L. Vattuone

Switching from molecular to dissociative adsorption with vibrational energy: ethylene on Ag(0 0 1)

We show that vibrational energy can be used to tune the chemisorption state of an adsorbed molecule. When dosing C2H4 with a supersonic molecular beam on Ag(0 0 1) we demonstrate by vibrational spectroscopy that, with increasing nozzle temperature, TN, firstly no stable adsorption occurs, then a p-bonded configuration is populated, switching finally to a more strongly, undissociated, di-r-bonded state. Direct dissociation is observed at still higher TN with the formation of dehydrogenated radicals. ” 2000 Elsevier Science B.V. All rights reserved.

Dynamics of the interaction of O2 with stepped and damaged Ag surfaces

The role of defects in catalytic reactions, especially those involving low rates and high degrees of selectivity, has been debated ever since the very early days of surface science. However, most studies on gas–surface interaction and chemisorption performed under controlled laboratory conditions have dealt so far with nearly perfect low-Miller-index surfaces, which are rather unlike the active powders employed as catalysts in industrial reactors. The failure in reproducing some chemical reactions, which occur readily under industrial conditions, was therefore often ascribed to the so-called structure gap separating the surface science approach from the real industrial conditions. Overcoming this limit without losing control over the experiment at the nanoscopic scale is therefore an issue of pivotal importance. It can be attacked by studying adsorption on either single-crystal surfaces damaged by ion bombardment or on surfaces aligned along high-Miller-index planes. In this paper we shall discuss O2 interaction with Ag(410), a vicinal surface of Ag(100) characterized by open (110)-like steps and by (100) terraces. The use of a supersonic molecular beam to dose O2 allowed us to gain information on the interaction of the gasphase molecules with steps and terraces separately, by selecting the angle of incidence and the impact energy. The open steps turned out to be active sites for dissociation, while flat Ag(100) planes are unreactive. For molecular adsorption a reduction in the activation barrier is observed at the steps, while the Ag(100) nanoterraces are much less reactive than wide (100) planes. The main results are confirmed by the preliminary investigation of O2/Ag(210).

Enhanced Chemical Reactivity of Pristine Graphene Interacting Strongly with a Substrate: Chemisorbed Carbon Monoxide on Graphene/Nickel(1 1 1)

Graphene is usually considered a chemically inert material. Theoretical studies of CO adsorption on free‐standing graphene predict quite low adsorption energies (<0.1 eV). However, we show here by vibrational spectroscopy and scanning tunneling microscopy that the nondissociative chemisorption of CO occurs at cold, pristine graphene grown on Ni(1 1 1). The CO adlayer remains stable up to 125 K, although some coverage survives flashes to 225 K. This unexpected result is explained qualitatively by the modification of the density of states close to the Fermi energy induced by the relatively strong graphene–substrate interaction. The value of the adsorption energy allows us to estimate an equilibrium coverage of the order of 0.1 monolayers at 10 mbar pressure, which thus paves the way for the use of graphene as a catalytically active support under realistic conditions.

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

Chemisorption dynamics in the presence of well defined surface defects

Publisher Summary This chapter discusses the concept of chemisorption dynamics in the presence of well-defined surface defects. It describes the active role of defects in the dynamics of gas–surface interactions. In particular in the chapter, the case of oxygen (O 2 ) and ethylene (C 2 H 4 ) adsorbed on silver (Ag) surfaces is analyzed , either flat or with a high density of open steps, finding that some processes are enabled by the presence of defects. For both gases, open steps were indeed proved to remove the adsorption barriers for chemisorption. For O 2 /Ag, moreover, a pathway leading to population of subsurface sites was also found. The chapter is dedicated to studies performed on surfaces with well defined defects and dosing the reactants with supersonic molecular beams, allowing to select the energy and the angle at which the molecules collide with the surface.

Influence of growing conditions on the reactivity of Ni supported graphene towards CO

Free standing graphene is chemically inert but, as recently demonstrated, CO chemisorption occurs at low crystal temperature on the single layer grown by ethene dehydrogenation on Ni(111). Such layer is inhomogeneous since different phases coexist, the relative abundance of which depends on the growth conditions. Here we show by X ray photoemission and high resolution electron energy loss spectroscopies that the attained CO coverage depends strongly on the relative weight of the different phases as well as on the concentration of carbon in the Ni subsurface region. Our data show that the chemical reactivity is hampered by the carbon content in the substrate. The correlation between the amount of adsorbed CO and the weight of the different graphene phases indicates that the top-fcc configuration is the most reactive.

The 12th International Conference on Vibrations at Surfaces (VAS 12) (Erice, 20?26 July 2007)

The 12th International Conference on Vibrations at Surfaces (VAS 12) took place from 20–26 July 2007 as an event of the International School of Solid State Physics at the Ettore Majorana Foundation and Centre for Scientific Culture, Erice (Italy). The format and special environment of the conference have contributed to its transition from a traditional, medium-size conference into a more effective workshop, with a series of lectures reporting the most recent developments in the field, two poster sessions presenting recent results and even works in progress being discussed. The papers collected in this issue cover the highlights of the conference very thoroughly. Quite a few novel aspects concerning vibrations at surfaces are represented here, for example: new aspects in surface phonon spectroscopy, such as the very recent progress in inelastic x-ray scattering, the first observation of the boson peak in disordered surfaces, progress in the theory of atom scattering inelastic resonances, the action spectroscopy, the study of polycrystalline surfaces with electron energy-loss spectroscopy etc; parallel developments in experimental vibrational studies of adsorbed phases, either inorganic or organic, with those in ab initio theoretical simulations; the theory of enhanced electron--phonon interaction in low dimensions (2D and 1D); the extension from the traditional realm of surface vibrations and spectroscopy to other aspects of surface dynamics, like friction and various nonlinear effects, and to relevant dynamical phenomena occurring at interfaces. Other novelties presented at the conference, but already published in recent issues of the Journal of Physics: Condensed Matter, are also worth mentioning: the spin-echo spectroscopy with 3He allowing for slow-dynamics spectroscopy at very high, unprecedented resolutions (2007 J. Phys.: Cond. Matter 19 300301 and 305010; the first demonstration of dissociative surface trapping of molecules (2007 J. Phys.: Cond. Matter 19 305003; the discovery of optical surface phonons in metals, solving a quarter of a century old controversy about surface acoustic resonances (2007 J. Phys.: Cond. Matter 19 305011). Future development of the VAS conference series could involve extending it to new areas directly involving surface vibrations which have traditionally been covered by other scientific communities. These are nonlinear optics (second-harmonic generation, femtosecond pump and probe experiments), surface acoustic waves (SAW) in THz domains with extension to dispersion effects and optical phonons, THz SAW applications to sensors and other devices, etc. The mature field of surface vibrations has many new branches into a wide range of applicative, mostly nanotechnological areas. The present VAS edition was intended to renew the conference and stimulate its evolution into new challenging directions. We believe that this special issue of the Journal of Physics: Condensed Matter will meet with the same large consensus gained at the 12th International Conference on Vibrations at Surfaces, and will foster new progress in the fields of surface dynamical phenomena and their applications. The next International Conference on Vibrations at Surfaces (VAS 13) will take place in the fall of 2009 in Orlando, Florida, and will be chaired by Professor Talat S Rahman, University of Central Florida. We gratefully acknowledge the Ettore Majorana Foundation and Centre for Scientific Culture, Erice (Italy) and its staff for excellent hospitality and support, and SPECS and Varian for financial aid.