Exploring the 3D Nano and Atomic World: Coherent Diffractive Imaging and Atomic Electron Tomography

Abstract:  The discovery and analysis of X-ray diffraction from crystals by Max von Laue, William Henry Bragg and William Lawrence Bragg in 1912 marked the birth of crystallography. Over the last century, crystallography has been fundamental to the development of many fields of science. However, many samples in physics, chemistry, materials science, nanoscience, geology, and biology are non-crystalline, and thus their 3D structures are not accessible by traditional crystallography. Overcoming this hurdle has required the development of new structure determination methods. In this talk, I will present two methods that can go beyond crystallography: coherent diffractive imaging (CDI) and atomic electron tomography (AET). In CDI, the diffraction pattern of a non-crystalline sample or a nanocrystal is first measured and then directly phased to obtain an image. The well-known phase problem is solved by combining the oversampling method with iterative algorithms. In the first part of the talk, I will briefly discuss the principle of CDI and highlight its capability of direct observation of 3D topological spin textures and their interactions in a ferromagnetic superlattice. In the second part of the talk, I will present a general tomographic method, termed AET, for 3D structure determination of crystal defects and disordered materials at the single atomic level. By combining advanced electron microscopes with powerful computational algorithms, AET has been used to reveal the 3D atomic structure of crystal defects and chemical order/disorder and to precisely localize the 3D coordinates of individual atoms in materials without assuming crystallinity. The experimentally measured coordinates can then be used as direct input for quantum mechanical calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. As coherent X-ray sources and powerful electron microscopes are under rapid development around the world, we expect that CDI and AET will find broad applications in both the physical and biological sciences.

 

Bio: Jianwei (John) Miao is professor in the Department of Physics & Astronomy and California NanoSystems Institute at UCLA. He is an internationally renowned pioneer in the development of novel imaging methods with X-rays and electrons. He performed the seminal experiment of extending X-ray crystallography to allow structural determination of non-crystalline specimens in 1999, which is known as coherent diffractive imaging (CDI), lensless or computational microscopy. CDI methods such as plane-wave CDI, ptychography (i.e., scanning CDI) and Bragg CDI have been broadly implemented using synchrotron radiation, X-ray free electron lasers, high harmonic generation, optical and electron microscopy. In 2012, he applied CDI algorithms to pioneer atomic electron tomography (AET) for 3D structure determination of materials without assuming crystallinity. He has since performed several groundbreaking AET experiments to image a wide range of crystal defects with unprecedented 3D detail. In 2019, he developed 4D AET to observe crystal nucleation at atomic resolution, showing early stage nucleation results contradict classical nucleation theory. More recently, he advanced AET to solve a long-standing grand challenge in the physical sciences – determining the 3D atomic structure of amorphous solids for the first time.

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