Speaker
Description
The atomic-scale surface structure often determines a material’s properties in natural processes and industrial applications. Yet it remains unresolved for many materials, particularly insulating oxides and minerals, which are abundant in the Earth’s crust. Noncontact atomic
force microscopy (nc-AFM) is ideally suited to address this challenge. It enables atomically resolved imaging of insulating surfaces and, with appropriate tip functionalization, chemical identification of individual surface atoms.
In recent work [1,2], we applied this approach to sapphire, Al$_2$O$_3$(0001), a prototypical insulating oxide surface. Directly resolving the positions of individual Al and O atoms disproved the long-standing assumption that the reconstructed surface is metallic. Instead, the
extensive reconstruction preserves overall Al$_2$O$_3$ stoichiometry and is driven by increased coordination of surface Al cations [1]. In complementary work, we showed that the nominally unreconstructed Al$_2$O$_3$(0001) surface is inhomogeneous and rough, rather than a uniform bulk termination [2]. The versatility and non-perturbative nature of nc-AFM are further demonstrated through atomic-scale studies of water at mineral surfaces. On silver iodide (AgI), a widely used ice-nucleating material in cloud seeding, we showed that its polar basal surfaces reconstruct and that their atomic structure governs whether water forms an extended epitaxial ice layer or three-dimensional clusters [3]. Together, these examples demonstrate how nc-AFM can reveal the true atomic structure of insulating surfaces and link it to their stability, reactivity, and function.
[1] J.I. Hütner, et al., Science 385, 1241–1244 (2024)
[2] J.I. Hütner-Reisch, et al., Nature Communications 17, 4692 (2026)
[3] J.I. Hütner, et al., Science Advances 11, 44, eaea2378 (2025)