Controlled Synthesis of Ag2S,Ag2Se,and Ag Nanofibers by Using a General Sacrificial Template and Their Application in Electronic Device Fabrication

Nanofiber bundles of Ag₂S, Ag₂Se, and Ag have been successfully synthesized by making use of Ag₂C₂O₄ template nanofiber bundles, utilizing both anion‐exchange and redox reactions. The obtained bundles were polycrystalline nanofibers composed of nanoparticles in which the precursor morphology was well‐preserved, indicating that Ag₂C₂O₄ nanofiber bundles acted as a general sacrificial template for the synthesis of silver‐based semiconductor and metal nanofibers. Dispersing media and transforming reactants were found to be key factors influencing the chemical transformation in the system. In particular, separate single‐crystalline Ag nanofibers were obtained via a nontemplate route when ascorbic acid was used as a relatively weak reductant. An electrical transfer and switching device was built with the obtained Ag₂S and Ag nanofiber bundles, utilizing the unique ion‐conductor nature of Ag₂S and revealing their potential applications in electronics.     

Light‐Emitting Rubrene Nanowire Arrays: A Comparison with Rubrene Single Crystals

This is a report on a new method of growth of a light‐emitting rubrene nanowires array with diameters of 200 ± 10 nm by using organic vapor transport through Al₂O₃ nanoporous templates. Nanometer‐scale laser confocal microscope (LCM) photoluminescence (PL) spectra and crystalline structures of the rubrene nanowires are compared with those of rubrene single crystals prepared with the same experimental conditions without the template. In the LCM PL spectra it is observed that the PL spectra and intensity varies with the detecting positions because of the crystal growth characteristics of the rubrene molecules. A single rubrene nanowire has a wider LCM PL band width than that of the rubrene single crystal. This may originate from the light emissions of the mixed polarized bands due to additional new crystallinity in the formation of the nanowires. From the current–voltage characteristic curves, the semiconducting nature of both the rubrene nanowires and single crystals is observed.     

Single‐Crystalline LiMn2O4 Nanotubes Synthesized Via Template‐Engaged Reaction as Cathodes for High‐Power Lithium Ion Batteries

Single‐crystalline nanotubes of spinel LiMn₂O₄ with a diameter of about 600 nm, a wall thickness of about 200 nm and a length of 1–4 μm have been synthesized via a template‐engaged reaction using β‐MnO₂ nanotubes as a self‐sacrifice template. In this fabrication, a minimal structural reorganization can be responsible for the chemical transformation from [001]‐oriented β‐MnO₂ template to [110]‐oriented LiMn₂O₄. Galvanostatic charge/discharge measurements indicate that the nanotubes exhibit superior high‐rate capabilities and good cycling stability. About 70% of its initial capacity can be retained after 1500 cycles at 5 C rate. Importantly, the tubular nanostructures and the single‐crystalline nature of the most LiMn₂O₄ nanotubes are also well preserved after prolonged charge/discharge cycling at a relatively high current density, indicating good structural stability of the single‐crystalline nanotubes during lithium intercalation/deintercalation process. As is confirmed from Raman spectra analyses, no evident microstructural changes occur upon long‐term cycling. These results reveal that single‐crystalline nanotubes of LiMn₂O₄ will be one of the most promising cathode materials for high‐power lithium ion batteries.     

General Synthesis of Single‐Crystal Tungstate Nanorods/Nanowires: A Facile,Low‐Temperature Solution Approach

The general large‐scale synthesis of a family of single‐crystalline transition metal tungstate nanorods/nanowires is easily realized by a hydrothermal crystallization technique under mild conditions using cheap and simple inorganic salts as precursors. Uniform tungstate nanorods/nanowires such as MWO₄ (M = Zn, Mn, Fe), Bi₂WO₆, Ag₂WO₄, and Ag₂W₂O₇ with diameters of 20–40 nm, lengths of up to micrometers, and controlled aspect ratios can be readily obtained by hydrothermal transformation and recrystallization of amorphous particulates. This novel and efficient pathway toward various kinds of related low‐dimensional tungstate nanocrystals under mild conditions could open new opportunities for further investigating the novel properties of tungstate materials.     

One‐Step Solvent‐Free Synthesis and Characterization of Zn1−xMnxSe@C Nanorods and Nanowires

The carbon‐encapsulated, Mn‐doped ZnSe (Zn_1−xMn_xSe@C) nanowires, nanorods, and nanoparticles are synthesized by the solvent‐free, one‐step RAPET (reactions under autogenic pressure at elevated temperature) approach. The aspect ratio of the nanowires/nanorods is altered according to the Mn/Zn atomic ratio, with the maximum being observed for Mn/Zn = 1:20. A 10–20 nm amorphous carbon shell is evidenced from electron microscopy analysis. The replacement of Zn by Mn in the Zn_1−xMn_xSe lattice is confirmed by the hyperfine splitting values in the electron paramagnetic resonance (EPR) experiments. Raman experiments reveal that the Zn_1−xMn_xSe core is highly crystalline, while the shell consists of disordered graphitic carbon. Variable‐temperature cathodoluminescence measurements are performed for all samples and show distinct ZnSe near‐band‐edge and Mn‐related emissions. An intense and broad Mn‐related emission at the largest Mn alloy composition of 19.9% is further consistent with an efficient incorporation of Mn within the host ZnSe lattice. The formation of the core/shell nanowires and nanorods in the absence of any template or structure‐directing agent is controlled kinetically by the Zn_1−xMn_xSe nucleus formation and subsequent carbon encapsulation. Mn replaces Zn mainly in the (111) plane and catalyzes the nanowire growth in the [111] direction.     

Large‐Scale Synthesis of Long Crystalline Cu2‐xSe Nanowire Bundles by Water‐Evaporation‐Induced Self‐Assembly and Their Application in Gas Sensing

By a facile water evaporation process without adding any directing agent, Cu_2‐xSe nanowire bundles with diameters of 100–300 nm and lengths up to hundreds of micrometers, which comprise crystalline nanowires with diameters of 5–8 nm, are obtained. Experiments reveal the initial formation/stacking of CuSe nanoplates and the subsequent transformation to the Cu_2‐xSe nanowire bundles. A water‐evaporation‐induced self‐assembly (WEISA) mechanism is proposed, which highlights the driving force of evaporation in promoting the nanoplate stacking, CuSe‐to‐Cu_2‐xSe transformation and the growth/bundling of the Cu_2‐xSe nanowires. The simplicity, benignancy, scalability, and high‐yield of the synthesis of this important nanowire material herald its numerous applications. As one example, the use of the Cu_2‐xSe nanowire bundles as a photoluminescence‐type sensor of humidity is demonstrated, which shows good sensitivity, ideal linearity, quick response/recovery and long lifetime in a very wide humidity range at room temperature.     

Stoichiometry Controlled,Single‐Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane

Thermoelectric Bi₂Te₃ based bulk materials are widely used for solid‐state refrigeration and power‐generation at room temperature. For low‐dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi₂Te₃ nanowires, thin films, and nanostructured bulk materials has become an important and active field of research. Stoichiometric Bi₂Te₃ nanowires with diameters of 50–80 nm and a length of 56 μm are grown by a potential‐pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix. By transmission electron microscopy (TEM), dark‐field images together with electron diffraction reveal single‐crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high‐accuracy, quantitative enegy‐dispersive X‐ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi₂Te₃ nanowires. X‐ray and electron diffraction reveal growth along the [110] and [210] directions and the c axis of the Bi₂Te₃ structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi₂Te₃ nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.     

Nanostructured TiO2 and Its Application in Lithium‐Ion Storage

Titania nanorods and nanowires are synthesized via a hydrothermal reaction of amorphous TiO₂ in alkaline NaOH, followed by ion exchange in HCl aqueous solution, and dehydration at 400 °C. Although the hydrothermal treatment produces three different particle morphologies depending on the reaction time (nanosheets, nanorods, and nanowires), the products exhibit the same crystal structure. Ion exchange of Na₂Ti₃O₇ in HCl aqueous solution brings about a phase change to H₂Ti₃O₇, but there is no change in the particle morphology. Dehydration of the nanostructured H₂Ti₃O₇ leads to two types of crystal structure—anatase TiO₂ for the nanorods, and TiO₂–B for the nanowires—although no significant difference is found in the morphology of the products even after dehydration. The nanorods are 40–50 nm in length and 10 nm in diameter, whereas the nanowires are several micrometers in length and tens to hundreds of nanometers in thickness. In‐situ X‐ray diffraction revealed the formation of anatase TiO₂ from the TiO₂–B above 450 °C. This finding implies that the phase transformation occurs rather slowly for the TiO₂–B nanowires due to the larger particle size and higher crystallinity of H₂Ti₃O₇. Tests with Li‐metal half cells indicated that the anatase TiO₂ nanorods are more favorable for the storage and release of Li ions because of their greater surface area than the TiO₂–B nanowires.     

Synthesis of sulfur-doped ZnO nanowires by electrochemical deposition

Sulfur-doped zinc oxide (ZnO) nanowires have been successfully synthesized by an electric field-assisted electrochemical deposition in porous anodized aluminum oxide template at room temperature. X-ray diffraction and the selected area electron diffraction results show that the as-synthesized nanowires are single crystalline and have a highly preferential orientation. Transmission electron microscopy observations indicate that the nanowires are uniform with an average diameter of 70 nm and length up to several tens of micrometers. X-ray photoelectron spectroscopy further reveals the presence of S in the ZnO nanowires. Room-temperature photoluminescence is observed in the doped ZnO nanowires, which exhibits a violet emission and blue emissions besides the typical photoluminescence spectrum of a single crystal ZnO.     

ZnO近紫外波长纳米激光器的研究

随着纳米科技的兴起,纳米激光的研究成为了又一个新的重要课题.ZnO纳米微晶有两种结构可以产生随机激光,一是六角柱形蜂窝状微晶结构,二是颗粒粉末状结构,产生的近紫外激光波长是387.5 nm,光泵浦阈值是50 kW/cm2.采用气相输运的催化外延晶体生长过程来制备ZnO纳米线阵列构成的光致纳米激光器,激光波长383 nm,线宽仅为0.3 nm,光泵浦阈值是40 kW/cm2.     

Vertically Well‐Aligned ZnO Nanowires on c‐Al2O3 and GaN Substrates by Au Catalyst

In this letter, we report that vertically well‐aligned ZnO nanowires were grown on GaN epilayers and c‐plane sapphire via a vapor‐liquid‐solid process by introducing a 3 nm Au thin film as a catalyst. In our experiments, epitaxially grown ZnO nanowires on Au‐coated GaN were vertically well‐aligned, while nanowires normally tilted from the surface when grown on Au‐coated c‐Al₂O₃ substrates. However, pre‐growth annealing of the Au thin layer on c‐Al₂O₃ resulted in the growth of well‐aligned nanowires in a normal surface direction. High‐resolution transmission electron microscopy measurements showed that the grown nanowires have a hexagonal c‐axis orientation with a single‐crystalline structure.     

Synthesis and Room‐Temperature Ultraviolet Photoluminescence Properties of Zirconia Nanowires

This paper reports the synthesis of tetragonal zirconia nanowires using template method. An as‐prepared sample was characterized by scanning and transmission electron microscopy. It was found that the as‐prepared materials were tetragonal zirconia nanowires with average diameters of ca. 80 nm and length of over 10 μm. The Raman spectrum showed peaks at 120, 461, and 629 cm^–1, which are attributed to the E_g, E_g, and B_1g phonon modes of the tetragonal zirconia structure, respectively. The UV‐vis absorption spectrum showed an absorption peak at 232.5 nm (5.33 eV in photon energy). Photoluminescence (PL) spectra of zirconia nanowires showed a strong emission peak at ca. 388 nm at room temperature, which is attributed to the ionized oxygen vacancy in the zirconia nanowires system.     

Laser‐Ablation Growth and Optical Properties of Wide and Long Single‐Crystal SnO2 Ribbons

Wide and long ribbons of single‐crystalline SnO₂ have been achieved via laser ablation of a SnO₂ target. Transmission electron microscopy (TEM) shows the as‐grown SnO₂ ribbons are structurally perfect and uniform, with widths of 300–500 nm, thicknesses of 30–50 nm (width‐to‐thickness ratio of ～ 10), and lengths ranging from several hundreds of micrometers to the order of millimeters. X‐ray diffraction (XRD) pattern and energy‐dispersive X‐ray spectroscopy (EDS) spectral analysis indicate that the ribbons have the phase structure and chemical composition of the rutile form of SnO₂. Selected‐area electron diffraction (SAED) patterns and high‐resolution TEM images reveal that the ribbons are single crystals and grow along the [100] crystal direction. Photoluminescence measurements show that the synthesized SnO₂ ribbons have one strong emission band at ～ 605 nm and a red‐shift of ～ 30 nm, as compared to standard SnO₂ powder, which may be attributed to crystal defects and residual strains accommodated during the growth of the ribbons.     

A New Route to Large‐Scale Synthesis of Silicon Nanowires in Ultrahigh Vacuum

An approach for the large‐scale synthesis of high‐purity silicon nanowires (SiNWs) in ultrahigh vacuum is presented. A mixture of Si and SiO₂ is evaporated by an electron beam, and the growth temperature is 700 °C, which is much lower than those used for other oxide‐assisted growths. A new type of single‐crystal SiNWs, with [221] orientation, is thus synthesized. Moreover, it is experimentally demonstrated that SiO intermediates are formed in the process, and the nanowires are obtained via a disproportionation reaction of 2SiO → Si + SiO₂. A growth mechanism is proposed and the critical factors for the formation of 1D nanowires are also determined. The approach is particularly compatible with the mature Si‐based technology, and is favorable for device integration and practical applications.     

Highly Ordered Mesoporous Crystalline MoSe2 Material with Efficient Visible‐Light‐Driven Photocatalytic Activity and Enhanced Lithium Storage Performance

Highly ordered mesoporous crystalline MoSe₂ is synthesized using mesoporous silica SBA‐15 as a hard template via a nanocasting strategy. Selenium powder and phosphomolybdic acid (H₃PMo₁₂O₄₀) are used as Se and Mo sources, respectively. The obtained products have a highly ordered hexagonal mesostructure and a rod‐like particle morphology, analogous to the mother template SBA‐15. The UV‐vis‐NIR spectrum of the material shows a strong light absorption throughout the entire visible wavelength region. The direct bandgap is estimated to be 1.37 eV. The high surface area MoSe₂ mesostructure shows remarkable photocatalytic activity for the degradation of rhodamine B, a model organic dye, in aqueous solution under visible light irradiation. In addition, the synthesized mesoporous MoSe₂ possess a reversible lithium storage capacity of 630 mAh g^?1 for at least 35 cycles without any notable decrease. The rate performance of mesoporous MoSe₂ is much better than that of analogously synthesized mesoporous MoS₂, making it a promising anode for the lithium ion battery.     

Direct Synthesis of Large‐Scale WTe2 Thin Films with Low Thermal Conductivity

Large‐scale, polycrystalline WTe₂ thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H₂Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single‐crystalline telluride nanoflakes. Through‐plane and in‐plane thermal conductivities of single‐crystal WTe₂ flakes and polycrystalline WTe₂ thin films are measured for the first time. Nanoscale grains and disorder in WTe₂ thin films suppress the in‐plane thermal conductivity of WTe₂ greatly, which is at least 7.5 times lower than that of the single‐crystalline flakes.     

Inside Front Cover: Green Synthesis and Property Characterization of Single‐Crystalline Perovskite Fluoride Nanorods (Adv. Funct. Mater. 1/2008)

The synthesis of single‐crystalline, cubic perovskite KMnF₃ and NH₄MnF₃ nanorods, and their rare‐earth‐ion‐doped analogues with reproducible shapes and sizes, has been realized using a modified template‐directed approach, report Stanislaus Wong and co‐workers on p. 103. The properties of the nanorods and their as‐doped counterparts suggest their practical incorporation into functional nanometer‐scale devices with applications in a number of fields. The cover shows the crystal structure of perovskite fluorides overlaid on a SEM image of as‐prepared KMnF₃ nanorods with diameters measuring around 50 nm. The generalized green synthesis of single‐crystalline KMnF₃ and NH₄MnF₃ nanorods as well as of their rare‐earth ion doped analogues, possessing reproducible shape and controllable size, has been achieved using a modified template‐directed approach under ambient room‐temperature conditions, with simple inorganic salts as functional precursors. Extensive characterization of the resulting nanorods has been performed using diffraction, electron microscopy, optical spectroscopy, as well as magnetic techniques. We have studied the antiferromagnetism of as‐prepared ternary metal fluoride nanorods as well as the luminescence of their as‐doped counterparts. Our collective data suggest the possibility of the incorporation of these high‐quality, chemically pure materials into functional nanoscale devices with various potential applications that exploit the interesting optomagnetic properties of these systems.     

Microstructure and Superconductivity of La1.85Sr0.15CuO4 Nanowire Arrays

Superconducting La_1.85Sr_0.15CuO₄ nanowire arrays are successfully synthesized through a sol–gel method combined with porous alumina as a morphology‐directing hard template for the first time. The morphology, structure, and composition of the as‐prepared nanowire arrays are characterized by field‐emission scanning electron microscopy, transmission electron microscopy, high‐resolution transmission electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy. These experimental results indicate that the nanowires are well crystallized with an approximately uniform diameter of about 30 nm. The superconducting transition temperature T_c (ca. 30 K) of the annealed nanowires is lower than that in bulk La_1.85Sr_0.15CuO₄. It is suggested that this superconductivity suppression is derived from the weakening of in‐plane hybridization in the nanowire system.     

Ultrahigh Responsivity of Ternary Sb–Bi–Se Nanowire Photodetectors

High‐quality single‐crystalline ternary (Sb_1‐xBi_x)₂Se₃ nanowires (NWs) (x = 0–0.88) are synthesized by chemical vapor deposition. Nanowires with x from 0 to 0.75 are indexed as an orthorhombic structure. With increasing Bi incorporation ratio, (Sb_1‐xBi_x)₂Se₃ NWs exhibit remarkable photoresponsivities, which originate from growing surface Se vacancies and augmented oxygen chemisorptions. Notably, spectra responsivity and external quantum efficiency of an (Sb_0.44Bi_0.56)₂Se₃ NW photodetector reach as high as ≈8261.4 A/W and ≈1.6 × 10⁶ %, respectively. Those excellent performances unambiguously demonstrate that Sb–Bi–Se NWs are promising for the utilizations of high‐sensitivity and high‐speed photodetectors and photoelectronic switches.     

From Mesostructured Wurtzite ZnS‐Nanowire/Amine Nanocomposites to ZnS Nanowires Exhibiting Quantum Size Effects: A Mild‐Solution Chemistry Approach

Mesostructured wurtzite ZnS‐nanowire‐bundle/amine nanocomposites displaying remarkable quantum size effects are synthesized by using a mild‐solution reaction using different amines, such as n‐butylamine, ethylamine, and tetraethylenepentamine, Zn(NO₃)₂·6 H₂O, and CS(NH₂)₂ or Na₂S·9 H₂O as the precursors at temperatures ranging from room temperature to 180 °C. A possible mechanism for the shape‐controlled growth of ZnS nanowires and nanocomposites is proposed. Increasing the reaction temperature or dispersing the composite in acetic acid or NaOH solution leads to the destruction of the periodic structure and the formation of individual wurtzite nanowires and their aggregates. The nanowire/amine composites and individual wurtzite nanowires both display obvious quantum size effects. Strong band‐edge emission is observed for the wurtzite ZnS nanowires after removal of the amine. The optical properties of these nanocomposites and nanowires are strongly related to the preparation conditions and can be finely tuned. This technique provides a unique approach for fabricating highly oriented wurtzite ZnS semiconductor nanowires, and can potentially be extended to other semiconducting systems.