Brett A. Sexton

A vibrational and TDS study of the adsorption of pyrrole, furan and thiophene on Cu(100): evidence for π-bonded and inclined species

Abstract Vibrational (EELS) and thermal desorption spectra (TDS) have been measured for the heterocyclic 5-membered ring molecules pyrrole (C4H4NH), furan (C4H4O) and thiophene (C4H4S) adsorbed on Cu(100) at 85–300 K. Adsorption is molecular and reversible with no evidence for dissociation. At low exposures, each molecule is weakly π-bonded to the CU(100) surface with the molecular ring parallel to the surface plane. The vibrational frequencies are essentially unperturbed relative to the condensed phases. At higher exposures, TDS and EELS results suggest two different models for adsorption prior to the onset of multilayer formation. Pyrrole and furan form a stable second layer (bi-layer) in which the molecular rings are inclined toward the surface normal. The bi-layer and the π-bonded monolayer can be separately isolated by thermal treatment. Thiophene, however, appears to increase the monolayer coverage by “compression” on the Cu(100) surface, with the average tilt angle of the rings changing with coverage. At higher exposures at 85 K, multilayer condensation is initiated for all molecules. Although the molecular frequency shifts for these adsorbed molecules are very small relative to the condensed phases, evidence for the different adsorption models is obtained from TDS studies and by application of the surface dipole selection rule. The case for the structure of the bi-layer of furan on Cu(100) is strongly supported by the known low temperature crystal structure in which alternate layers of rings are inclined to one another. The weak nature of these adsorption systems and lack of dissociation is consistent with previous work on weak molecular adsorption on copper surfaces.

A vibrational and TDS study of sulfur adsorbates on Cu(100): Evidence for CH3S species

Abstract Vibrational (EELS) and TDS data for methyl mercaptan (CH 3 SH), dimethyl sulfide (CH 3 ) 2 S and dimethyl disulfide (CH 3 S) 2 are analyzed to determine the nature of the adsorption states on Cu(100). Dimethyl sulfide is reversibly adsorbed on Cu(100); no dissociation (Cue5f8S bond breaking) was found. By contrast, methyl mercaptan and dimethyl disulfide dissociate below 300 K to form adsorbed CH 3 S (methyl mercaptide) species. Depending on the coverage, two orientations of methyl mercaptide are found: linear and bent. The two different orientations can be distinguished via the surface dipole selection rule by different intensities of the methyl rocking and deformation vibrations. By contrast with the methoxy species, which on Cu(100) decomposes to formaldehyde, no H 2 C=S is liberated during decomposition of CH 3 S. The mercaptide is stable to ∼ 350 K, but decomposes at higher temperatures to form adsorbed sulfur and recombinant methane, hydrogen and ethane. The methane appears to be formed by methyl-hydrogen recombination when the C-S bond scission occurs. TDS results show that sulfur released from the decomposition poisons the surface toward further adsorption. In addition, the selectivity toward methane versus ethane can be altered by pre-titrating the adsorbed hydrogen with oxygen, thereby changing the relative methyl-hydrogen and methyl-methyl recombination probabilities.

Quantitative comparison of preparation methodologies for x-ray fluorescence microscopy of brain tissue

X-ray fluorescence microscopy (XFM) facilitates high-sensitivity quantitative imaging of trace metals at high spatial resolution over large sample areas and can be applied to a diverse range of biological samples. Accurate determination of elemental content from recorded spectra requires proper calibration of the XFM instrument under the relevant operating conditions. Here, we describe the manufacture, characterization, and utilization of multi-element thin-film reference foils for use in calibration of XFM measurements of biological and other specimens. We have used these internal standards to assess the two-dimensional distribution of trace metals in a thin tissue section of a rat hippocampus. The data used in this study was acquired at the XFM beamline of the Australian Synchrotron using a new 384-element array detector (Maia) and at beamline 2-ID-E at the Advanced Photon Source. Post-processing of samples by different fixation techniques was investigated, with the conclusion that differences in solvent type and sample handling can significantly alter elemental content. The present study highlights the quantitative capability, high statistical power, and versatility of the XFM technique for mapping trace metals in biological samples, e.g., brain tissue samples in order to help understand neurological processes, especially when implemented in conjunction with a high-performance detector such as Maia.

Refractive microlens array for wave-front analysis in the medium to hard x-ray range

We report an alternative approach to x-ray wave-front analysis that uses a refractive microlens array as a Shack-Hartmann sensor. The sensor was manufactured by self-assembly and electroplating techniques and is suitable for high-resolution wave-front analysis of medium to hard x rays. We demonstrate its effectiveness at an x-ray energy of 3 keV for analysis of x-ray wave-front perturbations caused by microscopic objects. The sensor has potential advantages over other methods for x-ray phase imaging and will also be useful for the characterization of x-ray beams and optics.

STM investigation of galena surfaces in air

Abstract The scanning tunnelling microscope has been used to resolve the surface microstructure of natural galena (PbS), cleaved in air. Large atomically flat (001) planes with single and multiple atom layer steps are common. Cubic (100) facets and dislocations are observed. Surfaces exposed to air for several days have also been imaged and results are compared with oxide depth measurements by XPS. We present evidence for discontinuous oxide layers on these air-exposed surfaces, which may have some implications in the flotation chemistry of minerals of this type.

Scanning probe microscope analysis of microstructures in optically variable devices

The fine-scale features of optically variable devices (OVDs) fabricated in resist by electron beam lithography have been examined using scanning probe microscopy (SPM). These features have included patterns of gratings, micro-text and geometrical images. Scanning probe microscopy has provided information on the groove angle, depth of profile and spatial frequency of the features as determined by the details of processing of the image. The OVD patterns formed in EBR-9 and X-AR-P 7400 resists exhibited a more rounded profile with a lower side-wall angle than in ZEP-7000 and PMMA resists.

Surface atomic structure of the GaAs(001)(2 × 4) reconstruction

Abstract Atomic resolution STM images of the (2 × 4) reconstruction, are presented which show the existence of at least four distinct surface structures. Two of these correspond to the α and β phases previously modelled from RHEED experiments. The second layer structure within the missing row of the α -(2 × 4) phase is clearly resolved in these images Evidence is presented of dimer twist as part of the surface energy minimisation process of one of the α phases. We also show images of a previously unreported structure that displays (2 × 4) symmetry.

Micron-scale magnetic structures for atom optics

Abstract Magnetic structures with periodicities on the scale of a micron have been constructed by magneto-optical recording on perpendicularly magnetised TbFeCo film with a focussed diode laser beam in the presence of an external field, and by microfabricating perpendicularly magnetised grooved CoCr structures. Strong reflection signals and predominantly specular reflection have been realised for beams of laser-cooled caesium atoms normally incident on both types of structure. Preliminary measurements of the specularity for a grooved a=1 xa0μm magnetic structure are particularly encouraging.

Hand‐held Device for Monitoring Dissolved Organics in Fresh and Recycled Water on ppb Levels

New hand‐held robust UV255 sensor for the control of dissolved organic carbon (DOC) in water has been developed by CSIRO. The device is designed for compact operation and simplified circuitry. UV 255 sensor exhibited a good liner response at low concentration range of DOC (<5u2009ppm) and has shown the detection limit less than 10 ppb of DOC. The current design allows for immersion of the sensor into a solution. The sensor can also be modified for flow‐through applications by allowing water to flow through side arms. New sensor discriminates readily between various types of potable and recycled water.