Breaking the Phonon Bottleneck for Holes in Semiconductor Quantum Dots

Cooney, Ryan R.; Sewall, Samuel L.; Anderson, Kevin E. H.; Dias, Eva A.; Kambhampati, Patanjali

doi:10.1103/physrevlett.98.177403

Cited by 197 publications

(382 citation statements)

References 32 publications

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“…16 Understanding of surface processes is also important for controlling carrier multiplication (CM), a process in which two or more electronÀhole pairs are generated from a single absorbed photon, promising higher efficiency of QD-based solar cells than is currently possible. 17 Recent time-resolved experiments have demonstrated that radiative quantum yield, 18À20 exciton lifetime, 21 relaxation rates, 19,22,23 and the efficiency of the CM 24 are strongly affected by the type of ligands passivating the QD surface. Various contradicting data on CM efficiency reported in recent literature 25À28 may also be a result of uncontrolled differences in QD surface structure and treatment.…”

Section: Introductionmentioning

confidence: 99%

Electronic Structure of Ligated CdSe Clusters: Dependence on DFT Methodology

et al. 2011

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Simulations of ligated semiconductor quantum dots (QDs) and their physical properties, such as morphologies, QDÀligand interactions, electronic structures, and optical transitions, are expected to be very sensitive to computational methodology. We utilize Density Functional Theory (DFT) and systematically study how the choice of density functional, atom-localized basis set, and a solvent affects the physical properties of the Cd 33 Se 33 cluster ligated with a trimethylphosphine oxide ligand. We have found that qualitative performance of all exchange-correlation (XC) functionals is relatively similar in predicting strong QDÀligand binding energy (∼1 eV). Additionally, all functionals predict shorter CdÀSe bond lengths on the QD surface than in its core, revealing the nature and degree of QD surface reconstruction. For proper modeling of geometries and QDÀligand interactions, however, augmentation of even a moderately sized basis set with polarization functions (e.g., LANL2DZ* and 6-31G*) is very important. A polar solvent has very significant implications for the ligand binding energy, decreasing it to 0.2À0.5 eV. However, the solvent model has a minor effect on the optoelectronic properties, resulting in persistent blue shifts up to ∼0.3 eV of the low-energy optical transitions. For obtaining reasonable energy gaps and optical transition energies, hybrid XC functionals augmented by a long-range HartreeÀFock orbital exchange have to be applied.

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Section: Introductionmentioning

confidence: 99%

Electronic Structure of Ligated CdSe Clusters: Dependence on DFT Methodology

et al. 2011

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“…The excited hole can then relax efficiently within the densely spaced valance band levels. 41 Figure 3BII and CII). In the non-adiabatic limit, the total ET rate is the sum of Auger-assisted ET rates to these product states, which is given by:…”

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confidence: 99%

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

et al. 2014

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Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remains unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe and CdTe) to molecular acceptors (anthraquinone, methylviologen and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ~ 0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model, in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Frank-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes.

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“…8,9 In particular, hot electrons could relax rapidly by transferring energy to holes, which often have a greater effective mass and thus smaller energy spacing, through Auger-like processes followed by phonon-assisted relaxation 10,11 or nonadiabatic channels involving surface ligands. 12,13 Trap states 14,15 in the quantum dots and highfrequency vibrational modes in surface ligands 16 provide additional relaxation pathways to promote rapid carrier cooling. It has been demonstrated that by carefully designing multiple layers of heterostructures around colloidal CdSe quantum dots and suppressing these pathways, the lifetimes of hot carriers could be increased by 3 orders of magnitude up to 1 ns.…”

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confidence: 99%

Slow Hot-Carrier Relaxation in Colloidal Graphene Quantum Dots

et al. 2010

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Reducing hot-carrier relaxation rates is of great significance in overcoming energy loss that fundamentally limits the efficiency of solar energy utilization. Semiconductor quantum dots are expected to have much slower carrier cooling because the spacing between their discrete electronic levels is much larger than phonon energy. However, the slower carrier cooling is difficult to observe due to the existence of many competing relaxation pathways. Here we show that carrier cooling in colloidal graphene quantum dots can be 2 orders of magnitude slower than in bulk materials, which could enable harvesting of hot charge carriers to improve the efficiency of solar energy conversion.KEYWORDS Graphene, quantum dots, energy relaxation, carrier cooling, quantum confinement W hen the size of a semiconductor crystal is reduced to approach the exciton Bohr radius of the bulk material, the limited volume significantly modifies electron distribution, resulting in size-dependent properties such as bandgap and energy relaxation dynamics.1-3 This phenomenon, known as "quantum confinement", has been investigated in many semiconductor materials and led to practical applications such as bioimaging, lasing, photovoltaics, and light-emitting diodes. For these electro-optical applications of quantum dots, relaxation of high excited states (or cooling of hot carriers) is of central importance and thus has been extensively studied. [4][5][6] Because of the large electronic energy spacing in quantum confined systems in comparison with phonon frequencies, phonon-assisted relaxation between discrete electronic states requires emission of multiple phonons to conserve energy. Since the phonon-assisted relaxation is the primary mechanism for carrier cooling in bulk semiconductors, it was expected that carrier cooling in quantum dots should be significantly slower than in bulk semiconductors because of the low probability of multiphonon processes (i.e., "phonon bottleneck"). 5,7 In reality, however, in quantum dots there exist alternative relaxation mechanisms efficient enough to cause subpicosecond carrier cooling that is not significantly slower than that in bulk materials. 8,9 In particular, hot electrons could relax rapidly by transferring energy to holes, which often have a greater effective mass and thus smaller energy spacing, through Auger-like processes followed by phonon-assisted relaxation 10,11 or nonadiabatic channels involving surface ligands.12,13 Trap states 14,15 in the quantum dots and highfrequency vibrational modes in surface ligands 16 provide additional relaxation pathways to promote rapid carrier cooling. It has been demonstrated that by carefully designing multiple layers of heterostructures around colloidal CdSe quantum dots and suppressing these pathways, the lifetimes of hot carriers could be increased by 3 orders of magnitude up to 1 ns.9 With this approach, electrons and holes were spatially separated by the heterostructures to reduce their energy transfer. Epitaxial growth of the heterostructures and use ...

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Breaking the Phonon Bottleneck for Holes in Semiconductor Quantum Dots

Cited by 197 publications

References 32 publications

Electronic Structure of Ligated CdSe Clusters: Dependence on DFT Methodology

Electronic Structure of Ligated CdSe Clusters: Dependence on DFT Methodology

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

Slow Hot-Carrier Relaxation in Colloidal Graphene Quantum Dots

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