Observation of Photoinduced Charge Transfer in Novel Luminescent CdSe Quantum Dot–CePO₄:Tb Metal Oxide Nanowire Composite Heterostructures

We report on the synthesis, structural characterization, and intrinsic charge transfer processes associated with novel luminescent zero-dimensional (0D) CdSe nanocrystal–one-dimensional (1D) CePO₄:Tb nanowire composite heterostructures. Specifically, ∼4 nm CdSe quantum dots (QDs) have been successfully anchored onto high-aspect ratio CePO₄:Tb nanowires, measuring ∼65 nm in diameter and ∼2 μm in length. Composite formation was confirmed by high-resolution transmission microscopy, energy-dispersive X-ray spectroscopy mapping, and confocal microscopy. Photoluminescence (PL) spectra, emission decay, and optical absorption of these nanoscale heterostructures were collected and compared with those of single, discrete CdSe QDs and CePO₄:Tb nanowires. We show that our composite heterostructure evinces both PL quenching and a shorter average lifetime as compared with unbound CdSe QDs and CePO₄:Tb nanowires. We propose that a photoinduced 0D–1D charge transfer process occurs between CdSe and CePO₄:Tb and that it represents the predominant mechanism, accounting for the observed PL quenching and shorter lifetimes noted in our composite heterostructures. Data are additionally explained in the context of the inherent energy level alignments of both CdSe QDs and CePO₄:Tb nanowires

Electro-Optical Device with Tunable Transparency Using Colloidal Core/Shell Nanoparticles

Suspended particle devices (SPDs) adapted for controlling the transmission of electromagnetic radiation have become an area of considerable focus for smart window technology due to their desirable properties, such as instant and precise light control and cost-effectiveness. Here, we demonstrate a SPD with tunable transparency in the visible regime using colloidal assemblies of nanoparticles. The observed transparency using ZnS/SiO₂ core/shell colloidal nanoparticles is dynamically tunable in response to an external electric field with increased transparency when applied voltage increases. The observed transparency change is attributed to structural ordering of nanoparticle assemblies and thereby modifies the photonic band structures, as confirmed by the finite-difference time-domain simulations of Maxwell’s equations. The transparency of the device can also be manipulated by changing the particle size and the device thickness. In addition to transparency, structural colorations and their dynamic tunability are demonstrated using α-Fe₂O₃/SiO₂ core/shell nanomaterials, resulting from the combination of inherent optical properties of α-Fe₂O₃/SiO₂ nanomaterials and coloration due to their tunable structural particle assemblies in response to electric stimuli

Electro-Optical Device with Tunable Transparency Using Colloidal Core/Shell Nanoparticles

Suspended particle devices (SPDs) adapted for controlling the transmission of electromagnetic radiation have become an area of considerable focus for smart window technology due to their desirable properties, such as instant and precise light control and cost-effectiveness. Here, we demonstrate a SPD with tunable transparency in the visible regime using colloidal assemblies of nanoparticles. The observed transparency using ZnS/SiO₂ core/shell colloidal nanoparticles is dynamically tunable in response to an external electric field with increased transparency when applied voltage increases. The observed transparency change is attributed to structural ordering of nanoparticle assemblies and thereby modifies the photonic band structures, as confirmed by the finite-difference time-domain simulations of Maxwell’s equations. The transparency of the device can also be manipulated by changing the particle size and the device thickness. In addition to transparency, structural colorations and their dynamic tunability are demonstrated using α-Fe₂O₃/SiO₂ core/shell nanomaterials, resulting from the combination of inherent optical properties of α-Fe₂O₃/SiO₂ nanomaterials and coloration due to their tunable structural particle assemblies in response to electric stimuli

Electro-Optical Device with Tunable Transparency Using Colloidal Core/Shell Nanoparticles

Suspended particle devices (SPDs) adapted for controlling the transmission of electromagnetic radiation have become an area of considerable focus for smart window technology due to their desirable properties, such as instant and precise light control and cost-effectiveness. Here, we demonstrate a SPD with tunable transparency in the visible regime using colloidal assemblies of nanoparticles. The observed transparency using ZnS/SiO₂ core/shell colloidal nanoparticles is dynamically tunable in response to an external electric field with increased transparency when applied voltage increases. The observed transparency change is attributed to structural ordering of nanoparticle assemblies and thereby modifies the photonic band structures, as confirmed by the finite-difference time-domain simulations of Maxwell’s equations. The transparency of the device can also be manipulated by changing the particle size and the device thickness. In addition to transparency, structural colorations and their dynamic tunability are demonstrated using α-Fe₂O₃/SiO₂ core/shell nanomaterials, resulting from the combination of inherent optical properties of α-Fe₂O₃/SiO₂ nanomaterials and coloration due to their tunable structural particle assemblies in response to electric stimuli

Synthesis, Characterization, and Formation Mechanism of Crystalline Cu and Ni Metallic Nanowires under Ambient, Seedless, Surfactantless Conditions

In this report, crystalline elemental Cu and Ni nanowires have been successfully synthesized through a simplistic, malleable, solution-based protocol involving the utilization of a U-tube double diffusion apparatus under ambient conditions. The nanowires prepared within the 50 and 200 nm template membrane pore channels maintain diameters ranging from ∼90–230 nm with lengths attaining the micrometer scale. To mitigate for the unwanted but very facile oxidation of these nanomaterials to their oxide analogues, our synthesis mechanism relies on a carefully calibrated reaction between the corresponding metal precursor solution and an aqueous reducing agent solution, resulting in the production of pure, monodisperse metallic nanostructures. These as-prepared nanowires were subsequently characterized from an applications’ perspective so as to investigate their optical and photocatalytic properties

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

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 CaW_1–<i>x</i>­Mo_<i>x</i>O₄ (0 ≤ “<i>x</i>” ≤ 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–<i>x</i>­Mo_<i>x</i>O₄:Eu³⁺ (0 ≤ “<i>x</i>” ≤ 1) solid-solution composite nanowires increase with increasing Mo component (“<i>x</i>”). We note a clear dependence of luminescence output upon nanowire chemical composition with our 1D CaW_1–<i>x</i>Mo_<i>x</i>O₄:Eu³⁺ (0 ≤ “<i>x</i>” ≤ 1) evincing the highest photoluminescence (PL) output at “<i>x</i>” = 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–<i>x</i>Mo_<i>x</i>­O₄:Eu³⁺ (“<i>x</i>” = 0.8)–0D QD composite nanoscale heterostructures. Our results show that CaW_1–<i>x</i>­Mo_<i>x</i>O₄:Eu³⁺ (“<i>x</i>” = 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 CaW_1–<i>x</i>Mo_<i>x</i>O₄:Eu³⁺ (“<i>x</i>” = 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 CaW_1–<i>x</i>­Mo_<i>x</i>O₄:Eu³⁺ (“<i>x</i>” = 0.8) to both CdSe and CdS QDs, an assertion supported by complementary near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements

Probing the Dependence of Electron Transfer on Size and Coverage in Carbon Nanotube–Quantum Dot Heterostructures

As a model system for understanding charge transfer in novel architectural designs for solar cells, double-walled carbon nanotube (DWNT)–CdSe quantum dot (QD) (QDs with average diameters of 2.3, 3.0, and 4.1 nm) heterostructures have been fabricated. The individual nanoscale building blocks were successfully attached and combined using a hole-trapping thiol linker molecule, i.e., 4-mercaptophenol (MTH), through a facile, noncovalent π–π stacking attachment strategy. Transmission electron microscopy confirmed the attachment of QDs onto the external surfaces of the DWNTs. We herein demonstrate a meaningful and unique combination of near-edge X-ray absorption fine structure (NEXAFS) and Raman spectroscopies bolstered by complementary electrical transport measurements in order to elucidate the synergistic interactions between CdSe QDs and DWNTs, which are facilitated by the bridging MTH molecules that can scavenge photoinduced holes and potentially mediate electron redistribution between the conduction bands in CdSe QDs and the C 2p-derived states of the DWNTs. Specifically, we correlated evidence of charge transfer as manifested by (i) changes in the NEXAFS intensities of π* resonance in the C <i>K</i>-edge and Cd <i>M</i>₃-edge spectra, (ii) a perceptible outer tube G-band downshift in frequency in Raman spectra, as well as (iii) alterations in the threshold characteristics present in transport data as a function of CdSe QD deposition onto the DWNT surface. In particular, the separate effects of (i) varying QD sizes and (ii) QD coverage densities on the electron transfer were independently studied

Observation of Ferroelectricity and Structure-Dependent Magnetic Behavior in Novel One-Dimensional Motifs of Pure, Crystalline Yttrium Manganese Oxides

Multiferroic materials, such as nanostructured <i>h</i>-YMnO₃, are expected to fulfill a crucial role as active components of technological devices, particularly for information storage. Herein, we report on the template mediated sol–gel synthesis of unique one-dimensional nanostructured motifs of hexagonal phase YMnO₃, possessing a space group of <i>P</i>6₃<i>cm</i>. We found that the inherent morphology of the as-obtained <i>h</i>-YMnO₃ nanostructures was directly impacted by the chemical composition of the employed membrane. Specifically, the use of anodic alumina and polycarbonate templates promoted nanotube and nanowire formation, respectively. Isolated polycrystalline nanotubes and single crystalline nanowires possessed diameters of 276 ± 52 nm, composed of 17 nm particulate constituent grains, and 125 ± 21 nm, respectively, with lengths of up to several microns. The structures and compositions of all our as-prepared products were probed by XRD, SEM, HRTEM, EXAFS, XANES, SAED, and far-IR spectroscopy. In the specific case of nanowires, we determined that the growth direction was mainly along the <i>c</i>-axis and that discrete, individual structures gave rise to expected ferroelectric behavior. Overall, our YMnO₃ samples evinced the onset of a spin-glass transition at 41 ± 1 K for both templateless bulk control and nanowire samples but at 26 ± 3 K for nanotubes. Interestingly, only the as-synthesized crystalline nanotubular mesh gave rise to noticeably enhanced magnetic properties (i.e., a higher magnetic moment of 3.0 μ_B/Mn) as well as a lower spin-glass transition temperature, attributable to a smaller constituent crystallite size. Therefore, this work not only demonstrates our ability to generate viable one-dimensional nanostructures of a significant and commercially relevant metal oxide but also contributes to an understanding of structure–property correlations in these systems

Correlating Size and Composition-Dependent Effects with Magnetic, Mössbauer, and Pair Distribution Function Measurements in a Family of Catalytically Active Ferrite Nanoparticles

The magnetic spinel ferrites, MFe₂O₄ (wherein “M” = a divalent metal ion such as but not limited to Mn, Co, Zn, and Ni), represent a unique class of magnetic materials in which the rational introduction of different “M”s can yield correspondingly unique and interesting magnetic behaviors. Herein we present a generalized hydrothermal method for the synthesis of single-crystalline ferrite nanoparticles with M = Mg, Fe, Co, Ni, Cu, and Zn, respectively, which can be systematically and efficaciously produced simply by changing the metal precursor. Our protocol can moreover lead to reproducible size control by judicious selection of various surfactants. As such, we have probed the effects of both (i) size and (ii) chemical composition upon the magnetic properties of these nanomaterials using complementary magnetometry and Mössbauer spectroscopy techniques. The structure of the samples was confirmed by atomic pair distribution function analysis of X-ray and electron powder diffraction data as a function of particle size. These materials retain the bulk spinel structure to the smallest size (i.e., 3 nm). In addition, we have explored the catalytic potential of our ferrites as both (a) magnetically recoverable photocatalysts and (b) biological catalysts and noted that many of our as-prepared ferrite systems evinced intrinsically higher activities as compared with their iron oxide analogues