Graham C. Smith

The determination of depth profiles from angle-dependent XPS using maximum entropy data analysis

Abstract The principles of maximum entropy have been used to determine elemental depth profiles from simulated angle-dependent XPS data. The theory of maximum entropy data analysis as applied to angle-dependent XPS is presented, and brief details are given of the novel encoding needed for this application. The method overcomes the ambiguity of solutions that is encountered when reconstructions are attempted from real data with noise. This is done by maximising the overall logarithmic probability function obtained by optimising the balance between maximum entropy and minimum chi-squared. The overall scaling of the noise in the data is determined, confidence limits are assigned to reconstructions from individual data sets, and the relative probabilities of different prior models are found.

Surface analytical science and automotive lubrication

This paper discusses the role surface analytical science has played in understanding the complex phenomena occurring during lubrication of modern internal combustion engines. The interactions between the multi-functional lubricant additives and the various metallic surfaces present in the tribological contacts are complex and lead to the formation of inhomogenous multi-layer protective surface films. Surface analysis by ultra-high vacuum (UHV) techniques such as x-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry has allowed model structures for these films to be derived through which the behaviour of the contact can be understood. Non-UHV probes have elucidated the role played by the soluble, partly reacted or degraded film precursors that occupy the interfacial region between the solid surface and the liquid lubricant, and have allowed the mechanical properties of the films to be investigated on a micro- and nano-scale. The use of surface analytical techniques to study interactions between lubricant species and combustion soot is also discussed.

Efficient solar cells are more stable: the impact of polymer molecular weight on performance of organic photovoltaics

The principle remaining challenge in the research area of organic photovoltaic (OPV) materials is to develop solar cells that combine high efficiency, stability and reproducibility. Here, we demonstrate an experimental strategy which has successfully addressed this challenge. We produced a number of samples of the highly efficient PTB7 polymer with various molecular weights (Mn ∼ 40–220k). OPV cells fabricated with this polymer demonstrated significant improvement of the cell efficiency (by ∼90% relative) and lifetime (by ∼300% relative) with the Mn increase. We attribute these effects to the lower density of recombination centers (persistent radical defects revealed by EPR spectroscopy) and better photoactive layer morphology in the samples with higher Mn. Relevance of the observed correlation between the OPV efficiency and stability is discussed.

3D Printed Graphene Based Energy Storage Devices

3D printing technology provides a unique platform for rapid prototyping of numerous applications due to its ability to produce low cost 3D printed platforms. Herein, a graphene-based polylactic acid filament (graphene/PLA) has been 3D printed to fabricate a range of 3D disc electrode (3DE) configurations using a conventional RepRap fused deposition moulding (FDM) 3D printer, which requires no further modification/ex-situ curing step. To provide proof-of-concept, these 3D printed electrode architectures are characterised both electrochemically and physicochemically and are advantageously applied as freestanding anodes within Li-ion batteries and as solid-state supercapacitors. These freestanding anodes neglect the requirement for a current collector, thus offering a simplistic and cheaper alternative to traditional Li-ion based setups. Additionally, the ability of these devices’ to electrochemically produce hydrogen via the hydrogen evolution reaction (HER) as an alternative to currently utilised platinum based electrodes (with in electrolysers) is also performed. The 3DE demonstrates an unexpectedly high catalytic activity towards the HER (−0.46 V vs. SCE) upon the 1000th cycle, such potential is the closest observed to the desired value of platinum at (−0.25 V vs. SCE). We subsequently suggest that 3D printing of graphene-based conductive filaments allows for the simple fabrication of energy storage devices with bespoke and conceptual designs to be realised.

Multi-technique surface analytical studies of automotive anti-wear films

Abstract A multi-technique study of model automotive anti-wear films using SEM with quantitative electron microprobe analysis, reflection–absorption IR spectroscopy, SIMS imaging and XPS depth profiling is described. Constrained simulation of the XPS depth profiles allowed model cross-sections of the film structures to be constructed. With zinc dialkyl dithiophosphate as an anti-wear additive, the anti-wear films were found to have a thin inorganic mixed sulphide/oxide layer immediately above the Fe substrate. This formed the base for a thicker Zn-containing polyphosphate-like overlayer. Addition of detergent and dispersant to the lubricant formulation resulted in thicker, more patchy films, with a clearer differentiation between film and substrate.

Surface Analysis by Electron Spectroscopy

In AES and XPS, electrons are emitted from the sample as a consequence of electron or X-ray irradiation respectively, and are subsequently energy-analysed and detected. For electrons in the energy range 100 to 1000 electron volts (eV), the distance that may be travelled before undergoing an inelastic collision, known as the inelastic mean free path, may be typically of the order of 2–3 nm. This distance corresponds to perhaps 10 atom layers in most materials, and it is this that gives the techniques their surface specificity. Experimentally, this inelastic mean free path is very difficult to measure and, in practice, a parameter known as the attenuation length, which also includes the effect of elastic scattering, is determined instead. Figure 2.1, from the work of Seah and Dench (1979), shows a compilation of measured attenuation length data for elements. These data are primarily derived from thin overlayer experiments in which the structure of the overlayers was usually not well characterised, with the result that the average of the compilation is systematically low. Nevertheless, a broad minimum in the energy range of interest is seen which rises at both high and low energies. More recent estimations of the attenuation lengths of Auger electrons and X-ray photoelectrons are discussed in Chapter 5.

Multi-spectral scanning Auger microscopy of tribological surfaces

Abstract The novel surface analysis technique of multi-spectral Auger microscopy (MULSAM) has been used to study simultaneously and at high resolution the chemical and topographic variations across wear scars generated with model oil formulations. This is the first time this technique has been applied to anything other than well-characterised samples from the semiconductor industry. The MULSAM image manipulations prove sufficiently robust to allow meaningful work on these extremely non-ideal specimens. The work leads to new insights into the composition and spatial distribution of anti-wear films. In particular, the results show significant correlations between the surface topography and the local chemical nature of the anti-wear film.

Interfacial microanalysis of rubber–tyre-cord adhesion and the influence of cobalt

Abstract The effect of cobalt-containing adhesion promoters on the structure and morphology of rubber–brass and rubber–tyre-cord interfaces before and after ageing has been investigated by X-ray photoelectron spectroscopy (XPS) depth profiling, glancing incidence X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect the cobalt adhesion promoters had upon the interface morphology as they suppressed the growth of crystalline dendrites normally associated with the ageing process was imaged in TEM using samples prepared by the focused ion beam (FIB) milling technique. XPS depth profiling through the interfaces revealed that different types of adhesion promoter influenced the amount and distribution of cobalt ions in the bonding layer. XRD demonstrated the influence that cobalt had upon the structure of the interface and subsequent crystallinity, with a lesser degree of crystallinity being associated with better adhesion performance. From the results a model for the effect of the Co chemistry of the adhesion promotor has been developed.

Surface studies of oil-seal degradation

Fluoroelastomers are frequently used as engine oil-seal materials. Under certain test conditions specific fluoroelastomers may show degradation of mechanical properties. A range of fluoroelastomers of different chemical composition have been aged in simple oil/additive blends and in oil formulations equivalent to commercial blends. These were then examined using XPS, SEM/EPMA and XRD to elucidate the physical and chemical changes associated with degradation. The interaction is shown to proceed through amine catalysed post-curing of the constituent polymers. These reactions promote defluorination, embrittlement and cracking of elastomers with a consequent decline in tensile properties as fracture failure mechanisms dominate performance. Degradation of these materials was found, even in the most extreme case, to be limited to the near-surface region of the samples, to a depth of less than approximately 50 μm. Degradation was reduced in elastomers with a higher fluorine level, higher terpolymer content, and a greater extent of cross-linking.

Pencil It in: Exploring the Feasibility of Hand-Drawn Pencil Electrochemical Sensors and Their Direct Comparison to Screen-Printed Electrodes.

We explore the fabrication, physicochemical characterisation (SEM, Raman, EDX and XPS) and electrochemical application of hand-drawn pencil electrodes (PDEs) upon an ultra-flexible polyester substrate; investigating the number of draws (used for their fabrication), the pencil grade utilised (HB to 9B) and the electrochemical properties of an array of batches (i.e, pencil boxes). Electrochemical characterisation of the PDEs, using different batches of HB grade pencils, is undertaken using several inner- and outer-sphere redox probes and is critically compared to screen-printed electrodes (SPEs). Proof-of-concept is demonstrated for the electrochemical sensing of dopamine and acetaminophen using PDEs, which are found to exhibit competitive limits of detection (3σ) upon comparison to SPEs. Nonetheless, it is important to note that a clear lack of reproducibility was demonstrated when utilising these PDEs fabricated using the HB pencils from different batches. We also explore the suitability and feasibility of a pencil-drawn reference electrode compared to screen-printed alternatives, to see if one can draw the entire sensing platform. This article reports a critical assessment of these PDEs against that of its screen-printed competitors, questioning the overall feasibility of PDEs’ implementation as a sensing platform.