Porous silicon templates for electrodeposition of nanostructures

We report the fabrication and characterization of porous silicon templates for electrodeposition of high aspect ratio one-dimensional metallic nanostructures (nanowires/nanoparticles) in them. Even though nanostructures/nanowires in the past have been fabricated in alumina, polymer or silica templates, the advantages of this approach are the possibility for seamless integration of nanostructures with other silicon components, and silicon based sensors because of better physical and electrical interconnection between the nanostructure and the silicon substrate. In this work, fabrication and characterization of nanowires/nanostructures such as single-segment Ni–Fe and Au and two-segment Ni–Fe/Au electrodeposited in the porous silicon template are presented. The templates with ordered and controlled nanometer-sized pores, 40 nm and 290 nm in diameter, were created through porous Si etching. The morphology, composition and structural characteristics of the template and of the single-segment Ni–Fe and Au and two-segment Ni–Fe/Au nanostructures of diameter 275±25 nm, length up to 100 μm and pitch of 1 μm were analyzed using scanning electron microscopy and X-ray diffraction techniques. The micrographs confirm that the plating parameters have a strong influence on morphology and composition of the structures. Further, the Ni–Fe images show the formation of both vertical and branched nanowires along with nanoparticles, from breakage/discontinuous growth of nanowires. Ni–Fe nanostructures were further analyzed for temperature-dependent magnetization and magnetization vs. magnetic field measurements using a commercial physical property measurement system. They reveal no magnetic anisotropy of the nanostructures probably due to a balance between ‘reduced’ shape anisotropy from branched and rough pore surfaces and magnetocrystalline anisotropy. PACS 61.46.+w; 75.75.+a; 81.07.-b; 81.16.Be     

Synthesis, morphologies and Raman-scattering spectra of crystalline stannic oxide nanowires

SnO₂ nanowires were synthesized using a direct gas reaction route and were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and Raman-scattering spectroscopy. XRD, SEM, SAED and HRTEM indicated that the products were tetragonal SnO₂ nanowires with diameters of 10–50 nm. The nanowires were single crystal and solid inside. Dendritic nanowires were observed for the first time. Three vibrational modes were observed in the Raman spectra of the samples. Received: 7 January 2002 / Accepted: 11 April 2002 / Published online: 19 July 2002     

Functional nanowires of tin oxide

Quasi-one-dimensional nanostructures of tin oxide were produced in controlled conditions through condensation from the vapor phase. The preparation was assisted by noble metal catalysts and uniform single-crystalline nanowires were produced. The nucleation of nanowires was achieved at 470 °C, owing to the vapor–liquid–solid growth mechanism activated by the catalytic Pt clusters. The peculiar microstructural properties of these semiconducting metal oxide nanostructures will be summarized. The high aspect ratio and the high degree of crystallinity achieved for the nanowires foresee their functional exploitation. PACS 61.46.Hk; 81.07.Bc; 81.16.Hc; 68.37.Lp     

Thermal evaporation synthesis of zinc oxide nanowires

High quality ZnO nanowires were synthesized at high temperature without using heterogenous catalysts. The nanowires had a uniform prismatic shape and were grown in a cacti-like morphology. Characterizations of the products by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy showed that the ZnO nanowires were single crystalline and of high purity. An intensive exciton emission was observed around 3.25 eV from the ZnO nanowires at room temperature. The growth mechanism was discussed based on the experimental conditions and the ZnO crystal growth habits. This growth method can be used to prepare other metal oxide nanowires. PACS 61.46.+w; 81.16.-c; 81.07.De, 81.05.Hd     

Indium tin oxide nanowires growth by dc sputtering

Indium tin oxide nanowires have been grown by dc sputtering on different substrates without the use of catalysts or oblique deposition. The nanowire length was of the order of several μm, while their diameter was ∼50–100 nm. Small side branches on the nanowires were frequently observed. The nanowires were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The growth mechanism of the nanowires is discussed.     

Electrochemical deposition and characterization of Cu2O nanowires

Cu₂O nanowires were successfully synthesized by an electrochemical method using an alumina membrane as template through precise control of the pH value of the electrolyte. The deposition process was monitored by the time–current curve. Characterization was performed by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The growth directions of the Cu₂O nanowires were determined and the possible growth mechanism is discussed. PACS 68.37.Lp; 81.07.-b; 81.16.Be     

Catalyst-free growth of single-crystalline alumina nanowire arrays

Alumina nanowires were synthesized on a large-area silicon substrate via simple thermal evaporation of a mixture of aluminum and alumina powders. The microstructure of the as-grown alumina nanowires was characterized using X-ray diffraction and transmission electron microscopy techniques. These nanowires usually have a straight morphology and are single crystalline with the wire axis parallel to the (001) direction. Arrays of the alumina nanowires were also observed grown on the surface of many large particles. The catalyst-free growth of the alumina nanowires was explained in the framework of vapor–solid growth. PACS 81.07.-b; 81.16.-c; 81.07.Bc     

Synthesis and characterization of silicon-doped gallium oxide nanowires for optoelectronic UV applications

Silicon-doped gallium oxide nanowires have been synthesized by thermal methods using either a mixture of gallium oxide and silicon powders or metallic gallium with silicon powder as precursor materials. The growth mechanism has been found to be a vapour–liquid–solid (VLS) or vapour–solid (VS) process, respectively, depending on the precursor used. In the former case, silicon oxide droplets at the end of the nanowires have been observed. Their possible role during the growth of the nanostructures is discussed. Structural and morphological characterization of the doped nanowires has been performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show a high crystalline quality and a uniform distribution of silicon along the nanowires. Room temperature cathodoluminescence (CL) in the SEM shows that slight variations in the composed UV–blue emission band appear due to the influence of Si impurities in the oxygen vacancy defect structure.     

Polyol-thermal synthesis of silver nanowires for Hg<Superscript>2+</Superscript> sensing detection

This study demonstrates a facile but effective polyol-thermal reaction method for the synthesis of silver nanowires in autoclaves (160–180 °C). By this approach, the generated silver nanowires show an average diameter of ~40 nm and length up to tens of micrometers with a high yield and potential for large-scale production. To achieve shape- and size-controlled Ag nanowires, several experimental parameters were investigated and optimized, including surface controller(s), molar ratio of surfactant(s) to silver ions, temperature, and concentration of reactants. The structure and composition of silver nanowires were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. In particular, the twinned crystal structure observed in both spherical particles and nanowires was analyzed by HRTEM technique, and the possible formation and growth mechanisms were discussed. The optical property of the as-prepared product was measured by ultraviolet–visible (UV–vis) spectroscopy. The sensing detection of metal ions (e.g., Hg²⁺) using the obtained silver nanowires in aqueous media was finally investigated.     

Large-scale synthesis of In2O3 nanowires

In₂O₃ nanowires have been successfully fabricated on a large scale from indium particles by thermal evaporation at 1030 °C. The as-synthesized products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show that these nanowires are uniform with diameters of about 60–120 nm and lengths of about 15–25 μm. XRD and selected-area electron diffraction analysis together indicate that these In₂O₃ nanowires crystallize in a cubic structure of the bixbyite Mn₂O₃ (I) type (also called the C-type rare-earth oxide structure). The growth mechanism of these nanowires is also discussed. Received: 29 June 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

Impact of growth temperature on the crystal habits,forms and structures of VO<Subscript>2</Subscript> nanocrystals

We investigated the impact of the process temperature on the habits, forms and crystal structure of VO₂ nanocrystals grown by a vapor-transport method on (0001) quartz substrates. Four distinct growth regimes were discerned: orthorhombic nanowires, sheets, hemispheres, and nanowires with a monoclinic structure. The nanostructures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). I/V characterization of individual nanowires was enabled by Ti/Au contact formation via electron beam lithography and lift-off techniques. The expected metal–insulator transition (MIT) was found in monoclinic VO₂ nanowires.     

Applications of electron microscopy to the characterization of semiconductor nanowires

We review our current progress on semiconductor nanowires of β-Ga₂O₃, Si and GaN. These nanowires were grown using both vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. Using transmission electron microscopy (TEM) we studied their morphological, compositional and structural characteristics. Here we survey the general morphologies, growth directions and a variety of defect structures found in our samples. We also outline a method to determine the nanowire growth direction using TEM, and present an overview of device fabrication and assembly methods developed using these nanowires. PACS 61.14.-x; 81.07.-b; 61.14.Lj; 81.05.-t     

Synthesis and characterization of cadmium hydroxide nanowires by arc discharge method in de-ionized water

In this study, Cd(OH)₂ nanowires have been synthesized by using arc discharge method in de-ionized water. The morphology and properties of the Cd(OH)₂ nanowires were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and UV–Vis spectroscopy. TEM observations revealed that Cd(OH)₂ nanowires were abundant morphology in synthesized nanostructures, and the diameter of the Cd(OH)₂ nanowires ranges from 5 to 40 nm with several micrometers of length. In addition, the width of nanowires is not uniform and varies throughout the nanowire. XRD analysis revealed that the Cd(OH)₂ nanowires grow along [001] direction. Furthermore, hexagonal- and irregular-shaped Cd(OH)₂ nanoplates were synthesized during arc discharge. It was obtained that required arc current is 50 A for the effective and large scale production of Cd(OH)₂ nanowires. Furthermore, the optical properties of the nanowires have been characterized by UV–Vis spectra. By the means of the optical studies, the direct band gap of Cd(OH)₂ nanowires was found to be 4.0 eV with strong quantum size effect. It is also shown that a simple and cheap method which does not require relatively expensive vacuum and laser equipment stipulates an economical alternative for the synthesis of Cd(OH)₂ nanowires.     

Large scale synthesis of silicon nanowires

Artificial nanostructures (Samuelson et al., Physica E 21:560–567, 2004; Xia et al., Adv Mater 15:353–389, 2003) show promise for the organization of functional materials (Huck and Samuelson, Nanotechnology 14:NIL_5–NIL_8, 2003) to create nanoelectronic (Mizuta and Oda, Science 279:208–211, 2008) or nano-optical devices (Mazur et al.; Tanemura et al., Synthesis, Optical Properties and Functional Applications of ZnO Nano-materials: A Review, 1–3:58–63, 2008). However, in most manufacturing recipes described so far, nanostructures are synthesized in solution and/or uncontrolled deposition results in random arrangements; this makes it difficult to measure the properties of attached nanodevices or to integrate them with conventionally fabricated microcircuitry. Here, we describe a fully CMOS compatible process technology for mass manufacture of polysilicon nanowires by the CVD (chemical vapor deposition) method. The large scale production of nanowires could successfully be synthesized on silicon (100) substrates. However, the method presented here can successfully be employed with all technologically useful substrates with good adhesion for silicon such as SiO₂, diamond-like carbon or III–V semiconductors. This opens up the possibility for the fabrication of strain-sensitive and defect-sensitive optoelectronic devices on the optimum III–V substrate (Fonstad et al.). Finally, scanning electron microscopy (SEM) was used to characterize the as-synthesized nanowires and energy-filtered transmission electron microscopy (EFTEM) and scanning transmission electron microscopy (STEM) analysis were used to determine the nanowire composition.     

Nanostructure size evolution during Au-catalysed growth by carbo-thermal evaporation of well-aligned ZnO nanowires on (100)Si

We report the structural and morphological properties of well-aligned ZnO nanowires grown at 750 °C on Au-deposited and annealed (100)Si substrates using carbo-thermal evaporation. As-grown nanowires are made of wurtzite ZnO, have cylindrical shape and carry droplet-like nanoparticles (NPs) at their tips, as expected for vapour–liquid–solid (VLS) growth. Grazing incidence X-ray diffraction measurements demonstrate that the NPs are made of pure fcc Au. No secondary Au/Zn alloy phases were detected. Bragg diffraction patterns confirmed that the nanowires were grown with their crystal c-axes parallel to the [100] direction of Si (i.e. normal to the substrate surface), while Au NPs are mostly (111)-oriented. The diameter distribution of ZnO nanowires mimics that of the Au NPs at their tips. A quantitative study of the nanostructure size distribution after sequential annealing and growth steps evidences the occurrence of three nanoscale processes: (i) Ostwald ripening and/or coalescence of Au NPs before nanowire nucleation, (ii) Au-catalysed VLS nucleation and axial growth of ZnO nanowires and (iii) radial growth of nanowires by a vapour–solid process. These processes originate the NP and nanowire size evolution during the experiments. The present findings are interpreted in terms of Zn vapour pressure changes during carbo-thermal evaporation. PACS 61.46.+w; 68.65.-k; 81.16.Dn     

低温条件下单晶氮化铝纳米线生长机理的研究

在25mL的不锈钢反应釜中，利用无水三氯化铝与叠氮化钠在无溶剂的条件下直接反应，成功地合成出了单晶氮化铝纳米线，反应温度为450℃，有效反应时间为24h.高分辨率透射电子显微镜测试结果显示，纳米线多为长直线状外貌特征，直径在40—60nm范围内，最大长度可达几个微米.高分辨率电子衍射和X射线衍射结果都表明，多数纳米线为六方结构，也有少量呈现面心立方结构.同时，提出了长直线状六方和面心立方单晶氮化铝纳米线的生长机理的假设，并对六方单晶氮化铝纳米线生长方向的人工控制也进行了讨论. 关键词： 六方单晶氮化铝 纳米线 X射线衍射 透射电子显微镜     

Fabrication of two types of one-dimensional Si–C nanostructures by laser ablation

Two types of one-dimensional (1D) nanostructures—amorphous silicon carbide (SiC) nanowires, 5–30 nm thick and 0.5–2 μm long, and carbon nanotubes (CNTs) filled completely with crystalline SiC nanowires, 10–60 nm thick and 2–20 μm long—were synthesized by the laser ablation of carbon-silicon targets in the presence of high-pressure Ar gas up to 0.9 MPa. All the CNTs checked by transmission electron microscopy contained SiC, and no unfilled CNTs were produced. We discuss the growth of the two nanostructures based on the formation of molten Si–C composite particles and their instabilities leading to the precipitation of Si and C.     

Sol–gel template synthesis and characterization of LaCoO3 nanowires

Densely packed LaCoO₃ nanowires of the rare-earth perovskite-type composite oxide were synthesized within a porous anodic aluminum oxide (AAO) template by means of the sol–gel method using nitrate as raw the material and citric acid as the chelating agent. The results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the LaCoO₃ nanowires possessed a uniform length and diameter, which were controlled by the thickness and the pore diameter of the applied AAO template, respectively. The results of X-ray diffraction (XRD) and the selected area electron diffraction (SAED) indicated that the LaCoO₃ nanowires had a rhombohedral perovskite-type crystal structure. Furthermore, X-ray photoelectron spectroscopy (XPS) demonstrated that LaCoO₃ nanowires were formed. Finally, the formation mechanism of nanowires was also discussed. PACS 61.66.Fn; 61.46.-W; 81.20.Fw     

Large-scale synthesis and photoluminescence properties of?SiC networks

By using a novel and low-cost microwave method, three-dimensional SiC networks have been synthesized in large-scale. The composition and structural features of the product were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The results show that the SiC networks consist of nanocable X-junction and Y-junction. Some nanocables are composed of 3C–SiC multicore encapsulated in single amorphous SiO₂ shell. The SiC networks emitted stable violet–blue light around 380 nm under 325-nm excitation. Compared to the emission peak of the SiC networks after etched and the reported results of 3C–SiC nanowires, the emission peak of the SiC networks shows significant blueshift. The origin of the photoluminescence for the SiC networks could be due to two possible reasons: the central crystalline SiC nanowires and amorphous SiO₂ shell. A two-step growth mechanism of the SiC networks was proposed based on the experimental characterizations. The successful synthesis of SiC networks is an important step in the development of SiC-based electronic devices and circuits.     

Growth and use of metal nanocrystal assemblies on high-density silicon nanowires formed by chemical vapor deposition

In this paper, we describe the growth and potential application of metal nanocrystal assemblies on metal-catalyzed, CVD-grown silicon nanowires (SiNWs). The nanowires are decorated by chemical assembly of closely spaced (1–5 nm) Ag (30–100 nm diameter) and Au (5–25 nm diameter) nanocrystals formed from solutions of AgNO₃ and NaAuCl₄·2H₂O, respectively. The formation and growth of metal nanocrystals is believed to involve the galvanic reduction of metal ions from solution and the subsequent oxidation of available Si-hydride sites on the surfaces of the nanowires. A native oxide layer suppresses formation of metal nanocrystals; adding HF to the ionic solutions significantly increases the density of nanocrystals on the surfaces of the nanowires. The nanocrystals coating the nanowires were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction. Ag nanocrystals on the nanowires afford sensitive detection of Rhodamine 6G (R6G) molecules in the 100 picomolar–micromolar range by surface enhanced Raman spectroscopy. In addition, Au nanocrystals formed on selected surfaces of a substrate of arbitrary shape can serve as effective nuclei for localized nanowire growth. PACS 81.07.b; 81.15.Gh