Femtosecond<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>1</mml:mn><mml:mi mathvariant="italic">P</mml:mi></mml:math>-to-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>1</mml:mn><mml:mi mathvariant="italic">S</mml:mi></mml:math>Electron Relaxation in Strongly Confined Semiconductor Nanocrystals

Klimov, Victor I.; McBranch, D.

doi:10.1103/physrevlett.80.4028

Cited by 475 publications

(395 citation statements)

References 24 publications

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“…47,48 Spectrally resolved TA dynamics exhibit a photoinduced absorption feature on the low-energy side of the 1S exciton transient bleach feature, which has a lifetime that is dictated by intraband relaxation of an excited exciton into the 1S state. 30 This photoinduced absorption feature originates from Coulomb-interaction-induced shift of the 1S transition in the presence of a photoexcited exciton (the "biexciton shift" effect). 47,48 The amplitude of this feature normalized by the amplitude of the 1S exciton bleach directly relates to the magnitude of the shift (inset of Figure 4b) and, therefore, can be used to elucidate the X-X interaction energy.…”

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confidence: 99%

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

2007

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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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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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“…Hence, a direct comparison between carrier multiplication in bulk and quantum dots requires assessment of bulk carriermultiplication efficiencies on a similar, ultrafast, timescale (see Supplementary Information). Whereas time-resolved optical and infrared spectroscopies are ideally suited to probe carrier populations in colloidal quantum dots 2,5,7,8,10,11,14,15,[17][18][19][22][23][24] , light of terahertz frequencies interacts strongly with free carriers and allows for the direct characterization of carrier density and mobility [25][26][27] . Here, we quantify carrier multiplication in bulk PbSe and PbS on ultrafast timescales using terahertz time-domain spectroscopy 26 (THz-TDS).…”

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Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

Pijpers¹,

Ulbricht²,

Tielrooij³

et al. 2009

Nature Phys

257

362

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One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells 1 . Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions 1-3 . Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carriermultiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.Carrier multiplication is the process in which the absorption of a single, high-energy photon results in the generation of two or more electron-hole pairs. The excess energy of the initially excited electron is used to excite a second electron over the bandgap, rather than being converted into heat through sequential phonon emission. Carrier multiplication is important for the operation for high-speed electronic devices 4 , but is especially relevant for solar cells 1 , because relaxation of hot carriers through phonon emission is a common loss mechanism in bulk semiconductor solar cells. In this context, semiconductor quantum dots are promising building blocks for future solar cells 1 . In addition to the size-tunability of the quantum-dot optical properties, the carrier-multiplication efficiency in quantum dots was reported to be much higher than in bulk materials, where the process is generally referred to as impact ionization. It has been argued that carrier multiplication is more efficient in nanostructured semiconductors owing to quantum-confinement effects causing (1) a slowing of the phonon-mediated relaxation channel 1 and (2) enhanced Coulomb interactions 2 , resulting from forced overlap between wavefunctions and reduced dielectric screening at the quantum-dot surface 3 . In recent years, several femtosecond spectroscopy studies have revealed highly efficient carrier multiplication in PbSe and PbS (refs 2, 5-9) (refs 18, 19) quantum dots. In initial studies, carrier-multiplication efficiencies may have been overestimated owing to several experimental complications, including too high excitation fluences (generating multiple carriers by sequential absorption of multiple photons), lack of stirring of quantum-dot suspensions (causing photo-induced charging) and sample-to-sample variability 19 . Furthermore, recent tight-binding calculations 20 suggest carrier multiplication in quantum dots is not only not enhanced relative to bulk, but is actually lower. Answering the...

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Femtosecond1P-to-1SElectron Relaxation in Strongly Confined Semiconductor Nanocrystals

Cited by 475 publications

References 24 publications

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

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

Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

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