X-PEEMmeasurements were carried out on beamline BL05B2 at the
National Synchrotron Radiation Research Centre (NSRRC) in Hsinchu,
Taiwan. BL05B2 beamline uses an elliptically polarized undulator
(EPU5) with a spherical-grating monochromator, yielding very high
photon flux (2 × 1012 photons S−1 at 800 eV in a 0.4 mm × 0.2 mm
spot) with spatial resolution better than 50 nm [15,16]. The resolution
of the images collected hereinwas chosen to clearly showthe individual
particleswith a large enough field of view to simultaneously collect data
from multiple particles at once. The image resolution was typically between
150 and 200 nm. When synchrotron light is shone on a sample,
the resulting photoelectrons emitted by the sample are accelerated towards
an electron lens column and projected onto an aluminiumcoated
YAG crystal screen. The real-time, sample surface imageswere acquired
by a CCD detector mounted behind the screen in total electron yield
(TEY) mode. The analysis chamber vacuum was held at ultrahigh vacuum(
10−10 Torr).Movie stacks of images were acquired while scanning
the desired photon energy range across the Cu and Fe L-absorption
edge, so that each pixel in the stacked image contained a complete
NEXAFS spectrum. NEXAFS spectra were analysed from 20 × 20 pixel
regions with an acceptable signal-to-noise ratio. Copper and iron spectra
were calibrated using metallic standards at position 932.6 eV for
Cu L-edge [17] and 707.7 eV for Fe L-edge [18]. All spectra presented
here were normalised to a linear pre-edge fit.