Highlights Surface Science Lab

Fine tuning of graphene-metal adhesion by surface alloys  



  Controlling the graphene adhesion to the substrate requires a thorough understanding of the physical mechanisms underlying the graphene-metal interaction.This is a key step towards the establishment of graphene-based technologies and the development of graphene heterostructures in novel electrical and mechanical devices. It is well known, in fact, that the coupling between supported graphene (GR) and the substrate affects a number of properties of GR-based devices, such as the electromechanical, thermal and optical properties, as well as  the electronic transport and the contact resistance. Several methods, such as the choice of the substrate, the intercalation of adspecies or the creation of an oxide buffer layer, have proven effective in manipulating the graphene–metal interaction, but not to a precisely controllable extent.Moreover, these methods come with some significant drawbacks. Here we show that bimetallic surface alloying provides a viable route for governing the GR-metal interaction by selectively choosing  the elemental composition of the surface alloy. This concept is illustrated by characterising the properties of graphene on a model PtRu surface alloy on Ru(0001), with Pt concentrations ranging from 0 to 50%. Pt and Ru traditionally stand out as two model examples of weakly and strongly interacting substrates, respectively. In fact, while GR interacts very weakly with Pt -as reflected in the almost flat morphology of the C layer-, a strong coupling has been observed for GR/Ru(0001), leading to a significant corrugation of the moiré superstructure.

  Our study was conducted via a multidisciplinary approach, combining a range of experimental techniques (high-energy resolution core level Photoemission Spectroscopy, Low Energy Electron Diffraction and Low Energy Electron Microscopy) and state-of-the-art DFT calculations.Our results show that the progressive increase of the Pt content in the surface alloy is associated with a gradual lifting of graphene from the substrate, which results from the Pt-induced carbon orbital rehybridization. Alloying is also found to affect the growth mode and the morphology of graphene, which is strongly corrugated on bare Ru but becomes flat at a Pt coverage of 50%. Our work is the proof of concept that the employment of binary surface alloys, which are used in many areas of materials science, can provide an unprecedented tool to selectively manipulate the GR-metal adhesion. The proposed method can be readily extended to a range of supports, thus opening the way to a full tunability of the graphene-substrate interaction.

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Fine-tuning of graphene metal adhesion by surface alloys;
D. Alfe', M. Pozzo, E. Miniussi, S. Gunther, P. Lacovig, S. Lizzit, R. Larciprete, B. Santos Burgos, T.O. Menteş¸ M.Á. Niño, A. Locatelli, and A. Baraldi;
Sci. Rep. in press.



Destruction in the deep valleys
 

Since its first isolation, graphene has attracted a staggering interest due to its outstanding properties, ranging from its electrical conductivity to its mechanical, optical and chemical properties. This unique two-dimensional material may be therefore exploited for a wealth of industrial applications in particular when supported on suitable solid surfaces. However, due to its interaction with the substrate, graphene exhibits distinct properties with respect to free-standing graphene. The interaction strength with the substrate directly affects the corrugation of the carbon film, thus resulting in different thermal stability and heat conductivity.
In this work we have investigated epitaxial graphene grown on Re(0001), which can be considered as a model strongly interacting system. Our innovative approach exploits the combined use of synchrotron based experimental techniques available at the SuperESCA and Nanospectroscopy and density functional theory calculations.  Our results show that the graphene layer is strongly corrugated, with nanometer scale periodic mounds and valleys.

However, in contrast with what observed on other metals, graphene on Re(0001) dissolves at high temperature, despite the intrinsic thermal stability of the carbon network. Noteworthy, we could locate the onset of C-C bond breaking in the strongly interacting regions of the graphene sheet, where C atoms are closer to the substrate. The mechanism of GR breaking involves the presence of C vacancies, which quickly migrate to the strongly interacting regions where subsequent C-C bond breakup takes place.

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Thermal Stability of Corrugated Epitaxial Graphene Grown on Re(0001);
E. Miniussi, M. Pozzo, A. Baraldi, E. Vesselli, R. R. Zhan, G. Comelli, T.O. Menteş¸ M.Á. Niño, A. Locatelli, S. Lizzit, and D. Alfe';
Phys. Rev. Lett. 106, 216101 (2011).



Details of the structure at the interface

The knowledge of the atomic positions at metal-oxide junctions is a prerequisite for the rational design of ultrathin oxide films. The determination of atomic arrangement at the interfaces is a key feature for the use of oxide coatings as protective layers and for tuning the electronic properties of the oxide supported active metal clusters, which is a very important issue in order to engineer new catalysts. However, the breaking of the bulk periodicity often results in a large atomic rearrangement, confined within few Å from the interface, which is extremely difficult to characterize experimentally. Because of the large lattice mismatch, the periodic structural modifications extend laterally over few nanometers, thus strongly limiting the applicability of diffraction techniques based on long-range order due to the large dimensions of the unit cells. The possibility to separate nonequivalent chemical species in x-ray photoelectron diffraction measurements allowed us to unveil a large modification of the alumina ultrathin epitaxial oxide film grown on Ni3Al (111).

In order to determine the alumina structure we have compared the experimental photoelectron diffraction modulation functions of chemically nonequivalent Al and O species with multiple-scattering simulations.The remarkable outcome of this study is the ejection of the alloy first-layer Al atoms towards the ultrathin oxide layer.
Our findings provide the evidence for the formation of a new Al intermediate metallic layer at the metal-oxide interface. The formation of this new interface structure is crucial for the explanation of several properties of the aluminum oxide films.

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Large Interlayer Relaxation at a Metal-Oxide Interface:The Case of a Supported Ultrathin Alumina Film;
Erik Vesselli, Alessandro Baraldi, Silvano Lizzit, and Giovanni Comelli
Phys. Rev. Lett. 105, 046102 (2011).



Under-Coordinated Atoms at Pt-Rh Bimetallic Surfaces

The electronic structure and chemical reactivity changes of highly under-coordinated Rh atoms on a Pt(111) surface versus coordination number were studied by a combination of synchrotron radiation core level photoemission spectroscopy and density functional theory. The properties of Rh adatoms are strongly modified by the Pt substrate, when compared with the equivalent atomic configuration in the homo-metallic environment. A remarkable linear relationship is found between core level shifted spectral components, originating from different atomic geometrical structures, and the corresponding induced d-band center shifts. This finding strongly underscores the relevance of core level shifts as reliable experimental descriptors of chemical reactivity, also for under-coordinated atoms in bimetallic Transition Metal systems.


 

The availability of an experimental tool capable of detecting local surface alloy configurations with enhanced chemical reactivity, opens up the possibility to investigate more complex systems with high density of under-coordinated atoms, such as nanostructured multicomponent alloys surfaces and bimetallic nanoclusters.

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Enhanced Chemical Reactivity of Under-Coordinated Atoms at Pt-Rh
Bimetallic Surfaces: A Spectroscopic Characterization; Alessandro Baraldi, Laura Bianchettin, Stefano de Gironcoli, Erik Vesselli, Silvano Lizzit,
Luca Petaccia, Giovanni Comelli, and Renzo Rosei;
J. Phys. Chem. C 15, 3378 (2011).




The surface structure of Rh oxide

Ultra-thin oxide films have recently attracted great interest. Consistent research efforts are focused on the characterization of their morphological and electronic properties, which are extensively studied in order to establish a link with their chemical reactivity trends.
In particular, the peculiar properties of transition metal (TM) oxide films can be exploited for tailoring more efficient and cheap catalysts. Indeed, it is well known that, under heterogeneous catalytic oxidation reaction conditions, new oxide phases may be stabilized on TM surfaces, thus significantly modifying the reactivity of the active catalyst. By using XPD we measured one- and two-dimensional angular scans of the O1s and Rh3d5/2 core level shifted components

 

 which were compared to the corresponding multiple-scattering simulations of the modulation amplitudes, yielding a quantitative structural evaluation. We found a relaxation of both the in-plane and out-of-plane coordinates, yielding significant improvement of the reliability factor. In advance, we present novel insights into the structural details of the oxide film on the basis of a thorough parallelism with the bulk RhO2 rutile phase.

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The Rh oxide ultrathin film on Rh(100): an x-ray photoelectron diffraction study;
Rong Rong Zhan, Erik Vesselli, Alessandro Baraldi, Silvano Lizzit,
and Giovanni Comelli
J. Chem. Phys. 133, 214701 (2010)



 



Self-assembled Rh nanoclusters on graphene template

Several studies have clearly demonstrated that atomic aggregates in the nanometer size range, that is, formed by dozens or hundreds of atoms, present remarkably different properties with respect to their bulk crystalline counterparts. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms.
We have presented a thorough study on graphene-supported Rh nanocluster assemblies grown in register with the template surface of graphene/Ir(111) and their geometry dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory.


 

 By carefully selecting the Rh coverage and the annealing temperature, we demonstrate the possibility to control the density of edge and facet atoms and the nanocluster arrangement in superlattices. In particular, growth conditions of the nanoclusters exhibit a remarkably high degree of crystallinity.
 By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters.
We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms.

The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism.
Our approach gives access to detailed information on fundamental properties at the nanoscale and may help pave the road for attractive technological applications.

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Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template;
Alberto Cavallin, Monica Pozzo, Cristina Africh, Alessandro Baraldi, Erik Vesselli, Carlo Dri, Giovanni Comelli, Rosanna Larciprete, Paolo Lacovig, Silvano Lizzit, and Dario Alfe`;
ACSNano 6, 3034 (2012).

  

  

Ultima modifica il Lunedì, 21 Dicembre 2020 11:53