Semiconductor nanocrystallites (quantum dots, QDs) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. We are interested in synthesis, spectroscopic studies, application in optoelectronic devices, and biological imaging of quantum dots.

Synthesis

Our group has a long history of synthesizing high-quality colloidal semiconductor nanomaterials for a wide range of optoelectronic applications. Current material classes include InAs- and iron oxide-based nanoparticles for optical and magnetic resonance imaging applications, InP QDs for lighting, lead halide perovskite QDs for quantum light generation. Employing strategies such as precursor optimization, ligand engineering and inorganic shell growth, we design a diverse collection of robust and functional nanomaterials and study their co-assembly into more complex architectures. Efforts also include supramolecular approaches to form J-aggregates.

Spectroscopy

Broadly, our group develops and uses new single molecule spectroscopic techniques to study the spectral dynamics and many-body physics in nanoscale emitters. Recent research has focused on the higher-order multiexciton dynamics in colloidal quantum dots and spectral coherence measurements of single colloidal quantum dots and quantum emitters in 2D van der Waals materials. Ongoing efforts further include translation of single molecule spectroscopy into the short-wave infrared using superconducting nanowire single photon detectors (SNSPDs) and the study of exciton transport in J aggregates. Together, these studies inform materials development and their applications in optoelectronics and optical contrast agents in biological imaging.

Optoelectronic Devices

Wavelength interconversion bears great promise in harnessing the solar spectrum to its full extent. Down-conversion via singlet exciton fission can convert single high-energy photon into two lower-energy triplet excitons, reducing the thermalization energy loss of the solar cells in excess of their bandgaps. Triplet exciton fusion-based upconversion can enable solar cells to harvest sub-bandgap photons by upconverting two lower-energy photons into a high-energy photon. Both downconversion and upconversion could potentially increase the efficiency of a conventional single-junction beyond the Shockley-Queisser limit if critical efficiency-limitations can be overcome. We investigate the fundamental energy transfer mechanisms across hybrid interfaces that can guide the interfacial design and the device engineering of singlet fission/triplet fusion materials for coupling to PV devices. In addition, current research on organic/inorganic hybrid perovskites focuses on understanding the role of the interface on device performance and interface engineering strategies.

Biological Imaging

Imaging with short-wave infrared (SWIR, 1000-2000 nm) light has great potential for visualizing biological structures previously undetectable with visible and near-infrared light. The minimal autofluorescence of biological tissue in the SWIR leads to increased sensitivity, while the significantly reduced light attenuation from scattering and from absorption, by the blood and other structures, enables imaging with high spatiotemporal resolution and penetration depth. Our group develops new optical imaging probes like SWIR-emitting InAs QDs and dyes as well as imaging techniques that harness the unique transmission of SWIR light through tissue. Currently, our research focuses on tracking cellular migration and angiogenesis as well as technology development towards optical brain-imaging. These studies are complemented with fundamental work on the penetration depth and scattering properties of SWIR light in biological tissue.