ATOMIC SCALE INVESTIGATIONS OF MODEL CATALYSTS FOR OXIDATION REACTIONS

2002/2003XVI Ciclo1973Versione digitalizzata della tesi di dottorato cartacea

Strain release at the graphene-Ni(100) interface investigated by in-situ and operando scanning tunnelling microscopy

Interface strain can significantly influence the mechanical, electronic and magnetic properties of low- dimensional materials. Here we investigated by scanning tunneling microscopy how the stress intro- duced by a mismatched interface affects the structure of a growing graphene (Gr) layer on a Ni(100) surface in real time during the process. Strain release appears to be the main factor governing morphology, with the interplay of two simultaneous driving forces: on the one side the need to obtain two-dimensional best registry with the substrate, via formation of moire patterns, on the other side the requirement of optimal one-dimensional in-plane matching with the transforming nickel carbide layer, achieved by local rotation of the growing Gr flake. Our work suggests the possibility of tuning the local properties of two-dimensional films at the nanoscale through exploitation of strain at a one-dimensional interface

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

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

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

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

Imaging on-surface hierarchical assembly of chiral supramolecular networks

The bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,10′-dibromo-9,9′-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling

Towards the FAIRification of Scanning Tunneling Microscopy Images

ABSTRACTIn this paper, we describe the data management practices and services developed for making FAIR compliant a scientific archive of Scanning Tunneling Microscopy (STM) images. As a first step, we extracted the instrument metadata of each image of the dataset to create a structured database. We then enriched these metadata with information on the structure and composition of the surface by means of a pipeline that leverages human annotation, machine learning techniques, and instrument metadata filtering. To visually explore both images and metadata, as well as to improve the accessibility and usability of the dataset, we developed “STM explorer” as a web service integrated within the Trieste Advanced Data services (TriDAS) website. On top of these data services and tools, we propose an implementation of the W3C PROV standard to describe provenance metadata of STM images

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