C. B. Carter

Growth and Sintering of Fullerene Nanotubes

Carbon nanotubes produced in arcs have been found to have the form of multiwalled fullerenes, at least over short lengths. Sintering of the tubes to each other is the predominant source of defects that limit the utility of these otherwise perfect fullerene structures. The use of a water-cooled copper cathode minimized such defects, permitting nanotubes longer than 40 micrometers to be attached to macroscopic electrodes and extracted from the bulk deposit. A detailed mechanism that features the high electric field at (and field-emission from) open nanotube tips exposed to the arc plasma, and consequent positive feedback effects from the neutral gas and plasma, is proposed for tube growth in such arcs.

Superhard silicon nanospheres

Abstract Successful deposition and mechanical probing of nearly spherical, defect-free silicon nanospheres has been accomplished. The results show silicon at this length scale to be up to four times harder than bulk silicon. Detailed measurements of plasticity evolution and the corresponding hardening response in normally brittle silicon is possible in these small volumes. Based upon a proposed length scale related to the size of nanospheres in the 20– 50 nm radii range, a prediction of observed hardnesses in the range of 20– 50 GPa is made. The ramifications of this to computational materials science studies on identical volumes are discussed.

The stacking-fault energy of nickel

Abstract The weak-beam method of electron microscopy has been used to study dis sociated edge dislocations and faulted dipoles in nickel. Micrographs have been compared with many-beam computed image profiles to deduce values for the dimensions of the defects, and these results are then used in conjunction with anisotropic elasticity theory to obtain the stacking-fault energy, γ, or an upper limit, γmax. The interpretation of these results is discussed and it is concluded that the true value of γ is most likely to lie in the range γ ∼120–130 mJ m−2.

On the stacking-fault energies of copper alloys

Abstract The stacking-fault energies of a number of copper alloys with electron-atom ratios less than or equal to 1·10 have been measured from observations of single dissociated dislocations imaged...

Air-stable full-visible-spectrum emission from silicon nanocrystals synthesized by an all-gas-phase plasma approach

A novel dual-plasma system has been developed to combine the synthesis of silicon nanocrystals (Si-NCs), the etching to controllably tailor the Si-NC size, and the surface functionalization of Si-NCs into one simple all-gas-phase process. Si-NCs are synthesized in SiH(4)-based plasma; they then travel through CF(4)-based plasma, where Si-NCs are etched and passivated by C and F. The resulting Si-NCs exhibit air-stable emission across the full visible spectrum. Structural and optical characterization indicates that the emission in the red-to-green range is based on the recombination of quantum-confined excitons in Si-NCs, while the blue emission originates from defect states. The quantum yields of stabilized photoluminescence from Si-NCs range from 16% at the red end to 1% at the blue end.

Steps and the structure of the (0001) α-alumina surface

Abstract The initial stages of facet formation on the (0001) α-alumina surface are explored through annealing vicinal single crystals for different lengths of time and characterizing the surface with atomic force microscopy. Faceting of the (0001) surface begins with the formation of 0.2 nm or ∼ c 6 high steps ( c = 1.3 nm). These steps show a preference for existing as pairs; the pairs then bunch together and form facets which are typically multiples of c in height. In this fashion, single facets and faceted domains initially exist together on the same surface. A faceted terrace-and-step morphology develops as the facet domains coalesce with longer annealing times.

Defect structure and intermixing of ion-implanted AlxGa1−xAs/GaAs superlattices

Cross‐sectional transmission electron microscopy and Raman spectroscopy have been used to study the defect structure and intermixing of annealed ion‐implanted Al0.3Ga0.7As/GaAs superlattices. The results show clearly that the amount and depth of superlattice layer intermixing depends on the ion mass. In superlattices that retain their structure after implantation and annealing, the distribution of defect clusters (primarily interstitial loops) is inhomogeneous; most defect clusters are nucleated in the GaAs layers. Examination of unannealed superlattice samples reveals that ion beam damage occurs preferentially in the GaAs layers.

Direct TEM determination of intrinsic and extrinsic stacking fault energies of silicon

Abstract The intrinsic and extrinsic stacking fault energies of silicon have been determined from images of double ribbons obtained using the weak–beam method of electron microscopy. The ribbons occurred in distorted regions of small–angle twist boundaries on {111} planes prepared by welding. The results are compared with values obtained from isolated dislocations in the screw and edge orientation in the same sample, which were found to be consistently lower than the values obtained from double ribbons. It is found that, contrary to other recent work, the ratio γMin γMex is actually only ∼ 14% greater than unity.

The identification of thin amorphous films at grain-boundaries in Al2O3

The presence of amorphous grain-boundary phases in ceramic materials can significantly influence their properties. Such grain-boundary films can be identified by the dark-field diffuse scattering technique, the Fresnel fringe technique, and analytical electron microscopy (energy-dispersive spectroscopy). However, spectrum artefacts can present major problems for the use of such techniques. Specifically, grain-boundary grooving, surface damage of the specimen and silicon contamination are shown experimentally to arise from ion-milling during the preparation of TEM specimens. It is experimentally shown that, with the above techniques, these artefacts can cause grain-boundaries in commercial alumina specimens to appear to contain glassy phases. The ambiguity in interpreting the results from the use of each of these techniques is discussed in detail.

Antiphase boundaries in GaAs

Antiphase boundaries in GaAs have been produced by growing the GaAs on {001} Ge substrates. The GaAs was grown by the technique of organometallic vapor phase epitaxy to a thickness in excess of 1 μm. The antiphase boundaries are shown to be faceted with facets parallel to the {110} planes being particularly common. The rigid‐body translation at the different facet planes is shown to be small for the {110} planes but it can be large for other facet planes.