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We report on spatially resolved electron energy-loss spectroscopy studies of optical modes in individual star-shaped gold nanoparticles. We studied different morphologies, ranging from a spheroid to well-developed nanostars. For each shape, essentially two groups of modes are appearing: the first one is localized around the core of the nanostars and has an energy slightly less than the quasi-static dipolar mode of a gold sphere (about 2.2 eV); in the second group, the modes are localized at the end of the nanostar tips, with varying energies depending on the geometry of each tip and with energy down to 1.2 eV. The localization of the tip modes is interpreted with the help of boundary element methods simulations.
Using electron energy loss spectroscopy in a 100 kV VG
scanning transmission electron microscope we study nitrogen doped carbon
nanotubes grown via electron cyclotron resonance (ECR) microwave plasma
techniques. The process is controlled by direct current (dc) biasing the
grid separating the ECR source and the substrate. We show that plasma
induced sputtering of the ECR source wall (stainless steel) can lead to
significant iron and chromium contamination of growth samples. We identify
various Fe, Cr, Ni nitride phases, and propose a growth model based on
nitridation-induced metal segregation of steel based FeCrN alloys. Trace Cr
doping of nanotube catalysts appears a promising route for introducing large
nitrogen concentrations into both single and multi-walled nanotubes and may
accelerate nanotube growth rates.
The effect of annealing in diluted oxygen on the structural characteristics of thin silicon dioxide layers with embedded Si nanocrystals fabricated by ultra-low energy ion implantation (1 keV) is reported. The nanocrystal characteristics (size, density, coverage) have been measured by spatially resolved Electron Energy Loss Spectroscopy using the spectrum-imaging mode of a Scanning Transmission Electron Microscope. Their evolution has been studied as a function of the annealing duration under N2+O2 at 900°C. An extended spherical Deal-Grove model for the self-limiting oxidation of embedded silicon nanocrystals has been carried out. It shows that stress effects, due to the deformation of the oxide, slows down the chemical oxidation rate and leads to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results.
We use High Resolution Electron Microscopy (HRTEM)
together with Electron Energy Loss Spectroscopy (EELS) to analyze
the crystallography and the chemical configuration of interfaces
in a state-of-the-art
tunnel junction. EELS indicates that manganese ions keep their
bulk valency up to the last atomic plane in contact with the
insulator. Tunnel magnetoresistance however decreases with
temperature faster than magnetisation in these samples.
Quantitative HRTEM reveals some local departures from chemical
abruptness at the interfaces, which could play a role in this
Scalability and performance of current flash memories can be improved substantially by novel devices based on Multi-Dot Floating Gate MOSFETs. The multi-dot layer in the very thin gate oxide can be fabricated CMOS-compatibly by ion beam synthesis (IBS). Here, we present both experimental and theoretical studies on IBS of multi-dot layers consisting of Si nanocrystals (NCs). The NCs are produced by ultra low energy Si+ ion implantation, which causes a high Si supersaturation in the shallow implantation region. During post-implantation annealing, this supersaturation leads to phase separation of the excess Si from the SiO2. Till now, the study of this phase separation suffered from the weak z contrast between Si and SiO2 phases in Transmission Electron Microscopy (TEM). Here, this imaging problem is solved by Energy Filtered Scanning Transmission Electron Microscopy (EFSTEM). Additionally, kinetic lattice Monte Carlo simulations of Si phase separation have been performed and compared with EFSTEM images. It has been predicted theoretically that the morphology of the multi-dot Si floating gate changes with increasing ion fluence from isolated, spherical NCs to percolated spinodal Si pattern. These patterns agree remarkably with EFSTEM images. However, the predicted fluence for spinodal pattern is lower than the experimental one. Because oxidants of the ambient atmosphere penetrate into the as-implanted SiO2, a substantial fraction of the implanted Si is lost due to oxidation.
We use High Resolution Electron Microscopy together with Electron Energy Loss Spectroscopy to analyze the crystallography and the chemical configuration of a Co/SrTiO3 interface in a Co/SrTiO3/La2/3Sr1/3MnO3 magnetic tunnel junction.
EELS fine structures on characteristic core edges are closely related to the bonding type and structural environment around the excited atoms. Consequently, they constitute useful hints for investigating the electronic properties at interfaces. One major difficulty then lies in the extraction of the information relevant to the involved atoms, from that due to neighbouring atoms on both sides of the selected interface seen end on by the impinging electrons. We have investigated possible ways to reach this goal, using the spectrum-line method, which consists in acquiring sequences of spectra at well defined increments (0.3 nm) while scanning a subnanometer electron probe (0.7 nm) across the interface. This approach constitutes one step forward with respect to the spatial-difference (SD) method which processes spectra acquired on two adjacent areas, one encompassing the interface, the other one on a reference area nearby. A normalized version (NSD) has been introduced to extend its field of application from grain boundaries to heterophase interfaces in order to enhance their specific ELNES contribution