Electronic and structural properties of graphene/metal interfaces

Onderzoek naar de interactie tussen grafeen en een ondersteunende metaallaag Grafeen is een koolstofverbinding met een tweedimensionale structuur; een vlak rooster waarin koolstofatomen in de vorm van een honingraat zijn gerangschikt. De verwachtingen voor toekomst zijn hooggespannen. Elektronica raakt meer en meer op koolstof gebaseerd en grafeen wordt gezien als een fundamenteel ingrediënt voor dergelijke toepassingen. Een beter begrip van de interacties tussen grafeen en de ondersteunende laag is dan nodig. Daarbij gaat het over de vraag hoe de eigenschappen van grafeen door deze interactie worden beïnvloed. Vooral de studie van raakvlakken tussen grafeen en metaal krijgt veel aandacht. De reden is de belangrijke rol van metalen in het groeiproces van grafeen (vooral chemische dampdepositie, CVD). Om bijvoorbeeld veldeffect transistoren (FET`s) gebaseerd op grafeen te realiseren, moeten de elektronische eigenschappen van het grafeenmembraan in contact met het metaal volledig gekarakteriseerd worden. In zijn proefschrift geeft Luca Bignardi een overzicht van de veranderingen van elektronische en structurele eigenschappen grafeen dat via de CVD-methode gegroeid is op technologisch relevante metalen zoals nikkel en koper. Bignardi bestudeerde bezette en onbezette elektronische toestanden en onderzocht de structurele eigenschappen van de raakvlakken. Hij gebruikte daarbij verschillende technieken zoals foto-elektronspectroscopie en scanning tunneling microscopy. In het onderzoek is ook veel aandacht besteed aan de optimalisering van groeiprocedures die de eigenschappen van grafeen kunnen beïnvloeden

A combined experimental and theoretical study of methyl acetoacetate adsorption on Ni{100}

The enantio-selective hydrogenation of methyl acetoacetate (MAA) over modified Ni-based catalysts is a key reaction in the understanding of enantioselective heterogeneous catalysis as it represents the only example of this class of reactions catalysed by base metals. Yet, there is very little molecular-level information available about the adsorption complex formed by the reactants on Ni surfaces. Here, we report a combined experimental and theoretical study of the adsorption of MAA on the Ni{100} surface. X-ray photoelectron spectroscopy shows that MAA forms stable multilayers at low temperatures, which desorb between 200 K and 220 K. At higher temperatures a single chemisorbed layer is formed, which decomposes between 300 K and 350 K. Density functional theory modelling predicts an enolate species with bidentate coordination as the most stable chemisorbed species. Comparison of photoelectron spectroscopy and X-ray absorption data with simulations using this adsorption model show good qualitative and quantitative agreement. The molecular plane is tilted with respect to the surface plane by about 50deg. This breaking of symmetry provides a mechanism for the enantio-selective hydrogenation

The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters

Iron oxide nanoclusters are of interest for a broad range of applications, but limited experimental information on their oxidation mechanism is available outside of the gas phase. Here, the oxidation of graphene-supported size-selected Fe-n clusters is studied using high-resolution X-ray Photoelectron Spectroscopy.Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe-n clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p(3/2) binding energy of metallic and oxidized clusters on the cluster size. Binding energies are also linked to chemical reactivity through the asymmetry parameter which is related to electron density of states at the Fermi energy. Upon oxidation, iron atoms in clusters reach the oxidation state Fe(II) and the absence of other oxidation states indicates a Fe-to-O ratio close to 1:1, in agreement with previous theoretical calculations and gas-phase experiments. Such knowledge can provide a basis for a better understanding of the behavior of iron oxide nanoclusters as supported catalysts

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study

The possibility of depositing precisely mass-selected Ag clusters (Ag-1, Ag-3, and Ag-7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d(5/2) core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag-7 deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level

Graphene growth from photo-polymerized bi-phenylthiol self-assembled monolayers

We present an enhanced methodology for the synthesis of graphene, from photo-polymerized self-assembled monolayers (SAMs) of 1,1ʹ-biphenyl-4-thiol on both electropolished and oxidized copper substrates. The SAMs were subjected to a two-step process involving light-induced polymerization followed by annealing in a vacuum furnace to yield the two-dimensional solid. Comprehensive characterization using contact angle measurements, X-ray photoelectron spectroscopy, and Raman spectroscopy, as well as scanning electron and transmission electron microscopy, provided conclusive evidence of growth of single-layer graphene. Notably, our findings revealed superior quality graphene on oxidized copper substrates compared to their electropolished counterparts, highlighting the impact of substrate choice on the quality of the resultant material

Thermolubricity of gas monolayers on graphene

Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. Graphene was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates

Early clinical experience with volumetric modulated arc therapy in head and neck cancer patients

<p>Abstract</p> <p>Background</p> <p>To report about early clinical experience in radiation treatment of head and neck cancer of different sites and histology by volumetric modulated arcs with the RapidArc technology.</p> <p>Methods</p> <p>During 2009, 45 patients were treated at Istituto Clinico Humanitas with RapidArc (28 males and 17 females, median age 65 years). Of these, 78% received concomitant chemotherapy. Thirty-six patients were treated as exclusive curative intent (group A), three as postoperative curative intent (group B) and six with sinonasal tumours (group C). Dose prescription was at Planning Target Volumes (PTV) with simultaneous integrated boost: 54.45Gy and 69.96Gy in 33 fractions (group A); 54.45Gy and 66Gy in 33 fractions (group B) and 55Gy in 25 fractions (group C).</p> <p>Results</p> <p>Concerning planning optimization strategies and constraints, as per PTV coverage, for all groups, D_98%> 95% and V_95%> 99%. As regards organs at risk, all planning objectives were respected, and this was correlated with observed acute toxicity rates. Only 28% of patients experienced G3 mucositis, 14% G3 dermitis 44% had G2 dysphagia. Nobody required feeding tubes to be placed during treatment. Acute toxicity is also related to chemotherapy. Two patients interrupted the course of radiotherapy because of a quick worsening of general clinical condition.</p> <p>Conclusions</p> <p>These preliminary results stated that volumetric modulated arc therapy in locally advanced head and neck cancers is feasible and effective, with acceptable toxicities.</p