Tailoring the photoluminescence of MgO nanowires using the Ag shell layers and nanoparticles

We fabricated MgO/Ag core-shell nanowires, where core nanowires were coated with a conformal layer of Ag via sputtering. By subsequent thermal annealing, we generated Ag nanoparticles. Photoluminescence (PL) studies revealed that Ag-shell coating decreased the intensity of blue emission, whereas that of UV emission remained almost invariant. The subsequent annealing has induced an additional green emission band. We discuss here the possible mechanism by which the PL properties have been tailored by Ag-layer coating and by forming nanoparticles. This approach can be applied to tailor and improve the optical properties of different composite nanostructures.     

Tailoring the morphology of electrodeposited ZnO and its photoluminescence properties

High density ZnO columnar films with well-aligned and well-perpendicular to the surface of film were electrodeposited on ITO substrates by using an electrolyte consisting of a mix of water and organic solvent namely dimethylsulfoxide (DMSO). The effect of mixing ratio of water and DMSO on the growth of film has been examined critically. SEM images have shown that well-oriented ZnO quasi-nano columns were formed perpendicular to the substrate. At the same time we found there are three kinds of competitions for growth of ZnO crystalmorphology i.e. column, rod and needle like. The needle like morphology has high density with well-aligned structure. The reasons for the growth of films of different morphology and their photoluminescence (PL) properties have been presented and discussed. It has been found that the three-dimensional (3D) ordered ZnO structure exhibits high intensity PL band which may shift their position and intensity with the varying conditions of depositions.     

Synthesis and photoluminescence of ZnO nanowires/nanorods

We report growth of ZnO nanowires on various substrates using a vapour phase transport method and show that the growth mechanism is vapour-liquid-solid growth. We present photoluminescence data for samples grown on a-plane sapphire at room and low temperatures indicating that the optical quality of these structures is potentially excellent, with intense emission and narrow bound exciton linewidths. The intensity decays rapidly with increasing temperature, indicating a strong temperature-activated non-radiative mechanism whose origin is unclear. We observe a high energy excitonic emission close to the band edge which we assign to the “surface” exciton in ZnO at ∼3.368 eV. This assignment is consistent with the large surface to volume ratio of the nanowire systems under consideration and also indicates that this large ratio has a significant effect on the luminescence even at low temperatures. These surface effects may also be responsible for the rapid decay of the luminescence with increasing temperature via a temperature-activated surface recombination. The nanowire systems appear to offer the prospect of extremely efficient excitonic emission for device applications, and we note that one of the important aspects of achieving this potential will be control of the surface effects via passivation or other means.     

Mn掺杂ZnO纳米线的拉曼散射和光致发光特性

研究了不同Mn掺杂含量的ZnO纳米线在室温条件下的拉曼散射和光致发光性能,发现Mn掺杂入ZnO后引入了部分应力,在其拉曼光谱中表现出拉曼峰的位置发生偏移,Mn的掺杂含量越高,峰偏移得越明显.Mn的掺杂对ZnO纳米线的发光性能也有影响,尽管掺杂后仍保持有较为明显的紫外发光峰,但是,随着Mn含量的增加,紫外发光峰的强度降低,并且半峰宽逐渐增大.此外,Mn的掺杂明显地改变了ZnO紫外发光峰的位置.     

Pd-nanoparticle-decorated ZnO nanowires: ultraviolet photosensitivity and photoluminescence properties

ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O(2) molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.     

Enhanced photoluminescence and photoconductivity of ZnO nanowires with sputtered Zn

We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the effect of Zn diffusion on the photoluminescence and photoconduction properties of ZnO NWs. Elemental mapping clearly indicates higher Zn concentration in the NWs due to diffusion of Zn. The Zn-sputtered NWs show an enhanced ultraviolet emission with 7 nm red shift. Since the ionization energy of Zni is 51 meV, the enhanced PL emission with a red shift is correlated to the coupling between free exciton and zinc interstitials (Zni) defects. The photocurrent transients show almost 20 times more photocurrent generation in Zn/ZnO NWs compared to the as-grown NWs. In contrast, the thin film shows no significant change in the photoluminescence and photoconductivity. Based on the photoconductivity and photoluminescence results, we predict that Zn diffusion in the NWs occurs easily compared to the films because of the smaller dimensions of the NWs.     

Growth of ZnO nanowires on a patterned Au substrate

ZnO nanowires were site-selectively deposited on the catalytic gold-patterned substrates using a vapor transport process at low temperature. We observed that Au-Zn phase played an important role in initiating the ZnO nucleation, which was identified from the TEM observation of the interface between the substrate and ZnO nanowires. And then further growth of the nanowires on the ZnO surface was driven by the reaction of the Zn vapor and O₂ gas in vapor-solid growth mode. Finally, it was concluded that the site-specific deposition of ZnO nanowires was ascribed to the vapor-liquid-solid mechanism of Au, Zn vapor, and Au-Zn phases at the initial stage of ZnO nucleation.     

花状纳米氧化锌颗粒的制备及其光学性能研究

以硬脂酸锌和过氧化氢为原料，采用油相法一步合成出一种特殊的氧化锌花状纳米结构。利用透射电镜、高分辨电镜、X射线衍射等对其形貌和结构的分析表明：花状氧化锌纳米颗粒结构为六方晶相，大小约为30nm。吸收光谱和荧光光谱测量显示该纳米结构有显著的近带边紫光发射特性，对应的缺陷发光强度较弱，表明这一纳米结构有较好的光学特性，因此在生物荧光标记方面有着潜在的应用价值。     

Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles

Electrochemical deposition of ZnO nanorods having a diameter of 80-150 nm and length ~ 2 μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100 s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375 nm and a broad defect-related emission band centered at ~ 596 nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50 s. The enhancement factor reached a maximum value for the Au sputtering time of 50 s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity.     

Sn催化的Ga掺杂ZnO纳米线的制备及其光致发光性能研究

采用化学气相沉积的方法,以Sn粉为催化剂制备出大长径比的Ga掺杂ZnO纳米线。采用扫描电子显微镜观察制备的产物,发现样品为直径约25～90nm的纳米线。通过比较不同Ga掺杂含量样品的室温光致发光谱,发现一定掺杂含量的Ga可以提高ZnO纳米线的紫外发光强度,同时,Ga的掺杂也会引起ZnO紫外发光峰的蓝移。随着Ga含量的增加,蓝移程度越来越小,甚至发生红移。Sn的引入只对Ga掺杂ZnO纳米线的蓝绿光有贡献。     

Anionic surfactant-assisted hydrothermal synthesis of high-aspect-ratio ZnO nanowires and their photoluminescence property

ZnO nanowires with high-aspect-ratio of up to ca. 600 were synthesized in a quaternary reverse microemulsion containing sodium dodecyl sulfate (SDS) / water / heptane / n-hexane via a hydrothermal method. SDS, as an anionic surfactant, plays an important role in the formation of morphologies. Subsequently, we studied lots of key influencing factors including the molar ratio (w) value of NaOH to Zn(OAc)₂, the reaction temperature, and the instance without the quaternary reverse microemulsion. The selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) reveal the single-crystal nature of the ZnO nanowires. The morphologies and crystalline structure of the as-obtained products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and powder X-ray diffraction (XRD), respectively. Through this route, we can obtain a mass of products and the method is both convenient and reproducible. Finally, we measured the photoluminescence (PL) spectra and found that the ZnO nanowires exhibited green-orange emission at 525 nm and short ultraviolet emission at 380 nm and the ZnO nanomaterials with different aspect ratio (length to diameter) (L / D) showed PL intensity disciplinary change. Aiming at this phenomenon, we propose a reasonable mechanism to explain the PL spectra of the ZnO nanomaterials in detail.     

Structural,photoluminescence and picosecond nonlinear optical effect of In-doped ZnO nanowires

In-doped ZnO (IZO) nanowires were grown using the chemical vapour deposition method. The IZO nanowires have been characterized by scanning electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and UV–Visible spectroscopy. The PL results demonstrated a larger band-gap for the IZO nanowires in comparison to the undoped ZnO. Two major emission peaks were observed for the IZO nanowires, one originated from the free exciton recombination (ultraviolet emission) and another possibly related to the deep-level emission (visible emission). Furthermore, the nonlinear optical characteristic of the nanowires was studied using picosecond Z-scan technique. The experimental results show that the two and three-photon absorption coefficient of samples were able to be observed. These studies make the promising potential applications of the samples in the development of multifunctional all-optical devices.     

Surface-plasmon-enhanced photoluminescence of CdS nanoparticles with Au/SiO2 nanocomposites

To examine the effect of metal nanostructures on the photoluminescence of CdS nanoparticles, Au/SiO₂ nanocomposites were prepared with back-reflection geometry. The contribution from the prolonged optical-path lengths was excluded, and the luminescence enhancement was attributed only to surface-plasmon resonance. The optimum nanostructures for the PL enhancement were examined, and correlated with the wavelength of the surface-plasmon peak of the Au/SiO₂ nanocomposites and the PL emission peak of CdS nanoparticles.     

Carcinoembryonic antigen admittance biosensor based on Au and ZnO nanoparticles using FFT admittance voltammetry

In this work, a highly sensitive carcinoembryonic antigen fast Fourier transform admittance biosensor is introduced. The proposed biosensor is based on bilayer films of ZnO/Au nanoparticles as an immobilization matrix. These layers are prepared by self-assembly and deposition method on a gold electrode surface, respectively. Carcinoembryonic antibody (anti-CEA) was immobilized on gold nanoparticles and positively charged horseradish peroxidase (HRP) was used to block sites against nonspecific binding. The admittance biosensor was developed based on fast Fourier transform continuous square wave voltammetry, which produces a sensitive, fast (less than 20 s) and reliable response for determination of carcinoembryonic antigen. The technique was applied as a detector in a flow injection system. The admittances reduction current of the biosensor decreases linearly in two concentrations ranges of CEA from 0.1 to 70 ng/mL and from 70 to 200 ng/mL with a detection limit of 0.01 ng/mL in presence of 0.5 mM H(2)O(2) as an eluent solution.     

Ethanol and hydrogen sensors based on ZnO nanoparticles and nanowires

Hydrogen sensing characteristics of thick films of nanoparticles of ZnO and 3% Mn or Co doped ZnO with and without impregnation of 1% Pt have been investigated over a wide concentration range. The sensor characteristics of the Pt impregnated ZnO and 3% Mn or Co-doped ZnO nanoparticles are found to be excellent with the sensitivity reaching values in the 1000-1500 range for 1000 ppm of H2 at 125 degrees C or lower. The sensitivity is quite high even for 10 ppm of H2. The response and recovery times are generally less than 10 s in the region of high sensitivity. These films exhibit very good sensing characteristics for ethanol, the sensitivity being well above 2000 at 50 degrees C for 1000 ppm of vapour. Nanowires of Pt-impregnated ZnO also exhibit satisfactory sensing characteristics, but the values of the sensitivity are somewhat lower. Humidity has negligible effect on the performance of these sensors.     

Facile template-free fabrication of olive-like ZnO nanoparticles and their photoluminescence properties

A facile template-free solvothermal approach was applied to synthesize olive-like ZnO nanoparticles with an average diameter of about 300 nm and an average length of about 600 nm. XRD, TEM, SEM, SAED, EDX and PL spectra were employed to characterize the crystal phase, morphologies, the chemical compositions, and optical properties of the ZnO nanostructure. The experimental results showed the as-obtained ZnO was single-crystalline nanostructure and the concentration of CH₃COO⁻ solution played a key role in controlling the morphology of ZnO. The growth mechanism of ZnO was tentatively investigated. Besides, the olive-like ZnO nanoparticles exhibit a very strong ultraviolet emission centered at 383 nm and a weak green luminescence emission at around 522 nm.     

Optical and photoluminescence studies of gold nanoparticles embedded ZnO thin films

Gold nanoparticles (AuNPs) embedded ZnO thin films were prepared by sandwiching a thin thermally evaporated Au film between two sputtered ZnO films. The films were characterized by high resolution transmission electron microscopy (HRTEM), glancing angle X-ray diffraction (GXRD), optical absorption and photoluminescence (PL) measurements. GXRD data exhibited peaks which were attributed to the reflections from various ZnO and Au planes. Size dependence of the plasmon absorption was studied by forming nanoparticles with various sizes. Optical absorption spectra showed strong absorption due to localized surface plasmons at about 608, 638 and 676 nm for films having average AuNPs sizes of 27, 40 and 67 nm respectively. AuNPs embedded ZnO film showed a strong reduction in the intensity of photoluminescence, which was prominent in the case of pure ZnO film. The rise in temperature at a single nanoparticle site was calculated to be 22 K for a particle size of 80 nm.     

非催化剂法制备ZnO纳米线阵列的近带边高分辨变温光致发光性能研究

采用CVD法在a-面蓝宝石衬底上制备了ZnO籽晶层,然后在籽晶层上用碳热还原法制备了高质量的ZnO纳米线阵列。发现生长后的ZnO纳米线阵列具有良好的近带边发光性能,束缚激子发光峰半峰宽<500!eV。通过变温PL测试,确定3.37737eV的发光峰为自由激子发光,分别通过Vina模型和Vashni模型对其进行拟合,发现在60K以下Vashni模型的拟合相当好,而在60～150K,Vina模型的拟合效果更优。3.29eV处的峰为DAP峰,计算可得样品中的施主浓度为1.8×1018cm-3。     

The novel,one step and facile synthesis of ZnO nanoparticles using heteropolyoxometalates and their photoluminescence behavior

A novel and facile wet chemical method is presented to synthesize zinc oxide nanoparticles (NPs) under ambient atmosphere and temperature. Keggin type heteropolyoxometalate (H₃[PW₁₂O₄₀]) was used as stabilizer and the effect of stirring time and amount of H₃[PW₁₂O₄₀] (HPW) were studied. XRD and TEM techniques were applied for the morphological and structural characterizations of NPs. Size of nanoparticles were determined using TEM, Scherrer’s formula as well as effective mass approximation. The results of these three methods are in good agreements and revealed single hexagonal zincite type crystalline with average particle size in the range of 3–15 nm. Photoluminescence behavior of the prepared sample shows a strong orange to red emission centred at about 620–635 nm, a green emission at around 550 nm and broad UV emission at around 400 nm.