Our research is focused on fundamental problems in nanoscience and how they impact the application of nanoscale materials to solar energy harvesting. Our approach integrates the design and synthesis of novel nanomaterials with detailed electronic spectroscopy in order to reveal how such materials interact with light. The group welcomes a broad spectrum of scientists, with interests ranging from synthetic chemistry to femtosecond spectroscopy.
One of the defining themes in nanoscience is the control of physical properties of a material (such as its electronic structure) through solution-phase synthesis that produces nanostructures of well-defined composition, size, and shape. Our synthetic efforts are directed at creating complex nanomaterials that incorporate the properties necessary for solar energy applications, such as optimized light absorption and spatial separation of photoexcited charges.
Time-resolved electronic spectroscopy allows us to directly probe the behavior of excited electrons and holes created when a material absorbs sunlight. Events such as charge separation, transfer, recombination, and trapping determine the efficiency of solar energy harvesting. We are interested in mapping out the dynamics of such events to understand how to improve the design of next generation solar materials.
Excited State Dynamics in Semiconductor Nanocrystals
Semiconductor nanocrystals (NCs) are remarkable materials that have unique and tunable optical and electronic properties due to the quantum confinement of charge carriers. A NC of one composition can be easily modified in shape, size, and surface functionalization to tune its excited-state properties for a wide range of applications. Additionally, the surface-capping ligands allow these systems to form colloidal suspensions in a variety of chemical environments. Through the use of ultrafast spectroscopic techniques and analytical or numerical modeling, our group investigates the behavior of photoexcited charge carriers in colloidal semiconductor nanocrystals and how these behaviors are influenced by factors such as the nanocrystal material, size, shape, and surface chemistry.
The Motion of Trapped Holes in Cd-Chalcogenide NCs
The dynamics of photoexcited holes that become spatially localized (trapped) on nanocrystal surfaces are poorly understood as they remain difficult to study due to their weak spectroscopic signature. However, studying and controlling the photoexcited hole is crucial to processes such as electron-hole recombination and charge transfer, especially in the context of oxidation photochemistry. Our group, in collaboration with the Eaves group (CU) showed that these holes form small polarons, trapped to chalcogen atoms on the surface. Contrary to the conventional picture that trapped holes are static, we found that they are mobile. At room temperature, trapped holes undergo random walk diffusion through a sequence of incoherent hops along the particle surface. The diffusion coefficient is small, and therefore, trapped hole motion could have profound implications for oxidation chemistry in NCs and semiconductors more generally.
The Effect of Surface-Capping Ligands on NC Excited States
The ligands on the surface of a colloidal semiconductor NC influence how its excited state is produced, how it relaxes, and how it couples to the environment. For example, our group has shown that CdTe quantum dots capped with inorganic ligands (such as Se2-) exhibit severely damped modulations in the transient absorption (TA) kinetics of their excited-state decay in comparison to native aliphatic ligand-capped quantum dots. This indicates that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals to their environment. In addition, we’ve also shown that CdS NCs with two common organic ligands become reduced under illumination, likely by hole transfer to the ligand which results in subsequent ligand dissociation. This generates long-lived reduced NCs, providing opportunities for efficient electron transfer.
Integration of Light Absorbers and Redox Catalysts for Light-Driven Multielectron Chemistry
The combination of the light-harvesting properties of colloidal semiconductor NCs and the catalytic properties of redox enzymes has emerged as a versatile platform to drive a variety of multi-electron reactions with light. This strategy is inspired by photosynthesis, in which light absorption is coupled to enzyme catalysis via electron transfers. In these NC-enzyme architectures, NCs absorb light and photoexcited electrons transfer to enzymes, thereby driving enzyme catalysis with light. We adapt NC structural properties toward productive NC-enzyme interactions, controlling the electron pathways and the impact on overall catalytic rate and efficiency. This light-driven control and investigation of enzymatic catalysis is particularly insightful for reactions that are difficult to perform cost-effectively by artificial means, such as the production of transportation fuels, fertilizer, and other value-added compounds.
Photoexcited charge transfer from a NC to an enzyme is a critical first step for these reactions, as the efficiency of this step determines the upper limit on the photochemical activity of the system. The charge transfer efficiency, in turn, depends on its competition with other NC excited state relaxation pathways. An understanding of the parameters that govern the interfacial charge transfer can therefore lead to better control of the rate and efficiency of catalysis. We use ultrafast time-resolved spectroscopy and kinetic modeling to measure the rates and efficiencies of NC-to-enzyme electron transfer.
Compositionally Complex Nanomaterials
The elemental composition of nanocrystals can be controlled to change their electronic structure (e.g., band gap) with important implications for photocatalytic and optoelectronic applications. However, in compositionally-complex materials the relationships between nanoscale composition and electronic structure are not always clear. Our group studies novel, compositionally-complex nanocrystals with controllable heterogeneity at the nanoscale. We characterize these materials by time-resolved spectroscopic techniques and spatially-resolved electron microscopy techniques. This work helps us reveal rich relationships between local composition and electronic structure, informing the development of novel nanocrystals with enhanced performance.
