Four-wave-mixing imaging and carrier dynamics of PbS colloidal quantum dots

Masia, Francesco; Moreels, Iwan; Hens, Zeger; Langbein, Wolfgang Werner; Borri, Paola

doi:10.1103/physrevb.82.155302

Cited by 13 publications

(7 citation statements)

References 55 publications

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“…Measurements are taken for τ 23 = 2 ps to avoid nonresonant nonlinear effects at time overlap and in the HHVV polarization configuration of P 1 , P 2 , P 3 , and reference to suppress background from detector nonlinearities. 10 A change in τ 23 only changes the total FWM amplitude due to the decay of the density grating but does not modify the dynamics versus τ 12 (see dotted line in Fig. 2 for τ 23 = 100 ps).…”

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“…10 Furthermore, multicarrier effects, such as charged excitons or multiexcitons, give rise to a population decay on the 10-100-ps timescale due to Auger recombination, 14 which is not observed in the density dynamics. We found that such processes are important for time-averaged excitation intensities >1 KW/cm 2 A possible mechanism responsible for a fast dephasing of the ZPL in PbS CQDs is the phonon-assisted thermalization between the different fine-structure states of the excitonic ground state, which are split by intervalley scattering and exchange interaction and have different angular momentum projections.…”

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Exciton dephasing in lead sulfide quantum dots byX-point phonons

et al. 2011

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Using a sensitive transient four-wave mixing technique in heterodyne detection, we measured the ground-state excitonic dephasing in PbS colloidal quantum dots of 4-6-nm diameter in the temperature range from 5 to 100 K. We observe an ultrafast (<100 fs) initial decay followed by a component of about 300 fs at 5 K, which becomes faster with decreasing dot size and increasing temperature. This dynamics is attributed to pure dephasing of phonon-assisted transitions in the peculiar band structure of PbS, which allows coupling to phonons with wave vectors both near the Brillouin zone center and near the (100) zone edge, the X point. We estimate an upper limit of 7% for the weight of the zero-phonon line with 2.6-ps spin-flip limited dephasing. The dephasing time inversely proportional to the homogeneous linewidth of an optical transition is a fundamental physical quantity in light-matter interaction. It was recognized about a decade ago that semiconductor quantum dots (QDs), in which carriers are confined in all three directions, exhibit a peculiar coupling of excitons with phonons as a result of the local lattice distortion associated with the excitonic excitation.1,2 This coupling gives rise to an excitonic homogeneous line shape at low temperatures consisting of a Lorentzian zero-phonon line (ZPL) superimposed onto a broad band of phonon-assisted transitions and, in turn, a strongly nonexponential dephasing. 2Among the various types of semiconductor quantum dots, PbX (X = S, Se) colloidal QDs (CQDs) have attracted much interest in recent literature due to their emission in the near infrared and their peculiar band structure. The effective masses of electrons and holes in these system are similar and relatively small, resulting in an exciton Bohr radius (46 nm for PbSe and 18 nm for PbS) that is large compared to the typical QD sizes. Hence PbX dots are considered a model system for QDs with strong confinement. Moreover, both the maximum of the valence band and the minimum of the conduction band are at the L point of the Brillouin zone, resulting in a 64-fold-degenerate ground-state exciton compared to the eightfold degeneracy in spherical QDs with a direct band gap at the point (e.g., InAs, InP, CdS, CdSe). 3 Furthermore the exciton radiative lifetime is in the microsecond range, 3 considerably longer than in other QDs, opening the prospect of ultralong radiative-lifetime limited dephasing times.While the homogeneous line shape of excitons in III-V and II-VI QDs, epitaxially grown and colloidal, has been measured and calculated extensively in the literature, 1,2,4,5 to date there have been only few works on PbX CQDs. Low-temperature ensemble photoluminescence (PL) gave an upper limit of 30 meV for the homogeneous linewidth in PbSe CQDs. Single-dot PL spectroscopy, 1 commonly used to determine the homogeneous linewidth, is limited by the small radiative emission rate and the high detector noise in the near-infrared wavelength region of PbX emission. 7 Moreover, spectral diffusion, a variation of the QD transition f...

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Exciton dephasing in lead sulfide quantum dots byX-point phonons

et al. 2011

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“…[1][2][3][4] While most studies have focused on colloidal nanoparticles of lead sulfide, 5 work on isolated (gas phase) clusters of lead sulfide, (PbS) n , has been relatively scarce. Recently, in a combined experimental and theoretical study 6 we explored the geometric structures and electron affinities of gas-phase lead sulfide clusters, (PbS) n , ranging in size from n = 1-15.…”

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(PbS)32: A baby crystal

Kiran

Kandalam

Rallabandi

et al. 2012

The Journal of Chemical Physics

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Theoretical calculations based on density functional theory have found (PbS)(32) to be the smallest cubic cluster for which its inner (PbS)(4) core enjoys bulk-like coordination. Cubic (PbS)(32) is thus a "baby crystal," i.e., the smallest cluster, exhibiting sixfold coordination, that can be replicated to obtain the bulk crystal. The calculated dimensions of the (PbS)(32) cluster further provide a rubric for understanding the pattern of aggregation when (PbS)(32) clusters are deposited on a suitable surface, i.e., the formation of square and rectangular, crystalline nano-blocks with predictable dimensions. Experiments in which mass-selected (PbS)(32) clusters were soft-landed onto a highly ordered pyrolytic graphite surface and the resulting aggregates imaged by scanning tunneling microscopy provide evidence in direct support of the computational results.

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“…For these conditions, relaxation times in the QD GS as short as 2 ps have been reported. 19,47 With our quasi-resonant excitation with very little excess energy, the process dominating the carrier dynamics is an energetic rearrangement with the GS exciton manifold of the QDs.…”

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Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

et al. 2017

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For possible applications of colloidal nanocrystals in optoelectronics and nanophotonics, it is of high interest to study their response at low excitation intensity with high repetition rates, as switching energies in the pJ/bit to sub-pJ/bit range are targeted. We develop a sensitive pump-probe method to study the carrier dynamics in colloidal PbS/CdS quantum dots deposited on a silicon nitride waveguide after excitation by laser pulses with an average energy of few pJ/pulse. We combine an amplitude modulation of the pump pulse with phase-sensitive heterodyne detection. This approach permits to use co-linearly propagating co-polarized pulses. The method allows resolving transmission changes of the order of 10−5 and phase changes of arcseconds. We find a modulation on a sub-nanosecond time scale caused by Auger processes and biexciton decay in the quantum dots. With ground state lifetimes exceeding 1 μs, these processes become important for possible realizations of opto-electronic switching and modulation based on colloidal quantum dots emitting in the telecommunication wavelength regime.

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Four-wave-mixing imaging and carrier dynamics of PbS colloidal quantum dots

Cited by 13 publications

References 55 publications

Exciton dephasing in lead sulfide quantum dots byX-point phonons

Exciton dephasing in lead sulfide quantum dots byX-point phonons

(PbS)32: A baby crystal

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

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