Exciton Relaxation And Electron Transfer Dynamics Of Semiconductor Quantum Dots

Download or read book Exciton Relaxation and Electron Transfer Dynamics of Semiconductor Quantum Dots written by Cunming Liu and published by . This book was released on 2015 with total page 177 pages. Available in PDF, EPUB and Kindle. Book excerpt: "Quantum dots (QDs) exhibit unique electronic and optical properties arising from their three-dimensional confinement and strongly enhanced coulomb interactions. Developing a detailed understanding of their exciton relaxation dynamics is important not only for the sake of exploring the fundamental physics of quantum confinement processes, but also for their potential applications. Ultrafast transient absorption (TA) spectroscopy, which has been proven as a powerful tool to explore the relaxation dynamics of excitons, was employed to characterize the hot single/multiexciton relaxation dynamics at the first four exciton states 1Se-1S3/2 (1S), 1Se-2S3/2 (2S), 1Pe-1P3/2 (1P) and 1Se-2S1/2 (2SH) of CdSe/CdZnS QDs. We found all exciton states can accomplish their initialization in sub-ps timescales. And, the 1S single exciton mainly relaxes to surface trapping state with a lifetime of 0.90 ł 0.02 ns. A 74 ł 2 ps Auger recombination occurs to nonradiatively annihilate 1S multiexctions under higher pump fluence. We also observed for the first time that the 2S hot hole can relax via two possible pathways-intraband multiple phonon coupling (2S3/2-1P3/2 or 2S3/2-1S3/2) and the intrinsic defect trapping, with a lifetime of 7 ł 2 ps. Owing to the overlap between highly energetic 1P and 2SH exciton states, their probed dynamics both show the characteristics of 1P and 2SH exciton states. Surprisingly, an ultra-short component of 8 ł 1 ps, directly associated with the Auger recombination at the 1P exciton state, was discovered under higher pump fluence. After finishing the studies of exciton relaxation inside QDs, ultrafast TA spectroscopy was further applied to explore the electron transferring outside from QDs. By employing a novel photocatalytic system consisting of water-soluble CdSe QDs and Ni-dihydrolipoic acid (Ni-DHLA) catalyst, which has represented a robust photocatalysis of H2 from water, the photoinduced electron transfer (ET) dynamics between QDs and catalyst, one of most primary steps during H2 generation, was studied. We found that smaller bare CdSe QDs exhibit a better ET performance with an amplitude-weighted average lifetime of 69 ł 2 ps. And, nearly ~90% of TA signal in the presence of Ni-DHLA can be attributed to the ET process. CdS shelling on the bare CdSe QDs worsens the ET between QDs and Ni-DHLA catalysts. Under higher pump fluence, the ET performance with CdSe/CdS core-shell QDs is improved because of the Auger recombination in CdSe cores strongly suppressed by CdS shelling, which is contrary with bare CdSe QDs. These studies may supply potential ways to optimize the QDs sensitized photocatalytic systems for H2 production. Finally, by virtue of effective mass approximation (EMA) method and Marcus theory, the mechanism of ET between CdSe QDs and Ni-DHLA catalysts was clarified in theory. Additionally, photoluminescence (PL) quenching and time-resolved PL measurements were used to demonstrate that the main pathway for ET between QDs and Ni-DHLA catalysts is through dynamic collisions. As Ni-DHLA concentration increases, the static attachment between QDs and Ni-DHLA catalyst molecules can occur to further speed up the ET process from QDs to Ni-DHLA catalysts."--Pages vi-vii.