J. C. Girard

On slip band features and crack initiation in fatigued 316L austenitic stainless steel: Part 1: Analysis by electron back-scattered diffraction and atomic force microscopy

Electron back-scattered diffraction (EBSD) and atomic force microscopy (AFM) have been used to study surface slip features on 316L austenitic stainless steel polycrystals tested in the low cycle fatigue range. EBSD investigations allow activated slip planes to be identified for each grain and the local inclination of these slip planes to the surface to be calculated. AFM allows the height of steps induced at the surface along slip bands to be measured and the local morphology of extrusions to be characterized at a nanometer scale. In this study, both techniques are used on the same surface in order to combine crystallographic and topographic information. Based on the results, a schematic model of the slip band emergence is proposed.

Structure and properties of carbon onion layers deposited onto various substrates

120 keV carbon ions implantations at high fluences (0.5–8×1017 ions cm−2) were performed at elevated temperature (⩾500 °C) in silver layers deposited on various substrates (Si (100), 304 L stainless steel, and pure fused silica). Spherical carbon onions (3–15 nm in diameter) were so produced in the silver layers. A pure carbon onion thin film deposited on the substrate was obtained after annealing in vacuum. Atomic force microscopy and high-resolution transmission electron microscopy experiments were performed to characterize the structure of the thin films. Optical transmittance spectra of carbon onion layers deposited onto silica substrates revealed two absorption peaks centered at 220–230 nm and at 265 nm that were attributed to the presence of carbon onions and residual disordered graphitic carbon, respectively. Tribological experiments performed on silver–carbon onions composite thin films revealed that the friction coefficient is close to that of a pure silver film (0.2) but with much better wear be...

STM study of the nucleation and annealing of ion bombardment induced defects on Cu(100)

The morphology of the (100) face of copper after sputtering with 600 eV Ar+ ions has been investigated by scanning tunneling microscopy (STM) as a function of ion beam flux and fluence. This process generates vacancy and adatom islands bounded by monatomic steps. These islands exhibit their equilibrium shape, which can be described as a square with rounded corners. At low flux, vacancy as well as adatom islands coexist and the removal mode is three-dimensional, whereas at high flux, adatom islands disappear and the removal mode becomes two-dimensional, that is layer-by-layer. Moreover, the observation of distinct vacancy or adatom islands depleted zones in the vicinity of pre-existing monatomic steps demonstrates that the kinetics of adsorption of vacancies or adatoms on a step depends on the side by which they reach the edge. The energy barriers which control these processes also affect the interlayer mass transport. These observations can be related in a consistent way if the height of these barriers is assumed to decrease for steps bounding small-sized islands.

Study of the mechanical properties of ceramic materials by the nanoindentation technique

A nanoindentation device has been developed allowing the application of forces ranging between a few micronewtons and 100 milinewtons with nanoindentation capabilities. The apparatus offers also very accurate positioning in three directions through the use of a high resolution tridirectional piezoelectric actuator. One of the main interests of the device is to allow fast and precise measurements in normal laboratory environment, making nanoindentation testing as simple to use as conventional hardness ones. After a short presentation of the device and its working principle and of some of the methods used to derive the mechanical properties of materials, some results concerning the nanoscale plasticity in some ceramic materials are presented. Special features as unloading pop-out in silicon, associated to a reversible densification process, and pop-in or yield points during loading, in GaAs and MgO, are pointed out.

Flower-Shaped Domains and Wrinkles in Trilayer Epitaxial Graphene on Silicon Carbide

Trilayer graphene is of particular interest to the 2D materials community because of its unique tunable electronic structure. However, to date, there is a lack of fundamental understanding of the properties of epitaxial trilayer graphene on silicon carbide. Here, following successful synthesis of large-area uniform trilayer graphene, atomic force microscopy (AFM) showed that the trilayer graphene on 6H-SiC(0001) was uniform over a large scale. Additionally, distinct defects, identified as flower-shaped domains and isolated wrinkle structures, were observed randomly on the surface using scanning tunneling microscopy and spectroscopy (STM/STS). These carbon nanostructures formed during growth, has different structural and electronic properties when compared with the adjacent flat regions of the graphene. Finally, using low temperature STM/STS at 4K, we found that the isolated wrinkles showed an irreversible rotational motion between two 60° configurations at different densities of states.

Evidence for Flat Bands near the Fermi Level in Epitaxial Rhombohedral Multilayer Graphene

The stacking order of multilayer graphene has a profound influence on its electronic properties. In particular, it has been predicted that a rhombohedral stacking sequence displays a very flat conducting surface state: the longer the sequence, the flatter the band. In such a flat band, the role of electron-electron correlation is enhanced, possibly resulting in high Tc superconductivity, magnetic order, or charge density wave order. Here we demonstrate that rhombohedral multilayers are easily obtained by epitaxial growth on 3C-SiC(111) on a 2° off-axis 6H-SiC(0001). The resulting samples contain rhombohedral sequences of five layers on 70% of the surface. We confirm the presence of the flat band at the Fermi level by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy, in close agreement with the predictions of density functional theory calculations.

Atomic force microscopy imaging of dried or living bacteria

Atomic force microscopy (AFM) was used to obtain micrographs of dried bacteria in air, and of living ones in their culture medium. Images of dried bacteria were very similar to images obtained elsewhere by the much more complicated cryoetching preparation technique for transmission electron microscopy. Living bacteria were immobilized on a poly-L-lysine film, and directly observed in their culture medium at a resolution unattainable by any other technique applicable to living material. The images were similar to those obtained in scanning electron microscopy where the specimen must be fixed, dried and coated with conductive material, and as a result, no longer viable.

Structured illumination fluorescence microscopy with distorted excitations using a filtered blind-SIM algorithm.

Structured illumination microscopy (SIM) is a powerful technique for obtaining super-resolved fluorescence maps of samples, but it is very sensitive to aberrations or misalignments affecting the excitation patterns. Here, we present a reconstruction algorithm that is able to process SIM data even if the illuminations are strongly distorted. The approach is an extension of the recent blind-SIM technique, which reconstructs simultaneously the sample and the excitation patterns without a priori information on the latter. Our algorithm was checked on synthetic and experimental data using distorted and nondistorted illuminations. The reconstructions were similar to that obtained by up-to-date SIM methods when the illuminations were periodic and remained artifact-free when the illuminations were strongly distorted.

Plasticity study of deformed materials by in situ atomic force microscopy

The emerging of a single dislocation at a surface creates a step with a height equal to the Burgers vector component normal to the surface. By coupling an atomic force microscope with a tensile tester, the fine slip line structure can be analyzed. This equipment is particularly suitable to follow the course of plastic flow from the emergence of the first few dislocations from bulk crystal to the stages of work hardening. The motivations, instrumentation and results on LiF single crystal are described.

Study by atomic force microscopy of elementary deformation mechanisms involved in low load indentations in MgO crystals

Abstract Nanoindentations have been performed at very low load in (001) MgO single crystals. The surface deformation has been investigated by atomic force microscopy, and the rosette arms pattern, induced by the glide of only a few dislocations, has been observed. Moreover, surrounding steps, oriented along the ⟨100⟩ directions, have been identified and associated with the early stage of the formation of the square shaped slip lines pattern commonly observed in the case of microindentation performed under very high loads. The fine structure of these surrounding steps has been resolved clearly, and a mechanism has been proposed based upon interaction between dislocations belonging, respectively, to edge and screw rosette arms and upon the activation of a ⟨011⟩(211) secondary glide system.