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Nanostructured materials—conjugated organic molecules, colloidal quantum dots, and other low-dimensional semiconductors—offer novel ways to solve practical challenges surrounding the detection, generation, and movement of light and charge. Accordingly, they are being investigated for optoelectronic applications ranging from 3rd-generation photovoltaics, to biological imaging, to cost-effective cameras in the short-wave infrared (λ:1‒3µm).
Excitons mediate the interaction of light with these nano-scale systems, giving rise to their attractive optical properties. As a result, spectroscopy is the natural tool to gain insight—in particular, transient broad-band spectroscopies, as key materials exhibit rich spectra and rapid (femtoseconds-milliseconds) dynamics.
In the Wilson Lab, we use spectroscopy to reveal and understand the properties of excitonic materials. We then exploit this knowledge to build novel optoelectronic devices—particularly those that allow functionality in the short-wave infrared (SWIR), as new research opportunities are arising driven by practical applications (sensing, emission), improved research equipment (SWIR APDs, ultrafast techniques), and a rising tide of SWIR-active materials.
Excitons mediate the interaction of light with nano-scale semiconductors, and coupling between excitonic sites gives rise to emergent behaviour. Building on past work, we will use cutting-edge spectroscopy to lead the study of exciton fission & fusion—phenomena in organic materials which exploit spin-dependent energy levels to trade exciton number for exciton energy. A second theme will be exciton dynamics in close-packed nanocrystalline films, where we can synthetically tune site-to-site coupling to explore the transition from stable excitons to free carriers in a disordered landscape.
Energy Transfer in Hybrid Systems:
Spectroscopic studies of dipole-less (i.e. Dexter) exciton transfer between colloidal nanocrystals and organic films will reveal the mechanistic origin of the phenomenon’s surprising efficiency, and guide efforts to fabricate practical fusion- and fission-based up- and down-converter devices. The capabilities developed during this effort will enable studies of many hybrid interfaces, for example between two-dimensional semiconductors and quantum dots. Here, weak coupling could achieve sensitized absorption and emission, while strong coupling could create hybrid, localized states.
Our fundamental investigations draw vitality from their technological potential, and we will strive to apply our findings to create novel thin-film devices. For example, the integration of an efficient spin-mixing SWIR absorber (i.e. colloidal nanocrystals) and a fusion-capable, visible-emitting organic film could enable efficient, low-intensity photon upconversion. The ability to efficiently interconvert low-intensity light between the visible and infrared would be an enabling technology—particularly for applications such as 3rd-generation photovoltaics, biological imaging, and cost-effective cameras in the short-wave infrared.
…so what’s an ‘exciton’, anyhow?
The definition of an exciton has become overloaded, so that the word can mean subtly different things to different people. However, two unifying characteristics are: 1) excitons are an excited state of a material, and 2) they are charge-neutral. Speaking loosely, the language of excitons becomes useful whenever it’s more appropriate to consider a quasi-particle consisting of a spatially-correlated electron and hole, than to keep track of the two constituent fermions separately.
Excitons are important because they frequently mediate the non-dissipative interaction of light with nano-scale materials—they are the ‘energy packet’ created when these nontraditional semiconductors absorb a photon. As a result, excitons are the lifeblood of optoelectronic devices—by managing their lifecycle, we can generate light, liberate charge, or drive chemistry at a reaction center. Particularly, unlike photons, excitons interact strongly with each other. Accordingly, multi-excitonic effects are significant, even dominant, at low concentrations of photoexcitations—the kind that are readily encountered under ambient conditions.