CDW in FeCl4−—Graphite Intercalation Compounds Studied by STM

Samples of graphite electrochemically intercalated with FeCl4 − in CH3NO2 solution are studied by scanning tunneling microscopy (STM) from a submicrometer to atomic scale. A new long range triangular modulation of the charge density is observed on the surface. This modulation is superimposed on the usual graphite periodicity. This new structure is tentatively attributed to a charge density wave (CDW) resulting from the charge transfer between the intercalated species and the graphite host

STM Observations at the Atomic Scale of a Tilt Grain Sub-Boundary on Highly Oriented Pyrolytic Graphite

We report here the first observations at the atomic scale of a symmetrical tilt grain sub-boundary with a STM. The edge dislocations observed at the atomic scale along the boundary line can be understood in the frame of geometrical models. These STM images also reveal a local modification of the charge density near the Fermi level in the junction area.Des observations à l'échelle atomique d'un sous-joint de grains de flexion ont été réalisées pour la première fois par microscopie à effet tunnel (STM). Des modèles géométriques ont permis d'interpréter le réseau de dislocations mis en évidence sur la ligne de jonction. Ces images STM révèlent également la modification locale de la densité volumique de charge près du niveau de Fermi dans la zone de jonction

Temperature dependence of infrared conductivity of manganites Pr

The temperature dependence of the reflectivity spectra of three manganites ceramics with compositions Pr0.7Ca0.3MnO3, Pr0.7Ca0.25Sr0.05MnO3 and Pr0.7Ca0.1Sr0.2MnO3 has been investigated by infrared reflectivity spectroscopy in the wave number range 0.005-1.1 eV. The compound Pr0.7Ca0.25Sr0.05MnO3 which shows the largest conductivity jump at the ferromagnetic-paramagnetic phase transition has been studied in details. The optical conductivity of this compound is deduced from the best fit to reflectivity spectra of a "double-damping Drude" model, itself derived from the factorized form of the dielectric function. Excellent agreement with Kramers-Kronig transformation is reported. The model allows in particular to discriminate the contributions to the optical conductivity of trapped charges (polarons) and mobile charge carriers

AFM and STM Studies of the Carbonization and Graphitization of Polyimide Films

Kapton polyimide and high-modulus polyimide (PPT) films were carbonized and graphitized at various temperatures from 600 to 3000-degrees-C. Their surface was studied by atomic-force microscopy and/or by scanning tunneling microscopy in order to follow the modification of the large-scale morphology and the atomic structure as a function of the heat-treatment temperature (HTT). On the pristine Kapton films, the local order of the molecules is brought to the fore. With increasing HTT (600 to 1000-degrees-C) the structure becomes more disordered while at larger scale a bumpy morphology appears. During graphitization, the bumpy morphology gradually disappears and is replaced by graphitized terraces whose size increases with HTT. At atomic scale, it is shown that the graphene layers progressively grow for HTT higher than 1800-degrees-C. On the films treated between 1800 and 2400-degrees-C, graphene layers containing point defects are imaged and (square-root 3 X square-root 3)R 30-degrees superstructures are observed near large defects. On the samples treated at 2400 and 2600-degrees-C, moire patterns are observed and are attributed to stacking faults (turbostratic structure)