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Cantilever Dynamic Analysis in Tapping Mode Atomic Force Microscopy

Deng, W (2015) Cantilever Dynamic Analysis in Tapping Mode Atomic Force Microscopy. Doctoral thesis, Liverpool John Moores University.

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Tapping mode atomic force microscopy (AFM) is becoming popular in the area of biology, as well as for polymer and semiconductor materials science. Unlike other AFM techniques, it only makes intermittent contact with the sample, which largely reduces any potential surface damage to soft materials, like cells and polymer. Moreover, phase image can also be obtained from tapping mode AFM besides height and amplitude image. Phase images provide extra information on the test sample in comparison with height and amplitude images.
Studies have been carried out to investigate the contributions to phase shifts in tapping mode AFM using point-mass model. Results showed that the phase shift is independent of the Young’s modulus of the material; the phase shift only changes when energy dissipation occurs, such as the case with adhesion hysteresis and viscosity. However, the simple point mass model can only study the first order vibration mode of the AFM cantilever. Moreover, it does not take into account geometrical effects of the tip and the cantilever.
However, correct interpretation of phase images still poses a significant challenge to the AFM community. In this study, the cantilever’s dynamic behaviour in tapping mode AFM is studied through a three dimensional finite element method. A rectangular silicon cantilever with the dimensions 240 μm length, 30 μm width, 2.7 μm thick and with a silicon tip radius of 9nm, are used in the simulation. The cantilever dimensions are the same as those of the Olympus model AC240TS cantilevers used in AFM experiments. The material properties of the silicon cantilever as used in the simulation again match those of the real cantilevers and are defined as: Young’s modulus of 170 GPa and a Poisson’s ratio of 0.28. A piezo actuator is attached to the cantilever. A sinusoidal voltage is subsequently applied to the piezo actuator in order to vibrate the cantilever.
The cantilever’s dynamic displacement responses are firstly obtained via simulation under different tip-sample separations and for different tip-sample interaction forces, such as elastic force, adhesion force, viscosity force and the van der Waals force, which correspond to the cantilever’s action upon various different representative computer-generated test samples. Simulated results show that the dynamic cantilever displacement response consists of three states: free vibration, a transition state and a stable state. Phase shift, transition time, stable amplitude and frequency changes are then analysed from the dynamic displacement responses that are obtained. The phase shift of free vibration is 90o. It is found that under pure repulsive force, the phase shifts are above 90o, while the phase shifts are below 90o under pure attractive force. Also, attractive forces have the ability to decrease the phase shifts. When different interaction forces are coupled together, depending on the strength of the attractive forces, the phase shifts may suddenly drop below 90o.
Finally, experiments are carried out on a real AFM system to support the findings of the simulations. Olympus model AC240TS cantilever is used in the experiment, while polyurethane (PU), polyvinyl chloride (PVC) and parafilm are used as test sample. Phase shifts were recorded by changing the setpoint ratio (setpoint ratio = setpoint amplitude/free amplitude). The phase shifts were recorded from set-point amplitudes varying from 43.6nm to 4.36nm, which have the similar trend as simulation results.

Item Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Divisions: General Engineering Research Institute
Date Deposited: 19 Oct 2016 14:05
Last Modified: 28 Apr 2017 11:45
Supervisors: Zhang, Guangming and Harvey, David
URI: http://researchonline.ljmu.ac.uk/id/eprint/4349

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