fabrication of optoelectronic devices operating in the blue and UV regions of the electromagnetic spectrum. Using a novel UHV TEM with in-situ MBE the early stages of the growth of AIN on (0001) α-Al₂O₃ substrates has been studied. Growth of device quality GaN on sapphire substrates typically involves nitridation of the substrate followed by the deposition of an AIN buffer layer. We present an overview of our instrument and some results from a preliminary investigation of the sapphire nitridation process.

We have examined carbon-supported PtRu₅ specimens by a transmission electron microscopy, including a novel STEM-based mass-spectroscopic technique, which gives the approximate number of atoms of individual nano-sized clusters. The combination of these techniques demonstrate that the PtRu₅ clusters are oblate on the carbon support.

The development of effective new tools for structural characterization of disordered materials and systems is becoming increasingly important as such tools provide the key to understanding, and ultimately controlling, their properties. The relatively novel technique of correlograph analysis (i.e., the approach of calculating angular autocorrelations within diffraction patterns) promises unique advantages for probing the local symmetries of disordered structures. Because correlograph analysis examines a component of the high-order four-body correlation function, it is more sensitive to medium-range ordering than conventional diffraction methods. As a follow-up of our previous publication, where we studied thin samples of sputtered amorphous silicon, we describe here the practical experimental method and common systematic errors of electron correlograph analysis. Using both experimental data and numerical simulations, we demonstrate that reliable structural information about the sample can only be extracted from the mean correlograph averaged over a sufficient number of individual results.

Consideration of atomic ordering beyond just pairs of atoms shows that amorphous silicon is better modeled as paracrystalline material than as a disordered network.

Unlike flat sheets, crumpled paper balls have both high free volume and high compressive strength, and can tightly pack without significantly reducing the area of accessible surface. Such properties would be highly desirable for sheet-like materials such as graphene, since they tend to aggregate in solution and restack in the solid state, making their properties highly dependent on the material processing history. Here we report the synthesis of crumpled graphene balls by capillary compression in rapidly evaporating aerosol droplets. The crumpled particles are stabilized by locally folded, π–π stacked ridges as a result of plastic deformation, and do not unfold or collapse during common processing steps. In addition, they are remarkably aggregation-resistant in either solution or solid state, and remain largely intact and redispersible after chemical treatments, wet processing, annealing, and even pelletizing at high pressure. For example, upon compression at 55 MPa, the regular flat graphene sheets turn into nondispersible chunks with drastically reduced surface area by 84%, while the crumpled graphene particles can still maintain 45% of their original surface area and remain readily dispersible in common solvents. Therefore, crumpled particles could help to standardize graphene-based materials by delivering more stable properties such as high surface area and solution processability regardless of material processing history. This should greatly benefit applications using bulk quantities of graphene, such as in energy storage or conversion devices. As a proof of concept, we demonstrate that microbial fuel electrodes modified by the crumpled particles indeed outperform those modified with their flat counterparts.