Influence of ordered adsorbates on atomic and molecular diffusion on solid surfaces: the case of oxygen passivated Fe(001)

Picone1, D. Giannotti1, M. Riva1, A. Calloni1, G. Bussetti1, G. Fratesi2, M. Finazzi1, L. Duò1, F. Ciccacci1 and A. Brambilla1

1.Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

2.Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy

The diffusion of atomic and molecular species on solid surfaces is a fundamental phenomenon  that drives a large variety of physical and chemical processes. In the field of epitaxial growth, the self-assembly of metallic nanosized clusters on either oxide or metallic substrates is exploited to produce nanopatterned materials in parallel bottom-up approaches [1]. In this frame, adatom diffusion is of paramount importance, since the kinetic constraints typically determine the final morphology of the self-assembled nanostructures. Considering  heterogeneous catalysis, adatom diffusion is particularly relevant in cases where spill-over effects operate and the reaction proceeds through different steps, mediated by spatially separated active sites [2]: the atomic diffusivity can therefore tune the rate of the chemical reactions. On the other hand, in catalysts consisting in oxide-supported metal clusters, sintering due to mass transfer induced by atoms detaching from smaller clusters and diffusing to larger ones (Ostwald ripening) often leads to catalyst deactivation.

Various factors affect surface diffusion, such as isotropic strain, mesoscopic relaxations and adatom-adatom long range interactions. Among the factors affecting diffusivity, the presence of foreign species adsorbed on the surface might play an important role. Such species can be either light elements, such as atomic oxygen, or heavy elements, such as Pb and Bi, or graphene.

We will analyze the effects that a single layer of oxygen atoms adsorbed on the Fe(001) surface has on the diffusivity of transition metal atoms and fullerene molecules (C60). Combining Scanning Tunneling Microscopy and Density Functional Theory, we will show that the oxygen overlayer dramatically changes both the interlayer and intralayer diffusivity of atoms deposited on Fe(001) [3,4,5].  In addition, the influence of oxygen on the morphology of C60/Fe(001) will be discussed, showing that C60 follows an hybrid nucleation path in between diffusion-mediated growth and ballistic deposition.

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[2] W. C. Conner and J. L. Falconer, Spillover in Heterogeneous Catalysis Chem. Rev. 95, 759 (1995).

[3] A. Picone, M. Riva, A. Brambilla, A. Calloni, G. Bussetti, M. Finazzi, F. Ciccacci, L. Duò, “Metal-oxide reacted interfaces: a microscopic view” Surf. Sci. Rep. 71, 32-76 (2016).

[4] A. Picone, A. Riva, M. Fratesi, G. Brambilla, A. Bussetti, G. Finazzi, M. Duò, L. Ciccacci, F. Enhanced Atom Mobility on  the Surface of a Metastable Film. Phys. Rev. Lett. 113, 046102 (2014).

[5] M. Riva, A. Picone, D. Giannotti, A. Brambilla, G.  Fratesi, G. Bussetti, L. Duò, F. Ciccacci, M. Finazzi, Mesoscopic Organization of Cobalt thin Films on Clean and Oxygen-saturated Fe(001) Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 92, 115434 (2015).