Ni/Ni3C core-shell nanochains and its magnetic properties: one-step synthesis at low temperature

One-dimensional Ni/Ni3C core-shell nanoball chains with an average diameter by around 30 nm were synthesized by means of a mild chemical solution method using a soft template of trioctylphosphine oxide (TOPO). It was revealed that the uniform Ni nanochains were capped with Ni3C thin shells by about 1-4 nm in thickness and each Ni core consists of polygrains. The coercivity of the core-shell nanochains is much enhanced (600 Oe at 5 K) and comparable with single Ni nanowires due to the one-dimensional shape anisotropy. Deriving from the distinctive structure of Ni core and Ni 3C shell, this architecture may possess a possible bifunctionality. This unique architecture is also useful for the study on the magnetization reversal mechanism of one-dimensional magnetic nanostructure.     

Highly luminescent blue emitting CdS/ZnS core/shell quantum dots via a single-molecular precursor for shell growth

Highly luminescent blue-emitting CdS/ZnS core/shell quantum dots (QDs) were synthesized in N-oleoylmorpholine by two facile steps: first, the CdS core QDs were prepared via a simple one-pot method involving a direct reaction of Cd precursor cadmium stearate and S precursor S powder in solvent N-oleoylmorpholine; second, ZnS shells were successively overcoated on CdS core through the decomposition of single molecular precursor zinc diethyldithiocarbamate. The thickness of shell was precisely tuned by controlling drip feed speed and amount of shell precursor. The obtained CdS/ZnS core/shell QDs showed the maximum photoluminescent quantum yield of 54.8% and narrow spectra bandwidth, exhibiting high monodispersity, good color purity and long fluorescent lifetimes. The CdS/ZnS core/shell QDs with tunable emission wavelength of 424–470 nm were obtained by controlling the thickness of ZnS shell overgrown on different-sized CdS QDs, which are promising materials for blue light-emitting devices.     

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.     

炭化聚甲基丙烯酸甲酯／聚丙烯腈核壳聚合物制备炭纳米空心球

采用炭化聚甲基丙烯酸甲酯(PMMA)/聚丙烯腈(PAN)核壳聚合物的方法制备了炭纳米空心球.以两步无皂乳液聚合法制备了PMMA/PAN核壳粒子:首先以间歇无皂乳聚合法制备出直径约200 nm的PMMA粒子乳液,再以其作为种子乳液,以饥饿滴定法在PMMA外表而聚合一层厚度约30 nm的PAN外壳.将制备的PMMA/PAN乳液冷冻干燥后,分别经过250℃预氧化及1000℃炭化工艺,制备了炭纳米空心球.透射电镜结果显示所有核壳粒子均炭化成空心球并呈现交联状态.     

Photoluminescence properties of single crystalline ZnO/CdS core/shell one-dimensional nanostructures

ZnO/CdS core/shell one-dimensional nanostructures were synthesized using ZnO nanorod arrays as templates, which were fabricated by a vapor transport process. CdS shells with various thicknesses were epitaxially grown on the ZnO nanorod arrays by metal organic chemical vapor deposition. Selected area electron diffraction measurement revealed that both ZnO cores and CdS shells were single crystalline growing along the c-axis. The photoluminescence properties of the ZnO/CdS core/shell nanostructures were also varied with different CdS shell thicknesses. A carrier transition process from ZnO to CdS was assumed to induce the enhancement of CdS photoluminescence.     

One-pot/three-step synthesis of zinc-blende CdSe/CdS core/shell nanocrystals with thick shells

A one-pot/three-step synthetic scheme was developed for phase-pure epitaxy of CdS shells on zinc-blende CdSe nanocrystals to yield shells with up to sixteen monolayers.The key parameters for the epitaxy were identified,including the core nanocrystal concentration,solvent type/composition,quality of the core nanocrystals,epitaxial growth temperature,type/concentration of ligands,and composition of the precursors.Most of these key parameters were not influential when the synthetic goal was thin-shell CdSe/CdS core/shell nanocrystals.The finalized synthetic scheme was reproducible at an almost quantitative level in terms of the crystal structure,shell thickness,and optical properties.     

Transport modulation in Ge/Si core/shell nanowires through controlled synthesis of doped Si shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.     

Preparation and characterization of bio-compatible Fe3O4@Polydopamine spheres with core/shell nanostructure

Bio-compatible and superparamagnetic Fe₃O₄@Polydopamine nanocomposites with well-defined core/shell nanostructures have been successfully synthesized by using an in situ self-polymerization method. The size of the core/shell product can be controlled by varying the size of the central Fe₃O₄ core, and different thicknesses of the PDA shells are obtained by tuning the dopamine monomer concentration. The morphology, phase composition and crystallinity of the as-prepared nanocomposites have been characterized by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). The 5-dimethylthiazol-2-yl-2,5-diphenyl-tetrazolium bromide (MTT) cytotoxic analysis shows that the product is biocompatible, with a low toxicity. These unique core/shell materials would be applied in catalyst supports or drug delivery.     

Luminescent properties of ZnS:Mn2+/ZnS core/shell nanocrystals

High-quality ZnS:Mn2+/ZnS core/shell nanocrystals (NCs) with a core crystal diameter of 6.1 nm and 1.15 nm thick shells were synthesized via a high-boiling solvent process. The energy levels of the conduction band and valance band are estimated to be -3.2 eV and -6.8 eV by cyclic voltammetry and ultraviolet-visible (UV-vis) absorption spectra. The ZnS:Mn2+/ZnS NC emission peak is primarily located at 580 nm under 310 nm light excitation, originating from the charge transition from 4T1 to 6A1 within the 3d5 configuration of the Mn2+ ion. Based on ZnS:Mn2+/ZnS NCs as the active layer electroluminescent devices, the emission peak mainly locates at 460 nm with one shoulder emission peaking at 580 nm. The photoluminescence and electroluminescence properties of ZnS:Mn2+/ZnS NCs are investigated in the view of charge carrier injection and energy level alignment.     

Dark-red-emitting CdTe0.5Se0.5/Cd0.5Zn0.5S quantum dots: Effect of chemicals on properties

CdTe_0.5Se_0.5/Cd_0.5Zn_0.5S core/shell quantum dots (QDs) with a tunable photoluminescence (PL) range from yellow to dark red (up to a PL peak wavelength of 683 nm) were fabricated using various reaction systems. The core/shell QDs created in the reaction solution of trioctylamine (TOA) and oleic acid (OA) at 300 °C exhibited narrow PL spectra and a related low PL efficiency (38%). In contrast, the core/shell QDs prepared in the solution of 1-octadecene (ODE) and hexadecylamine (HDA) at 200 °C revealed a high PL efficiency (70%) and broad PL spectra. This phenomenon is ascribed that the precursor of Cd, reaction temperature, solvents, and ligands affected the formation process of the shell. The slow growth rate of the shell in the solution of ODE and HDA made QDs with a high PL efficiency. Metal acetate salts without reaction with HDA led to the core/shell QDs with a broad size distribution.     

Synthesis and magnetic properties of FePt nanoparticles with hard nonmagnetic shells

Chemically synthesized FePt nanoparticles were coated with nonmagnetic SiO2 and MnO shells by sol-gel and polyol processes. TEM images show that the FePt/SiO2 nanoparticles exhibit a thick spherical shell. The size and morphology of the MnO shell can be controlled by changing the reaction temperature, the molar ratio of surfactants/Mn(acac)2, and/or the concentration of precursor. The morphology of the MnO shell can be either spherical-like or cubic-like, depending on whether the molar ratio of surfactants/Mn(acac)2 is less than or larger than 2. From XRD measurements, the spherical core/shell nanoparticles exhibit 3D random crystallographic orientation, while the cubic core/shell nanoparticles prefer (200) texture. The magnetic moment of FePt particles can be enhanced by coating with SiO2 and MnO shells. Furthermore, the agglomeration of FePt particles upon the thermal annealing can be significantly inhibited with SiO2 and MnO shells.     

Simple synthesis of ZrO2/SiOx core/shell nanofibers using ZrSi2 with gallium

ZrO₂/SiO_x core/shell nanofibers with diameter ~ 50 nm were synthesized by the thermal oxidation of ZrSi₂ substrates with gallium. The crystalline ZrO₂ cores were grown with amorphous SiO_x shells. It is proposed that the growth of crystalline ZrO₂ core was guided by the prior supersaturation of Zr species in the molten gallium film, whereas the amorphous SiO_x shell could be attributed to the deposition of SiO vapor on the surface of ZrO₂ core. In addition, the ZrO₂/SiO_x core/shell nanofibers show a wide visible photoluminescence (PL) emission at 480 nm, which should originate from the SiO_x shells.     

Characterization of highly luminescent LaPO4:Eu3+/LaPO4 one-dimensional core/shell heterostructures

Highly luminescent LaPO4:Eu3+/LaPO4 one-dimensional (1D) core/shell heterostructures were successfully synthesized by a mild and simple self-aggregation process under refluxing or hydrothermal conditions. The resulting 1D core/shell heterostructures were characterized using a variety of techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS) to demonstrate successful coating by the crystalline LaPO4. In addition, a possible formation mechanism for this core/shell heterostructure was proposed. Finally, the photoluminescence property of the LaPO4:Eu3+/LaPO4 1D core/shell heterostructures was investigated in detail, which illustrates that the core/shell heterostructures remarkably increase the luminescence efficiency because the LaPO4 shells effectively eliminate surface trap-states and suppress the energy quenching in the energy-transfer processes.     

Photoluminescence polarization in strained GaN/AlGaN core/shell nanowires

The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (μ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.     

Fabrication and size-dependent optical properties of copper/lophine core/shell nanocomposites

Copper/lophine core/shell nanocomposites were fabricated through the self-assembly of lophine on the surfaces of copper nanostructures. The growth of the lophine shells was induced by the cooperation of coordination and several other noncovalent interactions. The dimensions, structures, and morphologies of the nanocomposites can be readily controlled by just altering the size and morphology of the copper core. The formation mechanism of the core/shell nanocomposites was put forward on the basis of a series of characterizations. The optical properties of the shell lophine layer can be tuned by changing the size of the copper core.     

Multifunctional nanoparticles for dual imaging

For imaging with different modalities, labels, which provide contrast for all modalities, are required. Colloidal nanoparticles composed out of an inorganic core and a polymer shell offer progress in this direction. Both, the core and the polymer shell, can be synthesized to be fluorescent, magnetic, or radioactive. When different cores are combined with different polymer shells, different types of particles for dual imaging can be obtained, as for example, fluorescent cores with radioactive polymer shells. Properties and perspectives of such nanoparticles for multimodal imaging are discussed.     

Fe2o3 shell growth on Pt nanoparticles

Fe2O3 shells have been synthesized around Pt cores to create Pt@Fe2O3 core-shell nanoparticles. The synthesis conditions allow control of the shell shape and allow the preparation of both hexagonal shells and spherical shells. 2D cross-sectional TEM images show that the cores are not positioned at the centers of the shells. By rotating the nanoparticles and monitoring the apparent motions of the cores in the 2D cross-sectional images, it is possible to determine quantitatively the radial position of the Pt core with respect to the center of the Fe2O3 shell. The distribution of core positions within the core-shell structures is bimodal. These observations suggest that the Fe2O3 shells grow on the Pt cores by a nucleation process, rather than layer-by-layer growth.     

Core/multishell nanowire heterostructures as multicolor, high-efficiency light-emitting diodes

We report the growth and characterization of core/multishell nanowire radial heterostructures, and their implementation as efficient and synthetically tunable multicolor nanophotonic sources. Core/multishell nanowires were prepared by metal-organic chemical vapor deposition with an n-GaN core and InxGa1-xN/GaN/p-AlGaN/p-GaN shells, where variation of indium mole fraction is used to tune emission wavelength. Cross-sectional transmission electron microscopy studies reveal that the core/multishell nanowires are dislocation-free single crystals with a triangular morphology. Energy-dispersive X-ray spectroscopy clearly shows shells with distinct chemical compositions, and quantitatively confirms that the thickness and composition of individual shells can be well controlled during synthesis. Electrical measurements show that the p-AlGaN/p-GaN shell structure yields reproducible hole conduction, and electroluminescence measurements demonstrate that in forward bias the core/multishell nanowires function as light-emitting diodes, with tunable emission from 365 to 600 nm and high quantum efficiencies. The ability to synthesize rationally III-nitride core/multishell nanowire heterostructures opens up significant potential for integrated nanoscale photonic systems, including multicolor lasers.     

Synthesis of core-shell nanostructures of Co3O4@SiO2 with controlled shell thickness (5-20 nm) and hollow shells of silica

Synthesis of uniform silica shell over Co3O4 nanoparticles was carried out using the colloidal solutions of Tergitol and cyclohexane. The shell could be controlled to a thickness of up to 20 nm by varying different parameters such as the amount of tetraethylorthosilicate, concentration of Co3O4 nanoparticles, reaction time and the presence of water and 1-octanol. Control of the amount of water (required for hydrolysis) appears to be the key factor for controlling the shell thickness. The methodology used is suitable to form shell over nanoparticles (present in powder form; synthesized at high temperature) which have high degree of agglomeration. Hollow shells of silica were obtained by the dissolution of the oxide core of Co3O4@SiO2 core-shell nanostructures. The composition of these core-shell nanostructures was confirmed by high-resolution transmission electron microscopy and elemental mapping by energy dispersive X-ray analysis. The hollow shells were characterized by using TEM, EDX and IR. Electron paramagnetic resonance studies of the core-shell nanostructures indicate the presence of free radicals on silica shell due to the presence of dangling bonds in the silica. Increase in the magnetic susceptibility was observed for these core-shell nanostructures.