This project uses ultrafast lasers to make ?movies? of light energy traveling between tiny semiconductor nanoparticles known as quantum dots. These quantum dots are efficient light absorbers and potentially useful for devices such as solar cells or light emitting diodes. In a device, quantum dots must be closely packed together in order to facilitate energy transfer from one quantum dot to the next. As a result, the distance between quantum dots and the speed at which energy migrates has a strong effect on device efficiency. By imaging how energy moves in quantum dot solids with different packings, this research aims to provide guidelines for designing structures for efficient light energy harvesting through control of energy migration rates. To achieve these goals, synthetic strategies are developed to control the size of the quantum dots and the chemical linkers used to connect them. Then, these quantum dots are used as ?artificial atoms? and assembled into well-ordered structures known as superlattices. The research team also develops microscopy techniques to record fast energy transfer events with a resolution of 10 femtoseconds (a femtosecond is one quadrillionth of a second) and to image energy migration distances with a resolution of 50 nanometers (a nanometer is one billionth of a meter). The research and educational activities are integrated to educate the next generation of solar energy researchers through K-12, undergraduate, and graduate science education. The PIs partner with Science Express and AP Fridays at Purdue to involve K-12 students in a cutting-edge laboratory environment. Results from this project are utilized to educate the general public on new solar energy technologies using podcast and video media.
Long-range exciton transport and coherence in colloidal quantum dot solids are highly desirable for their optoelectronic and quantum information applications. However, exciton transport in colloidal quantum dot solids thus far has been almost exclusively realized in the incoherent regime with excitons localized in individual quantum dots due to the relatively large energetic disorder compared to the electronic coupling strength in these systems. Incoherent transport of localized excitons presents a major limitation in obtaining long-range coherence and transport. In this project, strong dipole-dipole interactions between perovskite quantum dots are leveraged to enhance transport by promoting coherent motion of delocalized excitons. The team elucidates transport mechanisms by employing ultrafast coherent microscopy with ~10 fs time resolution and better than 50 nm spatial resolution. Exciton propagation distance, delocalization length, and coherent transport contributions are measured as a function of temperature. The extent of exciton delocalization is systematically controlled by varying inter-particle distance, electronic coupling, and dimensionality using ligand chemistry. The project provides guidelines for designing excitonic materials using perovskite quantum dots as building blocks.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
