Development of High-Sensitivity AFM Using the Second Resonance Mode of a Silicon Cantilever―High-Speed Imaging of PTCDA Chemical Structures―

Atomic force microscopy (AFM) with CO-functionalized tips has revolutionized the visualization of molecular structures, yet it remains limited by slow scan speeds and a reliance on constant-height operation. In this article, we demonstrate a significant advancement by operating a silicon cantilever in its second resonance mode, leveraging its high f0/k ratio (where f0 and k denote the resonance frequency and spring constant, respectively) to enhance force sensitivity. This platform achieves robust chemical-structure imaging at speeds up to 200 nm/s―over 60 times faster than previous reports. Additionally, by utilizing large oscillation amplitudes to maintain monotonic frequency-shift feedback, we successfully resolve molecular bonds directly in topographic mode. These breakthroughs expand the frontiers of AFM, enabling the structural analysis of complex 3D molecules and the real-time tracking of dynamic on-surface reactions.