Matteo Dalmiglio

Transfer-Free Electrical Insulation of Epitaxial Graphene from its Metal Substrate

High-quality, large-area epitaxial graphene can be grown on metal surfaces, but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a stepwise intercalation of silicon and oxygen, and the eventual formation of a SiO(2) layer between the graphene and the metal. We follow the reaction steps by X-ray photoemission spectroscopy and demonstrate the electrical insulation using a nanoscale multipoint probe technique.

Energetics and Hierarchical Interactions of Metal–Phthalocyanines Adsorbed on Graphene/Ir(111)

The adsorption of metal-phthalocyanine (MPc) layers (M = Fe, Co, Cu) assembled on graphene/Ir(111) is studied by means of temperature-programmed X-ray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). The balance between interaction forces among the organometallic molecules and the underlying graphene gives rise to flat-lying molecular layers, weakly interacting with the underlying graphene. Further MPc layers pile up face-on onto the first layer, up to a few nanometers thickness, as deduced by NEXAFS. The FePc, CoPc, and CuPc multilayers present comparable desorption temperatures, compatible with molecule-molecule interactions dominated by van der Waals forces between the π-conjugated macrocycles. The MPc single layers desorb from graphene/Ir at higher temperatures. The CuPc single layer desorbs at lower temperature than the FePc and CoPc single layers, suggesting a higher adsorption energy of the FePc and CoPc single layers on graphene/Ir with respect to CuPc, with increasing molecule-substrate interaction in the order E(CuPc) < E(FePc) ~ E(CoPc).

Revealing the adsorption mechanisms of nitroxides on ultrapure, metallicity-sorted carbon nanotubes.

Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO2 and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. Franck–Condon satellites, hitherto undetected in nanotube–NOx systems, were resolved in the N 1s X-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1s photoemission signal is due to the C=O species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature.

Spatially Resolved Chemical Characterization with Scanning Photoemission Spectromicroscopy: Towards Near‐Ambient‐Pressure Experiments

The major experimental challenges in investigations of heterogeneous catalysis are the morphologically complex and dynamic micro‐ and nanosystems and the exploration of events that occur at the catalyst surface, which determine the catalyst activity and selectivity. Modern‐day investigations of catalytic reactions require a multitechnique experimental and computational approach, in which each tool provides specific and complementary information. The unique combination of surface and chemical sensitivity has ranked X‐ray photoelectron spectroscopy (XPS) as one of the most important experimental methods for the characterization of catalytic systems. After its invention, more than half a century ago, a revolutionary step in XPS development was the addition of sub‐micrometer lateral resolution intermediate between light microscopy and electron microscopy. XPS microscopes have responded to the increasing demands on nanotechnology to have access to the local chemical composition, electronic and magnetic structure, and reorganization processes at morphologically complex surfaces and interfaces. The high spatial resolution in XPS microscopes is achieved by two different approaches, magnification of the image of the irradiated surface area (X‐ray photoemission electron microscopy) or demagnification of the incident photon beam by using X‐ray focusing optics (scanning photoemission microscopy; SPEM). In the present article, using selected examples, we demonstrate the capabilities of SPEM for studies relevant to catalysis, and we will discuss the next steps in the ongoing development. In the first part, we present the successful characterization of the oxidation of (i) polycrystalline PtRh particles and (ii) Pd thin films that decorate carbon nanotubes. In the second part, we describe two new setups, developed at Elettra, to overcome the “pressure gap” for photoemission spectromicroscopy experiments, which is the major limitation in the exploration of “real world” catalytic reactions. The first measurements of core‐level photoemission spectroscopy and imaging obtained with spatial resolution of the order of 100 nm at near‐ambient pressure are presented.

Electron resist behavior of Pd hexadecanethiolate examined using X-ray photoelectron spectroscopy with nanometric lateral resolution.

Electron resist behavior of Pd hexadecanethiolate is studied by varying the e-dosage from 2-280 muC.cm(-2). The e-beam exposed resist is characterized using energy dispersive spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy with nanometric lateral resolution. Electron beam exposure causes defects in the alkyl chain of the thiolate, giving the required solubility contrast during the developing step, thus qualifying the precursor as an e-beam resist. On exposure to the e-beam, the reduction of Pd(2+) to Pd(0) is observed, and the reduction increases with increasing e-dosage. The resist is highly sensitive, with the estimated sensitivity being 32 muC.cm(-2). Thermolysis at 250 degrees C leads to the formation of Pd nanoparticles, demonstrating the essential feature of a direct write resist for conducting patterns.

Disentangling Vacancy Oxidation on Metallicity-Sorted Carbon Nanotubes

Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O2 and N2, which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O2 doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all of the SWCNT’s vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first-principles density functional theory calculated Kohn–Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups o...