Electron Extraction Dynamics in CdSe and CdSe/CdS/ZnS Quantum Dots Adsorbed with Methyl Viologen

Abstract

Semiconductor quantum dots (QDs) composed of multiple components are playing an important role in solar energy conversion as light harvesting materials. Electron extraction dynamics in CdSe and core/shell CdSe/CdS/ZnS colloidal QDs are studied by femtosecond transient absorption spectroscopy in this Article. Our study demonstrates that, in the presence of the commonly used electron acceptor, methyl viologen (MV2+), electrons in the 1S state of CdSe QDs can be effectively extracted with a time constant less than 150 fs. With regard to type I core/shell CdSe/CdS/ZnS QDs, 400 nm excitation will mainly populate the CdS first, due to its large absorption cross section at around that wavelength. Electrons from the conduction band of CdS then can be directly extracted by MV2+ before transferring to core CdSe. Therefore, MV2+ can serve as an efficient bridge to extract electrons from the shell of type I QDs. As compared to the bare QDs, core/shell QDs have slower charge separation and much slower recombination rates. Thus, the core/shell QDs are beneficial for designing solar cells.

Hot electron transfer in Zn–Ag–In–Te nanocrystal–methyl viologen complexes enhanced with higher-energy photon excitation

“…The obtained rise-time was shortened by the complex formation: the risetimes of ZAITe NCs and ZAITe NC-MV 2+ complexes, s rise (-ZAITe) and s rise (ZAITe-MV 2+ ), were 450 fs and 220 fs, respectively. Similar rise-time shortening was reported for CdSe NC-MV 2+ complexes 9,36 and CdSe-Au hetero NPs 17 as direct evidence for the transfer of hot electrons from NCs to acceptors, MV 2+ or Au NPs, respectively. Furthermore, it was reported that hot electrons in CdSe NCs were relaxed to the band-edge state with a lifetime of a few hundred femtoseconds to several picoseconds, being comparable to the time range of TA signal evolution shown in Fig.…”

“…Charge separation dynamics with the use of various electron acceptors has been reported in many papers. [5][6][7][8][9][10][11][12][13][14][15][16][17] For example Banin et al reported that hybrid nanostructures of Au-tipped CdS nanorods with a larger Au size exhibited a higher probability for charge separation of multiple excitons formed in the CdS nanocrystals, improving the photocatalytic activity for H 2 generation. 15 The rate of photoinduced electron transfer from CdSe NCs to methyl viologen (MV 2+ ) was shown to be dependent on the particle morphology: plate-like CdSe NCs exhibited a slower rate than that of spherical or rod-shaped CdSe NCs due to the weak electronic coupling along the short and long axes in the lateral dimension.…”

Quantum confinement-induced enhanced nonlinearity and carrier lifetime modulation in two-dimensional tin sulfide

“…In addition, the proportion of surface defects will also increase with decrease in the thickness, to some extent, modifying the carrier dynamics. These kinds of carriers slowing down phenomenon have been investigated in conventional QDs and two-dimensional films [57,59,60]. The slowed carrier cooling facilitates the carrier multiplication [23] and hot-electron transfer [12], thus enabling the higher efficiency of energy use in photovoltaic devices.…”

Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer

“…Reducing the material dimensionality to achieve quantum confinement from one (two-dimensional material) to all three (quantum dot) dimensions is believed to slow hot-carrier cooling, since the enlarged energy spacing arising from quantum confinement causes a ‘phonon bottleneck'6. Nevertheless, the initial hot-carrier extraction efficiency in typical quasi-zero-dimensional semiconductor quantum dot systems at room temperature is still low.…”