Structure of Excited-State Transitions of Individual Semiconductor Nanocrystals Probed by Photoluminescence Excitation Spectroscopy

Htoon, Han; Cox, P.J.; Klimov, Victor I.

doi:10.1103/physrevlett.93.187402

Cited by 50 publications

(42 citation statements)

References 15 publications

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“…Indeed, the levels close to the emission energy retain the indirect nature of the bulk 25 -1 band gap (only ∼10 −3 admixture of the component), while at higher energies a strong intermixing of X and states occurs (up to 30%). This situation is different from direct band-gap quantum dots, where strong direct band-gap related absorption peaks are located right next to the emission line [12][13][14].…”

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confidence: 69%

“…So far only ensemble studies were performed on the absorption spectrum of Si nanocrystals by photoluminescence excitation (PLE) or transmission methods [10,11], preventing us from observing single Si nanodot features. PLE of individual quantum dots was demonstrated for direct band-gap materials [12][13][14], but it is much more difficult to perform on single Si nanocrystals due to their low emission rate, stemming from ∼μs exciton lifetimes [15]. At the same time, understanding the electronic structure of Si nanocrystals * Corresponding author: ilyas@kth.se relevant for light absorption is central to their application as phosphors [16], biolabels [17], sensitizers [18], downshifters [19], or photon multipliers [20].…”

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confidence: 99%

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Single-dot absorption spectroscopy and theory of silicon nanocrystals

et al. 2016

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confidence: 69%

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confidence: 99%

Single-dot absorption spectroscopy and theory of silicon nanocrystals

et al. 2016

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“…The low concentration of 6; 4 tubes in our samples of less than 1 per 500 m 2 makes it highly unlikely that the emission lines result from more than a single 6; 4 tube. This indicates that spectral variations are caused by local changes in the dielectric function along individual tubes [9,10].…”

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confidence: 97%

“…Ambiguities arising from the interpretation of ensemble measurements with contributions from different tube species, nonresonant optical transients, as well as the sample heterogeneity regarding length and impurity distributions thus call for an investigation on the single tube level. Single tube PL measurements avoid the averaging of decay dynamics in ensembles of different structures n; m and within subensembles of tubes having the same n; m for which nonuniform emission properties have been observed for micelle encapsulated tubes [6,9,10]. Similarly, a distribution of excited state lifetimes and PL quantum yields may be expected even for a single tube species.…”

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Exponential Decay Lifetimes of Excitons in Individual Single-Walled Carbon Nanotubes

et al. 2005

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The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same n; m type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a lowfrequency phonon mode.

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“…3, which shows hole cooling times inferred from THz data as a function of hole excess energy, for electronic excitation into the cold 1S e and hot 1P e states. These relaxation times describe the rate at which the lowest energy hole state is occupied, limited by the slowest (final [27]) step in the overall relaxation process.…”

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Direct Observation of Electron-to-Hole Energy Transfer in CdSe Quantum Dots

et al. 2006

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We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots. Time-resolved luminescence and terahertz spectroscopy reveal that the rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes the first direct, quantitative measurement of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 0:15 ps time scale. DOI: 10.1103/PhysRevLett.96.057408 PACS numbers: 78.67.Hc, 65.80.+n, 73.21.La, 73.22.Dj Semiconductor quantum dots (QDs) exhibiting strong quantum confinement of electrons provide a rare opportunity to study the fundamental properties of electronic excitations in a size regime between the atomic and bulk limits. Electron confinement gives rise to discrete energy levels [with (1S e ; 1P e ; . . . ) electron and (1S 3=2 ; 1P 3=2 ; . . . ) hole states] [1], rather than the continuum of states present in bulk semiconductors. In both bulk and QD semiconductors, absorption of a photon generates ''hot'' charges with energies in excess of the band edge. While in bulk materials cooling occurs readily through sequential one-phonon emission, the electron energy level spacing in QDs (typically hundreds of meV) is large compared to typical longitudinal optical phonon frequencies (25 meV), and electron cooling via coupling to phonons is expected to be slow. Accordingly, it has been proposed that cooling in QDs is hindered by a so-called ''phonon bottleneck'' [2], though cooling rates comparable to those in bulk have been observed [3][4][5][6], suggesting that other effects may prevail. The measurement and understanding of the decay dynamics of the different electron states is also of technological importance: QDs are increasingly finding applications as the active component in single-photon emitters, lightemitting diodes [7], photovoltaic cells [8,9], lasers [10], and photon up-converters; for all these applications, knowledge and manipulation of the decay dynamics of the hot and cold electron-hole (exciton) states is a prerequisite.In the prototypical case of CdSe QDs, the radiative lifetime of the lowest exciton 1S 3=2 1S e cold state has been studied extensively by time-resolved luminescence measurements [11]. However, when the hot 1P 3=2 1P e exciton is generated through optical excitation, this state is not observed in the emission spectrum [1,11]. This means that nonradiative processes, i.e., electron cooling (1P e ! 1S e ) and hole cooling (1P 3=2 ! 1S 3=2 ), compete effectively with radiative decay of the hot exciton. Thus, fast electron and/or hole cooling processes must occur, in contradiction with the predicted phonon bottleneck. To explain the apparent fast cooling, an Auger-like mechanism has been proposed [5,6,12 -14], in which efficient transfer of energy from electrons to holes occurs followed by relatively fast hole relaxation through the more closely spaced valence levels [15]. Other explanations...

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Structure of Excited-State Transitions of Individual Semiconductor Nanocrystals Probed by Photoluminescence Excitation Spectroscopy

Cited by 50 publications

References 15 publications

Single-dot absorption spectroscopy and theory of silicon nanocrystals

Single-dot absorption spectroscopy and theory of silicon nanocrystals

Exponential Decay Lifetimes of Excitons in Individual Single-Walled Carbon Nanotubes

Direct Observation of Electron-to-Hole Energy Transfer in CdSe Quantum Dots

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