In-liquid frequency modulation atomic force microscopy (FM-AFM) has been used for visualizing subnanometer-scale surface structures of minerals, organic thin films and biological systems. In addition, three-dimensional atomic force microscopy (3D-AFM) has been developed by combining it with a three-dimensional (3D) tip scanning method. This method enabled the visualization of 3D distributions of water (i.e. hydration structures) and flexible molecular chains at subnanometer-scale resolution. While these applications highlighted the unique capabilities of FM-AFM, its force resolution, speed and stability are not necessarily at a satisfactory level for practical applications. Recently, there have been significant advancements in these fundamental performances. The force resolution was dramatically improved by using a small cantilever, which enabled the imaging of a 3D hydration structure even in pure water and made it possible to directly compare experimental results with simulated ones. In addition, the improved force resolution allowed the enhancement of imaging speed without compromising spatial resolution. To achieve this goal, efforts have been made for improving bandwidth, resonance frequency and/or latency of various components, including a high-speed phase-locked loop (PLL) circuit. With these improvements, now atomic-resolution in-liquid FM-AFM imaging can be performed at ~1 s/frame. Furthermore, a Si-coating method was found to improve stability and reproducibility of atomic-resolution imaging owing to formation of a stable hydration structure on a tip apex. These improvements have opened up new possibilities of atomic-scale studies on solid-liquid interfacial phenomena by in-liquid FM-AFM.

中文翻译：

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液体调频原子力显微镜基本性能的改进

液内调频原子力显微镜 (FM-AFM) 已用于可视化矿物、有机薄膜和生物系统的亚纳米级表面结构。此外，通过将其与三维 (3D) 尖端扫描方法相结合，开发了三维原子力显微镜 (3D-AFM)。这种方法能够以亚纳米级分辨率可视化水（即水合结构）和柔性分子链的 3D 分布。虽然这些应用突出了 FM-AFM 的独特能力，但其力分辨率、速度和稳定性不一定处于实际应用的令人满意的水平。最近，这些基本性能有了显着的进步。通过使用小悬臂，力分辨率显着提高，这使得即使在纯水中也能对 3D 水合结构进行成像，并且可以直接将实验结果与模拟结果进行比较。此外，改进的力分辨率允许在不影响空间分辨率的情况下提高成像速度。为了实现这一目标，已经做出努力来提高各种组件的带宽、谐振频率和/或延迟，包括高速锁相环 (PLL) 电路。通过这些改进，现在可以以 ~1 秒/帧的速度执行原子分辨率的液体 FM-AFM 成像。此外，由于在尖端形成稳定的水合结构，发现 Si 涂层方法可以提高原子分辨率成像的稳定性和再现性。