Are Chemical Bonds Truly Observable in Real Space using Atomic Force Microscopy?

The development of molecule-functionalized high-resolution atomic force microscopy (HR-AFM) has ushered in a new era, enabling the direct visualization of small molecules and various types of surfaces, including metals, insulators, and semiconductors. This advancement has also facilitated the tracking of specific chemical reactions. Furthermore, our previous work has showcased HR-AFM's remarkable capabilities in breaking chemical bonds[1], distinguishing functional groups[2,3], heteroatoms[4], and even electron orbital signatures[5]. However, a fundamental question at the core of AFM imaging principles - whether chemical bonds are truly observable in real space (or if the "chemical bonds" seen in AFM images are directly interoperable) - remains a subject of ongoing debate.

Our present study addresses this contentious issue by designing a meticulously crafted experiment and employing our unique real-space pseudopotential density functional theory-based AFM simulation method[6]. Our findings reveal that while relatively strong covalent bonds can indeed be observed in real space, it is also possible to detect a "chemical bond" connecting two entirely isolated atoms. Our work[7] is a key reference for effectively distinguishing features resembling chemical bonds in AFM images and advances this field of study.