Mitigating surface charging in XPS using an in-situ sub-nanometer gold coating technique
Surface charging in electron spectroscopy for chemical analysis (ESCA) detrimentally affects the analytical accuracy, and the common charge neutralization techniques struggle to handle the heterogeneous surface charging which plagues ESCA studies of many high-impact composites having different electrically resistive constituents. To facilitate the research of such composites, this report articulates the scientific causes of heterogeneous surface charging plaguing such research and discloses technical details in mitigating the problems by an in-situ sub-nanometer gold coating technique. Briefly, the report describes a method of translating the common in-situ sputter-cleaning facility in an X-ray photoelectron spectroscopy (XPS) system to a means for the precise deposition of a few gold monolayers onto a sample. Such an ultrathin gold overlayer guarantees the presence of a uniform surface potential that is virtually equal to spectrometer-ground and thereby relieves any heterogeneous surface charging problems in XPS. The Au 4f 7/2 peak of the overlayer is also a more accurate ESCA/XPS reference. Further, the report clarifies that the in-situ deposition of gold and the XPS monitoring of the deposition is essential. An arbitrary thick gold overlayer may block almost all XPS signals from the sample, and too little gold deposition fails the purpose of mitigating heterogeneous surface charging. In short, the method supports particularly studies of composites with heterogeneities in electrical conductivity .
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