Ultrafast Exciton Dynamics in CdSe Quantum Dots Studied from Bleaching Recovery and Fluorescence Transients

Abstract

We have performed ultrafast absorption bleach recovery and fluorescence upconversion measurements ( approximately 100 fs time resolution) for three CdSe samples, with nanoparticle diameters of 2.7, 2.9, and 4.3 nm. The two types of experiments provide complementary information regarding the contributions of the different processes involved in the fast relaxation of electrons and holes in the CdSe quantum dots. Transient absorption and emission experiments were conducted for the 1S [1Se-1S3/2(h)] transition, 1S(e) and 1S3/2(h) representing the lowest electron (e) and hole (h) levels. The bleach recovery of the 1S transition shows a approximately 400-500 fs initial rise, which is followed by a size-dependent approximately 10-90 ps decay and finally a long-lived (approximately ns) decay. The fluorescence upconversion signal for the 1S transition shows quite different temporal behavior: a two times slower rise time (approximately 700-1000 fs) and, when the fluorescence upconversion signal has risen to about 20% of its maximum intensity, the signal displays a slight leveling off (bend), followed by a continued rise until the maximum intensity is reached. This bend is well reproducible and power and concentration independent. Simulations show that the bend in the rise is caused by a very fast decay component with a typical time of about 230-430 fs. Considering that the 1S quantum dot excitation is comprised of five exciton substates (F=+/-2, +/-1L, 0L, +/-1U, and 0U), we attribute the disparity in the rise of the bleaching and emission transients to the results from the dynamics of the different excitons involved in respectively the bleaching and fluorescence experiments. More specifically, in transient absorption, population changes of the F=+/-1U excitons are probed, in emission population effects for the F=+/-2 ("dark") and the F=+/-1L ("bright") exciton states are monitored. It is discussed that the fast (approximately 400-500 fs) rise of the bleach recovery is representative of the feeding of the F=+/-1U exciton (by filling of the 1S(e) electron level) and that the slower (approximately 700-1000 fs) feeding of the emissive +/-2, +/-1L excitons is determined by the relaxation of the hole levels within the 1S3/2 fine structure. Finally, the approximately 230-430 fs component, typical of the bend in the fluorescence transient, is attributed to the thermalization of the close-lying +/-2 ("dark") and +/-1L ("bright") excitons.

Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles

“…In a QDSSC, hot electrons will only contribute to overall photocurrent if the rate constant of electron transfer is much greater than that of carrier cooling (k ET ≫ k cooling ). The rate at which carrier cooling occurs has been investigated previously using first excitonic peak rise times in ultrafast transient absorption measurements, and has been found to be subpicosecond in nature (23,(35)(36)(37). To demonstrate typical k cooling rates in the CdSe QDs utilized in this study, we synthesized 10 small batches of QDs with diameters ranging from 2.1 to 5.6 nm.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…These effects are beyond the scope of this review. The interested reader is referred to a number of publications addressing this topic in detail [36][37][38][39][40][41][42][43][44][45][46][47][48]. Phonons (i.e., lattice vibrations) have a pervasive role in semiconductors, and therefore coupling of charge carriers and excitons to phonons plays a decisive role in a wide range of properties [49].…”

“…The interaction between phonons and excitons in nanoscale semiconductors is expected to differ from that in bulk materials due to both quantum confinement effects on the exciton energy levels and dimensional confinement of phonon modes (i.e., the phonon wavelength cannot be larger than the NC size) [49]. Coupling of photogenerated carriers to phonons provides an important energy relaxation pathway, thus being essential to a number of photophysical processes in semiconductor NCs (e.g., exciton relaxation dynamics, carrier cooling, thermal transport) [42,[50][51][52][53]. Moreover, coupling to acoustic phonon modes determines the homogeneous linewidths of optical transitions [54,55], while coupling to optical phonon modes has been observed to relax selection rules at low temperatures, yielding distinct phonon-assisted transitions (the so-called phonon replicas) [56,57].…”

Optical Properties of Spherical Colloidal Nanocrystals

“…These properties have been investigated for the first time by Wang et al (Wang et al, 2006) who showed the carrier relaxation from bright to dark and surface defect states. Since such kind of relaxation is predicted to be faster than the natural radiative emission lifetime (less than 100 ps) the role of ± 1 U and ± 1 L bright intrinsic states (see Section 2 and Efros et al, 1996 for details) in presence of emitting surface states has been only postulated (Bawendi et al, 1992;Jungnickel & Henneberger, 1996).…”