  In this work, Pt-Ti core-shell nanoparticles (NP) of 2 nm to 3 nm in size and
30000 u \pm 1500 u as specified single particle mass, deposited on flat silicon
substrates by means of a mass-selected cluster beam source, were used for the
investigation of the modification of the X-Ray Standing Wave (XSW) field
intensity with increasing NP surface coverage. The focus of the investigation
is on the determination of the range of validity of the undisturbed flat
surface approach of the XSW intensity in dependence of the actual coverage rate
of the surface. Therefore, the nanoparticles were characterized using
reference-free grazing incidence X-ray fluorescence analysis (GIXRF) employing
radiometrically calibrated instrumentation. In addition, near-edge X-ray
absorption fine structure (NEXAFS) measurements were performed to investigate
the binding state of titanium in the core-shell nanoparticles which was found
to be amorphous TiO2. The combination of GIXRF measurements and of the
calculated XSW field intensities allow for a quantification of the core-shell
nanoparticle surface coverage. For six different samples, the peak surface
coverage could be determined to vary from 7 % to 130 % of a complete
monolayer-equivalent coverage. A result of the current investigation is that
core-shell nanoparticles modify the intensity distribution of the XSW field
with increasing surface coverage. This experimental result is in line with
calculated XSW field intensity distributions at different surface coverages
using an effective density approach.
