masonlw

Many Microscopy techniques have been developed to explore atomic structures of material and biological specimens at a nano-scale. Coherent Diffractive Imaging (CDI) and Atomic Electron Tomography (AET) are the most powerful among these techniques. Mathematics, especially variational optimization, also follows and supports microscopy, helping to solve various image processing problems such as deblurring and denoising. In our latest research, variational methods can help obtain super-resolution ptychography images, marking a substantial improvement in computational microscopy and bypassing the resolution limit. While sub-pixel shifting and structured probes are two crucial keys to the super-resolution problem, total variation and l1 regularization play a significant role in reconstruction. In another research project, we explore the magnetic vector fields created in the vacancy between atoms of a magnetized material. Our analysis shows that the vector tomography needs in-plane rotation and constraint support to guarantee reconstruction. The sparsity inducing l1 minimization is the right tool to generate highly accurate support. Our variational methods have successfully solved a wide range of image processing problems. We will continue to utilize these techniques again in our upcoming research.

Magnetic systems exhibiting chirality have gained significant attention over the past decade. From enabling the efficient movement of magnetic domain walls to stabilizing the topological magnetic quasiparticles known as skyrmions, chirality and chiral interactions may play a central role in the next generation of magnetic memory and computing schemes. The possible responses of chiral magnetic systems to stimuli such as magnetic fields, electrical currents, light, and thermal gradients is at the forefront of thin-film magnetismresearch [1]. I will briefly introduce the underlying interactions that give rise to chiral magnetism, the emergence of topology and discuss some of the surprising systems where chirality and topology are emerging.  I will focus on the various techniques that can be combined to image and understand the three-dimensional magnetic structures that emerge.
[1]. Hellman et al., Reviews of Modern Physics 89, 025006 (2017).

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.

Atomically thin or two-dimensional (2D) materials can be assembled into bespoke heterostructures to produce some extraordinary physical phenomena. Likewise, these highly tunable materials are useful platforms for exploring fundamental questions of interfacial chemical/electrochemical reactivity. One exciting and relatively recent example is the formation of moiré superlattices from azimuthally misoriented (twisted) layers. These moiré superlattices result in flat bands that lead to an array of correlated electronic phases. However, in these systems, complex strain relaxation can also strongly influence the fragile electronic states of the material. Precise characterization of these materials and their properties is therefore critical to the understanding of the behavior of these novel moiré materials (and 2D heterostructures in general). In this talk, I will discuss how spontaneous mechanical deformations (atomic reconstruction) and resultant intralayer strain fields at moiré superlattices of twisted bilayer graphene have been quantitatively imaged using 4D-STEM Bragg interferometry. I will also discuss the impact of these mechanical deformations to the electronic band structure of these moiré superlattices and the correlated electronic phases they host. The talk will then explore how various degrees of freedom that are unique to 2D materials may be used to tailor interfacial chemistry at well-defined mesoscopic electrodes and the outlook for new paradigms of functional materials for energy conversion and low-power electronic devices.

Uncovering structure/function relationships in condensed phase electronic materials is increasingly important for applications from solar energy to quantum optoelectronics. In this talk, we will explore multimodal microscopy to the role of microscopic heterogeneity and defects in emerging semiconductors, focusing on halide perovskites as a system that is not only technologically important, but also scientifically challenging. Halide perovskites are soft, and easily damaged by low doses of electrons. They are also complicated mixed conductors, exhibiting electronic and ionic relaxation dynamics that vary across many orders of magnitude in time and space. To probe these systems, we combine hyperspectral optical spectroscopy, electron microscopy, scanning probe microscopy, and data science tools to understand and control defects for applications in photovoltaics, showing how surface and grain boundary passivation enables the growth of films that approach thermodynamic efficiency limits for performance, while also problem the role of processing additives on microstructure and performance.