How Isolated Are the Electronic States of the Core in Core/Shell Nanoparticles?

Abstract

We investigated how isolated are the electronic states of the core in a core-shell (c/s) nanoparticles (NPs) from the surface, when the particles are self-assembled on Au substrates via a dithiol (DT) organic linker. Applying photoemission spectroscopy the electronic states of CdSe core only and CdSe/ZnS c/s NPs were compared. The results indicate that in the c/s NPs the HOMO interacts strongly with electronic states in the Au substrate and is pinned at the same energies, relative to the Fermi level, as the core only NPs. When the capping molecules of the NPs were replaced with thiolated molecules, an interaction between the thiol groups and the electronic states of the NPs was observed that depends on the properties of the NPs studied. Thiols binding to the NPs induce the formation of surface trap states. However, while for the core only CdSe NPs the LUMO states are strongly coupled to the surface traps, independent of their size, this coupling is size dependent in the case of the CdSe/ZnS c/s NPs. For a large core, the LUMO is decoupled from the surface trap states. When the core is small enough, the LUMO is delocalized and interacts with these states.

Horizontal versus vertical charge and energy transfer in hybrid assemblies of semiconductor nanoparticles

“…MT linkers were found to quench the PL signal by about 75%, and the quenching was independent of the linker length. This effect is well known for CdSe NPs; it results from the formation of surface traps owing to the thiol binding [28–31]. The PL quenching, which is independent of the MT length, indicates that the number of traps is the same for all linkers, and hence, the number of adsorbed molecules per particle is independent of the length of the linker.…”

Dipolar Molecular Capping in Quantum Dot-Sensitized Oxides: Fermi Level Pinning Precludes Tuning Donor–Acceptor Energetics

“…Pinning of donor–acceptor energetics constitutes a generic interfacial phenomenon, which has been observed for chemically and morphologically distinct strongly coupled interfaces ( e.g ., QD/oxide systems, − QDs/metal contact, bulk semiconductor/semiconductor and semiconductor/liquid interfaces, metal/organic polymer interfaces, , etc. ).…”

Influence of the core size on biexciton quantum yield of giant CdSe/CdS nanocrystals

“…This model is further supported by recent publications that point out to a rather substantial dependence of the electron wavefunction's penetration outside of the CdSe core as function of the core size. 44 It was shown that the probability to nd an electron outside of the core decreases 4-fold when the NC diameter only changes from d ¼ 2.3 nm to d ¼ 2.8 nm. 45 Such strong core size dependence can certainly contribute to the trap-assisted Auger mechanism affecting biexcitonic quantum yield, especially for small core/thin shell nanocrystals.…”