On the STM imaging contrast of graphite: towards a “true’' atomic resolution

Different phenomena observed in the high-resolution images of graphite by scanning tunneling microscopy (STM) or atomic force microscopy (AFM) such as the asymmetry in the charge density of neighboring carbon atoms in a hexagon, the high corrugation amplitudes and the apparent absence of point defects has led to a controversial discussion since the first published STM images of graphite. Different theoretical concepts and hypotheses have been developed to explain these phenomena. Despite these efforts a generally accepted interpretation is still lacking. In this paper we discuss a possible imaging mechanism based on mechanical considerations. Forces acting between tip and sample are taken into account to explain the image contrast. We present for the first time a direct atomic resolution of the graphite hexagonal structure by transmission electron microscopy (HRTEM), revealing the expected hexagonal array of atoms and the existence of several types of defects. We discuss the possibility that the STM image of graphite is a result of convolution of the electronic properties and the atomic hardness of graphite

Determination of the Mean Absorption Potential of Si for Electrons by Energy Loss Spectroscopy

Complex structure potentials of silicon for 111 systematic diffraction were measured by convergent beam electron diffraction technique (Kossel ‐ Möllenstedt pattern). The imaginary mean potential and its different components were determined directly by measuring the energy loss spectra of transmitted elctrons by means of an imaging filter (GIF) installed in the electron microscope. The imaginary mean inner potential was estimated to be 0.63 eV. The component of the imaginary inner potential due to plasmon excitation was estimated to be 0.53 eV. Some low indexed Fourier components were determined by matching intensity simulations to Kossel‐Möllenstedt patterns. The absorption potential due to thermal vibration was found to be in the order of the full Einstein model

Elemental carbon as catalytic material: Recent trends and perspectives

Elemental carbon plays a role in catalysis in several modifications which are based upon the sp2 connectivity. Supports for catalytic materials, catalysts in its own right and carbonaceous deposits can be understood in their reactivity by applying the concept of chemical anisotropy. The paper will introduce this concept and describe several case studies in which carbon of different anisotropy will be investigated. The spectrum of defined carbon materials available for such studies has increased considerably in the last years with the advent of nanocarbons characterised by the incorporation of non-six-membered carbon rings (NSMCR) into the planar graphene network causing bending of the carbon sheets. Catalytic test reactions used in this work are the selective oxidation of methanol, the decomposition of NO and the hydrogenation of CO. Metals supported in a geometrically defined way are copper and ruthenium. Methods of investigation are photoemission (UPS) and photoabsorption (XAS) spectroscopies, electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed desorption (TPD) and temperature-programmed reaction spectroscopy (TPRS)