The sample was mounted as slurry on carbon tape and measurements
were obtained with a Kratos Axis Ultra spectrometer using Al
Kα X-ray source operating at 15 kV and 15 mA. The pressure in the
analyser chamber was 10−9 Torr. The angle of the sample surface
to the detector was set at 90°, providing an analysis depth of up to
10 atomic layers for S and 4 atomic layers for Cu and Fe. The
spectrometer was calibrated using the Cu 2p line at 932.6 eV and
standardized against the Au 4f line at 84.0 eV. The Kratos charge
neutralizer systemwas used on all specimenswith a filament current
of 2.1 A and a charge balance voltage of 3.6 V. Broad scans were
collected at a pass energy of 160 eV and a step size of 0.7 eV. High
resolution Cu 2p and Fe 2p core level regions were collected using
40 eV pass energy and 0.05 eV step size. All spectra were fitted
with Shirley backgrounds [19]. Each spectrum was calibrated with
respect to a C 1s value of 284.8 eV.