Ultrafast Electron−Hole Dynamics in Core/Shell CdSe/CdS Dot/Rod Nanocrystals

Abstract

We investigated the transient bleaching and absorption of the asymmetric core/shell CdSe/CdS nanorods using the pump-probe technique. We observed ultrafast carrier relaxation and identified hole localization dynamics with 650 +/- 80 fs time constant. Upon pumping the CdSe core, we found an intense bleaching signal in the CdS spectral region, which we assigned to the delocalization of the electronic wave function on the basis of envelope-function theoretical calculations.

Luminescence Studies of Individual Quantum Dot Photocatalysts

“…Visible light is absorbed by the system, creating electron wave functions that are delocalized throughout the CdS rod and hole wave functions that are three-dimensionally confined to the CdSe quantum dot (with fast hole localization, in ∼650 fs). 9 When a Pt tip is added, the route of electrons transfer from the seeded rod to Pt competes effectively enough with the route of radiative emission. Consequently, with this design, the electrons are separated from the holes via three distinct components and by a tunable physical length and are subsequently available for reductive processes.…”

Low Threshold, Amplified Spontaneous Emission from Core‐Seeded Semiconductor Nanotetrapods Incorporated into a Sol–Gel Matrix

“…[ 22 ] Conversely, for larger CdSe cores possessing a Type I band alignment with the CdS arms, stimulated emission in the CdSe core rapidly depletes the population of excitons photogenerated in CdS since both stimulated emission and CdS carrier relaxation into the CdSe core are extremely fast processes. [ 23 ] Given that the achievement of stimulated emission in the CdSe core occurs at the expense of exciton buildup in the CdS shell, simultaneous ASE from both moieties is decidedly diffi cult, and our many attempts to achieve concurrent dual wavelength ASE from both the core and arms were without success. While an independent detailed study of the complex exciton dynamics of manybody interactions between multiple excitons generated in the CdSe core and CdS arms is clearly warranted here, it is foreseeable that large-core CdSe seeded CdS tetrapods with suffi ciently long arms such that exciton relaxation to the CdSe core does not extend across a signifi cant fraction of the arm length may exhibit dual wavelength ASE from both the core and its arms, provided other requirements on parameters such as quantum yield and volume fraction are satisfi ed.…”

Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states

“…Secondly, the absorption cross section of the CdS arms in the ultraviolet (UV) spectrum, where optical excitation occurs, is over 300 × larger than that of CdSe. 32 Efficient relaxation of excitons generated in the arms to the core therefore enables the generation of very high excitation densities within the cores, [33][34][35] leading to the formation of emissive multiexciton states. [35][36][37][38][39] In addition, the greater level of symmetry of the tetrapods compared to nanorods suggests that separating electron and hole of the exciton should be more facile in the former than in the latter: Only nanorods with preferential orientation in the electric field will allow exciton storage, whereas for the tetrapods, all electric field orientations should lead to carrier separation.…”

“…This observation further indicates that the quenching induced by carrier separation requires trapping of the excited carriers in localized states on timescales that exceed those of ultrafast carrier thermalization and multiexciton recombination. 33,35 Consequently, exciton storage at these field strengths is not an instantaneous electrostatic effect, in contrast to, for example, the quantum-confined Stark effect. 50 Rather, the external field promotes the ultimate localization of carriers to long-lived nonradiative states without drastically changing the ultrafast thermalization dynamics.…”