Our research interests are aligned along the following themes:
Much of our research program is directed at understanding the interplay between geometric and electronic structure at interfaces as well as in solid-state materials and to examine how this translates to functional properties. Our research thus spans the range from materials synthesis, mechanistic understanding of crystal growth processes, and structural characterization to device integration and mechanistic studies of catalysis and intercalation phenomena. We further seek to translate fundamental understanding of interfaces and materials to develop functional thin films and devices for a wide range of applications ranging from Mott memory to thermochromic window coatings and thin films for the corrosion protection of steel.
1. Colossal Metal—Insulator Transition Materials
Perhaps the most ubiquitous example of a phase transition is the melting of ice to liquid water. In our research, we are especially interested in solid—solid phase transformations that can be as dramatic in terms of changes in physical properties and are quite significant for technological applications. In particular, we are interested in phase transitions that are accompanied by abrupt changes in electronic properties (metal—insulator transitions, charge ordering—delocalization, charge density waves). In much of our research, we use binary and ternary vanadium oxides as our primary test cases to explore the influence of finite size and doping on metal—insulator phase transitions as well as to extract design principles for stabilization of new charge ordering motifs in mixed-valence solid-state compounds. This approach has enabled the rational design of novel ternary vanadium bronze phases characterized by colossal metal insulator transitions that span several orders of magnitude.
A rather practical application of the pronounced above-room temperature metal—insulator transitions of VO2 and ternary vanadium oxide bronzes is as dynamic glazing components for "smart" window coatings designed to improve the energy efficiency of buildings. The infrared part of the solar spectrum is primarily responsible for the heating of interiors (solar heat gain). The metallic phase of vanadium oxides tend to be infrared reflective and preclude solar heat gain, thereby preventing the heating of interiors at high ambient temperatures; however, at low temperatures, a phase transition to the insulating phase occurs and being infrared transparent, this phase permits solar radiation to heat the interiors.
Our primary goal in this area is to develop a detailed understanding of the structural phase progression and alterations in local geometric and electronic structure as strongly correlated materials undergo electronic phase transitions. Fundamental mechanistic understanding of the phase transitions is used to inform the design of novel doping schemes and entirely new compositions that exhibit colossal metal—insulator transitions. We have enjoyed considerable success with this rational design approach, obtaining precise control over charge ordering motifs and leading to the discovery of numerous compounds exhibiting colossal metal—insulator transitions in close proximity to room temperature. Ongoing research focuses on designing quantum corral structures based on precise and tunable separation of charge along 1D tunnels and layers of vanadium oxide bronze structures.
2. Measuring and Mapping Electronic Structure
A major thrust of our research program involves examination of chemical bonding at interfaces of graphene with metals, dielectrics, and organic molecules through the development of polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM) tools in conjunction with electrical transport measurements and density functional theory modeling. In a similar vein, we are using these techniques to study lithiation mapping processes in Ag2VO2PO4 and V2O5 to inform the design of cathode materials for Li-ion batteries.
Using polarized NEXAFS in conjunction with STXM allowed us to map local electronic corrugations of free-standing graphene grown via chemical vapor deposition (Nat. Commun. 2011, 2, 372/1-8), and to detect locally doped regions. The corrugations and doped regions represent some of the most formidable impediments that need to be resolved for manifestation of true Dirac physics in graphene.
In other work, we observed the substrate hybridization of the graphene Π cloud with dz2 orbitals of Cu and Ni substrates when graphene is grown atop these metals by chemical vapor deposition (J. Phys. Chem. Lett. 2010, 1, 1247-1253; Chem. Sci. 2013, 4, 494-502). In recent work, we deployed principal component analysis to scanning transmission X-ray microscopy data acquired for graphene oxide and mapped out the chemical group distribution at the edges and interior domains of graphene oxide (J. Phys. Chem. Lett. 2013, 4, 3144-3151). Our ongoing research thrusts are focused on (a) mapping lithiation pathways in cathode materials for Li-ion batteries; (b) developing an orbital-specific description of mechanisms of metal—insulator transitions; and (c) electronic structure mapping of dopant-induced inhomogeneities in graphene.
3. Solution-Phase Routes to Oxide and Oxyhalide Nanocrystals
We attempt to use low-temperature solution-chemistry starting with metal—organic precursors to capture different polymorphs and morphologies of metal oxides and oxyhalides. We have focused primarily on mixed metal HfxZr1-xO2 and CexHf1-xO2 as well as REOX (RE: rare earth; X: halide) nanocrystals from the non-hydrolytic condensation of metal alkoxides and metal chlorides. By varying steric and electronic parameters on added ligands and metal alkoxide precursors as well as precursor addition rates we are able to achieve very precise control of the stoichiometry, phase, and size of the nanocrystals. Most notably, the kinetics of the reaction can be altered based on choice of the precursor to trap specific polymorphs at room temperature. In recent work, we've shown that the tetragonal to monoclinic phase transition in nanorods of hafnium dioxide induces ferroelastic domain formation, yielding a "bar-code"-like pattern of twin variants. This synthetic approach has also been extended to facilitate ligand exchange and condensation yielding ternary rare-earth oxyhalide nanocrystals in the matlockite phase. The potential applicability of solid solution REOCl nanostructures as phosphors is illustrated by demonstrating the upconversion of near-infrared illumination to green and red emission by Er3+:GdOCl nanocrystals. Our ongoing research is focused on synthesis, doping, and fundamental physical property measurements of thin nanosheets of layered REOX phases and on developing high-refractive-index and high-dielectric constant films of mixed metal oxides. Another area of research seeks to correlate the steric and electronic properties of ligands and precursors to the eventual morphologies and dimensions of the nanocrystals.
4. Nanocomposites and Coatings
We are interested in developing polymer/nanoparticle hybrid coatings for corrosion protection of steels as well as metal matrix and polymer composites for "light and strong" applications. We are particularly interested in the use of nanoscale magnesium and graphene in conjunction with carbon nanotubes to fabricate composites that inhibit corrosion of low alloy steels at low coating thicknesses. Our modular design of coatings is based on an "active—passive" approach incorporating both barrier-type performance as well as electrochemically active components that disrupt the formation of corrosion cells on the metal surface. Tailoring the surface functionalization of the nanoparticle fillers allows for good dispersion within polymer matrices, which is critical for corrosion inhibition as well as for mechanical properties of the composites.
B.S., 2000, St. Stephen's College
Ph.D., 2004, State University of New York at Stony Brook
Postdoctoral Fellow 2004-2007, Columbia University