Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

Tetrakis(4‐sulfonatophenyl)porphyrin‐Directed Assembly of Gold Nanocrystals: Tailoring the Plasmon Coupling Through Controllable Gap Distances

It is known that universality and controllability over nanocrystal orientation must be accomplished to facilitate the potential applications of metal nanocrystals in the areas of photonics, electronics, and optics. The facile fabrication of linear chains of Au nanorods and bifurcated junctions of nanorods/nanospheres is achieved via the crosslinking of H‐type tetrakis(4‐sulfonatophenyl)porphyrin aggregates in solution. The tuning of the plasmon coupling between the Au nanocrystals is demonstrated by varying the porphyrin concentration and thus the interparticle gap distances. Finite‐difference time‐domain calculations show that the red shift of the plasmon band exhibits a nearly exponential decay with increasing interparticle gap distances, thus giving rise to a “plasmon ruler equation.” The gap distances determined according to this equation agree well with the experimental observations and further confirm the porphyrin‐directed assembly process. The interaction mechanism between the Au nanorods and porphyrins is further investigated by a biological procedure using the dark‐field light scattering technique.     

Size-induced variations in lattice dimension, photoluminescence, and photocatalytic activity of ZnO nanorods

Highly crystalline ZnO nanorods with diameters ranging from 5 to 57 nm were prepared by a seed-mediated solution method. With a diameter reduction, the lattice volume of ZnO nanorods increased and c/a ratio decreased, in apparent contradiction to what was observed in spherical ZnO nanocrystals. All ZnO nanorods showed a strong yellow emission without the UV or green emissions that had been observed for ZnO nanostructures prepared by other methods. For larger diameters, the yellow emission exhibited an abnormal red shift, which was associated with the lattice variations in the nanoscale structure and the resulting band modifications. The size-induced band modifications were also confirmed by the photocatalytic activity of ZnO nanorods, which have an optimum diameter (approximately 30 nm) for the photodegradation of Rhodamine B dye solution.     

Synthesis of CdS-Au2S-Au hybrid dendritic nanostructures

The hybrid CdS-Au₂S-Au dendritic nanocrystals were synthesized in toluene solution at 70 °C. UV-vis and photoluminescence (PL) spectra recorded the optical properties of hybrid nanostructures, which showed an obvious blue shift relative to the absorption peak of CdS dendritic nanocrystals. The initial CdS dendritic nanocrystals exhibited band gap and trap state emission, both of which were quenched by Au parts. Analysis of the hybrid nanostructures by XRD shows the presence of appreciable amounts of Au₂S, indicating that the chemical process involving cation exchanges between Au⁺ ions and Cd²⁺ ions was found.     

Tailoring the photoluminescence of ZnO nanowires using Au nanoparticles

Rational design of hybrid nanostructures through attaching nanowires with nanoparticles is an effective route to enhance the existing functionalities or to explore new ones. We carry out a systematic investigation on the photoluminescence of ZnO nanowire-Au nanoparticle hybrid nanostructures synthesized by attaching Au nanoparticles onto ZnO nanowires. Citrate-stabilized 40?nm Au nanoparticles effectively quench the green emission and enhance the UV emission of the ZnO nanowires, which is consistent with the wavelength-dependent generation of surface plasmon. The UV/green emission intensity ratio could be reversibly and reproducibly tailored by attaching/detaching Au nanoparticles. This enhancement of UV emission diminishes if the Au nanoparticles are coated with a polymer layer. We also find that the orange-red emission of the ZnO nanowires is related to the excess oxygen on the ZnO surface, and it is also tunable via annealing and surface modifications.     

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.     

Understanding the Photothermal Conversion Efficiency of Gold Nanocrystals

Plasmon‐based photothermal therapy is one of the most intriguing applications of noble metal nanostructures. The photothermal conversion efficiency is an essential parameter in practically realizing this application. The effects of the plasmon resonance wavelength, particle volume, shell coating, and assembly on the photothermal conversion efficiencies of Au nanocrystals are systematically studied by directly measuring the temperature of Au nanocrystal solutions with a thermocouple and analyzed on the basis of energy balance. The temperature of Au nanocrystal solutions reaches the maximum at ～75°C when the plasmon resonance wavelength of Au nanocrystals is equal to the illumination laser wavelength. For Au nanocrystals with similar shapes, the larger the nanocrystal, the smaller the photothermal conversion efficiency becomes. The photothermal conversion can also be controlled by shell coating and assembly through the change in the plasmon resonance energy of Au nanocrystals. Moreover, coating Au nanocrystals with semiconductor materials that have band gap energies smaller than the illumination laser energy can improve the photothermal conversion efficiency owing to the presence of an additional light absorption channel.     

Plasmon Modes Induced by Anisotropic Gap Opening in Au@Cu2O Nanorods

Integration of semiconductors with noble metals to form heteronanostructures can give rise to many interesting plasmonic and electronic properties. A number of such heteronanostructures have been demonstrated comprising noble metals and n‐type semiconductors, such as TiO₂, ZnO, SnO₂, Fe₃O₄, and CuO. In contrast, reports on heteronanostructures made of noble metals and p‐type semiconductors are scarce. Cu₂O is an unintentional p‐type semiconductor with unique properties. Here, the uniform coating of Cu₂O on two types of Au nanorods and systematic studies of the plasmonic properties of the resultant core–shell heteronanostructures are reported. One type of Au nanorods is prepared by seed‐mediated growth, and the other is obtained by oxidation of the as‐prepared Au nanorods. The (Au nanorod)@Cu₂O nanostructures produced from the as‐prepared nanorods exhibit two transverse plasmon peaks, whereas those derived from the oxidized nanorods display only one transverse plasmon peak. Through electrodynamic simulations the additional transverse plasmon peak is found to originate from a discontinuous gap formed at the side of the as‐prepared nanorods. The existence of the gap is verified and its formation mechanism is unraveled with additional experiments. The results will be useful for designing metal–semiconductor heteronanostructures with desired plasmonic properties and therefore also for exploring plasmon‐enhanced applications in photocatalysis, solar‐energy harvesting, and biotechnologies.     

Au–ZnO hybrid nanostructures prepared by electro-exploding wire technique: Raman signal enhancement and photoluminescence emission quenching

Electro-exploding wire (EEW) technique was employed to prepare ZnO and Au–ZnO hybrid nanoparticles. Average size of the prepared ZnO nanoparticles is found to be 3.8 nm and uniform throughout. These ultrafine ZnO nanoparticles are found to agglomerate around the highly surface active Au nanoparticles. It also acts as a stabilizer for the Au nanoparticles by avoiding self agglomeration. The hybrid nanocrystals show strong crystallinity of face-centered cubic and hexagonal wurtzite structure of gold (Au) and zinc oxide (ZnO), respectively. Presence of Au₃Zn in pristine sample is a clear indication of a strong interaction between ZnO and Au systems. The hybrid system shows strong enhancement in the ZnO Raman signals and quenching in the visible Photoluminescence (PL) emission. Energy-dependent PL analysis shows the dominance of the surface defects over the bulk contribution in these ultrafine ZnO and Au–ZnO hybrid nanostructures.     

Large scale synthesis and enhanced field emission properties from hybrid CuO/ZnO nanorod

Upper-directionally grown nanorods were synthesized on a large scale by a simple method of direct heating of Cu foil in air. Hybrid CuO/ZnO nanorods were fabricated by ZnO thin film coating using magnetron sputtering. Field emission (FE) measurements of CuO and hybrid CuO/ZnO nanorod films show that they have turn-on field of 3.81 and 3.24 V/microm and a current density of 0.39 and 1.1 microA/cm2 under an applied field of about 6.6 V/microm, respectively. By comparing X-ray photoelectron spectroscopy analysis and the FE properties of two types of samples, we concluded that the narrowing of band gap due to the change of electron binding energy of hybrid CuO/ZnO nanorods effectively improved FE.     

Morphological transformations induced by Co impurity in ZnO nanostructures prepared by rf-sputtering and their physical properties

Growth of uniform and vertically well aligned nanorods is a difficult process and becomes more complicated in case of ZnO nanorods on silicon (Si) substrate due to thermal instability of the Si substrate and large lattice mismatch (~?40%) between the substrate and the ZnO nanorods array. Growth of ZnO nanorods assisted by metal ion via rf-sputtering is a good technique; however, it needs many parameters to be controlled for desired growth and morphology of nanostructures. In this work, we report the morphological transformations of ZnO nanostructured thin film by simply controlling the concentration of Cobalt (Co) impurity in sputtering target. With the introduction of Co ions in ZnO matrix, the initial coalescence grain structure (pyramidal morphology) changes into columnar grains and as the concentration of Co ions increases further, a highly oriented ZnO nanorods array is obtained. The possible mechanism with the help of schematic diagram is also proposed for the morphological transformation of ZnO nanostructures. The vertically aligned nanorods show good optical properties as well as robust ferromagnetism at room temperatures. It has also been observed that with the dopant conc. increasing there was a significant decrease in the band gap energy. The structure and morphology of rf-sputtered nanostructured thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. Interestingly, with Co conc. increasing in ZnO matrix results in decreasing LO modes in Raman spectroscopy. It can have strong influence on the magnetic properties of the material. The good optical and strong ferromagnetic properties of the ZnO nanorods, suggest its possible applications in the fields of lasers, spintronics and medical applications.     

Enhanced visible-light-responsive photocatalytic property of CdS and PbS sensitized ZnO nanocomposite photocatalysts

CdS and PbS nanoparticles sensitized ZnO nanorods were synthesized by successive ionic layer adsorption and reaction method. The photocatalytic activity of different structures was evaluated by photocatalytic degeneration yield of methyl orange. Co-sensitization of CdS and PbS nanoparticles on ZnO nanorods showed enhanced photocatalytic activity due to its response at visible light area and the stepwise band gap constructed in ZnO/CdS/PbS nanostructures.     

Enhanced luminescence of Ag-decorated ZnO nanorods

Ag-decorated ZnO nanorods were synthesized by thermal evaporation of a mixture of ZnO and graphite powders at 900 °C followed by wet Ag coating and thermal annealing. The ZnO nanorods had a rod-like morphology with a relatively uniform width and length. The widths and lengths of the nanorods ranged from 50 to 300 nm and up to a few hundred micrometers, respectively. The diameters of the Ag particles on the nanorods ranged from 10 to 100 nm. The dependence of the photoluminescence properties of Ag-decorated ZnO nanorods on the postannealing atmosphere was examined. Annealing resulted in an increase and decrease in the near band edge (NBE) and deep level (DL) emission intensities of Ag-coated ZnO nanorods, respectively, whereas both the NBE and DL emission intensities of uncoated ZnO nanorods were increased by annealing. The intensity ratio of NBE emission to DL emission of the Ag-coated ZnO nanorods was increased ~15-fold by hydrogen annealing. The underlying mechanism for NBE emission enhancement and DL emission suppression of Ag-coated ZnO nanorods by postannealing is discussed based on the surface plasmon resonance effect of Ag.     

Optoelectronics behaviour of ZnO nanorods for UV detection

In this study ZnO nanorods have been synthesized by a chemical precipitation method. The room temperature UV–Vis absorption spectra of the ZnO nanorods indicated two absorption peaks in the UV region, one in the near UV region and the other attributed to the band gap of ZnO. The Photoluminescence spectra of ZnO nanorods show two emission bands, one ultraviolet emission band at 378 nm and the other in the defect related yellow emission band near 550 nm. The stimulated yellow luminescence of ZnO nanorods were affected by the synthesis time and annealing temperature. The same ZnO nanorods were deposited onto the ITO substrate to form a UV photoconductive detector. The ratio of the UV photogenerated current to dark current was as high as nine times under 3 V bias. Hence, these nanorods can be promising materials in the use of UV radiation detection.     

Fabrication and characterization of Mg-doped ZnO nanorod arrays

Photoelectronic characteristics are investigated in well-aligned MgO-coated ZnO nanorods (MgO/ZnO nanocables) grown on Si substrates buffered with ZnO film at a low temperature by solution techniques. Transmission electron microscopy shows that a rough surface was observed for the MgO-coated ZnO nanorods due to deposition of MgO nanoparticles on the surface of the ZnO nanorods. However, after annealed at high temperatures, the surface of the MgO-coated ZnO nanorods was flattened to form Mg-doped ZnO nanorods. Photoluminescence spectra of Mg-doped ZnO nanorods displayed a blue shift of the near-band-edge emission with increasing annealing temperature indicative of an increase in the band gap of the MgZnO alloy due to diffusion of the Mg atoms into the ZnO nanorods. In contrast, no blue shift was detected for the samples annealed in H2/N2 (5%/95%) reduction atmosphere but a blue emission was detected at 800 degrees C, indicating that MgO diffusion process may produce a new luminescent center to emit the blue emission in H2/N2 reduction atmosphere.     

A template-free alcoholthermal route to Ti(Sn)-doped ZnO nanorods

Ti(Sn)-doped single-crystalline ZnO nanorods with an average diameter of 20 nm and length up to nearly 1 μm were synthesized by a facile ultrasonic irradiation-assisted alcoholthermal method without involving any templates. Photoluminescence spectra of the Ti-doped ZnO nanorods were measured at room temperature and three emitting bands, being a violet emission at 400-415 nm, a blue band at 450-470 nm and a green band at around 550 nm, were detected. The emission intensities of the Ti-doped ZnO nanorods enhance gradually with increasing the doping concentrations. As to the Sn-doped ZnO nanorods, the green emission shifts to 540 nm and the emission intensities increase first but decrease later with increasing the doping concentrations.     

高质量四脚状ZnO纳米结构的制备及其影响因素

采用汽相传输法制备高质量四脚状ZnO纳米结构(T-ZnO).以ZnO纳米晶为同质催 化剂,在锌源-衬底间添加蒸汽过滤器,研究过滤器和纳米晶对产物组成和形态的影响,以及 T-ZnO的结构特征.实验结果表明,T-ZnO为纤锌矿结构,分布均匀、尺寸一致,由四个柱 状纳米棒和一个球形核心组成.纳米棒直径约为60nm,沿<0001>晶向生长.过滤器的使用可 有效地去除产物中颗粒,而ZnO纳米晶的使用明显降低其尺寸,提高其均匀性.     

Low-temperature growth and optical properties of Ce-doped ZnO nanorods

Ce-doped ZnO nanorod arrays were grown on zinc foils by a hydrothermal method at 180°C. The effects of Ce-doping on the structure and optical properties of ZnO nanorods were investigated in detail. The characterisation of the rod array with X-ray diffraction and X-ray photoelectron spectroscopy indicated that Ce³⁺ ions were incorporated into the ZnO lattices. There were no diffraction peaks of Ce or cerium oxide in the pattern. From UV-Vis spectra, we observed a red shift in the wavelength of absorption and decreased band gap due to the Ce ion incorporation in ZnO. The photoluminescence integrated intensity ratio of the UV emission to the deep-level green emission (I _UV/I _DLE) was 1.25 and 2.87, for ZnO and Ce-doped ZnO nanorods, respectively, which shows a great promise for the Ce-doped ZnO nanorods with applications in optoelectronic devices.     

Seed‐Induced Growth of Flower‐Like Au–Ni–ZnO Metal–Semiconductor Hybrid Nanocrystals for Photocatalytic Applications

The combination of metal and semiconductor components in nanoscale to form a hybrid nanocrystal provides an important approach for achieving advanced functional materials with special optical, magnetic and photocatalytic functionalities. Here, a facile solution method is reported for the synthesis of Au–Ni–ZnO metal–semiconductor hybrid nanocrystals with a flower‐like morphology and multifunctional properties. This synthetic strategy uses noble and magnetic metal Au@Ni nanocrystal seeds formed in situ to induce the heteroepitaxial growth of semiconducting ZnO nanopyramids onto the surface of metal cores. Evidence of epitaxial growth of ZnO{0001} facets on Ni {111} facets is observed on the heterojunction, even though there is a large lattice mismatch between the semiconducting and magnetic components. Adjustment of the amount of Au and Ni precursors can control the size and composition of the metal core, and consequently modify the surface plasmon resonance (SPR) and magnetic properties. Room‐temperature superparamagnetic properties can be achieved by tuning the size of Ni core. The as‐prepared Au–Ni–ZnO nanocrystals are strongly photocatalytic and can be separated and re‐cycled by virtue of their magnetic properties. The simultaneous combination of plasmonic, semiconducting and magnetic components within a single hybrid nanocrystal furnishes it multifunctionalities that may find wide potential applications.     

Structural and optical properties of single-crystalline ZnO nanorods grown on silicon by thermal evaporation

The growth of perfectly hexagonal-shaped ZnO nanorods, with Zn-terminated (0001) facets bounded with [Formula: see text] surfaces, has been performed on nickel-coated Si(100) substrate via thermal evaporation using metallic zinc powder and oxygen. Detailed structural investigations confirmed that the synthesized nanorods are single crystalline with the wurtzite hexagonal phase and preferentially grow along the c-axis direction. Raman spectra of the as-grown ZnO nanorods showed an optical-phonon E(2) mode at 438?cm(-1), indicating that as-grown nanostructures are in good crystallinity with the wurtzite hexagonal phase. The ZnO nanorods were found to show strong band edge emission with very weak or no deep-level emission, as shown by photoluminescence measurements. The clear observation of free excitons at low temperatures (13-50?K) indicates that the as-grown ZnO nanorods are of high quality.