P.L. Wincott

Scanning tunnelling microscopy of suspended graphene

Suspended graphene has been studied by STM for the first time. Atomic resolution on mono- and bi-layer graphene samples has been obtained after ridding the graphene surface of contamination via high-temperature annealing. Static local corrugations (ripples) have been observed on both types of structures.

Controlled cobalt doping in biogenic magnetite nanoparticles

Cobalt-doped magnetite (CoxFe3 −xO4) nanoparticles have been produced through the microbial reduction of cobalt–iron oxyhydroxide by the bacterium Geobacter sulfurreducens. The materials produced, as measured by superconducting quantum interference device magnetometry, X-ray magnetic circular dichroism, Mössbauer spectroscopy, etc., show dramatic increases in coercivity with increasing cobalt content without a major decrease in overall saturation magnetization. Structural and magnetization analyses reveal a reduction in particle size to less than 4 nm at the highest Co content, combined with an increase in the effective anisotropy of the magnetic nanoparticles. The potential use of these biogenic nanoparticles in aqueous suspensions for magnetic hyperthermia applications is demonstrated. Further analysis of the distribution of cations within the ferrite spinel indicates that the cobalt is predominantly incorporated in octahedral coordination, achieved by the substitution of Fe2+ site with Co2+, with up to 17 per cent Co substituted into tetrahedral sites.

Incorporation of sulphur into the (111) surface of nickel

Abstract With the aim of monitoring coverage-dependent adsorbate coordination, we have studied the polarisation-dependent S K-edge SEXAFS of p(2 × 2)S and ( 5 3 × 2) S overlayers on Ni(111). The data indicate S bonding to the three-fold hollow sites in the p(2 × 2)S phase, with a SiNi bond distance of 2.23 ± 0.02 A. In the higher coverage, ( 5 3 × 2) S phase the S-Ni distance is 2.27 ± 0.02 A. The Ni (111)(5 3 × 2) S results are compared with those expected on the basis of the three proposed structural models of this surface. The data are consistent with only one of these models, in which S occupies rectangular hollow sites in a pseudo-Ni(100)c(2 × 2)S overlayer.

Electrochemical and Surface Analytical Studies of Enargite in Acid Solution

The electrochemical oxidation and reduction of the surface of natural enargite (Cu3AsS4) was investigated in 0.1 M HCl solution using cyclic voltammetry and chronoamperometry. Surface analysis by ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy, together with aqueous phase analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES), were used to aid in the interpretation of the electrochemical behavior of this complex system. XPS analyses on enargite oxidized at potentials >0.2 V (SCE) detected the presence of Cull surface species, with associated sulfate and chloride. At potentials 0.2 V (SCE), whereas dissolved arsenic concentrations were negligible over the entire potential range investigated. Raman spectroscopy provided additional evidence of elemental sulfur formation at oxidizing potentials at which sulfate forming reactions occurred in parallel. The sulfur was responsible for an active-passive transition observed at ca. 0.3 V (SCE) in voltammograms

Photoemission studies of single crystal CuO(100)

Photoemission studies of a cleaved CuO single crystal have been carried out, using station 6.2 at the CLRC Daresbury Laboratory. The (100) face orientation produced by anvil cleaving was determined using Laue back-reflection and LEED. Resonant photoemission measurements at the Cu 3pCu 3d threshold were undertaken and these are compared with existing data from oxidized copper films, and with theoretical configuration interaction calculations for the resonance process. Angle-resolved measurements of the valence band structure have also been performed at different photon energies along three crystal axes, and these are mapped onto the 3D structure of the monoclinic material. This allows general comparison with existing momentum-resolved calculations. The validity of comparisons with both localized and delocalized descriptions of the electronic structure of this highly correlated oxide is discussed.

NEXAFS fingerprinting of TiO2(110)-SO2

Abstract Polarisation-dependent S K-edge NEXAFS has been used to monitor the progression of the thermally-activated reaction between TiO 2 (110) and SO 2 .SO 2 chemisorbs at 105 K, reacting to form a surface sulphate-like species as the substrate is heated to ⩾ 135 K. This is the final reaction product, having a coverage of 0.1 ML up to at least 450 K. A sulphite-like species is identified as an intermediate in this reaction, having a maximum concentration of ∼ 0.2 ML at 155 K. Structural models are suggested in which SO 2 initially bonds to Ti sites, subsequently forming complex anion-like species by incorporation into the raised row of bridging O atoms.

Electronic structure of Pt overlayers on (1 × 3) reconstructed TiO2(100) surfaces

Abstract The effect of Pt atoms on the Ti 3d emission of Ti3+ states at (1 × 3) reconstructed TiO2(100) surfaces was studied by means of valence band photoemission spectroscopy using synchrotron radiation with excitation energies between 30 and 120 eV. A clean (1 × 3) reconstructed TiO2(100) surface was prepared by Ar+ sputtering and annealing at T = 870 K as checked with low energy electron diffraction. Subsequently, Pt was evaporated in situ using a resistively heated Pt filament. Evaporation was performed stepwise up to a nominal coverage of 1.8 monolayers (ML) as checked with Auger electron spectroscopy (AES). The Pt 4f core levels are shifted to higher binding energies with respect to Pt metal, indicating the presence of small Pt clusters. Difference spectra show a dublet between the TiO2 valence band maximum and the Fermi energy which is attributed to Pt 5d states. We found that the position and the intensity of the Ti 3d band gap emission were not significantly altered upon evaporation of Pt. In addition, the valence band maximum of TiO2 remains unchanged. These findings indicate the absence of an electronic charge transfer due to the high work function of TiO2(100)-(1 × 3) of 5.4 eV, in line with a simple metal-semiconductor contact theory.

Electronic structure effects of potassium adsorption on TiO2(100)

Potassium adsorption and oxygen co-adsorption on TiO 2 (100) have been studied with resonance photoemission and surface core-level shift spectroscopy. K-induced band-gap states are observed at 0.9±0.1 eV, arising from charge transfer from K4s into Ti3d polaron states. Corresponding Ti2p and 3p surface core-level shifts or -1.8 eV are observed, consistent with reduction of surface cations, nominally to Ti 3+ . Oxygen adsorption on the K pre-covered surface is found to largely reserve the charge transfer

Origin of the photon induced Cl+ yield from Si(111)7 × 7-Cl at the Cl and Si K-edges

Abstract The yield of Cl + ions from Si(111)7 × 7-Cl has been monitored through both the Cl and Si K-edges. The Cl K-edge ion yield EXAFS suggests a majority site origin, the data producing the same result as Auger yield SEXAFS: an atop geometry with a Cl-Si bond length of 2.00 ± 0.02 A. The Cl + -yield data recorded at the Si K-edge are identical to bulk Si EXAFS and the desorption mechanism at this edge is concluded to be X-ray induced electron stimulated desorption (XESD).

X-ray and electron beam modification of thiophene overlayers on TiO2(100)1 × 1 and 1 × 3

The effects of electron and X-ray beams on thiophene overlayers on TiO2(100) 1 × 1 and 1 × 3 surfaces have been investigated using AES, UPS and XPS. Mg Kα X-rays were found to polymerise a thiophene multilayer condensed at 120 K. The evidence points to a substrate-secondary-electron mediated process. A 3 keV electron beam also modifies a condensed thiophene overlayer, probably by polymerisation.