Interaction of carbon monoxide with Cu nanoclusters grown on alumina surface

The present work addresses the interaction of carbon monoxide with copper nanoclusters supported on an ultrathin alumina film grown on the Ni3Al(111) termination, acting as a template for a highly ordered nucleation. Through accurate quantum-mechanical calculations combined with experimental data, it has been found that the dissociation of carbon monoxide occurs at the copper nanoclusters, at variance with extended surfaces. The detailed mechanism is explained at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and carbon monoxide coverage. The small size of the nanoclusters allows to achieve an exceptionally high local concentration of molecules at the cluster surface, considerably higher than the saturation limit for the single crystal surfaces. The high coverage facilitates the dissociation of the molecules, accompanied by carbon incorporation into the particles. We discuss the possibility of using other transition metals for an optimal seeding of the supported nanoparticles. In agreement with empirical findings, Pd is confirmed to be the best choice for a highly ordered nucleation

Band offsets and stability of BeTe/ZnSe (100) heterojunctions

We present ab-initio studies of band offsets, formation energy, and stability of (100) heterojunctions between (Zn,Be)(Se,Te) zincblende compounds, and in particular of the lattice-matched BeTe/ZnSe interface. Equal band offsets are found at Be/Se and Zn/Te abrupt interfaces, as well as at mixed interfaces, in agreement with the established understanding of band offsets at isovalent heterojunctions. Thermodynamical arguments suggest that islands of non-nominal composition may form at the interface, causing offset variations over about 0.8 eV depending on growth conditions. Our findings reconcile recent experiments on BeTe/ZnSe with the accepted theoretical description.Comment: RevTeX 5 pages, 3 embedded figure

Cross-sectional imaging of sharp Si interlayers embedded in gallium arsenide

We investigate the electronic properties of the (110) cross-sectional surface of Si-doped GaAs using first-principles techniques. We focus on doping configurations with an equal concentration of Si impurities in cationic and anionic sites, such as occurring in a self-compensating doping regime. In particular we study a bilayer of Si atoms uniformly distributed over two consecutive (001) atomic layers. The simulated cross-sectional scanning tunneling microscopy images show a bright signal at negative bias, which is strongly attenuated when the bias is reversed. This scenario is consistent with experimental results which had been attributed to hitherto unidentified Si complexes.Comment: 10 pages, 3 figure

Implementing the Use of Energy Bar Charts in the Framework of an Early Physics approach

Introductory college courses use the Multiple Representations (MR) method for teaching/learning energy processes. It helps students understand concepts which are challenging to learn, like energy, and to solve related problems. Although this method is well-recognised in the context of Physics Education and researchers, it is less known by high school teachers because of its limited use in Physics textbooks. We report a recent experience where we accompanied teachers in their Pedagogical Content Knowledge (PCK) revision and in the building of an innovative way of teaching using conceptual fragmentation. The assessment confirmed the teaching efficiency of using Multiple Representations tools such as Energy Bar Charts

Temperature-Driven Changes of the Graphene Edge Structure on Ni(111): Substrate vs Hydrogen Passivation

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 \ub0C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges

Operando atomic-scale study of graphene CVD growth at steps of polycrystalline nickel

An operando investigation of graphene growth on (100) grains of polycrystalline nickel (Ni) surfaces was performed by means of variable-temperature scanning tunneling microscopy complemented by density functional theory simulations. A clear description of the atomistic mechanisms ruling the graphene expansion process at the stepped regions of the substrate is provided, showing that different routes can be followed, depending on the height of the steps to be crossed. When a growing graphene flake reaches a monoatomic step, it extends jointly with the underlying Ni layer; for higher Ni edges, a different process, involving step retraction and graphene landing, becomes active. At step bunches, the latter mechanism leads to a peculiar \u2018staircase formation\u2019 behavior, where terraces of equal width form under the overgrowing graphene, driven by a balance in the energy cost between C\u2013Ni bond formation and stress accumulation in the carbon layer. Our results represent a step towards bridging the material gap in searching new strategies and methods for the optimization of chemical vapor deposition graphene production on polycrystalline metal surfaces

Spectroscopic fingerprints of iron-coordinated cobalt and iron porphyrin layers on graphene

Achieving design capabilities of monolayer 2D functional catalysts represents a challenging perspective. Coordinated single metal atom sites can offer tailored electronic configuration, ligation geometries, chemical activity and selectivity, together with stability. We report spectroscopic evidence of the formation of a 2D metal-organic framework supported by a single graphene sheet in which coordination among Tetra-Pyridyl-Porphyrins (TPyPs) is spontaneously obtained by exploiting single iron atoms. The spectroscopic characterization, together with ab initio methods, reveals that metal inter-molecular coordination occurs via the terminal nitrogen atoms contained in the pyridinic residues of adjacent TPyPs. Interestingly, the peripheral coordination of metal atoms is found to affect the electronic configuration of the porphyrins core. Due to the chemical stability of the supporting graphene layer, its weak interaction with the metal-organic framework, and the known electrochemical activity of the latter, this system represents an optimal candidate for the design and engineering of prototype 2D electrocatalytic materials

1D selective confinement and diffusion of metal atoms on graphene

The role of moiré graphene superstructures in favoring confined adsorption of different metal atoms is an intriguing problem not yet completely solved. Graphene (G) grown on Ni(100) forms a striped moiré pattern of valleys, where G approaches the nickel substrate and interacts with it rather strongly, and ridges, where G stays far away from the substrate and acts almost free-standing. Combining density functional theory (DFT) calculations and scanning-tunneling microscopy (STM) measurements, we show that this peculiar moiré constitutes a regular nanostructured template on a 2D support, confining in 1D trails single metal atoms and few atoms clusters. DFT calculations show that the confinement is selective and highly dependent on the atomic species, with some species preferring to adsorb on ridges and the other showing preference for valleys. Co and Au adsorbates, for instance, have opposite behavior, as predicted by DFT and observed by STM. The origin of such disparate behavior is traced back to the electrostatic interaction between the charged adsorbate and the nickel surface. Moreover, the selectivity is not restricted to the adsorption process only, but persists as adsorbate starts its diffusion, resulting in unidirectional mass transport on a continuous 2D support. These findings hold great promise for exploiting tailored nanostructured templates in a wide range of potential applications involving mass transport along element-specific routes

Role of defects in the electronic properties of amorphous/crystalline Si interface

The mechanism determining the band alignment of the amorphous/crystalline Si heterostructures is addressed with direct atomistic simulations of the interface performed using a hierarchical combination of various computational schemes ranging from classical model-potential molecular dynamics to ab-initio methods. We found that in coordination defect-free samples the band alignment is almost vanishing and independent on interface details. In defect-rich samples, instead, the band alignment is sizeably different with respect to the defect-free case, but, remarkably, almost independent on the concentration of defects. We rationalize these findings within the theory of semiconductor interfaces.Comment: 4 pages in two-column format, 2 postscript figures include

CO on supported Cu nanoclusters: Coverage and finite size contributions to the formation of carbide via the boudouard process

The interaction of carbon monoxide with an ordered array of copper nanoclusters was investigated under ultrahigh vacuum conditions by means of in situ X-ray photoelectron spectroscopy in combination with density functional theory calculations. The Cu clusters were supported on an alumina template grown on the Ni3Al(111) termination. Adsorption and dissociation of carbon monoxide occur at the copper clusters, yielding accumulation of carbidic carbon at the metal particles through the Boudouard process. The involved mechanisms are investigated at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and of local CO coverage. It is found that the high coverage of CO at the cluster surface, which considerably exceeds that achievable on single crystal surfaces, facilitates the metal restructuring and the reaction, yielding carbon incorporation into the bulk of the particles