J. Van Landuyt

Holotomography: Quantitative phase tomography with micrometer resolution using hard synchrotron radiation x rays

Because the refractive index for hard x rays is slightly different from unity, the optical phase of a beam is affected by transmission through an object. Phase images can be obtained with extreme instrumental simplicity by simple propagation provided the beam is coherent. But, unlike absorption, the phase is not simply related to image brightness. A holographic reconstruction procedure combining images taken at different distances from the specimen was developed. It results in quantitative phase mapping and, through association with three-dimensional reconstruction, in holotomography, the complete three-dimensional mapping of the density in a sample. This tool in the characterization of materials at the micrometer scale is uniquely suited to samples with low absorption contrast and radiation-sensitive systems.

The study of carbon nanotubules produced by catalytic method

Abstract Catalytic methods for the production of carbon nanotubules have been developed based on the decomposition of acetylene on well-dispersed metal particles strongly adsorbed on a support. Cobalt on silica was found to be the best catalyst—support combination for the production of graphitic tubules. The method for the catalyst preparation and the reaction conditions were optimized. Straight and coiled carbon tubules were obtained with inner and outer diameter of 3–7 and 15–20 nm, respectively, and up to 300 μm in length. These nanotubules were not coated by amorphous carbon. Traces of amorphous carbon could be removed by hydrogen. High resolution electron microscopy images and electron diffraction patterns of the straight nanotubules were similar to those obtained by the arc-discharge method. Coiled nanotubules were revealed by TEM to be regular polygonized helices where the bends are caused by pairs of pentagon-heptagon carbon rings among the hexagonal network.

Carbon nano-tubes; their formation process and observation by electron microscopy

Abstract Carbon nano-tubes have been investigated thoroughly by transmission electron microscopy. Non-helical tubes are not exceptional and mixing with helical tubes with various helicities is often observed within the same tubule. The helical nature of the tubes is thought to be actually required by lattice continuity during the formation of successive seamless concentric tubes. Besides, introducing dislocations is also a possible accommodation mechanism for the strains produced between the successive tube surfaces. Reciprocal space constructions have been accomplished for explaining the diffraction effects for non-helical tubes as well as for helical ones. Well-resolved high-resolution images are obtained in particular for the non-helical tubes and the image features could be confirmed by computer simulations. A possible growth process for the successive concentric tubes is proposed.

The Texture of Catalytically Grown Coil-Shaped Carbon Nanotubules

The growth of micron-size carbon fibres from thermal decomposition of hydrocarbons catalyzed by a metal has been widely studied. Coil-shaped fibres often grow among straight or twisted filaments. Their internal structure has not been studied in detail as yet. In the present work, the thermal cracking of acetylene on Co nanoparticles dispersed on porous silica has produced relatively well graphitized hollow nanotubules, including straight filaments and regular helices. The small diameter of the coiled tubules and the absence of an amorphous coating allowed a determination of their texture by transmission electron microscopy (TEM). The coiled tubules consist of regularly polygonized, coaxial graphene tubes whose angular bends are aligned. The bends are probably caused by the occurrence along the helix of pairs of pentagon-heptagon carbon rings in the hexagonal network. Such a structure was recently predicted to be a thermodynamically stable topology for helical, single-sheet carbon tubes. A molecular model, consistent with theoretical predictions on how to connect cylindrical tubule segments, is provided.

Nanocrystalline diamond films : transmission electron microscopy and Raman spectroscopy characterization

Abstract The structure of fine-grained diamond films with crystallite size as small as 10–50 nm has been studied with cross-section transmission electron microscopy and Raman spectroscopy. The nanocrystalline films of 1 to 2 μm thickness were grown on Si substrates by d.c. plasma chemical vapor deposition in Ar/CH4/H2 gas mixtures, with methane concentration CH4/(H2+CH4) varied from 3 to 100%. The substrates were seeded with 5 nm diamond powder to enhance the nucleation density. Submicron thick nanocrystalline films were also grown as a first layer for successive growth of large-grain films. The films demonstrate a kind of columnar growth even for the case of grains with a size of a few tens of nanometers. The crystallites showed many imperfections, twins on (111) planes being the dominant defect type. The twin density is very high, but often they are only a few atomic layers wide. A buffer layer of β-SiC with a thickness up to 400 nm can be observed at the diamond/silicon interface. In addition to the diamond phase, Raman spectroscopy revealed disordered sp3 and sp2 carbon phases, presumably located at grain boundaries. High intrinsic stresses in the film prevent the observation of the size-induced low-frequency shift of the fundamental diamond peak at 1332 cm−1. The appearance of fingerprints of CN vibrations and distorted diamond in Raman spectra is discussed.

The structure of different phases of pure C70 crystals

Abstract Single crystal of pure C70 are grown from the vapour phase and the structure and morphology of these crystals is studied. By means of X-ray diffraction and TEM measurements five different phases are observed. The observed phases are (from high to low temperatures) fcc, rhombohedral, ideal hcp (c/a=1.63), deformed hcp (c/a=1.82) and a monoclinic phase. The occurrence of these different phases and the phase transitions is accounted for in a simple model. For the monoclinic structure a model for the stacking of the orientationally ordered molecules in the lattice is proposed. For both the hcp and fcc phases a Lennard-Jones type interaction potential is used to calculate bond strengths, lattice energies and the theoretical morphology.

3-D vacancy ordered superstructures in “homogeneous” YBa2Cu3O7-δ

Abstract High resolution electron microscopy and electron diffraction techniques were applied to study structural aspects of YBa 2 Cu 3 O 7- δ for varying δ, i.e., oxygen contents. Particular attention was paid to the superstructures arising from oxygen vacancy ordering, three types of specimens were investigated: constant stoichiometry, cooled vacuum annealed and microscope heated samples. The most stable vacancy ordered structure appears to be the orthorhombic II or 2 a 0 structure occurring in a wide range of compositions around δ =0.5. Several other superstructures were found in the electron beam heated and rapidly cooled samples which reveal apart from 2 a 0 , also 3 a 0 , 4 a 0 and 5 a 0 structures as well as 2 a 0 ×2 a 0 and 2√2 a 0 ×2√2 a 0 structures. These phases are presumably only transient. Attention is also paid to the interpretation of diffuse intensity lines in the diffraction patterns of beam heated and rapidly cooled samples which could be interpreted in terms of the cluster theory as being due to a transition state which can be considered as a precursor to the orthorhombic II structure. Crystallographic models are proposed for the various superstructures observed in this study as related to the oxygen contents. The order-disorder transition is found to occur in stages with increasing temperature. One observes a loss of correlation of the stacking of the 2-D patterns in successive layers, disordering of the chain separation, followed by disordering within the chains, leading finally to a random distribution of the remaining oxygen atoms in the CuO 1- δ layers.

Diffraction effects due to shear structures: A new method for determining the shear vector

Abstract A survey is given of the diffraction effects associated with the shear structures and a theoretical analysis is made of these effects. From the main results a method is deduced which enables a full crystallographic analysis of a shear structure from purely geometrical features of the diffraction pattern. The method is applied to shear structures associated with deviations from stoichiometry in rutile. The displacement vector associated with the shear process is determined.

Direct observation of laser-induced crystallization of a-C : H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm2. At fluences below 100 mJ/cm2 an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (≈400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm2. The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.

In situ transmission electron microscopy study of Ni silicide phases formed on (001) Si active lines

The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400 °C when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800 °C, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni2Si is never observed.The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400 °C when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800 °C, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni2Si is never observed.