Ph.D. Materials Science, Northwestern University
M.Sc. Engineering Science, The Pennsylvania State University
B.S. Engineering Physics, Rose-Hulman Institute of Technology
I work in scanning probe microscopy, studying the physical electronic, and magnetic properties of single atoms and atomic scale impurities in solid materials.
At Radboud University, during my postdoc, I studied the atomic scale properties of the layered semiconductor black phosphorus (BP), which became the platform for my early career Veni grant from the… read more
At Radboud University, during my postdoc, I studied the atomic scale properties of the layered semiconductor black phosphorus (BP), which became the platform for my early career Veni grant from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (or Dutch research council). By developing protocols to handle the extremely air-sensitive BP crystals, I was the first to spectroscopically characterize intrinsic impurities and revealed shallow acceptor states, related to the frequently observed p-type doping of the material. Using the ordering of extrinsic charge impurities, I later revealed the that free-carriers induced at the surface of BP using extrinsic donor impurities lead to the development of strongly anisotropic screening; as this technique utilized the thermodynamic equilibrium of a statistical ensemble of atoms at low temperatures, I was able to detect differences in effective screening as a function of direction in real-space.
Finally, a significant portion of my recent work has been directed toward extrinsic magnetic impurities in BP. In this direction, I showed that a theoretically predicted switchable, bi-stable valency could be observed in single cobalt adatoms on BP. With the help of theory, we showed that the valencies are stabilized by the substrate screening of Co 4s and 3d orbitals. Furthermore, each valency has a different magnetic moment due to the reorganization of charge within the Co 3d orbitals. More recently, I have shown that the concept of valency bistability can be extended to other 3d transition metal atoms on BP (manuscript under preparation).
After learning how to manipulate Co atoms on BP, I studied the valency dynamics in coupled cobalt atoms on black phosphorus. Contrary to all previously observed atomic spin ensembles, the coupled cobalt atoms favor neither parallel nor antiparallel valency alignment. I used this unique characteristic, in conjunction with the anisotropic electronic characteristics we previously discerned, to construct an atomic scale synapse from just three Co atoms on BP. Scaling the ensemble up to a total of seven atoms enabled the demonstration of a stochastic neural network, the so-called Boltzmann machine, in this atomic-scale cluster. Finally, due to the distributed geometry the Boltzmann machine, I was able to demonstrate that the physical Boltzmann machine could move beyond a simple machine learning model, to directly mimic the learning characteristics in the human brain. With two naturally separated time scales, one for computation and one for learning, this material presents the possibility to develop schemes for autonomous learning based on the in-materio evolution of the system to external stimuli. I have also developed approaches with heterogeneous atomic ensembles to non-locally gate the atomic valency.
I began my Ph.D. work in the laboratories of Prof. Mark Hersam and Dr. Nathan Guisinger as a National Science Foundation Graduate Fellow. My work focused on realizing an atomically pristine interface between graphene and a noble metal surface, which I accomplished via ultra- high vacuum molecular beam epitaxy on single crystal Ag(111). Owing to the passive, non- catalytic role of the substrate during growth, this approach ultimately provided a powerful general framework to explore the growth of other novel 2D materials, culminating in the discovery of a novel 2D boron allotrope which has no naturally occurring bulk counterpart. I also extended this strategy to realize ultra-clean lateral and vertical heterostructures between these 2D materials under ultra-high vacuum conditions, a process then extended by another coworker in my group. Having observed the influence of the metallic substrate on the 2D materials from these works, I turned my attention to graphene on semiconducting germanium substrates. With these new substrates, I showed how the symmetry and surface chemistry influenced the graphene growth kinetics, ultimately leading to the bottom-up growth of globally, aligned ultra-narrow graphene nanoribbons. I then studied the influence of the covalent Ge bond configurations on the electronic and mechanical properties of the graphene and found that graphene effectuated a new surface reconstruction of Ge(110).
That expertise was the basis for my successful Marie Curie Individual Fellowship proposal to drive emergent phase transitions at monolayer transition metal dichalcogenide (TMD) interfaces within the group of Prof. Dr. Khajetoorians at Radboud University. In conducting this work, I have participated in the design, fabrication, and commissioning of an ultra-high vacuum (UHV) chemical vapor deposition (CVD) chamber for the growth of monolayer TMDs. Working in a multi-university collaboration, we have used this system to grow a V2S3 monolayer on Au(111) which shows non-dispersive electronic order near the Fermi-level related to an interplay between the moiré pattern and the electronic structure of the V2S3. In an extension of this project, I have also investigated the electronic and magnetic properties of single cobalt atoms on monolayer WS2 via scanning tunneling microscopy/spectroscopy (STM/STS), x-ray absorption spectroscopy, and x-ray circular magnetic dichroism (manuscript under preparation).
While developing the UHV CVD chamber at Radboud, I also worked in a team to develop the capacity to collect light out of a scanning tunneling microscope tip-sample junction. After the constructing an ex-situ optical system, this team successfully observed light emission from a single molecule. Furthermore, we have since demonstrated, for the first time, that tip-induced nanocavity plasmons can instigate the motion of a single molecule, without direct tunneling into molecular orbitals.
My future research will be directed toward understanding the versatility of orbital memory - what stabilizes it and why? I will also be looking into the nature of the coupling between valency states in different atoms on the surface of a material and then working to translate that knowledge into complex ensembles with greater control of these interatomic interactions.
I am also particularly interested in the use of atomic lattices to perform analog quantum simulations of a variety of many body Hamiltonians.
Finally, I am interested in the synthesis and characterization of layered van der Waals magnets.