No abstract

Microscopists have always pursued the development of an instrument that combines topography and materials properties analyses at the highest resolution. The measurement of the tiny amount of energy dissipated by a vibrating tip in the proximity of the sample surface has provided atomic force microscopes with a robust and versatile method to determine the morphology and the compositional variations of surfaces in their natural environment. Applications in biology, polymer science and microelectronics illustrate the potential of phase-imaging force microscopy for nanoscale analysis.

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in free-standing graphene layers.

Graphite vaporization provides an uncontrolled yet efficient means of producing fullerene molecules. However, some fullerene derivatives or unusual fullerene species might only be accessible through rational and controlled synthesis methods. Recently, such an approach has been used to produce isolable amounts of the fullerene C(60) from commercially available starting materials. But the overall process required 11 steps to generate a suitable polycyclic aromatic precursor molecule, which was then dehydrogenated in the gas phase with a yield of only about one per cent. Here we report the formation of C(60) and the triazafullerene C(57)N(3) from aromatic precursors using a highly efficient surface-catalysed cyclodehydrogenation process. We find that after deposition onto a platinum (111) surface and heating to 750 K, the precursors are transformed into the corresponding fullerene and triazafullerene molecules with about 100 per cent yield. We expect that this approach will allow the production of a range of other fullerenes and heterofullerenes, once suitable precursors are available. Also, if the process is carried out in an atmosphere containing guest species, it might even allow the encapsulation of atoms or small molecules to form endohedral fullerenes.

The Sn͞Ge(111) interface has been investigated across the 3 3 3 ! p 3 3 p 3 R30 ± phase transition using core level and valence band photoemission spectroscopies. We find, both above and below the transition, two different components in the Sn 4d core level and a band splitting in the surface state crossing the Fermi energy. Theoretical calculations show that these two effects are due to the existence of two structurally different kinds of Sn atoms that fluctuate at room temperature between two positions and are stabilized in a 3 3 3 structure at low temperature. [S0031-9007(98)

In situ soft X-ray absorption spectroscopy (XAS) was employed to study the adsorption and dissociation of carbon monoxide molecules on cobalt nanoparticles with sizes ranging from 4 to 15 nm. The majority of CO molecules adsorb molecularly on the surface of the nanoparticles, but some undergo dissociative adsorption, leading to oxide species on the surface of the nanoparticles. We found that the tendency of CO to undergo dissociation depends critically on the size of the Co nanoparticles. Indeed, CO molecules dissociate much more efficiently on the larger nanoparticles (15 nm) than on the smaller particles (4 nm). We further observed a strong increase in the dissociation rate of adsorbed CO upon exposure to hydrogen, clearly demonstrating that the CO dissociation on cobalt nanoparticles is assisted by hydrogen. Our results suggest that the ability of cobalt nanoparticles to dissociate hydrogen is the main parameter determining the reactivity of cobalt nanoparticles in Fischer-Tropsch synthesis.

The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface barrier is analyzed using weak chemisorption theory. The electronic structure of the uncoupled PTCDA molecule and of the metal surface is calculated. Then, the induced density of interface states is obtained as a function of these two electronic structures and the interaction between both systems. This induced density of states is found to be large enough (even if the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital. We conclude that the metal/PTCDA interface molecular level alignment is due to the electrostatic dipole created by the charge transfer between the two solids.Electronic materials made of molecular films are a fast developing field, with many potential applications in organicbased devices. Designing new organic-based materials requires a detailed understanding of the different processes occurring in these devices. In particular, metal/organic and semiconductor/organic interface barriers play a decisive role [1,2]. However, the formation of barriers is not yet well understood.In the Schottky-Mott model of metal/organic interfaces, it is assumed that no interface dipole is formed at the junction, and that the position of molecular levels with respect to the metal Fermi level is defined by vacuum level alignment. This situation was disproved by Narioka et al.[3] who, using ultra-violet photoemission spectroscopy (UPS), found large interface dipoles (∼ 0.5 − 1.0eV ) at several metal/organic interfaces. Independent data by Hill et al.[4] confirmed this conclusion. Various mechanisms are believed to operate simultaneously at these interfaces, and several models have been advanced [1,2]. Metal-molecule chemical reaction has been seen to create interface gap states that pin the Fermi level [5], a situation that is analogous to that described by the Unified Defect Model proposed for inorganic semiconductor/metal interfaces [6]. Compression of the metal surface electronic tail by adsorbed molecules, leading to vacuum level interface shift (the "pillow" effect), has also been proposed as a general metal/organic interface mechanism [7,8,9].In this letter, we explore the first application to a metal/organic interface of the Induced Density of Interface (or virtual) States (IDIS) Model [10]. We study a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface and analyze how the chemical interaction between the organic molecule and the metal creates an IDIS in the organic energy gap. Our calculations show that, although the chemical interaction is weak, the IDIS is large enough that a Charge Neutrality Level (CNL) of the organic molecule can be defined. Our results show that the interface Fermi level E F is pinned at the CNL, a situation similar to that described for the formation of Schottky barriers at conventional semiconductor/metal junctions.In this theoretical analysis, we study the metal/PTCDA interact...

Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic-force microscopy on Si͑111͒ surfaces. At the distance of closest approach between the tip and the surface, there is an onset of covalent chemical bonding between the dangling bonds of the tip and the surface. Displacement curves and lateral scans on the surface show that this interaction energy and force are comparable to the macroscopic Van der Waals interaction. However, the covalent interaction completely dominates the force gradients probed in the experiments. Hence, this covalent interaction is responsible for the atomic resolution obtained on reactive surfaces and it should play a role in improving the resolution in other systems. Our results provide a clear understanding of a number of issues such as ͑i͒ the experimental difficulty in achieving stable operation, ͑ii͒ the quality of the images obtained in different experiments and the role of tip preparations and ͑iii͒ recently observed discontinuities in the force gradient curves. ͓S0163-1829͑98͒08336-2͔