The role of carbon precursor on carbon nanotube chirality in floating catalyst chemical vapour deposition.

We have studied the influence of different carbon precursors (methane, ethanol and toluene) on the type, diameter and chiral angle distributions of carbon nanotubes (CNTs) grown with the floating catalyst technique in a horizontal gas-flow reactor. Using electron diffraction to study their atomic structures, we found that ethanol and toluene precursors gave high single-wall CNT yields (92% and 89% respectively), with narrow diameter distributions: 1.1 nm to 1.7 nm (ethanol); 1.3 nm to 2.1 nm (toluene), with a propensity for armchair-type chiral angles. In contrast, methane-grown CNTs gave high double-wall CNT yields (75%) with broader diameter populations: 1.2 to 4.6 nm (inner CNT) and 2.2 to 5.3 nm (outer CNT) with a more uniform spread of chiral angles, but weakly peaked around 15 to 20 degrees. These observations agree with known growth models. However, double-wall CNTs grown with toluene showed an unusually narrow interlayer spacing of 0.286 ± 0.003 nm with suggestions of large, 20° to 25°, differences between inner and outer CNT chiral angles. Methane gave a large interlayer spacing (0.385 ± 0.002 nm) with suggestions of small 5° to 10° inter-tube chirality correlations.

High-angle triple-axis specimen holder for three-dimensional diffraction contrast imaging in transmission electron microscopy.

Electron tomography requires a wide angular range of specimen-tilt for a reliable three-dimensional (3D) reconstruction. Although specimen holders are commercially available for tomography, they have several limitations, including tilting capability in only one or two axes at most, e.g. tilt-rotate. For amorphous specimens, the image contrast depends on mass and thickness only and the single-tilt holder is adequate for most tomographic image acquisitions. On the other hand, for crystalline materials where image contrast is strongly dependent on diffraction conditions, current commercially available tomography holders are inadequate, because they lack tilt capability in all three orthogonal axes needed to maintain a constant diffraction condition over the whole tilt range. We have developed a high-angle triple-axis (HATA) tomography specimen holder capable of high-angle tilting for the primary horizontal axis with tilting capability in the other (orthogonal) horizontal and vertical axes. This allows the user to trim the specimen tilt to obtain the desired diffraction condition over the whole tilt range of the tomography series. To demonstrate its capabilities, we have used this triple-axis tomography holder with a dual-axis tilt series (the specimen was rotated by 90° ex-situ between series) to obtain tomographic reconstructions of dislocation arrangements in plastically deformed austenitic steel foils.

Atom probe tomography and transmission electron microscopy of a Mg-doped AlGaN/GaN superlattice.

The electronic characteristics of semiconductor-based devices are greatly affected by the local dopant atom distribution. In Mg-doped GaN, the clustering of dopants at structural defects has been widely reported, and can significantly affect p-type conductivity. We have studied a Mg-doped AlGaN/GaN superlattice using transmission electron microscopy (TEM) and atom probe tomography (APT). Pyramidal inversion domains were observed in the TEM and the compositional variations of the dopant atoms associated with those defects have been studied using APT. Rarely has APT been used to assess the compositional variations present due to structural defects in semiconductors. Here, TEM and APT are used in a complementary fashion, and the strengths and weaknesses of the two techniques are compared.

High-resolution three-dimensional imaging of dislocations.

Dislocations and their interactions govern the properties of many materials, ranging from work hardening in metals to device pathology in semiconductor laser diodes. However, conventional electron micrographs are simply two-dimensional projections of three-dimensional (3D) structures, and even stereo microscopy cannot reveal the true 3D complexity of defect structures. Here, we describe an electron tomographic method that yields 3D reconstructions of dislocation networks with a spatial resolution three orders of magnitude better than previous work. We illustrate the method's success with a study of dislocations in a GaN epilayer, where dislocation densities of 1010 per square centimeter are common.