Several subsurface imaging methods based on atomic force microscopy (AFM) linear nanomechanical mapping, namely contact resonance (CR), bimodal and harmonic AFMs, are investigated and compared. Their respective subsurface detection capability is estimated and evaluated on a model specimen, which is prepared by embedding SiO2 microparticles in a PDMS elastomer. The measured CR frequency, bimodal and harmonic amplitudes are related to local mechanical properties by analyzing cantilever dynamics and further linked to subsurface depths of the particles by finite element analysis. The maximum detectable depths are obtained from the apparent particle diameters in subsurface image channels via employing a simple geometrical model. Under common experimental settings, results demonstrate that the depth limits reach up to about 812 nm, 212 nm and 127 nm for CR, bimodal and harmonic AFM modes, respectively. The depth sensitivity can be tuned and optimized by using either different cantilever eigenmodes in CR-AFM or spectroscopy analysis in bimodal and harmonic AFMs. The three imaging methods have their own suitable application situations. The comparisons can advance a further step into understanding the subsurface image contrast via AFM mechanical sensing.

中文翻译：

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基于原子力显微镜机械传感的埋在聚合物基体中的刚性颗粒的亚表面成像

研究和比较了几种基于原子力显微镜 (AFM) 线性纳米力学映射的亚表面成像方法，即接触共振 (CR)、双峰和谐波 AFM。它们各自的地下探测能力是在模型样本上进行估计和评估的，该样本是通过将 SiO2 微粒嵌入到 PDMS 弹性体中制备的。通过分析悬臂动力学，测得的 CR 频率、双峰和谐波幅度与局部机械性能相关，并通过有限元分析进一步与颗粒的地下深度相关联。通过采用简单的几何模型，从地下图像通道中的表观粒径获得最大可检测深度。在常见的实验设置下，结果表明深度限制达到约 812 nm，CR、双峰和谐波 AFM 模式分别为 212 nm 和 127 nm。通过在 CR-AFM 中使用不同的悬臂本征模式或在双峰和谐波 AFM 中使用光谱分析，可以调整和优化深度灵敏度。三种成像方式各有适合的应用场合。这些比较可以进一步推进通过 AFM 机械传感了解地下图像对比度的步骤。