Measuring the Dynamics of Excitons in 1D Semiconductor Quantum Wires with Quantum State Resolution

Washington University, Saint Louis MO

Investigators

Abstract

Nontechnical Description: In photovoltaic (PV) devices it would be ideal for every photon that impinges on the device to be absorbed and for at least one electron and one hole to be separately collected as current and electricity with little or no loss of energy within the system, regardless of the color of light or energy of the photon absorbed. PV devices that incorporate semiconductor nanoparticles (NPs) as the absorbing medium offer the advantages of tunable absorption energies through size control and large absorption efficiencies. When light with energy in excess of the band gap of the semiconductor NPs is absorbed, the photogenerated electrons and holes relax down to the lowest-energy states. The efficiency for the charge carrier relaxation depends on numerous factors, including the densities of electron and hole states, the roles and rates of different energy-transfer mechanisms, the temperature, and the chemical environment of the NPs. The research team is synthesizing NPs with varying sizes, shapes, semiconductor materials, and chemically passivated surfaces. Several optical spectroscopy and microscopy techniques are being implemented, and new models are being developed to characterize the relaxation dynamics in the semiconductor NPs. The ultimate goals of the research activities are to not only characterize the relaxation dynamics of the carriers, but to develop novel nanostructures with properties that are optimal for the light-to-electricity conversion of PV devices. The research project is highly interdisciplinary, and graduate and undergraduate students, especially those from underrepresented backgrounds, are gaining the expertise needed to become the next generation of scientists. The educational mission of the principal investigator extends beyond the research team as educational videos on the physics of light and on the importance of alternative energy sources are being developed and disseminated to the public and local schools. Technical Description: The relaxation dynamics and efficiencies of photogenerated electrons and holes in semiconductor nanoparticles (NPs) ultimately limit the yields of photovoltaic devices that incorporate NPs as the absorbing medium. The goals of the research are to accurately characterize the intraband relaxation dynamics (IRD) and mechanisms for carrier relaxation in semiconductor NPs. Specific research activities that include nanoparticle synthesis, electron microcopy and imaging, and optical spectroscopy in both the time- and frequency-domains are characterizing the IRD of the electrons and holes. The team is paying particular attention to the roles of dimensionality and the densities of states on the rates and efficiencies of electron and hole relaxation to the band edge. Carriers in one-dimensional semiconductor quantum wires (QWs) and belts (QBs) can have translational kinetic energy and delocalization along their lengths. This dimensionality gives rise to a continuum of states that can be accessed during carrier relaxation. These one-dimensional NPs contrast those of widely studied zero-dimensional quantum dot (QD) and two-dimensional quantum platelet (QP) systems. Time-resolved transient absorption experiments are performed on NPs with contrasting dimensionality to identify the role of the states, kinetic energy, and momentum on the carrier IRD. A new model, quantum-state renormalization is being developed to help unravel the dynamics from complicated transient absorption spectra recorded on ensembles of the NPs. The efforts are complemented through a long-standing collaboration with the synthetic group of William E. Buhro (Wash. U.) and a new collaboration with the ultrafast spectroscopy group of Martin Zanni (U. Wisconsin). 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.