Bragg Interference Research Articles

Quantitative criteria for the detection and characterization of nanocrystals from high-resolution electron microscopy images

Abstract A new method of high-resolution electron microscopy image analysis is presented which improves considerably the visibility of nanocrystals in amorphous materials. The technique is based on individual Bragg filtering of the lattice fringes in the image. The significant feature is that the amplitude and phase of the fringes as functions of position are calculated separately. This is similar to a holographic reconstruction with the Bragg interference representing the carrier wave. The phase information is shown to be particularly robust with respect to noise due to amorphous material. The images so produced are in a form suitable for statistical analysis, and criteria for judging quantitatively the extent of the crystal are proposed. The application of the technique to a wide range of similar problems concerning partially ordered materials is discussed.

Angular dependence of electronically allowed nuclear bragg reflection of Mössbauer radiation

A typical angular integrated Mossbauer reflection spectrum of an electronically allowed nuclear Bragg reflection was analysed. Mossbauer reflection spectra were measured with high angular resolution. They reveal almost pure nuclear scattering far off Bragg and strong nuclear-electronic interference in the scattering near and at Bragg. The spectra show a pronounced asymmetry with respect to the Bragg position, which is caused by refraction and by interference of nuclear and electronic scattering.

Surface oxidation and reduction of small platinum particles observed by in-situ X-ray diffraction

A new method is presented, in which the constructive Bragg interference intensity of small metallic clusters during the formation or decay of a surface compound is measured by an in situ X-ray technique. First results for a series of platinum catalysts supported on silica gel show intensity losses on the 111-peak of up to 40% under oxygen following a standard pretreatment. These losses are restored reversibly under hydrogen. The average thickness of the disrupted surface layer ranges between 0.9 to 4.0 A depending on the treatment temperature and platinum crystallite size.

X-Ray Interference Measurements of Ultrathin Semiconductor Layers

AbstractWe present various x-ray diffraction phenomena from semiconductor hetero-epitaxial layers. Each of these phenomena gives useful information on the layers. Knowing what to look for in the x-ray rocking curve (XRC) can make this nondestructive technique a very powerful tool for characterization of a few A-several g.tm thick layers We discuss the use of individual Bragg peak, diffraction fringe, and interference structure to obtain layer information. We particularly emphasize the use of x-ray interference in studying buried strained quantum well or quantum barrier layers. We present experimental rocking curves of an AlGaAs/GaAs double heterojunction laser structure and GaInAs/GaAs strained layer superlattices in both <001> and <111> orientations.

Indirect determination of the incoherent scattering in disordered carbon materials

An indirect method of determining the intensity of the incoherent X-ray scattering component of the total scattering intensity from a disordered carbon material is presented. It utilizes the incoherent scattering from a crystalline carbon sample (standard) measured at a diffraction angle 2θ far from a Bragg interference maximum, and requires only knowledge of the linear absorption coefficient μ and density ρ of the material. The results obtained for angles 2θ in the range 100–160° show that the direct and indirect determinations of the incoherent scattering are in excellent agreement. The indirect method takes a simple form when the disordered and crystalline carbon sample thicknesses, t, are optimized either for small-angle scattering, μt = 1, or for wide-range diffraction, exp [−μt/sin θ] ≃ 0.

On the interpretation of Total Current Spectroscopy (TCS) spectra from MoS2crystals

Abstract Total Current Spectroscopy (TCS) spectra from MoS2 (0001) face for three different angles of incidence of the primary beam with respect to the c-axis as well as TCS spectrum from an edge surface cut perpendicularly to the (0001) face of a molybdenite crystal are given. Energy positions of the TCS structure are found to be independent of the variations in the angle of incidence of the primary beam and also of the change of crystal face. From this it is concluded that the fine structure in the TCS spectra from molybdenite crystal for the primary energies studied is due to electron-electron scattering and not to Bragg interference effects.