Electronic structure and dynamics

The electronic level structure and dynamics underpin many fundamental and applied aspects of nanocrystal quantum dot behavior. Current investigations  mostly focus on the mid-infrared mercury chalcogenide quantum dots developed by our group, and an understanding of the structure and dynamics in these materials is critical for improving infrared nanocrystal devices. Due to their small energy gaps, the mercury chalcogenides also provide a unique platform to investigate basic and general aspects of carrier relaxation in nanostructures. These studies extensively utilize a variety of mid-infrared time-resolved and static spectroscopies in conjunction with spectroelectrochemistry, microscopy, transport measurements and analytical theory.

At the electronic structure level we are particularly interested in the effects of oxidation and reduction on the optical spectroscopy. Addition of electrons (n-doping), for example, bleaches the valence-conduction transition (analogous to the HOMO-LUMO transition in molecules) and induces new optical transitions within the conduction band. These conduction band states are unoccupied and difficult to study in neutral quantum dots, yet they contain essential information about the effects of particle shape, charging and other perturbations on the level structure. Recent discoveries include spin-orbit coupling which depends on the particle shape, linewidths which depend on the number of electrons in the system, and the unique roles played by surface chemistry in governing the quantum dot redox potential. 

The coupling of quantum dot excited states to the nanocrystal environment (ligands, surface defects, host material etc.) governs the nonradiative relaxation, and minimizing this relaxation is critical in many practical applications. Understanding and controlling nonradiative relaxation in the mid-infrared is a particularly rich problem because electronic and vibrational transitions have similar energies in this regime. This enables unique physical processes such as polaron formation and decay, and near-field energy transfer between electrons and surface ligands which do not operate in the visible and near-infrared. We are actively studying the relaxation mechanisms across mid-infrared energy gaps by examining the effects of particle size, shell growth, ligand and host material on the carrier dynamics.

When quantum dots contain multiple excited electrons, a process known as Auger relaxation occurs. This process involves one electron relaxing to its ground state by transferring its energy to another in the same quantum dot, and Auger relaxation is detrimental yet ubiquitous in practical situations such as lasers, LEDs and photodetectors. Although the Auger mechanism is well-understood in bulk crystalline semiconductors, there are many unknown aspects of the mechanism in quantum dots. Recent experiments on HgTe and HgSe have highlighted mechanistic differences between bulk and nanoscale Auger processes, and illuminated new mechanisms for the suppression of Auger relaxation.

Selected publications (dynamics)
Selected publications (structure)