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

Towards an Early Physics approach for secondary students

Some traditional approaches to teaching Physics at the secondary level of instruction have disclosed their limits, especially in distance learning. A consequence of such limits seems to be a somewhat diffused lack of students' scientific abilities, mainly caused by their learning difficulties. To overcome the shortcomings of tradition, we stimulated some teachers to get involved in a new teaching approach to develop their awareness of these limits and difficulties and exploit their PCK (Pedagogical Content Knowledge). This approach explores and intercepts the main learning features and needs in the first years of Physics studies. For that reason and the analogy in Math Education, we named it Early Physics

Site-Dependent Oxidation States of Single Cobalt Atoms in a Porphyrin-Based Monolayer on Graphene

We investigate a layer of cobalt tetrapyridyl porphyrins (CoTPyPs) self-assembled on an almost freestanding graphene (GR) sheet supported by Ir(111) with complementary experimental techniques and density functional theory (DFT) ab initio simulations. Beside the metal atoms enclosed within the porphyrin macrocycles, additional Co atoms can be accommodated at the molecular network’s interstice via physical vapor deposition and can bind up to four adjacent molecules. Therefore, such a system presents two metallic sites, both tetra-coordinated to nitrogen atoms. At the same time, a rearrangement of the network occurs depending on the coverage of such additional atoms. The bare CoTPyPs arrange themselves on GR in an almost hexagonal close-packed pattern with alternating orientations. The addition of extra Co atoms causes a dramatic transformation in the network. At full peripheral metal coverage (i.e., one additional Co per CoTPyP), the network drastically changes becoming almost square. Intermediate coverages display different peculiar patterns characterized by unique chiral structures. Importantly, our DFT calculations reveal a remarkable effect on the system’s work function attributed to the presence of these additional metal atoms, despite their extremely small amount even at full coverage (less than 2% of a monolayer with respect to the number of carbon atoms in the GR sheet). Furthermore, we report a different behavior of the two Co sites showing different oxidation states and molecular orbital occupations

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

Exceptionally Stable Cobalt Nanoclusters on Functionalized Graphene

To improve reactivity and achieve a higher material efficiency, catalysts are often used in the form of clusters with nanometer dimensions, down to single atoms. Since the corresponding properties are highly structure-dependent, a suitable support is thus required to ensure cluster stability during operating conditions. Herein, an efficient method to stabilize cobalt nanoclusters on graphene grown on nickel substrates, exploiting the anchoring effect of nickel atoms incorporated in the carbon network is presented. The anchored nanoclusters are studied by in situ variable temperature scanning tunneling microscopy at different temperatures and upon gas exposure. Cluster stability upon annealing up to 200 °C and upon CO exposure at least up to 1 × 10−6 mbar CO partial pressure is demonstrated. Moreover, the dimensions of the cobalt nanoclusters remain surprisingly small (<3 nm diameter) with a narrow size distribution. Density functional theory calculations demonstrate that the interplay between the low diffusion barrier on graphene on nickel and the strong anchoring effect of the nickel atoms leads to the increased stability and size selectivity of these clusters. This anchoring technique is expected to be applicable also to other cases, with clear advantages for transition metals that are usually difficult to stabilize

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