Core-shell Nanowire Arrays Research Articles

Controllable growth and optical properties of ZnO nanostructures on Si nanowire arrays

In this paper, two types of hybrid semiconductors made up of silicon nanowires and ZnO nanostructures, namely Si/ZnO core–shell nanowire arrays and ZnO quantum dots (QDs)-decorated Si nanowire arrays, have been prepared by combining metal-assisted wet-chemical etching and metal–organic chemical vapor deposition (MOCVD). We demonstrate that ZnO QDs and thin ZnO layers can be grown on Si nanowires in a controlled manner by varying growth parameters including working pressure and growth time. Meanwhile, porous silicon and porous Si/ZnO nanowire arrays have also been fabricated. The morphology and optical properties of both hybrid nanostructures have been carefully investigated for their potential applications in nanowire optoelectronics. A quantum confinement effect in ZnO QDs was confirmed by the blue-shifted photoluminescence. Porous Si/ZnO core–shell nanowires display a very broad emission band throughout the entire visible light range.

ZnO/CdTe and ZnO/CdS core-shell nanowire arrays for extremely thin absorber solar cells

Abstract In this study we report on ZnO/CdTe core-shell nanostructures fabrication for extremely thin absorber (Eta)-solar cells. Successive ion layer adsorption and reaction (SILAR) and close space sublimation (CSS) methods were used to sensitize electrodeposited ZnO nanowires by CdS and CdTe. The close morphological investigation showed that CdTe thin layer deposited by CSS method did not uniformly cover the ZnO nanowires (NWs). On the tip of the NWs, nanoparticles with a 20 nm diameter were observed whereas some areas were not covered, which could affect the solar cell efficiency. ZnO/CdS nanowires arrays were prepared by SILAR technique and were compared to those of ZnO/CdTe. The obtained uniform layers (∼ 25 nm thick) on the entire ZnO NWs surface allowed us to consider SILAR method as a good alternative for deposition of conformal extremely thin films. The adsorption edge up to 800 nm for CdTe compared to 500 nm for CdS, foresees the ZnO/CdTe core/shell structure favourable to achieve good photovoltaic performances. Enhanced optical properties of the nanostuctured electrodes over the planar counterparts proved the effect of the increased surface area.

Vertically aligned ZnO/amorphous-Si core–shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core–shell nanowire arrays (ZnOnanowires coated with amorphous silicon) through chemical vapor deposition. Thecore–shell heterostructured nanowires possessed uniform morphology and the thickness ofthe amorphous silicon shells could be controlled easily by tuning the deposition durationand temperature. The core–shell heterostructured nanowires exhibited enhancedantireflection and absorption performance as well as tunable PL properties. Becausethe individual ZnO/a-Si nanowires showed p-type characteristics and the ZnOcores were n-type semiconductors, the core–shell nanowires formed p–n junctionsnaturally.

Fabrication of Ni/Al2O3 Core–Shell Nanowire Arrays in Porous Alumina Membranes

Abstract Ordered Ni/Al2O3 core–shell nanowire arrays were prepared in porous alumina membrane via alternating current electrodeposition and chemical etching methods. The shape of the inner Ni core is columnar with a diameter of 50 nm. The shape of the outer Al2O3 shell is a regular hexagon and the well thickness is about 40 nm. This novel structured material can be used as a media for magnetic storage and high-temperature nanoelectronic devices.

Growth and characterization of type-II ZnO/ZnTe core-shell nanowire arrays for solar cell applications

Large area, well-aligned type-II ZnO/ZnTe core-shell nanowire arrays have been fabricated on an a-plane sapphire substrate. The ZnO nanowires were grown in a furnace by chemical vapor deposition with gold as catalyst and then were coated with a ZnTe shell on the ZnO nanowires surface by a metal-organic chemical deposition chamber. The morphology and size distribution of the ZnO/ZnTe core-shell nanowire arrays were studied by scanning electron microscopy (SEM) and the crystal structure was examined by x-ray diffraction (XRD). Transmission measurement was used to study the optical properties of the core-shell nanowires. The results indicated that the ZnO/ZnTe core-shell nanowire arrays have good crystalline quality. In addition, it was found that the nanowire arrays have good light absorption characteristics and these properties make it suitable for making photovoltaic devices.

Selective-area growth of vertically aligned GaAs and GaAs/AlGaAs core–shell nanowireson Si(111) substrate

We report on selective-area growth of vertically aligned GaAs nanowires on Si(111)substrate. Modification of the initial Si(111) surface by pretreatment under anAsH3 atmosphere and low-temperature growth of GaAs were important for controlling thegrowth orientations of the GaAs nanowire on the Si(111) surface. We also found that thesize of openings strongly affected the growth morphology of GaAs nanowires on Si(111).Small diameter openings reduced the antiphase defects and improved the optical propertiesin the GaAs nanowires. Moreover, we realized coherent growth without misfitdislocation at the GaAs/Si interface. Finally, we demonstrated fabrication of aGaAs/AlGaAs core–shell nanowire array on a Si surface and revealed that theluminescence intensity was markedly enhanced by passivation effects. These results arepromising for future III–V nanowire-based optoelectronic integration on Si platforms.

Three-dimensional electrodes for dye-sensitized solar cells: synthesis of indium–tin-oxide nanowire arraysand ITO/TiO2 core–shell nanowire arrays by electrophoretic deposition

Dye-sensitized solar cells (DSSCs) show promise as a cheaper alternative to silicon-basedphotovoltaics for specialized applications, provided conversion efficiency can be maximizedand production costs minimized. This study demonstrates that arrays of nanowires can beformed by wet-chemical methods for use as three-dimensional (3D) electrodes inDSSCs, thereby improving photoelectric conversion efficiency. Two approaches wereemployed to create the arrays of ITO (indium–tin-oxide) nanowires or arrays ofITO/TiO2 core–shell nanowires; both methods were based on electrophoreticdeposition (EPD) within a polycarbonate template. The 3D electrodesfor solar cells were constructed by using a doctor-blade for coatingTiO2 layers ontothe ITO or ITO/TiO2 nanowire arrays. A photoelectric conversion efficiency as high as 4.3% was achieved inthe DSSCs made from ITO nanowires; this performance was better than that ofITO/TiO2 core–shell nanowiresor pristine TiO2 films. Cyclic voltammetry confirmed that the reaction current was significantly enhancedwhen a 3D ITO-nanowire electrode was used. Better separation of charge carriers andimproved charge transport, due to the enlarged interfacial area, are thought to bethe major advantages of using 3D nanowire electrodes for the optimization ofDSSCs.

Catalyst-free synthesis and cathodoluminescent properties of ZnO nanobranches on Si nanowire backbones

We report the catalyst-free synthesis of ZnO nanobranches on Si nanowires using metalorganic chemical vapor deposition. The formation of single-crystalline ZnO nanobranches on Si nanowire backbones has been confirmed by lattice resolved transmission electron microscopy. Depending on the growth parameters, especially the growth temperature, the morphology and size of the ZnO nanobranches evolved from nanothorn-shaped (at 350 °C) to nanoneedle-shaped structures (at 500 °C). When the growth temperature was further increased to 800 °C, thin ZnO nanowire branches grew out of the Si nanowire backbones coated with thin ZnO shells, whereas no ZnO branch was formed on bare Si nanowires due to limited nucleation. The growth behavior was further exploited to fabricate ZnO/Si nanowire networks by growing the ZnO nanowires selectively on laterally aligned Si–ZnO core-shell nanowire arrays. In addition, cathodoluminescent properties of ZnO nanobranches on Si nanowire backbones are discussed with respect to position and size.

Pd/Pt core–shell nanowire arrays as highly effective electrocatalysts for methanol electrooxidation in direct methanol fuel cells

Highly ordered Pd/Pt–core–shell nanowire arrays (Pd/Pt NWAs) have been prepared by anodized aluminum oxide (AAO) template-electrodeposition and magnetron sputtering methods. Pd/Pt NWA electrode shows a very high electrochemical active surface area and high electrocatalytic activity for the methanol electrooxidation in acid medium for direct methanol fuel cells (DMFCs). The mass specific anodic peak current density is 756.7 mA mg −1 Pt for the methanol oxidation on the Pd/Pt NWA electrode, an increase by a factor of four as compared to conventional E-TEK PtRu/C electrocatalysts. The mechanism of the significant enhancement of the Pd/Pt core/shell NWA nanostructure in the efficiency and electrocatalytic activity of Pt for the methanol electrooxidation in acid medium is discussed.

Synthesis of ordered arrays of multiferroic NiFe2O4-Pb(Zr0.52Ti0.48)O3 core-shell nanowires

A synthesis method was developed for producing core-shell nanowire arrays, which involved a combination of a modified sol-gel process, electrochemical deposition, and subsequent oxidization in anodized nanoporous alumina membranes. This method was applied to generate ordered arrays of one dimensional multiferroic NiFe2O4 core and Pb(Zr0.52Ti0.48)O3 (PZT) shell nanostructures. Extensive microstructural, magnetic, and ferroelectric characterizations confirmed that the regular arrays of core-shell multiferroic nanostructures were composed of a spinel NiFe2O4 core and perovskite PZT shell. This synthesis method can be readily extended to prepare different core-shell nanowire arrays and is expected to pave the way for one dimensional core-shell nanowire arrays.

Electrochemical fabrication and magnetic properties of highly ordered silver–nickel core-shell nanowires

Ordered silver–nickel core-shell nanowire arrays were successfully fabricated by electrodeposition. The ordered silver nanowire arrays embedded in a porous alumina template were first fabricated from an aqueous solution of Ag(NO 3) 2 and Ac(NH 3). After removing out the template, the obtained silver nanowire arrays were subsequently electrodeposited with nickel at 1.6–2.6 V and 60 °C using the electrolyte composed of NiSO 4, NiCl 2 and H 3BO 3. Transmission electron microscopy observation revealed that a ∼15 nm thick nickel film was coated on the surface of the silver nanowires with about 200 nm in diameter. It was found that the silver nanowires with nickel coating showed enhanced magnetic properties in comparison to that of pure silver nanowires. The magnetic force microscope image of silver–nickel core-shell nanowires showed magnetic domain state. In addition, the hysteresis loops of the silver–nickel nanowire arrays showed a coercive field of 180 Oe, almost independent of the applied magnetic field. However, it was observed that a larger magnetic domain was found in parallel direction than that in perpendicular direction.