Julia E. Fulghum

Na impurity chemistry in photovoltaic CIGS thin films: Investigation with x-ray photoelectron spectroscopy

Thermal processing of Cu(In1−xGax)Se2 thin-films grown as part of photovoltaic devices on soda-lime glass leads to the incorporation of Na impurity atoms in the Cu(In1−xGax)Se2. Na contamination increases the photovoltaic efficiency of Cu(In1−xGax)Se2-based devices. The purpose of this investigation is to develop a model for the chemistry of Na in Cu(In1−xGax)Se2 in an effort to understand how it improves performance. An analysis of x-ray photoelectron spectroscopy data shows that the Na concentration is ∼0.1 at. % in the bulk of Cu(In1−xGax)Se2 thin films and that the Na is bound to Se. The authors propose a model invoking the replacement of column III elements by Na during the growth of Cu(In1−xGax)Se2 thin films. Na on In and Ga sites would act as acceptor states to enhance photovoltaic device performance.

Light-Induced Biocidal Action of Conjugated Polyelectrolytes Supported on Colloids

A series of water soluble, cationic conjugated polyelectrolytes (CPEs) with backbones based on a poly(phenylene ethynylene) repeat unit structure and tetraakylammonium side groups exhibit a profound light-induced biocidal effect. The present study examines the biocidal activity of the CPEs, correlating this activity with the photophysical properties of the polymers. The photophysical properties of the CPEs are studied in solution, and the results demonstrate that direct excitation produces a triplet excited-state in moderate yield, and the triplet is shown to be effective at sensitizing the production of singlet oxygen. Using the polymers in a format where they are physisorbed or covalently grafted to the surface of colloidal silica particles (5 and 30 microm diameter), we demonstrate that they exhibit light-activated biocidal activity, effectively killing Cobetia marina and Pseudomonas aeruginosa. The light-induced biocidal activity is also correlated with a requirement for oxygen suggesting that interfacial generation of singlet oxygen is the crucial step in the light-induced biocidal activity.

Direct observations of aluminosilicate weathering in the hyporheic zone of an Antarctic Dry Valley stream

Abstract This study focused on chemical weathering and bacterial ecology in the hyporheic zone of Green Creek, a McMurdo Dry Valley (Antarctica) stream. An in situ microcosm approach was used to observe dissolution features on the basal-plane surface of muscovite mica. Four mica chips were buried in December 1999 and dug up 39 d later. Atomic force microscopy (AFM) of the basal-plane surfaces revealed small, anhedral ∼10-A-deep etch pits covering ∼4% of the surfaces, from which an approximate basal-plane dissolution rate of 8.3 × 10−18 mol muscovite cm−2 s−1 was calculated (on the basis of the geometric surface area) for the study period. This is an integrated initial dissolution rate on a fresh surface exposed for a relatively brief period over the austral summer and should not be compared directly to other long-term field rates. The observation of weathering features on mica agrees with previous stream- and watershed-scale studies in the Dry Valleys, which have demonstrated that weathering occurs where liquid water is present, despite the cold temperatures. AFM imaging of mica surfaces revealed biofilms including numerous small (

Predictive modeling of electrocatalyst structure based on structure-to-property correlations of x-ray photoelectron spectroscopic and electrochemical measurements.

Chemical structure and catalytic activity of nonplatinum porphyrin-based electrocatalyst for oxygen reduction is characterized by combination of X-ray photoelectron spectroscopy (XPS) and rotating disk electrode. The goal of the study is to show how modifications in the molecular structure affect catalytic characteristics and how to use these structural modifications in a purposeful manner to increase catalytic activity. Initial correlation of structure to electrochemical performance is achieved through the application of principal component analysis (PCA) to curve-fits of high-resolution XPS spectra combined with results of electrochemical measurements. Furthermore, a predictive model that describes this correlation is build using the combination of genetic algorithm (GA) and multiple linear regression (MLR). Based on structure-to-property correlations, two types of active sites responsible for the catalytic activity, i.e., Co associated with pyropolymer and Co particles covered by oxide layer, are determined, and a dual-site for oxygen reduction on cobalt porphyrins is hypothesized, allowing for designing a catalyst structure with optimal performance characteristics.

Enhancement of Kaolinite Dissolution by an Aerobic Pseudomonas mendocina Bacterium

This research focused on whether bacteria living in aerobic environments where Fe is often a limiting nutrient could access Fe associated with the clay mineral kaolinite. Kaolinite is one of the most abundant clays at the Earths surface, and it often contains trace quantities of Fe as surface precipitates, accessory minerals, and structural substitutions. We hypothesized that aerobic bacteria may enhance kaolinite dissolution as a means of obtaining associated Fe. To test this hypothesis, we conducted microbial growth experiments in the presence of an aerobic Pseudomonas mendocinaThis research focused on whether bacteria living in aerobic environments where Fe is often a limiting nutrient could access Fe associated with the clay mineral kaolinite. Kaolinite is one of the most abundant clays at the Earths surface, and it often contains trace quantities of Fe as surface precipitates, accessory minerals, and structural substitutions. We hypothesized that aerobic bacteria may enhance kaolinite dissolution as a means of obtaining associated Fe. To test this hypothesis, we conducted microbial growth experiments in the presence of an aerobic Pseudomonas mendocina

Identification of chemical components in XPS spectra and images using multivariate statistical analysis methods

Abstract A variety of data analysis methods can be used to enhance the information obtained from a measurement, or to simplify extraction of significant components from large data sets. Much work is needed to improve the quantification and interpretation of XPS spectra and images from complex organics. Multivariate analysis (MVA) is increasingly used for applications in electron spectroscopy to aid the analyst in interpreting the vast amount of information yielded by spectroscopic techniques. In general, the goals of MVA are to determine the number of components present, identify the chemical components, and quantify component concentrations in the mixture. Principal component analysis (PCA) is frequently used to determine the number of mathematical components which describe the data set. These mathematical components must then be related to chemically meaningful components. Various approaches to solve rotational ambiguities of spectral resolution, including local rank method (EFA), pure variables method (Simplisma) and multivariate curve resolution (MCR), are tested in the determination of chemical components from XPS data. Limitations associated with the resolution of a single matrix are shown to be partially or completely overcome when several related matrices are treated together. The test data sets contain XPS images or spectra acquired from blends of poly(vinyl chloride), PVC, and poly(methyl methacrylate), PMMA. The PVC degrades rapidly upon exposure to the X-ray beam. Spectra and images from the blend, acquired as a function of time, provide the multi-dimensional data sets for algorithm evaluation. In addition to spectral resolution, multivariate image analysis methods, such as principal component analysis, are used to extract maps of the pure components from an images-to-spectra data set.

Dissolution of well and poorly ordered kaolinites by an aerobic bacterium

Abstract Previous research by our group (e.g., [Chem. Geol. 132 (1996) 25; Geochim. Cosmochim. Acta 64 (2000) 1363]) has shown that an aerobic Pseudomonas mendocina bacterium enhances Fe(hydr)oxide dissolution in order to obtain Fe under Fe-limited conditions. The P. mendocina is incapable of utilizing Fe as a terminal electron acceptor and requires several orders of magnitude lower Fe concentrations than do dissimilatory Fe reducing bacteria. The research reported here compared the effects of the P. mendocina on dissolution of well and poorly ordered Clay Minerals Society Source Clay kaolinites KGa-1b and KGa-2, respectively, under Fe-limited conditions. KGa-1b and KGa-2 contain 0.04 and 0.94 bulk wt.% Fe, respectively, and their surface Fe/Si atomic ratios=0.008 and 0.012. Following strong cleaning of the kaolinites in 5.8 M HCl at 85 °C, the surface Fe/Si atomic ratios decreased to 0.004 and 0.008, respectively. Both kaolinites also developed a Si-enriched surface precipitate upon strong cleaning. Because the P. mendocina take up Fe, we could not measure Fe release from the kaolinite directly, but rather had to monitor it indirectly by comparing microbial populations sizes under Fe-limited growth conditions. We found that microbial growth on uncleaned, weakly cleaned, and strongly cleaned kaolinites increased with the amount of Fe readily available to organic ligands as estimated by dissolution in 0.001 M oxalate (pH 3). This suggests that it is the amount of readily accessible Fe that controls Fe acquisition and hence microbial growth. The trend is based on only a relatively small range of kaolinite Fe contents, and the research thus needs to be expanded to include kaolinites with a broader range of bulk and surface Fe concentrations. Significant enhancement of Al release was observed in the presence of the bacteria, along with generally some enhancement of Si release. This enhancement of kaolinite dissolution could be related to an observed pH increase from ∼7–8 to ∼9 in the presence of the bacteria and/or to production of Al chelating agents. The P. mendocina produce a variety of organic exudates, including siderophores [Chem. Geol. 132 (1996) 25; Geomicrobiology (2001b)], and further studies into the effects of the siderophores on Al complexation and on kaolinite dissolution are ongoing.

Differential charging in XPS. Part III. A comparison of charging in thin polymer overlayers on conducting and non‐conducting substrates

Previous work has used small-area and imaging XPS to show that differential charging in bulk insulators develops as a result of non-uniform x-ray flux across the surface, causing lateral differential charging. Charging in heterogeneous samples can be further affected by the local sample environment and sample mounting. The current work extends these studies through an analysis of differential charging effects in thin overlayers on conducting and insulating substrates. The charging observed in PnBMA overlayers on indium tin oxide, glass, Ag and Al is discussed as a function of substrate conductivity and photoelectron cross-sections. Substrate conductivity is the most significant factor in determining the magnitude of the overlayer charging observed when no charge compensation is utilized. Differential charging in the PnBMA overlayer was used to image a patterned substrate containing insulating and conducting areas.

RECENT DEVELOPMENTS IN HIGH ENERGY AND SPATIAL RESOLUTION ANALYSIS OF POLYMERS BY XPS

Abstract Historically, obtaining useful XPS spectra from polymers has been perceived to be difficult. Problems have included low signal intensities from monochromatic sources, poor spatial resolution, and difficulties with charge compensation. Recent improvements in XPS instrumentation now make it possible to routinely analyze samples which previously would have been considered challenging or impossible. This is particularly true for multi-component polymer samples, which can be difficult to characterize using only large-area spectroscopy. The status of current XPS instrumentation is overviewed, with a variety of examples from polymer analysis. Examples include the use of imaging XPS in the analysis of PVC/PMMA blends, quantification using the valence band region, chemical derivatization of PVMK using hydrazine and trifluorophenylmethyl hydrazine, and high spatial resolution analysis of fiber mats. The importance of high spatial resolution analyses in the evaluation of heterogeneous samples is discussed.

Differential Charging in XPS. Part II: Sample Mounting and X‐ray Flux Effects on Heterogeneous Samples

Factors which affect photoelectron peak positions on heterogeneous samples are evaluated using patterned indium tin oxide (ITO) on glass. Local sample environment, sample mounting and spatial variation in x-ray flux are all shown to contribute to differential charging effects if no charge neutralization is used. X-ray photoelectron spectroscopy data acquisition using both Mg Kα and monochromatic Al Kα x-ray sources shows that confusing and potentially misleading results can be obtained if samples are mounted such that only part of the surface is grounded to the spectrometer. In this case, multiple photoelectron peaks were observed for each photoelectron line of each element in the sample. However, more significant differential charging artifacts were observed from the insulating glass than from the ITO. The proximal causes of the photoelectron peak shifts were evaluated using imaging XPS in conjunction with small-area spectroscopy.