原子力显微镜探针悬臂几何结构变化对高次谐波信息增强的研究

Investigation of enhancing higher harmonics by changing the shape of atomic force microscope cantilever

轻敲模式原子力显微镜高次谐波信号包含待测样品表面纳米力学特性等方面的信息,但是传统原子力显微镜的高次谐波信号非常微弱.里兹法证明在探针悬臂的特定位置打孔可以实现探针的内共振从而增强高次谐波信号强度.本文通过有限元仿真计算获得探针第一共振频、第二共振频及其比值随着孔的尺寸和位置变化的规律.在实验上通过聚焦离子束在探针悬臂上打孔使其第二共振频约为第一共振频的6倍,提高了第6次谐波信号的信噪比,并在实验室研制的高次谐波成像实验装置上获得了6次谐波图像.

Higher harmonics of tapping-mode atomic force microscope carries information about the mechanical properties of the sample on a nanometer scale. Unfortunately, the vibration amplitudes of traditional atomic force microscope （AFM） cantilever at higher harmonics are too small for practical AFM imaging. Ritz method demonstrates that specific cutout on the cantilever can realize internal resonance to enhance higher harmonics. In this paper, by COMSOL finite element simulation, the laws for fundamental frequency, second resonance frequency and their ratio each as a function of the size of the cutout and the position of the cutout on the cantilever are achieved. Using focused ion beam to hole the cantilever makes the second resonance frequency close to 6 times that of the fundamental frequency and also the 6th harmonic enhanced. Moreover, we obtain the image of the 6th harmonic on our home-made higher harmonic system.