Probing structure-induced optical behavior in a new class of self-activated luminescent 0D/1D CaWO₄ metal oxide-CdSe nanocrystal composite heterostructures

In this report, we synthesize and characterize the structural and optical properties of novel heterostructures composed of (i) semiconducting nanocrystalline CdSe quantum dots (QDs) coupled with (ii) both one- and zero-dimensional (1D and 0D) motifs of self-activated luminescent CaWO₄ metal oxides. Specifically, ∼4 nm CdSe QDs have been anchored onto (i) high-aspect ratio 1D nanowires, measuring ∼230 nm in diameter and ∼3 μm in length, as well as onto (ii) crystalline 0D nanoparticles (possessing an average diameter of ∼80 nm) of CaWO₄ through the mediation of 3-mercaptopropionic acid (MPA) as a connecting linker. Composite formation was confirmed by complementary electron microscopy and spectroscopy (i.e., IR and Raman) data. In terms of luminescent properties, our results show that our 1D and 0D heterostructures evince photoluminescence (PL) quenching and shortened PL lifetimes of CaWO₄ as compared with unbound CaWO₄. We propose that a photoinduced electron transfer process occurs from CaWO₄ to CdSe QDs, a scenario which has been confirmed by NEXAFS measurements and which highlights a decrease in the number of unoccupied orbitals in the conduction bands of CdSe QDs. By contrast, the PL signature and lifetimes of MPA-capped CdSe QDs within these heterostructures do not exhibit noticeable changes as compared with unbound MPA-capped CdSe QDs. The striking difference in optical behavior between CaWO₄ nanostructures and CdSe QDs within our heterostructures can be correlated with the relative positions of their conduction and valence energy band levels. In addition, the PL quenching behaviors for CaWO₄ within the heterostructure configuration were examined by systematically varying (i) the quantities and coverage densities of immobilized CdSe QDs as well as (ii) the intrinsic morphology (and by extension, the inherent crystallite size) of CaWO₄ itself.