Interfacial Electron Transfer from Single CdSe/ZnS Quantum Dots to TiO2 Nanoparticles: Linker-Length Dependence at the Single-Molecule Level
Date Issued
2012
Date
2012
Author(s)
Chang, Yu-Pin
Abstract
Single molecule fluorescence spectroscopy (SMS) which combined home-made confocal microscopy and time-correlated single photon counting (TCSPC) system was used to record the fluorescence trajectories and to calculate lifetimes of single semiconductor quantum dot (QD) at the single-molecule level. Electron transfer (ET) rate from single QD to TiO2 film via bifunctional linkers were obtained. CdSe/ZnS QDs were tethered to TiO2 film through bifunctional mercaptoalkanoic acid (MAAs), the interparticle distance of which is controlled by the linker length. The fabricated QD-TiO2 heterodimers provide a model system for the single-molecule exploration of photoinduced electron transfer between QDs and charge acceptors, which is essential process in quantum dot-sensitized solar cells (QDSSCs). The trajectories and lifetimes were obtained by SMS with an excitation of 448 nm pulsed laser. The differences of lifetimes provide strong evidence for chain-length-dependent ET efficiency. ET rate constants were determined to be 2.8x107, 1.9x107, and 3.5x106 s-1 for the chain-length of 1.5, 6.2 and 13.8 Ȧ, the quantum yield were also determined to be 67.4, 45.8 and 7.8 %. The results showed that ET rate constants decreased with length of linkers increased. We further calculate electron coupling elements (HDA) between QDs and TiO2 using the semiclassical Marcus expression. The HDA were determined to be 4.07, 3.82 and 1.58 cm-1 of three different chain lengths, and be close to our prediction.
We further tethered QDs by linkers with aromatic spacer. The results suggest that the presence of π orbitals accelerates ET rates between QDs and TiO2. We attributed the rapid ET rates to enhancement of electronic coupling strength.
Finally, we examined the size-effect on QD-MAA-TiO2 systems. The results suggest that the size-dependence of ET dynamics is still valid regardless of the existence of linkers.
Subjects
Single Molecule Spectroscopy
Single Quantum Dot
Titanium Oxide
Electron Transfer
Heterodimers
Fluorescence Lifetime
Type
thesis
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