Exciton dephasing in lead sulfide quantum dots byX-point phonons

Abstract

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...

Measurement of electronic splitting in PbS quantum dots by two-dimensional nonlinear spectroscopy

“…After 100 fs, the 2D spectrum does not show significant changes for at least 3 ps, the maximum population time measured. This timescale is consistent with recent exciton dephasing measurements of 4-6 nm diameter PbS quantum dots at 5 K using heterodyne-detected transient grating (TG) 21 . The physical mechanism leading to the ultrafast initial decay of the TG signal was attributed to phonon-assisted optical transitions.…”

Quantum beats due to excitonic ground-state splitting in colloidal quantum dots

“…The signal decayed as exp(−4τ /T 2 ) with T 2 ∼ 440f s. From the measured dephasing time T 2 a homogeneous linewidth of ∼ 3 meV can be obtained, in agreement with previous measurements 13 . Low energy acoustic phonons can efficiently scatter excitons in the highly degenerate spin states within the manifold in PbS quantum dots and are generally thought to be the mechanism behind the rapid dephasing in colloidal quantum dots [13][14][15][16][17][18] .…”

Coherent Spectroscopy of Multiple Excitons in Colloidal Nanocrystal Quantum Dots

“…The widely used technique to measure coherent properties of excitons is a four‐wave mixing technique that employs nonlinear optical responses . The conventional coherent spectroscopy has been usually employed at cryogenic temperatures.…”

“…The widely used technique to measure coherent properties of excitons is a four-wave mixing technique that employs nonlinear optical responses. [45][46][47][48][49][50][51][52][53][54][55][56][57] The conventional coherent spectroscopy has been usually employed at cryogenic temperatures. In view of the actual operating temperatures of QD devices, i. e., near room temperature, [20,21,58,59] the conventional coherent spectroscopy is not suitable to measure the excitonic coherent properties in colloidal QDs at room temperature.…”