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 Eu 3+ -activated alkaline-earth metal tungstate/molybdate solid-solution composite CaW 1−xMo x O 4 (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 CaW 1−x Mo x O 4 :Eu 3+ (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 CaW 1−x Mo x O 4 :Eu 3+ (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 CaW 1−x Mo x O 4 :Eu 3+ ("x" = 0.8)−0D QD composite nanoscale heterostructures. Our results show that CaW 1−x Mo x O 4 :Eu 3+ ("x" = 0.8) nanowires give rise to PL quenching when CdSe QDs and CdS QDs are anchored onto the surfaces of 1D CaWO 4 −CaMoO 4 :Eu 3+ nanowires. The observed PL quenching is especially pronounced in CaW 1−x Mo x O 4 :Eu 3+ ("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 Eu 3+ 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 CaW 1−x Mo x O 4 :Eu 3+ ("x" = 0.8) to both CdSe and CdS QDs, an assertion supported by complementary near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements.

Eu3+-based luminescence ratiometric thermometry

“…The 535 nm PL band arose from the 5 D 1 -7 F 1 transition of Eu 3+ , and the 580, 593, 615, 650 and 700 nm PL bands were attributed, respectively, to the 5 D 0 -7 F J (J ¼ 0, 1, 2, 3 and 4) transitions of Eu 3+ . [34][35][36][37][38][39] The 615 nm PL band originating from the hypersensitive transition was the most intense band, a result similar to that reported previously. [34][35][36][37][38][39] By monitoring this PL band, the excitation spectrum was collected ( Fig.…”

“…The attributions of these bands have been described previously. [34][35][36][37][38][39] According to a proposed model, upon excitation at 405 nm, the Eu 3+ ions at the ground state absorbed these photons and adopted each a high-energy excited state (Fig. 2b).…”

Chemically Tunable, All‐Inorganic‐Based White‐Light Emitting 0D–1D Heterostructures

“…[32,36,37,[72][73][74][75][76][77][78][79][80][81][82][83][84] In a typical experiment in order to create heterostructures with a molar compositional ratio of [ZnS:Mn 2+ ]:[AET-capped CdSe QDs]) of 1:1.5, 3 µmol (0.3 mg) of ZnS:Mn 2+ in 1 mL of EtOH was added to 4.5 µmol (0.9 mg) of AET-capped CdSe QDs in 2 mL of EtOH. [32,36,37,[72][73][74][75][76][77][78][79][80][81][82][83][84] In a typical experiment in order to create heterostructures with a molar compositional ratio of [ZnS:Mn 2+ ]:[AET-capped CdSe QDs]) of 1:1.5, 3 µmol (0.3 mg) of ZnS:Mn 2+ in 1 mL of EtOH was added to 4.5 µmol (0.9 mg) of AET-capped CdSe QDs in 2 mL of EtOH.…”

“…As for the complementary choice of 0D QDs, CdSe QDs possess interesting size-dependent absorption behavior, and are desirable, because of their very high quantum yield, good photostability, and narrow band emission. [26][27][28] As such, in general, multidimensional nanoscale-based 0D-1D heterostructures are potentially significant as versatile building blocks for future applications, such as but not limited to photocatalysis, [29,30] energy storage, [31] photovoltaics, [32][33][34][35] optoelectronics, [32,[36][37][38] molecular electronics and computational devices, [39][40][41][42][43] sensors, [44] biological imaging, [45,46] as well as drug delivery and therapy. [25] The resulting 1D-0D composite motif not only yields a sizeable interfacial area for "two-way" optoelectronic interactions between the individual CdSe QD and ZnS constituent units but also provides for the possibility for unique electron transport pathways.…”

Probing charge transfer in a novel class of luminescent perovskite-based heterostructures composed of quantum dots bound to RE-activated CaTiO3 phosphors

“…Both of the Pr: CaTiO 3 -CdSe and Eu: CaTiO 3 -CdSe heterostructures were formed using a procedure developed by our group and which has been previously utilized and reported for the creation of composite-based systems. [31][32][33][34] According to Figure 6, the CdSe quantum dots immobilized onto the outer surfaces of the CaTiO 3 exist as either small aggregates or clusters of several dots, as opposed to individual QDs. This phenomenon has also been observed in our previous studies, involving ligand exchange and the formation of associated heterostructures.…”