Caroline M. Whelan

Kinetic parameters of CO adsorbed on Pt(111) studied by in situ high resolution x-ray photoelectron spectroscopy

The kinetics of the adsorption system CO/Pt(111) has been studied by time-resolved high-resolution x-ray photoelectron spectroscopy using third generation synchrotron radiation at BESSY II. CO is dosed by a supersonic molecular beam device which allows for a high sample pressure (here up to 10−6u2009mbar) and a fast switching of the pressure. The site-specific occupation of CO adsorbed on on-top and bridge sites is determined quantitatively from Cu200a1s spectra, which can be taken with a minimum collection time of 1.5 s per spectrum. Based on the observation of thermal equilibrium between the two sites, we perform a phenomenological analysis of the data, assuming a constant binding energy difference ΔE. From the on-top/bridge occupation ratio as a function of coverage obtained by uptake measurements we extract a value of ΔE=41u2009meV. With the same ansatz, ΔE is calculated from temperature-dependent measurements at a constant coverage. Finally, determination of the coverage during isothermal desorption is used to o...

IN-SITU CORE-LEVEL PHOTOELECTRON SPECTROSCOPY OF ADSORBATES ON SURFACES INVOLVING A MOLECULAR BEAM — GENERAL SETUP AND FIRST EXPERIMENTS

In this contribution we introduce a new apparatus combining high-resolution photoelectron spectroscopy and supersonic molecular beam techniques designed to follow simple surface reactions like adsorption, desorption or oxidation in situ in a time- and temperature-resolved manner. Using high brightness synchrotron radiation high resolution core-level spectra can be obtained in less than 3 s. The molecular beam allows for local pressures of up to 1 · 10-5 mbar and translational energies of the molecules of up to 2 eV in the case of CO. Preliminary examples of CO adsorption on Pt(111) are shown, where kinetic parameters can be derived from the coverage dependence of the different adsorption sites.

Effect of oxygen rf-plasma on electronic properties of CNTs

Multi-wall carbon nanotubes (CNTs) were modified by an oxygen radio frequency plasma treatment, the appropriate duration of which was determined from post-treatment high-resolution transmission electron microscopy (HRTEM) images. A comparison of HRTEM results from gold-decorated pristine and plasma treated CNTs showed that the plasma treatment improves the uniformity of the distribution of surface defects. X-ray photoelectron spectroscopy confirms that oxygen atoms attach to the surface of CNTs. Changes in the C 1s photoemission signal point towards a loss of delocalization of valence electrons, confirmed by the modifications of valence band spectra, and consistent with currently accepted theory.

Self-assembly of mechanically interlocked and threaded rings : a HREELS and XPS study of thiol-functionalised catenane and rotaxane molecules on Au(111)

Abstract Thiol-functionalised catenane and rotaxane thin films were investigated in order to understand the self-assembly of such complex molecules on Au(111). Adsorption from the liquid phase at 300xa0K leads to the formation of overlayers without long-range order, as evidenced by high-resolution electron energy-loss spectroscopy (HREELS). As expected for thiol adsorption, the sulfur 2p binding energies determined by X-ray photoelectron spectroscopy (XPS) are consistent with the formation of thiolate surface intermediates. The properties of these films are explored as a function of annealing. Changes in vibrational spectra such as the emergence of a Auue5f8O band and variations in core-level binding energies and intensities reveal molecular rearrangement due to partial desorption. In addition, based on coherent domain sizes estimated by the angular width of the elastic beam intensity, annealing promotes long-range order within the adlayers.

The thermal chemistry of saturated layers of acetylene and ethylene on Ni(100) studied by in situ synchrotron x-ray photoelectron spectroscopy

Temperature-programmed x-ray photoelectron spectroscopy was used to study the thermal chemistry of acetylene (C2H2) and ethylene (C2H4) on Ni(100) in the temperature range 90–530 K. The use of a third generation synchrotron light source facilitated the measurement of high-resolution Cu200a1s photoelectron spectra within a few seconds, approaching the ideal of real-time analysis. In a quantitative and quasi-continuous manner, the thermal dehydrogenation pathways are followed. For the acetylene decomposition, acetylide (CCH) and methylidyne (CH) are confirmed as intermediates. For the dehydrogenation of ethylene, a vinyl species (C2H3) is observed. Using the fingerprint capabilities of x-ray photoelectron spectroscopy, acetylene can be identified as the subsequent dehydrogenation product. Upon further heating, acetylide and methylidyne are successively formed on the surface, as in the decomposition experiment starting with acetylene adsorbed at 100 K. For both systems carbidic carbon is formed as the final dehy...

A fast x-ray photoelectron spectroscopy study of the adsorption and temperature-dependent decomposition of propene on Ni(100)

Using synchrotron radiation, the adsorption and decomposition of propene (C3H6) on the Ni(100) surface has been investigated in situ by time-resolved and temperature-programmed x-ray photoelectron spectroscopy. At 105 K, high-resolution C 1s spectra indicate precursor mediated occupation of a single adsorption state from submonolayer to monolayer coverage with evidence of adsorbate–adsorbate interactions and dispersed phase growth. High exposures lead to the formation of multilayers which desorb above 105 K leaving a chemisorbed monolayer. Between 105 and 150 K, a shift of the binding energies in the C 1s spectra is attributed to the transition from π- to di-σ-bonded propene. An abrupt change in the photoemission spectra occurs at 200 K due to the conversion of di-σ-bonded propene to a C3 intermediate containing a methyl group. Formation of this C2HxCH3 surface species is complete at 300 K and is immediately proceeded by dehydrogenation to carbidic carbon which is the final decomposition product above 370 K.

Experimental and theoretical study of the adsorption of fumaramide [2]rotaxane on Au(111) and Ag(111) surfaces

Thin films of fumaramide [2]rotaxane, a mechanically interlocked molecule composed of a macrocycle and a thread in a bead and thread configuration, were prepared by vapor deposition on both Ag(111) and Au(111) substrates. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy were used to characterize monolayer and bulklike multilayer films. XPS determination of the relative amounts of carbon, nitrogen, and oxygen indicates that the molecule adsorbs intact. On both metal surfaces, molecules in the first adsorbed layer show an additional component in the C 1s XPS line attributed to chemisorption via amide groups. Molecular-dynamics simulation indicates that the molecule orients two of its eight phenyl rings, one from the macrocycle and one from the thread, in a parallel bonding geometry with respect to the metal surfaces, leaving three amide groups very close to the substrate. In the case of fumaramide [2]rotaxane adsorption on Au(111), the presence of certain out-of-plane phenyl ring and Au-O vibrational modes points to such bonding and a preferential molecular orientation. The theoretical and experimental results imply that the three-dimensional intermolecular configuration permits chemisorption at low coverage to be driven by interactions between the three amide functions of fumaramide [2]rotaxane and the Ag(111) or Au(111) surface.

Interaction Forces Between a Glass Surface and Silica-Modified PMMA-Based Abrasives for CMP Measured by Colloidal AFM

Interaction forces between a glass surface and two types of silica-coated poly(methyl methacrylate) (PMMA)-based terpolymer abrasive particles were investigated using colloidal atomic force microscopy (AFM) and correlated with relevant chemical mechanical planarization (CMP) and post-CMP parameters. Based on the average values of the pull-off force vs pH, the interaction between a glass bead and the composites can be considered as a silica-glass interaction, as confirmed by ζ potential measurements. The increase in repulsive force with increasing pH corresponds to a decrease in composite-glass adhesion. For pH between 2 and 7, the electrostatic attractive forces between polymer particles and silica surface are responsible for high particle counts.

Sealing of porous low-k dielectrics

The ongoing evolution from SiO 2 to insulator materials with lower dielectric constant, k, through the introduction of pores, brings new challenges in terms of processing and reliability. Porosity enhances penetration of undesired chemical species. A method of sealing microporous low-k dielectric chemical vapor deposited silicon oxycarbide films using UV-ozone induced oxidation has been investigated. The film thickness, refractive index, porosity, pore size, and sealing as a function of exposure time have been characterized by ellipsometry. The films are sealed without modification of the underlying porosity, an essential first step in integrating porous materials.

A HREELS study of benzenethiol adsorption on Au(111) and Au(322)

Adsorption of benzenethiol on a Au(111)--reconstructed surface at 300 K leads to formation of molecular long range disordered overlayers without lifting of the surface reconstruction. Absence of the S–H stretching vibration, both in and out of specular scattering geometry, along with identification of modes typical of benzene-like systems, indicates formation of a thiolate intermediate at both saturation and submonolayer coverage. Comparison of HREELS band intensities as a function of coverage in the specular direction indicates that no major coverage-dependent reorientation occurs on Au(111) . Consideration of relative band intensities, use of dipole selection rules and comparison with previous studies of benzene and benzenethiol adsorption on transition metal surfaces indicate a largely flat-lying bonding geometry at both monolayer and submonolayer coverages. In contrast, adsorption on Au(322), a surface consisting of monoatomic (100) steps separating (111) microterraces five unit cells wide, leads to formation of an ordered monolayer. The adsorbed intermediate on Au(322) is benzenethiolate at both low and high coverage. At low coverages the molecule is strongly inclined with respect to the (111) micro-terraces while at higher coverages adsorbates adopt a more flat-lying geometry. This unusual behavior may be due to coverage-dependent occupation of step and terrace sites with a strongly inclined species formed at step sites, preferentially occupied at low coverage, and a second flat-lying state becoming occupied within the monolayer. Alternatively, both step- and terrace-bound thiolate are strongly inclined at low exposures, with the two species undergoing an orientational transition beyond a critical coverage to form a more flat-lying intermediate. This study shows clear evidence for step-induced ordering of the thiolate intermediate and the ability of steps to promote a molecular reorientation.