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We report a periodically patterned organization of hierarchical ZnO twin-crystals on a planar substrate in a biomimetic solution process. The deposition of the porous complex ZnO structure on specific sites of the substrate is achieved by introduction of the substrate surface modification and gelatin in a hydrothermal crystallization of ZnO., (This journal is © The Royal Society of Chemistry 2012)

A new and general approach based on vapor-phase transport technique using Au-coated plant cell walls has been developed to synthesize patterned ZnO nanostructures. Nanowires, nanodendrites and nanotowers were fabricated by adsorption of different metallic ions on plant cell walls. It is shown that plant cell wall can serve as a well-defined template to grow patterned nanostructures. Using transmission electron microscope and Raman spectroscopy, the structural characteristic of the nanostructures were investigated, exhibiting good crystallinity and hexagonal symmetry of the nanomaterials. Quite interestingly, the shape of the nanostructures can be controlled by the metallic ions adsorbed on plant cell walls. Without metallic ions, a homogeneous distribution of nanowires was obtained. On the other hand, with Ni+2 or Fe+3 ions, nanodendrites and nanotowers were observed, respectively. Our approach provides a low cost method that opens up new possibilities for the growth of patterned nanomaterials with desired shapes.

Based on the concept of optical enhancement in nanoparticle-mediated coupling, a novel composite consisting of ZnO nanorods and ZnO nanoparticles has been synthesized and characterized. The ZnO nanoparticles can be regarded as light-coupling media to increase the absorption cross section of excitation light as well as the emission extraction efficiency due to the relaxation of momentum selection rule. With our devised ZnO nanostructures, the band gap emission of ZnO nanorods can be enhanced by up to 8.5 times. The strategy of optical enhancement provided here should be very useful in designing many other nanostructures for high efficiency optoelectronic devices. [ABSTRACT FROM AUTHOR]

Nonlinear optical properties of ZnO nanorods (NRs) are strongly influenced by its dimension and aspect ratio. Size-dependent second harmonic generation (SHG) in ZnO NRs has been investigated with polarized excitations recently. However, detailed description to the SHG dependency with NR dimensions has not yet been given. In this paper, the relationship between rod diameter/length and corresponding χ(2) values based on Lorentz local field is established, both theoretically and experimentally, for the first time. Theoretically, Lorentz local field induced spectral red shift and the consequent dielectric constant modification is used to elucidate the size effect for χ(2) under the condition that both excitation and SHG wavelengths are far from the band gap. Experimentally, χ(2) of ZnO nanorods with various sizes is measured via Maker fringe technique, and the results fit well to our theory. [ABSTRACT FROM AUTHOR]

A new type of pyroelectric field effect transistor based on a composite consisting of single zinc oxide nanowire and lead zirconate titanate (ZnO NW–PZT) has been developed. Under infrared (IR) laser illumination, the transconductance of the ZnO NW can be modulated by optothermal gating. The drain current can be increased or decreased by IR illumination depending on the polarization orientation of the Pb(Zr0.3Ti0.7)O3 (PZT) substrate. Furthermore, by combining the photocurrent behavior in the UV range and the optothermal gating effect in the IR range, the wide spectrum of response of current by light offers a variety of opportunities for nanoscale optoelectronic devices. [ABSTRACT FROM AUTHOR]

We report the influence of carrier reflector and back surface field generated by doped graphene on n-ZnO nanoridges/p-silicon photodetectors and silicon solar cells. It is found that the p-type graphene not only acts as an electron blocking layer, but also helps the collection of photogenerated holes. Quite surprisingly, the on/off ratio of the photodetector with the insertion of doped graphene can be increased by up to 40 times. Moreover, we demonstrate that typical silicon solar cells with the doped graphene, the cell efficiency can be enhanced by about 20%. Our approach would expand numerous applications for graphene-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract: We report a simple fabrication process for the growth of nanotip-decorated ZnO nanobottles with an enhanced field emission property. Well-aligned ZnO nanobottles grown on ITO-coated glass substrates were synthesized by the thermal evaporation technique with the up-side down arrangement of substrate compared with the common method. The hydrothermal growth was carried out to decorate the nanobottles with ZnO nanotips. We demonstrate that the nanobottles decorated with nanotips show a much better field emission property. The improved field emission is attributed to the different local field enhancement factors at the nanobottles tops. Our results confirm the importance of the top morphology of a material in field emission. [Copyright &y& Elsevier]

Abstract: A simple two-step approach to produce highly ordered ZnO nanotube arrays is introduced. In the first step, Zn nanotubes were prepared by the electrodeposition of Zn into the pores of anodic alumina membrane (AAM), and then they were oxidized in air and converted into ZnO nanotubes. This approach is very simple, molecular anchor free, and is capable of growing ZnO nanotubes at low temperature. The ZnO nanotubes synthesized by this method exhibit an intense and sharp cathodoluminescence at 378nm and a weak defect emission at about 500nm at room temperature, indicating good quality of the prepared samples. Our approach is very general, which provides an alternative method for preparing highly ordered metal oxide nanotube arrays with good quality at relatively low temperatures. [Copyright &y& Elsevier]

In this study, we theoretically and experimentally demonstrate high performance of antireflection (AR) coating composed of the ZnO nanorods (NRs) and TiO 2 layers applied on InGaP/GaAs/Ge triple-junction solar cells. The high performance of this AR coating is due to the realization of a smooth gradient profile of refractive index fabricated by only two physical layers. First, due to the inherent inhomogeneous-nanoporous geometry along the surface normal, the ZnO NRs are reasonable to be considered as discrete multiple optical layers with low scattering loss, leading to the index smoothly increasing from air ambient toward down to the solar cells. Second, to compensate the gap of index between ZnO and solar cells, an additional TiO 2 layer with index in between is hence necessary to insert under ZnO NRs, significantly further reducing the Fresnel reflection loss of the entire device. The ZnO NRs/ TiO 2 layer shows a low wavelength-averaged (solar spectrum weighted) reflectance of 6.00% (4.78%) over a wide spectral range of λ =380–1800 nm, and exhibits a hydrophobic surface with a water contact angle of 128.2°. At device level, we compared the photovoltaic performance of solar cells with and without AR coating, the short-circuit density ( J SC ) is enhanced by 31.8% and 23.8% for solar cells integrated with the ZnO NRs/TiO 2 layer and conventional SiO 2 /TiO 2 double-layer AR (DLAR) coating, respectively. Under a very large incident angle of solar illumination ( θ =80°), the ZnO NRs/TiO 2 layer remains static with J SC enhancement of 35.2%, whereas the J SC enhancement of conventional DLAR coating drops down to 9.4%. In addition, the ZnO NRs/TiO 2 layer barely affects the open-circuit voltage and fill factor of the solar device. Therefore, the proposed ZnO NRs/TiO 2 layer with a smoother graded refractive index change is highly promising for the AR coating applications for the next-generation solar cells. [ABSTRACT FROM AUTHOR]

Abstract: The interface property has been one of the critical issues in developing hybrid polymer/metal oxide solar cells. We synthesize a conjugated oligomer, an amine- and bromine-terminated 3-hexyl thiophene (O3HT-(Br)NH2), to modify the ZnO-nanorod (ZnO-NR) surface in hybrid polymer/ZnO-NR photovoltaic cells. This oligomer is of the same repeat unit structure as and ∼1/12 the contour length of that of the light-harvesting polymer. In addition to passivate the NR surface, the presence of this conjugated oligomer enhances the electron mobility, and drives larger hole density toward the anodic surface for collection. The improved charge transport property of the hybrid is presumably a result of modulating the nanomorphology of the bi-carrier transport network induced by the conjugated oligomer. As a result, there is a large enhancement in photocurrent and photovoltage leading to an improved device performance of ∼35%. [Copyright &y& Elsevier]

Abstract: Both the performance and stability are the two major tasks for scientists to tackle in polymer solar cells. We introduce 3-hydroxyflavone (3-HF), a free-radical scavenging agent, as an additive in poly(3-hexythiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM) blend forming new photoactive material, which is used as an active layer in inverted solar cell devices. It was found that the addition of an adequate amount of 3-HF enhances the photocurrent, which is due to the improved carrier mobility. A power conversion efficiency (PCE) of the device has improved from 2.57% to 3.05%. In addition, 3-HF can further stabilize the inverted solar cell performance with a 85% retaining of its original PCE over a month in ambient condition as compared to the additive-free device of a value of 57%. Thus, incorporating 3-HF as an additive is an effective and simple method to approach the goal of high PCE while maintaining stable ambient operation. [Copyright &y& Elsevier]

Abstract: We study the effects of bifunctional linker on the optical properties of ZnO nanocolumn-linker-CdSe quantum dots heterostructure. The CdSe quantum dots are anchored on the surface of ZnO nanocolumns through either aliphatic linker of 3-aminopropyl trimethoxysilane (APS) or aromatic linker of p-aminophenyl trimethoxysilane (APhS). X-ray photoelectron spectroscopy is used to confirm the bifunctional linker bound onto CdSe quantum dots and onto the ZnO nanocolumns. The TEM study reveals a CdSe quantum dot shell of about 15nm coated on the ZnO nanocolumns. The photoluminance (PL) spectroscopy and time-resolved PL spectroscopy of ZnO nanocolumn-linker-CdSe quantum dots reflects that the photo-induced electron transfer across the interface of ZnO and CdSe through the aromatic APhS is more efficient than the aliphatic APS. This study demonstrates that through the usage of appropriate surface linker, the charge transfer rate across the interfaces of donor/acceptor (D/A) heterostructure can be improved for potential photovoltaic cell applications. [Copyright &y& Elsevier]

Abstract: The electrical property of the organic–inorganic heterojunctions is one of the important factors affecting the performance of hybrid photovoltaic devices. We introduce a good electron conductor, C60, to modify the surface of ZnO-nanorod arrays in a ZnO-nanorod/poly(3-hexylthiophene):(6,6)-phenyl C61 butyric acid methyl ester (ZnO/P3HT:PCBM) hybrid photovoltaic structure. We found that there is ∼40% and ∼15% increment in the short circuit current (J SC ) and open circuit voltage (V OC ), respectively, for the device after C60 modification. By probing the carrier dynamics and the surface property of ZnO-nanorods, the presence of the C60 layer assists the exciton separation and passivates part of the defect states on the ZnO-nanorod surface. The charge transport property at the ZnO/polymer blend interface is, therefore, improved. As a result, higher charge concentration can transfer from the polymer blend to the ZnO-nanorods more effectively and subsequently travel to electrodes leading to the improved performance in the photovoltaic device. [ABSTRACT FROM AUTHOR]

A new and general approach to achieving efficient electrically driven light emission from a Si-based nano p-n junction array is introduced. A wafer-scale array of p-type silicon nanotips were formed by a single-step self-masked dry etching process, which is compatible with current semiconductor technologies. On top of the silicon nanotip array, a layer of n-type ZnO film was grown by pulsed laser deposition. Both the narrow line width of 10 nm in cathodoluminescence spectra and the appearance of multiphonon Raman spectra up to the fourth order indicate the excellent quality of the ZnO film. The turn-on voltage of our ZnO/Si nanotip array is found to be ∼2.4 V, which is 2 times smaller than its thin film counterpart. Moreover, electroluminescence (EL) from our ZnO/Si nanotips array light-emitting diode (LED) has been demonstrated. Our results could open up new possibilities to integrate silicon-based optoelectronic devices, such as highly efficient LEDs, with standard Si ultralarge-scale integrated technology. [ABSTRACT FROM AUTHOR]