Nanotribological surface characterization by frequency modulated torsional resonance mode AFM

The aim of this work is to develop an experimental method to
measure in-plane surface properties on the nanometer scale by
torsional resonance mode atomic force microscopy and to understand
the underlying system dynamics. The invention of the atomic force
microscope (AFM) and the advances in development of new AFM based
techniques have significantly enhanced the capability to probe
surface properties with nanometer resolution. However, most of
these techniques are based on a flexural oscillation of the force
sensing cantilever which are sensitive to forces perpendicular to
the surface. Therefore, there is a need for highly sensitive
measurement methods for the characterization of in-plane
properties. To this end, scanning shear force measurements with an
AFM provide access to surface properties such as friction, shear
stiffness, and other tribological surface properties with nanometer
resolution. Dynamic atomic force microscopy utilizes the frequency
response of the cantilever-probe assembly to reveal nanomechanical
properties of the surface. The frequency response function of a
cantilever in torsional motion was investigated by using a
numerical model based on the finite element method (FEM). We
demonstrated that the vibration of the cantilever in a torsional
oscillation mode is highly sensitive to lateral elastic
(conservative) and visco-elastic (non-conservative) in-plane
material properties, thus, mapping of these properties is possible
in the so-called torsional resonance mode AFM (TR-mode). The
theoretical results were then validated by implementing a frequency
modulation (FM) detection technique to torsion mode AFM. This
method allows for measuring both conservative and non-conservative
interactions. By monitoring changes of the resonant frequency and
the oscillation amplitude, we were able to map elastic properties
and dissipation caused by the tip-sample interaction. During
approach and retract cycles, we observed a slight negative detuning
of the torsional resonance frequency, depending on the tilt angle
between the oscillation plane and the surface before contact to the
HOPG surface. This angle leads to a mixing of in-plane (horizontal)
and out-of-plane (vertical) sample properties. These findings have
a significant implication for the imaging process and the
adjustment of the microscope and may not be ignored when
interpreting frequency shift or energy dissipation measurements. To
elucidate the sensitivity of the frequency modulated torsional
resonance mode AFM (FM-TR-AFM) for the energy dissipation
measurement, different types of samples such as a compliant
material (block copolymer), a mineral (chlorite) and a
macromolecule (DNA) were investigated. The measurement of energy
dissipation on these specimens indicated that the TR-AFM images
reveal a clear difference for the domains which have different
mechanical properties. Simultaneously a topographic and a chemical
contrast are obtained by recording the detuning and the dissipation
signal caused by the tip-surface interaction. Using FM-TR-AFM
spectroscopically, we investigated frequency shift versus distance
curves on the homopolymer polystyrene (PS). Depending on the
molecular weight, the frequency detuning curve displayed two
distinct regions. Firstly, a rather compliant surface layer was
probed; secondly, the less mobile bulk of the polymer was sensed by
the oscillatory motion of the tip. The high sensitivity of this
technique to mechanical in-plane properties suggests that it can be
used to discriminate different chemical properties (e.g. wetting)
of the material by simultaneously measuring energy dissipation and
surface topography.