Force spectroscopy using bimodal atomic force microscopy
Author
Aksoy, Mehmet Deniz
Advisor
Atalar, Abdullah
Date
2010Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Show full item recordAbstract
In atomic force microscopy (AFM) achieving compositional contrast while
mapping topographical features is a challenging task. Conventional single mode
frequency and amplitude modulation AFM techniques are sensitive to the properties
of the tip sample interaction, however in the absence of additional information
channels, compositional features such as elasticity and density cannot be
distinguished from topographical variations. To tackle this problem bimodal excitation
techniques are introduced. In bimodal amplitude modulation AFM, sensitivity
to compositional features improves by recording the phase of the higher
order vibrations, while the topography is acquired using the amplitude of the first
order vibrations. Increased sensitivity to mechanical properties allows imaging
delicate samples such as organic molecules using gentle forces.
In this thesis we propose a force spectroscopy technique in which two modes
of a cantilever are excited in such a way that the amplitudes of the components of
the vibration stay constant. Presence of the force field modulates the properties of the primarily bi-harmonic vibration of the cantilever, which is, in our case,
the instantaneous frequencies of vibration modes. The frequency shift of the first
mode remains sensitive to topographical variation, whereas the frequency shift
of the higher mode samples the gradient of the tip sample forces and allows us
to extract the tip sample interaction as a function of separation within a single
cycle of the slow oscillation.
We provide an analytic treatment of the proposed scheme and confirm our
predictions by numerical simulations. We present an analysis of the sensitivity
of higher mode frequency shifts to compositional features in the presence of
thermal and sensor noise. We demonstrate that the method is suitable for the
fast acquisition of contact properties, especially in vacuum environment where
the large quality factor of the cantilever limits the available bandwidth of the
amplitude modulation techniques. Finally we investigate phase shifts in bimodal
amplitude modulation AFM using the developed formalism and show that phase
contrast can be optimized by solving a simpler problem in single mode amplitude
modulation AFM.
Keywords
Atomic Force MicroscopyForce Spectroscopy
Amplitude Modulation Atomic Force Microscopy
Atomic Force Microscopy
Frequency Modulation
Bimodal Excitation
Bimodal Imaging
Dynamic Atomic Force Microscopy
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