Structural and electronic properties of graphene nanoflakes on Au(111) and Ag(111)

We investigate the electronic properties of graphene nanoflakes on Ag(111) and Au(111) surfaces by means of scanning tunneling microscopy and spectroscopy as well as density functional theory calculations. Quasiparticle interference mapping allows for the clear distinction of substrate-derived contributions in scattering and those originating from graphene nanoflakes. Our analysis shows that the parabolic dispersion of Au(111) and Ag(111) surface states remains unchanged with the band minimum shifted to higher energies for the regions of the metal surface covered by graphene, reflecting a rather weak interaction between graphene and the metal surface. The analysis of graphene-related scattering on single nanoflakes yields a linear dispersion relation E(k), with a slight p-doping for graphene/Au(111) and a larger n-doping for graphene/Ag(111). The obtained experimental data (doping level, band dispersions around EF, and Fermi velocity) are very well reproduced within DFT-D2/D3 approaches, which provide a detailed insight into the site-specific interaction between graphene and the underlying substrate

Ordered Arrays of Size-Selected Oxide Nanoparticles

A bottom-up approach to produce a long-range ordered superlattice of monodisperse and isomorphic metal-oxide nanoparticles (NP) supported onto an oxide substrate is demonstrated. The synthetic strategy consists of self-assembling metallic NP on an ultrathin nanopatterned aluminum oxide template followed by a morphology-conserving oxidation process, and is exemplified in the case of Ni, but is generally applicable to a wide range of metallic systems. Both fully oxidized and core-shell metal-metal-oxide particles are synthesized, up to 3-4 nm in diameter, and characterized via spectroscopic and theoretical tools. This opens up a new avenue for probing unit and ensemble effects on the properties of oxide materials in the nanoscale regime

Charge transport in a single molecule transistor probed by scanning tunneling microscopy

We report on the scanning tunneling microscopy/spectroscopy (STM/STS) study of cobalt phthalocyanine (CoPc) molecules deposited onto a back-gated graphene device. We observe a clear gate voltage ( V g ) dependence of the energy position of the features originating from the molecular states. Based on the analysis of the energy shifts of the molecular features upon tuning  V g , we are able to determine the nature of the electronic states that lead to a gapped differential conductance. Our measurements show that capacitive couplings of comparable strengths exist between the CoPc molecule and the STM tip as well as between CoPc and graphene, thus facilitating electronic transport involving only unoccupied molecular states for both tunneling bias polarities. These findings provide novel information on the interaction between graphene and organic molecules and are of importance for further studies, which envisage the realization of single molecule transistors with non-metallic electrodes

Multiplet features and magnetic properties of Fe on Cu(111): From single atoms to small clusters

The observation of sharp atomiclike multiplet features is unexpected for individual 3d atoms adsorbed on transition-metal surfaces. However, we show by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism that individual Fe atoms on Cu(111) exhibit such features. They are reminiscent of a low degree of hybridization, similar to 3d atoms adsorbed on alkali-metal surfaces. We determine the spin, orbital, and total magnetic moments, as well as magnetic anisotropy energy for the individual Fe atoms and for small Fe clusters that we form by increasing the coverage. The multiplet features are smoothened and the orbital moment rapidly decreases with increasing cluster size. For Fe monomers, comparison with density functional theory and multiplet calculations reveals a d(7) electronic configuration, owing to the transfer of one electron from the 4s to the 3d states

Two-Orbital Kondo Screening in a Self-Assembled Metal Organic Complex

Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl dicarbonitrile molecules exhibit strongly spatial anisotropic Kondo features with directionally dependent Kondo temperatures and line shapes, as evidenced by scanning tunneling spectroscopy. First-principles calculations find nearly full polarization for the half-filled Fe 3d(xz) and 3d(yz) orbitals, which therefore can give rise to Kondo screening with the experimentally observed directional dependence and distinct Kondo temperatures. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements confirm that the spin in both channels is effectively Kondo-screened. At ideal Fe coverage, these two-orbital Kondo impurities are arranged in a self-assembled honeycomb superlattice

Fe self-organization on stepped MgO surfaces

In this work we present a study of Fe self-organization on thesurface of stepped MgO as a function of deposition temperatureand thickness. The Fe morphology results in being very muchinfluenced by the presence on the MgO terraces of a roughnessdue to exposure of the sample to the air. Because of the additionalsites introduced by the MgO roughness, at room temperature Fenucleates indistinctly on steps and terraces. Dramatic changes inthe Fe nucleation appear on increasing the deposition temperatureto 800 K. Here Fe diffuses on the MgO terraces and 40% of theclusters nucleate on step edges. This effect is more evident ondecreasing the amount of Fe deposit.' 2008 Elsevier Ltd. All rights reserved

Electrospray Deposition and Magnetic Properties of Prototypical Molecular Magnets

The controlled deposition, characterization and manipulation of single molecule magnets (SMMs) on surfaces is one of the crucial points to be addressed with regard to their possible implementation as functional units in future electronic and spintronic devices. Profound understanding of molecule-molecule and molecule-substrate interactions is required as well as unraveling their effect on the molecular electronic and magnetic properties. Local measurement techniques like scanning tunneling microscopy (STM) together with ensemble-averaging methods like X-ray absorption spectroscopy (XAS) have been proven to set up an appropriate frame to study these materials. The majority of these studies deal with SMMs that exhibit rather simple structures with mostly only one magnetic ion. The situation gets more complicated when it comes to larger polynuclear compounds that can be quite fragile with respect to surface deposition or not easy to organize on surfaces due to their bulky ligand shell. Here, we provide an overview of our results on successful deposition of polynuclear SMMs on functional surfaces by employing the electrospray ion beam deposition method. For two prototypes in the field, Mn12-ac and Fe4H, we obtain highly ordered submonolayers on functional surfaces and elucidate the electronic coupling to the respective substrates using scanning tunneling spectroscopy (STS). New results for Mn12-ac on graphene/Ir(111) and for Fe4H on Au(111) are compared to previous studies on a decoupling graphene layer. X-ray magnetic circular dichroism (XMCD) measurements on submonolayers of uniformly aligned Fe4 molecules on both substrates reveal its robust magnetism, showing magnetic anisotropy values similar to bulk.publishe

Growth and study of 3d Ni nanoparticles films deposited on inert substrates

The interest in metal nanostructured films has grown in the last years because of their fascinating physical properties and their potentiality in various applications, like magnetic recording industry and tribology. As an example, smaller particles are required in order to realize advanced magnetic memory units. This request constitutes an important challenge, because of the super-paramagnetic limit for the density of recorded bits, which makes conventional recording media with three-dimensional particles unstable [1]. We performed a study of 3d (Cu, Ni and Fe) metal nanoparticles films grown on oxidized Si and MgO inert substrates. The deposition has been obtained by making use of a recently developed experimental system, with a gas aggregation cluster source, a quadrupole mass filter and a deposition chamber [2,3]. The source can produce high flux of particles with variable size distribution in a range from 1 to 10 nm. We produced films of Cu, Ni and Fe particles of different size (from 3 to 5 nm average diameter) and thickness values, from 1 to 10 nm corresponding to regimes where the nanoparticles are sparse or they constitute a nanostructured film. The samples have been characterized ex-situ with scanning electron microscopy, atom force microscopy, scanning tunneling microscopy and electron spectroscopy techniques to check morphology and composition. Magneto Optical Kerr Effect (MOKE) experiments have been performed on Ni grown on MgO(100) to investigate the role played by the magnetic dipole interaction between the particles. Results have shown that samples are paramagnetic at Room Temperature. We also performed a low temperature MOKE experiment on a 7 nm thick film of Ni nanoparticles with 5 nm average diameter grown on MgO(100), and we obtained a ferromagnetic hysteresis at T=100 K, revealing magnetic blocking at this temperature. [1] see, for instance S. D. Bader, Rev. of Mod. Phys. 78 (2006) 1 and ref.s therein. [2] C. Binns, SUrf. Sci. Rep. 44 (2001) 1. [3] C. Binns et al., J. Phys. D: Appl Phys. 38 (2005) R357

Morphology-induced magnetic phase transitions in Fe deposits on MgO films investigated withXMCD and STM

The interplay between magnetic properties and morphology of thin iron deposits on MgO films grown onAg001 was investigated by means of x-ray magnetic circular dichroism and scanning tunneling microscopySTM measurements as a function of the Fe thickness and after postgrowth thermal treatments. The as-grownFe deposits display a sharp transition as a function of the Fe thickness, corresponding to the development offerromagnetism at around 4.5 ML. The ferromagnetic phase can be turned into a superparamagnetic phase bymeans of a thermal treatment. STM measurements allowed us to ascribe the onset of ferromagnetism to thetransition from a three-dimensional to a two-dimensional growth mode of the iron deposit and to explain thesuperparamagnetic behavior in the annealed film as due to the formation of a collection of well-separatedsquarediron particles. Moreover, using the particle shape and size distribution measured by STM, we calculateda value of the magnetic anisotropy of the Fe particles, which is 1 order of magnitude larger than the bulkiron one. This increase is mainly ascribed to the role of surface anisotropy in the Fe nanoparticles and also therole of the dipolar interactions between particles is discussed