A hybridized self-powered textile for simultaneously collecting solar energy and random body motion energy was demonstrated.

Seamlessly joint graphene-nanotube 3D architectures were created by one-step CVD for efficient energy conversion and storage.

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A Touchy Subject

The ability to hold a glass being filled with water without dropping it depends on our ability to touch objects and to know the correct pressure to exert. Thus, for robotics or artificial skin design, methods are needed for sensitive pressure detection. Wu et al. (p. 952, published online 25 April) designed a device based on an array of zinc oxide nanowires that generate a small voltage when flexed that could be translated into a pressure signal. The device has a pressure-sensing range of up to 30 kPa, comparable to the 10 to 40 kPa range of a human finger.

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Growth of vertically aligned ZnO nanowire arrays has been extensively studied on a variety of important semiconductor substrates, such as SiC and GaN. Systematic experiments were carried out to investigate the effect of growth parameters to the quality of the nanowires. In addition, the growth of nanowalls connecting individual aligned nanowires was studied and a growth mechanism was proposed. These conductive and interconnected nanowalls are indispensable for nanodevices to be fabricated on nonconductive substrates for serving as a common electrode. Finally, these nanowire arrays have been integrated as ultra violet detectors, which show good optical performance.

α-MoO₃nanobelts are successfully lithiated by a secondary reaction. The capacity retention rate of lithiated MoO₃ nanobelts is 92 % after 15 cycles, whereas the non-lithiated nanobelts retain only 60 % (see figure). The conductivity is increased by close to two orders of magnitudes after lithiation suggesting that Li⁺ ions have been introduced into the MoO₃ layers during lithiation.

A novel heterostructured ZnO/ZnS ring structure has been synthesized. The formation process of the ring is attributed to the strain induced bending that arises mainly from the lattice mismatch between ZnO and ZnS. This growth model is in consistent to the electrostatic model proposed previously about the formation of nanorings, nanosprings and nanohelices, but it shows the dominant contribution from interface strain in the ring formation for such a special case. The ring structure of ZnO/ZnS provides an ideal candidate for investigate optoelectronics of ring structured II-VI semiconductors.

The large-strain plasticity (LSP) of single-crystalline silicon nanowires (Si NWs) observed in situ at room temperature by axial tension experiments carried out in an ultrahigh-resolution electron microscope is reported. The LSP is demonstrated to result in a fourfold reduction in NW diameter before fracture (see figure), which is three orders of magnitude higher than that of bulk Si.

An effective in situ field-emission measurement system, in which the distance between the emitting source and counter electrode can be accurately measured and finely controlled by setting up a second stage inside a scanning electron microscopy chamber, is developed. It is found that nanowires with a density between 60 and 80 μm^–2 (see figure) and of ca. 1 μm in length give the highest emitting current density.

A ZnO nanowire behaves like a rectifier under bending strain, as demonstrated by its current–voltage characteristics (see graph). This is interpreted with the consideration of a piezoelectricity-induced potential energy barrier at the interface of the conductive tip and nanowire (see schematic). Under appropriate bending and voltage control, each NW could correspond to a device element for random-access-memory, diode, and force-sensor applications.

This article introduces the fundamental principle of nanopiezotronics, which utilizes the coupled piezoelectric and semiconducting properties of nanowires and nanobelts for designing and fabricating electronic devices and components, such as field-effect transistors and diodes. The physics of nanopiezotronics is based on the principle of a nanowire nanogenerator that converts mechanical energy into electric energy. It is anticipated to have a wide range of applications in electromechanical coupled electronics, sensing, harvesting/ recycling energy from the environment, and self-powered nanosystems.

A facile composite-hydroxide-mediated synthesis method is adopted to prepare ultralong, single-crystalline, hexagonal structured La(OH)₃ nanobelts. The detectable conductive behavior of a single La(OH)₃ nanobelt promises their potential application in sensors. La₂O₃ nanobelts can be obtained by calcination of the La(OH)₃ nanobelts. Both types of nanobelts fluoresce with purple light under UV excitation, which may be used in biological labeling.

Nanowire arrays of piezoelectric and semiconductive ZnO grown on flexible plastic substrates have been successfully demonstrated for converting mechanical energy into electrical energy using a conductive atomic force microscope (see figure). Piezoelectric power generators using such arrays might be able to harvest energy from their environment, providing flexible power sources with potential applications in implantable biosensors, self-powered electronic devices, and more.

RuO₂nanowires and RuO₂/TiO₂core/shell nanowires are synthesized by reactive sputtering. The core/shell nanowires are obtained by coating RuO₂ nanowires with TiO₂. The mechanical, optical, electrical and optoelectronic properties of the nanowires are characterized, and indicate several potential applications of the nanowires.

The size- and morphology-controlled growth of ZnO nanowire (NW) arrays is potentially of interest for the design of advanced catalysts and nanodevices. By adjusting the reaction temperature, shelled structures of ZnO made of bunched ZnO NW arrays are prepared, grown out of metallic Zn microspheres through a wet-chemical route in a closed Teflon reactor. In this process, ZnO NWs are nucleated and subsequently grown into NWs on the surfaces of the microspheres as well as in strong alkali solution under the condition of the pre-existence of zincate (ZnO₂^2–) ions. At a higher temperature (200 °C), three different types of bunched ZnO NW or sub-micrometer rodlike (SMR) aggregates are observed. At room temperature, however, the bunched ZnO NW arrays are found only to occur on the Zn microsphere surface, while double-pyramid-shaped or rhombus-shaped ZnO particles are formed in solution. The ZnO NWs exhibit an ultrathin structure with a length of ca.  500 nm and a diameter of ca. 10 nm. The phenomenon may be well understood by the temperature-dependent growth process involved in different nucleation sources. A growth mechanism has been proposed in which the degree of ZnO₂^2–saturation in the reaction solution plays a key role in controlling the nucleation and growth of the ZnO NWs or SMRs as well as in oxidizing the metallic Zn microspheres. Based on this consideration, ultrathin ZnO NW cluster arrays on the Zn microspheres are successfully obtained. Raman spectroscopy and photoluminescence measurements of the ultrathin ZnO NW cluster arrays have also been performed.

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The shapes of noble metal nanocrystals (NCs) are usually defined by polyhedra that are enclosed by {111} and {100} facets, such as cubes, tetrahedra, and octahedra. Platinum NCs of unusual tetrahexahedral (THH) shape were prepared at high yield by an electrochemical treatment of Pt nanospheres supported on glassy carbon by a square-wave potential. The single-crystal THH NC is enclosed by 24 high-index facets such as {730}, {210}, and/or {520} surfaces that have a large density of atomic steps and dangling bonds. These high-energy surfaces are stable thermally (to 800°C) and chemically and exhibit much enhanced (up to 400%) catalytic activity for equivalent Pt surface areas for electro-oxidation of small organic fuels such as formic acid and ethanol.

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We have developed a nanowire nanogenerator that is driven by an ultrasonic wave to produce continuous direct-current output. The nanogenerator was fabricated with vertically aligned zinc oxide nanowire arrays that were placed beneath a zigzag metal electrode with a small gap. The wave drives the electrode up and down to bend and/or vibrate the nanowires. A piezoelectric-semiconducting coupling process converts mechanical energy into electricity. The zigzag electrode acts as an array of parallel integrated metal tips that simultaneously and continuously create, collect, and output electricity from all of the nanowires. The approach presents an adaptable, mobile, and cost-effective technology for harvesting energy from the environment, and it offers a potential solution for powering nanodevices and nanosystems.

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Ceria nanoparticles are one of the key abrasive materials for chemical-mechanical planarization of advanced integrated circuits. However, ceria nanoparticles synthesized by existing techniques are irregularly faceted, and they scratch the silicon wafers and increase defect concentrations. We developed an approach for large-scale synthesis of single-crystal ceria nanospheres that can reduce the polishing defects by 80% and increase the silica removal rate by 50%, facilitating precise and reliable mass-manufacturing of chips for nanoelectronics. We doped the ceria system with titanium, using flame temperatures that facilitate crystallization of the ceria yet retain the titania in a molten state. In conjunction with molecular dynamics simulation, we show that under these conditions, the inner ceria core evolves in a single-crystal spherical shape without faceting, because throughout the crystallization it is completely encapsulated by a molten 1- to 2-nanometer shell of titania that, in liquid state, minimizes the surface energy. The principle demonstrated here could be applied to other oxide systems.

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We have converted nanoscale mechanical energy into electrical energy by means of piezoelectric zinc oxide nanowire (NW) arrays. The aligned NWs are deflected with a conductive atomic force microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zinc oxide creates a strain field and charge separation across the NW as a result of its bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. The efficiency of the NW-based piezoelectric power generator is estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices.

The biodegradability and biocompatibility of ZnO wires in biofluid has been studied by investigating the interaction of the wires with deionized water, ammonia, NaOH solution, and horse blood serum. The results show that ZnO can be dissolved into mineral ions by these liquids within a few hours (see figure) and that ZnO is a bio-safe, nontoxic material for in vivo biosensing and biodetection.

Well-aligned aluminum nitride nanorods with hairy surfaces (see Figure) are a new hierarchical nanostructure with promising field-emission properties. The columnar nanostructures are produced by a vapor–solid process. Their cathodoluminescence spectrum reveals an intense emission peak with a low turn-on field of 3.8 V μm^–1, suggesting potential applications in optoelectronic nanodevices.

The low melting point of Zn and the high melting point of ZnO, as well as their hexagonal crystal structures, present great advantages for designing and fabricating various metal/semiconductor core/shell nanostructures. By controlling the kinetics in the Zn and ZnO system, the lower-energy facets, and the oxidation rates of different surfaces, we can control the fabrication of Zn/ZnO core/shell single-crystal, polycrystalline, and mesoporous nanodisks, as well as a variety of ZnO nanotubes. The oxidation of a Zn nano-object leads to the formation of Zn/ZnO core/shell nanodisks. A lower oxidation temperature results in the formation of a single-crystal-like Zn/ZnO core/shell structure, while a higher oxidation temperature leads to the formation of textured and even polycrystalline nanostructures. A re-sublimation process of Zn in the core leaves a ZnO shell structure. This is an approach for synthesizing metal/semiconductor core/shell or composite nanostructures. This article offers a detailed description of the kinetics controlling the procedures, the nanostructures obtained, their morphological and crystal structures, and their formation mechanisms.

Growth of aligned and uniform α-Fe₂O₃ nanowire (NW) arrays has been achieved by a vapor–solid process. The experimental conditions, such as type of substrate, local growth and geometrical environment, gas-flow rate, and growth temperature, under which the high density α-Fe₂O₃ NW arrays can be grown by a vapor–solid route via the tip-growth mechanism have been systematically investigated. The density of the α-Fe₂O₃ NWs can be enhanced by increasing the concentration of Ni atoms inside the alloy substrate. The synthesized temperature can be as low as 400 °C. Fe₃O₄ NWs can be produced by converting α-Fe₂O₃ NWs in a reducing atmosphere at 450 °C. The transformation of phase and structure have been observed by in situ transmission electron microscopy. The magnetic and field-emission properties of the NWs indicate their potential applications in nanodevices.

Abstract

Polar surface induced asymmetric growth of single-side teethed ZnO nanocombs was attributed to the self-catalysis of the Zn-terminated (0 0 0 1) surface (Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502). In this Letter, nanocombs of ZnO with double-sided teeth have been observed. This symmetric growth of the fish-ribbon like teeth has been identified due to the existence of an inversion domain boundary along the ribbon, so that both side surfaces of the ribbon are terminated with the chemically active Zn-(0 0 0 1) plane. A model is also given about the formation of ∼110° double-sided nanocombs based on the nucleus composed of multiply twinned pyramids. The data show that the Zn-terminated (0 0 0 1) surface is responsible for the formation of the teeth, while the oxygen-terminated ($000\overline{1}$) surface is chemically inactive and does not grow teeth.

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Polycrystalline iron phosphide coated iron oxide and hollow iron phosphide nanoparticles were synthesized by sonichemistry. Structure analysis indicated that the diameters of these nanoparticles were less than 14 nm, and the core–shell and hollow nanoparticles are Fe₃O₄–FeP and FeP, respectively. Magnetic measurement demonstrated that both the core–shell Fe₃O₄–FeP and hollow FeP nanoparticles exhibited ferromagnetic behaviors.

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Voltammetric properties of electrodes made of multiple-walled carbon nanotubes (MWNTs) with different diameters were investigated by cyclic voltammetry. The results indicate that electrodes made of smaller MWNTs exhibit better voltammetric properties. In addition, electrodes made of mixed MWNTs with large diameter distribution possess the best voltammetric properties. The phenomenon can be explained by the filled-in frame structures formed on the electrode surface.

Twists and turns: ZnS nanohelices have been synthesized in high yield by using a simple vapor deposition process. These nanohelices also have Y-shaped branched secondary growths, which always point toward the inside of the helix. The formation of these hierarchical structures is discussed based on structural information.

Vielfach verdreht: Mit einer einfachen Abscheidung aus der Dampfphase wurden ZnS-Nanohelices in hoher Ausbeute hergestellt. Diese Nanohelices weisen zusätzliches Y-förmiges sekundäres Wachstum auf, das immer ins Innere der Helix weist. Die Bildung dieser hierarchischen Strukturen wird auf der Grundlage von Strukturinformationen diskutiert.

Abstract

A previously unknown rigid helical structure of zinc oxide consisting of a superlattice-structured nanobelt was formed spontaneously in a vapor-solid growth process. Starting from a single-crystal stiff nanoribbon dominated by the c-plane polar surfaces, an abrupt structural transformation into the superlattice-structured nanobelt led to the formation of a uniform nanohelix due to a rigid lattice rotation or twisting. The nanohelix was made of two types of alternating and periodically distributed long crystal stripes, which were oriented with their c axes perpendicular to each other. The nanohelix terminated by transforming into a single-crystal nanobelt dominated by nonpolar () surfaces. The nanohelix could be manipulated, and its elastic properties were measured, which suggests possible uses in electromechanically coupled sensors, transducers, and resonators.

By coating patterned and aligned ZnO nanorod arrays with TiO₂, a bottom–up process is used to fabricate a 2D photonic crystal (PC). The as-synthesized 2D PC slab shows a highly ordered air-hole array (see Figure) and exhibits a photonic bandgap that agrees reasonably well with the theoretically calculated value.

Large-scale, single-crystalline, cubic-structured tungsten oxide (WO_3–δ) nanowire networks (see Figure) have been synthesized by the thermal evaporation of tungsten metal powder in the presence of oxygen. The formation of ordered planar oxygen vacancies is suggested to be the driving mechanism for the formation of these interpenetrative nanowire networks.

One-dimensional (1D) nanostructures of CdSe have been found to exhibit morphologies of nanowires, nanobelts, and nanosaws, but their synthesis is by trial and error. To meet the needs of large-scale, controlled, and designed synthesis of nanostructures, it is imperative to systematically find experimental conditions under which the desired nanostructures are synthesized reproducibly, in large quantity, and with controlled morphology. This article reports the first systematic study on the growth of 1D CdSe nanostructures by a vapor–liquid–solid (VLS) process by varying a wide range of experimental conditions. Over 150 experiments have been conducted to investigate the morphology dependence of three different types of nanostructures: nanowires, nanobelts, and nanosaws, over various substrate temperatures and pressures. The results of this work yield a road map for the controlled growth of 1D CdSe nanostructures. This research serves as a guidance and “menu” for scaling up of the synthesis of CdSe nanostructures. This is a key step towards the controlled synthesis of nanostructures to meet the needs of many industrial applications of nanomanufacturing.

Fraktale Hämatit-Dendrite mit Tannenzweig-Struktur (siehe Bild) wurden durch eine Hydrothermalbehandlung von K₃[Fe(CN)₆] erhalten. Die dendritischen, manchmal auch schneeflockenförmigen Einkristalle bilden sich in einem bislang unbekannten Prozess durch schnelles Wachstum entlang von sechs kristallographisch äquivalenten Richtungen.

Fractal hematite dendrites with structures resembling pine trees (see picture) were obtained by hydrothermal treatment of K₃[Fe(CN)₆]. They are formed by a novel process in which fast growth along six crystallographically equivalent directions forms dendritic or snowflakelike single-crystal structures, which differs from the mechanisms reported for other fractal structures.

Abstract

Freestanding single-crystal complete nanorings of zinc oxide were formed via a spontaneous self-coiling process during the growth of polar nanobelts. The nanoring appeared to be initiated by circular folding of a nanobelt, caused by long-range electrostatic interaction. Coaxial and uniradial loop-by-loop winding of the nanobelt formed a complete ring. Short-range chemical bonding among the loops resulted in a single-crystal structure. The self-coiling is likely to be driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. Zinc oxide nanorings formed by self-coiling of nanobelts may be useful for investigating polar surface–induced growth processes, fundamental physics phenomena, and nanoscale devices.

High-porosity, single-crystal ZnO nanowires have been synthesized for the first time using a solid–vapor process on a Si substrate. The thermal decomposition of ZnO and the formation of a Zn₂SiO₄ network on the nanowire surface are the keys for forming the high-porosity interior. The structures exhibit a very high surface-to-volume ratio and are expected to have high absorptive capacity and chemical selectivity that could be used for filter and sensor systems.

A wurtzite-structured CdSe nanosaw (see Figure) is formed by a two-step process: a fast-growth process along [010] creates the main ribbon, while a subsequent side-growth process along [0001] creates the one-sided teeth. The growth of the teeth is suggested to be a combined result of secondary epitaxial nucleation processes resulting from a zinc-blende–wurtzite phase transformation and the self-catalytic effect of the Cd-terminated (0001) surface.

Zinc oxide, an important semiconducting and piezoelectric material, has three key characteristics. First, it is a semiconductor, with a direct bandgap of 3.37 eV and a large excitation binding energy (60 meV), and exhibits near-UV emission and transparent conductivity. Secondly, due to its non-centrosymmetric symmetry, it is piezoelectric, which is a key phenomenon in building electro-mechanical coupled sensors and transducers. Finally, ZnO is bio-safe and bio-compatible, and can be used for biomedical applications without coating. With these unique advantages, ZnO is one of the most important nanomaterials for integration with microsystems and biotechnology. Structurally, due to the three types of fastest growth directions—<0001>, <010>, and <20>—as well as the ±(0001) polar surfaces, a diverse group of ZnO nanostructures have been grown in our laboratory. These include nanocombs, nanosaws, nanosprings, nanorings, nanobows, and nanopropellers. This article reviews our recent progress in the synthesis and characterization of polar-surface-induced ZnO nanostructures, their growth mechanisms, and possible applications as sensors, transducers, and resonators. It is suggested that ZnO could be the next most important nanomaterial after carbon nanotubes.

Abstract

Bulk crystals of ZnS usually take the zinc blende structure. However, the vapor deposited one-dimensional ZnS nanostructures normally take the metastable wurtzite structure. This Letter investigates the conditions under which the formed phase can be controlled between zinc blende and wurtzite in nanomaterials synthesis. The formation of pure zinc blende structured ZnS nanobelts is related not only to the lower deposition temperature (680–750 °C), but also to the size of the Au catalyst particle (<50 nm). Pure wurtzite structured ZnS nanobelts are synthesized in a relatively high deposition temperature (>750 °C) disregard the size of the Au catalyst particles.

Abstract

Wurtzite structured ZnS nanoribbons have been synthesized by a catalyst-free solid–vapor deposition technique. The nanoribbon has a saw-teeth shape, and the nanosaw is formed by a two-step process: a fast growth along a-axis forms the body of the saw; a subsequent growth along c-axis creates the teeth. The one-sided teeth structure is suggested to be the self-catalyzed growth of the Zn-terminated (0 0 0 1) surface, while the oxygen-terminated (0 0 0 ) surface is relatively chemically inactive. The growth of ‘feather’-like structure of ZnS is also reported.

Die Flüssigphasensynthese von einkristallinen ZnO-Scheiben und -Ringen (siehe Bild) in hohen Ausbeuten gelingt mit anionischen Tensiden als Templaten bei nur 70–90 °C. Über die Wachstumstemperatur und das molare Verhältnis der Reagentien lässt sich die Reaktion so steuern, dass Scheiben oder Ringe bevorzugt erhalten werden. Auf der Grundlage von Strukturinformationen aus SEM und TEM wird ein Wachstumsmechanismus vorgeschlagen.

Solution-phase synthesis of single-crystal ZnO disks and rings was achieved in high yield at low temperature (70–90 °C) by using an anionic surfactant as a template. The reaction can be controlled by means of the growth temperature and the molar ratio of reagents to favor formation of disks or rings. A growth mechanism is proposed on the basis of structural information provided by SEM and TEM.

Novel nanostructures of semiconducting oxides are reviewed here. It is shown that nanobelts, nanowires, and nanodiskettes of materials such as zinc oxide, gallium oxide, silica, and tin oxide can be fabricated using a vapor-phase evaporation method. Two applications of these materials—in field effect transistors and as gas sensors—are highlighted.

High-resolution transmission electron microscopy (HRTEM) is one of the most powerful tools used for characterizing nanomaterials, and it is indispensable for nanotechnology. This paper reviews some of the most recent developments in electron microscopy techniques for characterizing nanomaterials. The review covers the following areas: in-situ microscopy for studying dynamic shape transformation of nanocrystals; in-situ nanoscale property measurements on the mechanical, electrical and field emission properties of nanotubes/nanowires; environmental microscopy for direct observation of surface reactions; aberration-free angstrom-resolution imaging of light elements (such as oxygen and lithium); high-angle annular-dark-field scanning transmission electron microscopy (STEM); imaging of atom clusters with atomic resolution chemical information; electron holography of magnetic materials; and high-spatial resolution electron energy-loss spectroscopy (EELS) for nanoscale electronic and chemical analysis. It is demonstrated that the picometer-scale science provided by HRTEM is the foundation of nanometer-scale technology.

Functional oxides are the fundamentals of smart devices. This article reviews novel nanostructures of functional oxides, including nanobelts, nanowires, nanosheets, and nanodiskettes, that have been synthesized in the authors’ laboratory. Among the group of ZnO, SnO₂, In₂O₃, Ga₂O₃, CdO, and PbO₂, which belong to different crystallographic systems and structures, a generic nanobelt structure has been synthesized. The nanobelts are single crystalline and dislocation-free, and their surfaces are atomically flat. The oxides are semiconductors, and have been used for fabrication of nanodevices such as field-effect transistors and gas sensors. Taking SnO₂ and SnO as examples, other types of novel nanostructures are illustrated. Their growth, phase transformation, and stability are discussed. The nanobelts and related nanostructures are a unique group that is likely to have important applications in electronic, optical, sensor, and optoelectronic nanodevices.

One-dimensional (1D) nanostructures have numerous potential applications in science and engineering. Nanocomposites made of nanowires, such as carbon nanotubes, are likely to decrease material’s density and increase its strength,^[1] which are of critical importance to space technology. To investigate the uniqueness offered by these materials, new techniques must be developed to quantitatively measure the properties of individual wire-like structures whose structures are well characterized by electron microscopy techniques, because their properties may sensitively depend on their geometrical shape/configurations and crystal as well as surface structures. Within the framework of in-situ TEM we have recently developed a novel approach that relies on electric field induced mechanical resonance for measuring the properties of individual wire-like structures, such as Young’s modulus, electron field emission, tip work function, and electrical quantum conductance. This is a new technique that provides the properties of a single nanowire with well characterized.

A new approach to the characterization of the mechanical and electrical properties of individual nanowires and nanotubes is demonstrated by in-situ transmission electron microscopy (TEM). The technique allows a one-to-one correlation between the structure and properties of the nanowires. Recent developments include the determination of the Young's modulii of carbon nanotubes and semiconductor nanowires, femtogram nanobalance of a single fine particle, field emission of carbon nanotubes, and quantum ballistic conductance in carbon nanotubes.