ZnO纳米线阵列的图形化生长

采用光刻和射频磁控溅射技术在Si衬底上制备了图形化的ZnO种子层薄膜。分别采用气相榆运和水热合成法，制备了最小单元为30μm的图形化的ZnO纳米线阵列。X射线衍射（XRD）分析显示单晶纳米线阵列具有高度的c轴[001]择优取向生长性质，从扫描电子显微镜（SEM）照片看出，阵列图形完整清晰，边缘整齐，纳米线阵列在室温下光致发光（PL）谱线中在380hm左右具有强烈的紫外发射峰，可见光区域发射峰得到了抑制，证明ZnO纳米线阵列氧空位缺陷少，晶体质量高。     

Vertical ZnO nanowire growth on metal substrates

Vertical growth of ZnO nanowires is usually achieved on lattice-matched substrates such as ZnO or sapphire using various vapor transport techniques. Accomplishing this on silicon substrates requires thick ZnO buffer layers. Here we demonstrate growth of vertical ZnO nanowires on FeCrAl substrates. The pre-annealing prior to growth appears to preferentially segregate Al and O to the surface, thus leading to a self-forming, thin pseudo-buffer layer, which then results in vertical nanowire growth as on sapphire substrates. Metal substrates are more suitable and cheaper than others for applications in piezoelectric devices, and thin self-forming layers can also reduce interfacial resistance to electrical and thermal conduction.     

Temperature dependant structural and electrical properties of ZnO nanowire networks

In this paper, we report a successful growth of zinc oxide nanowire networks by simple thermal evaporation process using metallic zinc powder in the presence of oxygen. The morphological investigations of the synthesized nanowire networks are conducted by using field emission scanning electron microscopy (FESEM) which reveals that the grown products are in high-density over the whole substrate surface and possessing nanowire networks like structures. The structural and compositional properties of the grown nanowire networks are analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), respectively which confirm that the synthesized products are well-crystalline, with wurtzite hexagonal phase ZnO. The as-grown ZnO nanowire networks grown on silicon substrate are utilized to fabricate n-ZnO/p-Si heterojunction diode and presented in this paper. The I-V characteristics of the fabricated heterojunction diode at different temperatures (77 K-477 K) are also investigated. High values of quality factor, which are obtained from this study, indicate a non-ideal behavior of the fabricated device. The mean barrier height of -0.84 eV is also estimated and presented in this paper.     

p-Type ZnO nanowire arrays

Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on alpha-sapphire substrates by using N2O as a dopant source via vapor-liquid-solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1-2) x 10(18) cm(-3) and a field-effect mobility of 10-17 cm2 V(-2) s(-1). Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type ZnO NWs with durable and controlled transport properties is important for fabrication of nanoscale electronic and optoelectronic devices.     

Cu掺杂ZnO纳米粒子的光学性能及铁磁性

利用溶胶凝胶法制备了纳米结构的Cu掺杂ZnO基稀磁半导体,通过X射线衍射分析表明,样品为纯相ZnO纤锌矿结构,磁性测量表明样品在室温下呈室温铁磁性,铁磁性来源为氧化锌晶格中的缺陷与Cu2+离子之间的交换作用。室温光致发光(PL)谱观察到紫外带边和可见光区两个发射峰,且随着Cu掺杂量增加,紫外峰淬灭,可见峰发射增强。     

A high-sensitivity,fast-response,rapid-recovery UV photodetector fabricated based on catalyst-free growth of ZnO nanowire networks on glass substrate

Here, we report for the first time the fabrication of metal–semiconductor–metal ultraviolet photodetector based on catalyst-free growth of ZnO nanowire networks on ITO seeds/glass substrates by thermal evaporation method. The morphological, structural, and optical properties of the sample were studied by using field emission scanning electron microscopy, X-ray diffraction, photoluminescence, and UV–Vis spectrophotometer. Upon exposure to 365 nm light (1.5 mW/cm²) at five-bias voltage, the device showed 2.32 × 10³ sensitivity. In addition, the photocurrent was 1.79 × 10⁻⁴ A, and the internal gain of the photodetector was 24.2. The response and the recovery times were calculated to be 3.9 and 2.6 s, respectively, upon illumination to a pulse UV light (365 nm, 1.5 mW/cm²) at five-bias voltage. All of these results demonstrate that this high-quality detector can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications.     

Room-temperature ferromagnetic properties of Cu-doped ZnO rod arrays

We have investigated properties of the Cu-doped ZnO crystalline film synthesized by the hydrothermal method. X-ray diffraction and X-ray photoelectron spectroscopy results provide the evidence that Cu^2 + is incorporated into the ZnO lattices. Photoluminescence spectrum of the rod arrays shows that the UV emission peak shifts a little to lower energy and its intensity decreased. There are another two emission peaks centred in blue and green regions because of the incorporation of Cu^2 + ions. The rod arrays have exhibited room-temperature ferromagnetic behaviour with the remanence of 0·926 × 10^− 3 emu/cm³. We suggest that the exchange interaction between local spin-polarized electrons (such as the electrons of Cu^2 + ions) and conductive electrons is the cause of room-temperature ferromagnetism.     

Optical and magnetic properties of Cu-doped ZnO nanoparticles

Cu-doped ZnO nanoparticles were synthesized by a simple chemical method at low temperature with Cu:Zn atomic ratio from 0 to 5 %. The synthesis process was based on the hydrolysis of zinc acetate dehydrate and copper acetate tetrahydrate heated under reflux to 65 °C using methanol as a solvent. X-ray diffraction (XRD) analysis reveals that the Cu-doped ZnO crystallize in a wurtzite structure with a change of crystal size from 12 nm for undoped ZnO to 5 nm for Cu-doped ZnO. These nano size crystallites of Cu doped ZnO self-organized into microspheres. The XRD patterns, Scanning electron microscopy and transmission electron microscopy micrographs of doping of Cu in ZnO confirmed the formation of microspheres and indicated that the Cu²⁺ is successfully substituted into the ZnO host structure of the Zn²⁺ site. Cu doping shifts the absorption onset to blue from 373 to 350 nm, indicating an increase in the band gap from 3.33 to 3.55 eV. A relative increase in the intensity of the deep trap emission of Cu-doped ZnO is observed when increasing the concentration of Cu. Magnetic measurements indicate that Cu-doped ZnO samples are ferromagnetic at room temperature except pure ZnO.     

Ferromagnetism in Cu-doped ZnO nanoparticles at room temperature

Cu-doped ZnO nanoparticles were successfully synthesized and structurally characterized by using X-ray diffraction (XRD) and transmission electron microscope (TEM). XRD shows that Zn_1−xCu_xO (x ≤ 0.04) samples are single phase with the ZnO-like wurtzite structure, while the secondary phase Cu is observed in Zn_0.95Cu_0.05O sample. Magnetic measurements indicated that Zn_1−xCu_xO (x = 0.02, 0.03, 0.04) are ferromagnetic at room temperature and the magnetic moment per Cu atom decreased with increasing Cu concentration. XRD, TEM and X-ray photoelectron spectroscopy (XPS) analysis revealed that no ferromagnetic-related secondary phase was detected. The origin of the ferromagnetism in Zn_1−xCu_xO (x ≤ 0.04) was mainly due to Cu ions substituted into the ZnO lattice.     

Catalyst size limitation in vapor-liquid-solid ZnO nanowire growth using pulsed laser deposition

Using a pulsed laser deposition system in a plane reflector configuration, we have grown single crystal ZnO nanowires using different gold catalyst quantities and different local particle fluxes. The number of ZnO nanowire over the square micrometer substrate surface proved to depend on both gold quantity and particle flux, in specific conditions. If for a “low particle flux” the nanowire density seems to strongly depend on the gold quantity, for a “high particle flux”, the dependence is no longer observable. The results were interpreted in terms of the catalyst absorbed particle concentration dependence on the catalyst droplet size and incident particle flux.     

Control of ZnO nanowire growth and optical properties in a vapor deposition process

A vapor deposition method was applied to synthesize zinc oxide(ZnO) nanowires and nanorods with diameter from 40 nm to 500 nm, length from 1 μm to 70 μm by adjusting the flow rate of argon, oxygen and the pressure during growth. Results of scanning electron microscopy(SEM) and high resolution transmission electron microscopy(TEM) proved the hexagonal wurtzite structure of the synthesized ZnO nanowires or nanorods, which grow along the 0001 direction. The results show that the growth conditions strongly impact the morphology, growth rate and optical properties of the ZnO nanostructures.The ZnO nanowires with small diameters tend to show stronger ultraviolet(UV) light emission from the electron-hole recombination near band edge in photoluminescence(PL), while those with larger diameters tend to exhibit PL spectra dominated by the broad green light emission due to the defects.     

A ZnO nanowire vacuum pressure sensor

In this study, we report the growth and characterization of lateral ZnO nanowires (NWs) on ZnO:Ga/glass templates. Using x-ray diffraction and micro-Raman spectroscopy, it was found that crystal quality of the as-grown ZnO NWs is good. It was also found that the average length and average diameter of the laterally grown ZnO NWs were 5?μm and 30?nm, respectively. A vacuum pressure sensor was then fabricated using a single NW bridging across two electrodes. By measuring the current-voltage characteristics of the samples at low pressure, we found that the currents were of 17, 34.28, 57.37 and 96.06?nA for the ZnO NW measured at 1 × 10(-3)?Torr, 1 × 10(-4)?Torr, 3 × 10(-5)?Torr and 5 × 10(-6)?Torr, respectively. These values suggest that the laterally grown ZnO NWs prepared in this study are potentially useful for vacuum pressure sensing.     

Transparent conducting silver nanowire networks

We present a transparent conducting electrode composed of a periodic two-dimensional network of silver nanowires. Networks of Ag nanowires are made with wire diameters of 45-110 nm and a pitch of 500, 700, and 1000 nm. Anomalous optical transmission is observed, with an averaged transmission up to 91% for the best transmitting network and sheet resistances as low as 6.5 Ω/sq for the best conducting network. Our most dilute networks show lower sheet resistance and higher optical transmittance than an 80 nm thick layer of ITO sputtered on glass. By comparing measurements and simulations, we identify four distinct physical phenomena that govern the transmission of light through the networks: all related to the excitation of localized surface plasmons and surface plasmon polaritons on the wires. The insights given in this paper provide the key guidelines for designing high-transmittance and low-resistance nanowire electrodes for optoelectronic devices, including thin-film solar cells. For the latter, we discuss the general design principles to use the nanowire electrodes also as a light trapping scheme.     

Position-controlled interconnected InAs nanowire networks

We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or "nanotrees". A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.     

Hyperbranched lead selenide nanowire networks

Lead chalcogenide nanostructures are good potential candidates for applications in multiexciton solar cells, infrared photodetectors, and electroluminescence devices. Here we report the synthesis and electrical measurements of hyperbranched PbSe nanowire networks. Hyperbranched PbSe nanowire networks are synthesized via a vapor-liquid-solid (VLS) mechanism. The branching is induced by continuously feeding the PbSe reactant with the vapor of a low-melting-point metal catalyst including In, Ga, and Bi. The branches show very regular orientation relationships: either perpendicular or parallel to each other. The diameter of the individual NWs depends on the size of the catalyst droplets, which can be controlled by the catalyst vapor pressure. Significantly, the hyperbranched networks can be grown epitaxially on NaCl, a low-cost substrate for future device array applications. Electrical measurements across branched NWs show the evolution of charge carrier transport with distance and degree of branching.     

Synergetic nanowire growth

Interest in nanowires continues to grow because they hold the promise of monolithic integration of high-performance semiconductors with new functionality into existing silicon technology. Most nanowires are grown using vapour-liquid-solid growth, and despite many years of study this growth mechanism remains under lively debate. In particular, the role of the metal particle is unclear. For instance, contradictory results have been reported on the effect of particle size on nanowire growth rate. Additionally, nanowire growth from a patterned array of catalysts has shown that small wire-to-wire spacing leads to materials competition and a reduction in growth rates. Here, we report on a counterintuitive synergetic effect resulting in an increase of the growth rate for decreasing wire-to-wire distance. We show that the growth rate is proportional to the catalyst area fraction. The effect has its origin in the catalytic decomposition of precursors and is applicable to a variety of nanowire materials and growth techniques.     

Effects of dopants on magnetic properties of Cu-doped ZnO thin films

We report the observation of room temperature ferromagnetism (FM) in Cu-doped ZnO (ZnO: Cu) thin films synthesized by sol–gel technique. While donor Al³⁺ cations are introduced into the ZnO: Cu films, the saturation magnetization decreases rapidly. Cu 2p core-level X-ray photoelectronic spectra demonstrate that the FM is strongly correlated with Cu²⁺ cations (3d ⁹ configuration). To further study the relationship between the FM and acceptors, Na⁺ cations are also introduced into the ZnO: Cu films to increase the saturation magnetization. The enhanced FM in the (Cu, Na)-codoped ZnO is suggested to be due to the hybridization between delocalized holes and spin-split Cu 3d states.     

Chemical sensing with ZnO nanowire field-effect transistor

Zinc oxide nanowires are configured as n-channel FETs. These transistors are implemented as chemical sensors for detection of various chemical gases. It is observed that the nanowire conductance is reduced when it is exposed to oxygen, nitrogen dioxide, ammonia gases at room temperature. Its ammonia sensing behavior is observed to switch from oxidizing to reducing when temperature is increased to 500 K. This effect is mainly attributed to the temperature dependent Fermi level shift. In addition, carbon monoxide is found to increase the nanowire conductance in the presence of oxygen. Furthermore, the detection sensitivity dependence on the nanowire radius is presented.