B. Jouffrey

Long ropes of boron nitride nanotubes grown by a continuous laser heating

A continuous CO2 laser beam was focused on a surface of hexagonal boron nitride under low nitrogen pressure, inducing a stiff radial temperature gradient, rising locally over the dissociation temperature. Boron nitride nanotubes and faceted boron nitride onions were found on the heated surface, segregated in specific zones. Nanotubes were grown in macroscopic quantity, stoichiometric, well crystallized, mostly with few layers, and self-assembled in ropes as long as 40 μm. The cavity of boron nitride onions was often occupied by a boron nanocrystal. The recombination of boron and nitrogen both in plasma and on surface is suggested for the nucleation process.

The orientation dependent simulation of ELNES

We describe a program that allows the simulation of energy-loss near edge structure (ELNES). As an extension to the WIEN97 package (a full potential linearized augmented plane wave package for calculating crystal properties) [1] it permits to separate different contributions to the inelastic scattering cross section according to the character of the final state, explicitly taking into account projection onto scattering vector and integration over collection and convergence angle. Thus the program facilitates analysis of ELNES under precisely defined experimental conditions, and allows the investigation of anisotropic effects in ELNES from crystal structures. Dipole-allowed as well as dipole-forbidden transitions can be analyzed with this program.

High resolution electron microscopy study of Guinier-Preston (GP1) zones in AlCu based alloys

Abstract Guinier-Preston (GP1) zones in the model alloy Al-1.84 at. %Cu and in the industrial alloy Al1.68Cu1.10Mg0.33Mn at.% (Aa 2017A) were studied by means of high resolution electron microscopy. The experimental images are interpreted using multislice simulations. The majority of GP1 zones consist of atomic monolayers, the disc diameter ranging from 4 to 10 nm. In the binary alloy, double layer GP1 zones and unstable disc shaped particles on {111} planes were also found. The contrast of the GP1 zones is strongly dependent on their size, on the foil thickness, on the defocus of the objective lens, less sensitive to the Cu content and much less sensitive to the position of the GP zone in the depth of the foil.

The physical significance of the mixed dynamic form factor

We show that the mixed dynamic form factor for inelastic scattering of fast electrons in crystals is closely related to the density matrix of the probe electron and to that of the scatterer. With this insight it is possible to calculate both energy filtered diffraction patterns and energy spectroscopic high-resolution images. As an example we discuss the Si-K and -L edges.

Synthesis of SWNTs and MWNTs by a molten salt (NaCl) method

A method to obtain the single-walled and multi-walled carbon nanotubes is presented. It consists of electrolytic conversion of graphite to carbon nanotubes in fused NaCl at 810 °C. The experimental conditions are given. The filling of the nanotubes is also possible if elements are added in the salt solution.

Orientation dependence of ionization edges in EELS.

Anisotropy in the density of unoccupied states can be detected in the fine structure of ionization edges in angle-resolved EELS. It is shown that in a crystal an interference term occurs in the inelastic signal, and how it relates to electron channeling and site selection. The combination of orientation and site selection induces subtle variations in the ELNES. It is shown how this technique can be used to analyze local anisotropy related to the point group of the target atom. A second example shows how to extract non-dipole transitions at small scattering angles.

The mixed dynamic form factor for atomic core-level excitations in interferometric electron-energy-loss experiments

Abstract The mixed dynamic form factor (MDFF) is the basic quantity in inelastic electron scattering experiments. Although it is often dominated by the usual dynamic form factor, it can in principle be measured in interferometry-like inelastic electron scattering experiments which have gained increasing popularity. We therefore derive an expression for the MDFF for atomic core-level excitations and discuss it in detail. Simulations for silicon, using Hartree-Slater-like atomic wave functions, illustrate the results. Beyond the theoretical interest, two practical applications to the measurement of the MDFF are envisaged

The copper content of Guinier-Preston (GP1) zones in Al-1.84 at% Cu alloy

Abstract The (Guinier–Preston) GP1 zones were studied by means of high resolution electron microscopy and contrast simulations. In the 〈110〉 zone axis orientation, shearing of a GP1 zone by a dislocation has been observed. Different models of monolayer GP1 zones—containing 100, 80, 60, 50 and 40% Cu (balance of Al)—were used for the multislice simulations. According to the contrast analysis of throughfocus series of experimental images and other experimental data, the GP1 zones with the composition ranging from 40 to 100% Cu coexist. A simple model of a displacement field around the GP1 zone was used in simulations in order to interpret the atomic column displacement measured on some experimental micrographs.

Plasmons and Related Excitations

The Inelastic Scattering Cross-Section. There is a clear distinction between energy loss to inner-shell electrons and to valence or conduction electrons. The reason for the different treatment of these two processes is essentially that, in the former case, the initial state has a sharp energy while in the latter, there is a range of energy within the valence or conduction band. Orbitals of inner shell electrons show almost no overlap between neighbouring sites, so the exchange integral is negligibly small, leading to an extremely small bandwidth. The K-band in Na has a width of 2 × 10-19 eV, and a K-electron in Na jumps roughly once a week to a neighbouring site [3.1]. Those electrons — loyal to their atoms — are well described within an atomic model, which means that energy loss to inner-shell electrons can be treated within atomic theory. (Strictly speaking, this is true only for final states far above the Fermi level. When the excited inner shell electron, after interaction with the fast beam electron, occupies states slightly above the Fermi energy, the density of unoccupied states is mirrored as near edge structure, a typical solid state effect.)

Channeling, localization and the density matrix in inelastic electron scattering

Building on the relationship between the mixed dynamic form factor (MDFF) for inelastic electron scattering and the one-particle density matrices of the initial and final states of the scatterer it is shown that the MDFF contains information both on the spatial density and the spatial coherence of excitations. We discuss how the MDFF can-at least partly-be measured in scattering geometries invoking channeling conditions. Therefrom, the localization of inelastic events can be determined.