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Biomedical subjects

B Jouffrey

Publications and source records attributed to B Jouffrey.

18 recordsLinked to original sources
PubMed2006

ELNES at magic angle conditions.

If one needs to cancel the effects of the anisotropy of the sample in a EELS experiment in the TEM, a particular couple of values for the collection and convergence angle must be used, called magic angle conditions (MAC). Recent developments in the theory have shown that a full relativistic treatment is mandatory to correctly describe this effect and that the MAC are strongly dependent on the acceleration voltage. We show how the analytical formula can be derived and give the exact analytical solution for the MAC which can then be easily applied to every practical case. We show the consequences of the energy dependence of the magic angle and that the parallelity of the beam will be the limiting factor for high acceleration voltages while for low acceleration voltages the contribution coming from Bragg spots may make it impossible to reach MAC.

Journal Article↗
PubMed2006

Polarization dependence in ELNES: influence of probe convergence, collector aperture and electron beam incidence angle.

The differential scattering cross section in electron energy loss near edge spectroscopy (ELNES) generally depends on the orientation of the Q wave vector transferred from the incident electron to an atomic core electron. In the case where the excited atom belongs to a threefold, fourfold or sixfold main rotation axis, the dipole cross section depends on the angle of Q with respect to this axis. In this paper, we restrict to this situation called dichroism. Furthermore, if we take into account the relativistic effects due to the high incident electron velocity, this dipole cross section also depends on the angle of Q with respect to the electron beam axis. It is due to these dependences that the shape of measured electron energy loss spectra varies with the electron beam incidence, the collector aperture, the incident beam convergence and the incident electron energy. The existence of a particular beam incidence angle for which the scattering cross section becomes independent of collection and beam convergence semi-angles is clearly underscored. Conversely, it is shown that EELS spectra do not depend on the beam incidence angle for a set of particular values of collection and convergence semi-angles. Particularly, in the case of a parallel incident beam, there is a collection semi-angle (often called magic angle) for which the cross section becomes independent of the beam orientation. This magic angle depends on the incident beam kinetic energy. If the incident electron velocity V is small compared with the light velocity c, this magic angle is about 3.975theta(E) (theta(E) is the scattering angle). It decreases to 0 when V approaches c. These results are illustrated in the case of the K boron edge in the boron nitride.

Journal Article↗
PubMed2004

HREM, FIM and tomographic atom probe investigation of Guinier-Preston zones in an Al-1.54at% Cu alloy.

An investigation of Guinier-Preston zones in Al-1.54at% Cu alloy annealed for 30h at 100 degrees C was carried out on the same monocrystal with complementary techniques of high-resolution electron microscopy (HREM) and tomographic atom probe-field ion microscopy (TAP-FIM). HREM results show that majority of GP1 zones are monolayers 1-9nm in size. However, some GP2 zones and particles in an intermediate state of growth between GP1 and GP2 stage were also found. From TAP results it follows that GP1 zones with different Cu concentrations ranging from 40% up to 100% Cu coexist. The residual solid solution is very heterogeneous. In the vicinity of GP particles the Cu content in the matrix falls down to zero, the solid solution in other regions contains from 0.7 to 1at% Cu.

Journal Article↗
PubMed2004

The magic angle: a solved mystery.

We resolve the long-standing mysterious discrepancy between the experimental magic angle in EELS--approximately 2theta(E)--and the quantum mechanical prediction of approximately 4theta(E). A relativistic approach surpassing the usually applied kinematic correction yields a magic angle close to the experimental value. The reason is that the relativistic correction of the inelastic scattering cross section in anisotropic systems is significantly higher than in isotropic ones.

Algorithms↗
PubMed2003

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.

Journal Article↗
PubMed2002

The separation of surface and bulk contributions in ELNES spectra.

We present a method to separate surface from volume contributions in the fine structure of ionization edges in electron energy loss spectrometry (ELNES). It is based on spectra taken at two positions with different surface-to-volume ratio. Contrary to the similar spatial difference method it uses well defined scaling factors, allowing an estimate of the errors propagated into the result.

Journal Article↗
PubMed2002

Separation of pure elemental and oxygen influenced signal in ELNES.

The energy loss near edge structure (ELNES) of many elements is strongly influenced by the presence of oxygen or other elements at surfaces, grain boundaries, or in the bulk material. The presented investigation deals mainly with the influence of oxygen at the surface. A method for the separation of both, the pure bulk signal and the oxidized surface signal, was evaluated and tested on Al, Cu, Mg, and Si. A comparison of experimental data with ab initio bandstructure calculations and other proofs of the accuracy of ELNES separation are presented. Influences of error propagations were tested and are exemplarily given for Al and Si.

Journal Article↗
PubMed2001

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.

Journal Article↗
PubMed2000

Observation of ionization in a crystal interferometer

We present a new interferometric setup where interference of a fast probe electron affects the ionization cross section of an atom. Interference is detected in the intensity of the inelastically scattered electrons at the Bragg scattering angle in transmission. The crystal serves both as a target for core ionization and as a beam-splitting and phase-shifting device.

Journal Article↗
PubMed2000

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.

Journal Article↗
PubMed2000

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.

Journal Article↗
PubMed1992

Degradation, amorphization, and recrystallization of ion bombarded Si(111) surfaces studied by in situ reflection electron microscopy and reflection high energy electron diffraction techniques.

In this paper we report the effect of noble gas ions bombardment on the degradation of atomically flat Si(111) surfaces at room and high (400 degrees C-600 degrees C) temperatures. Reflection high energy electron diffraction (RHEED) and reflection electron microscopy (REM) have been used to characterize the topography and structure of the as-implanted and post annealed surface layers. It is shown that the fading of the specularly reflected beam is not directly related to the amorphization of the surface. This experimental study has also evidenced the difficulties one meets to regrow a defect-free material after amorphization by noble gas bombardment. For high temperature for which the amorphization is not possible, the surface loses its stepped structure and turns into a monocrystalline but atomically rough surface. This roughness is a function of substrate temperature.

Argon↗
PubMed1976

Kodirex for high resolution electron microscopy.

Measurement of the important performance parameters shows that Kodak Kodirex film is more suitable than conventional ones for dark-field transmission electron microscopy of molecules at very high mignification. Results are cited for 120 kV; but the relation ship is valid up to 3 MV.

Merbromin↗