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.

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.

Plasmon spectroscopy of the thinnest possible membrane, a single layer of carbon atoms: graphene, has been carried out in conjunction with ab initio calculations of the low loss function. We observe and +-surface plasmon modes in free-standing single sheets at 4.7 and 14.6 eV, which are substantially redshifted from their values in graphite. These modes are in very good agreement with the theoretical spectra, which find theand + in-plane modes of graphene at 4.8 and 14.5 eV. We also find that there is little loss caused by out-of-plane modes for energies less than about 10 eV.

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 p ! p* transition and the p 1 s 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.Recent measurements of the remarkable electronic properties of graphene have resulted in intense research activity on twodimensional (2D) crystals [1][2][3][4][5] . Unlike most materials in condensed matter physics, where the Schrödinger equation can be used to describe their electronic properties, for graphene the charge carriers mimic relativistic particles and can thus be described using the Dirac equation 3 . The ability of extended 2D structures to exist is the subject of a long-standing theoretical debate, and it has previously been suggested that 2D films embedded in three-dimensional (3D) space can be stabilized by out-of-plane undulations 6,7 . Elucidating the atomic structure of graphene may seem blindingly obvious at first consideration, but, given that it is necessarily an 'imperfect' 2D crystal, it offers insight in three important ways. First, direct imaging of atoms combined with energy-loss spectroscopy provides further corroboration of the existence of areas of free-standing monolayers of carbon atoms. Second, revealing the atomic structure of the edges of graphene and the fundamental topological defects within adds insight to the stability issues, as does the characterization of the surface contamination believed to consist mainly of hydrocarbons ubiquitously found on graphene. This last point may also provide clues as to certain limitations in the electronic behaviour of graphene films.Evidence of the existence of free-standing graphene has been obtained from electron diffraction experiments 3 , which, in this case, was averaged over approximately a square micrometre of material. Recently, others have presented defect configurations in suspended graphene using bright-field phase contrast 8 . The appearance of atomic structure in phase contrast in the case of 3D crystals is not immediately interpretable, and even in 2D crystals, is sensitive to focusing conditions. However, the atomic lattice seen in high-angle annular dark-field (HAADF) images acquired in a scanning transmission electron microscope (STEM)...