Steve J. Spencer

Quantitative XPS: I. Analysis of X-ray photoelectron intensities from elemental data in a digital photoelectron database

Abstract An analysis of the correlation of theoretical predictions for photoelectron intensities is made with experimental data from an XPS digital database for 46 solid elements measured using a spectrometer with calibrated intensity and energy scales. This analysis covers single element samples measured for Al and Mg Kα X-rays. The spectral data are for widescans at 1 eV energy intervals with kinetic energies from 200 to 1506 eV using Al X-rays and to 1273 eV using Mg X-rays. In addition are narrow scans around the photoelectron peaks at 0.1 eV energy intervals. All spectra have the instrument intensity/energy response function removed so that the peak areas are proportional to the number of electrons emitted per steradian per incident Kα photon. Correlations are made for the ionisation cross sections of Scofield and the inelastic mean free paths given by the TPP-2M formula. The correlations are excellent, apart from a factor which may be associated with the background removal arising from the use of the Tougaard Universal cross section. These correlations lead directly to pure element relative sensitivity factors suitable for quantitative analysis. General equations are also provided to extract values for a new form of relative sensitivity factor for an average matrix. These average matrix relative sensitivity factors lead to simpler equations involving matrix factors that are effectively unity instead of the traditional values in the range 0.3 to 3.0.

Argon Cluster Ion Beams for Organic Depth Profiling: Results from a VAMAS Interlaboratory Study

The depth profiling of organic materials with argon cluster ion sputtering has recently become widely available with several manufacturers of surface analytical instrumentation producing sources suitable for surface analysis. In this work, we assess the performance of argon cluster sources in an interlaboratory study under the auspices of VAMAS (Versailles Project on Advanced Materials and Standards). The results are compared to a previous study that focused on C(60)(q+) cluster sources using similar reference materials. Four laboratories participated using time-of-flight secondary-ion mass spectrometry for analysis, three of them using argon cluster sputtering sources and one using a C(60)(+) cluster source. The samples used for the study were organic multilayer reference materials consisting of a ∼400-nm-thick Irganox 1010 matrix with ∼1 nm marker layers of Irganox 3114 at depths of ∼50, 100, 200, and 300 nm. In accordance with a previous report, argon cluster sputtering is shown to provide effectively constant sputtering yields through these reference materials. The work additionally demonstrates that molecular secondary ions may be used to monitor the depth profile and depth resolutions approaching a full width at half maximum (fwhm) of 5 nm can be achieved. The participants employed energies of 2.5 and 5 keV for the argon clusters, and both the sputtering yields and depth resolutions are similar to those extrapolated from C(60)(+) cluster sputtering data. In contrast to C(60)(+) cluster sputtering, however, a negligible variation in sputtering yield with depth was observed and the repeatability of the sputtering yields obtained by two participants was better than 1%. We observe that, with argon cluster sputtering, the position of the marker layers may change by up to 3 nm, depending on which secondary ion is used to monitor the material in these layers, which is an effect not previously visible with C(60)(+) cluster sputtering. We also note that electron irradiation, used for charge compensation, can induce molecular damage to areas of the reference samples well beyond the analyzed region that significantly affects molecular secondary-ion intensities in the initial stages of a depth profile in these materials.

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.

XPS: binding energy calibration of electron spectrometers 4—assessment of effects for different x-ray sources, analyser resolutions, angles of emission and overall uncertainties

A detailed analysis is made of the binding energy calibration of X-ray photoelectron spectrometers when using monochromated Al Kα x-rays or unmonochromated Al or Mg Kα x-rays. The binding energies of the peaks for Cu 2p3/2, Ag 3d5/2 and Au 4f7/2, as well as for the Ni Fermi edge, are measured at high resolution using monochromated Al Kα x-rays. The apparent binding energy shifts of the peaks are then calculated for this source, and also for the Al and Mg unmonochromated x-ray sources, using full synthetic Kα x-ray structures, as a function of Gaussian spectrometer energy resolutions in the range 0.2–1.5 eV. For all three x-ray sources, the relative binding energies for the Cu 2p3/2 and Au 4f7/2 peaks are contained within ±0.015 eV but the effects for Ag 3d5/2 are stronger and the containment range must be increased to ±0.026 eV. Further data and calculations are provided for surface core-level shifts and here it is found necessary to restrict emission angles to 56° for all the peak separations to be restricted to the above range of ±0.026 eV. Other instrumental effects may give rise to additional larger or smaller effects. Non-optimized settings for monochromators can show further shifts of up to ±0.2 eV. The uncertainties associated with the above calibration are then analyzed to show how the uncertainty at 95% confidence varies across the binding energy range. Example calculations show that seven repeats of both the Cu 2p3/2 and Au 4f7/2 binding energies may be used to define the peak repeatability and that one or two measurements can then be made for each calibration peak to define the calibration. The precise number of measurements to be used depends on the peak energy repeatability and the required confidence limits for the calibration. In practical situations, however, it is likely that the greatest uncertainty in the binding energy scale arises from the drift in the electronics between calibrations.

Plasma deposited metal Schiff-base compounds as antimicrobials

This paper details the synthesis, characterisation including crystal structure, and testing for antimicrobial efficacy of two compounds: a zinc centred bis(N-allylsalicylideneiminato)-zinc (ZSB) and its known copper analogue, bis(N-allylsalicylideneiminato)-copper (CSB). Differences in antimicrobial efficacy of the two compounds were observed, suggesting possible mechanisms for antimicrobial activity. The ZSB system was plasma deposited under various pulse conditions onto non-woven fabric, and the antimicrobial efficacy of the resultant film measured for Staphylococcus aureus and Pseudomonas aeruginosa. These results suggest the potential utility of this compound as an effective antimicrobial thin film, and confirm the critical role the ligands play in effecting antimicrobial activity.

Transforming bilayer MoS2 into single-layer with strong photoluminescence using UV-ozone oxidation

The use of single-layer MoS2 in light emitting devices requires innovative methods to enhance its low photoluminescence (PL) quantum yield. In this work, we report that single-layer MoS2 with a strong PL can be prepared by oxidizing bilayer MoS2 using UV-ozone oxidation. We show that as compared to mechanically-exfoliated single-layer MoS2, the PL intensity of the single-layer MoS2 prepared by UV-ozone oxidation is enhanced by 20–30 times. We demonstrate that the PL intensity of both neutral excitons and trions (charged excitons) can be greatly enhanced in the oxidized MoS2 samples. These results provide novel insights into the PL enhancement of single-layer MoS2.

Cones formed during sputtering of InP and their use in defining AFM tip shapes

Small structures, formed on InP surfaces during sputtering, cause loss of depth resolution in sputter-depth profiles but may be conveniently incorporated into a method for studying AFM tip shapes to define resolution in AFM images. The sputtered structures formed here are filaments, often called cones, whose indium tips have a radius of about 10 nm. By sputtering with argon ions in the energy range, 4 keV to 8 keV, it is shown that the height of the filaments is critically dependent on the sample temperature. At room temperature, or below, the height is very small but, at 260°C, they grow to 200 nm. An Arrhenius plot for several temperatures indicates growth, probably by a stress-induced diffusion mechanism driven by charging of the indium cap by the ion beam. AFM images of these structures may be averaged to give reliable pseudo-reconstructions of the AFM tip.

Covalent Carbene Functionalization of Graphene: Toward Chemical Band-Gap Manipulation.

In this work, we employ dibromocarbene (DBC) radicals to covalently functionalize solution exfoliated graphene via the formation of dibromocyclopropyl adducts. This is achieved using a basic aqueous/organic biphasic reaction mixture to decompose the DBC precursor, bromoform, in conjunction with a phase-transfer catalyst to facilitate ylide formation and carbene migration to graphene substrates. DBC-functionalized graphene (DBC-graphene) was characterized using a range of spectroscopic and analytical techniques to confirm the covalent nature of functionalization. Modified optical and electronic properties of DBC-graphene were investigated using UV-vis spectroscopy, analysis of electrical I-V transport properties, and noncontact terahertz time-domain spectroscopy. The implications of carbene functionalization of graphene are considered in the context of scalable radical functionalization methodologies for bulk-scale graphene processing and controlled band-gap manipulation of graphene.

Extending the plasmonic lifetime of tip-enhanced Raman spectroscopy probes

Tip-enhanced Raman spectroscopy (TERS) is an emerging technique for simultaneous mapping of chemical composition and topography of a surface at the nanoscale. However, rapid degradation of TERS probes, especially those coated with silver, is a major bottleneck to the widespread uptake of this technique and severely prohibits the success of many TERS experiments. In this work, we carry out a systematic time-series study of the plasmonic degradation of Ag-coated TERS probes under different environmental conditions and demonstrate that a low oxygen (<1 ppm) and a low moisture (<1 ppm) environment can significantly improve the plasmonic lifetime of TERS probes from a few hours to a few months. Furthermore, using X-ray photoelectron spectroscopy (XPS) measurements on Ag nanoparticles we show that the rapid plasmonic degradation of Ag-coated TERS probes can be correlated to surface oxide formation. Finally, we present practical guidelines for the effective use and storage of TERS probes to improve their plasmonic lifetime based on the results of this study.

Sputter-induced cone and filament formation on InP and AFM tip shape determination

The structures formed on the InP(100) surface by sputtering with fluences of the order of 1021 ions m−2 of 4–8 keV argon are analysed by scanning electron microscopy, atomic force microscopy (AFM) and Auger electron spectroscopy. Well-defined cones are formed if small traces of polymeric coatings are deposited on the surface. However, for clean surfaces at room temperature, the sputtered surface may be cone-free. Surfaces that are sputtered at above ambient temperature exhibit a dense filamentary growth, the heights of which increase in magnitude as the temperature rises. For the conditions used in this work, filaments 200 nm high occur at 260 °C. The mechanism for this growth is thought to arise from stress through capacitative effects arising from charging of an indium cap at the top of the filament by the ion beam. At elevated temperatures, diffusive effects occur to reduce this strain by causing elongation of the filament. Samples may be heated by the power deposited from the ion beam so that filaments occur on samples that would otherwise be at room temperature. By heating the InP in the temperature range 100–180 °C, filaments 30–80 nm high may be grown, which are excellent for imaging the structure at the ends of AFM tips. New software, which averages the images of several individual filaments, allows the tip structure to be monitored, wear effects to be diagnosed and worn tips to be rejected. The samples can be used routinely, in-between other samples, to allow diagnosis directly without the need to use other forms of microscopy. Copyright