Stewart, David M.
Materials Science and Engineering
Maryland Energy Innovation Institute
I am a cross disciplinary scientist with over ten years of experience in micro fabrication, electrochemical systems, and basic materials chemistry and physics. Recently, I have been combining modeling and experiments to study electrochemical interfaces, kinetics, and stress-electrochemistry coupling. I am lead-PI on a DOE-funded collaborative program investigating how interface thermodynamics impact electrochemical reactions and point defect distributions. Additionally, I direct several projects on solid-state electrochemistry, materials synthesis, and surface analysis using thin film structures.
Beyond research, I enjoy teaching physics, chemistry, and nanofabrication, and mentoring a diverse group of students. I have advised over a dozen Ph.D. and undergraduate students and developed outreach programs to promote STEM education among younger students. I have been invited to present my work at conferences both domestically and internationally.
My research focuses on understanding and improving the materials at the heart of next-generation energy storage and computing devices, such as solid-state batteries and ion-based electronics. A central challenge in these technologies is that the interfaces between materials — the boundaries where one material meets another — are chemically unstable and poorly understood, which limits device performance and lifetime. To study these interfaces, we design and build specialized thin-film devices using fabrication techniques common to the microelectronics industry, such as physical vapor deposition and atomic layer deposition.
These miniaturized model systems allow us to isolate and measure phenomena that are otherwise impossible to observe directly. Using techniques such as electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and Raman microscopy, we can track how ions and electrons move across these interfaces, how defects form and evolve, and how mechanical stress interacts with electrochemical processes. The insights gained from this work inform computational models that can ultimately guide the design of safer, longer-lasting batteries and more energy-efficient computing hardware. A longer-term goal is to extend these methods to more earth-abundant materials, such as sodium and magnesium, to improve the sustainability of future energy technologies.
Research Projects
Thin Film Platforms and Strategies to Advance Fundamental Science for Solid-State Ionic Materials
A continuation of the Platforms program below, but taking it in new directions. Directing several projects with collaborators and advisees, all in the vein of using thin film structures to study fundamental phenomena in solid-state batteries:
- Transport at nanoionic interfaces: charge carrier populations and associated fields can be dramatically altered near interfaces, leading to orders of magnitude changes in the charge transport. This project aims to use multilayer thin films with layer thicknesses below the Debye length, and combine transport measurements parallel and perpendicular to the interfaces to unravel anisotropic charge mobility and the impacts of interfaces on macroscopic charge transport.
- Stress induced diffusion: with the coupling of electric, chemical, and mechanical energy fields there is a prediction that stress will enhance ion diffusion and also redirect it. We are studying different ways to apply external stress and electric fields to direct ion transport and validate the theoretical predictions.
Beyond-Li Battery Materials
This Vertically Integrated Project Team is composed of undergraduates from varying departments and across class levels. We are investigating the fundamental materials science of energy storage devices built around earth-abundant alternatives to lithium, with a focus on sodium (Na) and magnesium (Mg). Using vanadium oxide (V₂O₅) as a model host material, the team explores a unifying scientific and engineering approach called electrochemical phase engineering — a method of tuning a material's crystal structure and properties through controlled electrochemical and thermal treatments — to unlock new possibilities for next-generation batteries. Thin film fabrication and surface science techniques adapted from the microelectronics industry are combined with traditional electrochemical methods for a research experience that develops skills valued across the semiconductor, energy storage, and clean energy industries.
Discovering Ion-Electron Interactions at Electrochemical Interfaces Using Depth-Resolved Electron Microscopy
The goal is to reveal and understand how ion movement and its coupling to free electron spatial distributions creates interfacial dipoles and mobile, charged defects that form barriers to charge transport and phase changes, which are challenging to measure and model. This requires (1) creative fabrication of model devices that enables (2) in operando, 3D measurements correlating electronic, ionic, and phase distributions on a nanoscale and (3) a close collaboration with theory for modeling the intertwined ion-electronic interactions and ion transport process.
Summer Research Experiences in Renewable and Sustainable Energy Technology
The propagation of Li dendrites through grain boundaries plagues solid-state batteries, while controlling the plating and stripping process is essential to cycle efficiency in advanced cells. By fabricating interlayers with well-defined properties between solid electrolytes, we study deposition and transport of Li along homo- and heterogeneous boundaries. Under applied stress, plating/stripping and the propagation of Li dendrites can be dynamically altered, giving insight into new mechanical modeling efforts.
Thin Film Platforms for Solid-State Ionic Devices
Directing several projects with collaborators and advisees, all in the vein of using thin film structures to study fundamental phenomena in solid-state batteries:
- Electrochemo-mechanical coupling: by combining multiphysics modeling and novel experimental approaches, we are seeing new couplings between stress and electrochemical fields and the effects on bulk ion transport. Experiments using Raman-strain microscopy uncovered diffusion lengths 100x longer in stressed Si anodes and explained canonical results with new modeling techniques confirmed by experiment. Models of discharge dynamics in 3D structures similarly revealed much longer diffusion lengths than imagined, but which are restricted by stress from nanoconfinement.
- Solid-state ionic interfaces: by taking a systematic, surface science approach to interface formation, we investigate the buried boundary between electrode and electrolyte, which controls most aspects of battery performance. We use patterning techniques to form an array of film stacks together, each exposing different interfaces of the battery for characterization. Cathodoluminescence studies of band bending at heterojunctions revealed the coupling between electron and Li+ ion equilibria, and physical connections to the electrochemical concept of charge transfer impedance. These studies further inform modeling of advanced architectures in thrust 1.
- Diffusion and phase evolution: in solid materials, diffusion processes are often modeled without consideration for phase changes, dynamic material properties, or the activity of a large number of charged defects (ions). Experimental measurements have shown that changes in conductivity lead to ion distributions that deviate strongly from simple diffusion models. We are using patterning to create samples which force ion diffusion over 10-100s of microns that can be monitored in situ by phase changes and electrochemical measurements, which measures how phase inhomogeneities are created and migrate, and how material properties thus vary spatially.
Nanostructures for Electrical Energy Storage
Developed and characterized ALD thin film electrodes based on SnO2 as anodes for 3D microbatteries. Found a process to mingle SnO2 and Sn3N4 depositions to produce SnOxNy films of varying composition. Thin films had greater reversibility as Li+ electrodes than bulk examples (80% retention over 500 cycles as opposed to 40%), and the cycle stability was found to smoothly improve with increasing N:O ratio, while most other electrochemical properties were unchanged. This provides a high-performance, conformal anode for 3D batteries, as demonstrated in our published papers, and in ongoing work to this day.
Also used multiphysics modeling to study different micro-battery architectures, and projected performance of various design choices for both experiments and manufacturability. Models showed competitive performance with conventional cells and unveiled new avenues of research into stress-electrochemistry coupling and confinement effects on transport. These models are helping a spin-off company begin commercializing 3D battery technology developed in this group.
ZrB2 and h-BN Thin Films for Harsh, High Temperature Environment Sensors
For my Ph.D. work, I deposited thin film ZrB2 samples by e-beam co-evaporation, and the electrical, chemical, crystal phase, and morphological stability was studied under air, vacuum, and inert atmosphere annealing up to 1200 °C. We also analyzed the performance of environmental barriers at these temperatures, including amorphous Al2O3 by ALD and high crystallinity h-BN coatings by reactive sputtering with substrate biasing. With collaborators in electrical engineering, candidate thin film stacks were fabricated into piezoelectric sensors on YSZ substrates for high temperature sensors. Rapid oxidation of the bare ZrB2 led to quick failure in air, but in vacuum or inert atmosphere the films survived thermal cycling between 23–850 °C for a total of 45 hours at 850 °C. ALD Al2O3 was found to be a better oxidation barrier, but thermal expansion mismatch induced too large a stress for the films to remain intact. Devices using ZrB2 may hold the most promise in sensors for spacecraft, especially due to its radiation hardness.
Predictions of Exciton Lifetimes in Quantum Dot Solar Cells
I performed density functional theory calculations on several Si nanoparticle structures with and without Ag adsorbate clusters and dopants. Photo-absorption strength and exciton lifetimes were compared for different structures, and improved solar cell efficiency (2x improvement in lifetime) was predicted for a combination of larger Ag adsorbate clusters on doped, amorphous QDs.
Teaching
ENMA 465: Microprocessing Materials
ENMA 466: Advanced Materials Fabrication Laboratory
VIPS 208: Beyond Li Battery Materials
see Google Scholar for a relatively up to date publication list: https://scholar.google.com/citations?user=R206dyYAAAAJ