Research Interests

Overview

My research interests focus on imaging as many atoms as possible to solve important scientific problems. Here is a list of some of the research areas I'm interested in:

Electron ptychography of light elements and radiation-sensitive materials at the atomic scale.
Deciphering the atomic structure of amorphous materials using atomic electron tomography.
Imaging the active layer of solar cell materials.
Enabling atomic-resolution imaging from your desk.

More atoms more problems...

...to be solved! The advent of fast pixelated detectors for scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging with high sensitivity using ptychography. This method has expanded the applications of atomic-resolution imaging to beam-sensitive materials containing light elements, such as zeolites, metal-organic frameworks and lithium-ion battery cathodes. However, there is currently a resolution/field of view trade-off: most atomic-resolution images span only tens of nanometers, providing characterization for statistically negligible regions of a material. Furthermore, once the imaging region is extended to micron fields of view, the resolution is typically reduced. I am interested in extending the field of view of electron ptychography while preserving atomic resolution using highly defocused probes combined with specialized optical conditions.

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 (PCTF - see video on right). By using a large defocus value, the signal-to-noise ratio increases for specific spatial frequencies. This enables us to image significantly larger regions of the sample using relatively low electron dose levels.

Video of contrast transfer in electron ptychography

Atomic electron tomography

Atomic electron tomography (AET) routines involve acquiring electron microscopy data of a sample at various tilt angles, and using a computational algorithm to determine its three-dimensional structure at the atomic scale. This has allowed researchers to experimentally determine the 3D atomic structure of non-crystalline materials. For example, AET has been implemented to (a) correlate the structural defects of 2D materials with their optoelectronic properties, (b) decipher the chemical order and disorder in catalytic nanoparticles, and (c) determine the structure of amorphous materials. My research interests involve extending AET to (1) geometries which are currently not supported, such as large (50nm) nanoparticles, and (2) lighter element species using phase-imaging techniques such as electron ptychography - a method known as ptychographic AET (pAET).

Solar cell degradation

The development of robust, efficient solar cells has never been more crucial to civilization than it is today, and the characterization of active layers of solar cells (ALSCs) is critical to understanding their performance. For example, atomic-scale defects can grow from a single irregularity in an atomic pattern to micron-sized cracks or pores, degrading the performance of the solar cell. Thus, understanding the early onset of degradation, from the atomic scale to the micron scale, is crucial to the development of robust solar cells. Many of the most promising ALSCs comprise organic components (hybrid organic-inorganic perovskites) which are extremely sensitive to air, moisture and the illuminating electrons in the microscope. I am interested in combating this challenge by implementing x-ray and electron characterization methods which can decipher the air- and moisture-induced degradation of ALSCs without damaging the material during data collection. This will involve combining data acquisition under cryogenic conditions with Bragg crystallography and phase retrieval algorithms. By providing a characterization feedback loop to solar cell manufacturers, the speed of advancement can be enhanced significantly.

By combining data acquisition, ptychography and statistical analysis, one can provide valuable metrics to help scientists develop new and improved solar cells.

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 phase retrieval 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 electron 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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