Investigation on the photoconductive behaviors of an individual AlN nanowire under different excited lights

ABSTRACT: Ultra-long AlN nanowire arrays are prepared by chemical vapor deposition, and the photoconductive performances of individual nanowires are investigated in our self-built measurement system. Individual ultra-long AlN nanowire (UAN) exhibits a clear photoconductive effect under different excited lights. We attribute the positive photocurrent response of individual UAN to the dominant molecular sensitization effect. It is found that they have a much faster response speed (a rise and decay time of about 1 ms), higher photocurrent response (2.7 [MULTIPLICATION SIGN] 106), and more reproductive working performance (the photocurrent fluctuation is lower than 2%) in the air environment. Their better photoconductive performances are comparable to many nanostructures, which are suggested to be a candidate for building promising photosensitive nanodevices in the future.     

Structure-dependent growth control in nanowire synthesis via on-film formation of nanowires

On-film formation of nanowires, termed OFF-ON, is a novel synthetic approach that produces high-quality, single-crystalline nanowires of interest. This versatile method utilizes stress-induced atomic mass flow along grain boundaries in the polycrystalline film to form nanowires. Consequently, controlling the magnitude of the stress induced in the films and the microstructure of the films is important in OFF-ON. In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density. We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing. A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth. The OFF-ON method can be routinely used to grow nanowires from a variety of materials by tuning the material-dependent growth parameters.     

Catalyst-free in situ synthesis of ZrC nanowires with excellent thermal stability

No catalyst-assisted zirconium carbide nanowires (ZrCNWs) were successfully synthesized by thermal evaporation and in situ reaction using carbon nanotubes (CNTs) and ZrCl₄ as carbon and zirconium sources. Vapor-Solid (VS) mechanism was proposed to explain the growth process of ZrCNWs. The ZrCNWs prepared at 1400°C have a single crystal structure with the diameters of 130-290 nm. The axial growth direction of ZrCNWs is [111] and the yield is 76.5 wt%. After heat treatment at 1700°C, 2100°C, and 2450°C, the diameter of ZrCNWs increased and their aspect ratio decreased. By simulating the thermal stress of a single ZrCNW, it is speculated that the coarsening and fracture of ZrCNWs are caused by the overall slip of the atomic layers and the stress concentration at the dislocations.     

Macro-quantity synthesis of AlN nanowires via combined technique of arc plasma jet and thermal treatment

A two-step approach for macro-synthesis of AlN nanowires was developed in this study. The Al nano-particles were first prepared by directly injecting a rough Al powder (20–50 μm in size) into N₂ thermal plasma jet, subsequently the formed nano-particles were heated at 1100 °C in N₂ atmosphere for 3 h, and finally at least 31% of the as-heated intermediate was transformed into the separable AlN nanowires. It provides a new route employing nano-particles as precursors to fabricate nanowires, and this achievement is also a significant work for large-scale production and commercial application of AlN nanowires.     

Bismuth-catalyzed synthesis of ZnO nanowires and their photoluminescence properties

By means of heating a mixture of Zn and Bi powders, we have successfully prepared ZnO nanowires. Since the produced nanowires correspond to the pure hexagonal ZnO phase with no catalytic nanoparticles being observed at the tips, the growth is dominated by the base-growth mechanism, in which Bi species at the bottom of the nanowires played a catalytic role. Photoluminescence (PL) analysis of indicated that the integrated intensity ratio of the 2.5 eV-band to the UV band increased with increased growth temperature. An increase of the Bi₂O₃ phase itself, Bi₂O₃ phase-induced oxygen vacancies, and an increase of oxygen vacancies in ZnO with increasing growth temperature were responsible for the intensification and the appearance of the 2.5 eV-band.     

High-yield synthesis of ZnO nanowire arrays and their opto-electrical properties

In this article, ZnO nanostructures were synthesized via the hydrothermal method which used ZnCl(2) and HMTA mixed solution as the precursor. A multistep growth was adopted to improve the growth restriction of a closed system, not only the length but also the aspect ratio were increased with steps of growth, and the shape of nanorods maintained integrity. Furthermore, photoluminescence spectra which have the near-band-edge-emission (～3.37 eV) and defect-related emission show the optical properties of ZnO nanostructures. The defect-related emission intensity was greatly enhanced with the increasing surface area of ZnO nanowires. The level of the OH group was attributed to the yellow-light emission (～580 nm) and the red-shift phenomenon. In addition, we fabricated two types of ultraviolet photodetectors: a single nanowire device and a nanowire-array device, operating at a low bias (less than 5 mV). With the lower energy consumption and the weaker persistent photoconductive effect, our ultraviolet photodetectors have better performance, exhibiting a short response time and higher sensitivity.     

梯度共聚物的可控制备及其性能

梯度共聚物是近年来伴随着活性聚合方法而发展起来的一种新型共聚物,其特点在于单体单元组成沿着分子链方向逐渐变化,链结构界于常见的无规共聚物和嵌段共聚物之间。本文从梯度共聚物的结构特点入手,总结了其可控制备方法、表征手段、物化性质以及应用前景。基于共聚动力学模型控制单体加料速率的半连续活性/可控自由基聚合可实现梯度共聚物的结构定制,基于多步单体进料方式的RAFT乳液聚合则由于其简单和高效将成为梯度共聚物可控制备的重要方法。梯度共聚物的自组装行为和微观聚集态不同于嵌段共聚物,表现出独特的界面活性、热学特性和力学性能,组成梯度结构有望成为调控高分子材料性能的新参数,梯度共聚物有望在乳化剂、相相容剂、阻尼材料、多形状记忆材料等领域得到应用。     

Growth and physical properties of individual single-walled carbon nanotubes

We have developed a versatile catalyst-assisted chemical vapor deposition (CVD) technique for the synthesis of single-walled carbon nanotubes (SWNTs) from discrete nickel nanoparticles (average diameter of 4.7 ± 1.4 nm). Atomic force microscopy (AFM), transmission electron microscopy (TEM) and micro-Raman spectroscopy are used to characterize these as-grown nanotubes. Using a conventional set-up, we are able to produce isolated SWNTs with a narrow diameter distribution (1.5 ± 0.5 nm). The advantages of such a versatile CVD method for the study of physical properties at the single nanotube level are illustrated by means of two prospective studies on vibrational and electrostatic properties of SWNTs.     

六方相三氧化钨纳米线的制备及其光催化性能研究

以Na2WO4.2H2O和HCl为原料,K2C2O4和K2SO4分别作为结构导向剂,通过水热法在150℃下反应12 h,制备出六方相WO3纳米线。对比了结构导向对产物结构和形貌的影响,研究了WO3在紫外光照射下降解亚甲基蓝溶液的光催化活性。结果表明,以K2C2O4为结构导向时制备的WO3纳米线具有更高的比表面积和较好的光催化活性。     

A computational and experimental investigation of the mechanical properties of single ZnTe nanowires

One-dimensional nanostructures such as ZnTe, CdTe, Bi(2)Te(3) and others have attracted much attention in recent years for their potential in thermoelectric devices among other applications. A better understanding of their mechanical properties is important for the design of devices. A combined experimental and computational approach has been used here to investigate the size effects on the Young's modulus of ZnTe nanowires (NWs). The mechanical properties of individual ZnTe nanowires in a wide diameter range (50-230 nm) were experimentally measured inside a high resolution transmission electron microscope using an atomic force microscope probe with the ability to record in situ continuous force-displacement curves. The in situ observations showed that ZnTe NWs are flexible nanostructures with the ability to withstand relatively high buckling forces without becoming fractured. The Young's modulus is found to be independent of nanowire diameter in the investigated range, in contrast to reported results for ZnO NWs and carbon nanotubes where the modulus increases with a decrease in diameter. Molecular dynamics simulations performed for nanowires with diameters less than 20 nm show limited size dependence for diameters smaller than 5 nm. The surface atoms present lower Young's modulus according to the simulations and the limited size dependency of the cylindrical ZnTe NWs is attributed to the short range covalent interactions.     

Copper nanowire/PA6 composites prepared by in situ polymerization and its properties

Most magnetic composite materials have a serious sedimentation problem because of their large density. When the nanomaterials are combined with magnetic materials, the density would become larger. In this article, polystyrene (PS) beads were used as core to synthesize raspberry-like PS-Fe₃O₄@TiO₂ particles (PFTPs) by a facile method, which could float on the water. The loaded Fe₃O₄@TiO₂ particles were characterized by X-ray diffraction (XRD), which could confirm the anatase phase TiO₂ coated on Fe₃O₄ nano-particles. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) confirmed the presence of acrylic acid in reaction media could effectively tune the morphology of resulting composite particles between “core-shell” and “raspberry-like”. The data of Vibrating sample magnetometer (VSM) indicated the change in saturation magnetization before and after TiO₂ coated, the introduction of PS could further influence the saturation magnetization of Fe₃O₄. Lastly, we proved the sedimentation stability of PFTPs via capturing the photographs of them in water and oil.     

Catalytic synthesis of SiC nanowires in an open system

SiC nanowires (NWs) are usually synthesized in a closed vacuum reaction system which limits the yield of SiC NWs. In this work, SiC NWs and carbon nanotubes were synthesized in an open tube furnace at 1550°C with Si powder as silicon sources, ethanol as carbon sources and ferrocene as catalyst. The as-synthesized products were ultralong β-SiC NWs with the diameter about 80-100 nm and the length up to several tens micrometers. The diameter of the carbon nanotubes was about 20-30 nm. The carbon nanotube yarns about 20 cm in length were obtained at the end of the tube furnace. The growth mechanism of SiC NWs and carbon nanotubes were proposed. Compared with the traditional synthetic techniques in the high vacuum closed system, the novel synthesis method in the open system provided a new approach to the synthesis of SiC NWs.     

Controlled synthesis of a large fraction of metallic single-walled carbon nanotube and semiconducting carbon nanowire networks

Controlled synthesis of both single-walled carbon nanotube and carbon nanowire networks using the same CVD reactor and Fe/Al(2)O(3) catalyst by slightly altering the hydrogenation and temperature conditions is demonstrated. Structural, bonding and electrical characterization using SEM, TEM, Raman spectroscopy, and temperature-dependent resistivity measurements suggest that the nanotubes are of a high quality and a large fraction (well above the common 33% and possibly up to 75%) of them are metallic. On the other hand, the carbon nanowires are amorphous and semiconducting and feature a controlled sp(2)/sp(3) ratio. The growth mechanism which is based on the catalyst nanoisland analysis by AFM and takes into account the hydrogenation and temperature control effects explains the observed switch-over of the nanostructure growth modes. These results are important to achieve the ultimate control of chirality, structure, and conductivity of one-dimensional all-carbon networks.     

Controlled synthesis of uniform silver nanowires with high aspect ratios in aqueous solutions of gemini surfactant

A simple solution-phase approach has been demonstrated for the large-scale synthesis of silver nanowires with diameters in the range of 15–25 nm, and lengths usually in the range of tens of micrometers. In the presence of gemini surfactant 1,3-bis(cetyldimethylammonium) propane dibromide (16-3-16), the growth of silver could be directed into a highly anisotropic mode to form uniform nanowires with aspect ratios up to about 2,000. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), X-ray powder diffraction (XRD), electron diffraction (ED), and UV-vis absorption spectroscopy, were used to characterize the as-prepared silver nanowires, indicating the formation of a highly pure phase, good crystallinity, as well as a uniform diameter.     

Study of the synthesis and physical properties of fire-resistant polyurethane ionomers

Fire-resistant polyurethane ionomers successfully synthesized at our laboratory have been proved by infrared spectra. In aqueous solution, the surface tension for fire-resistant polyurethane ionomers made by toluene diisocyanate or isophorone diisocyanate was found to decrease substantially with increasing concentration of 1,4-bis(2-hydroxyethyl)piperazine but to increase gradually with increasing phenylphosphonic acid or 1,4-bis(2-hydroxyethyl)piperazine with the phenylphosphonic acid concentration used to prepare these ionomers. For 1,4-bis(2-hydroxyethyl)piperazine-based polyurethane ionomers made by toluene diisocyanate or isophorone diisocyanate in aqueous solution, the number-average particle sizes of these ionomers decrease drastically with increasing 1,4-bis(2-hydroxyethyl)piperazine concentration, as a result of intramolecular interaction. On the other hand, for both phenylphosphonic acid and 1,4-bis(2-hydroxyethyl)piperazine with phenylphosphonic acid-based polyurethane ionomers, their average particle size increased with increasing phenylphosphonic acid or 1,4-bis(2-hydroxyethyl)piperazine with phenylphosphonic acid concentration. This may be attributed to the results of intermolecular interaction. Obviously, the limiting oxygen index values are seen to be higher for the isophorone diisocyanate type than for the toluene diisocyanate type of fire-resistant polyurethane ionomer. In fact, our experimental results suggest that the isophorone diisocyanate types of fire-resistant polyurethane ionomers provide good fire-resistance. For self-cured films of fire-resistant polyurethane ionomers, the tensile strength at breaking point increases with increasing the concentration of fire retardants, but the elongation at breaking point for these ionomers, on the other hand, appears to decrease with increasing concentration of fire retardants. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 19–26, 1998     

Effect of the compatibilizer on the physical properties of biaxially deformed in situ composites

The effect of a compatibilizer (poly(ester imide), PE^sI) on the biaxial deformation of a thermotropic liquid crystalline polymer (TLCP, poly(ester amide)) in a poly(ether imide) and the properties of biaxially deformed in situ composites were studied. The compatibilizer improved dispersion of the TLCP and the adhesion between TLCP and the matrix. The properties of blown films were affected by the amount of the compatibilizer used. The morphology evidently shows that ca. 0.6 wt% PEsI provides the best morphology when 10 wt% VB phase is included. The mechanical properties, especially in the hoop direction, were significantly improved for the compatibilized films compared to uncompatibilized one. The impact strength of a compatibilized blend film with 0.6 wt% PEsI was almost twice that of an uncompatibilized blend film.     

不同堆积密度的氟锆酸钾结构性能研究

以氧氯化锆、氯化钾及氢氟酸为原料生产六氟锆酸钾(K2Zr F6)的过程中会得到不同堆积密度的氟锆酸钾,其使用效果不同。测定三种不同堆积密度氟锆酸钾的Zr含量、熔点,通过X射线多晶粉末衍射分析(XRD)、扫描电镜(SEM)对其结构形貌进行研究。结果表明,随堆积密度的增大,氟锆酸钾的Zr含量相对降低,熔点升高;不同堆积密度氟锆酸钾的晶型结构、微观形貌存在差异。     

不同堆积密度的氟锆酸钾结构性能研究

以氧氯化锆、氯化钾及氢氟酸为原料生产六氟锆酸钾(K2Zr F6)的过程中会得到不同堆积密度的氟锆酸钾,其使用效果不同。测定三种不同堆积密度氟锆酸钾的Zr含量、熔点,通过X射线多晶粉末衍射分析(XRD)、扫描电镜(SEM)对其结构形貌进行研究。结果表明,随堆积密度的增大,氟锆酸钾的Zr含量相对降低,熔点升高;不同堆积密度氟锆酸钾的晶型结构、微观形貌存在差异。     

Preparation, characterization, physical properties, and photoconducting behaviour of anthracene derivative nanowires

Organic nanowires of 9,10-dibromoanthracene (DBA) and 9,10-dicyanoanthracene (DCNA) were obtained by adding the THF solution of DBA/DCNA into water containing P123 surfactants. The as-prepared nanowires were characterized by UV-vis, fluorescence spectra, Field Emission Scanning Electron Microscopy (FESEM), and Transmission Electron Microscopy (TEM). We found that DBA and DCNA nanowires emitted green light rather than blue light for molecules in THF solution. The red-shift UV and fluorescent spectra of DBA and DCNA nanowires implied that these nanowires were formed through J-aggregation. The photoconducting study of DBA/DCNA nanowire-based network on rGO/SiO(2)/Si shows different photocurrent behaviors upon irradiation, which displayed that electron transfer from DCNA nanowire to rGO was stronger than that of DBA nanowires to rGO.     

In situ synthesis and hydrogen storage properties of PdNi alloy nanoparticles in an ordered mesoporous carbon template

Organized mesoporous carbon has been used as a nanoreactor to prepare PdNi metallic particles using an incipient wetness method starting from Pd and Ni salts. The final composite material consists of nanosized metallic particles of an alloy with composition Pd_0.60Ni_0.40 highly dispersed within the carbon host structure. The thermodynamic hydrogenation properties of both the PdNi-free OMC and the Pd_0.60Ni_0.40-OMC composite have been determined by hydrogen isotherm sorption measurements. The introduction of the palladium–nickel alloy into the carbon matrix does not increase the hydrogen storage capacity at 77 K and 2 MPa, since the hydrogen uptake is mainly attributed to physisorption on the carbon surface. However, at room temperature and moderate pressure (0.5 MPa), the filling of the OMC with nanocrystalline Pd_0.60Ni_0.40 results in larger hydrogen uptake than that of the PdNi-free OMC.