  Atomic Force Microscopy (AFM) and Ultrasonic Force Microscopy (UFM) have been
applied to the characterization of composite samples formed by SrTiO 3 (STO)
nanoparticles (NPs) and polyvinyl alcohol (PVA). The morphological features of
the STO NPs were much better resolved in UFM than in contact-mode AFM
topography. For high STO concentrations the individual STO NPs formed
nanoclusters, which gathered in microaggregates. The STO aggregates, covered by
PVA, exhibited no AFM frictional contrast, but were clearly distinguished from
the PVA matrix using UFM. Similar aggregation was observed for NPs in the
composite samples than for NPs deposited on top of a flat silicon substrate
from a milliQ water solution in the absence of polymer. In the hybrid films,
most STO nanoparticles typically presented a lower UFM contrast than the PVA
matrix, even though stiffer sample regions such as STO should give rise to a
higher UFM contrast. STO NPs with intermediate contrast were characterized by
an UFM halo of lower contrast at the PVA/STO interface. The results may be
explained by considering that ultrasound is effectively damped on the nanometer
scale at PVA/ STO interfaces. According to our data, the nanoscale ultrasonic
response at the PVA/STO interface plays a fundamental role in the UFM image
contrast.
