A. J. Harvey

Formation of N- containing C-nanotubes and nanofibres by carbon resistive heating under high nitrogen pressure

Abstract A variety of nanocarbons of different shapes and compositions (C, C–N) has been formed using a modified high isostatic pressure (HIP) apparatus. It was found that the presence of nitrogen in the background gas increases evaporation of graphite in the hot zone of the heater and increases the yield of carbon nanostructures. Carbon deposits formed by different gas pressures and temperatures were characterised using transmission electron microscopy and electron energy loss analytical microscopy.

Assessment of electron energy-loss spectroscopy below 5 eV in semiconductor materials in a VG STEM

Abstract The possibilities of obtaining information about interband scattering processes from electron energy-loss spectra, taken in a VG601UX scanning transmission microscope, are investigated. With the help of precise simulations of the zero-loss peak it is feasible to process, extract and analyse data in the electron volt regime. The accuracy of the results is restricted predominantly by instrumental limitations. It is possible to extract band gaps of > 2 eV (e.g. for GaN) correctly. In the case of band gaps between 1 and 2eV (i.e. in GaAs), the uncertainty is larger, since the tail of the zero-loss peak interferes with the interband losses, and further refinements in the data analysis as well as the measurement techniques are discussed. ‘Band-gap mapping’ in the VG601UX can be carried out with a spatial resolution of a few nanometres.

Transmission electron microscopy studies of nanofibers formed on Fe7C3-carbide

Abstract Carbon nanofibers have been formed on iron details of the high isostatic pressure (HIP) apparatus. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques proved the existence of faceted catalyst particles found in the middle part of fibers and allowed identification of their lattice and composition as the pseudo-hexagonal crystalline lattice of Fe7C3. Two systems of arc-reflections with 0.335-nm spacing were ascribed to two different orientations of graphite layers and a trace analysis showed that the growth direction of the fiber coincides with the [210]-direction of pseudo-hexagonal Fe7C3-lattice. This direction is compared with directions of fibers formed on bcc-Fe and Fe3C.

Electron energy loss studies of dislocations in GaN thin films

We present studies of the effects of dislocations in epitaxial GaN films on the low loss and the core loss electron energy loss spectrum. Electron microscopy samples were prepared in cross-sectional and plan-view geometry and investigations carried out in a dedicated cold field emission scanning transmission electron microscope. We consider the relative dislocation signal strength in both orientations on grounds of geometric considerations, and from comparison with the experimental signal, deduce that the scattering cross section for bulk and dislocation related scattering in the core loss energy regime are similar. The low loss results suggest that the scattering cross sections for dislocation related scattering in the band-gap regime are significantly smaller than for bulk scattering processes, also the localization of scattering events in this energy regime is an order of magnitude less than in core loss spectroscopy. The experimental low loss spectra provide evidence for dislocation related energy sta...

SUPPRESSION OF WAVY GROWTH IN METALORGANIC VAPOR PHASE EPITAXY GROWN GAINAS/INP SUPERLATTICES

The effects of growth temperature, V–III partial pressure ratio in the gas phase, and the nature of the carrier gas on the morphology of lattice matched InGaAs/InP multiple quantum well stacks are investigated. Preliminary results of an extension of this study to strained InGaAs/InP systems are presented.

UV-Vis plasmon studies of metal nanoparticles

Previous studies of the surface plasmon resonance (SPR) from alloy nanoparticles of gold and silver have focused on bulk studies of the nanoparticles in solution. Here, we present a selection of data observed from individual alloy nanoparticles using highly spatially resolved electron energy-loss spectroscopy (EELS). In order to resolve the SPR in both the pure gold and Au-Ag alloy nanoparticles, we successfully applied the Richardson-Lucy deconvolution routine. We found that resonances observed from different sized alloy nanoparticles do not follow the same trends in frequency variation as their pure metal equivalent. Additionally, we do not consistently observe a single SPR peak, as one might expect from an alloy nanoparticle. This indicates that alloy nanoparticles, produced by the simultaneous reduction of metal salts, cannot be considered truly homogeneous.

Dislocation-induced electronic states and point-defect atmospheres evidenced by electron energy loss imaging

Electronic band gap states connected with individual dislocations in diamond and GaN are revealed, using highly spatially resolved electron energy loss (EEL) spectrum mapping. Comparison with calculations of low EEL spectra from first-principle methods allows the identification of the joint density of states of different dislocation core types. Also presented is evidence for instances where point defects/impurities have accumulated in the strain field or segregated to the core of dislocations.

Nucleation of wavy growth modes in quantum well stacks of III-V compound alloys

Abstract The influence of strain and temperature on the development of 3D island growth patterns (wavy layer growth) and the establishment of a defined modulation period in epitaxially grown multiple quantum well structures of the III–V compound alloys is discussed. Experimental evidence for a previously derived theoretical roughening transition curve, which separates 2D monolayer from 3D island growth is presented. In this paper we furthermore derive the f2 ∼ λ−1 relationship between misfit strain f and modulation wavelength λ, and give experimental evidence. Ultra-high resolution scanning transmission electron microscopy (STEM) analysis is performed in the above alloys to investigate the occurrence of alloy inhomogeneities. In parallel, highly dislocated regions in the multiple quantum well (MQW) stacks are systematically studied by transmission electron microscopy (TEM). The latter two investigations are undertaken in order to establish possible nucleation sites and to better understand the nucleation process for the 3D growth mode.

Temperature and strain dependence of the roughening transition in III‐V semiconductor and SiGe epitaxial growth

Experimental and theoretical evidence is given for the occurrence of a temperature and strain dependent roughening transition from two dimensional (2‐D) monolayer to 3‐D island growth in strained III‐V compound ternary alloys and GeSi. For sufficiently large strain energy values the shape of the transition curve was found to follow a T∼e−2f relationship, as predicted from classical nucleation theory arguments, where T is the growth temperature and ef the areal misfit strain energy. The asymptotic behavior in the zero strain energy regime could be reproduced by an empirical curve of a more complex expression. The transition curve appears to separate routes of strain relief in the above systems, which were found to predominantly follow 3‐D island formation in the higher, and misfit dislocation formation in the lower growth temperature/strain regime.

Electron microscopy and electron energy loss spectroscopy studies of carbon fiber formation at Fe catalysts

Nanofibers were produced by the chemical vapor deposition method in the presence of Fe-catalyst particles at temperatures of 700–850 K by disproportionation of CO. Electron diffraction and high resolution electron imaging as well as electron energy loss spectroscopy and x-ray analysis techniques were used to study the nanofiber formation in different places at the catalyst surface. The particles enclosed by the fibers were found to be Fe7C3 and Fe3C. Crystallographic relationships between deposit and particle were established. The structural properties of the deposit were found to be dependent on the position at the particle surface. Graphitic growth was favored at certain facets. At the particle tip graphene sheet formation competed with Fe2O3 oxide formation. We also report the occurrence of a low loss feature between 3 and 4 eV, concurrent with the dispersion of an interface plasmon at the graphite/Fe2O3 interface.