Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals

Klimov, Victor I.

doi:10.1146/annurev.physchem.58.032806.104537

Cited by 893 publications

(1,463 citation statements)

References 80 publications

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“…The recombination of biexcitons can produce a subnanosecond decay component in absorption transients similar to the one shown for the untreated sample 33. In this experiment, biexcitons can be produced only by the absorption of two photons by a CQD, since the photon energy used is less than twice the band gap, i.e., below the threshold for MEG 26.…”

Section: Resultssupporting

confidence: 56%

Ultrafast Charge Dynamics in Trap‐Free and Surface‐Trapping Colloidal Quantum Dots

et al. 2015

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Ultrafast transient absorption spectroscopy is used to study subnanosecond charge dynamics in CdTe colloidal quantum dots. After treatment with chloride ions, these can become free of surface traps that produce nonradiative recombination. A comparison between these dots and the same dots before treatment enables new insights into the effect of surface trapping on ultrafast charge dynamics. The surface traps typically increase the rate of electron cooling by 70% and introduce a recombination pathway that depopulates the conduction band minimum of single excitons on a subnanosecond timescale, regardless of whether the sample is stirred or flowed. It is also shown that surface trapping significantly reduces the peak bleach obtained for a particular pump fluence, which has important implications for the interpretation of transient absorption data, including the estimation of absorption cross‐sections and multiple exciton generation yields.

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Section: Resultssupporting

confidence: 56%

Ultrafast Charge Dynamics in Trap‐Free and Surface‐Trapping Colloidal Quantum Dots

et al. 2015

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“…A photoinduced absorption that is redshifted from the 1S bleach is clearly observable and transient dynamics measured at the amplitude maxima of the bleach (B m ) and induced absorption (A m ) (shown in Figure 4b) show that the decay of the induced absorption feature corresponds with the appearance of the 1S bleach feature, typical of a biexciton shift. From analysis of the ratio A m /B m and the line width of the 1S transition, 47,48 we obtain a value of ∆ XX ) 16 meV, which is somewhat larger than the values reported in ref 9 but might be expected given that the value reported here is for a smaller size InAs NQD core than in prior reports. Thus, the biexciton binding energy in InAs NQDs studied here represents ∼1% of the 1S exciton energy and, therefore, will not produce a notable deviation of the CM onset below the expected threshold of ∼2E g .…”

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confidence: 74%

“…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. 47,48 Shown in Figure 4a is the TA spectrum measured for a pump delay that corresponds to the maximum transient signal amplitude (pump delay <1 ps) for an InAs-CdSe coreshell with a core diameter of 2.8 nm using low-intensity 1.77 eV excitation. A photoinduced absorption that is redshifted from the 1S bleach is clearly observable and transient dynamics measured at the amplitude maxima of the bleach (B m ) and induced absorption (A m ) (shown in Figure 4b) show that the decay of the induced absorption feature corresponds with the appearance of the 1S bleach feature, typical of a biexciton shift.…”

mentioning

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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“…As such, they are promising materials for a variety of electro-optic applications, such as LEDs and photovoltaic devices. Knowledge of the processes immediately following photoexcitation is therefore essential, and there has been much interest recently in exciton dynamics in QDs [see Klimov (2007), and references therein], especially in exciton decay, exciton cooling, multiexciton dynamics, and the possible formation of multiexcitons by carrier multiplication.…”

Section: Quantum Dotsmentioning

confidence: 99%

“…Many studies of carrier dynamics in semiconductor quantum dots have been performed using ultrafast optical spectroscopy (Klimov, 2007). Most of these studies, typically based on transient absorption or time-resolved fluorescence, have focused on interband transitions in the optical range.…”

Section: Quantum Dotsmentioning

confidence: 99%

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

et al. 2011

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Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals

Cited by 893 publications

References 80 publications

Ultrafast Charge Dynamics in Trap‐Free and Surface‐Trapping Colloidal Quantum Dots

Ultrafast Charge Dynamics in Trap‐Free and Surface‐Trapping Colloidal Quantum Dots

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

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

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