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)

• Towards Bright Mid-infrared Emitters: Thick-shell N-type HgSe/CdS Nanocrystals J. Am. Chem. Soc. 2021
• Multicarrier Dynamics in Quantum Dots Chem. Rev. 2021
• Auger suppression in n-type HgSe quantum dots ACS Nano 2019
• Slow Auger relaxation in HgTe colloidal quantum dots J. Phys. Chem. Lett. 2018
• Evidence for the role of holes in blinking: negative and oxidized CdSe/CdS quantum dots ACS Nano 2012
• Reduced damping of surface plasmons at low temperatures Phys. Rev. B 2009
• Trion decay in colloidal quantum dots ACS Nano 2009
• Slow electron cooling in colloidal quantum dots Science 2008
• Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands J. Chem. Phys. 2005
• Characterizing quantum-dot blinking using noise power spectra Appl. Phys. Lett. 2004
• Light emission and amplification in charged CdSe quantum dots J. Phys. Chem B 2004
• Intraband hole burning of colloidal quantum dots Phys. Rev. B 2001
• Intraband relaxation in CdSe quantum dots Phys. Rev. B 1999

Selected publications (structure)

• Multicarrier Dynamics in Quantum Dots Chem. Rev. 2021
• Shape-controlled HgTe quantum dots and reduced spin-orbit splitting in the tetrahedral shape J. Phys. Chem. Lett. 2020
• Conduction band fine structure in colloidal HgTe quantum dots ACS Nano 2018
• Reversible electrochemistry of mercury chalcogenide quantum dot films ACS Nano 2017
• HgS and HgS/CdS quantum dots with infrared intraband transitions and the emergence of a surface plasmon J. Phys. Chem. C 2016
• Air-stable n-doped HgS quantum dots J. Phys. Chem. Lett. 2014
• A mirage study of CdSe colloidal quantum dot films, Urbach tail, and surface states J. Chem. Phys. 2012
• Intraband spectroscopy and band offsets of colloidal II-VI core/shell structures J. Chem. Phys. 2007
• Interband and intraband optical studies of PbSe quantum dots J. Phys. Chem. B 2002
• Electrochromic nanocrystal quantum dots Science 2001
• n-type colloidal semiconductor nanocrystals Nature 2000
• Intraband transitions in semiconductor nanocrystals Appl. Phys. Lett. 1999
• Polar CdSe nanocrystals: Implications for electronic structure J. Chem. Phys. 1997
• Size-dependent two-photon excitation spectroscopy of CdSe nanocrystals Phys. Rev. B 1996