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|Title: ||Growth Morphology, Electronic Properties and Interaction of Organo-Metallic Molecules Adsorbed on Graphene|
|Authors: ||SCARDAMAGLIA, MATTIA|
|Tutor: ||Mariani, Carlo|
|Issue Date: ||17-Dec-2012|
|Abstract: ||Since the isolation of a single graphene flake in 2004, the extraordinary electronic, optical and mechanical properties of this one-atom-thick material emerged, attracting a continuous rising interest both from fundamental and technological perspectives.
Within this research field, a key challenge is to move from model structures to more complex configurations: being graphene a surface without bulk, its properties are very sensitive to local perturbations, but the countless possibilities of modifying graphene properties by suitable functionalization procedures with atoms or molecules are still largely unexplored. The possibility to control and tailor the electronic, morphological and transport properties of graphene, by influencing its band parameters and interaction strength, is a requirement in the perspective of nano-devices fabrication. Furthermore, the effect that graphene itself could have on the molecular or metal nanostructure deposited onto it is an
intriguing ambivalence of the graphene functionalization. However, reproducibility and reliability is still an issue.
An important help in the formation of ordered and reproducible nano-architectures on graphene comes from the epitaxial growth of graphene on hexagonally close-packed surfaces of transition metals: the similar, but slightly mismatched, lattice structure of these two materials leads to the formation of an ordered modulation of the graphene sheet, also known as moiré pattern. This superstructure, whose actual geometrical and chemical corrugation depends on the degree of interaction with the substrate, can be used as a template for the formation of ordered arrays of adsorbates. Using epitaxial graphene with a nanoscale controlled corrugation leads to the opportunity to create regular 1D and 2D architectures by atom or molecular adsorption in order to tailor the electronic, magnetic or transport properties of these low-dimensional structures. A fascinating perspective is to study arrays of size-selected magnetic nano-clusters or systems comprising a single magnetic atoms embedded in an organic frame in interaction with the graphene-metal support to elucidate their structural, electronic and magnetic properties: self-assembling of nano-sized architectures is attractive for basic investigations as well as for device applications. In particular, supramolecular ordered assembly of metalorganic molecules on graphene is a suitable way to obtain regular nano-architectures with the metallic atoms ordered in a spin network.
This Thesis lies within the scientific background described so far and constitutes a first step on the way of the research in this field: the aim of this work is addressed to the control and the tune of the electronic and, in future, the magnetic properties of regular arrays of metal-phthalocyanines with a single magnetic atom embedded in an organic frame in interaction with the nano-patterned graphene-metal support.
By investigating the effect on the graphene and molecular properties exploiting several spectroscopic techniques we aim to solve the puzzling problem of the balance between molecule-molecule and molecule-substrate interaction.
Within this context, the in situ growth of graphene is one of the target of this work, that consists in an effort to design a suitable system adaptable to the present experimental stage.
Among the several substrate on which graphene can be grown, the hexagonally close-packed (111) surface of iridium has revealed to be the best compromise between interaction and natural corrugation. Graphene on Ir(111) reveals all the electronic properties of free standing graphene almost unaffected by the interaction with the substrate, the weak hybridization is testified by a slight p-doping of the Dirac cone. Ir(111) is therefore a good substrate leading to a quasi-free standing graphene sheet with the remarkable advantage of the formation of a highly-ordered hexagonal modulated moiré superstructure generated by the slight lattice mismatch between the lattice constant of these two materials, testified by both STM and LEED measurements. Furthermore, a high stability and a low degree of degradation of supported graphene after air exposure has been demonstrated by fully recovering of the as-grown properties by annealing treatments.
The moiré structure has been thus exploited as a template to fabricate unique bidimensional molecular networks of metal-phthalocyanines (FePc, CoPc and CuPc). The main effect of the adsorption of a FePc single-layer onto the graphene sheet is a light electron doping, which counterbalances the original p-doping of graphene/Ir, bringing the graphene sheet to a quasi-ideal condition.
By means of X-ray absorption spectroscopy we demonstrated the flat lying geometry of the molecular layers adsorbed on graphene. The organic macrocycle of the molecules presents a weak interaction with the substrate, since no core-level lineshape changes are observed from single-layer stage to thin-film. However, subtle differences between molecules arise: the difference in the benzene and pyrrole carbon atom energy shift indicates a minor deformation of CuPc in direct contact with graphene with respect to FePc and CoPc.
Thermal-programmed desorption experiments have revealed to be a powerful tool to investigate the interaction strength between molecules and substrate. The different behavior of CuPc with respect to FePc and CoPc is obtained by a low desorption temperature of the molecular single-layer adsorbed on graphene, consistent with a pure van der Waals interaction. FePc and CoPc instead show a high thermal stability, sign of a stronger interaction than a pure van der Waals. The reason of this difference lies in the different occupation of the 3d shell of the central metal atom of the MPcs: moving from molecules with mostly unfilled 3d shell, as FePc and CoPc, to molecules with more filled 3d shell, as CuPc, the metallic character of the molecular orbital is progressively reduced. Thus open shell MPcs at the SL-stage are more inclined to interact with the supporting
We determine also the growth mode of the molecular thin-film, which is consistent with a Stranski-Krastanov growth. This is the case of a balance between molecule-molecule and molecule-substrate interaction, since after the completion of the first monolayer an island growth takes place. This is an important achievement, which indicates a template-driven grown at the single-layer stage, different from the case of MPc adsorbed on graphite (HOPG) forming island even at low coverage. The difference with graphite is further highlighted in a thermal desorption experiment comparing the desorption temperature of adsorbed FePc onto the two substrates: it is definitively higher on graphene than on HOPG.
An important role played by graphene is the decoupling action between the underlying Ir metal substrate and the adsorbed molecules. The surface states of iridium are preserved upon adsorption of molecular thin-film when the graphene sheet is in between them, while direct adsorption onto the metal totally quenches these structures already at low coverage. Not only the substrate’s surface properties are preserved, indeed graphene seems to induce an enhancement of the magnetic dichroism for a single-layer of FePc; non trivial behavior of the orbital and spin moments as a function of coverage has been determined. In the perspective to design ordered nano-architectures of molecules in which a single magnetic atom is embedded in an organic frame to exploit an ordered spin network, the moiré template of graphene/Ir(111) is thus a perfect candidate, trapping the molecules without corrupting their properties and, at the same time, leaving graphene almost unaffected by the metallic substrate.
The puzzling problem is then a subtle balance between molecule-molecule and molecule-substrate interaction, where the morphology of the moiré pattern is a fundamental parameter but it is not the only driver of the interaction, since even the d-state occupation of the central metal atom of the MPc molecule determines the interaction strength. The flat monolayer of weakly interacting MPc molecules is then an ideal channel for conduction and doping by using substituted phthalocyanines, rendering it an organic buffer layer decoupled from the underlying metal.
A natural prosecution of this work is to extend of the study to other MPcs with different magnetic metallic center in order to discriminate the relationship between the spin state and molecular orbitals influenced by the graphene sheet. This study provides a procedure protocol to identify the best conditions to control the graphene properties in different environment and to define the best suitable substrate to design ordered nano-architectures.|
|Appears in PhD:||SCIENZA DEI MATERIALI|
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