Spatial confinement can create relaxation bottlenecks by mismatch between electronic and vibrational frequencies. This hypothesis motivated discovery of multiple excitons, which could greatly enhance the efficiency of quantum dot (QD) solar cells. Surprisingly, recent experiments showed no bottleneck. Our time-domain ab initio study of the electron-phonon dynamics rationalizes the fast relaxation in PbSe and CdSe QDs, which have substantially different electronic properties. Atom fluctuations and surface effects lift degeneracies and create dense distributions of electronic levels at all but the lowest energies, while confinement enhances the electron-phonon coupling. The analysis applies to nanomaterials in general, modifying the fundamental view on the electron-phonon interaction.

Quantum confinement can dramatically slow down electron-phonon relaxation in nanoclusters. Known as the phonon bottleneck, the effect remains elusive. Using a state-of-the-art time-domain ab initio approach, we model the observed bottleneck in CdSe quantum dots and show that it occurs under quantum Zeno conditions. Decoherence in the electronic subsystem, induced by elastic electron-phonon scattering, should be significantly faster than inelastic scattering. Achieved with multiphonon relaxation, the phonon bottleneck is broken by Auger processes and structural defects, rationalizing experimental difficulties.

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The phonon-induced relaxation dynamics of charge carriers in a PbSe quantum dot is studied for the first time by ab initio density functional theory in the time-domain. The picosecond time scale of the relaxation and the absence of the phonon bottleneck are rationalized by relatively high electron and hole state densities. While many of these states show only weak optical activity, most of them participate in the electron-vibrational relaxation. Our simulations demonstrate that the slight asymmetry in the electron and hole band structure is sufficient to allow symmetry-forbidden S-P, P-S, etc. transitions, which are seen in the experimental absorption spectra. The relaxation is nonexponential, in agreement with the strongly non-Lorentzian spectral line shapes observed in experiments. The energy exchanged during individual transitions is typically greater than the characteristic phonon energy, indicating that the transitions are multiphonon. Both electrons and holes interact better with low-frequency acoustic phonons, rather than higher frequency optical modes. Holes decay only slightly faster than electrons, rendering the hole-assisted Auger relaxation pathways inefficient. This relatively symmetric vibrational relaxation of electrons and holes proceeds on a picosecond time scale, much slower than the ultrafast highly efficient carrier multiplication that was reported recently in relation to improved solar power conversion.

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Colloidal quantum dots (QDs) are near-ideal nanomaterials for energy conversion and lighting technologies. However, their photophysics exhibits supreme sensitivity to surface passivation and defects, of which control is problematic. The role of passivating ligands in photodynamics remains questionable and is a focus of ongoing research. The optically forbidden nature of surface-associated states makes direct measurements on them challenging. Therefore, computational modeling is imperative for insights into surface passivation and its impact on light-driven processes in QDs. This Account discusses challenges and recent progress in understanding surface effects on the photophysics of QDs addressed via quantum-chemical calculations. We overview different methods, including the effective mass approximation (EMA), time-dependent density functional theory (TDDFT), and multiconfiguration approaches, considering their strengths and weaknesses relevant to modeling of QDs with a complicated surface. We focus on CdSe, PbSe, and Si QDs, where calculations successfully explain experimental trends sensitive to surface defects, doping, and ligands. We show that the EMA accurately describes both linear and nonlinear optical properties of large-sized CdSe QDs (>2.5 nm), while TDDFT is required for smaller QDs where surface effects dominate. Both approaches confirm efficient two-photon absorption enabling applications of QDs as nonlinear optical materials. TDDFT also describes the effects of morphology on the optical response of QDs: the photophysics of stoichiometric, magic-sized XY (X = Cd, Pb; Y = S, Se) QDs is less sensitive to their passivation compared with nonstoichiometric XY QDs. In the latter, surface-driven optically inactive midgap states can be eliminated by anionic ligands, explaining the better emission of metal-enriched QDs compared with nonmetal-enriched QDs. Ideal passivation of magic-sized QDs by amines and phosphine oxides leaves lower-energy transitions intact, while thiol derivatives add ligand-localized trap states to the band gap. Depending on its position, any loss of ligand from the QD's surface also introduces electron or hole traps, decreasing the QD's luminescence. TDDFT investigations of QD-ligand and QD-QD interactions provide an explanation of experimentally detected enhancement of blinking on-times in closely packed Si QDs and establish favorable conditions for hole transfer from the photoexcited CdSe QD to metal-organic dyes. While TDDFT well describes qualitative trends in optical response to stoichiometry and ligand modifications of QDs, it is unable to calculate highly correlated electronic states like biexcitons and magnetic-dopant-derived states. In these cases, multiconfiguration methods are applied to small nanoclusters and the results are extrapolated to larger-sized QDs, providing reasonable explanations of experimental observables. For light-driven dynamics, the electron-phonon couplings are important, and nonadiabatic dynamics (NAD) is applied. NAD based on first-principles calculations...

Charge transfer photoinduced by steady light absorption on a silicon surface leads to formation of a surface photovoltage (SPV). The dependence of this voltage on the structure of surface adsorbates and on the wavelength of light is studied with a combination of ab initio electronic structure calculations and the reduced density matrix for the open excited system. Our derivations provide time averages of surface electric dipoles, which follow from a time-dependent density matrix (TDDM) treatment using a steady state solution for the TDDM equations of motion. Ab initio calculations have been carried out in a basis set of Kohn-Sham orbitals obtained by a density functional treatment using atomic pseudopotentials. Applications have been done to a H-terminated Si(111) surface and for adsorbed Ag, with surface coverage ranging from 0 to 3/24 of a monolayer. Calculations done also for amorphous Si agree with measured values of the SPV versus incident photon frequency for H-terminated a-Si. Surface adsorbates are found to enhance light absorption and facilitate electronic charge transfer at the surface. Specifically, Ag clusters add electronic states in the energy gap area, provide stronger absorption in the IR and visible spectral regions, and open up additional pathways for surface charge transfer. Our treatment can be implemented for a wide class of photoelectronic materials relevant to solar energy capture.

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si6H12) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.