Studies of surface chemistry in semiconductor quantum dots
Studies of surface chemistry in semiconductor quantum dots. In many respects, colloidal QDs are near-ideal materials for solar energy conversion and lighting technologies. However, QDs exhibit frustrating sensitivity of their electronic and optical properties to the surface passivation and chemical environment. As such, surface chemistry plays a crucial role in defining the working functions of QDs critical for technological applications. We study the effect of QD surfaces in light driven physico-chemical processes via developing and practically adopting reliable quantum chemical methodologies capable of modeling photo-induced dynamics and spectroscopic observables in the nanosize systems and their interfaces. Dr. Kilina’s research specifically focuses on a systematic theoretical analysis of the effect that a soft layer of organic ligands or/and impurities and defects on the QD surface have on the electronic and optical spectra, charge transfer and energy dissipation in QDs.
As a first step towards these goals, Dr. Kilina’s group has performed the benchmark of used methodologies. They investigated how the calculated photophysical properties of ligated QDs are affected by differences in methodological choices, e.g., various functionals, basis sets, and solvent models and identified the most robust approach by comparing to available experimental data (J. Phys. Chem. C 2011, 115, 15793; Nanoscale, 2012, 4, 904). Simultaneously, they have continued to develop NA dynamics and validated approximations and apply this methodology in describing phonon-mediated dynamics in pristine, ligated, and doped QDs (J. Phys. Chem. C, 2011, 115, 21641). Combining the NA time-domain DFT with the time-dependent density matrix technique, they have developed a new approach allowing for identification of conditions affecting radiative and nonradiative pathways of energy relaxation, as well as to simulate, for the first time, the time-resolved emission spectra of the co-doped Si QD (J. Phys. Chem. Lett. 2013, 4, 2906). Together with physicists from NDSU and University of South Dakota, Dr. Kilina’s group also investigated three types of Si nanosystems, a QD, one-dimensional chains of QDs and a nanowire, to determine which form – QD arrays or nanowire – maximizes the absorption and emission of light and how that effectiveness is affected by the interaction between QDs and the disorder in their structures. Surprisingly, the structural disorder in the amorphous QDs results in better light absorption at lower energies compared to crystalline-based nanomaterials. These studies are published in the Journal of Renewable and Sustainable Energy (J. Renew. Sustain. Energ. 2013, 5, 043120) and have been chosen for potential press interest and highlighted in the Press Release of American Institute of Physics in September 2013.
Working in collaboration with scientists from Los Alamos National Lab and University of Rochester, Dr. Kilina revealed the role of surface ligands in ultrafast loss of photoexcitation to heat in QDs. Quantum confinement can significantly slow down electron-phonon relaxation in QDs. Known as phonon-bottleneck, the effect remains elusive. Multiple opinions exist about the origin and conditions of the phonon bottleneck in nanostructures. We have found that at high excitation energies (> 2.5 of gap energy), surface ligands significantly increase electron-phonon mediated relaxation rates. This is because ligands introduce manifold of hybridized orbitals delocalized over both the QD and ligand atoms. While hybridized states are not optically active and do not contribute to absorption, they are strongly coupled to high frequency vibrations of ligands and, thus, open new channels for energy relaxation resulting in ultrafast photoexcitation relaxation rates in QDs. These findings on the mechanism of the ligand contribution to intraband carrier relaxation are published in ACS Nano (ACS Nano 2012, 6, 6515). This work provides theoreticians with energy-resolved relaxation rates across the broad excitation region and the dependence on the ligand type, which can be used further for analytical models. This work also intends to stimulate experimental efforts in controlling the energy relaxation mechanisms that holds an important promise for improvements of the solar energy conversion.
