Enhanced Atom Mobility on the Surface of a Metastable Film

Abstract

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization. DOI: 10.1103/PhysRevLett.113.046102 PACS numbers: 68.43.Jk, 68.55.J-, 81.15.Aa Atomic diffusion on solid surfaces is a ubiquitous phenomenon that plays a fundamental role in determining a large variety of physical and chemical processes. An example is represented by the self-assembly of metallic nanosized clusters stabilized on either oxide or metallic substrates, which is exploited to produce nanopatterned materials in fast and parallel bottom-up approaches [1,2]. In this frame, adatom diffusion is of paramount importance, since the kinetic constraints typically determine the final morphology of the self-assembled nanostructures. In the field of heterogeneous catalysis, adatom diffusion is particularly relevant in cases where spill-over effects intervene and the reaction proceeds through different steps mediated by spatially separated active sites [3][4][5]: an enhanced atomic diffusivity can therefore boost the rate of the chemical reactions. On the other hand, in oxide supported metal clusters, catalysis, sintering due to Ostwald ripening, induced by atoms detaching from smaller clusters and diffusing to larger ones, often leads to catalyst deactivation [6].Various factors have been recognized to influence the diffusivity of atomic species on solid surfaces, for instance, (i) isotropic two-dimensional strain in heteroepitaxial systems, with an in-plane lattice mismatch between the film and the substrate [7,8]; (ii) mesoscopic strain, where the relaxations induced by the already nucleated islands significantly affect atomic motion [9]; (iii) availability of small amounts of foreign atoms (surfactants) that, being adsorbed on the substrate prior to the film deposition, can affect the intra-and/or interlayer mass transport [10-13]; (iv) presence of molecular species (chemisorbed or in the gas phase), such as, for instance, water [3] or CO [14]; and (v) long-range adatom-adatom interactions [15][16][17]. More recently, the enhancement of surface diffusion with acoustic standing waves has been proposed as a method for surface nanostructuring by driving atomic motion [18].In this Letter, we report on a remarkable increase of atomic diffusion with the thickness of an ultrathin metastable film, namely, tetragonally distorted body-centered (bct) Co grown on Fe(001). Previous qu...

Electronic structure and magnetism of strained bcc phases across the fcc to bcc transition in ultrathin Fe films

“…The QUAN-TUM ESPRESSO package [37] was used for the calculations, within plane waves and ultrasoft pseudopotentials. The Fe pseudopotential was generated starting from scalar-relativistic all-electron atomic calculations and using nonlinear core corrections, and has been extensively studied in our previous works [38][39][40]. We used a kinetic energy cutoff of 45 Ry for the wave functions and 200 Ry for the effective potential and the charge density.…”

Self-organized nano-structuring of CoO islands on Fe(001)

“…A 5 monolayers (ML) thick buffer layer of metallic Co was grown onto the Fe(001)-p(1 × 1)O surface. In such a thickness range, Co is known to grow pseudomorphically on the Fe substrate [33,34], with a body centered tetragonal (bct) structure and the same inplane lattice parameter as bulk Fe (a Fe = 0.287 nm). The growth of thin Co films on Fe(001)-p(1 × 1)O is known to proceed in a layer-by-layer (also known as Frank-van der Merwe) mode, as demonstrated by former studies [33,35] and confirmed by our STM observations [36].…”

Magnetism in thin Cr films grown on Fe(001)-p(1×1)O: a spin-resolved investigation of single and multi-layers

“…8 Our recent contributions to the field range from the study of different AF materials, such as 3d metal oxides, 9 to the addition of controlled amounts of oxygen to the growth recipes of a large number of magnetic heterostructures. [10][11][12][13] Regardless of its actual composition (either Fe or Cr), the presence of a passivating layer of oxygen induces deep modifications in the structural equilibrium of the surface, which usually relaxes by increasing the distance between the topmost layer and the subsurface ones.…”