Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals

Klimov, Victor I.

doi:10.1021/jp9944132

Cited by 959 publications

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“…Such derivative-like behavior around the peak of the 1S bleach indicates the influence of the biexciton effect, whereby the initial photoexcitation induces a red-shift in the absorbing state seen by the probe photon. As previously observed for CdSe QDs, 17 the recovery of the PA observed at red-shifted probe wavelengths correlates to the cooling time from higher excited states to the 1S h -1S e state; thus, the biexciton effect appears to be strongest for higher excited states than for completely relaxed band-edge excitons. The biexciton effect obscures the pure state-filling dynamics, and thus may complicate the direct observation of the quantum beating predicted by the coherent superposition model.…”

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confidence: 79%

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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confidence: 79%

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

et al. 2005

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“…In particular, the measured biexciton recombination times, 4 attributed to Auger processes, are one order of magnitude faster in InSb CQDs than observed [5][6][7] and predicted 8 in materials such as CdSe, whereas the electron cooling times, also attributed to Auger decay, are in line with those commonly found in nanostructures. 5,6,[8][9][10][11][12] These observations raise the question of whether, unlike in the case of other materials, different (intra-band and inter-band) non-radiative decay processes may be governed by different (i.e., Auger and non-Auger) mechanisms in InSb CQDs. (The unexpectedly small interlevel energy spacings within the conduction band of InSb CQDs resulting from very recent scanning tunneling spectroscopy and atomic force microscopy measurements 13 also remain unexplained.…”

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confidence: 80%

Exciton Dynamics in InSb Colloidal Quantum Dots

Sills

Harrison

Califano

2015

J. Phys. Chem. Lett.

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Extraordinarily fast biexciton decay times and unexpectedly large optical gaps are two striking features observed in InSb colloidal quantum dots that have remained so far unexplained. The former, should its origin be identified as an Auger recombination process, would have important implications regarding carrier multiplication efficiency, suggesting these nanostructures as potentially ideal active materials in photovoltaic devices. The latter could offer new insights into the factors that influence the electronic structure, and consequently the optical properties, of systems with reduced dimensionality, and provide additional means to fine tune them. Using the state-of-the-art atomistic semiempirical pseudopotential method we unveil the surprising origins of these features and show that a comprehensive explanation for these properties requires delving deep into the atomistic detail of these nanostructures and is, therefore, outside the reach of less sophisticated, albeit more popular, theoretical approaches.

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“…In the case when multiexcitons are generated at high pump fluences via a traditional mechanism of sequential absorption of multiple photons, it is important to account for nonlinearity of the 1S TA response as a function of the average number of excitons per NQD. 23 However, when multiexcitons are produced via CM, the nonlinearity is insignificant if the average exciton multiplicity (defined as QE/100%) does not exceed the degeneracy of the 1S level. 6 The latter was the case in the present experiments, in which the measured QEs did not exceed 200%.…”

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confidence: 99%

“…The transient absorbance (-∆R) divided by the ground-state absorbance (R 0 ) exhibits initial linear growth with increasing pump intensity until the calculated number of photogenerated electron-hole pairs (excitons), N eh , begins to exceed 1 (the absorption cross section is calculated 22,23 to be σ 1.55 eV (cm 2 ) ) 8.9 × 10 -17 [r(nm)] 3 ). For N eh > 1, -∆R/R 0 begins to saturate at a value of ∼0.5.…”

mentioning

confidence: 99%

“…For N eh > 1, -∆R/R 0 begins to saturate at a value of ∼0.5. Saturation is typical of other NQD materials and is attributed to filling of the 1S electron state, 23,25,28 which is characterized by 2-fold degeneracy for InAs NQDs. The saturation value of |∆R/R 0 | is theoretically expected to approach 1, but often does not, possibly as a result of photoinduced absorption.…”

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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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Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals

Cited by 959 publications

References 60 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

Exciton Dynamics in InSb Colloidal Quantum Dots

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

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