Enhanced charge storage capability of Ge/GeO(2) core/shell nanostructure

A Ge/GeO(2) core/shell nanostructure embedded in an Al(2)O(3) gate dielectrics matrix was produced. A?larger memory window with good data retention was observed in the fabricated metal-insulator-semiconductor (MIS) capacitor for Ge/GeO(2) core/shell nanoparticles compared to Ge nanoparticles only, which is due to the high percentage of defects located on the surface and grain boundaries of the GeO(2) shell. We believe that the findings presented here provide physical insight and offer useful guidelines to controllably modify the charge storage properties of indirect semiconductors through defect engineering.     

水热法制备Ge/SiOx纳米电缆

以混合的氧化锗粉和硅粉为原料,采用水热法在高温高压下制备出具有核-壳同轴结构的Ge／SiOx纳米电缆。扫描和透射电镜研究表明这种Ge／SiOx纳米同轴电缆的产量高,直径分布均匀,长度可达微米级,并证实其为非晶态SiOx包裹Ge内核的核-壳结构。Ge芯线沿着[211]方向生长。Ge／SiOx纳米同轴电缆的生长过程遵循气-液-固和氧化物辅助生长机制,与原料中GeO2与Si的比率有关。     

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.     

Synthesis and luminescence properties of ZnS:Mn/ZnS core/shell nanorod structures

Mn-doped ZnS nanorods synthesized by solvothermal method were successfully coated with ZnS shells of various thicknesses. The powder X-ray diffraction (XRD) measurements showed the ZnS:Mn nanorods were wurtzite structure with preferential orientation along c-axis. Transmission electron microscopy images (TEM) revealed that the ZnS shells formed from small particles, growing along a-axis orientation, which was proved by the XRD measurements. Room temperature photoluminescence (PL) spectra showed that the intensity of Mn emission first increased and then decreased with the thickening of the ZnS shells. The effects of ZnS shells on the luminescence properties of ZnS:Mn nanorods is discussed.     

Growth and structural characterization of InN/In2O3 coaxial nanocables

A synthesis route to coaxial nanocables has been developed using low dissociation temperature of InN and high melting point of In₂O₃. InN/In₂O₃ coaxial nanocables were successfully synthesized by a reaction of the mixture of In and In₂O₃ powder with a constant flowing ammonia atmosphere through two-stage. High-resolution transmission electron microscopy observations demonstrated that the nanocable consists of wurtzite InN nanowire core, In₂O₃ outer shell separated in the radial direction. The InN/In₂O₃ coaxial nanocables were 20–80 nm in diameter and up to several tens of micrometers in length. It is proposed that the low dissociation temperature property of InN material acts as an important role to form In₂O₃ protecting layers which coated the surface of InN nanowires and high melting point of In₂O₃ prevent InN to further dissociate. The method results in high purity, and high reproducibility of the InN/In₂O₃ nanocable-structures.     

Self-organized growth of Si/Silica/Er2Si2O7 core-shell nanowire heterostructures and their luminescence

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.     

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.     

Performance analysis of a Ge/Si core/shell nanowire field-effect transistor

We ana/lyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp3d5s* tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 and 85% of the ballistic limit. For this approximately 15 nm diameter Ge nanowire, we also find that 14-18 modes are occupied at room temperature under ON-current conditions with ION/IOFF = 100. To observe true one-dimensional transport in a 110 Ge nanowire transistor, the nanowire diameter would have to be less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on a common basis nanowire transistors of various materials and structures.     

Alteration of amorphous Bi4Ge3O12 by Tl2O and Na2O

Ternary thallium and sodium bismuth germanate glasses were prepared and their densities refractive indices, and infra-red spectra obtained. The effect of univalent cations on the stability of the arrangement of decoupled GeO₄ tetrahedra in amorphous Bi₄Ge₃O₁₂ was compared with the effect of divalent cations. The molar volumes of glasses with the nominal mol% composition 20 M₂O(MO).20 Bi₂O₃.60 GeO₂ are directly related to the size and charge type of M^z+. However, the molar volumes of such glasses are inversely related to the ionic potential (z/r) of M^z+. The infra-red spectra of these ternary glasses exhibitv _Ge?O shifts that reflect the presence of both isolated and small clusters of GeO₄ tetrahedra compared to amorphous Bi₄Ge₃O₁₂. This slight increase in the degree of polymerization appears to be directly related to the ionic potential of M^z+.     

CdS/ZnO核壳结构纳米微粒的合成及其发射光谱研究

采用单分子前驱体热分解的方法合成了单分散CdS纳米晶,以CdS纳米晶作为核,在CTAB辅助下,对其表面进行修饰,荧光光谱表明CdS/ZnO核壳结构被成功合成。考查了温度对包覆的影响,结果表明,随着温度的升高晶体结晶越好,包覆越来越完全,ZnO包覆在CdS纳米晶的表面而掩盖了CdS纳米晶的缺陷,使得缺陷发光减弱而带隙发光增强。     

Synthesis of highly photo-stable CuInS2/ZnS core/shell quantum dots

CuInS₂ quantum dots are considered near-ideal fluorophores based on their bright emission and low toxicity. However, CuInS₂ quantum dots are still bothered by their sensitivity to surface chemistry and chemical environment. Traditionally, the CIS QDs require an additional coating process to be encapsulated inside silica sphere or organic polymer. Up till now, few works have been made concerning improving the intrinsic stability of CIS QDs. In an effort to improve the stability of CuInS₂ quantum dots, we came up with a new method by increasing the ZnS shell thickness. These QDs were characterized by photoluminescence, HRTEM, XRD and XRF analysis. We investigated the influence of ZnS shell thickness on the ambient stability of CIS/ZnS QDs. The results demonstrated that a thicker ZnS shell helped significantly improve both photostability and chemical stability of the QDs. Finally, the thick shell QDs were dispersed into transparent polymer matrix and fabricated into a LED device, which also gave much more stability compared with conventional QDs.     

Depolymerization of GeO2 and GeO2 · Sb2O3 glasses by Bi2O3

Glasses that contain at least 60 mol% GeO₂ were prepared in the Bi₂O₃ · GeO₂ and Bi₂O₃ · Sb₂O₃ · GeO₂ systems. Their densities, refractive indices, and infra-red spectra were recorded. Negative molar volume deviations and positive refraction deviations occur for all of the binary glasses. These create deviations for the 60 to 80 mol % GeO₂ ternary glasses that indicate non-ideal mixing when Sb³⁺ substitutes for Bi³⁺. Also, the main Ge-O stretching vibration shifts to as low as 695 cm^?1 for the Bi₂O₃-rich binary and ternary glasses. All of these findings show that Bi₂O₃ more effectively depolymerizes GeO₂ than does Sb₂O₃. The probable structural reasons for this behaviour are discussed.     

Synthesis and characterization of silica core/nano-ferrite shell particles

Core/shell particles were synthesized by assembling oppositely charged ferrite (Fe₃O₄ or NiFe₂O₄) nanoparticles on the surface of monodispersed silica core particles (having size ～0.4 μm) prepared by hydrolysis and condensation of tetraethylortosilicate. Optimal conditions for synthesis of silica core/nano-Fe₃O₄ shell particles were found at pH ～ 5.4. The obtained particles have superparamagnetic behavior above a blocking temperature of ≈25 K, which make them very attractive for a broad range of biomedical and bioengineering applications. Incorporation of nickel into ferrite structure could not be achieved at lower pH value, so functionalization of core particles was required. Incorporation of nickel into ferrite structure was successful at pH above 7, however at higher pH the formation rate of nickel–ferrite particles becomes very fast and the self-aggregation dominates the competing formation of the nickel–ferrite shell. Because of that the self-aggregation was prevented by surface modification of nickel–ferrite nanoparticles with citric acid before their deposition on the functionalized silica core and homogenous and continuous NiFe₂O₄ shell was finally obtained.     

Continuous white luminescence of ZnS:Pb2+ and core/shell ZnS:Pb2+/ZnS nanocrystals

High-quality water-soluble ZnS:Pb2+ nanocrystals were synthesized via a simple chemical codepositing method. The as-synthesized ZnS:Pb2+ nanocrystals show high monodispersity and crystallinity with a narrow size distribution (3.2 +/- 0.4 nm). ZnS:Pb2+/ZnS core/shell structures were also obtained by coating a ZnS shell displaying significantly enhanced photoluminescence (PL) intensity and photostability. For the ZnS:Pb2+/ZnS samples the position of emission spectrum shows a red-shift of approximately 10 nm, which produces a fairly pure white emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31, 0.33). These phenomena are explained by a model of multiple Pb2+ luminescent centers.     

CuO/Ta2O5 core/shell nanoparticles produced by arc-discharge in water

The formation of novel nanoparticles, i.e. CuO/Ta₂O₅ core/shell, via anodic arc-discharge in water is reported. Typical core diameters and shell thicknesses are about 20 and 5 nm, respectively. Both core and shell are crystalline. The resulting core/shell-structured nanopowder exhibits dielectric behavior.     

Nickel oxide/polypyrrole/silver nanocomposites with core/shell/shell structure: Synthesis,characterization and their electrochemical behaviour with antimicrobial activities

Magnetic and conducting Nickel oxide–polypyrrole (NiO/PPy) nanoparticles with core–shell structure were prepared in the presence of Nickel oxide (NiO) in aqueous solution containing sodium dodecyl benzenesulfonate (SDBS) as a surfactant as well as dopant. A stable dispersion of silver (Ag) nanoparticles was synthesized by chemical (citrate reduction) method. NiO/PPy nanocomposites were added to the Ag colloid under stirring. Ag nanoparticles could be electrostatically attracted on the surface of NiO/PPy nanocomposites, leading to formation of NiO/PPy/Ag nanocomposites with core/shell/shell structure. The morphology, structure, particle size and composition of the products were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, cyclic voltammetry (CV) and current–voltage (I–V) analysis. The resultant nanocomposites have the good conductivity and excellent electrochemical and catalytic properties of PPy and Ag nanoparticles. Furthermore, the nanocomposites showed excellent antibacterial behaviour due to the presence of Ag nanoparticles in the composite. The thermal stability of NiO–PPy as well as NiO/PPy/Ag nanocomposites was higher than that of pristine PPy. Studies of IR spectra suggest that the increased thermal stability may be due to interactions between NiO and Ag nanoparticles with the PPy backbone.     

Synthesis of ZnSe quantum dots and ZnSe-ZnS core/shell nanostructures

Colloidal ZnSe nanocrystals were synthesized in hot mixtures of long-chain alkylamines, fatty acids, and alkylphosphines. It was possible to tune the size of nanocrystals by varying the reaction time. Transmission electron microscope images showed the presence of spherical ZnSe nanocrystals and X-ray diffraction pattern of ZnSe nanocrystals showed the existence of both the crystalline phase, namely, wurtzite and zinc blende. The ZnSe nanocrystals were then passivated with higher band gap ZnS; this lead to a 2.6-fold enhancement in the integrated photoluminescence intensity of ZnSe nanocrystals. We also synthesized the reverse type core/shell ZnS/ZnSe nanocrystals. These exhibited a significant red shift in the absorption edge after coating with a thin ZnSe shell.     

Observation of hole accumulation in Ge/Si core/shell nanowires using off-axis electron holography

Hole accumulation in Ge/Si core/shell nanowires (NWs) has been observed and quantified using off-axis electron holography and other electron microscopy techniques. The epitaxial [110]-oriented Ge/Si core/shell NWs were grown on Si (111) substrates by chemical vapor deposition through the vapor-liquid-solid growth mechanism. High-angle annular-dark-field scanning transmission electron microscopy images and off-axis electron holograms were obtained from specific NWs. The excess phase shifts measured by electron holography across the NWs indicated the presence of holes inside the Ge cores. Calculations based on a simplified coaxial cylindrical model gave hole densities of (0.4 ± 0.2) /nm(3) in the core regions.     

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.%.