Third-Eigenmode Atomic Force Microscopy for Resolving Solvation Structures and Structural Defects in Organic Semiconductor Films under Ambient Conditions

Organic semiconductor molecules are key elements for the development of next-generation flexible and lightweight electronic devices. Their performance is largely governed by molecular arrangement within thin films, whereas environmental sensing is governed by surface functionality and mediated by the interfacial structure at the solid-gas interface. However, experimental tools capable of accessing such information with molecular-scale resolution remain scarce. Here, we employ third-eigenmode atomic force microscopy (AFM) with moderately compliant cantilevers to directly probe the interfacial solvation structure of the liquid layer formed at the interface between small-molecule organic semiconductor thin films and the ambient gas environment. This approach also enables true molecular-resolution imaging across molecular steps, providing detailed insight into interplanar molecular organization and crystal growth mechanisms. Overall, our results establish third-eigenmode AFM as a versatile methodology for high-resolution characterization of molecular packing and for elucidating the relationship between surface functionality and interfacial structure in organic semiconductor materials.