双模态振幅调制原子力显微术对比度反转研究

Investigation on Contrast Inversion in Bimodal Amplitude-modulation Atomic Force Microscopy

双模态振幅调制原子力显微术广泛应用于微纳米力学成像,然而成像过程中可能存在的对比度反转问题会造成成像结果的难以理解和解释.将有限差分法和同相正交法相结合,采用数值方法研究了不同力常数探针、样品组分力学性能以及成像参数设置对双模态振幅调制原子力显微术二阶模态振幅和相位对比度反转的影响.研究结果显示,对于硬探针,不同粘度系数组分上二阶模态振幅和相位对比度随弹性模量增加无反转产生.探针在不同弹性模量组分上二阶模态振幅对比度随粘度系数增加会产生反转,而二阶模态相位对比度则无反转产生.对于软探针,组分弹性模量或粘度系数增加会造成相互作用区转变,使探针响应产生跳变.软探针在组分弹性模量较高时二阶模态相位对不同粘度系数的对比度以及在粘度系数较小时对不同弹性模量组分的对比度均较硬探针更低.两种探针在不同弹性模量组分上二阶模态振幅对比度随着二阶模态自由振幅的增加会发生反转,而不同粘度系数下的对比度则未出现反转.此外,二阶模态自由振幅越小,则二阶模态相位对不同弹性模量或粘度系数组分的对比度越高.二阶模态振幅或相位对比度在不同相互作用区可能会发生反转,成像时应使相互作用区处于排斥区,可提高对比度.

Bimodal amplitude-modulation atomic force microscopy(AM-AFM) is widely used in micro/nanoscale mechanical imaging. However, the contrast inversion which probably appears during imaging makes it difficult to understand the imaging results. Combining the finite difference method with the in-phase and quadrature method, the influences of the selection of probes with different force constants, the mechanical properties of the sample components, and the settings of imaging parameters on contrast inversion of the amplitude and phase of the second mode are investigated numerically in bimodal AM-AFM. Results show that for a stiff probe, the contrasts of the amplitude and phase of the second mode for components with different viscosity coefficients would not reverse with increasing elastic modulus. The contrast of the amplitude of the second mode for components with different moduli would be inverted with increasing viscosity coefficients, while contrast inversion of the phase of the second mode will not occur. For a soft probe, the increase of elastic modulus or viscosity coefficient would result in the transition of the interaction regimes, giving rise to discontinuous jumps of the response. For a soft cantilever, the contrast of the phase of the second mode for components with a higher modulus and different viscosity coefficients or the contrast of the phase of the second mode for components with a small viscosity coefficient and different moduli is lower compared with that of a stiff cantilever. The contrast of the amplitude of the second mode for components with different elastic moduli will reverse as the free amplitude of the second mode increases. However, the contrast at different viscosity coefficients does not reverse. Furthermore, the smaller is the free amplitude of the second mode, the higher is the contrast of the phase of the second mode to the elastic moduli or viscosity coefficients of the components. The amplitude or phase contrast of the second mode may be inverted in different interaction regimes, and the interaction regime should be kept in the repulsive regime to achieve a higher imaging contrast.