Interfaces and boundaries between materials, phases, and structural motifs are ubiquitous in our natural and manufactured world. Changes in the atomic and electronic structure at interfaces lead to unique properties such as modified excited state lifetimes, charge transport mechanisms, and interfacial electronic states. In materials, boundaries and interfaces can create unique electronic states that modify charge transfer or facilitate charge recombination. This influences material functionality, particularly in non-equilibrium systems such as photovoltaics, electrochemical cells, and heterogeneous catalysis. In my research group, we aim to investigate how the nanoscale and mesoscale structure of materials, including the presence of grain boundaries and interfaces, influences electronic structure, excited state dynamics, chemical reactivity, and their ultimate functionality.
Drawing electronic structure on the nanoscale using switchable molecular interfaces
Photoemission electron microscopy can image the nanoscale optical selection rules of 2D materials with <55 nm spatial resolution. Here we can see that certain laser polarizations can preferentially excite the edges of black phosphorus compared to the flake interior. (https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03849)
Two dimensional materials hold phenomenal promise in upending the scaling problem in electronic devices and offer unique physics and chemistry critical for next generation energy and information conversion and transport. Realizing the full potential of 2D materials in electronic devices, quantum information, and chemical catalysis, however, is currently roadblocked by a time-intensive and expensive reproducibility problem. This is down to the twin challenges of 2D material synthesis and strong sensitivity of 2D material electronic structure to nanoscale variations in morphology/dielectric environment. In the King Lab, we seek to completely map the connection between nanoscale morphology and 2D materials functionality and offset the reproducibility problem. We aim to uncover how and why atomic and nanoscale variations in 2D materials determine their electronic structure, dynamics, and functionality; and to develop a rational route using molecular interfaces to control this deterministic connection for adaptive functionalities such as charge transport and ferroelectricity.
The properties and dynamics of buried interfaces are at the heart of material and energy transformation, ranging from energy transfer between a battery electrode and a solid or liquid electrolyte to the surface chemistry that can occur at a solid-liquid interface. It is well known that the surface properties of materials can be distinct from those of a bulk material. At material interfaces unique surface electronic states can exist, band-bending can alter the energy of conduction and valence bands relative to the Fermi level, and even a material’s crystal structure can be significantly modified.
These interfacial electronic states are often the “gatekeepers” for charge transfer and reactivity at buried interfaces but selectively investigating their electronic structure and ultrafast dynamics has proved difficult due to a dearth of experimental methods to probe them selectively. The group is developing a time-resolved phase-stabilized electronic sum-frequency generation spectrometer to probe the electronic structure and ultrafast dynamics of buried interfaces in situ.
Time-resolved SFG spectroscopy will be used to selectively probe, for example, the electronic structure and dynamics of a solid liquid interface and the role of interface states (IS) the reactivity of the interface.
Enhancing MXene catalysis on the nanoscale
Layered transition metal carbides and nitrides, colloquially called MXenes, are promising new 2D materials for catalysis, particularly for activation of N2 and CO2. Trial and error efforts to optimize MXene catalytic efficiency and selectivity, however, have failed to resolve the simultaneous contribution from both intrinsic material and defects to functionality for the rational design of MXenes for catalysis. In the King Lab, we aim to use nanoscale imaging of MXene electronic structure, dynamics, morphology, N2 and CO2 binding location, and N2 and CO2 activation dynamics to identify the roles of intrinsic material versus defects in N2 and CO2 activation. We will then modify the contributions from intrinsic versus defect sites to N2 and CO2 activation by enhancing catalysis in nanoscale regions with intentional plasmonic fields.
Using TR-PEEM we aim to image the nanoscale electronic structure of MXenes and the mechanisms of CO2 and N2 bond activation.
We are grateful for funding from the following sources: