Mechanisms Of Energy Dissipation In Tapping Mode Atomic Force Microscopy

Microscope technology started from the optical microscope in the 16 th century,and gradually developed to the electron microscope,scanning tunneling microscope,atomic force microscope(AFM)and so on in the 20 th century.The development of microscopy technology has also witnessed the desire of scientists to explore the micro world.Atomic force microscopy uses a cantilever probe to explore the microscopic world.In tapping mode,the energy dissipation influenced both the phase image and the quality factor.The energy dissipation generated during the contact between the probe and the sample reflects the characteristics of the sample surface and the phase image.Secondly,reducing the energy dissipation can improve quality factor,the imaging accuracy.Energy dissipation can be caused by many factors in the vibration process of the probe and in the contact with the sample.Clarifying the impacts of various dissipation mechanisms is a process of identifying false and finding the truth.Based on microscale contact models and cantilever dynamic models,this dissertation studied the energy dissipation between probe and sample caused by adhesion,plastic deformation,liquid bridge,air damping and other factors.The main contents and results are as follows:(1)Dynamic characteristics of the probe were studied on the basis of vibration theory and finite element simulation.The point mass model and Euler-Bernoulli beam model were used to describe the vibration characteristics of the probe.Hereafter,the equivalence of the two models was proved.The relationship between energy dissipation and phase image and quality factor in tapping mode was illustrated.By means of the improved AFM system for oscilloscope monitoring and ANSYS software simulation of forced vibration response of cantilever beam under one side limitation,it was proposed for the first time that the steady-state vibration response of the cantilever beam can remain approximately sinusoidal after the probe slightly touches the sample surface.Truncation phenomenon in frequency sweep experiment was explained.(2)The effect of adhesion between probe and sample on energy dissipation was quantitatively studied.The characteristics of several kinds of classical contact models were analyzed.The suitable contact models were given according to the actual conditions of AFM test.The JKR contact model was used to describe the loading-unloading curve of probe and sample under external force without considering the capillary force.The relationship between the unstable point of AFM force curve and the corresponding position of JKR curve was given.The influence of the cantilever beam stiffness and the elastic modulus of the sample on the change of separation force was analyzed.The above method for determining the unstable position of separation was proved to be reasonable by force curve experiment,and the energy dissipation caused by adhesion in the contact-separation process was calculated quantitatively.Finally,a contact model was established considering the roughness of samples,and the relationship between roughness variation and energy dissipation during contact separation was given.(3)The energy dissipation caused by plastic deformation was studied using the plastic contact theory and numerical simulation.It was proposed that when the probe contacts the sample,the surface of the sample will undergo plastic deformation only under the action of surface force.During contact separation,the loading stage can be described by the full plastic contact state of M-P contact model,and the unloading stage can be described by the JKR model.According to the characteristics of probe-sample contact in tapping mode and in contact mode,the influence mode of plastic deformation was given.The influences of tip size and yield limit on AFM measurement results were discussed.It was pointed out that the relationship between measurement accuracy and sample protection should be coordinated when selecting probe.(4)Three models of liquid bridge generation are given,including liquid film extrusion model,liquid film flow model and capillary condensation model.The volume of liquid bridge generated by each model under different humidity was given.Each model was used to analyze the required time scale to generate balance bridge.Combined with AFM contact mode,force curve mode and tapping mode,the time characteristics of the probe in contact with the sample was given under different operating modes.The corresponding main mechanism of liquid bridge was given.By studying the relationship between the volume of the liquid bridge and energy dissipation,the range of energy dissipation caused by the liquid bridge in AFM tapping mode under different ambient humidity was obtained.(5)The influence of air viscous damping on AFM system was studied.A scheme to calculate the energy dissipation caused by viscous damping was proposed.The experimental results showed that the pressure film damping effect has a significant effect on the vibration of the probe.The mechanisms of the pressure film damping on the three different types of probe were discussed in the frequency sweeping experiments using self-made microsphere probe,conventional probe and tipless probe.The influences of probe angle,tip height and micron ball on film damping were discussed.The pressure film damping of different probes was simplified to establish models.The accuracy of the simplified models were verified by comparing the theoretical calculation and experimental results.The energy dissipation value caused by pressure film damping of AFM in tapping mode was calculated.Some reasonable suggestions for structural design of probe were given.The innovation of this dissertation lies in that the action mechanism and energy dissipation mechanisms in AFM are given using theoretical calculation and experiment.It provides a basis for further checking the test system to improve the test accuracy.It also provides theoretical support for operators to choose various operating parameters and settings when using AFM.