Spatio-temporal dynamics of coupled electrons and holes in nanosize CdSe-CdS semiconductor tetrapods

We report on coupled electron-hole transfer, Coulomb drag, in CdSe/CdS semiconductor nanotetrapods. We demonstrate that photoexcited holes can either be transferred to the CdSe core or become trapped in one of the CdS arms. By combining time-resolved pump-probe and photoluminescence measurements we investigate how the Coulomb potential drags the electron to the hole localization site. As supported by effective-mass calculations taking into account Coulomb effects we conclude that the hole dynamics determines the fate of the electron in a coupled dynamics.

Figure 1

(Color online) Calculated electron and hole wave functions in CdSe/CdS tetrapods in the presence of Coulomb interactions for different states of the hole. (a) Energy diagram plotted over a spatial coordinate going along the axis of one of the tetrapod arms. The arrows indicate the real-space dynamics of the electron and hole to the core (red arrow) or to a trapping site (blue arrow). (b) Electron and hole wave functions at the band edge of the structure with the hole confined in the CdSe. (c) Electron and hole at the band-edge states of CdS before relaxation of the hole to the CdSe core. (d) The hole wave function is localized in a low-energy trapping site in one of the CdS arms, resulting in the localization of the electron wave function to the same arm.

Figure 2

(Color online) [(a) and (b)] Transient differential transmission and [(c) and (d)] photoluminescence decays for S1 (left column) and S2 (right column) at $T=5\text{K}$. [(a) and (b)] Rise and decay of charge-carrier populations monitored by the normalized bleaching of CdS states (blue) and the bleaching of the CdSe core states (red). [(c) and (d)] Normalized PL emission transients measured at 1.9 eV (red) for S1 and S2, respectively. The dashed line in (c) and (d) shows the instrument response in time. The respective insets show the steady-state PL spectra. The weak CdS emission of S2 has been rescaled by a factor of ten.

Figure 3

(Color online) Low-temperature photoluminescence spectrum of a single S2 tetrapod showing emission from the CdSe (1.9 eV) and CdS (2.64 eV). The inset illustrates polarization anisotropy measured in emission recording the CdS PL as a function of the angle of rotation of a polarizer.