Erik Dujardin

Production, properties and potential of graphene

This review on graphene, a one-atom thick, two-dimensional sheet of carbon atoms, starts with a general description of the graphene electronic structure as well as a basic experimental toolkit for identifying and handling this material. Owing to the versatility of graphene properties and projected applications, several production techniques are summarized, ranging from the mechanical exfoliation of high-quality graphene to the direct growth on carbides or metal substrates and from the chemical routes using graphene oxide to the newly developed approach at the molecular level. The most promising and appealing properties of graphene are summarized from an exponentially growing literature, with a particular attention to matching production methods to characteristics and to applications. In particular, we report on the high carrier mobility value in suspended and annealed samples for electronic devices, on the thickness-dependent optical transparency and, in the mechanical section, on the high robustness and full integration of graphene in sensing device applications. Finally, we emphasize on the high potential of graphene not only as a post-silicon materials for CMOS device application but more ambitiously as a platform for post-CMOS molecular architecture in electronic information processing.

Growth and form of gold nanorods prepared by seed-mediated, surfactant-directed synthesis

Gold nanorods were prepared via a seed-mediated sequential growth process involving the use of citrate-stabilised seed crystals and their subsequent growth in a series of reaction solutions containing [AuCl4]−, ascorbic acid and the cationic surfactant cetyltrimethylammonuim bromide (CTAB). Electron diffraction analysis and HRTEM images of mature nanorods showed superpositions of two specific pairs of crystallographic zones, either and or and , which were consistent with a cyclic penta-twinned crystal with five {111} twin boundaries arranged radially to the [110] direction of elongation. The nanorods have an idealised 3-D prismatic morphology with ten {111} end faces and five {100} or {110} side faces, or both. TEM studies of crystals at various stages of growth indicated that the seed crystals are initially transformed by growth and aggregation into decahedral penta-twinned crystals, 4% of which become elongated when a fresh reaction solution is added, whilst the remaining twins grow isometrically. Reiteration of this procedure increases the length of the existing nanorods, induces further transformation of isometric particles to produce a second (and third) population of shorter, wider nanorods, and increases the size of the isometric crystals. The data indicate that symmetry breaking in fcc metallic structures to produce anisotropic nanoparticles is based on an intrinsic structural mechanism (twinning) that is subsequently modulated extrinsically during growth in solution by specific adsorption of AuI–surfactant complexes on the side faces/edges of the isometric penta-twinned crystals and which is responsible for the preferential growth along the common [110] axis. We propose that the coupling of multiple twinning and habit modification is a general mechanism that applies to other experimental procedures (electrochemical, inverse micellar media) currently used to prepare metallic nanoparticles with a high aspect ratio.

Graphitic cones and the nucleation of curved carbon surfaces

The nucleation and growth of curved carbon structures, such asfullerenes, nanotubes and soot, are still not well understood. Avariety of models have been proposed, and it seems clear that the occurrence of pentagons, which yield 60° disclination defects in the hexagonal graphitic network, is a key element in the puzzle. The problem of nucleation has been complicated by the great variety of structures observed in any one sample. Here we report an unusual carbon sample generated by pyrolysis of hydrocarbons, consisting entirely of graphitic microstructures with total disclinations that are multiples of +60°. The disclination of each structure corresponds to the presence of a given number of pentagons in the seed from which it grew: disks (no pentagons), five types of cones (one to five pentagons), of which only one was known previously, and open tubes (six pentagons). Statistical analysis of these domains shows some unexpected features, which suggest that entropy plays a dominant role in the formation of disclinations. Furthermore, the total disclination of a domain is determined mainly at the nucleation stage.

Bio-inspired Materials Chemistry

Progress reports are a new type of article in Advanced Materials dealing with the hottest current topics, and providing renders with a critically selected overview of important progress in these fields. It is not intendedthat the articles be comprehensive, but rather insightful, selective, critical, opinionated, and even visionary. We have approached scientists we believe are at the very forefront of these fields to contribute the articles, which will appear on an annual basis. The article below describes the latest advances in bio-insplred materials chemistry.

Few-layer graphene on SiC, pyrolitic graphite, and graphene: A Raman scattering study

To show the similarities between exfoliated graphene and epitaxial few layer graphite (FLG) layers, we present micro-Raman scattering measurements on three different graphite-based materials: micro-structured Highly Oriented Pyrolytic Graphite (HOPG) disks with heights in the 20-2 nm range, exfoliated graphene monolayer, and FLG epitaxially grown on carbon terminated 4H-silicon carbide (4H-SiC) substrates. We show that despite the fact the FLG layers are composed of many layers, the band structure of FLG epitaxially grown on 4H-SiC substrate must be composed of simple electronic bands as witnessed by a single component, Lorentzian shaped, double resonance Raman feature.

Synthesis of mesoporous silica by sol–gel mineralisation of cellulose nanorod nematic suspensions

Mesoporous silica was synthesised by sol–gel mineralisation using nematic liquid crystalline templates consisting of partially ordered suspensions of cellulose rod-like nanocrystals, ca. 145 × 13 nm in size. The nanorods were prepared by acid hydrolysis of cellulose powder and concentrated droplets evaporated onto glass slides to form nematic liquid crystals. Addition of an aqueous alkaline solution of pre-hydrolysed tetramethoxysilane to the droplets resulted in a birefringent cellulose–silica composite that was subsequently calcined at 400 °C for 2 h. Removal of the cellulose nanorod template produced a birefringent silica replica that exhibited patterned mesoporosity due to the presence of co-aligned cylindrical pores, approximately 15 nm in diameter and 10 nm in wall thickness. TEM studies suggest that a chiral imprint of the helically ordered cellulose nanorods was imposed on the silica structure, although further studies are required to confirm these preliminary observations. As cellulose nanorods can be prepared from renewable, inexpensive sources, they offer a cost-effective, environmentally benign route to the template-directed synthesis of mesoporous materials.

Structure and Oxidation Patterns of Carbon Nanotubes

We discuss the oxidation of carbon nanotubes and how it is affected by structure and geometry. While graphite is known to oxidize primarily at defects to create etch pits, nanotubes have additional structural features such as high curvature, helicity, and contain five and seven membered rings which modify the initiation and propagation of oxidation. Oxidation does not necessarily start at the tip of the tubes, and there are pronounced differential oxidation rates between layers which depend on the helicity of the individual shells.

Self-assembled switches based on electroactuated multiwalled nanotubes

A fabrication process for nanoelectromechanical systems (NEMS) based on multiwalled carbon nanotubes (CNTs) suspended across metallic trenches is described. The process is versatile and allows the production of CNT-NEMS with singly or doubly clamped nanotubes at an adjustable height above a bottom electrode. When a voltage bias is applied between the nanotube and the bottom electrode, the devices act as very efficient electrical switches. Surface functionalization of the bottom electrode with a self-assembled monolayer is implemented to improve the switching reversibility. Moreover, it opens opportunities to use these CNT-NEMS as a vertical cross-bar junction for molecular electronics studies.

DNA-driven self-assembly of gold nanorods

Specific organization of gold nanorods into anisotropic 3D-aggregates is obtained by DNA hybridisation.

Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes

We analytically and numerically analyze the fluorescence decay rate of a quantum emitter placed in the vicinity of a spherical metallic particle of mesoscopic size (i.e with dimensions comparable to the emission wavelength). We discuss the efficiency of the radiative decay rate and non-radiative coupling to the particle as well as their distance dependence. The electromagnetic coupling mechanisms between the emitter and the particle are investigated by analyzing the role of the plasmon modes and their nature (dipole, multipole or interface mode). We demonstrate that near-field coupling can be expressed in a simple form verifying the optical theorem for each particle modes.