Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots

Klimov, Victor I.; Mikhailovsky, Alexander; McBranch, D.; Leatherdale, C. A.; Bawendi, Moungi G.

doi:10.1126/science.287.5455.1011

Cited by 1,371 publications

(1,872 citation statements)

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“…The average number of photons absorbed per QD follows Poisson statistics such that N eh ) J p σ a , where J p is the photon fluence and σ a is the absorption cross section at the excitation photon energy. 23 To study MEG resulting from absorption of a single photon, we maintain a constant N eh value ensuring that the vast majority of photoexcited QDs have absorbed just one photon (e.g., for N eh ) 0.25, < 14% of photoexcited QDs absorb more than one photon). The details describing how we determine the MEG QYs from the TA data are presented in the online Supporting Information.…”

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confidence: 99%

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

et al. 2005

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We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.

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Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

et al. 2005

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“…In other NQD materials in which Auger recombination is well-established, a direct relationship between biexciton lifetime (τ XX ) and NQD volume has been observed. 1,23,24 Thus, to establish that the fast relaxation seen in TA dynamics at high pump intensity is due to Auger recombination, we measured pump-power dependent TA dynamics for a variety of InAs NQD sizes. However, because the utilized synthetic route to InAs NQDs provides undesirably broad NQD size/shape distributions (∼13% size dispersion is typical), which introduces uncertainty in the NQD dimensions, here we consider the relation of biexciton lifetime to the NQD confinement energy E c ) E g (NQD) -E g (bulk).…”

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Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

2007

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Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron−hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2E g ). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III−V semiconductors should exhibit a CM threshold near the apparent 2E g limit. Further, we discuss the possibility of achieving sub-2E g CM thresholds through strong exciton−exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2E g .Carrier Multiplication Thresholds in Bulk and Nanocrystalline Semiconductors. It has recently been established that carrier multiplication (CM), the production of multiple excitons following absorption of a single photon of sufficient energy, is efficient within the range of solar photon energies in semiconductor nanocrystal quantum dots (NQDs). [1][2][3][4][5][6][7][8][9] This finding is important because CM can potentially provide increased power conversion efficiency in low-cost, singlejunction photovoltaics via an enhanced photocurrent. 10,11 To obtain optimum photovoltaic power conversion efficiency using a single semiconductor material with energy gap E g , it is desirable that the CM onset energy is minimal and that the CM efficiency above the onset is as large as is allowed by energy conservation.According to energy conservation, the apparent minimal photon energy that is required to activate CM (pω CM ) is simply 2E g . However, the CM thresholds observed in bulk materials are significantly larger than this value. 12,13 In bulk semiconductors, the generally accepted mechanism for multiexciton generation is impact ionization in which a highenergy electron (or a hole) transfers its energy to a valenceband electron that is excited across the energy gap. 14 The CM threshold in this case is determined not only by conservation of energy but also by conservation of translational momentum, which increases the threshold above 2E g . For example, using a free-particle approximation 15,16 and applying both energy and momentum conservation laws, one can obtain that the CM threshold is approximately 4E g (Figure 1a).In NQDs, translational momentum conservation is relaxed. Therefore, one might expect that the CM threshold could reach the 2E g limit as defined by energy conservation. However, available experimental data indicate that pω CM is still greater than twice the energy gap in such materials. For example, in PbSe and PbS NQDs, it ...

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“…[9][10][11][12][13][14][15][16] Auger effects have been invoked to explain fluorescence and bleaching dynamics in ultrafast spectroscopy measurements. [17][18][19][20] Ionization processes are often referred to in the context of low densities and slow dynamics, such as the reversible PL quenching of single NQDs, manifested by "off" periods in the fluorescence under continuous-wave excitation. 8,21,22 However, the effect of the fast Auger recombination on the comparatively slow ionization processes and the implication on the PL dynamics of NQDs have not been treated so far, neither experimentally nor theoretically.…”

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“…To experimentally observe this effect, we combine time-resolved PL, which has previously been described in the framework of Auger autoionization, 10,13 and high-density excitation above the one e-h population threshold, the effects of which have thus far only been considered on ultrafast time scales using PL upconversion and transient absorption. 17 The quantum dots investigated in this study are CdSe/ZnS core/shell nanocrystals stabilized by hexadecylamine/trioctylphosphine/trioctylphosphine oxide ligands. The NQDs are of three different sizes, with mean core radii of 1.25, 1.55, and 2.45 nm.…”

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Interplay between Auger and Ionization Processes in Nanocrystal Quantum Dots

Kraus¹,

Lagoudakis²,

Müller³

et al. 2005

J. Phys. Chem. B

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We study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density. Our results suggest that Auger recombination accelerates ionization processes that lead to the occupation of dark, nonemissive nanocrystal states. A model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.The optoelectronic properties of semiconductor nanocrystal quantum dots (NQDs) bear great relevance to fundamental research in semiconductor physics. 1,2 Moreover, the tunability of their photoluminescence (PL) from ultraviolet to near-infrared wavelengths and the large absorption cross-section under nonresonant excitation render these nanostructures promising elements of future applications, e.g., in light-emitting diodes, biophysical fluorescence markers, and for biomedical sensitization purposes. [3][4][5] It is therefore important to understand the nature of spontaneous emission and competing nonradiative decay channels in these materials.The most significant nonradiative processes that reduce the quantum efficiency of NQDs are thermal ionization or tunneling of a carrier to a trap state and Auger effects. In semiconductor nanostructures with strong quantum confinement, the Auger effects are greatly enhanced by increased carrier-carrier interactions, 6 whereas thermal ionization is usually hindered by the increased spacing between successive energy states. The energy released from an Auger recombination process is given to another carrier in a single scattering event, usually with the participation of a phonon, whereas thermal ionization is a multiscattering process of a carrier with several phonons. 7 In the case of Auger autoionization or other ionization processes, the excess of a charge carrier in the NQD quenches the radiative recombination of the remaining electron-hole (e-h) pairs. 8 The general importance of Auger effects and ionization processes on the fluorescence and photoinduced absorption dynamics of NQDs has been discussed in several studies using different timeresolved techniques and rather different excitation densities. [9][10][11][12][13][14][15][16] Auger effects have been invoked to explain fluorescence and bleaching dynamics in ultrafast spectroscopy measurements. [17][18][19][20] Ionization processes are often referred to in the context of low densities and slow dynamics, such as the reversible PL quenching of single NQDs, manifested by "off" periods in the fluorescence under continuous-wave excitation. 8,21,22 However, the effect of the fast Auger recombination on the comparatively slow ionization processes and the implication on the PL dynamics of NQDs have not been treated so far, neither experimentally nor theoretically.In this letter, we study the correlation between ion...

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Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots

Cited by 1,371 publications

References 15 publications

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

Interplay between Auger and Ionization Processes in Nanocrystal Quantum Dots

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