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Overview
Overview
My research interests focus on experimentally determining the 3D structure of functional materials. I am particularly interested in determining the atomic structure of quantum materials and the nanoscale structure of integrated circuits. I am an expert in computational imaging methods for X-ray and electron microscopy, such as ptychography and tomography, and have extensive experimental expertise in X-ray and electron microscopy. My research projects include:
Multiscale, multimodal structure determination of functional materials
Determining local order in disordered and quantum materials using atomic electron tomography
Nanoscale 3D imaging of extended structures
Efficient atomic-resolution ptychography
More information on each project can be found below.
Multiscale, multimodal structure determination of functional materials
Multiscale, multimodal structure determination of functional materials
The rapid development of functional materials, increasingly enabled by automated synthesis, is creating a growing need for characterization methods that can resolve structure across relevant length scales. While diffraction and spectroscopy provide powerful ensemble-averaged measurements, the properties of many emerging materials are governed by nanoscale heterogeneity, local atomic arrangements, and buried interfaces. These features can be difficult to access with conventional electron microscopy, particularly in beam-sensitive materials, systems containing light elements, and complex three-dimensional structures.
My research develops advanced electron and X-ray imaging methods to overcome these limitations and establish quantitative relationships between local structure and materials function. I use electron ptychography to improve sensitivity to light elements while reducing sample damage, combine high-resolution imaging with three-dimensional reconstruction to resolve local atomic configurations and interfaces, and integrate electron and X-ray microscopy to connect atomic-scale observations with statistically representative mesoscale structure. My work has progressed from fundamental studies of contrast formation in electron ptychography, to imaging beam-sensitive materials and buried interfaces, and now toward correlative multiscale imaging of functional materials.
Example 1: Multiscale 3D imaging of catalysts
Example 1: Multiscale 3D imaging of catalysts
Example 2: Multiscale 3D imaging of microelectronics
Example 2: Multiscale 3D imaging of microelectronics
Determining local order in disordered and quantum materials using atomic electron tomography
Determining local order in disordered and quantum materials using atomic electron tomography
Atomic electron tomography (AET) combines tilt-series electron microscopy with computational reconstruction to determine three-dimensional atomic structure. The technique has enabled direct measurement of local atomic arrangements in two-dimensional systems, catalytic nanoparticles, and materials with no long-range order. However
My research aims to extend AET to amorphous materials with no long-range order, and quantum materials which contain light-element species. In particular, my research involved combining AET with phase-sensitive methods such as electron ptychography, enabling ptychographic atomic electron tomography (pAET) with improved sensitivity to weakly scattering elements and challenging material systems.
Related Publications
Related Publications
Accurate determination of the 3D atomic structure of amorphous materials - Nature
Amorphous materials have great importance to next-generation microelectronics and quantum devices, and many of their functional properties are determined by their atomic-scale local order. However, determing its atomic structure atom-by-atom cannot be achieved by X-ray/electron diffraction or cryo-electron microscopy, owing to the lack of long range order. During my postdoctoral studies, I co-developed a workflow to determine the atomic coordinates of amorphous nanoparticles using atomic electron tomography (AET). This is the only method capable of experimentally determining the 3D atomic structure of disordered materials, and thus requires rigorous preprocessing, refinement and validation steps to faithfully represent the structure.
Three dimensional imaging of individual carbon atoms - Nature Nanotechnology, In Press.
Atomic electron tomography (AET) typically involves quantitative imaging via annular-dark-field scanning transmission electron microscopy (ADF-STEM). However, ADF-STEM does not provide enough contrast to see all atoms on the periodic table (Z>14). By combining ptychography, a highly sensitive phase-imaging technique, with AET, the atomic positions of light elements (i.e. carbon, oxygen, etc.) can be determined with high spatial precision. I co-developed a pipeline to accurately determine the position of individual atoms in twisted bilayer graphene using ptychographic atomic electron tomography (pAET), revealing every carbon atom in the bilayer system and revealing morphological textures that could not be observed with conventional imaging techniques.
Nanoscale 3D imaging of extended structures
Nanoscale 3D imaging of extended structures
Tomography is a crucial tool for resolving the 3D structure of cells, batteries, computer chips and more, but geometric constraints limit 3D imaging to a small fraction of microscopic samples. I am particularly interested in extending the capabilities of 3D imaging to incorporate many more sample geometries. One example of this is multislice electron ptychography, whereby nanoscale 3D imaging can be performed without tilting a sample. I used this technique to probe quantum interfaces that could not be imaged with tomography. Another technique is laminography, whereby a sample is rotated such that 3D imaging can be performed on extended planar samples. Finally, improving alignment procedures is crucial for ensuring a 3D structure is faithfully resolved. I am working on improving alignment and centering algorithms to make tomography universally applicable to all materials.
Related Publications
Related Publications
Three-dimensional structure of buried heterointerfaces revealed by multislice ptychography
The properties of van der Waals heterostructures are strongly influenced by the three-dimensional structure of their buried interfaces, which can govern behavior relevant to applications such as quantum sensing. Conventional electron microscopy primarily provides two-dimensional projections, while tomography of layered structures can be geometrically challenging. In this project, I performed multislice electron ptychography to recover depth-dependent structural information from a single projection by computational optical sectioning, achieving nanoscale axial resolution. This approach provides an efficient route to characterizing buried interfaces in complex nanoscale devices.
Efficient atomic-resolution ptychography
Efficient atomic-resolution ptychography
I am an expert in the field of electron ptychography, an advanced imaging technique which combines scanning transmission electron microscopy (STEM) experiments with computational imaging methods. Instead of using a lens to form an image, ptychography involves collecting a series of diffraction patterns and using a 'computational lens' (i.e. an algorithm) to reconstruct atomic-resolution images of materials. During my PhD studies, I studied the mechanisms by which contrast is transferred into images via electron ptychography. By varying the microscope parameters, there is a high level of tunability of the phase contrast transfer function, i.e. contrast versus feature size. This flexibility can be leveraged to image specific materials structures while minimizing the electron-induced sample damage, as demonstrated for zeolite frameworks during my PhD studies.
One of the challenges of electron ptychography is the gap between speed of data acquisition (a few seconds) and image reconstruction (weeks, months). I am currently interested in developing fast, user-friendly reconstruction algorithms to bridge the time gap between experiments and analysis, using a combination of AI methods such as deep-learning electron ptychography and analytical methods such as relaxed Wigner distribution deconvolution. The combination of AI-accelerated reconstruction with continued fundamental-level studies is crucial for high-throughput, atomic-resolution characterization of materials.
Related Publications
Related Publications
Atoms for all
Atoms for all
Electron microscopes have proven to be a powerful tool for deciphering the atomic structure of materials, but their high cost limits their availability to those with access to prosperous academic institutions, national laboratories and large industrial companies in high-income countries. Not only are electron microscopes expensive to both buy and operate, but their size (20 ft / 6 m high) requires them to be placed in specially designed laboratories which require significant investment, placing strict geographical constraints on atomic-scale characterization. In recent years, however, the combination of reasonably priced desktop scanning electron microscopes (DSEMs), fast pixelated detectors and advanced imaging algorithms have paved the way for atomic resolution imaging from an office desk. One of the main challenges of achieving this goal is the partial coherence associated with the low accelerating voltages of DSEMs (5-30 kV). I am interested in performing experiments on a DSEM adapted for transmission imaging, and using phase retrieval algorithms to generate atomic-resolution images. If successful, the accessibility of atomic-scale characterization should increase exponentially, accelerating and globalizing scientific advancement through electron microscopy.
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