Efficient field emission properties of ZnO (core)/graphite (shell) nanowires

In this work the field emission studies of a new type of field emitter, zinc oxide (ZnO) core/graphitic (g-C) shell nanowires are presented. The nanowires are synthesized by chemical vapor deposition of zinc acetate at 1300 °C Scanning and transmission electron microscopy characterization confirm high aspect ratio and novel core–shell morphology of the nanowires. Raman spectrum of the nanowires mat represents the characteristic Raman modes from g-C shell as well as from the ZnO core. A low turn on field of 2.75 V/μm and a high current density of 1.0 mA/cm² at 4.5 V/μm for ZnO/g-C nanowires ensure the superior field emission behavior compared to the bare ZnO nanowires.     

Colorimetric iodide recognition and sensing by citrate-stabilized core/shell Cu@Au nanoparticles

In the light of the significance and urgency for the recognition and sensing of anions specifically, especially those of biological relevance, herein, we wish to demonstrate a novel colorimetric avenue for highly selective iodide recognition and sensing using simple citrate-stabilized core/shell Cu@Au nanoparticles. No other ions than iodide can induce an appreciable color change of the Cu@Au nanoparticles solution from purple to red by transforming the interconnected, irregularly shaped nanoparticles to the single, separated, and nearly spherical ones, as confirmed by the transmission electron microscopy (TEM). On the basis of the optical spectra and TEM studies, a mechanism of iodide-induced aggregating/fusion, fragmentation, and reorganization of atoms is proposed. With this strategy, 6 μM (0.76 ppm) of iodide can be recognized within 20 min by naked-eye observation. This sensitive and selective colorimetric assay opens up a fresh insight of facile, rapid, and reliable detection of iodide and may find its future application in the analysis of the total iodine in edible salt as well as the clinical diagnosis of urinary iodide.     

Porous ternary complex metal oxide nanoparticles converted from core/shell nanoparticles

We demonstrate an easy and scalable low-temperature process to convert porous ternary complex metal oxide nanoparticles from solution-synthesized core/shell metal oxide nanoparticles by thermal annealing. The final products demonstrate superior electrochemical properties with a large capacity and high stability during fast charging/discharging cycles for potential applications as advanced lithium-ion battery (LIB) electrode materials. In addition, a new breakdown mechanism was observed on these novel electrode materials.

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Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H₂ as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).     

Facile electrochemical characterization of core/shell nanoparticles. Ag core/Ag(2)O shell structures

We report in this paper a facile approach for the formation and electrochemical characterization of silver-silver oxide core-shell nanoparticles (NPs). Thus, thermal treatment at temperatures between 200 and 360 degrees C of Ag NP, in the gas phase or in an organic solvent, has been used to achieve the formation Ag@Ag2O NP. The evidence of formation of such a core-shell structure was obtained by cyclic voltammetry using a Nafion modified electrode (where Nafion containing carbon particles is used as the matrix to encapsulate the core-shell NP). Initial positive scans measure free Ag. Initial negative scans measure Ag2O, with the following positive scan, compared to the initial one, providing a measure of "trapped" or core Ag. The results presented demonstrate the utility of this approach in characterizing core-shell structures, like Ag@Ag2O, which could be extended to other core-shell forms, such as bimetallic core-shell NP.     

Ag nanoparticles enhanced near-IR emission from Er3+ ions doped glasses

Vitreous materials containing rare-earth (RE) ions and metallic nanoparticles (NPs) attract considerable interest because the presence of the NPs may lead to an intensification of luminescence. In this work, the characteristics of 1.54 μm luminescence for the Er³⁺ ions doped bismuthate glasses containing Ag NPs were studied under 980 nm excitation. The surface plasmon resonance (SPR) band of Ag NPs appears from 500 to 1500 nm. Transmission electron microscopic (TEM) image reveals that the Ag NPs are dispersed homogeneously with the size from 2 to 7 nm. The strength parameters Ω_t(t = 2, 4, 6), spontaneous emission probability (A), radiative lifetime (τ) and stimulated emission section (σ_em) of Er³⁺ ions were calculated by the Judd–Ofelt theory. When the glass contains 0.2 wt% AgCl, the 1.54 μm fluorescence intensity of Er³⁺ reaches a maximum value, which is 7.2 times higher than that of glass without Ag NPs. The Ag NPs embedded glasses show significantly fluorescence enhancement of Er³⁺ ions by local field enhancement from SPR.     

A method for synthesizing the core (Ag)/shell (PSt) composite nanoparticles

Core-shell composite materials have been widely used in many fields. In this paper, the core (Ag)-shell (PSt) composite nanoparticles have been successfully fabricated in microemulsions at ambient pressure. Firstly, Ag nanoparticles with about 60-100 nm in diameters were synthesized by reducing silver nitrate by ascorbic acid, and then, styrene polymerized at the surface of Ag nanoparticles by K₂S₂O₄ initiator in microemulsion solutions. The Ag/PSt core-shell composite nanoparticles were identified by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and infrared spectra (IR). Results show that Ag-core nanoparticles were coated with ultra thin PSt shell with thickness of about 3-6 nm.     

Superparamagnetic maghemite/polyrhodanine core/shell nanoparticles: Synthesis and characterization

Maghemite nanoparticles (MNPs) with a thin layer of polyrhodanine (PRd) at the surface were synthesized via chemical oxidative polymerization of rhodanine monomer at the MNPs surface in the presence of ferric chloride as oxidant. X-ray diffraction (XRD) pattern gave direct evidence that the synthesized nanoparticles are crystalline maghemite of about 8 nm in size. Magnetization of the particles versus an applied magnetic field exhibited no hysteresis loop, indicated superparamagnetic behavior in the particles. Transmission electron microscopy (TEM) together with Fourier-transform infrared (FT-IR) spectroscopy were used to determine the morphology and the chemical structure of the magnetic core and the polymeric shell. Through the microscopy analysis the shell thickness was estimated to be about 1.5 nm, whereas through thermogravimetric analysis (TGA) it was estimated to be about 0.6 nm. Moreover inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements revealed that the oxidant residue in the polymer backbone is ca. 4 wt.%.     

Two-fold emission from the S-shell of PbSe/CdSe core/shell quantum dots

The optical properties of PbSe/CdSe core/shell quantum dots with core sizes smaller than 4 nm in the 5-300 K range are reported. The photoluminescence spectra show two peaks, which become increasingly separated in energy as the core diameter is reduced below 4 nm. It is shown that these peaks are due to intrinsic exciton transitions in each quantum dot, rather than emission from different quantum dot sub-ensembles. Most likely, the energy separation between the peaks is due to inter-valley coupling between the L-points of PbSe. The temperature dependence of the relative intensities of the peaks implies that the two emitting states are not in thermal equilibrium and that dark exciton states must play an important role.     

Silica shell/gold core nanoparticles: correlating shell thickness with the plasmonic red shift upon aggregation

Differences in the wavelengths of the surface plasmon band of gold nanoparticles (AuNP)--before and after particle aggregation--are widely used in bioanalytical assays. However, the gold surfaces in such bioassays can suffer from exchange and desorption of noncovalently bound ligands and from nonspecific adsorption of biomolecules. Silica shells on the surfaces of the gold can extend the available surface chemistries for bioconjugation and potentially avoid these issues. Therefore, silica was grown on gold surfaces using either hydrolysis/condensation of tetraethyl orthosilicate 1 under basic conditions or diglyceroxysilane 2 at neutral pH. The former precursor permitted slow, controlled growth of shells from about 1.7 to 4.3 nm thickness. By contrast, 3-4 nm thick silica shells formed within an hour using diglyceroxysilane; thinner or thicker shells were not readily available. Within the range of shell thicknesses synthesized, the presence of a silica shell on the gold nanoparticle did not significantly affect the absorbance maximum (~5 nm) of unaggregated particles. However, the change in absorbance wavelength upon aggregation of the particles was highly dependent on the thickness of the shell. With silica shells coating the AuNP, there was a significant decrease in the absorbance maximum of the aggregated particles, from ~578 to ~536 nm, as the shell thicknesses increased from ~1.7 to ~4.3 nm, because of increased distance between adjacent gold cores. These studies provide guidance for the development of colorimetric assays using silica-coated AuNP.     

DC magnetron sputtered polyaniline-HCl thin films for chemical sensing applications

Thin films of conducting polymers exhibit unique chemical and physical properties that render them integral parts in microelectronics, energy storage devices, and chemical sensors. Overall, polyaniline (PAni) doped in acidic media has shown metal-like electronic conductivity, though exact physical and chemical properties are dependent on the polymer structure and dopant type. Difficulties arising from poor processability render production of doped PAni thin films particularly challenging. In this contribution, DC magnetron sputtering, a physical vapor deposition technique, is applied to the preparation of conductive thin films of PAni doped with hydrochloric acid (PAni-HCl) in an effort to circumvent issues associated with conventional thin film preparation methods. Samples manufactured by the sputtering method are analyzed along with samples prepared by conventional drop-casting. Physical characterization (atomic force microscopy, AFM) confirm the presence of PAni-HCl and show that films exhibit a reduced roughness and potentially pinhole-free coverage of the substrate. Spectroscopic evidence (UV-vis, FT-IR, and X-ray photoelectron spectroscopy (XPS)) suggests that structural changes and loss of conductivity, not uncommon during PAni processing, does occur during the preparation process. Finally, the applicability of sputtered films to gas-phase sensing of NH(3) was investigated with surface plasmon resonance (SPR) spectroscopy and compared to previous contributions. In summary, sputtered PAni-HCl films exhibit quantifiable, reversible behavior upon exposure to NH(3) with a calculated LOD (by method) approaching 0.4 ppm NH(3) in dry air.     

Growth mode control for direct-gap core/shell Ge/GeSn nanowire light emission

Germanium-tin is a promising semiconductor alloy system for novel light emitting devices and optical sensors in the mid-IR region. For sufficiently high Sn compositions, the material has a direct band-gap near 0.5 eV, and could have applications either as a detector or an emitter. Although high Sn compositions have been achieved in Ge_1−xSn_x through a variety of growth strategies, the understanding of how chemical vapor deposition conditions affect Sn composition and optical properties in core–shell Ge/Ge_1−xSn_x nanowires is still lacking. In this study, gas precursor partial pressures and shell growth temperatures are systematically varied to provide guiding principles to overcome obstacles for higher Sn incorporation. We achieve a direct band-gap material using an elastically compliant Ge core nanowire substrate. In the course of the growth study, we demonstrate several findings regarding the Ge_1−xSn_x shell growth mechanism. First, we observe an H₂ passivation effect in which higher H₂ to SnCl₄ partial pressure ratio results in a concurrent increase in axial wire growth and decrease in radial growth. Second, we find that Ge_1−xSn_x shell growth in the studied CVD process is mass transport limited. Third, our results suggest that low shell growth temperature and high shell growth rate facilitate high Sn composition through metastable Sn solute trapping due to suppressed surface diffusion relative to the velocity of advancing shell surface steps. In this work, we demonstrate single nanowire photoluminescence at room temperature from core-shell Ge/Ge_0.88Sn_0.12 nanowires. Understanding the Ge_1−xSn_x shell growth mechanism via chemical vapor deposition (CVD) facilitates achieving minimal residual strain in the shell and the high crystalline quality and large Sn composition necessary for the observed optical properties. The results are universally applicable to Ge_1−xSn_x thin film epitaxy on compliant substrates including grown or etched nanowires, nanosheets, or free-standing 2D crystals.     

Synthesis and characterization of Fe doped CeO2 nanoparticles for pigmented ultraviolet filter applications

Iron doped CeO2 nanoparticles with doping concentrations between 0 and 30 mol% were synthesized by the co-precipitation method for potential application as a pigmented ultraviolet filtration material. Each sample was calcined in air and in argon. The iron solubility limit in the CeO2 lattice was found to be between 10 and 20 mol%. Raman spectroscopy results revealed that both iron doping and argon calcination increase the concentration of oxygen vacancies in the CeO2 lattice. Iron doping causes a blue-shift of the absorbance spectrum, which can be linked to the decreased crystallite size, as obtained by XRD peak broadening using the Scherrer formula. The undoped samples showed weak ferromagnetic behaviour whereas the doped samples were all paramagnetic.     

TL emission spectra from differently doped LiF:Mg detectors

There are two widely applied types of thermoluminescent detectors based on LiF:Mg luminophor: Lif:Mg,Ti and highly sensitive LiF:Mg,Cu,P. The role of luminescence centres in these materials is usually attributed to defects connected with, respectively, titanium and phosphorus dopants. In order to check how composition of dopants introduced into the LiF lattice influences emission spectra, measurements on a series of variously doped LiF:Mg samples were performed. Apart from LiF:Mg,Cu,P and LiF:Mg,Ti detectors with different concentration of activators, an experimental sample being a kind of a 'hybrid' between both standard materials was also prepared. It was synthesised with concentrations of magnesium and copper identical to those used for LiF:Mg,Cu,P preparation. but instead of phosphorus it was doped with titanium (LiF:Mg,Cu,Ti). The measurements of the emission spectra were performed by using a liquid nitrogen cooled CCD 1024E detector with an SP150 spectrograph. During the measurements the samples were placed inside a cryostat in a vacuum. Resulting data were numerically deconvoluted for individual peaks with respect to the wavelength and the temperature. The glow curve shape of this material resembles that of LiF:Mg,Cu,P, while sensitivity is at the level of LiF:Mg,Ti. Preliminary results indicate that emission of the LiF:Mg,Cu,Ti sample is similar to that of LiF:Mg,Cu,P rather than to LiF:Mg,Ti, showing a maximum for wavelengths well below 400 nm.     

Polyaniline films deposited by anodic polymerization: Properties and applications to chemical sensing

Polyaniline conductive thin films have been used for the detection of a number of important gases and vapors: organic solvents, ammonia, oxygen, hydrogen sulfide, nitrogen and sulfur oxides. The films can be produced by spin coating, thermal evaporation, the Langmuir–Blodgett technique and cyclic voltammetry. This paper presents preliminary results on acidity sensing with electrodeposited polyaniline layers. Polyaniline conductive thin films were prepared by anodic polymerization from an acidic solution of the monomer on two kinds of substrates: gold plated silicon and indium-tin oxide on glass. Ultraviolet-visible (UV-VIS) spectrometry of layers showed a maximum absorption peak around 800 nm for all the samples investigated (independent of preparation conditions) and revealed that the polymeric films were in the emeraldine base form, 18–25% protonated. The room-temperature in-plane d.c. conductivities of the polymer films were found to be between 4×10^–9 S cm^–1 and 9×10^–10 S cm^–1 (deposition rate approximately 4 m h^–1; film thickness 750–1100 nm). Immersion of the polyaniline films in dilute hydrochloric solution resulted in changes in the d.c. conductivity by up to nine orders of magnitude, reaching a value of 4×10^–2 S cm^–1 while immersed in the acidic solution. Humidity tests carried out by exposing polyaniline samples to water vapors changed the d.c. conductivity by one order of magnitude to 1.34×10^–8 S cm^–1.     

Synthesis and upconversion luminescence of NaYF4:Yb, Tm/TiO2 core/shell nanoparticles with controllable shell thickness

NaYF4:Yb, Tm/TiO2 core/shell nanoparticles were synthesized by a two-step method. First, the NaYF4:Yb, Tm nanocrystals were prepared using solvothermal technology; then, TiO2 shells were deposited on the nanocrystals by the hydrolysis of titanium ethoxide (TEOT) to form core/shell structures. By controlling the reaction time, we can adjust the thickness of TiO2 shell and thereby the weight percentage of TiO2 in the core/shell nanoparticles. The effect of shell thickness on the upconversion fluorescence of NaYF4:Yb, Tm nanocrystals was investigated in detail.     

Low-macroscopic field emission properties of wide bandgap copper aluminium oxide nanoparticles for low-power panel applications

Field emission properties of CuAlO(2) nanoparticles are reported for the first time, with a low turn-on field of approximately 2 V μm(-1) and field enhancement factor around 230. The field emission process follows the standard Fowler-Nordheim tunnelling of cold electron emission. The emission mechanism is found to be a combination of low electron affinity, internal nanostructure and large field enhancement at the low-dimensional emitter tips of the nanoparticles. The field emission properties are comparable to the conventional carbon-based field emitters, and thus can become alternative candidate for field emission devices for low-power panel applications.