Synthesis of Compositionally Defined Single-Crystalline Eu3+-Activated Molybdate–Tungstate Solid-Solution Composite Nanowires and Observation of Charge Transfer in a Novel Class of 1D CaMoO4–CaWO4:Eu3+–0D CdS/CdSe QD Nanoscale Heterostructures

Abstract

As a first step, we have synthesized and optically characterized a systematic series of one-dimensional (1D) single-crystalline Eu3+-activated alkaline-earth metal tungstate/molybdate solid-solution composite CaW1–xMoxO4 (0 ≤ “x” ≤ 1) nanowires of controllable chemical composition using a modified template-directed methodology under ambient room-temperature conditions. Extensive characterization of the resulting nanowires has been performed using X-ray diffraction, electron microscopy, and optical spectroscopy. The crystallite size and single crystallinity of as-prepared 1D CaW1–xMoxO4:Eu3+ (0 ≤ “x” ≤ 1) solid-solution composite nanowires increase with increasing Mo component (“x”). We note a clear dependence of luminescence output upon nanowire chemical composition with our 1D CaW1–xMoxO4:Eu3+ (0 ≤ “x” ≤ 1) evincing the highest photoluminescence (PL) output at “x” = 0.8, among samples tested. Subsequently, coupled with either zero-dimensional (0D) CdS or CdSe quantum dots (QDs), we successfully synthesized and observed charge transfer processes in 1D CaW1–xMoxO4:Eu3+ (“x” = 0.8)–0D QD composite nanoscale heterostructures. Our results show that CaW1–xMoxO4:Eu3+ (“x” = 0.8) nanowires give rise to PL quenching when CdSe QDs and CdS QDs are anchored onto the surfaces of 1D CaWO4–CaMoO4:Eu3+ nanowires. The observed PL quenching is especially pronounced in CaW1–xMoxO4:Eu3+ (“x” = 0.8)–0D CdSe QD heterostructures. Conversely, the PL output and lifetimes of CdSe and CdS QDs within these heterostructures are not noticeably altered as compared with unbound CdSe and CdS QDs. The differences in optical behavior between 1D Eu3+ activated tungstate and molybdate solid-solution nanowires and the semiconducting 0D QDs within our heterostructures can be correlated with the relative positions of their conduction and valence energy band levels. We propose that the PL quenching can be attributed to a photoinduced electron transfer process from CaW1–xMoxO4:Eu3+ (“x” = 0.8) to both CdSe and CdS QDs, an assertion supported by complementary near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements.