Towards atomic resolution EELS of anisotropic materials

Multivariate statistical analysis of electron energy-loss spectroscopy in anisotropic materials

Recently, an expression has been developed to take into account the complex dependence of the fine structure in core-level electron energy-loss spectroscopy (EELS) in anisotropic materials on specimen orientation and spectral collection conditions [Y. Sun, J. Yuan, Phys. Rev. B 71 (2005) 125109]. One application of this expression is the development of a phenomenological theory of magic-angle electron energy-loss spectroscopy (MAEELS), which can be used to extract the isotropically averaged spectral information for materials with arbitrary anisotropy. Here we use this expression to extract not only the isotropically averaged spectral information, but also the anisotropic spectral components, without the restriction of MAEELS. The application is based on a multivariate statistical analysis of core-level EELS for anisotropic materials. To demonstrate the applicability of this approach, we have conducted a study on a set of carbon K-edge spectra of multi-wall carbon nanotube (MWCNT) acquired with energy-loss spectroscopic profiling (ELSP) technique and successfully extracted both the averaged and dichroic spectral components of the wrapped graphite-like sheets. Our result shows that this can be a practical alternative to MAEELS for the study of electronic structure of anisotropic materials, in particular for those nanostructures made of layered materials.

Energy loss spectroscopic profiling across linear interfaces: the example of amorphous carbon superlattices

Energy loss spectroscopic profiling is a way to acquire, in parallel, spectroscopic information across a linear feature of interest, using a Gatan imaging filter (GIF) fitted to a transmission electron microscope (TEM). This technique is capable of translating the high spatial resolution of a bright field image into a sampling of the spectral information with similar resolution. Here we evaluate the contributions of chromatic aberration and the various acquisition parameters to the spatial sampling resolution of the spectral information, and show that this can reach 0.5 nm, in a system not ordinarily capable of forming electron probes smaller than 2 nm. We use this high spatial sampling resolution to study the plasmon energy variation across amorphous carbon superlattices, in order to extract information about their structure and electronic properties. By modelling the interaction of the relativistic incident electrons with a dielectric layer sandwiched between outer layers, we show that, due to the screening of the interfaces and at increased collection angles, the plasmon energy in the sandwiched layer can still be identified for layer thicknesses down to 5 A. This allows us to measure the change in the well bandgap as a function of well width and to interpret it in terms of the changes in the sp2 -fractions due to the deposition method, as measured from the carbon K-edges, and in terms of quantum confinement of the well wavefunction by the adjacent barriers.

Improved comparison of low energy loss spectra with band structure calculations: the example of BN filaments

Electron energy loss spectra have been recorded from BN filaments for energy losses between 2 and 50eV. They compare well with other BN shapes (nanotubes, sheets, etc). The interpretation of all the peaks in the spectra has been made via an ab initio calculation using the FLAPW code WIEN97 (A full potential linearized augmened plane wave package for calculating crystal properties, Karlheinz Schwarz, Techn. Universität Wien, Austria, 1999. ISBN 3-9501031-0-4). In order to fully simulate the observed spectra, the geometry particular to the EELS experiment has been taken into account making use of Wessjohann's relativistic formula for thin anisotropic slabs (Phys. Stat. Sol. B 77 (1976) 535). Furthermore, the effect of the convergent beam has been introduced into the simulation and has been proven to be an important parameter in obtaining the corresponding peak intensities.

Investigation of atomic structures of diamond-like amorphous carbon by electron energy loss spectroscopy

Research into amorphous carbon films has been developed to such an extent that the film property can be fine tuned to mimic that of the crystalline counterparts, be it diamond, graphite, or even fullerene-like. This flexibility makes such films ideal for a wide range of applications from anti-abrasive window coating to lubricating layers on the surface of magnetic hard-disk. Not only are their mechanical properties interesting, electrically the diamond-like amorphous carbon films are also easier to dope than crystalline diamond, making them potentially a better alternative to amorphous silicon for photovoltaic devices. We will show that electron energy loss spectroscopy, in particular the carbon 1s core absorption spectroscopy, has been instrumental in revealing the nature of the bonding between carbon atoms. Such information allows microstructure models to be developed for proper understanding of the observed properties and providing scientific basis for future improvement.