David C. Bell
Research Topics Summary
- Development of Low Voltage, Aberration Corrected Electron Microscopy
Low-voltage Aberration Corrected Electron Microscopy (LVHREM) has several advantages, including increased cross-sections for inelastic and elastic scattering, increased contrast per electron, improved spectroscopy efficiency, decreased delocalization effects and reduced knock-on damage. My group has shown that by using a source monochromator and modifying and re-tuning the Cs Corrector of a commercial instrument we are able to reduce the effect of chromatic aberration and achieve a usable, ~1 Ångström resolution at 40 keV.
One key to low voltage imaging is improving the contrast while retaining high-resolution; this opens up opportunities to examine a large range of atomically thin materials that are traditionally plagued by knock-on damage during imaging at higher accelerating voltages. We have demonstrated atomic imaging of Si and graphene at 40 keV without beam damage after 1hr of imaging. Now, we are enhancing our imaging capabilities (by addressing the chromatic aberration-limited resolution) and studying other experimental techniques (such as EELS) at extremely low accelerating voltages (<40 keV).
(David C. Bell, Chris J. Russo and Dmitry Kolmykov, 40 keV Atomic Resolution TEM, Ultramicroscopy. 114, 2012)
2. Basic Energy Sciences, Nano-Component Materials for Catalysis
Introduction of nanoparticles or nanowires into a macroscopic catalysis matrix material creates a nano-component material that takes advantage of the mesoscale properties. The resulting nano-component catalyst materials exhibit drastically different and enhanced properties as compared to the bulk individual constituents. An understanding of the reaction kinetics enables construction of innovative catalyst architectures with improved reaction selectivity under catalytic conditions.
By directly controlling assembly of novel nanoscale and mesoscale structures, new functionalities of advanced heterogeneous catalytic systems are possible. We utilize aberration corrected electron microscopy and atom probe tomography to probe the true nature of catalysis materials for iterating characterization with catalytic tests to improve the performance and stability of mesoporous catalysts.
For example, collaboration with Maria Flytzani-Stephanopoulos’s group at Tufts determined via aberration corrected STEM that alkali ions (sodium or potassium), added in small amounts, activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) used for producing H2. The use of an aberration corrected STEM for this work enabled the unambiguous determination of Pt catalyst size, some of which are just single atoms, and location within the matrix. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.
(Yanping Zhai et. al., “Alkali-stabilized, active Pt-OHx species for the water-gas shift reaction”, Science 329. no. 5999, 2010).
Current and future work is in developing correlative microscopy of mesoporous catalysts to link aberration-corrected TEM, STEM and Atom Probe Tomography (please see below in project 4).
This reported and ongoing work is funded by an Energy Frontier Research Center (EFRC) grant sponsored by the Department of Energy Office of Science.
3. Imaging and Analysis of Quantum Materials
Quantum electronic devices and systems are critically important to next generation electronics and devices for signal processing and computation, which exceed the limits of conventional semiconductors.
The main thrust of my research on quantum materials facilitates the characterization of newly developed quantum materials, such as graphene, topological insulators, and nitrogen vacancy (NV) centers in diamond with aberration corrected imaging and analytical tools, including atom probe tomography. My group has the highest resolution transmission electron microscope in the northeast of the United States, with sub- Ångström resolution and analytical capabilities at low voltages. Characterization of quantum materials allows properties to be explored and understood on the atomic level as well as feedback for further materials synthesis.
We have also applied aberration corrected electron energy loss spectrum imaging to image the plasmon resonances around tunable nano-structures.
(F. Von Cube, J. Niegemann, S. Irsen, D. C. Bell, and S. Linden, “Angular-resolved electron energy loss spectroscopy on a split-ring resonator”, Phys. Rev. B 89 2014).
The second thrust of my research in quantum materials involves in-situ electrical biasing measurements performed while directly imaging materials. By electrically biasing graphene devices with a low temperature-biasing holder, we can directly observe changes in electron flow while imaging a device. Given that graphene and h-BN are comprised of a single atomic layer of low-Z atoms, they represent ideal platforms for fundamental investigations of surface layer chemistry and active components in electronic device fabrication. We are currently investigating the use of graphene as a substrate for both catalysis and devices; combined with the biasing experiments, we are investigating how localized substrate charge affects surface chemistry and devices.
This research is funded by the Center for Integrated Quantum Materials an NSF Science and Technology Center.
4. Correlative Electron Microscopy-Atom Probe Characterization.
Applying analytical information derived from quantitative STEM (e.g. HAADF, EELS, and EDS) to understand material properties relies on matching of observed and theoretical models. STEM techniques are generally not sensitive to low-concentration dopants and impurity analysis, or to isotopic ratios within matrix materials, which can be crucial for semiconductors, nuclear materials, quantum devices and catalytic materials. This research project aims to understand why the calculated and measured analysis and imaging contrast mechanisms differ by significant amounts and to develop new quantitative approaches to analytical electron microscopy that can be applied to a range of problems.
In addition to improving detector systems on the STEM, using atom probe tomography to correlate with STEM data represents a promising option to; (1) enhance our understanding of the true nano- and micro-structure of materials, (2) provide chemical information as feedback to better understand mismatches between experimental analytical STEM data and theoretical models, and (3) yield data related directly to single atoms, low-concentration dopants, impurities, and isotopic ratios traditionally unavailable from pure STEM analysis.
We have developed techniques to correlate 3D atom probe tomography (APT) with aberration-corrected TEM, STEM, and analytical techniques, such as HAADF, EELS, and EDS.
(“Correlative STEM at the Atomic Scale: The Ultimate Materials Analysis Tool.” David C. Bell et. al. MRS Proceedings Fall 2009).
For instance, our current research has successfully correlated the nano-structure of a nanoporous gold matrix (derived from TEM) with the 3D nano-structure of the same system (obtained from APT) both before and after exposure to reaction conditions. These results directly indicated the coarsening of the nanogold matrix due to the reaction event.
As part of this project we have also developed methods of preparing glasses and powders for APT analysis. APT sample preparation is not straightforward for many samples such as powders and glasses, whereas for TEM/STEM preparation, the materials are simply cast on a grid. We have developed techniques for preparing glass for ATP analysis as well as modifying this technique to allow for encapsulation of powders in a matrix and allow them to be analyzed in the ATP.
(D.C. Bell et. al. "Microencapsulation Method for Atom Probe Analysis of Powders." Microscopy and Microanalysis 19, 2013).
5. Bio-Imaging, Drug delivery and DNA sequencing Research
My research has also included imaging biological and biomedical nanomaterials with aberration corrected and cryo-SEM and TEM.
In one ongoing project with a team from Boston Children’s Hospital, we are developing a foam suspension containing self-assembling, lipid-based microparticles encapsulating a core of pure oxygen gas for intravenous injection. We provide feedback via cryo-microscopy about the nature of the microparticles. When the micro particles are infused by intravenous injection into hypoxemic rabbits, arterial saturations increase within seconds to near-normal levels. The ability to administer oxygen and other gases directly to the bloodstream may represent a technique for short-term rescue of profoundly hypoxemic patients and to selectively augment oxygen delivery to at-risk organs.
(J. N. Kheir, et. al., “Oxygen Gas–Filled Microparticles Provide Intravenous Oxygen Delivery”, Sci Transl Med, 4:140, 2012).