Controlling the spatial location of photoexcited electrons in semiconductor CdSe/CdS core/shell nanorods

It is commonly assumed that after an electron-hole pair is created in a semiconductor by absorption of a photon the electron and hole rapidly relax to their respective lowest-energy states before recombining with one another. In semiconductor heterostructure nanocrystals, however, intraband relaxation can be inhibited to the point where recombination occurs primarily from an excited state. We demonstrate this effect using time-resolved optical measurements of CdSe/CdS core/shell nanorods. For nanorods with large CdSe cores, an electron photoexcited into the lowest-energy state in the core remains in the core, and an electron photoexcited into an excited state in the CdS shell remains in the shell, until the electron recombines with the hole. This provides a means of controlling the spatial location of photoexcited electrons by excitation energy. The control over electron localization is explained in terms of slow relaxation into the lowest-energy electron state in the nanorods, on time scales slower than electron-hole recombination. The observation of inhibited relaxation suggests that a simple picture of band alignment is insufficient for understanding charge separation in semiconductor heterostructures.

Figure 1

Transient spectra for CdSe/CdS core/shell nanorods (NRs). (a) Spectra for NRs with 2-nm cores (nanorod volume $V$ $=$ 570 nm${}^3$, length $L$ $=$ 43 nm, diameter $D=4.1$ nm). (b) Spectra for NRs with 5-nm cores ($V$ $=$ 550 nm${}^3$, $L$ $=$ 38 nm, $D$ $=$ 4.3 nm). The bottom panels show spectra for excitation of the lowest-energy, quasicore state with a pump-photon energy of 2.30 eV (for 2-nm cores) or 2.03 eV (for 5-nm cores) and for pump-probe delay times of 0.5, 1, 20, and 100 ps. The top panels show spectra for excitation of the CdS shell with a pump-photon energy of 3.10 eV and a pump-probe delay time of 2.0 ns. Also shown for comparison are transient spectra for bare CdSe nanocrystals with the same dimensions as the cores of the NRs. The CdSe spectra have been shifted downwards in energy by 0.26 eV (for 2-nm cores) and by 0.04 eV (for 5-nm cores), for easier comparison to the bottom panels; these shifts correspond to the reduction in the CdSe transition energies in the NRs as compared to the bare CdSe particles, based on their linear absorption spectra. The portions of the transient spectra near the pump-photon energies are dominated by scattered pump light, and have therefore been excluded from the graphs.

Figure 2

Transient kinetics for CdSe/CdS core/shell nanorods following excitation of the CdSe cores, for NRs with (a) 2-nm and (b) 5-nm cores. Kinetics are shown for different probe-photon energies around the bleach of CdS and CdSe in the bottom panels of Fig. 1, and are normalized by their value at 1 ps. The fits are step functions convolved with the 150-fs instrument response function.

Figure 3

(a) Emission spectra of CdSe/CdS core/shell nanorods when exciting the CdSe cores with a pump-photon energy of 2.57 eV. The three spectra on the left are for nanorods with 5-nm cores, and the three on the right are for nanorods with 2-nm cores. The nanorod volumes $V$ are 890, 560, 80, 860, 550, and 60 nm${}^3$, from the left to the right. (b) Photoluminescence decay rates as a function of emission energy, for the same nanorods as in (a). Decay rates are estimated as the inverse of the $1/e$ decay time of the time-resolved photoluminescence signal. Symbols are experimental data and solid lines are guides for the eye. The error bars represent 10$\%$ uncertainty.

Figure 4

Calculated electron densities for the two lowest-energy electron states, E1 and E2, in CdSe/CdS core/shell nanorods with (a) 2-nm and (b) 5-nm cores. Red indicates high density and blue indicates low density. The locations of the cores are indicated with white or black outlines. Also illustrated are the band diagrams of the nanorods and the processes involved in excitation into different electron states.

Figure 5

Calculated spectra for CdSe/CdS core/shell nanorods with (a) 2-nm and (b) 5-nm cores. The corresponding band structure diagrams and transitions are shown schematically in (c) and (d), respectively. Only the transitions from hole states to the two lowest-energy electron states, E1 or E2, are calculated. In (a) and (b), the thin black lines correspond to transitions calculated without considering electron-hole interactions, and the thick blue lines show the transitions shifted by estimated exciton binding energies. The red lines are calculated by broadening the shifted spectra by values corresponding to the luminescence linewidths. The dotted black lines are representative experimental transient-absorption spectra, reproduced from Fig. 1, for comparison to the theoretical spectra. The short dashed lines in (c) and (d) indicate that certain hole states represented by the long dashed lines actually correspond to groups of several, closely spaced states.