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双模态振幅调制原子力显微术相互作用区转变研究

INVESTIGATION ON TRANSITION BETWEEN TIP-SAMPLE INTERACTION REGIMES IN BIMODAL AMPLITUDE MODULATION ATOMIC FORCE MICROSCOPY

  • 摘要: 双模态振幅调制原子力显微术测试或成像过程中存在引力区和斥力区两种相互作用区. 开展双模态振幅调制原子力显微术针尖样品相互作用区转变的研究, 对于在特定作用区内成像的参数设置、相互作用区范围的控制, 以及对成像结果的正确理解和解释尤为重要. 将有限差分法和同相正交法相结合, 采用数值模拟方法研究了探针模态自由振幅大小设定、样品力学性能变化以及模态激励频率的设置对双模态振幅调制原子力显微术相互作用区转变的影响. 研究结果表明, 探针模态自由振幅之和越大, 则引力区向斥力区转变时的临界设定点越大, 使探针位于引力区的设定点的范围越小. 样品的弹性模量越大、黏度系数越小, 探针在接近样品过程中引力区向斥力区转变发生越早, 即引力区设定点的范围越小. 偏离自由共振频率对探针进行激励时, 引力区的范围均小于以自由共振频率激励时的引力区范围, 探针运动状态的突变并不一定对应相互作用区的转变, 且不能将相位值是否高于或低于90°作为判定探针位于引力区或斥力区的依据.

     

    Abstract: Bimodal amplitude modulation atomic force microscopy (AM-AFM) has two interaction regimes in measurements or imaging, i.e., the attractive regime and the repulsive regime. The investigation on the transition between the interaction regimes is critical for the setting of parameters for imaging in a specific interaction regime, the control of the ranges of the interaction regimes as well as the correct understanding and interpretation of the imaging results in bimodal AM-AFM. Combining finite difference method and the in-phase and quadrature method, the influences of the magnitudes setting of modal free resonance amplitudes, the variation of mechanical properties of the sample, and the setting of excitation frequencies on the transition between the interaction regimes in bimodal AM-AFM are studied by a numerical simulation. Results show that the higher the sum of the modal free resonance amplitudes, the larger the critical setpoint when the attractive regime transitions to the repulsive regime, i.e., the setpoint range of the attractive regime becomes smaller. The higher the elastic modulus and the smaller the viscosity coefficient of the sample, the earlier the attractive regime transitions to the repulsive regime during the approach of the probe to the sample surface. When the probe is excited at the frequencies away from the modal free resonance frequencies, the range of the attractive regime is smaller than that excited at the free resonance frequencies. The saltation of the state of motion of the probe does not necessarily correspond to the transition between the interaction regimes. Furthermore, the phase values are invalid to be employed to determine the attractive or repulsive regime by judging whether the phase value is higher or lower than 90°.

     

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