Bimodal Atomic Force Microscopy with a Torsional Eigenmode for Highly Accurate Imaging of Grain Orientation in Organic Thin Films

In organic electronics, the nature and spatial distribution of grains in polycrystalline thin films of small organic semiconductor molecules greatly impact the electronic properties of devices. Therefore, tools that accurately characterize organic films at the mesoscopic level are essential. To this end, we demonstrate here the power of a bimodal atomic force microscopy (AFM) with a torsional eigenmode for highly accurate imaging of grain orientations in organic thin films. The energy dissipated between the tip and sample during scanning depends on the in-plane crystalline orientation of each grain. This fact alters the cantilever torsional observables, allowing grain orientation recognition. Remarkably, bimodal AFM with the torsional eigenmode has important advantages, such as high sensitivity in the applied forces, true molecular resolution, and multiple parameters for regulating the image contrast, making it competitive with other well-established AFM methods for grain detection in organic thin films, namely, friction force microscopy and transverse shear microscopy.