Defects and interfaces play a crucial role in governing material properties, and their understanding remains an active area of research. Advanced electron microscopy techniques offer unparalleled insight into these phenomena. In this talk, I will present recent work on layered materials and heterostructures, emphasizing how electron microscopy enables key discoveries. Tin monoxide (SnO) is particularly intriguing as one of the few oxide materials exhibiting p-type semiconductivity. However, it is metastable and challenging to synthesize. We demonstrate that SnO can be stabilized through the formation of a thin orthorhombic SnO
2 layer—also a metastable phase—resulting in a unique case of mutual stabilization between two metastable oxides. The analogous lead compound, PbO, exists in two polymorphs (tetragonal and orthorhombic), both layered structures. We show that local strain can induce phase transformation between these polymorphs, with regions neardislocations in the tetragonal phase transforming into the orthorhombic form. Furthermore, local indentation can be used to deliberately trigger this transformation. Conversion chemistry based on anion/cation exchange offers a powerful route fortailoring nanocrystals and fabricating heterostructures. We have investigated multiple systems—including oxides, sulfides, and tellurides—where controlled lateral heterostructures are synthesized via simple chemical methods. These systems exhibitnotable enhancements in electrocatalytic and sensing performance. I will also describe a novel relaxation mechanism in lateral heterostructures of layered materials that motivated the development of a new HAADF-STEM approach using a defocused probe. The mechanism of image contrast and the insights gained through this method will be discussed. Finally, I will present our analysis of extended defects in reduced WO
3, particularly crystallographic shear planes, and their connection to memristive behavior in this system. Overall, the talk will highlight how complementary microscopy techniques can unravel the interplay between structure, defects, and functionality in complex materials and devices.
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