Nanometre-scale evidence for interfacial dissolution-reprecipitation control of silicate glass corrosion.

Silicate glasses are durable solids, and yet they are chemically unstable in contact with aqueous fluids-this has important implications for numerous industrial applications related to the corrosion resistance of glasses, or the biogeochemical weathering of volcanic glasses in seawater. The aqueous dissolution of synthetic and natural glasses results in the formation of a hydrated, cation-depleted near-surface alteration zone and, depending on alteration conditions, secondary crystalline phases on the surface. The long-standing accepted model of glass corrosion is based on diffusion-coupled hydration and selective cation release, producing a surface-altered zone. However, using a combination of advanced atomic-resolution analytical techniques, our data for the first time reveal that the structural and chemical interface between the pristine glass and altered zone is always extremely sharp, with gradients in the nanometre to sub-nanometre range. These findings support a new corrosion mechanism, interfacial dissolution-reprecipitation. Moreover, they also highlight the importance of using analytical methods with very high spatial and mass resolution for deciphering the nanometre-scale processes controlling corrosion. Our findings provide evidence that interfacial dissolution-reprecipitation may be a universal reaction mechanism that controls both silicate glass corrosion and mineral weathering.

The origin of long-period lattice spacings observed in iron-carbide nanowires encapsulated by multiwall carbon nanotubes.

Structures comprising single-crystal, iron-carbon-based nanowires encapsulated by multiwall carbon nanotubes self-organize on inert substrates exposed to the products of ferrocene pyrolysis at high temperature. The most commonly observed encapsulated phases are Fe3C, α-Fe, and γ-Fe. The observation of anomalously long-period lattice spacings in these nanowires has caused confusion since reflections from lattice spacings of ≥ 0.4 nm are kinematically forbidden for Fe3C, most of the rarely observed, less stable carbides, α-Fe, and g-Fe. Through high-resolution electron microscopy, selective area electron diffraction, and electron energy loss spectroscopy we demonstrate that the observed long-period lattice spacings of 0.49, 0.66, and 0.44 nm correspond to reflections from the (100), (010), and (001) planes of orthorhombic Fe3C (space group Pnma). Observation of these forbidden reflections results from dynamic scattering of the incident beam as first observed in bulk Fe3C crystals.With small amounts of beam tilt these reflections can have significant intensities for crystals containing glide planes such as Fe3C with space groups Pnma or Pbmn.

NaYF4:Pr3+ nanocrystals displaying photon cascade emission.

The quantum-cutting process is observed in nanocrystalline fluoride (NaYF(4)) doped with Pr(3+). The thermal decomposition synthesis method was used to synthesize the NaYF(4) nanocrystals with an average size of 42 nm. The morphological high resolution transmission electron microscopy (HRTEM), structural X-ray diffraction (XRD) and spectroscopy, with the use of synchrotron radiation, have been employed to characterize the material. The different excitation energies created different luminescence response of the NaYF(4):Pr(3+) nanocrystals. The quantum-cutting phenomenon has been observed under the direct excitation of the 4f5d bands of praseodymium. After excitation of the NaYF(4) matrix this process is quenched and part of the energy is released through the self-trapped excitons emission. The origin of the different types of emission transitions has been analyzed in detail.

Electron image series reconstruction of twin interfaces in InP superlattice nanowires.

The twin interface structure in twinning superlattice InP nanowires with zincblende structure has been investigated using electron exit wavefunction restoration from focal series images recorded on an aberration-corrected transmission electron microscope. By comparing the exit wavefunction phase with simulations from model structures, it was possible to determine the twin structure to be the ortho type with preserved In-P bonding order across the interface. The bending of the thin nanowires away from the intended 110 axis could be estimated locally from the calculated diffraction pattern, and this parameter was successfully taken into account in the simulations.

Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions.

We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water-surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with approximately 3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over approximately 6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelectron spectroscopy shows evidence of a small oxide population. Individual graphene flakes can be deposited onto mica by spray coating, allowing statistical analysis of flake size and thickness. Vacuum filtered films are reasonably conductive and are semitransparent. Further improvements may result in the development of cheap transparent conductors.

Electrical properties of self-assembled branched InAs nanowire junctions.

We investigate electrical properties of self-assembled branched InAs nanowires. The branched nanowires are catalytically grown using chemical beam epitaxy, and three-terminal nanoelectronic devices are fabricated from the branched nanowires using electron-beam lithography. We demonstrate that, in difference from conventional macroscopic junctions, the fabricated self-assembled nanowire junction devices exhibit tunable nonlinear electrical characteristics and a signature of ballistic electron transport. As an example of applications, we demonstrate that the self-assembled three-terminal nanowire junctions can be used to implement the functions of frequency mixing, multiplication, and phase-difference detection of input electrical signals at room temperature. Our results suggest a wide range of potential applications of branched semiconductor nanostructures in nanoelectronics.

Position-controlled interconnected InAs nanowire networks.

We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or "nanotrees". A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.

Au-free epitaxial growth of InAs nanowires.

III-V nanowires have been fabricated by metal-organic vapor-phase epitaxy without using Au or other metal particles as a catalyst. Instead, prior to growth, a thin SiOx layer is deposited on the substrates. Wires form on various III-V substrates as well as on Si. They are nontapered in thickness and exhibit a hexagonal cross-section. From high-resolution X-ray diffraction, the epitaxial relation between wires and substrates is demonstrated and their crystal structure is determined.

Structural properties of <111>B -oriented III-V nanowires.

Controlled growth of nanowires is an important, emerging research field with many applications in, for example, electronics, photonics, and life sciences. Nanowires of zinc blende crystal structure, grown in the <111>B direction, which is the favoured direction of growth, usually have a large number of twin-plane defects. Such defects limit the performance of optoelectronic nanowire-based devices. To investigate this defect formation, we examine GaP nanowires grown by metal-organic vapour-phase epitaxy. We show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets, resulting in a microfaceting of the nanowires. We discuss these findings in a nucleation context, where we present an idea on how the twin planes form. This investigation contributes to the understanding of defect formation in nanowires. One future prospect of such knowledge is to determine strategies on how to control the crystallinity of nanowires.

Growth and optical properties of strained GaAs-GaxIn 1-x P core-shell nanowires.

We have synthesized GaAs-Ga(x)In(1-x)P (0.34 < x < 0.69) core-shell nanowires by metal-organic vapor phase epitaxy. The nanowire core was grown Au-catalyzed at a low temperature (450 degrees C) where only little growth takes place on the side facets. The shell was added by growth at a higher temperature (600 degrees C), where the kinetic hindrance of the side facet growth is overcome. Photoluminescence measurements on individual nanowires at 5 K showed that the emission efficiency increased by 2 to 3 orders of magnitude compared to uncapped samples. Strain effects on the band gap of lattice mismatched core-shell nanowires were studied and confirmed by calculations based on deformation potential theory.