Abstract

Compared to simple core quantum dots (QDs), core/shell QDs offer an enhanced stability and tunability of the opti- cal and electronic properties. This difference essentially arises from the band alignment of the semiconductors that comprise the core/shell heterostructure. We study the band alignment in colloidal PbSe/CdSe core/shell quantum dots (QDs). First of all, the synthesis is improved by careful control over tempera- ture, [Pb]/[Cd] ratio and concentration for a large combination of core and shell sizes. The structural properties are studied using TEM, EDX and XRD. The optical and electronic proper- ties are probed using absorption and photoluminescence spec- troscopy. As compared to PbSe QDs, PbSe/CdSe QDs have a similar photoluminescence (PL) quantum yield, yet they show an enhanced Stokes shift between absorbance and PL.
Moreover, the luminescence lifetime increases drastically upon addition of a shell. These indirect indications of a staggered type-II band alignment are confirmed by the change of the PbSe/ CdSe PL upon addition of dodecanethiol and methylviologen, a hole and an electron scavenger for CdSe, respectively. We find that dodecanethiol enhances the PL, while methylviologen completely quenches the PL. Combined with calculations of the tunneling current, this demonstrates that the conduction-band electron wavefunction spreads over the CdSe shell while the valence-band hole wavefunction is confined to the PbSe core. This corresponds to a band alignment where the PbSe and CdSe conduction bands are in close proximity while the valence bands have a strong offset.

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