Insights in an eggshell

When living organisms produce crystalline materials, biological macromolecules have a key role in controlling crystal size, orientation and polymorph type. The complexities of this process lead to the formation of impossibly perfect objects, such as eggshells or pearls. Researchers in biomimetics, who seek to transfer the technology of the natural world into the engineering world, have analysed the amino acid sequence of eggshell-specific proteins looking for a relationship between their structure, the self-aggregation behaviour in solution, and the influence on calcite growth and habit modification (Angewandte Chemie International Edition http://dx.doi.org/10.1002/anie.200500261). Having identified a critical role in repetitions of (adjacent) charged amino acids, they designed seven slightly different peptides (which are short portions of proteins) with a sequence that resembles that of avian eggshell matrix proteins. Their experiments point to the rigidity of the peptide conformation and the ability to form aggregates as important factors for achieving crystals similar to those obtained with the parent protein.

The price of disorder

When materials are doped with non-stoichiometric atoms for example, to enhance the electric conductivity in semiconductors the random incorporation of the dopants in the crystal can lead to undesired modifications of the materials properties. High-temperature superconductors are commonly doped with oxygen to induce and modify their superconducting behaviour. By combining different imaging techniques that are selective for either oxygen dopants or superconducting properties of the material, Séamus Davis and colleagues (Science http://dx.doi.org/10.1126/science.1113095) observed a correlation between the location of the oxygen dopants within the crystal, and the variation in the strength of the superconducting state across the sample. Their findings suggest that the superconductor gap is predominantly increased in the vicinity of oxygen dopants. Furthermore, this non-uniformity is mirrored across the whole sample, where the superconductor electronic state showed a disorder similar to the random oxygen distribution. Although it may turn out to be difficult to achieve a more homogeneous oxygen distribution, and thus more uniform superconducting properties, these measurements identify the probable cause for the much-speculated electronic disorder in high-temperature superconductors.

Dynamic devices

High-resolution electron microscopy has traditionally been viewed as a static structural characterization tool, rather than a means of studying the dynamic properties of nanoscale materials and devices. However, placing a scanning tunnelling microscope (STM) probe inside a high-resolution transmission electron microscope (HRTEM) has recently enabled researchers to simultaneously perform atomic-scale imaging and electrical transport measurements. At the Microscopy and Microanalysis conference in Hawaii (31 July – 4 August 2005), John Cummings presented TEM studies of the mechanical and electrical properties of carbon nanotube devices (Microscopy and Analysis supplement 1; 2005). Cummings and colleagues have also used electron holography to study the electric-field distribution in nanotube field-emission devices, and are now developing techniques to fabricate nanoscale electronic devices on the surface of commercially available silicon nitride membranes, which could allow the investigation of a large variety of nanoscale systems to be studied during operation. The researchers are also expanding the analytical capabilities available inside a TEM to include thermal imaging, which would allow the investigation of the local generation and dissipation of heat during device operation.

Exciting enantioselective reactions

Photoinduced electron transfer (PET) is an essential step in the conversion of solar energy into chemical energy and can be used to synthesize complex organic molecules. During this process, light absorption generates molecules in excited electronic states susceptible to donating or accepting electrons, but the involvement of excited states makes it difficult to control the nature of the reaction products. Now Bauer and colleagues describe in Nature a catalytic PET reaction that proceeds with high yield and enantioselectivity (A. Bauer, F. Westkämper, S. Grimme & T. Bach Nature 436, 1139–1140; 2005). Using an electron-accepting organocatalyst that enforces a chiral environment on the substrate through hydrogen bonding, they obtain a reaction product consisting of up to 70% excess of one enantiomer with yields up to 64%. Mechanistic details of this PET-catalysed reaction have not been fully elucidated, but the concept of chirality multiplication by a hydrogen-bonded catalyst should be widely applicable to other photochemical reactions, and for general asymmetric synthesis.

Birth of a nanotube

Credit: Copyright (2005) APS

Chemical vapour deposition is a popular method for producing carbon nanotubes. Metallic nanoparticles attached to an appropriate substrate are exposed to a flow of gaseous hydrocarbons, from which carbonaceous deposits are formed. Previous studies into the development of nanotubes have either been static in nature or have introduced preformed structures — such as half-formed C60 molecules — that entice the onset of tube formation. Results of molecular dynamics simulations are presented by Jean-Yves Raty and co-workers (Physical Review Letters http://dx.doi.org/10.1103/PhysRevLett.95.096103) that show exactly how the nanotube structures 'self-assemble' from individual carbon atoms on the metallic nanoparticle catalysts. Raty and colleagues study two extreme initial conditions. In one case, individual carbon atoms are spaced evenly over the surface of the catalyst. In a second simulation, a partially formed diamond lattice is initially attached to the metal particle. In both cases, the carbon atoms diffuse across the surface of the catalyst until they bond with other carbon atoms. Once the surface of the metal is covered with a graphene sheet, the carbon structure begins to rise from the catalyst, forming the carbon nanotube from the bottom up.