Effect Of Excitation Frequency On Atomic Force Microscope Phase In Tapping Mode

Atomic force microscopy(AFM)is a powerful tool for studying the surface properties of materials.The core component of AFM is a scanning micro-cantilever beam,which is a force sensor.One end of the micro-cantilever beam is fixed and the other end is free.The end of the free end is provided with a needle tip that can detect the sample surface.When the AFM works in the tapping mode,the micro-cantilever beam will be excited to the vibration close to its resonant frequency,and the tip will swing with the micro-cantilever beam,and feedback the information of the sample surface through the change of the force between the tip and the sample surface.Phase imaging is an extended application of AFM developed in the tap mode to reveal structural information about surfaces at the nanoscale and to detect changes in surface composition,adhesion,hardness,friction,viscoelasticity and other properties.In this paper,the main research purpose is to obtain high quality phase image.By analyzing the dynamic model of TM-AFM micro-cantilever beam,the Euler-Bernoulli beam equation is used to solve the vibration mode.According to the vibration content,the dynamic model is simplified to the spring resonator model based on the bearing motion.The relationship between phase and excitation frequency is derived.According to the derived relationship,the experiment is designed.The phase scanning experiment is carried out on the mixed sample and the uniform sample.A number of control experiments were set near the first-order natural frequency of the micro-cantilever beam,and the excitation frequency was continuously approached until the first-order natural frequency was reached by changing the excitation frequency.After analyzing the experimental data,it is concluded that changing the excitation frequency of AFM cantilever beam in tapping mode will affect the contrast of phase imaging,which is consistent with the theory proposed previously.In the phase experiment of TM-AFM,the phase contrast increases first and then decreases rapidly when approaching the first-order natural frequency.There is a maximum phase difference,that is,the maximum phase contrast situation.Under the condition of this excitation frequency,the phase image is the clearest and the sample surface information is the most complete.Finally,it is found that the natural frequency of the probe changes as it approaches the sample in the phase scanning experiment.The scanning experiments of three different types of probe were designed under the condition of changing the distance between probe and sample based on it.The results show that the stiffness of probe is the reason for changing the first-order natural frequency of the micro-cantilever beam.