  A combination of experimental and numerical calculations on metallic silver
and platinum nanoparticles deposited on silica substrates is presented, with a
focus on the metal-substrate interactions. Experimentally, the nanoparticles
are elaborated under ultra-high vacuum and characterized by Grazing-Incidence
Small-Angle X-ray Scattering (GISAXS) and High Resolution Transmission
Electronic Microscopy (HRTEM) to determine their structure and morphology, and
in particular their aspect ratio (height/diameter) which quantifies the
metal-substrate interaction. Numerically, the interactions between the metal
and the silica species are modeled with the Lennard-Jones (12, 6) potential,
with two parameters for each metal and silica species. The geometric parameters
were found in the literature, while the energetic parameters were determined
from our experimental measurements of the aspect ratio. The parameters are:
{\sigma}_{Ag-O} = 0.278 nm, {\sigma}_{Ag-Si} = 0.329 nm, {\epsilon}_{Ag-O} = 75
meV, and {\epsilon}_{Ag-Si} = 13 meV for Ag-silica and {\sigma}_{Pt-O} = 0.273
nm, {\sigma}_{Pt-Si} = 0.324 nm, {\epsilon}_{Pt-O} = 110 meV, and
{\epsilon}_{Pt-Si} = 18 meV for Pt-silica. The proposed Ag-silica potential
reproduces quantitatively the unexpected experimental observation of the
variation of the aspect ratio for Ag nanoparticles larger than 5 nm, which has
been interpreted as a consequence of the silica roughness. The nanoparticle
orientation, structure and disorder are also considered. This metal-silica
potential for Ag and Pt should be helpful for further studies on pure metals as
well as their alloys.
