Ultrafast Exciton Dynamics in CdTe Nanocrystals and Core/Shell CdTe/CdS Nanocrystals

CdTe and CdTe/CdS core/shell nanocrystals (NCs) have been synthesized, structurally characterized, and studied using steady-state and transient optical methods. Time-resolved photoluminescence (PL) and transient absorption (TA) decays both showed exciton lifetimes in the tens of nanoseconds. The TA spectra of the CdTe NCs showed multiple bleaches, which have been assigned to the 1S(3/2)(h)-1S(e), 2S(3/2)(h)-1S(e), and 1P(3/2)(h)-1P(e) transitions. The spectral shifts of these bleaches with NC size and after shell deposition have been analyzed in the context of a quasi-type-II carrier distribution in the core/shell samples. The measured energy level spacings show clear evidence for electron delocalization and strong core-confinement of the hole in the core/shell NCs. In addition, the ultrafast evolution of these bleach features has been examined to extract electron cooling rates. For core and core/shell NCs of similar band gap, the arrival time of the electron in the 1S(e) level is approximately 40% longer for the core/shell than for the core NCs. The difference is attributed to reduced Auger rates in the core/shell NCs, where the electron and hole are spatially separated. Analysis of TA dynamics of CdTe cores also provides evidence for ultrafast hole trapping with a size-dependent rate constant. An additional bleach is observed in the core/shell samples at the blue end of the visible spectrum and is tentatively assigned to state filling of the lowest energy CdS transition due to penetration of the electron into the CdS conduction band. These observations provide a means to study carrier-resolved dynamics in these NCs using only the visible TA data.