Effects of passivation and postannealing on the photoluminescence properties of MgO/SiO2 core‐shell nanorods

MgO nanorods were grown by the thermal evaporation of Mg₃N₂ powders on the Si (100) substrate coated with a gold thin film. The MgO nanorods grown on the Si (100) substrate were a few tens of nanometers in diameter and up to a few hundreds of micrometers in length. MgO/SiO₂ core‐shell nanorods were also fabricated by the sputter‐deposition of SiO₂onto the MgO nanorods. Transmission electron microscopy (TEM) and X–ray diffraction (XRD) analysis results indicated that the cores and shells of the annealed core‐shell nanorods were a face‐centered cubic‐type single crystal MgO and amorphous SiO₂, respectively. The photoluminescence (PL) spectroscopy analysis results showed that SiO₂ coating slightly decreased the PL emission intensity of the MgO nanorods. The PL emission of the MgO/SiO₂ core‐shell nanorods was, however, found to be considerably enhanced by thermal annealing and strongly depends on the annealing atmosphere. The PL emission of the MgO/SiO₂ core‐shell nanorods was substantially enhanced in intensity by annealing in a reducing atmosphere, whereas it was slightly enhanced by annealing in an oxidative atmosphere. The origin of the PL enhancement by annealing in a reducing atmosphere is discussed with the aid of energy‐dispersive X‐ray spectroscopy analyses. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and photoluminescence properties of GaN/ZnO core‐shell nanowires with their shells sputtered

We have reported the preparation of ZnO‐coated GaN nanowires and investigated changes in the structural and photoluminescence (PL) properties by the application of a thermal annealing process. For fabricating the core‐shell nanowires, Zn target was used to sputter ZnO shell onto GaN core nanowires. X‐ray diffraction (XRD) analysis indicated that the annealed core‐shell nanowires clearly exhibited the ZnO as well as GaN phase. The transmissoin electron microscopy (TEM) investigation suggested that annealing has induced the crystallization of ZnO shell layer. We have carried out Gaussian deconvolution analysis for the measured PL spectra, revealing that the core GaN nanowires exhibited broad emission which consist of red, yellow, blue, and ultraviolet peaks. ZnO‐sputtering induced new peaks in the green region. Thermal annealing reduced the relative intensity of the green emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation,structure, and photoluminescence properties of Ga2O3/SnO2 coaxial nanowires

Ga₂O₃/SnO₂ coaxial nanowires were synthesized by thermal evaporation of GaN powders and then atomic layer deposition of SnO₂. Transmission electron microscopy (TEM) and X‐ray diffraction (XRD) analysis results indicate that the Ga₂O₃ cores and the SnO₂ shells of the coaxial nanowires after thermal annealing are single crystals with monoclinic and simple orthorhombic structures, respectively, although the SnO₂ shells are amorphous before annealing. Our results also show that photoluminescence (PL) emission can be enhanced by thermal annealing in an H₂/N₂ atmosphere. EDX concentration profile suggests that the enhancement in the bluish green emission is due to the increase in the concentration of the Ga vacancies in the cores during the H₂/N₂ annealing. On the other hand, a red emission is newly formed while the bluish green emission is degraded by annealing in an oxygen or nitrogen atmosphere (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,Raman, and photoluminescence properties of double‐shelled coaxial nanocables of In2O3 core with ZnO and AZO shells

We synthesized In₂O₃/ZnO/Al‐doped ZnO (AZO) core‐double shell nanowires, in which the inner shell (ZnO) and the outer shell (AZO) have been subsequently deposited on the core In₂O₃ nanowires. With their one‐dimensional morphology being preserved, the X‐ray diffraction (XRD), lattice‐resolved transmission electron microscopy (TEM) image, selected area electron diffraction, and Raman spectrum coincidentally revealed that the shell was comprised of hexagonal ZnO phase. In addition, TEM‐EDX investigation revealed the presence of Al elements in the shell region. The thermal annealing at 700 °C did not significantly change the nanowire morphology, however, the XRD spectrum indicated that the ZnO phase was crystallized by the annealing. PL spectrum of the 700 °C‐annealed In₂O₃/ZnO/AZO core‐double shell nanowires was comprised of three Gaussian bands at approximately 2.1 eV, 2.4 eV, and 3.0 eV, respectively. The integrated intensities of 2.1 eV‐, 2.4 eV‐, and 3.0 eV‐bands were decreased by the thermal annealing. This study will pave the road to the preparation and applicaition of double‐shelled nanowires. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Homoepitaxial growth and photoluminescence of self‐assembled In‐doped ZnS nanowire bundles

Self‐assembled In (Indium)‐doped ZnS nanowire bundles were synthesized via a thermal evaporation method without using any template. Vapor ‐ solid homoepitaxial growth was found to be the key reason for the formation of close‐packed nanowire bundles grown on the surface of microscale sphere‐shaped ZnS crystal. X‐ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM) analysis demonstrate that the In‐doped ZnS nanowires have the cubic structure, and there are numerous stacking faults along the <111> direction. Photoluminescence (PL) spectrum shows that the spectrum mainly includes two parts: a weak violet emission band centering at about 380 nm and a strong green emission band centering at about 510 nm. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of the emission from TeO2 nanorods by encapsulation with ZnO

TeO₂‐core/ZnO‐shell nanorods were synthesized by a two–step process comprising thermal evaporation of Te powders and atomic layer deposition of ZnO. Scanning electron microscopy images exhibit that the core‐shell nanorods are 50 ‐ 150 nm in diameter and up to a few tens of micrometers in length, respectively. Transmission electron microscopy and X‐ray diffraction analysis revealed that the cores and shells of the core‐shell nanorods were polycrystalline simple tetragonal TeO₂ and amorphous ZnO with ZnO nanocrystallites locally, respectively. Photoluminescence measurement revealed that the TeO₂ nanorods had a weak broad violet band at approximately 430 nm. The emission band was shifted to a yellowish green region (∼540 nm) by encapsulation of the nanorods with a ZnO thin film and the yellowish green emission from the TeO₂‐core/ZnO‐shell nanorods was enhanced significantly in intensity by increasing the shell layer thickness. The highest emission was obtained for 125 ALD cycles (ZnO coating layer thickness: ∼15 nm) and its intensity was much higher than that of the emission from the uncapsulated TeO₂ nanorods. The origin of the enhancement of the emission by the encapsulation is discussed in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Conversion from ZnO nanospindles into ZnO/ZnS core/shell composites and ZnS microspindles

The formation process of ZnO/ZnS core/shell microcomposites and ZnS microspindles prepared by the reaction of ZnO colloids and thioacetamide under hydrothermal conditions was investigated in detail by X‐ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy and selected‐area electron diffraction techniques. The precursors of spindlelike ZnO colloids were prepared by a hydrothermal method with the help of a surfactant. A growth mechanism was proposed to account for the formation of ZnO/ZnS core/shell microcomposites and ZnS microspindles. Luminescence measurement revealed that ZnO/ZnS core/shell microcomposites integrated the luminescence effect of ZnO and ZnS. The blue and green emissions were dramatically enhanced, while the orange emission disappeared. The results provide a good approach to tune the visible emission of the ZnO nanostructures by ZnS coating. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Catalyst‐free thermally‐evaporated growth and optical properties of ZnO nanowires on Si,GaN and sapphire substrates

Vertically well‐aligned zinc oxide nanowires (NWs) with high density were successfully synthesized on Si, sapphire and GaN/sapphire substrates by thermal evaporation of zinc powders without catalysts or additives. The growth behavior of ZnO NWs was strongly dependent on the substrate materials. The effects of the substrate position on the structures and properties of ZnO NWs were primarily discussed. The morphology and crystallinity of the resultant NWs were studied by scanning electron microscope, transmission electronic microscope and X‐ray diffraction. The photoluminescence (PL) characteristics of the ZnO NWs on the different substrates were studied. The results showed that the as‐grown ZnO NWs exhibit a sharp and strong ultraviolet emission at 3.27 eV and a very weak green emission at around 2.48 eV, indicating that the a‐synthesized NWs have excellent PL properties with good crystalline quality and can be an ideal candidate for making luminescent devices. By comparison of PL spectra, we revealed that the green‐to‐UV emission intensity ratios were considerably dependent on the substrate materials, which was explained by the difference in the structural morphology of the produced nanowires.     

Flux Bridgman growth and the influence of annealing temperatures on PL properties of ZnO single crystals

Transparent ZnO crystals were obtained by the flux Bridgman method from high temperature solution of 22 mol% ZnO‐78 mol% PbF₂ system. The influence of annealing temperatures on the photoluminescence (PL) of ZnO crystal was investigated. An ultraviolet emission peak at about 379 nm was observed in PL spectra and the peak position has a weak blueshift for annealed samples. A green band centered at 523 nm appeared in the annealed samples and its intensity enhanced with the increase of annealing temperatures, while the intensity of the ultraviolet peak decreased considerably. However, the ultraviolet emission peak became the strongest after annealing at 1000 °C. This phenomenon was considered to be associated with oxygen vacancy and F^‐ impurities induced by the PbF₂ flux. The results show that high temperature annealing in air seems helpful for improving the PL properties of ZnO crystal. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of copper germanate nanowires with good electrochemical sensing properties

Ternary copper germanate nanowires have been synthesized by the hydrothermal deposition route in an autoclave using germanium dioxide and copper oxide powders as the starting materials as well as the copper germanate nanowires have been investigated for cysteine sensing characteristics. The nanowires with the length of several dozens of micrometers are mainly composed of single crystalline orthorhombic CuGeO₃, which are supported by X‐ray diffraction, selected area electron diffraction, high‐resolution transmission electron microscopy and X‐ray photoelectron spectrum. Room temperature photoluminescence spectrum exhibits strong green light emission ability at 535.4 nm with a broad emission band. The copper germanate nanowires may be controlled grown by controlling the growth conditions of the nanowires. The nanowire growth process is explained using a solid state growth mechanism. The electrochemical analysis demonstrates that the copper germanate nanowires serve as a good electrochemical sensor with excellent electrochemical sensing ability for cysteine and catalytic ability for electron‐transfer process. The electrochemical result of the copper germanate nanowires presents an important advance toward the application for electrochemical sensors operating at room temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallography and cathodoluminescence of ultra‐long GaN nanowires

Ultra‐long GaN nanowires have been synthesized via a simple thermal evaporation process by heating mixed GaN and Ga₂O₃ powders in a conventional resistance furnace under ammonia gas at 1150 °C. The average length of GaN nanowires is estimated to be more than 100 μm after 30‐min growth, corresponding to a fast growth rate of more than 200 μm/h. Scanning electron microscope (SEM) observation indicated that the diameter of GaN nanowires was rather uniform along the growth direction and in the range of 100–200 nm. X‐ray diffraction (XRD) and transmission electron microscope (TEM) measurements confirmed that the GaN nanowires are crystalline wurtzite‐type hexagonal structure. Room‐temperature cathodoluminescence (CL) measurement indicated that an obvious red‐shift of the near band‐edge emission peak centered at 414 nm of the ultra‐long GaN nanowires and a wide shoulder in the range of 600–700 nm were observed. Possible reasons responsible for the red‐shift of the near band‐edge emission of the ultra‐long GaN nanowires was discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural investigation and electronic band transitions of nanostructured TiO2 thin films

Titanium dioxide (TiO₂) thin film was deposited on n‐Si (100) substrate by reactive DC magnetron sputtering system at 250 °C temperature. The deposited film was thermally treated for 3 h in the range of 400‐1000 °C by conventional thermal annealing (CTA) in air atmosphere. The effects of the annealing temperature on the structural and morphological properties of the films were investigated by X‐ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD measurements show that the rutile phase is the dominant crystalline phase for the film annealed at 800 °C. According to AFM results, the increased grain sizes indicate that the annealing improves the crystalline quality of the TiO₂ film. In addition, the formation of the interfacial SiO₂ layer between TiO₂ film and Si substrate was evaluated by the transmittance spectra obtained with FTIR spectrometer. The electronic band transitions of as‐deposited and annealed films were also studied by using photoluminescence (PL) spectroscopy at room temperature. The results show that the dislocation density and microstrain in the film were decreased by increasing annealing temperature for both anatase and rutile phases. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by CVD method

We report on microstructural, optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by a chemical vapor deposition (CVD) technique. Average diameters of the as grown (Zn,Mn)O nanorods and nanowires were ∼400 nm and ∼50 nm, respectively. X‐ray diffraction (XRD) and photoluminescence (PL) spectra provided the evidence that Mn was incorporated into ZnO lattice. PL spectra of the (Zn,Mn)O nanostructures showed shift in near band edge (NBE) emission at 396 nm together with a green band (GB) emission at 510 nm and a blue band (BB) emission at 460 nm. Magnetic measurements revealed mixed magnetic phases (ferromagnetic and antiferromagnetic) in the (Zn,Mn)O nanostructures. Vapor‐solid‐solid (VSS) mechanism was thought to be responsible for the growth of the nanostructures at low temperatures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ZnS-CdS核壳纳米微晶的制备与光学特性

采用微乳液法制备了核壳结构ZnS/CdS纳米微晶.以XRD、TEM表征其结构、粒度和形貌,UV、PL表征其光学性能.制得的纳米微晶近似呈球形,粒径4～5nm.研究了不同CdS壳层厚度的ZnS/CdS纳米微晶的光学性能,PL谱表明壳层CdS的修饰可减少ZnS的表面缺陷,表面态发射和非辐射跃迁减少,带边直接复合发光的几率增大,发光效率大大提高;在壳层CdS达到一定厚度时,PL谱却表现为CdS的特征发射,同时发现核心ZnS对壳层CdS的发光具有增强作用,提出了ZnS/CdS发光机理的能带模型.     

Controllable synthesis,characterization and optical properties of Ag@AgCl coaxial core‐shell nanocables

Core‐shell structures often exhibit improved physical and chemical properties. Developing a relatively general, facile, and low temperature synthetic approach for core‐shell structures with complex compositions is still a particularly challenging work. Here we report a general chemical conversion route to prepare high quality Ag@AgCl coaxial core‐shell nanocables via the redox reaction between Ag nanowires and FeCl₃ in solution. The powder X‐ray diffraction of the Ag@AgCl coaxial core‐shell nanocables shows additional diffraction peaks corresponding to AgCl crystals apart from the signals from the Ag nanowire cores. Scanning electron microscopy and transmission electron microscopy images of the Ag@AgCl coaxial core‐shell nanocables reveal that the Ag nanowires are coated with AgCl nanoparticles. The effect of the molar ratio of Fe:Ag on the morphology and optical absorption of the Ag@AgCl coaxial core‐shell nanocables is systematically investigated. The result shows that the optical absorption of Ag nanowires decreases gradually and that of AgCl nanoparticles improves gradually with the increase of the molar ratio of Fe:Ag. The formation process of the Ag@AgCl coaxial core‐shell nanocables has been discussed in detail. The present chemical conversion approach is expected to be employed in a broad range of applications to fabricate innovative core‐shell structures with different compositions and shapes for unique properties. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of semiconductor metal sulfide nanocrystals using microemulsion technique

The semiconductor nanocrystals ZnS, PbS, CdS and CuS were synthesized via microemulsion technique involving metal acetate, reducing agent (Na₂S) and Triton X‐100 as surfactant. Nanocrystals were characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The average size of ZnS, PbS, CdS and CuS nanocrystals were found to be 5.6 nm, 13.3 nm, 11.4 nm and 6.2 nm, respectively. Different parameters like surfactant (Triton X‐100) concentration, water‐to‐surfactant ratio (ω), precursor concentration [zinc acetate, (Zn(AC)₂], reducing agent concentration [sodium sulphide, (Na₂S)] were optimized to synthesize ZnS quantum dots.     

Epitaxial ZnS/Si core–shell nanowires and single-crystal silicon tube field-effect transistors

Epitaxial single-crystal ZnS/Si core–shell nanowires have been synthesized via a two-step thermal evaporation method. The epitaxial growth is due to the close match of crystal structure between zinc blende ZnS and diamond-like cubic Si. The nanowires have a uniform diameter of 80–200 nm and a length of several to several tens of micrometers. Single-crystal Si nanotubes can be obtained by chemical etching of the ZnS/Si core–shell structure. Characteristics of field-effect transistors (FETs) fabricated from the Si nanotubes suggests that the Si tubes show weak n-type semiconductivity with a mobility of about 3.7×10^?2 cm²/(V s), which is 1 order larger than that of intrinsic Si.     

Self‐template hydrothermal synthesis ZnS microspheres

Zinc sulfide (ZnS) microspheres were synthesized by a self‐template hydrothermal route using thiourea as sulphur source. The formation of these hollow spheres was mainly attributed to the oriented aggregation of ZnS nanocystals around the gas‐liquid interface between gas (H₂S, NH₃, or CO₂) and water followed by an Ostwald ripening process. The gas bubbles of H₂S, NH₃, or CO₂ produced during the reaction might play a soft‐template to form ZnS hollow microspheres. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), electron diffraction (ED), and photoluminescence (PL). The crystal structure of prepared ZnS microspheres is hexagonal phase polycrystalline. The average microspheres diameter is 1.5 ‐ 6 µm. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Controllable synthesis of novel In2O3 nanostructures and their field emission properties

Three kinds of novel indium oxide (In₂O₃) nanostructures, namely, nanorods, nanoflowers and nanowhiskers were synthesized on silicon substrate via a simple vapor‐phase transport method under atmospheric pressure. The In₂O₃nanostructures were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy‐dispersive X‐ray spectrometer (EDS) spectrum. The Raman spectra of these nanostructures showed four sharp scattering peaks centered at 308, 365, 522, and 628 cm^‐1, whose position and intensity were characteristic of standard Raman spectra for In₂O₃. The Room‐temperature photoluminescence (PL) spectra showed visible emissions centered around 576, 592, and 624 nm. Field emission measurements demonstrated that the nanoflowers possessed the best performance with a turn‐on field of 3.54 V/µm and a threshold field of 9.83 V/µm. And the field enhancement factors of these nanostructures are high enough for the application of field emission display devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cd2Ge2O6 nanowires grown by a simple hydrothermal route

Cd₂Ge₂O₆ nanowires have been synthesized by a simple hydrothermal route in the absence of any surfactants. The diameter and length of the Cd₂Ge₂O₆ nanowires with flat tips are 30‐300 nm and several dozens of micrometers, respectivley. X‐ray diffraction and high‐resolution transmission electron microscopy results show that the nanowires are composed of monoclinic Cd₂Ge₂O₆ phase. The growth condition dependence results show that the formation of the Cd₂Ge₂O₆ nanowires undergoes three morphological changes from spherical particles to nanorods, and finally to nanowires. The photoluminescence spectrum of the Cd₂Ge₂O₆ nanowires exhibits three fluorescence emission peaks centered at 422 nm, 490 nm and 528 nm showing the potential application for optical devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)