High-resolution manipulation of gold nanorods with an atomic force microscope.

The controlled manipulation and precise positioning of nanoparticles on surfaces is a critical requisite for studying interparticle interactions in various research fields including spintronics, plasmonics, and nanomagnetism. We present here a method where an atomic force microscope operating in vacuum is used to accurately rotate and displace CTAB-coated gold nanorods on silica surfaces. The method relies on operating an AFM in a bimodal way which includes both dynamic and contact modes. Moreover, the phase of the oscillating probe is used to monitor the nanoparticle trajectory, which amplitude variations are employed to evaluate the energy dissipation during manipulation. The nanoscale displacement modes involve nanorod in-plane rotation and sliding, but no rolling events. The transitions between these displacement modes depend on the angle between the scan axis direction and the nanorod long axis. The findings reveal the importance of mean tip-substrate distance and of oscillation amplitude of the tip. The role of substrate surface and of CTAB molecular bi-layer at nanorod surface is also discussed.

Interface bonding in silicon oxide nanocontacts: interaction potentials and force measurements.

The interface bonding between two silicon-oxide nanoscale surfaces has been studied as a function of atomic nature and size of contacting asperities. The binding forces obtained using various interaction potentials are compared with experimental force curves measured in vacuum with an atomic force microscope. In the limit of small nanocontacts (typically <103 nm2) measured with sensitive probes the bonding is found to be influenced by thermal-induced fluctuations. Using interface interactions described by Morse, embedded atom model, or Lennard-Jones potential within reaction rate theory, we investigate three bonding types of covalent and van der Waals nature. The comparison of numerical and experimental results reveals that a Lennard-Jones-like potential originating from van der Waals interactions captures the binding characteristics of dry silicon oxide nanocontacts, and likely of other nanoscale materials adsorbed on silicon oxide surfaces. The analyses reveal the importance of the dispersive surface energy and of the effective contact area which is altered by stretching speeds. The mean unbinding force is found to decrease as the contact spends time in the attractive regime. This contact weakening is featured by a negative aging coefficient which broadens and shifts the thermal-induced force distribution at low stretching speeds.

Nanoscale adhesion and sliding on biased semiconductors.

We present experimental and theoretical results on controlling nanoscale sliding friction and adhesion by electric fields on model contacts realized by bringing a conductive atomic force microscope tip into contact with the surface of a silicon-oxide/silicon wafer. We find that applying a bias voltage on silicon (or on the conductive tip) enables a noticeable control of the sliding forces. Two electrostatic interactions are identified as being relevant for the friction variation as a function of applied voltage. The first is a short-range electrostatic interaction between opposite charges localized at oxide-silicon/silicon and tip/silicon-oxide interfaces. This attractive interaction results from the high capacity of the oxide-semiconductor interface to change its charge density in response to a bias voltage. Various regimes of charging resulting from silicon electronic bands' alignment and deformation are evidenced. We mainly focused here on the strong charge accumulation and inversion domains. The second longer-range electrostatic interaction is between the voltage-induced bulk and surface charges of both tip and sample. This interaction decreases very slowly with the distance between tip and silicon surface, i.e. oxide thickness, and can be attractive or repulsive depending on voltage polarity. Our results demonstrate the possibility of controlling nanoscale friction/adhesion in nanoscale contacts involving semiconductors. These results are relevant for the operation of nanoscale devices or for on-surface nanomanipulation of metallic nanoparticles. We model the experimental results by adding an electrostatic energy contribution to the tip-surface binding energy, which translates into an increase or decrease of the normal force and ultimately of the sliding friction.

Stochastic stick-slip nanoscale friction on oxide surfaces.

The force needed to move a nanometer-scale contact on various oxide surfaces has been studied using an atomic force microscope and theoretical modeling. Force-distance traces unveil a stick-slip movement with erratic slip events separated by several nanometers. A linear scaling of friction force with normal load along with low pull-off forces reveals dispersive adhesive interactions at the interface. We model our findings by considering a variable Lennard-Jones-like interaction potential, which accounts for slip-induced variation of the effective contact area. The model explains the formation and fluctuation of stick-slip phases and provides guidelines for predicting transitions from stick-slip to continuous sliding on oxide surfaces.