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Semiconductor nanocrystals, known as quantum dots (QD), is a classic example of the quantum confinement effect that allows one to control system properties by varying system size rather than chemical composition. Conceived in the 1980s, QDs initiated the nanotechnology revolution, acknowledged by the Nobel Prize in Chemistry in 2023.
Electrons and holes, i.e., negative and positive charges, are weakly bound in semiconductors, forming excitons. If nanocrystal size becomes smaller than the exciton radius, electronic properties of the nanocrystal depend on system size. This is manifested in optical properties, e.g., emission color, as well as excitation dynamics. Our group initiated atomistic studies of excited state dynamics in QDs, establishing important concepts and facts that changed quantitative and qualitative thinking about QDs.
Often called artificial atoms, QDs exhibit atom-like spectra that can be classified using the hydrogen atom nomenclature: S-, P-, D- states. We showed that the atom-like spectra arise from a large number of transitions, whose optical selection rules are governed by S-, P-, D- type envelope functions. Because of the huge number of states involved, excited state dynamics in QDs is much more similar to that in bulk materials than atoms or molecules.
S. V. Kilina, A. J. Neukirch, B. F. Habenicht, D. S. Kilin, O. V. Prezhdo “Quantum Zeno effect rationalizes the phonon bottleneck in semiconductor quantum dots”, Phys. Rev. Lett., 110, 180404 (2013).
S. V. Kilina, D. S. Kilin, O. V. Prezhdo, “Breaking the phonon bottleneck in PbSe and CdSe quantum dots: time-domain density functional theory of charge carrier relaxation”, ACS-Nano, 3, 93 (2009).
Confinement of charges in QDs enhance their Coulomb interactions, enabling a variety of Auger-type processes, such as electron-hole energy exchange, and multiple exciton generation and annihilation. By performing atomistic modeling, we demonstrated a strong interplay of electron-electron and electron-vibrational interactions, and the importance of realistic QD models that include defects, dopants, ligands, core/shell interfaces, etc.
H. Zhu, Y. Yang, K. Hyeon-Deuk, M. Califano, N. Song, Y. Wang, W. Zhang, O. V. Prezhdo, T. Lian “Auger-assisted electron transfer from photoexcited semiconductor quantum dots”, Nano Lett., 14, 1263 (2014).
K. Hyeon-Deuk, O. V. Prezhdo “Multiple exciton generation and recombination dynamics in small Si and CdSe quantum dots: an ab initio time-domain study”, ACS Nano, 6, 1239 (2012).
S. A. Fischer, C. M. Isborn, O. V. Prezhdo, “Excited states and optical absorption of small semiconducting clusters: dopants, defects and charging”, Chem. Science, 2, 400 (2011).
QDs are much more complex than the simple models used initially to describe QD properties. Thus, QDs are often non-stoichiometric. Good QD emitters contain excess metal on the surface. QD surfaces are passivated by ligands or shells. These realistic aspects of QDs give rise to new phenomena that require an atomistic understanding provided by our simulations.
S. Gumber, O. Eniodunmo, S. A. Ivanov, S. Kilina, O. V. Prezhdo, D. Ghosh, S. Tretiak, “Hot carrier relaxation dynamics in non-stoichiometric CdSe quantum dots: computational insights”, J. Mater. Chem A., 11, 8256-8264 (2023).
S. Kilina, K. A. Velizhanin, S. Ivanov, O. V. Prezhdo, S. Tretiak “Surface ligands increase photoexcitation relaxation rates in CdSe quantum dots”, ACS Nano, 6, 6515 (2012).
H. Wei, C. M. Evans, B. D. Swartz, A. J. Neukirch, J. Young, O. V. Prezhdo, and T. D. Krauss “Colloidal semiconductor quantum dots with tunable surface composition”, Nano Lett., 12, 4465 (2012).
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