López-Guerra Enrique A, Somnath Suhas, Solares Santiago D, Jesse Stephen, Ferrini Gabriele
Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC, 20052, USA.
Department of Civil and Environmental Engineering, The George Washington University, Washington, DC, 20052, USA.
Sci Rep. 2019 Sep 3;9(1):12721. doi: 10.1038/s41598-019-49104-1.
Traditionally, dynamic atomic force microscopy (AFM) techniques are based on the analysis of the quasi-steady state response of the cantilever deflection in terms of Fourier analysis. Here we describe a technique that instead exploits the often disregarded transient response of the cantilever through a relatively modern mathematical tool, which has caused important developments in several scientific fields but that is still quite unknown in the AFM context: the wavelet analysis. This tool allows us to localize the time-varying spectral composition of the initial oscillations of the cantilever deflection when an impulsive excitation is given (as in the band excitation method), a mode that we call the few-cycle regime. We show that this regime encodes very meaningful information about the tip-sample interaction in a unique and extremely sensitive manner. We exploit this high sensitivity to gain detailed insight into multiple physical parameters that perturb the dynamics of the AFM probe, such as the tip radius, Hamaker constant, sample's elastic modulus and height of an adsorbed water layer. We validate these findings with experimental evidence and computational simulations and show a feasible path towards the simultaneous retrieval of multiple physical parameters.
传统上，动态原子力显微镜（AFM）技术基于通过傅里叶分析对悬臂梁偏转的准稳态响应进行分析。在此，我们描述一种技术，该技术转而通过一种相对现代的数学工具来利用悬臂梁常常被忽视的瞬态响应，这种数学工具在多个科学领域引发了重要进展，但在AFM领域仍然鲜为人知：小波分析。当施加脉冲激励时（如在带激励方法中），该工具使我们能够定位悬臂梁偏转初始振荡的时变频谱成分，我们将这种模式称为少周期 regime。我们表明，这种 regime 以独特且极其敏感的方式编码了关于针尖 - 样品相互作用的非常有意义的信息。我们利用这种高灵敏度来深入详细了解扰动AFM探针动力学的多个物理参数，例如针尖半径、哈梅克常数、样品的弹性模量以及吸附水层的高度。我们用实验证据和计算模拟验证了这些发现，并展示了一条同时获取多个物理参数的可行途径。
