Andrew Bleloch

High-resolution detection of Au catalyst atoms in Si nanowires

The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

Free-standing graphene at atomic resolution

Research interest in graphene, a two-dimensional crystal consisting of a single atomic plane of carbon atoms, has been driven by its extraordinary properties, including charge carriers that mimic ultra-relativistic elementary particles. Moreover, graphene exhibits ballistic electron transport on the submicrometre scale, even at room temperature, which has allowed the demonstration of graphene-based field-effect transistors and the observation of a room-temperature quantum Hall effect. Here we confirm the presence of free-standing, single-layer graphene with directly interpretable atomic-resolution imaging combined with the spatially resolved study of both the pi --> pi* transition and the pi + sigma plasmon. We also present atomic-scale observations of the morphology of free-standing graphene and explore the role of microstructural peculiarities that affect the stability of the sheets. We also follow the evolution and interaction of point defects and suggest a mechanism by which they form ring defects.

Macroscopic graphene membranes and their extraordinary stiffness.

The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal, and other characteristics of this one-atom-thick material. Here, we describe a new and highly reliable approach for making graphene membranes of a macroscopic size (currently up to 100 microm in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by only one side do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.

Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

Biochemical and Structural Insights into Bacterial Organelle Form and Biogenesis

Many heterotrophic bacteria have the ability to make polyhedral structures containing metabolic enzymes that are bounded by a unilamellar protein shell (metabolosomes or enterosomes). These bacterial organelles contain enzymes associated with a specific metabolic process (e.g. 1,2-propanediol or ethanolamine utilization). We show that the 21 gene regulon specifying the pdu organelle and propanediol utilization enzymes from Citrobacter freundii is fully functional when cloned in Escherichia coli, both producing metabolosomes and allowing propanediol utilization. Genetic manipulation of the level of specific shell proteins resulted in the formation of aberrantly shaped metabolosomes, providing evidence for their involvement as delimiting entities in the organelle. This is the first demonstration of complete recombinant metabolosome activity transferred in a single step and supports phylogenetic evidence that the pdu genes are readily horizontally transmissible. One of the predicted shell proteins (PduT) was found to have a novel Fe-S center formed between four protein subunits. The recombinant model will facilitate future experiments establishing the structure and assembly of these multiprotein assemblages and their fate when the specific metabolic function is no longer required.

3D morphology of the human hepatic ferritin mineral core: New evidence for a subunit structure revealed by single particle analysis of HAADF-STEM images

Ferritin, the major iron storage protein, has dual functions; it sequesters redox activity of intracellular iron and facilitates iron turn-over. Here we present high angle annular dark field (HAADF) images from individual hepatic ferritin cores within tissue sections, these images were obtained using spherical aberration corrected scanning transmission electron microscopy (STEM) under controlled electron fluence. HAADF images of the cores suggest a cubic morphology and a polycrystalline (ferrihydrite) subunit structure that is not evident in equivalent bright field images. By calibrating contrast levels in the HAADF images using quantitative electron energy loss spectroscopy, we have estimated the absolute iron content in any one core, and produced a three dimensional reconstruction of the average core morphology. The core is composed of up to eight subunits, consistent with the eight channels in the protein shell that deliver iron to the central cavity. We find no evidence of a crystallographic orientation relationship between core subunits. Our results confirm that the ferritin protein shell acts as a template for core morphology and within the core, small (∼2 nm), surface-disordered ferrihydrite subunits connect to leave a low density centre and a high surface area that would allow rapid turn-over of iron in biological systems.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking

We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.

Direct electron transfer to a metalloenzyme redox center coordinated to a monolayer-protected cluster.

A strategy for establishing electrical contact to the metal center of a redox metalloenzyme, galactose oxidase (GOase), by coordination of a linker attached to a monolayer-protected gold cluster is presented. The cluster-enzyme hybrid system was first prepared in solution and characterized by high-angle annular dark-field scanning transmission electron microscopy. Electrochemical communication between a gold electrode and GOase was achieved by first modifying the electrode surface with a biphenyl dithiol self-assembled monolayer followed by reaction with gold clusters capped with thioctic acid. GOase was then immobilized by replacement of the H(2)O molecule at the Cu(II) exogenous site by coordination of a carboxylate-terminated gold cluster. This chemical attachment ensured electrical contact between the redox center and the electrode, leading to direct mediatorless electron transfer to the protein. Hybrid systems can find applications in biosensors and biofuel cells and for studying electrochemically the catalytic mechanism of reactions for which free radicals and electron-transfer reactions are involved. The present results can be extended to other metalloenzymes.

Direct measurement of composition of buried quantum dots using aberration-corrected scanning transmission electron microscopy

The authors present a direct method to quantitatively measure the indium composition of buried InAs quantum dots embedded in a GaAs matrix. In this method, spatially resolved electron-energy-loss spectroscopy combined with aberration-corrected scanning transmission electron microscopy at atomic resolution was employed to measure compositional profiles across the center of a quantum dot and the adjacent wetting layer. The size and shape of the quantum dots were determined using the Z contrast in high angle annular dark field images. A substantial enrichment in indium at the top of the quantum dots was identified, which is consistent with theoretical predictions.

Modified mesoporous silicate MCM-41 materials: immobilised perruthenate—a new highly active heterogeneous oxidation catalyst for clean organic synthesis using molecular oxygen

A new oxidation catalyst containing perruthenate immobilised within MCM-41 has been prepared and used in the clean oxidation of alcohols to carbonyl compounds with molecular oxygen.