Controlled manipulation of nanoparticles : scanning probe measurements and modelling of trajectories and dissipative effects

Unintentional displacement of tiny nanoparticles, while imaging them on a surface, is a
common experience in atomic force microscopy (AFM). Understanding how to control and
turn this effect into a powerful method for studying the mobility of nano-objects is the
main objective of this thesis. This has several applications from fields ranging from environmental
control to drug delivery, besides the fundamental interest in nanomechanics.
Therefore, the starting point was the preparation of different kinds of nanoparticles on
a variety of bare and treated substrates. Nanoparticles and/or agglomerates of nanoparticles
were prepared on surfaces and characterized by AFM. The goal was to determine
the shape, size and/or size distribution of nanoparticles and the influence of the surface
chemistry and the morphology on the density of the particles. Besides the geometrical
properties, attempts were made to determine the forces between nanoparticles and
surfaces. For this purpose, a preparation process was developed to transfer and then
characterize particles on sensors. Also, Kelvin force imaging was done on the samples of
nanoparticles in order to determine the contact potential of the particles. Nanoparticles
with important commercial applications for e.g. TiO2, Al2O3, SiO2, ZrO2 and Au were
used for this study.
The next part of the thesis focuses on the manipulation of Au nanoparticles. A new
technique for controlled manipulation of nanospheres and asymmetric nanoparticles was
developed in the course of the thesis. The pathway of the tip was related to the trajectories
of the nanoparticles by an original collision model. In order to verify the model, experiments
were done by manipulating gold nanospheres on bare and nano-patterned SiO2
surfaces in tapping-mode AFM. Detailed analysis of particle trajectories due to impact
between the oscillating tip and particle (within one scan frame) was done. This model was
then extended for studying the effect of friction on the trajectories of the nanospheres.
Whenever the tip collides with the particle, the particle is displaced by a certain distance,
which depends upon the friction force between the particle and the surface. Modeling was
done to reproduce the trajectories of the nanoparticles at different values of friction. The
fluctuations, (and the apparent discontinuities) of the trajectories of the nanoparticles
were related to friction.
The effect of surface chemistry, temperature and environment on manipulation of the
nanoparticles is discussed in the third chapter of the thesis. For these studies, manipulation of Au nanoparticles either raw or coated with self-assembled monolayers ending
with a hydrophobic (methyl, -CH3) or hydrophilic group (hydroxyl, -OH) was done to
investigate the influence of the hydrophobicity of the coatings on the mobility of the
nanoparticles. The role of the environment and thermal activation was studied by performing
manipulation experiments in UHV and at different temperatures, ranging from
20 up to 150◦C. By measuring the phase shift while scanning, the threshold value of the
power dissipation needed for translating a single nanoparticle could be determined.
Lastly, the manipulation of asymmetric nanoparticles e.g. nanorods and flower shaped
nanoparticles was done. The main objective was to study the effects of external torques on
the motion of the nanoparticles. The collision model was extended to relate the rotational
and translational motion of these asymmetric structures to the pathway of the probing
tip. The trajectories of these particles were also simulated in this case. Corresponding,
AFM experiments were done by manipulating Au nanorods on Si. To study the case of
nanoflowers, antimony islands on HOPG were used.
In summary, preparation procedures for homogeneous nanoparticle samples are discussed.
AFM was used to determine the geometric characteristics and adhesion forces of the
nanoparticles. A new technique for controlled manipulation of nanoparticles was deduced
for spherical and asymmetrical particles where the focus was to try to understand how
to induce a well-defined direction of motion of the nanoparticles by adjusting the scan
pattern. Also, the factors effecting the manipulation of nanoparticles were studied.