AVS2015 Session AS+SS-ThA: Advances in 2D Chemical Mapping and Data Analysis

XPS imaging can be used to acquire chemical-state specific information with a spatial resolution of several microns. In response to perceived user interest, instrument manufacturers have put significant resources into developing chemical state XPS imaging and image processing capabilities. Current instrumentation allows for parallel image acquisition over a range of photoelectron energies, resulting in quantitative, lateral surface chemistry determinations. Although publications citing XPS continue to increase, XPS imaging contributes to only a small percentage of published work.

In this talk, we’ll present an overview of laboratory XPS imaging capabilities using a variety of examples to demonstrate the practical (and not-so-practical) experiments that are possible. Recent multivariate and multitechnique analysis applications, including Multivariate Auger Feature Imaging (MAFI) and XPS-Raman image correlation will be used to highlight current research utilizing XPS imaging. Results from a survey of instrument manufacturers, directors of XPS user facilities, and expert users will be presented, including speculation as to why the use of XPS imaging has not met expectations, recommendations for using XPS imaging and hopes for future developments.

X-ray photoelectron spectroscopy is widely used in determining surface chemistry of materials. Improvements in instrument sensitivity mean that spectra are routinely acquired from areas with diameters in the tens of microns, although most routine analysis is performed at much larger areas. The assumption is that the material and spectra are homogeneous over the area probed is often made although it may not be true. Information of the lateral distribution of elemental and chemical states on a surface can be probed using XPS imaging either at a single binding (kinetic) energy or over a narrow energy range corresponding to a core-level photoemission peak.

Multispectral XPS imaging, also referred to as spectromicroscopy, where a series of images incremented in energy such that each pixel contains a spectrum, is relatively new and under exploited for surface characterisation. An advantage of spectromicroscopy is that spectral information can be reconstructed from defined areas which are smaller than those possible with focused x-ray or virtual probe selected area XPS. This means that the reconstructed spectra are no longer averaged over the total area from which the image is acquired such that both sample and instrument dependent differences can be studied.

The 256 x 256 pixel multi-spectral image contains >65,500 spectra which is ideally suited to multivariate analysis. Development of data processing to support spectromicroscopy data reduction has been necessary and a number of approaches have been successfully applied in the characterisation of model and real-world samples[1-3]. Multivariate analysis can be used to classify regions of interest across the field of view and data can be partitioned such that chemical state, changes in peak position and background shape can be investigated. Here we detail the use of spectromicroscopy for the characterisation of complex materials including functionalised multiwall carbon nanotubes (MWCNT). This approach has allowed the considerable challenges of surface analysis of such materials to be addressed and has allowed the influence of signal from the substrate material to be removed from the MWCNT of interest.

[1] E.F. Smith, D. Briggs and N. Fairley Surf. Interface Anal. 2005, 38, 69-75

[2] J. Walton, N. Fairley Surf. Interface Anal. 2008, 40, 478 - 481

[3] A.J. Barlow, O. Scott, N. Sano and P.J. Cumpson Surf. Interface Anal. 2015, 47, 173-175

Surface structure and chemistry are properties that are crucial to the successful production and operation of numerous devices, materials and coatings. X-ray photoelectron spectroscopy (XPS) is an ideal tool for investigating these properties due to its inherent surface sensitivity, and ability to quantify the chemical states detected.

Whilst XPS is most often used for point analysis and/or depth profiling, it is also able to produce compositional maps of multi-phase materials. This is of particular use in scenarios where other surface science techniques are unsuitable, for example especially rough surfaces that cannot be imaged using SPM or surfaces with multiple phases of similar elemental composition, which cannot be differentiated by SEM-EDS. However the widespread implementation of XPS as a mapping tool has been hindered by the long acquisition times required.

Here we shall present the effects of increased x-ray performance, increased spectrometer sensitivity and modifications in data processing, i.e. both instrumental and software improvements, on the required acquisition time for XPS mapping. This is demonstrated using data from several samples, where multi-phase maps were acquired up to an order of magnitude more quickly than previously possible through implementation of these improvements.

First results from a new tandem imaging mass spectrometer will be presented. The unique TOF-TOF design allows the simultaneous collection of standard TOF-SIMS spectra and collision induced dissociation (CID) spectra of specifically selected precursors [1]. This new analytical capability maximizes the information content from a single acquisition and provides all data from the same analytical volume. The ability to acquire MS/MS data at the same primary ion beam repetition rate as used in conventional TOF-SIMS allows high speed image acquisition. The ability to unambiguously identify and image peaks above m/z 200 was applied to polymer additives and to the study of lipid composition changes in mouse spleen specimens infected with F. novicida.

[1] P.E. Larson, J.S. Hammond, R.M.A. Heeren and G.L. Fisher, Method and Apparatus to Provide Parallel Acquisition of MS/MS Data, U.S. Patent 20150090874, 02 April 2015.

The rate at which a battery can deliver energy is ultimately dominated by the ability or inability to effectively transport both ions and electrons throughout the electrodes. When charge transport is a limiting factor, material utilization within the battery becomes spatially inhomogeneous, reducing performance. Additionally, the material and architectural requirements for optimizing transport for both ions and electrons are not always synergistic, which can lead to design challenges. The effects of architecture on device performance are generally characterized by externally measured scalar quantities, such as cell potential or current, but these quantities do not reveal where within the electrode any problem may lie. There is a growing need to develop models which can accurately predict spatially resolved dynamics within battery electrodes, as well as experimental techniques to verify them, particularly as nanoscience produces more and more sophisticated electrode designs.

Here we show that chemical state mapping with X-ray photoelectron spectroscopy (XPS) is a powerful tool for revealing transport-limit-induced dynamics within battery electrodes, and connect surface science with electrochemical modeling. We examine the specific problem of facile ion transport but limited electronic transport, which often occurs in high aspect ratio electrodes made of low conductivity semiconductors or insulators. While characterizing complex structures using XPS is normally very challenging, it is possible to gain much more useful and accurate information when a model device is designed from the ground up to exploit the strengths of XPS. By fabricating battery chips in which the anticipated gradients of material utilization (i.e. the spatially varying amount of lithium intercalated) are laid out laterally on a flat substrate, we can clearly map chemical changes in the electrode as a function of distance from a current collector. By using transition metal oxide (M_xO_y) cathode materials, we are able to track the state of charge through local quantification of the reaction Mⁿ⁺ + e^- -> M^(n-1)+. Our data clearly reveal that as the applied current density increases, ion insertion activity is dramatically contracted towards the current collector, which leads to performance limitations at high rates. Importantly, we also use our spatially resolved data to validate the predictions of a sophisticated finite element multiphysics battery model. The visualization and understanding of design induced performance limits, as well as the validation of a predictive model, allow us to optimize the design of future high performance nanostructured battery electrodes.

Primary lithium batteries with hybrid carbon monofluoride–silver vanadium oxide (CFx–SVO) cathodes have become widely commercialized as power sources in implantable medical devices. Although CFx and SVO have been used separately as cathode materials, CFx–SVO hybrid cathodes have been developed to meet the increased energy-density, power, and longevity requirements specific to multiyear operation at physiological temperature. However, the microstructural basis for the performance characteristics has not been well understood, including chemical changes in the cathode materials as they discharge. This work presents a microstructural study of discharged cathode materials, aimed at identifying chemical and structural characteristics that can be related to the observed battery-performance characteristics. As a result, the relationships established can be used to improve the performance of future medical device technologies. Scanning electron microscopy with energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction were used to probe the cathodes chemistry and structure. Although a single analytical method cannot give a full picture of the cathode chemistry and microstructure, the combination of these complementary techniques makes it possible to develop a clearer picture of the structural and chemical changes that occur within the cathode as the battery discharges. The discharged cathodes of SVO, CFx, and SVO-CFx hybrid are compared, which demonstrates that the relatively gradual transformation of SVO that occurs in SVO-only cathodes is accelerated in the hybrid cathodes. Several trends will be shown including: (1) SVO loses its crystalline structure and silver content (replaced by lithium) with discharge; (2) CFx converts into carbon and LiF with discharge; (3) and the hybrid cathodes show steady conversion of CFx, accelerated conversion of SVO (as soon as 5% depth of dischage), and the beginning of LiF formation (as early as 10% depth of discharge).