Ordered dispersion of ZnO quantum dots in SiO2 matrix and its strong emission properties

Silica-Encapsulated Germania Colloids as 3D-Printable Glass Precursors

Core-shell colloids make attractive feedstocks for three-dimensional (3D) printing mixed oxide glass materials because they enable synthetic control of precursor dimensions and compositions, improving glass fabrication precision. Toward that end, we report the design and use of core-shell germania-silica (GeO₂-SiO₂) colloids and their use as precursors to fabricate GeO₂-SiO₂ glass monoliths by direct ink write (DIW) 3D printing. By this method, GeO₂ colloids were prepared in solution using sol-gel chemistry and formed oblong, raspberry-like agglomerates with ∼15 nm diameter primary particles that were predominantly amorphous but contained polycrystalline domains. An ∼15 nm encapsulating SiO₂ shell layer was formed directly on the GeO₂ core agglomerates to form core-shell GeO₂-SiO₂ colloids. For glass 3D printing, GeO₂-SiO₂ colloidal sols were formulated into a viscous ink by solvent exchange, printed into monoliths by DIW additive manufacturing, and sintered to transparent glasses. Characterization of the glass components demonstrates that the core-shell GeO₂-SiO₂ presents a feasible route to prepare quality, optically transparent low wt % GeO₂-SiO₂ glasses by DIW printing. Additionally, the results offer a novel, hybrid colloid approach to fabricating 3D-printed Ge-doped silica glass.

Flexible and color-tunable poly(ether ether ketone) co-coordinated with Eu(III) and Tb(III) fluorescent films: Thermal stability and luminescence property

In this work, a novel kind of carboxyl-containing poly(ether ether ketone) (PEEK-COOH) was successfully synthesized and carefully characterized. Based on PEEK-COOH as macromolecular ligands, a series of luminescent rare earth polymer complexes (PEEKn-Eux-Tb1-x) were prepared and made into flexible films. The effects of the ratio of carboxyl ligand to rare earth ions on fluorescence intensity of complexes were studied and discussed. When the molar ratio was 6:1, the fluorescence emission intensity reached a maximum. Furthermore, the relative emission intensity of Tb3+ and Eu3+ binary system depended on their ratio and the intensity of fluorescence emission peak exhibited continuous change. Because of this, the color of the prepared fluorescent films could range from bright green to fire red, and each color has high purity. The fluorescence lifetime of complexes ranged from 0.428 to 0.511 ms. Absolute quantum yield of PEEK-Eu0.4-Tb0.6 was 8.4%. Beside this, these fluorescence films exhibited a high thermal stability with 5 wt. % in the range of 240–250°C, and its tensile strength was about 28 MPa. In addition, PEEKn-Eux-Tb1-x films possessed good optical transparency. All the interesting results suggested the potential application of the luminescent rare earth polymer complexes in display devices and photo-electronic devices.

Effect of alkali bases on the synthesis of ZnO quantum dots

The surface-modified zinc oxide quantum dots (ZnO QDs) have broad application prospects in the field of biomedicine because of their good water solubility, dispersibility, and high fluorescence stability. The alkali bases play important roles in controlling the morphology, size distribution, dispersity, and fluorescence intensity of the synthesized ZnO QDs. In this article, ZnO QDs were synthesized to induce hydrolysis–condensation reaction. The influences of alkali bases (LiOH, NaOH, and KOH) and the ratio of n(Zn2+):n(OH−) on the properties of synthesized ZnO QDs were investigated. The results show that the particle size of the ZnO QDs prepared using LiOH and NaOH as raw materials are smaller than that using KOH. ZnO QDs prepared at the ratio of n(Zn2+):n(LiOH) = 1:1 have the best fluorescence performance and dispersibility.

Pulsed Plasma-Chemical Modification of SiO2 Nanopowder by ZnxOy Nanoparticles

This work presents the results of pulsed plasma-chemical modification of silicon dioxide nanopowder with zinc oxide nanoparticles (ZnO@SiO2). The obtained ZnO@SiO2 powders were characterized by transmission electron microscopy (TEM) and X-ray phase analysis. The size of the synthesized particles was in the range of 20–100[Formula: see text]nm. The photocatalytic characteristics of ZnO@SiO2 were studied. When exposed to ultraviolet radiation, the methylene blue (MB) decomposes efficiently. Two samples characterized by the content of silicon tetrachloride in the initial mixture were synthesized. The band gap estimated from the absorption spectra calculated from the diffuse reflectance spectra for these samples was 2.4[Formula: see text]eV and 2.95[Formula: see text]eV for indirect transitions and 3.03[Formula: see text]eV and 3.24[Formula: see text]eV for direct allowed transitions.

Spatial separation of electrons and holes for enhancing the gas-sensing property of a semiconductor: ZnO/ZnSnO3 nanorod arrays prepared by a hetero-epitaxial growth

The construction of semiconductor composites is known as a powerful method used to realize the spatial separation of electrons and the holes in them, which can result in more electrons or holes and increase the dispersion of oxygen ions ([Formula: see text] and O ^- ) (one of the most critical factors for their gas-sensing properties) on the surface of the semiconductor gas sensor. In this work, using 1D ZnO/ZnSnO₃ nanoarrays as an example, which are prepared through a hetero-epitaxial growing process to construct a chemically bonded interface, the above strategy to attain a better semiconductor gas-sensing property has been realized. Compared with single ZnSnO₃ nanotubes and no-matching ZnO/ZnSnO₃ nanoarrays gas sensors, it has been proven by x-ray photoelectron spectroscopy and photoluminescence spectrum examination that the as-obtained ZnO/ZnSnO₃ sensor showed a greatly increased quantity of active surface electrons with exceptional responses to trace target gases and much lower optimum working temperatures (less than about 170 °C). For example, the as-obtained ZnO/ZnSnO₃ sensor exhibited an obvious response and short response/recovery time (less than 10 s) towards trace H₂S gas (a detection limit down to 700 ppb). The high responses and dynamic repeatability observed in these sensors reveal that the strategy based on the as-presented electron and hole separation is reliable for improving the gas-sensing properties of semiconductors.

Energy Transfer Efficiency from ZnO-Nanocrystals to Eu3+ Ions Embedded in SiO₂ Film for Emission at 614 nm

In this work, we study the energy transfer mechanism from ZnO nanocrystals (ZnO-nc) to Eu³⁺ ions by fabricating thin-film samples of ZnO-nc and Eu³⁺ ions embedded in a SiO₂ matrix using the low-cost sol-gel technique. The time-resolved photoluminescence (TRPL) measurements from the samples were analyzed to understand the contribution of energy transfer from the various ZnO-nc emission centers to Eu³⁺ ions. The decay time obtained from the TRPL measurements was used to calculate the energy transfer efficiencies from the ZnO-nc emission centers, and these results were compared with the energy transfer efficiencies calculated from steady-state photoluminescence emission results. The results in this work show that high transfer efficiencies from the excitonic and Zn defect emission centers is mostly due to the energy transfer from ZnO-nc to Eu³⁺ ions which results in the radiative emission from the Eu³⁺ ions at 614 nm, while the energy transfer from the oxygen defect emissions is most probably due to the energy transfer from ZnO-nc to the new defects created due to the incorporation of the Eu³⁺ ions.

Study of energy transfer mechanism from ZnO nanocrystals to Eu(3+) ions

In this work, we investigate the efficient energy transfer occurring between ZnO nanocrystals (ZnO-nc) and europium (Eu(3+)) ions embedded in a SiO2 matrix prepared using the sol-gel technique. We show that a strong red emission was observed at 614 nm when the ZnO-nc were excited using a continuous optical excitation at 325 nm. This emission is due to the radiative (5)D0 → (7)F2 de-excitation of the Eu(3+) ions and has been conclusively shown to be due to the energy transfer from the excited ZnO-nc to the Eu(3+) ions. The photoluminescence excitation spectra are also examined in this work to confirm the energy transfer from ZnO-nc to the Eu(3+) ions. Furthermore, we study various de-excitation processes from the excited ZnO-nc and their contribution to the energy transfer to Eu(3+) ions. We also report the optimum fabrication process for maximum red emission at 614 nm from the samples where we show a strong dependence on the annealing temperature and the Eu(3+) concentration in the sample. The maximum red emission is observed with 12 mol% Eu(3+) annealed at 450 °C. This work provides a better understanding of the energy transfer mechanism from ZnO-nc to Eu(3+) ions and is important for applications in photonics, especially for light emitting devices.

Influence of Nanosized Silicon Oxide on the Luminescent Properties of ZnO Nanoparticles

For practical use of nanosized zinc oxide as the phosphor its luminescence quantum yields should be maximized. The aim of this work was to enhance luminescent properties of ZnO nanoparticles and obtain high-luminescent ZnO/SiO2composites using simpler approaches to colloidal synthesis. The luminescence intensity of zinc oxide nanoparticles was increased about 3 times by addition of silica nanocrystals to the source solutions during the synthesis of ZnO nanoparticles. Then the quantum yield of luminescence of the obtained ZnO/SiO2composites is more than 30%. Such an impact of silica is suggested to be caused by the distribution of ZnO nanocrystals on the surface of silica, which reduces the probability of separation of photogenerated charges between the zinc oxide nanoparticles of different sizes, and as a consequence, there is a significant increase of the luminescence intensity of ZnO nanoparticles. This way of increasing nano-ZnO luminescence intensity facilitates its use in a variety of devices, including optical ultraviolet and visible screens, luminescent markers, antibacterial coatings, luminescent solar concentrators, luminescent inks for security printing, and food packaging with abilities of informing consumers about the quality and safety of the packaged product.

Reagent concentration dependent variations in the stability and photoluminescence of silica-coated ZnO nanorods

ZnO nanorods produced hydrothermally, using different precursor concentrations, show strikingly different morphologies and photoluminescence properties after encapsulation in silica. These differences are traced to differences in the nanorod growth chemistry.

Arrays of nanorods composed of ZnO nanodots exhibiting enhanced UV emission and stability

A novel one-step coating and assembly approach for fabricating well-defined ZnO nanodot/SiO₂ nanorod arrays by hydrolysis-recrystallization growth from 1-D ZnO nanorods is described. The resultant composite nanorod arrays exhibit much enhanced UV emission efficiencies and excellent stability, and thus offer particular promise for application in UV emission devices operating in harsh environments.

Spectral and luminescent properties of ZnO–SiO2 core–shell nanoparticles with size-selected ZnO cores

Deposition of a SiO₂ shell on luminescent ZnO nanoparticles in dimethylsulfoxide precisely tunes the nanoparticle size from 3 to 6 nm.

Annealing temperature and environment effects on ZnO nanocrystals embedded in SiO2: a photoluminescence and TEM study

We report on efficient ZnO nanocrystal (ZnO-NC) emission in the near-UV region. We show that luminescence from ZnO nanocrystals embedded in a SiO2 matrix can vary significantly as a function of the annealing temperature from 450°C to 700°C. We manage to correlate the emission of the ZnO nanocrystals embedded in SiO2 thin films with transmission electron microscopy images in order to optimize the fabrication process. Emission can be explained using two main contributions, near-band-edge emission (UV range) and defect-related emissions (visible). Both contributions over 500°C are found to be size dependent in intensity due to a decrease of the absorption cross section. For the smallest-size nanocrystals, UV emission can only be accounted for using a blueshifted UV contribution as compared to the ZnO band gap. In order to further optimize the emission properties, we have studied different annealing atmospheres under oxygen and under argon gas. We conclude that a softer annealing temperature at 450°C but with longer annealing time under oxygen is the most preferable scenario in order to improve near-UV emission of the ZnO nanocrystals embedded in an SiO2 matrix.

In situ generation of well-dispersed ZnO quantum dots on electrospun silica nanotubes with high photocatalytic activity

The ZnO quantum dots-SiO(2) nanotubes (ZQDs-SNTs) nanocomposite was successfully fabricated by direct heat treatment of the electrospun zinc acetate/tetraethyl orthosilicate (TEOS)/polymer nanotubes (NTs). The results indicated that the ZnO quantum dots (ZQDs) with diameter about 3-5 nm were highly dispersed on the SiO(2) nanotubes (SNTs). And, there might be Zn-O-Si bonds between ZQDs and SiO(2) matrix, which formed interface states in the ZQDs-SNTs nanocomposite. The photocatalytic studies revealed that the ZQDs-SNTs nanocomposite exhibited high photocatalytic activity to degrade Rhodamine B (RB) under ultraviolet (UV) light irradiation, which might be ascribed to two reasons. The first one was the high dispersity of ZQDs; another one was the high separation efficiency of photogenerated electron-hole pairs due to the trap effect for photogenerated electrons of the interface states between ZQDs and SiO(2). During the photocatalytic reaction, the ZQDs-SNTs nanocomposite also exhibited high chemical stability in a wide range of pH values, which might be ascribed to the protective action of SiO(2) and the presence of Zn-O-Si bonds between ZQDs and SiO(2). Furthermore, the ZQDs-SNTs nanocomposites could be easily recycled because of their one-dimensional nanostructure property.

Fabrication and photoluminescent properties of ZnO/mesoporous silica composites templated by a chelating surfactant

A novel anionic surfactant-templated synthesis of ZnO/mesoporous silica nanocomposites has been carried out by using N-hexadecylethylenediamine triacetate (HED3A), a triprotic surfactant, as the structure-directing agent. The chelating template can capture zinc ions in solution and then direct the mesophase formation, enabling an amount of zinc oxide to be embedded in the porous silica matrix during calcination. With variation of the molar ratio of Zn(2+) to HED3A in the template, a series of composites with different doping amounts were obtained after the removal of organic components. The variation of the zinc ion concentration in the initial template solution induces an evolution of the silica mesophase, presumably due to the change in electronegativity of the HED3A headgroup caused by the chelating effect. Spectroscopic studies show a strong host-guest interaction between the silica pore walls and ultrafine ZnO nanoparticles. The photoluminescence properties of the resulting composites exhibit a size-dependent light emission and quantum-confinement effect of ZnO, accompanied by an infrequent violet emission originating from the ZnO-SiO(2) interface.