(6go) Electrochemical and Optoelectronic Transformations in Dynamic Semiconductor Nanomaterials

Research Interests: Next-generation energy technologies will rely on responsive semiconductor materials to rapidly convert and store energy, for applications including high-rate batteries, low-power electronics and efficient photovoltaics. These materials require facile exchange of charges and fields across interfaces and throughout a solid bulk. Solid-state transport – particularly of electrons and ions – often limits these technologies. One solution is to increase interfacial area, e.g. by thin-film processing or mesostructuring. This approach improves transport, but also reveals phenomena that only arise at nanometer scales (< 100 nm); i.e. interfacial charge storage, local focusing of light and tunable quantum phenomena. External stimuli modulate these properties through inter-related responses: for instance, electrochemical charging can induce distinct colorations depending on how ions accumulate around or within nanoparticles. Frontier technologies such as electrochromic ‘smart windows’ may be realized by exploiting these convoluted processes. Yet despite active research into the physics and synthesis of responsive nanomaterials, there are unresolved challenges to reach practical scales. Indeed, most devices are comprised of an ensemble of nanoparticles, and polydispersity in structure and composition is difficult to control. Moreover, techniques to observe dynamic transformations in nanomaterials are limited by spatial and temporal resolution, and convoluted by heterogeneous interfaces and phases.

I aim to connect local nanoscale properties to macroscopic behavior by linking rational synthesis of nanocrystalline ensembles to in situ characterization and analytic techniques. My research will confront these challenges at both the single particle and ensemble scales:

1. How do surface and bulk energetics affect the kinetics and phase behavior of nanoparticles during dynamic transformations, such as battery charging or photocarrier migration? This question delves deeper than a simple comparison of volumetric and surface energy. Metastable phases can be kinetically trapped during transformations, particularly in nanomaterials, and dynamic surface reconfiguration can lead to unexpected responses. I will manipulate particle size, shape, composition and surface chemistry to study these responses.
2. What effect does polydispersity in particle properties have on the macroscopic properties of nanomaterial ensembles? At rest, an ensemble relaxes to equilibrium. However, during dynamic transformations, slight variations in particle properties can dictate bulk kinetics and energetics. My studies will quantify and control polydispersity in ensembles to modulate bulk material properties.

Nanocrystalline ensembles can undergo rapid transformations that occur at sub-second time scales and across nanoscale interfaces, limiting the utility of many in situ characterizations. In situ optical spectroscopies can probe rapid coloration of semiconductor nanomaterials, with specificity to particular spectral features and spatial resolution approaching microns. Moreover, well-established optical spectroscopies are amenable to versatile in situ charging, excitation and other schemes. My group will combine these analytic strategies with precise colloidal synthesis and other techniques to investigate responsive nanomaterials for electrochemical and optoelectronic energy technologies.

Post-doctoral Research: During my post-doctoral research with Prof. Michael Chabinyc at UC Santa Barbara I have studied 2D layered hybrid perovskites for thin-film photovoltaics. These materials are tunable and robust analogues of 3D lead halide perovskites (e.g. MAPbI₃), and show exceptional promise for low-cost solar technologies. 2D perovskites are composed of atomically-thin layers of semiconducting lead halide sheets separated by insulating spacer molecules. The atomic layering of 2D perovskites creates similar properties to confined nanomaterials, namely quantum confinement with tunable optical and electronic properties, which extend throughout the bulk crystal. I investigated how the distribution of sheet thickness (i.e. how many lead atoms in each 2D layer) can be synthetically controlled, and how ensemble polydispersity in this layering affects bulk properties. I discovered through in situ optical and structural techniques that intermediate reactions skew the expected distribution of 2D layered phases. By carefully tuning reaction conditions, I revealed simple methods to control the distribution of layered phases – and decrease the concentration of detrimental defects – in thin-films. Building on these results, I then explored how accelerated reaction kinetics form colloidal dispersions of 2D perovskites. The insulating spacer molecules in these dispersions, composed of alkylammonium cations that ionically bond with lead halide sheets, dynamically interact with one another and with the solvent to direct colloidal structure. I discovered a distinction between alkylammonium cations that bridge adjacent lead iodide sheets into layered structures, and other spacer cations that act as terminating surfactants to stabilize the colloidal dispersion. This discovery provides a rational template to control the size, shape and structure of colloidal 2D perovskite nanoparticles, and demonstrates how ensemble dispersity can tune the bulk optoelectronic properties of practical devices.

Doctoral Research: My doctoral work with Prof. Delia Milliron at the University of Texas explored the electrochromic effect in metal oxide nanocrystal films. I discovered that distinct electrochemical charging processes, such as surface capacitance, ion insertion, or metal-insulator transitions, cause independent colorations in nanocrystal films. During my studies of TiO₂ nanocrystals, I resolved independent, reversible infrared and visible electrochromic responses by carefully controlling the electrochemical potential in a Li-ion electrolyte. My work showed that infrared coloration is caused by nanoscale confinement of free electrons during capacitive charging, leading to tunable plasmonic interactions with light. The small size of TiO₂ nanocrystals also allows small cations, such as Li⁺, to rapidly diffuse into interstitial voids in the crystal structure, inducing a visible darkening. My studies revealed that a single nanostructured material can control infrared and visible optical coloration for next-generation ‘smart window’ coatings. Moreover, my measurements of independent optical responses to charging demonstrate a sensitive in situ probe of local nanoscale transformations. I used the infrared plasmonic response to make fundamental discoveries of charge transport in anisotropic metal oxide nanocrystals, and exploited the visible coloration of lithiated TiO₂ particles to probe ion diffusion kinetics. These studies have revealed the potential for full-spectrum in situ optical spectroscopies to measure dynamic transformations in nanomaterial ensembles.

Teaching Interests: My goal as a teacher is to cultivate an engaged and responsive learning environment. As a graduate teaching assistant I helped my doctoral advisor build two chemical engineering courses from : an undergraduate survey course in materials science and a graduate course on applied solid-state physics for materials science. I employed tools that I acquired from curriculum design courses during my graduate studies to help incorporate simple feedback channels throughout both classes, including peer-directed projects, low-stakes quizzes and mid-semester surveys. As a mentor, I also strive for open communication and regular feedback to ensure a productive relationship. My work frequently recalls my core chemical engineering curriculum, and I will use my research experience as a teaching device within the canonical Chemical Engineering curriculum. I have been a consistent advocate of STEM education for under-represented and resource-limited communities, and as a faculty member I will continue to seek opportunities to advance inclusion and representation in science.

AIChE 2019 Presentation: “Spanning the Atomic to the Agglomerate Dimensions in Colloidal Dispersions of 2D Lead Halide Perovskites”, Session: Synthesis and Characterization of Materials for Optical and Electronic Applications, Nov. 14^th, 8:40AM.