carried out with a Kratos Axis Ultra DLD (Kratos Ltd., Manchester,
UK) using a monochromatic Al Kα X-ray source (1486.6 eV, emission
current: 10 mA, anode voltage: 15 kV). The instrument base pressure
remained below 8.0 × 10–10 Pa. The instrument
work function was calibrated to a binding energy of 84.0 eV for metallic
gold (Au 4f7/2). The charge neutralizer was used for all
analyses (filament current: 2.1 A, charge balance: 3.45 V, and filament
bias: 1.5 V). The charge neutralization was monitored with the help
of the C 1s peak for adventitious carbon. Survey spectra were acquired
at a pass energy of 80 eV with 20 sweeps and an energy step of 1 eV.
The high-resolution spectra were acquired at a pass energy of 20 eV
with 10 sweeps and an energy step of 0.1 eV. The analysis area was
300 μm × 700 μm. Data were processed using the commercial
software CasaXPS (version 2.3.16, Casa Software Ltd., Chichester,
UK). All spectra were recorded in the spectroscopy mode utilizing
the hybrid lens mode. For each sample, at least three independent
measurements were performed. The binding energies were calibrated
using the C 1s peak for adventitious carbon at a binding energy of
284.8 eV, with an associated error of ∼0.1–0.2 eV.38 (link) No argon ion sputter cleaning has been performed
prior to analysis.