Engineering the spin-flip limited exciton dephasing in colloidal CdSe/CdS quantum dots

Abstract

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin-flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin-flip mechanism. By reducing the electron-hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ∼110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied.

Figure 1

Left: Absorption spectra at room temperature of the synthesized CdSe/CdS QDs of different core diameters as indicated, having a 0, 2, or 9 ML shell thickness. Right: Transmission electron microscopy images of the corresponding samples with 4.6 nm core. Scale bar: 10 nm. Sketches of the core (red) and shell (green) structure are given for illustration.

Figure 2

Measured time-integrated FWM field amplitude versus delay between the first two pulses for different temperatures for the 5.7 nm core 2 ML shell CdSe/CdS QDs (top left) and at 5 K for different samples (bottom), as indicated. The dashed lines are exponential fits to the data. The dephasing time inferred from the longest decay component in the 5.7 nm core 9 ML shell sample is also indicated. Top right is a sketch of the three-beam FWM directional geometry.

Figure 3

ZPL weight Z and homogeneous line width fwhm of the ZPL (γ_ZPL) and of the acoustic phonon band (Γ_ph) versus temperature in all investigated samples. The inset is a sketch of the lower bright–dark exciton relaxation model. The lines onto γ_ZPL are fits to the data (see text).

Figure 4

Top: Exciton density dynamics measured by TI-FWM versus τ₂₃ at τ₁₂ = 0 on the 5.7 nm core 2 ML shell sample. Dashed lines are fits to the data (see text). The inset shows the decay rates inferred from the first two time constants in the multiexponential dynamics and the corresponding temperature activated fits. Bottom: Time-resolved PL dynamics versus temperature on the same sample. The inset shows the exciton recombination rate deduced from the long exponential PL decay, together with the calculated thermal average of the radiative recombination within the excitonic fine structure. Filled symbols show the recombination rate for the 5.7 nm core 9 ML thick shell QDs.

Figure 5

Left: TI-FWM field amplitude versus τ₂₃ for different τ₁₂ for the 5.7 nm core 2 ML shell sample at 10 K. The ratio between the dynamics at τ₁₂ = 1 ps and the dynamics at τ₁₂ = 0 is also shown (dotted line) together with its fit (dashed line). Right: Additional time constant τ_s (squares) inferred from the dynamics at τ₁₂ ≠ 0 is compared with the measured ZPL dephasing time on different samples as indicated. The relaxation time constant τ_rel deduced from τ_s is also shown (see text).