I was born and raised in the USA (Minnesota), obtaining my undergraduate degree from the University of Wisconsin – Madison in 2004. I then moved to Dresden, Germany where I obtained a Master’s degree (2007) and PhD (2011) in Materials Science on the topic “The Nanoscale Characterization of Functional Superconducing Thin Films.” I subsequently moved to Sweden, working as a post-doc at Uppsala University until 2016 on the topic of TEM Spectroscopy with an emphasis on the characterization of magnetic materials. In 2016, I moved to Stockholm University as an independent researcher.
I am primarily interested in method development for Transmission Electron Microscopy. I specialize in operating the TEM in scanning mode (STEM) to acquire spectroscopic information using Energy-Dispersive X-Ray Spectroscopy (EDX) and Electron Energy-Loss Spectroscopy (EELS). Some of my research topics include:
- The spectroscopy of nanoscale objects with up to atomic resolution
- Characterization of materials for energy applications such as thin film solar cells, magnetic materials, photo catalysts, Li-ion batteries, and high-temperature superconductors
- Statistical analysis of high-dimensional hyperspectral datasets.
In 2016, I was awarded a starting research grant from the Swedish Research Council entitled “Mapping magnetic moments with sub-nanometer resolution” (2016-05113). In this project, my collaborators and I are developing an experimental methodology that may allow for the visualization of quantitative magnetic moments from single atomic columns in ferromagnetic crystals. It works by making experimental modifications to an EELS technique known as Electron Magnetic Circular Dichroism (EMCD). These modifications involve the fabrication of a patterned aperture which sits in the post-specimen diffraction plane and filters electrons that have scattered in slightly different ways due to the presence of an uncompensated ferromagnetic field in the crystal into the upper and lower parts of the spectrometer CCD camera. The electrons are then dispersed according to their energy loss, revealing small variations in the fine structure superimposed over the ionization edges of the ferromagnetic element of interest. These features reveal modifications to the density of unoccupied states that can be used to infer magnetic behavior. The patterned aperture is critical to this project for two reasons. First, it has a symmetry that allows for the crystal being investigated to be tilted in such a way that one can resolve atomic columns, allowing us to probe their individual magnetic contributions. Second, the 2D filtering allows for the necessary information to be extracted simultaneous from each position of the finely-focused electron probe. This should allow for sub-atomic precision in these measurements. In addition to the technical advancements described above, this project has seen heavy development of data analysis methods, which will be necessary for the community as we search for these very weak signals.
In 2018, my colleagues Cheuk-Wai Tai, Hongyi Xu, and Tom Willhammar, and I were awarded a joint proposal from the Swedish Strategic Research (SSF) council entitled “A Multidimensional Toolkit for Modern Electron Microscopy” (ITM17-0301). In this project, we aim to develop a versatile, open-source hardware-based toolkit for the acquisition and analysis of STEM diffraction datasets. My primary involvement in this project is related to workpackage three, where we intend to extend the functionality of this toolkit to be relevant for momentum-resolved EELS experiments.
I urval från Stockholms universitets publikationsdatabas
Single-pass STEM-EMCD on a zone axis using a patterned aperture
2019. Thomas Thersleff (et al.). Scientific Reports 9Artikel
Measuring magnetic moments in ferromagnetic materials at atomic resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a (scanning) transmission electron microscope ((S)TEM). However, experimental and data processing hurdles currently hamper the realization of this goal. Experimentally, the sample must be tilted to a zone-axis orientation, yielding a complex distribution of magnetic scattering intensity, and the same sample region must be scanned multiple times with sub-atomic spatial registration necessary at each pass. Furthermore, the weak nature of the EMCD signal requires advanced data processing techniques to reliably detect and quantify the result. In this manuscript, we detail our experimental and data processing progress towards achieving single-pass zone-axis EMCD using a patterned aperture. First, we provide a comprehensive data acquisition and analysis strategy for this and other EMCD experiments that should scale down to atomic resolution experiments. Second, we demonstrate that, at low spatial resolution, promising EMCD candidate signals can be extracted, and that these are sensitive to both crystallographic orientation and momentum transfer.
Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope
2019. Devendra Negi (et al.). Physical Review Letters 122 (3)Artikel
We propose a magnetic measurement method utilizing a patterned post-sample aperture in a transmission electron microscope. While utilizing electron magnetic circular dichroism, the method circumvents previous needs to shape the electron probe to an electron vortex beam or astigmatic beam. The method can be implemented in standard scanning transmission electron microscopes by replacing the spectrometer entrance aperture with a specially shaped aperture, hereafter called ventilator aperture. The proposed setup is expected to work across the whole range of beam sizes -- from wide parallel beams down to atomic resolution magnetic spectrum imaging.
Quaternary Core-Shell Oxynitride Nanowire Photoanode Containing a Hole-Extraction Gradient for Photoelectrochemical Water Oxidation
2019. Zili Ma (et al.). ACS Applied Materials and Interfaces 11 (21), 19077-19086Artikel
A nanowire photoanode SrTaO2N, a semiconductor suitable for overall water-splitting with a band gap of 2.3 eV, was coated with functional overlayers to yield a core-shell structure while maintaining its one-dimensional morphology. The nanowires were grown hydrothermally on tantalum, and the perovskite-related oxynitride structure was obtained by nitridation. Three functional overlayers have been deposited on the nanowires to enhance the efficiency of photoelectrochemical (PEC) water oxidation. The deposition of TiOx protects the oxynitride from photocorrosion and suppresses charge-carrier recombination at the surface. Ni(OH)(x) acts a hole-storage layer and decreases the dark-current contribution. This leads to a significantly improved extraction of photogenerated holes to the electrode-electrolyte surface. The photocurrents can be increased by the deposition of a cobalt phosphate (CoPi) layer as a cocatalyst. The heterojunction nanowire photoanode generates a current density of 0.27 mA cm(-2) at 1.23 V vs the reversible hydrogen electrode (RHE) under simulated sunlight (AM 1.5G). Simultaneously, the dark-current contribution, a common problem for oxynitride photoanodes grown on metallic substrates, is almost completely minimized. This is the first report of a quaternary oxynitride nanowire photoanode in core-shell geometry containing functional overlayers for synergetic hole extraction and an electrocatalyst.
Mössbauerite as Iron-Only Layered Oxyhydroxide Catalyst for WO3 Photoanodes
2019. Michael Ertl (et al.). Inorganic Chemistry 58 (15), 9655-9662Artikel
Mössbauerite, a trivalent iron-only layered oxyhydroxide, has been recently identified as an electrocatalyst for water oxidation. We investigated the material as potential cocatalyst for photoelectrochemical water oxidation on semiconductor photoanodes. The band edge positions of mössbauerite were determined for the first time with a combination of Mott-Schottky analysis and UV-vis diffuse reflectance spectroscopy. The positive value of the Mott-Schottky slope and the flatband potential of 0.34 V vs reversible hydrogen electrode (RHE) identifies the material as an n-type semiconductor, but bare mössbauerite does not produce noticeable photocurrent during water oxidation. Type-II heterojunction formation by facile drop-casting with WO3 thin films yielded photoanodes with amended charge carrier separation and photocurrents up to 1.22 mA cm(-2) at 1.23 V vs RHE. Mössbauerite is capable of increasing the charge carrier separation at lower potential and improving the photocurrent during photoelectrochemical water oxidation. The rise in photocurrent of the mössbauerite-functionalized WO3 photoanode thus originates from improved charge carrier separation and augmented hole collection efficiency. Our results highlight the potential of mössbauerite as a second-phase catalyst for semiconductor electrodes.
Silicoaluminophosphate (SAPO)-Templated Activated Carbons
2019. Yunxiang Li (et al.).Artikel
Microporous activated carbon was prepared by depositing and pyrolyzing propylene within the microporous voids of SAPO-37 and subsequently removing the template by a treatment with HCl and NaOH. The carbon had a high surface area and large micropore and ultramicropore volumes. The yield, crystallinity, morphology, and adsorption properties compared well with those of a structurally related zeolite-Y-templated carbon. No HF was needed to remove the SAPO-37 template in contrast to the zeolite Y template, which could be of industrial importance.
Towards implementing hierarchical porous zeolitic imidazolate frameworks in dye-sensitized solar cells
2019. Hani Nasser Abdelhamid (et al.). Royal Society Open Science 6 (7)Artikel
A one-pot method for encapsulation of dye, which can be applied for dye-sensitized solar cells (DSSCs), and synthesis of hierarchical porous zeolitic imidazolate frameworks (ZIF-8), is reported. The size of the encapsulated dye tunes the mesoporosity and surface area of ZIF-8. The mesopore size, Langmuir surface area and pore volume are 15 nm, 960-1500 m(2). g(-1) and 0.36-0.61 cm(3). g(-1), respectively. After encapsulation into ZIF-8, the dyes show longer emission lifetimes (greater than 4-8-fold) as compared to the corresponding non-encapsulated dyes, due to suppression of aggregation, and torsional motions.
Localization of magnetic circular dichroic spectra in transmission electron microscopy experiments with atomic plane resolution
2017. Ján Rusz (et al.). Physical Review B 95 (17)Artikel
Inelastic electron scattering is a consequence of mostly Coulomb interaction between electrons in the sample and electron beam and, as such, it is a nonlocal event. In atomic resolution experiments, it thus opens the following question: How far is the origin of the inelastic scattering signal that is observed when the electron beam is passing nearby an atomic column or plane? We analyze computationally the delocalization of the magnetic signal in electron magnetic circular dichroism (EMCD) experiments in the so-called three-beam orientation, allowing one to image individual atomic planes. We compare the classical EMCD setup using the double-difference procedure (DD-EMCD) to a recently introduced atomic plane resolution EMCD (APR-EMCD) geometry, assuming the same probe size. We observe a strong localization of the EMCD signal to the closest atomic plane, confirming the potential of EMCD to study an evolution of magnetic properties near surfaces or interfaces with atomic plane resolution. The localization of the EMCD signal is remarkably higher than the localization of the nonmagnetic component of the inelastic scattering cross section. We also analyze double-channeling effects and find them particularly strong for the DD-EMCD method, while for APR-EMCD they appear to be minor. The DD-EMCD signal, on the other hand, appears to be more robust with respect to sample thickness than that of the APR-EMCD.
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