Facile route to high-density, ordered ZnO nanowire arrays and their photoluminescence properties

A simple template-directed wet chemistry route based on traditional thermal decomposition technique has been developed for the preparation of high-density, ordered ZnO nanowire arrays. The fabrication was performed at relative low temperature without involving complex procedures, sophisticated equipment and rigorous experiment conditions, thereby providing a straightforward and mild method to produce metal/metal oxide ordered nanostructures. The nanowire array system was evaluated by SEM, XRD, TEM and PL. A stable luminescence at 425 nm was present.     

Raman spectroscopy to probe residual stress in ZnO nanowire

Raman spectroscopic studies are performed to probe the stress along the length of a bent ZnO nanowire. The zone‐centre E₂^high optical phonon shows a systematic red shift as the junction point of the two arms of the nanowire is approached. The mechanism of the red shift is discussed on the basis of the tensile strain. From the red shift of the phonon peak position, the strain at different regions on the nanowire is estimated. Stress in the bent nanowire is also investigated using photoluminescence (PL) spectroscopy. Results of both Raman and PL study confirm that the bent nanowire is under tensile strain. Copyright © 2011 John Wiley & Sons, Ltd.     

Multiphoton ionization studies of amines with UV-VIS lasers

Three and four photon ionization studies at 355 and 532 nm are presented for the isomeric propylamines and n-pentylamine. Ionic fragmentation patterns and ion yields are presented as a function of laser wavelength and fluence. These time-of-flight (TOF) mass spectra are compared with both electron impact data and the TOF mass spectra of the previously reported butylamines. A brief discussion of the dynamics of the decomposition process is presented in light of the butylamine results and competitive ionic fragmentation pathways.     

Growth of self-assembled ZnO nanowire arrays

This paper reports on ZnO nanowires arrays synthesized using Sn as a catalyst. The Sn particles were produced from the reduction of SnO₂ powders via a vapour-solid growth process. Control of growth conditions led to the formation of ZnO nanowire arrays, radial nanowire ‘flowers’ and uniaxial fuzzy nanowires. ZnO nanowire–nanobelt junctions were also grown by changing the growth direction. As-grown nanowire arrays could be fundamental materials for investigating physical and chemical properties at nano-scale dimensions.     

Multiphoton excitation of molecules by single mode CW lasers

An intracavity laser interaction zone with molecules expanded in a molecular beam set-up permits multiphoton excitation under collision-free conditions by narrow band radiation. Two- and three-laser experiments are performed to study rapid adiabatic passage processes, stimulated de-excitation and hole burning effects. The detection of excitation occurs by means of a sensitive bolometric molecular beam detector. The essential information obtained for SF₆ concerns the vibrational bottleneck, the conservation of excited eigenstate characteristics for at least 1 ms, the ease with which about 15 photons can be deposited in the molecules with the help of two cw CO₂ lasers and the influence of simultaneous two-frequency radiation, yielding a significant extra-excitation. Other molecules where multiphoton excitation is observed utilizing the same set-up are CF₃I and CF₃Br.     

Study on the defects of ZnO nanowire

The room-temperature photoluminescence property of ZnO nanowires was studied. It showed an ultraviolet peak and a visible light band in the PL spectrum. Through Gaussian fitting, it was found that the visible light band can be divided into two peaks at 2.37 eV and 2.53 eV, which was originated from oxygen antisite and oxygen vacancy defects, respectively. After being exposed to air or post-annealed in oxygen ambience, aging effect was observed and the peak at 2.53 eV disappeared due to the removal of oxygen vacancy defects. Therefore, it is suggested that oxygen antisite and oxygen vacancy coexist in ZnO and induce visible light emission.     

Multiphoton ionization of atoms and ions by high-intensity X-ray lasers

Coulomb corrections to the action function and rate of multiphoton ionization of atoms and ions in a strong linearly polarized electromagnetic field are calculated for high values of the Keldysh adiabaticity parameter. The Coulomb corrections significantly increase the ionization rate for atoms (by several orders of magnitude). An interpolation formula proposed for ionization rate is valid for arbitrary values of the adiabaticity parameter. The high accuracy of the formula is confirmed by comparison with the results of numerical calculations. The general case of elliptic polarization of laser radiation is also considered.     

Characterization of ZnO nanowire field-effect transistors and exposed to ultraviolet radiation

A ZnO nanowire (NW) field-effect transistor (FET) is fabricated and characterized, and its characterization of ultraviolet radiation is also investigated. On the one hand, when the radiation time is 5~min, the radiation intensity increases to 5.1~μ W/cm², while the saturation drain current (I_\rm dss) of the nanowire FET decreases sharply from 560 to 320~nA. The field effect mobility (μ ) of the ZnO nanowire FET drops from 50.17 to 23.82~cm²/(V.s) at V_\rm DS=2.5~V, and the channel resistivity of the FET increases by a factor of 2. On the other hand, when the radiation intensity is 2.5~μ W/cm^2 , the DC performance of the FET does not change significantly with irradiation time (its performances at irradiation times of 5 and 20~min are almost the same); in particular, the I_\rm dss of NW FET only reduces by about 50~nA. Research is underway to reveal the intrinsic properties of suspended ZnO nanowires and to explore their device applications.     

ZnO nanowire array ultraviolet photodetectors with self-powered properties

A ZnO nanowire (NW) array ultraviolet photodetector (PD) with Pt Schottky contacts has been fabricated on a glass substrate. Under UV light illumination, this PD showed a high photo-to-dark current ratio of 892 at 30 V bias. Interestingly, it was also found that this PD had a high sensitivity of 475 without external bias. This phenomenon could be explained by the asymmetric Schottky barrier height (SBH) at the two ends causing different separation efficiency of photogenerated electron–hole pairs, which resulted in the formation of photocurrent. It is anticipated to have potential applications in self-powered UV detection field.     

Contact barriers in a single ZnO nanowire device

The contact potential between a single ZnO nanowire and Ti/Au contacts was estimated to be ∼30 meV by considering the Arrhenious plot of the two-probe resistance, the thermionic emission conduction, and the Fowler–Nordheim tunneling model. The net voltages applied to the contacts were calculated by subtracting the four-probe voltages from the two-probe voltages at the same currents. The activation energy of the four-probe resistance was about 2.4 mV which was 1/11th of that of the two-probe resistance. The Fowler–Nordheim plot clearly showed the crossover of the conduction mechanism from thermionic emission to tunneling regime as lowering the temperatures below T<100 K.     

The research on suspended ZnO nanowire field-effect transistor

This paper reports that a novel type of suspended ZnO nanowire field-effect transistors (FETs) were successfully fabricated using a photolithography process, and their electrical properties were characterized by I--V measurements. Single-crystalline ZnO nanowires were synthesized by a hydrothermal method, they were used as a suspended ZnO nanowire channel of back-gate field-effect transistors (FET). The fabricated suspended nanowire FETs showed a p-channel depletion mode, exhibited high on--off current ratio of ～10⁵. When V_DS=2.5 V, the peak transconductances of the suspended FETs were 0.396 μS, the oxide capacitance was found to be 1.547 fF, the pinch-off voltage V_TH was about 0.6 V, the electron mobility was on average 50.17 cm²/Vs. The resistivity of the ZnO nanowire channel was estimated to be 0.96× 10²Ω cm at V_GS = 0 V. These characteristics revealed that the suspended nanowire FET fabricated by the photolithography process had excellent performance. Better contacts between the ZnO nanowire and metal electrodes could be improved through annealing and metal deposition using a focused ion beam.     

Vertically integrated nanogenerator based on ZnO nanowire arrays

We report a technique to construct a vertically integrated nanogenerator (VI‐NG) based on ZnO nanowire (NW) arrays. The VI‐NG consists of nine single NGs connected mixed parallel and serial by a layer‐by‐layer stacking. For the single layer NG, the peak output voltage and current are 0.045 V and 2.5 nA, respectively. The VI‐NG produces an output power density of 2.8 nW/cm² with a peak output voltage of 0.15 V and output current of 7.2 nA. The vertical integration of the multi‐NG provides a feasible technique for effectively converting mechanical energies to electricity from environment. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Exciton polaritons confined in a ZnO nanowire cavity

Semiconductor nanowires of high purity and crystallinity hold promise as building blocks for miniaturized optoelectrical devices. Using scanning-excitation single-wire emission spectroscopy, with either a laser or an electron beam as a spatially resolved excitation source, we observe standing-wave exciton polaritons in ZnO nanowires at room temperature. The Rabi splitting between the polariton branches is more than 100 meV. The dispersion curve of the modes in the nanowire is substantially modified due to light-matter interaction. This finding forms a key aspect in understanding subwavelength guiding in these nanowires.     

Aligned growth of ZnO nanowires and lasing in single ZnO nanowire optical cavities

Ordered ZnO nanowire arrays have been fabricated in N₂ background gas by catalyst-free nanoparticle-assisted pulsed-laser deposition. A single ZnO nanowire was collected in an electrode gap by dielectrophoresis. Under the optical pumping above an exciting laser (λ= 355 nm) threshold of ∼ 334 kW/cm², ultraviolet lasing action in a single ZnO nanowire was observed at room temperature, indicating that the as-synthesized nanowires in pure N₂ background gas are of high quality. The crystalline facets of both ends of the nanowire acted to form an optical cavity. Therefore, the mode spacings corresponding to cavity lengths of the respective nanowires were observed in photoluminescence spectra. PACS 78.66.Hf; 81.07.Bc; 78.67.-n; 81.16.Mk     

Contact welding study of carbon nanotube with ZnO nanowire

Study of proton beam induced welding of multiwall carbon nanotubes (MWCNTs) with ZnO nanowires (NWs) has been carried out by proton (H⁺) beam irradiation. The samples were irradiated by 70-keV proton (H⁺) ion beams at different substrate temperatures. The irradiation-induced defects in CNTs and ZnO NWs were greatly reduced at elevated temperature. The crystalline structure of ZnO NWs and MWCNTs were found to remain stable after the irradiation at 700 K. As a preparation step, a coupling of two parallel ZnO NWs with irradiation has also been demonstrated. The welding mechanisms of MWCNTs and ZnO NWs were also been suggested. These two welding processes between same and distinct nanostructures to form homo- and hetero-junctions have provided an opportunity to mass produce interconnecting one-dimensional structures used for the manufacturing of future nanowire-based electronic circuits and devices.     

Co-sensitized quantum dot solar cell based on ZnO nanowire

An efficient photoelectrode is fabricated by sequentially assembled CdS and CdSe quantum dots (QDs) onto a ZnO-nanowire film. As revealed by UV-vis absorption spectrum and scanning electron microscopy (SEM), CdS and CdSe QDs can be effectively adsorbed on ZnO-nanowire array. Electrochemical impedance spectroscopy (EIS) measured demonstrates that the electron lifetime for ZnO/CdS/CdSe (13.8 ms) is calculated longer than that of ZnO/CdS device (6.2 ms), which indicates that interface charge recombination rate is reduced by sensitizing CdSe QDs. With broader light absorption range and longer electron lifetime, a power conversion efficiency of 1.42% is achieved for ZnO based CdS/CdSe co-sensitized solar cell under the illumination of one Sun (AM 1.5G, 100 mW cm⁻²).     

Surface reconstruction of ZnO nanowire arrays via solvent-evaporation-induced self-assembly

Vertically aligned ZnO nanowires (NWs) can be reconstructed on large scales by a solvent-evaporation-induced method. The morphologies of the nanowires are regulated by changing the concentration of the solution. Possible mechanism, which the compressive residual stresses and wires/wires self-attraction can be responsible for the surface reconstruction, is addressed. Furthermore, we compare the structural properties of ZnO nanowires before and after reconstructed.     

A templateless surfactant-free seedless aqueous route to single-crystalline ZnO nanowires synthesis

Zinc oxide (ZnO) nanowires (NWs) have been synthesized using zinc nitrate and hexamethylenetetramine by templateless, surfactant-free and seedless aqueous solution route. The morphology of ZnO NWs was considerably affected by growth time: a longer reaction time results in the formation of ZnO NWs. Structural analysis of the synthesized NWs showed an average diameter of 20–30 nm length of several micrometers and single-crystalline wurtzite hexagonal structure. Photoluminescence studies of ZnO NWs showed a strong green emission peak at 585 nm.     

片上制备横向结构ZnO纳米线阵列紫外探测器件

将纳米技术与传统的微电子工艺相结合, 片上制备了横向结构氧化锌(ZnO)纳米线阵列紫外探测器件, 纳米线由水热法直接自组织横向生长于叉指电极之间, 再除去斜向的多余纳米线, 其余工艺步骤与传统工艺相同. 分别尝试了铬(Cr)和金(Au)两种金属电极的器件结构: 由于Cr电极对其上纵向生长的纳米线有抑制作用, 导致横向生长纳米线长度可到达对侧电极, 光电响应方式为受表面氧离子吸附控制的光电导效应, 光电流大但增益低, 响应速度慢, 经二次电极加固, 纳米线根部与电极金属直接形成肖特基接触, 光电响应方式变为光伏效应, 增益和速度得到了极大改善; 由于Au电极对其上纵向生长的纳米线有催化作用, 导致溶质资源的竞争, 相同时间内横向生长的纳米线不能到达对侧, 而是交叉桥接, 但却形成了紫外光诱导的纳米线间势垒结高度调控机理, 得到的器件特性为最优, 在波长为365 nm的20 mW/cm²紫外光照下, 1 V电压时暗电流为10^-9 A, 光增益可达8×10⁵, 响应时间和恢复时间分别为1.1 s和1.3 s.     

Control of ZnO nanowire arrays by nanosphere lithography (NSL) on laser-produced ZnO substrates

Nanosphere lithography (NSL) is a successful technique for fabricating highly ordered arrays of ZnO nanowires typically on sapphire and GaN substrates. In this work, we investigate the use of thin ZnO films deposited on Si by pulsed laser deposition (PLD) as the substrate. This has a number of advantages over the alternatives above, including cost and potential scalability of production and it removes any issue of inadvertent n-type doping of nanowires by diffusion from the substrate. We demonstrate ordered arrays of ZnO nanowires, on ZnO-coated substrates by PLD, using a conventional NSL technique with gold as the catalyst. The nanowires were produced by vapor phase transport (VPT) growth in a tube furnace system and grew only on the areas pre-patterned by Au. We have also investigated the growth of ZnO nanowires using ZnO catalyst points deposited by PLD through an NSL mask on a bare silicon substrate.