Growth Morphology, Electronic Properties and Interaction of Organo-Metallic Molecules Adsorbed on Graphene

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 substrate. 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

Growth Morphology, Electronic Properties and Interaction of Organo-Metallic Molecules Adsorbed on Graphene

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 substrate. 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

Cooling dynamics and thermal interface resistance of glass-embedded metal nanoparticles

The cooling dynamics of glass-embedded noble metal nanoparticles with diameters ranging from 4 to 26 nm were studied using ultrafast pump-probe spectroscopy. Measurements were performed probing away from the surface plasmon resonance of the nanoparticles to avoid spurious effects due to glass heating around the particle. In these conditions, the time-domain data reflect the cooling kinetics of the nanoparticle. Cooling dynamics are shown to be controlled by both thermal resistance at the nanoparticule?glass interface, and heat diffusion in the glass matrix. Moreover, the interface conductances are deduced from the experiments and found to be correlated to the acoustic impedance mismatch at the metal/glass interface

Plasma fluorination of vertically aligned carbon nanotubes:functionalization and thermal stability

Grafting of fluorine species on carbon nanostructures has attracted interest due to the effective modification of physical and chemical properties of the starting materials. Various techniques have been employed to achieve a controlled fluorination yield; however, the effect of contaminants is rarely discussed, although they are often present. In the present work, the fluorination of vertically aligned multiwalled carbon nanotubes was performed using plasma treatment in a magnetron sputtering chamber with fluorine diluted in an argon atmosphere with an Ar/F2 ratio of 95:5. The effect of heavily diluted fluorine in the precursor gas mixture is investigated by evaluating the modifications in the nanotube structure and the electronic properties upon plasma treatment. The existence of oxygen-based grafted species is associated with background oxygen species present in the plasma chamber in addition to fluorine. The thermal stability and desorption process of the fluorine species grafted on the carbon nanotubes during the fluorine plasma treatment were evaluated by combining different spectroscopic techniques

Fluorination of vertically aligned carbon nanotubes:from CF4 plasma chemistry to surface functionalization

The surface chemistry of plasma fluorinated vertically aligned carbon nanotubes (vCNT) is correlated to the CF4 plasma chemical composition. The results obtained via FTIR and mass spectrometry are combined with the XPS and Raman analysis of the sample surface showing the dependence on different plasma parameters (power, time and distance from the plasma region) on the resulting fluorination. Photoemission and absorption spectroscopies are used to investigate the evolution of the electronic properties as a function of the fluorine content at the vCNT surface. The samples suffer a limited ageing effect, with a small loss of fluorine functionalities after two weeks in ambient conditions

In situ XPS of Competitive CO2/H2O Absorption in an Ionic Liquid

Superbasic ionic liquids (SBILs) are being investigated as potential CO2 gas capture agents, however, the presence of H2O in the flue stream can inhibit the uptake of CO2. In this study a thin film of the SBIL trihexyltetradecylphosphonium 1,2,4-triazolide ([P66614][124Triz]) was deposited onto rutile TiO2 (110) using in situ electrospray deposition and studied upon exposure to CO2 and H2O using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The molar uptake ratio of gas in the electrosprayed SBIL (ngas:nIL) was calculated to be 0.3:1 for CO2, 0.7:1 for H2O, and 0.9:1 for a CO2/ H2O mixture. NAP-XPS taken at two different depths reveals that the competitive absorption of CO2 and H2O in [P66614][124Triz] varies with sampling depth. A greater concentration of CO2 absorbs in the bulk layers, while more H2O adsorbs/absorbs at the surface. The presence of H2O in the gas mixture does not inhibit the absorption of CO2. Measurements taken during exposure and after the removal of gas indicate that CO2 absorbed in the bulk does so reversibly, whilst CO2 adsorbed/absorbed at the surface does so irreversibly. This is contrary to the fully reversible CO2 reaction shown for bulk ILs in literature and suggests that irreversible absorption of CO2 in our highly-structured thin films is largely attributed to reactions at the surface. This has potential implications on IL gas capture and thin film IL catalysis applications

Metal-free catalysis based on nitrogen-doped carbon nanomaterials: a photoelectron spectroscopy point of view

In this review, we discuss the use of doped carbon nanomaterials in catalysis, a subject that is currently intensively studied. The availability of carbon nanotubes since the 1990’s and of graphene ten years later prompted the development of novel nanotechnologies. We review this topic linking fundamental surface science to the field of catalysis giving a timely picture of the state of the art. The main scientific questions that material scientists have addressed in the last decades are described, in particular the enduring debate on the role of the different nitrogen functionalities in the catalytic activity of nitrogen-doped carbon nanotubes and graphene

Sonication-induced effects on carbon nanofibres in composite materials

The preparation and characterization of carbon nanofibre-gellan gum composite materials is presented. Electron microscopy analysis reveals that nanofibres are affected by sonolysis, i.e. fibre length reduces, while filling occurs. Spectroscopic analysis suggests that the nanofibres are modified during the preparation of the dispersions. It is shown that despite these effects, composite materials prepared using a short period of sonolysis (4 min) exhibit robust conductivity, strain at failure and Young\u27s modulus values of 35 ± 2 S cm−1, 20 ± 1% and 1.3 ± 0.3 MPa, respectively