School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100083, People's Republic of China. Key Laboratory of Micro-nano Measurement-Manipulation and Physics, Beihang University, Beijing 100083, People's Republic of China.
Nanotechnology. 2018 Sep 21;29(38):385702. doi: 10.1088/1361-6528/aad031. Epub 2018 Jun 29.
The tip motion of the dynamic atomic force microscope in liquids shows complex transient behaviors when using a low stiffness cantilever. The second flexural mode of the cantilever is momentarily excited. Multiple impacts between the tip and the sample might occur in one oscillation cycle. However, the commonly used Fourier transform method cannot provide time-related information about these transient features. To overcome this limitation, we apply the wavelet transform to perform the time-frequency analysis of the tip motion in liquids. The momentary excitation of the second mode and the phenomenon of multiple impacts are clearly shown in the time-frequency plane of the wavelet scalogram. The instantaneous frequencies and magnitudes of the second mode are extracted by the wavelet ridge analysis, which can provide quantitative estimations of the tip motion in the second mode. Moreover, the relations of the maximum instantaneous magnitude (MIM) to the amplitude setpoint and the Young's modulus of the sample surface are investigated. The results suggest that the MIM can be used to characterize the nanomechanical property of the sample surface at high amplitude setpoints.
在使用低刚度悬臂梁时，动态原子力显微镜的尖端运动在液体中表现出复杂的瞬态行为。悬臂梁的第二弯曲模态会被瞬间激发。在一个振动周期内，可能会发生多次尖端与样品之间的碰撞。然而，常用的傅里叶变换方法无法提供有关这些瞬态特征的时间相关信息。为了克服这一限制，我们应用小波变换对液体中尖端运动进行时频分析。第二模态的瞬时激发和多次碰撞现象在小波标度图的时频平面上清晰显示。通过小波脊分析提取第二模态的瞬时频率和幅度，可对第二模态下的尖端运动进行定量估计。此外，还研究了最大瞬时幅度（MIM）与振幅设定值和样品表面杨氏模量之间的关系。结果表明，在高振幅设定值下，MIM 可用于表征样品表面的纳米力学性能。
