Morphology and dopant-dependent optical characteristics of novel composite 1D and 3D-based heterostructures of CdSe nanocrystals and LaPO4:Re (Re = Eu, Ce, Tb) metal phosphate nanowires

Abstract

In this report, we synthesize and structurally characterize novel semiconducting nanoscale composite heterostructures composed of zero-dimensional (0D) CdSe nanocrystals coupled with both one- and three-dimensional (1D and 3D) rare earth metal-doped LaPO₄ metal phosphate materials. Subsequent optical characterization has demonstrated a clear dependence of the intrinsic charge and energy transfer processes in these systems on both (i) morphology and (ii) the presence of dopants. Specifically, ∼4.5 nm CdSe quantum dots (QDs) have been successfully anchored onto (a) high-aspect ratio rare-earth activated LaPO₄ nanowires, measuring ∼7 nm in diameter and ∼1.3 μm in length, prepared by a modified hydrothermal protocol, and (b) well-dispersed urchin-like 3D architectures of LaPO₄:Re (Re = Ce, Tb, Eu) (diameter ∼500 nm), fabricated using a large-scale, solution-precipitation approach in the presence of 6-mercaptohexanoic acid, used as a self-assembly facilitating agent. We have proposed a growth mechanism of our 3D sub-micron LaPO₄-based architectures, based on a detailed time-dependent scanning electron microscopy visualization study. In terms of properties, our results show that our 1D and 3D heterostructures evince both PL quenching and a shorter average lifetime of CdSe QDs as compared with unbound CdSe QDs. We suggest that a photo-induced charge transfer process occurs from CdSe QDs to LaPO₄:Eu through the mediation of water molecules in the intrinsic LaPO₄ structure. Conversely, analogous CdSe QD–3D LaPO₄:Eu heterostructures exhibit noticeably less PL quenching and longer lifetimes as compared with 1D composites since it appears that not only charge transfer from CdSe QDs to LaPO₄:Eu but also energy transfer from LaPO₄:Eu to CdSe QDs are substantially more efficient processes with 3D as compared with 1D heterostructures, possibly due to the nearly 3 times higher coverage density of QDs on the surfaces of the underlying 3D LaPO₄ motif, thereby contributing to its more effective absorption capability of LaPO₄:Eu emission. Moreover, the magnitude of the PL signal and the corresponding lifetimes in the CdSe QD (0D)–LaPO₄ (3D) heterostructures are dependent upon the rare-earth dopant tested itself. Data are additionally explained in the context of the inherent energy level alignments of both CdSe QDs and LaPO₄:Re (Re = Ce, Tb, and Eu) systems.