Probing interfacial water by H-sensitive and non-invasive scanning probe microscopy

Water/solid interfaces are a central theme across an incredibly broad range of scientific and technological processes. Scanning probe microscopy (SPM) has been extensively applied to probe interfacial water in many interdisciplinary fields. However, there exist two longstanding limitations in the past two decades, which makes SPM fall short compared with conventional spectroscopic methods. First, H atoms of water molecule are very small and light, so it is very difficult to image them directly; Second, the water molecules are linked by weak H bonds, and it is highly possible to disturb the fragile water structure during the imaging process. In order to overcome these two grand challenges, we have developed a new-generation SPM based on a qPlus sensor, which is sensitive to H and non-invasive to water structure [1]. The key lies in probing the high-order electrostatic force between the quadrupole-like CO-terminated tip and the polar water molecules at large tip-water distances. In this talk, I will first discuss the application of this technique to determine the microscopic structure of metastable water clusters [1]. In addition, we have unraveled the detailed atomic structures of ion hydrates at interfaces and discover a magic-number effect on the transport of ion hydrates [2]. Finally, I will show the ability of visualizing the growth of a two-dimensional ice in real space with atomic resolution, by capturing various metastable and intermediate structures during the ice growth at the ice edges [3].
[1] Peng et al., Nature Commun. 9, 122 (2018) .
[2] Peng et al., Nature 557, 701 (2018)
[3] Ma et al., Nature in press (2019)