(212c) Heterogeneity, Dynamic Ordering, and Ion Transport: Emerging Methods for Characterizing Complex Solids at the Nanoscale

Complex structure—characteristic of engineered nano- and micro-scale materials—often presents challenges to rigorous characterization using conventional methods, including microscopy and diffractometry. Here, emerging measurement and analysis methods are highlighted, primarily toward rationalizing the properties of functional materials for optoelectronic devices and renewable-energy applications. The advent of next-generation light sources and detector technologies have enabled new capabilities such as rapid or low-dose acquisition, pump-probe ultrafast modalities, and enhanced spatial resolution.

Using metal-halide perovskite semiconductors as case studies, we demonstrate how applied crystallography methods elucidate: (i) compositional heterogeneity and anomalous phase behavior in perovskite polycrystals (solution-processed thin films); (ii) intrinsically dynamic local order within the average high-symmetry structures of perovskite single crystals. The suite of synchrotron X-ray and neutron scattering methods serve to differentiate the static and dynamic disorder within these complex hybrid organic-inorganic crystals. Connections between measurement and calculations—particularly contrasting equilibrium elastic and energy-filtered inelastic scattering—will also be discussed toward atomic-scale modeling of complex materials.

Finally, scanning transmission electron microscopy (STEM) and electron nanoprobe diffraction (e.g., 4D-STEM) techniques are developed to characterize dynamic structure-transport relations. To investigate atomic-scale instabilities in the metal-halide perovskites, we used cryogenic 4D-STEM to directly map phase segregation due to photoinduced halide mobility. Further, room-temperature and high-temperature in situ 4D-STEM measurements elucidate transport mechanisms in graphite intercalation compounds for applications from battery electrodes to 2D electronics. With resolution limits realistically approaching the atomic scale, STEM-based methods to differentiate reversible and irreversible transformations, as well as diffusion-reaction boundaries, are proposed.