ZnO nanorods–polymer hybrid white light emitting diode grown on a disposable paper substrate

We demonstrate intrinsic white light emission from hybrid light emitting diodes fabricated using an inorganic–organic hybrid junction grown at 50 °C on a paper substrate. Cyclotene was first spin coated on the entire substrate to act as a surface barrier layer for water and other nutrient solutions. The active area of the fabricated light emitting diode (LED) consists of zinc oxide nanorods (ZnO NRs) and a poly(9,9‐dioctylfluorene) (PFO) conducting polymer layer. The fabricated LED shows clear rectifying behavior and a broad band electroluminescence (EL) peak covering the whole visible spectrum range from 420 nm to 780 nm. The color rendering index (CRI) was calculated to be 94 and the correlated color temperature (CCT) of the LED was 3660 K. The low process temperature and procedure in this work enables the use of paper substrate for the fabrication of low cost ZnO–polymer white LEDs for applications requiring flexible/disposable electronic devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrochemical synthesis of Cu/ZnO nanocomposite films and their efficient field emission behaviour

The Cu/ZnO nanocomposite films have been synthesized by cathodic electrodeposition and characterized using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), photoluminescence (PL) and field emission microscope (FEM). The XRD pattern shows a set of well defined diffraction peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. In addition, characteristic diffraction peaks corresponding to Cu and Zn are also observed. The SEM image shows formation of two-dimensional (2D) hexagonal sheets randomly distributed and aligned almost normal to the substrate. Uniformly distributed small clusters of Cu nanoparticles possessing average diameter of ∼25 nm, as revealed from the TEM image, are seen to be present on these 2D ZnO sheets. The selected area electron diffraction (SAED) image confirms the nanocrystalline nature of the Cu particles. From the field emission studies, carried out at the base pressure of ∼1 × 10⁻⁸ mbar, the turn-on field required for an emission current density of 0.1 μA/cm² is found to be 1.56 V/μm and emission current density of ∼100 μA/cm² has been drawn at an applied field of 3.12 V/μm. The Cu/ZnO nanocomposite film exhibits good emission current stability at the pre-set value of ∼10 μA over a duration of 5 h. The simplicity of the synthesis route coupled with the better emission properties propose the electrochemically synthesized Cu/ZnO nanocomposite film emitter as a promising electron source for high current density applications.     

Photoluminescence enhancement from hybrid structures of metallic single-walled carbon nanotube/ZnO films

We investigated the dependence of localized surface plasmon resonance (LSPR) coupled photoluminescence (PL) emission on the density of a metallic single-walled carbon nanotube (m-SWCNT). The m-SWCNTs of various densities were deposited on top of ZnO films by spin coating and filtration transfer method to form the hybrid structures. We observed PL enhancement from ZnO films deposited with spin coated m-SWCNT, comparing with pure ZnO film. The m-SWCNT acts as absorbers for the light emitted due to SPR. After resonant excitation, hot electrons in m-SWCNT are created in high energy states, which can then transfer from the m-SWCNT to the conduction band of the ZnO films. We discuss the relationship between the hot electron flow generated by internal photoemission and LSPR.     

Metal‐nanoparticle‐coating‐induced enhancement and weakening of resonant Raman scattering in ZnO: effect of surface electric field

Effects of Ag and Ti nanoparticle coatings on resonant Raman scattering in various ZnO thin films are presented. The longitudinal optical (LO) phonons, irrespective of the ZnO quality, exhibit an enhancement and a weakening by the Ag and Ti nanoparticle coatings, respectively. The enhancement (weakening) is always accompanied by a reduced (an increased) intensity ratio of the second to first‐order LO phonons, which can be associated with changes in the electron‐phonon coupling strength in the probed area of ZnO. Angle‐resolved X‐ray photoelectron spectroscopy provides evidence for the bending of the surface energy bands and their changes induced by the metal coatings. The effect of metal nanoparticle coatings on the Raman scattering of ZnO is thus attributed to the changes in the surface electric field. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhanced blue emission from ZnS–ZnO composites confined in SBA-15

A new optical material, the ZnO nanoparticles that are modified with ZnS and confined in SBA-15, has been prepared through the controllable sulfuration at relatively low temperature (40 °C) from the ZnO/SBA-15 composites. The precursor composites can be prepared through a novel path in which the zinc is well dispersed by directly grinding zinc nitrate into the as-synthesized SBA-15 occluded with template followed by calcination, and it is possible to control the conversion of ZnO to ZnS by adjusting the reaction time. The resulting samples are characterized by XRD, nitrogen adsorption–desorption and photoluminescence (PL) spectroscopy. ZnO–ZnS composites, mainly confined in the mesopore of SBA-15, exhibit dramatically enhanced blue emission at the expense of the UV emission.     

Magnetohydrodynamic flow and heat transfer of a hybrid nanofluid over a rotating disk by considering Arrhenius energy

This research work explores the effect of hybrid nanoparticles on the flow over a rotating disk by using an activation energy model. Here, we considered molybdenum disulfide and ferro sulfate as nanoparticles suspended in base fluid water. The magnetic field is pragmatic normal to the hybrid nanofluid flow direction. The derived nonlinear ordinary differential equations are nondimensionalized and worked out numerically with the help of Maple software by the RKF-45method. The scientific results for a non-dimensionalized equation are presented for both nanoparticle and hybrid nanoparticle case. Accoutrements of various predominant restrictions on flow and thermal fields are scanned. Computation estimation for friction factor, local Nusselt number and Sherwood number are also executed. Results reveal that the reduction of the heat transfer rate is greater in hybrid nanoparticles when compared to nanoparticles for increasing values of Eckert Number and the thermal field enhances for the enhanced values of volume fraction.     

Mg doping reduced full width at half maximum of the near-band-edge emission in Mg doped ZnO films

Sol--gol method was employed to synthesize Mg doped ZnO films on Si substrates. The annealing temperature-dependent structure and optical property of the produced samples were studied. An interesting result observed is that increasing Mg concentration in the studied samples induces the full width at half maximum (FWHM) of their near-band-edge (NBE) emission decrease and the defect related emission of the corresponding sample suppresses drastically. The possible mechanism of the observed result is discussed.     

Reversible quenching of luminescence in ZnO films by electric field action

We report the effect of the external electric field on the photoluminescence (PL) properties of ZnO films grown by a pulsed laser deposition method. The PL quenching of bound excitons under the electric field was attributed to a decrease in the capture cross section of the radiative centers. In addition, the change in the surface/grain boundaries charge induced a degradation of the 3.33 eV emission line over the whole sample, which remained even after voltage removal. Besides the PL degradation, this emission at 3.33 eV demonstrates the change in the thermal quenching process, where the activation energy of exciton detachment corresponds to its binding energy. All behaviors were restored to the initial state by application of the voltage with opposite polarity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Z箍缩驱动混合堆包层瞬态传热特性

Z箍缩驱动聚变-裂变混合能源堆(Z-FFR)以较长周期(10s)脉冲式运行,为实现3000 MW的热功率输出,单个脉冲需要产生的能量较大,包层和第一壁在强热冲击下的瞬态传热和温度特性是决定Z-FFR技术可行性的关键问题之一。通过理论计算,分析了在连续脉冲作用下包层和第一壁温度随时间的变化规律。同时以输出恒定的电功率为目标,提出了展平系统输出功率的简便方法,并分析了出口冷却剂温度的波动特性。结果表明材料最高温度均在安全限值内,第一壁表面瞬时高温层厚度约为0.5mm,系统输出功率波动幅度在-2.84%~+2.05%范围内。     

Z箍缩驱动混合堆包层瞬态传热特性

Z箍缩驱动聚变-裂变混合能源堆（Z-FFR）以较长周期（10 s）脉冲式运行,为实现3000 MW的热功率输出,单个脉冲需要产生的能量较大,包层和第一壁在强热冲击下的瞬态传热和温度特性是决定Z-FFR技术可行性的关键问题之一。通过理论计算,分析了在连续脉冲作用下包层和第一壁温度随时间的变化规律。同时以输出恒定的电功率为目标,提出了展平系统输出功率的简便方法,并分析了出口冷却剂温度的波动特性。结果表明材料最高温度均在安全限值内,第一壁表面瞬时高温层厚度约为0.5 mm,系统输出功率波动幅度在-2.84%~+2.05%范围内。     

变分法计算ZnO量子点中激子的基态能

ZnO是一种新型宽禁带直接带隙Ⅱ-Ⅵ族半导体材料,室温激子束缚能高达60meV,远大于室温热离化能(26meV),因此ZnO是适于室温或更高温度下使用的高效紫外发射材料。ZnO半导体量子点材料与体材料相比具有崭新的光电特性,特别在紫外激光器件方面,与ZnO的激子特性密切相关,因此理论上对ZnO量子点中激子的基态能(束缚能)的研究就显得十分必要。采用有效质量近似(EMA)方法,提出新的比较简单的尝试波函数,并用变分法对ZnO量子点的激子基态能进行了计算。将计算结果与我们用固态热分解法制备的ZnO量子点的实验结果进行了比较,发现与实验结果非常吻合;与Y．Kayanuma的理论计算结果进行了比较,二者的计算结果也基本一致。说明选取的尝试波函数简单有效,可用于计算其他半导体量子点材料,具有一定的实用价值。     

Angle-sensitive and fast photovoltage of silver nanocluster embeded ZnO thin films induced by 1.064-μm pulsed laser

Silver nanocluster embedded ZnO composite thin film was observed to have an angle-sensitive and fast photovoltaic effect in the angle range from –90o to 90o, its peak value and the polarity varied regularly with the angle of incidence of the 1.064-μ m pulsed Nd:YAG laser radiation onto the ZnO surface. Meanwhile, for each photovoltaic signal, its rising time reached ～2 ns with an open-circuit photovoltage of ～2 ns full width at half-maximum. This angle-sensitive fast photovoltaic effect is expected to put this composite film a candidate for angle-sensitive and fast photodetector.     

Fabrication of ZnO and its enhancement of charge injection and transport in hybrid organic/inorganic light emitting devices

ZnO nanocrystals were synthesized by hydrolysis in methanol. X-ray diffraction and photoluminescence spectra confirm that good crystallized ZnO nanoparticles were formed. Utilizing those ZnO nanoparticles and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), light emitting devices with indium tin oxide (ITO)/poly(3,4-oxyethyleneoxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS)/ZnO:MDMO-PPV/Al and ITO/PEDOT:PSS/MDMO-PPV/Al structures were fabricated. Electroluminescence (EL) spectra reveal that EL yield of hybrid MDMO-PPV and ZnO nanocrystals devices increased greatly as compared with pristine MDMO-PPV devices. The current-voltage characteristics indicate that addition of ZnO nanocrystals can facilitate electrical injection and charge transport. The decreased energy barrier to electron injection is responsible for the increased efficiency of electron injection.     

Core–Shell Nanoparticles Driven by Surface Energy Differences in the Co–Ag,W–Fe,and Mo–Co Systems

Core–shell nanoparticles are known to form in binary systems using a one‐step gas‐condensation deposition process where a large, positive enthalpy of mixing provides the driving force for phase separation and a difference in surface energy between component atoms creates a preferential surface phase leading to a core–shell structure. Here, core–shell nanoparticles have been observed in systems with enthalpy as low as ?5 kJ mol^?1 and a surface energy difference of 0.5 J m^?2 (Mo–Co). This suggests that surface energy dominates at the nanoscale and can lead to phase separation in nanoparticles. The compositions and size dependence of the core–shell structures are also compared and no core–shell structures are observed below a critical size of 8 nm.     

Investigation of electronic and optical properties in Al−Ga codoped ZnO thin films

Electronic and optical properties of Al−Ga codoped ZnO thin films were investigated by post-annealing. The lowest resistivity of the Al-Ga codoped ZnO films was observed from the 450 °C-annealed sample. The Fermi-level shift of the Al−Ga codoped ZnO film was ∼0.6 eV from x-ray photoelectron spectroscopy, and the widening of optical-bandgap in the Al−Ga codoped ZnO film was ∼0.3 eV. The correlations of optical-bandgap with Fermi-level shift and conduction band filling were suggested by schematic band diagrams.     

Birth of room‐temperature magnons and Raman line enhancement in ZnO nanostructures containing cobalt oxide

Cobalt (Co) addition and thermal annealing induced structural and vibrational properties of ZnO nanostructures were analysed. X‐ray diffraction pattern reveals that the nanostructures are in hexagonal wurtzite type and the formation of Co₃O₄. The Co ion induced morphology changes have been studied by high‐resolution scanning electron microscope images and energy dispersive spectroscopy measurements confirm the presence of Co ions. CoO‐related magnon excitation bands are emerged at room temperature for the Co‐added samples. There are no changes in the band positions of the Raman spectra of pure and Co‐added materials. Annealed sample exhibits the suppression of magnon bands and formation of Co₃O₄: ZnO composites. Raman line width and the electron phonon coupling constant are decreased with respect to the annealing temperature. The formation of Co₃O₄ : ZnO composite phases have further confirmed by infrared spectra. Copyright © 2013 John Wiley & Sons, Ltd.     

Amplification of Solar Energy Conversion in Quantum‐Confined CdSe‐Sensitized TiO2 Photonic Crystals by Trapping Light

Photonic effects amplifying solar energy conversion are reported in titania inverse opals sensitized with quantum‐confined CdSe films. TiO₂ inverse opals (i‐TiO₂‐o) and unstructured nanocrystalline TiO₂ (nc‐TiO₂) films are sensitized with CdSe deposited via successive ionic layer adsorption and reaction (SILAR) by generating Se^2? in situ under inert atmosphere, and the film absorbance is tuned by the number of SILAR cycles. Photonic effects are investigated while varying the i‐TiO₂‐o stop band position relative to CdSe films’ absorbance. i‐TiO₂‐o films with stop band at 700 and 560 nm are sensitized with CdSe having absorption edges at 600 and 650 nm thus tuning absorbance to the red and the blue of the stop band. Significant amplification in photon‐to‐current conversion efficiency is measured when CdSe films prepared via two cycles are adsorbed on i‐TiO₂‐o with a stop band at 700 nm, with a maximum average enhancement factor equal to 6.7 ± 1.6 at 640 nm, 60 nm to the blue of the stop band center, relative to nc‐TiO₂ sensitized with comparable CdSe amounts. The gain is observed over a wide frequency range to the blue of the stop band and is greatest when film absorbance was low. The photocurrent gain is not a result of differences in the rates of charge separation or charge transport, and occurs in the same frequency range where absorbance amplification is measured to the blue of the 700‐i‐TiO₂‐o stop band, and is thus attributed to slow light effects enhancing absorbance in the photonic crystal environment.     

Calculation of laser induced collisional energy transfer in the Sr-Ca system

Based on the four-state model of laser-induced collisional energy transfer,the cross section of the collisional energy transfer in the Sr-Ca system is obtained.Various factors,including field intensity,relative speed,and temperature,which influence the collisional cross section,are discussed for illustrating the features of the Sr-Ca laser-induced collisional energy transfer(LICET) process.The calculated results show that the LICET spectral profiles obviously become narrower when the laser field intensity i...     

Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.