Origin of functionality for functional materials at atomic scale

The physical properties of functional materials are closely related to the intrinsic symmetry and applied fields, wherein symmetry breaking under external fields generates functionalities for functional materials that underlie various technological applications. Understanding the origin of functionalities requires us to determine the precise atomic structure and electronic configurations of functional materials in the broken-symmetry states, thereby establishing the direct correlation between the structural details and functional properties of materials. The recent advances in aberration-corrected transmission electron microscopy (TEM) have enabled real-space imaging and spectroscopy at sub-angstrom resolution with extraordinary sensitivity to structure, chemistry, and bonding, providing great opportunities to bridge the gap between structure and functionality. Here we present case studies highlighting the utility of advanced TEM in understanding the origin of functionality for functional materials, alongside these examples is a discussion of the physical underpinnings of specific functional properties. We then conclude with a personal perspective on future directions as well as fundamental challenges.