Birck Nanotechnology Center and School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA.
Nanotechnology. 2011 Dec 2;22(48):485502. doi: 10.1088/0957-4484/22/48/485502. Epub 2011 Nov 9.
The use of a piezoelectric element (acoustic excitation) to vibrate the base of microcantilevers is a popular method for dynamic atomic force microscopy. In air or vacuum, the base motion is so small (relative to tip motion) that it can be neglected. However, in liquid environments the base motion can be large and cannot be neglected. Yet it cannot be directly observed in most AFMs. Therefore, in liquids, quantitative force and energy dissipation spectroscopy with acoustic AFM relies on theoretical formulae and models to estimate the magnitude of the base motion. However, such formulae can be inaccurate due to several effects. For example, a significant component of the piezo excitation does not mechanically excite the cantilever but rather transmits acoustic waves through the surrounding liquid, which in turn indirectly excites the cantilever. Moreover, resonances of the piezo, chip and holder can obscure the true cantilever dynamics even in well-designed liquid cells. Although some groups have tried to overcome these limitations (either by theory modification or better design of piezos and liquid cells), it is generally accepted that acoustic excitation is unsuitable for quantitative force and dissipation spectroscopy in liquids. In this paper the authors present a careful study of the base motion and excitation forces and propose a method by which quantitative analysis is in fact possible, thus opening this popular method for quantitative force and dissipation spectroscopy using dynamic AFM in liquids. This method is validated by experiments in water on mica using a scanning laser Doppler vibrometer, which can measure the actual base motion. Finally, the method is demonstrated by using small-amplitude dynamic AFM to extract the force gradients and dissipation on solvation shells of octamethylcyclotetrasiloxane (OMCTS) molecules on mica.
使用压电元件（声激励）来振动微悬臂梁的基底是动态原子力显微镜的一种常用方法。在空气或真空中，基底运动非常小（相对于尖端运动），可以忽略不计。然而，在液体环境中，基底运动可能很大，不能被忽略。然而，在大多数原子力显微镜中，基底运动无法直接观察到。因此，在液体中，使用声学原子力显微镜进行定量力和能量耗散光谱学依赖于理论公式和模型来估计基底运动的幅度。然而，由于多种效应，这些公式可能不准确。例如，压电激励的一个显著分量不是机械地激励悬臂梁，而是通过周围的液体传输声波，这反过来又间接激励悬臂梁。此外，即使在设计良好的液体池中，压电、芯片和支架的共振也会使真实的悬臂梁动力学变得模糊。尽管有些小组试图克服这些限制（通过理论修正或更好的压电和液体池设计），但人们普遍认为，声学激励不适合在液体中进行定量力和耗散光谱学。在本文中，作者对基底运动和激励力进行了仔细研究，并提出了一种实际上可以进行定量分析的方法，从而为在液体中使用动态原子力显微镜进行定量力和耗散光谱学打开了这一流行方法。该方法通过在云母上使用扫描激光多普勒测振仪在水中进行实验得到验证，该测振仪可以测量实际的基底运动。最后，通过使用小振幅动态原子力显微镜提取云母上八甲基环四硅氧烷（OMCTS）分子溶剂化壳的力梯度和耗散，验证了该方法。
