Effect of ammoniating temperature on microstructure and optical properties of one-dimensional GaN nanowires doped with magnesium

One-dimensional GaN nanowires doped with Mg element have been successfully prepared on Si (1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/Mg thin films, and the effect of the ammoniating temperatures on the microstructure and optical properties of the GaN nanowires was investigated in detail. X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), FT-IR spectrophotometer, Scanning electron microscope (SEM), high-resolution transmission electron microscope (TEM), and photoluminescence (PL) spectrum were carried out to characterize the microstructure, morphology, and optical properties of GaN nanowires. The results demonstrate that ammoniating temperature has a significant effect on microstructure, morphology and optical properties of GaN nanowires. GaN nanowires after ammoniation at 900 °C for 15 min are straight, smooth and of uniform thickness along spindle direction with the highest crystalline quality. The growth direction of these nanowires is parallel to [1 0 0] orientation.     

氨化Ga2O3 /Nb膜制备的GaN纳米线的光学和微观结构特性的研究

采用射频磁控溅射技术在硅衬底上制备Ga2O3/Nb薄膜,然后在900℃下于流动的氨气中进行氨化制备GaN纳米线.用X射线衍射(XRD)、透射电子显微镜(TEM)和高分辨透射电子显微镜详细分析了GaN纳米线的结构和形貌.结果表明:采用此方法得到的GaN纳米线有直的形态和光滑的表面,其纳米线的直径大约50nm,纳米线的长约几个微米.室温下以325nm波长的光激发样品表面,只显示出一个位于367 nm的很强的紫外发光峰.最后,简单讨论了GaN纳米线的生长机制.     

钽催化磁控溅射法制备GaN纳米线

利用磁控溅射技术通过氮化Ga2O3/Ta薄膜,合成大量的一维单晶纤锌矿型氮化镓纳米线.用X射线衍射、扫描电子显微镜、高分辨透射电子显微镜,选区电子衍射和光致发光谱对制备的氮化镓进行了表征.结果表明;制备的GaN纳米线是六方纤锌矿结构,其直径大约20～60 nm,其最大长度可达10 μm左右.室温下光致发光谱测试发现363 nm处的较强紫外发光峰.另外,简单讨论了氮化镓纳米线的生长机制.     

氨化Si基Ga2O3/V膜制备GaN纳米线

氨化硅基钒应变层氧化镓膜制备了大量氮化镓纳米线,X射线衍射、扫描电子显微镜和透射电子显微镜观察发现,纳米线具有十分光滑且干净的表面,其直径为20～60 nm左右,长度达到十几微米.高分辨透射电子显微镜和选区电子衍射分析结果表明,制备的氮化稼纳米线为六方纤锌矿结构.光致发光谱显示制备的氮化稼纳米线有良好的发光特性.另外,简单讨论了氮化稼纳米线的生长机制.     

Simple hydrothermal synthesis and sintering of Na0.5Bi0.5TiO3 nanowires

Single-crystalline Na_0.5Bi_0.5TiO₃ (NBT) nanowires, with diameters of 100 nm and lengths of about 4 μm, were synthesized by using a simple hydrothermal method. Phase composition, morphology and microstructure of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The effects of reaction temperature and reaction time on precipitation of the NBT nanowires were investigated. It was found that reaction time significantly influenced the growth behavior of the powders in the hydrothermal system. Based on the experimental results, the one-dimensional (1D) growth mechanism of the NBT was governed by a dissolution-recrystallization mechanism. NBT ceramics derived from the nanowires showed typical characteristics of relaxor ferroelectrics, with diffuseness exponent γ of as high as 1.73.     

Doped and de-doped polypyrrole nanowires by using a BMIMPF6 ionic liquid

We electrochemically synthesized π-conjugated polypyrrole (PPy) nanowires by using an environmentally stable and recyclable ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆), as a dopant, based on an anodic alumina oxide (Al₂O₃) nanoporous template. The de-doped states of the PPy nanowires were obtained from doped PPy nanowires through cyclic voltammetry (CV) using a solution of the catalyst in the BMIMPF₆ ionic liquid. We confirmed the de-doped states of PPy nanowires based on a decrease of the bipolaron peak of the ultraviolet and visible (UV–vis) absorbance spectra. The formation of PPy nanowires was visualized by using scanning electron microscopy and transmission electron microscopy. We studied the optical and structural properties of the doped and de-doped PPy nanowires by UV–vis absorbance and Fourier transform-infrared spectra. Our work included obtaining a laser confocal microscope Raman spectra for a single strand of the PPy nanowires.     

CVD growth of InGaN nanowires

In this paper, the chemical vapor deposition (CVD) growth of InGaN nanowires was systematically studied. The catalyst was Au and the starting materials were Ga, In and NH₃. The samples were characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron spectroscopy (TEM), and X-ray diffraction (XRD), etc. The influence of the growth temperatures, Au thicknesses, gas flowrates and Ga and In amount on the morphology and properties of InGaN nanowires was investigated. It is found that 600 °C is a suitable growth temperature. On the substrate with Au thickness of 150 Å, helical InGaN nanowires are obtained. The change of NH₃ partial pressure and Au thickness will result in the morphology change of the samples. An increase of Ga results in shorter InGaN nanowires while an increase of In amount will lead to longer InGaN nanowires. The morphology will also change when both the amount of In and Ga were increased or reduced without changing the ratio of Ga to In.     

TiO2/SiOx core-shell nanowires generated by heating the multilayered substrates

By heating Au/TiN/Si substrates, we fabricated TiO₂/SiO_x core-shell nanowires. By changing the thickness of predeposited Au layers, we demonstrated that the thickness of the Au layer needs to be optimized to obtain nanowires. High-resolution transmission electron microscopy image, X-ray diffraction spectrum, and selected area electron diffraction pattern coincidentally revealed that the resultant core nanowires had a tetragonal rutile structure of TiO₂, and the shell was comprised of amorphous SiO_x. The dominant growth mechanism was a base-growth mode, in which Au played a catalytic role, resulting in morphological changes with variation of the Au layer thickness. The TiO₂/SiO_x core-shell nanowires exhibited a broad photoluminescence emission band, which comprised four peaks centered at 1.54, 2.34, 2.67, and 2.99 eV, respectively. We expected that the 1.54 eV- and 2.34 eV-centered peaks arised from the TiO₂ core, whereas the 2.67 eV- and 2.34 eV-peaks were ascribed to both the TiO₂ core and the SiO_x shell.     

Synthesis of N-deficient GaN and its enhanced dielectric responses

The GaN powders were synthesized by the reaction of Ga₂O₃ and Li₃N and scanning transmission electron microscopy (STEM) analysis showed the as-prepared GaN were N-deficient with the N vacancies reaching as much as 21%. Besides single-phase of the hexagonal GaN, no other phase from impurities can be detected under the high-resolution transmission electron microscopy (HRTEM) observations. The room temperature (RT) frequency spectrums of the relative dielectric constants _r were measured and the N-deficient GaN exhibited at least twofold enhancement than that of GaN nanostructure materials, especially at low frequency range. Because of the great number of N vacancies (V_N), the rotation direction polarization (RDP) contributes mostly for the enhancement of _r in N-deficient GaN.     

Synthesis of ultralong Si3N4 nanowires by a simple thermal evaporation method

Large-scale vapor-solid synthesis of ultralong silicon nitride (Si3N4) nanowires was achieved by using simple thermal evaporation of mixture powders of active carbon and monoxide silicon. The products were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The results suggest that the silicon nitride nanowires have a smooth surface, with lengths of up to several hundreds of microns and diameters of 100-300 nm. A detailed study of both the chemical and structural composition was performed. Such ultralong silicon nitride nanowires demonstrate potential applications as materials for constructing nanoscale devices and as reinforcement in advanced composites.     

Annealing-induced enhancement of ferromagnetism in SnO2-core/Cu-shell coaxial nanowires

In this study, we have coated tin oxide (SnO₂) nanowires with a Cu shell layer via the sputtering method and subsequently investigated the effects of thermal annealing. The annealing-induced changes in morphologies, microstructures, and compositions of the resulting core-shell nanowires were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and energydispersive X-ray spectroscopy (EDX). The Cu shell layers were agglomerated to form clusters, which were mainly comprised of the Cu₂O phase. For the first time, a hysteresis loop indicating weak ferromagnetism was observed from the pure SnO₂ nanowires. Both the coercivity and the retentivity in the hysteresis loop were slightly increased by Cu-sputtering, indicating a very slight enhancement of ferromagnetism. Also, the ferromagnetic behavior was significantly enhanced by thermal annealing. We discuss the possible mechanisms of annealing-induced enhancement of ferromagnetism in the SiO₂/Cu core-shell nanowires, which include the generation of Cu₂O phase, Cu-doping into the SnO₂ lattice, and the generation of oxygen vacancies in SnO₂ core nanowires.     

Preparation and characterization of electro-spun RuO2-Ag2O composite nanowires for electrochemical capacitors

We synthesized RuO₂-Ag₂O composite nanowires by means of an electrospinning method and investigated the capacitance, high-rate performance, and cycle number dependence of the composite nanowire electrodes. In order to synthesize optimum RuO₂-Ag₂O composite nanowires, the relative mole ratio of Ag precursor to Ru precursor varied from ∼0.1 to ∼0.3. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy results show that crystalline RuO₂-Ag₂O composite nanowires (∼40-70 nm in diameter) are formed upon calcinations. Cyclic voltammetry results show that among the samples, the RuO₂-Ag₂O composite nanowires fabricated with the mole ratio of ∼0.2 give the highest capacitance, excellent high-rate performance, and excellent retention of capacity (∼97%).     

Decoration of In2O3 nanowires with BaTiO3 nanoparticles for enhancement of magnetic properties

We have fabricated the composite nanowires consisting of In₂O₃ core and BaTiO₃ shell, via a two-step method. The BaTiO₃ shell has been deposited on core In₂O₃ core nanowires by means of pulsed laser deposition technique. X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy were employed to analyze the structure and morphology of the products. While SEM confirmed that the products maintained the one dimensional morphology, the shell layer corresponded to the nanoparticles of the hexagonal BaTiO₃ phase. With bare In₂O₃ nanowires exhibiting weak ferromagnetism, the ferromagnetic behavior was enhanced by decorating the In₂O₃ nanowires with BaTiO₃ nanoparticles. The shell coating enhanced the ferromagnetic behavior at both 5 and 300 K, in terms of the increase of coercive field, remanent magnetization, and saturation magnetization values in the hysteresis curves. We suggested that the generation of BaTiO₃ nanoparticles and oxygen vacancies during the high-temperature shell coating process played a role in enhancing the ferromagnetic properties of the nanowires.     

均匀自支撑的二氧化钒纳米线的水热合成及相变特性研究

二氧化钒(VO₂)在接近室温时发生由半导体态向金属态的Mott相变,在智能窗和红外自适应伪装技术领域具有一定的应用前景。本文采用一种新颖的水热法制备均匀自支撑的VO₂纳米线。合成的纳米线的直径为150±30 nm,长度达到几十微米。通过X射线衍射、X射线光电子能谱、高分辨透射电镜和选取电子衍射等手段验证了高纯单斜相VO₂纳米线的成功制备。而且,VO₂纳米线的可逆相变性能采用差示量热扫描、变温XRD和变温Raman光谱进行了探究。结果表明：VO₂纳米线升温相变点为65.2 °C,磁滞回线宽度窄至6.5 °C,具有良好的可逆相变性。这些为VO₂纳米线的金属-半导体相变研究提供基础。     

Critical behavior and the absence of glass state in ferromagnetic La0.7Ca0.3CoO3 nanowires

We report a systematic study of the magnetic and thermodynamic properties of uniform La_0.7Ca_0.3CoO₃ nanowires prepared by a hydrothermal method, and compare them with the bulk counterpart. The nanowires, ∼59 nm in diameter, are single crystalline as revealed by electron microscopy. With decreasing temperature, the bulk La_0.7Ca_0.3CoO₃ shows a ferromagnetism followed by a reentrant glass state, whereas the nanowires show only a purely ferromagnetic ground state. The thermopower and specific heat results indicate that there exist enhanced spin and/or orbital fluctuations in the nanowires that could be responsible for the absence of the glass state. Critical behavior analysis shows that the nanowires fall into a three-dimensional Heisenberg ferromagnet class.     

Structural and compositional homogeneity of InAlN epitaxial layers nearly lattice-matched to GaN

A group of InAlN films was fabricated by molecular beam epitaxy and investigated by X-ray diffraction, transmission electron microscopy and element nano-analyses. All top In_xAl_1?xN layers have compositions around lateral lattice-matching to GaN (x ≈ 0.18) and are pseudomorphic. For a growth rate of 350 nm h^?1, each InAlN film separated into two sublayers with different In/Al-ratios. Micrographs reveal sharp transitions both at the InAlN/GaN and at the InAlN/InAlN interfaces. In contrast to these separated layers, an optimized epitaxy using an AlN interlayer and a lower growth rate, 100 nm h^?1, enabled the fabrication of a single-phase In_xAl_1?xN layer on GaN, homogeneous on a nanoscopic scale.     

Synthesis and growth mechanism of SiC/SiO2 nanochains by catalyst-free thermal evaporation method in Ar/CO atmosphere

SiC/SiO₂ nanochains were synthesized on a carbon fiber substrate by a catalyst-free thermal evaporation method in the Ar/CO atmosphere. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the as-synthesized SiC/SiO₂ nanochains are composed of single-crystalline SiC nanowires and amorphous SiO₂ beads. The introduction of CO can promote the formation of SiO₂, so that the SiC/SiO₂ nanochains are subsequently formed during cooling. In addition, the photoluminescence spectrum of SiC/SiO₂ nanochains showed a broad emission peak at around 350 nm, which is ascribed to the oxygen discrepancy in the SiO₂ beads as well as the SiC/SiO₂ interfacial effect. These findings can provide guidance for further study of the vapor growth of 1D SiC-based materials.     

Formation of Ag2S nanowires and Ag2S/CdS heterostructures via simple solvothermal route

In our present work, Ag₂S nanowires and Ag₂S/CdS heterostructures have been successfully prepared in anhydrous ethanol through a simple solvothermal route. From the transmission electron microscopy (TEM) analysis, it is found that the factors influenced the final product are the concentration of Ag⁺, reaction temperature, reaction time, and solvent. Ag₂S nanowires are formed by complete Ag⁺ cation exchange. Because of the selectivity for partial cation exchange, the reaction starts preferentially at the ends of the CdS nanowires to produce novel Ag₂S/CdS heterostructures. In addition, as the two end facets of wurtzite CdS nanowires are crystallographically nonequivalent, the produced Ag₂S/CdS heterostructures are asymmetric.     

High-temperature oxidation of Permalloy in air

Hematite (α-Fe₂O₃) nanowires were observed through directly annealing Ni₈₁Fe₁₉ foils at 600 °C for 120 min in atmosphere. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to characterise the nanowires. The results indicate that the growing mechanism includes iron segregation to the surface combined with an internal stress induced by the oxidation process.