Showing papers by "Zhifeng Ren published in 2003"

ZnO nanobridges and nanonails

Abstract: Wurtzite ZnO nanobridges and aligned nanonails have been synthesized by thermal vapor transport and condensation method. The nanobridges have two rows of c-axis ZnO nanorods epitaxailly grown on the edges of the {0001} plane of the ZnO nanobelt. Some variations of the nanobridges have also been observed. The ZnO nanonails, with crystalline cap and small diameter shafts, grow along the c-axis. The shape of the nanonail cap and shaft varies. The nanobridges have very low concentration of indium in the structure and the nanonails are pure ZnO. These materials have potential in applications such as optoelectronics, etc.

Photonic Crystals Based on Periodic Arrays of Aligned Carbon Nanotubes

Abstract: We demonstrate here that large area periodic arrays of well-aligned carbon nanotubes can be fabricated inexpensively on Ni dots made by the process of self-assembly nanosphere lithography. These periodic arrays appear colorful due to their efficient reflection and diffraction of visible light. In addition, due to their honeycomb lattice structure, these arrays can act as photonic band gap crystals in the visible frequency range. In this report, we present the initial exploration of the optical properties of such arrays. Here we show that these potential 2D photonic band gap crystal arrays might find very important applications in optoelectronics.

Field-emission studies on thin films of zinc oxide nanowires

Abstract: Studies on field emission (FE) from thin films of zinc oxide (ZnO) nanowires found that both the turn-on voltage and emission current density depend on the areal density of nanowires. The density of ZnO nanowires is controlled by the gold (Au) nanoparticle density deposited on the silicon substrates. The growth of ZnO nanowires was achieved by the thermal evaporation/condensation method. It is shown that the same screening effect observed on carbon nanotube field emitters also affects the FE from thin films of ZnO nanowires. Thin films with the lowest areal density of ZnO nanowires showed much better FE characteristics, comparable to that of carbon nanotubes. More importantly, the FE characteristics of ZnO nanowire thin film were further improved with annealing in hydrogen.

Large-quantity free-standing ZnO nanowires

Abstract: Large-quantity (grams) one-dimensional ZnO nanowires of different sizes have been synthesized by a simple thermal evaporation of ZnO powder in a tube furnace at a temperature controlled to 1000–1200 °C and pressure to 1–2 Torr air. A mixture of ZnO and graphite powder was used as the source. Fine graphite flakes were used to promote the growth. The graphite flakes are the key for large-quantity yield and were easily removed by oxidation in flowing O2 at about 700 °C for 1–3 h. The scanning- and transmission-electron-microscopic studies show that the diameter and length of the nanowires vary from 20 to 100 nm and 0.5 to 10 μm, respectively. Room temperature photoluminescence studies found that the luminescent intensity depends on the processing conditions. A reduced band edge ultraviolet (380 nm) and deep-band green (520 nm) emission have been observed for these nanowires. Most importantly, the method can be extended to any other oxide nanowires that will be the building block of future nanoscale devices.

Growth of large periodic arrays of carbon nanotubes

Abstract: Large periodic arrays of carbon nanotubes have been grown by plasma-enhanced hot filament chemical vapor deposition on periodic arrays of nickel dots that were prepared by polystyrene nanosphere lithography. A single layer of self-assembled polystyrene spheres was first uniformly deposited on a silicon wafer as a mask, and then electron beam vaporization was used to deposit a nickel layer through the mask. The size of and spacing between the nickel dots are tunable by varying the diameter of the polystyrene spheres, which consequently determines the diameter and site density of carbon nanotubes. The technique can be scaled up at much lower cost than electron beam lithography.

Nanoelectrode arrays based on low site density aligned carbon nanotubes

Abstract: Nanoelectrode arrays (NEAs) were fabricated from low site density aligned carbon nanotubes (CNTs). The CNTs were grown by plasma-enhanced chemical vapor deposition on Ni nanoparticles made by electrochemical deposition. Each nanotube is separated from the nearest neighbor by several micrometers. NEAs of 1 cm2 consisting of up to millions of individual nanoelectrodes, each with a diameter of 100 nm, were made by this nonlithography method. These carbon NEAs could be used as templates to fabricate other metal NEAs. Electrochemical characterization including cyclic voltammetry and square wave voltammetry were performed.

Dispersion and alignment of carbon nanotubes in polycarbonate

Abstract: Dispersion and alignment of carbon nanotubes in thermoplastic polymers such as polycarbonate have been studied. Dispersion was accomplished by mixing in a conical twin-screw extruder and alignment was carried out using a fiber-spinning apparatus. The effects of mixing time and fiber draw rates on dispersion and alignment were investigated. Uniform dispersions were produced with relatively short residence times in the extruder. Excellent alignment of carbon nanotubes in nanocomposite filaments was obtained when the fiber draw rate was greater than 70 m/min. The ability to closely control the dispersion and alignment of carbon nanotubes in polymers is expected to lead to the development of nanocomposites with desirable electronic and structural properties.

Density controlled carbon nanotube array electrodes

Abstract: CNT materials comprising aligned carbon nanotubes (CNTs) with pre-determined site densities, catalyst substrate materials for obtaining them and methods for forming aligned CNTs with controllable densities on such catalyst substrate materials are described. The fabrication of films comprising site-density controlled vertically aligned CNT arrays of the invention with variable field emission characteristics, whereby the filed emission properties of the films are controlled by independently varying the length of th CNTs in the aligned array within the film or by independently varying inter-tubule spacing of the CNTs within the array (site density) are disclosed. The fabrication of microelectrode arrays (MEAs) formed utilizing the carbon nanotrube material of the invention is also described.

Metal oxide nanostructures with hierarchical morphology

Abstract: The present invention relates generally to metal oxide materials with varied symmetrical nanostructure morphologies. In particular, the present invention provides metal oxide materials comprising one or more metallic oxides with three-dimensionally ordered nanostructural morphologies, including hierarchical morphologies. The present invention also provides methods for producing such metal oxide materials.

Method of producing a branched carbon nanotube for use with an atomic force microscope

Abstract: A method of producing a branched carbon nanotube (CNT) is disclosed. The branched CNT is used with an atomic force microscope having a cantilever and a tip and that is able to measure a surface of a substrate as well as an undercut feature of the substrate that protrudes from the surface. A catalytic material is deposited onto the tip of the microscope, and the catalytic material is subjected to chemical vapor deposition. This initiates growth of a primary branch of the branched carbon nanotube such that the primary branch extends from the tip. A secondary branch is then introduced to extend from the primary branch and produce the branched carbon nanotube. The primary branch interacts with the surface of the substrate and the secondary branch interacts with the undercut feature.

Individual free-standing carbon nanofibers addressable on the 50 nm scale

Abstract: We report on the fabrication of arrays of free-standing carbon nanofibers (CNFs) individually addressable on the 50 nm scale. The template for CNF growth consists of a set of tungsten leads patterned with a catalyst dot at the tip of each terminal. The fabrication process involves electron-beam lithography, projection photolithography, reactive ion etching, and dc plasma-enhanced chemical vapor deposition. Discharge power is found to drastically influence the morphology of CNFs grown off single catalyst dots.

Thermal Conductivity Reduction of SiGe Nanocomposites

Abstract: High figure of merit has been reported in superlattice structures in recent years mainly due to a large reduction in thermal conductivity, while maintaining or even enhancing the power factor. These findings suggest the possibility of using nanocomposites to reduce a material's thermal conductivity and thereby increasing the thermoelectric figure of merit. The current work reports on the thermal conductivity of various Si-Ge nanocomposites synthesized in a simple and straightforward process that allows one to exploit nanoscale physics while rapidly producing macroscale devices. Composite synthesis consisted of combining Si nanoparticles approximately 70 nm in diameter with micron-sized Ge particles in various atomic ratios via hotpressing to form macroscale samples. Room-temperature thermal conductivity was measured for each of the nanocomposite samples and compared with the values of SiGe bulk alloys. In this preliminary work, we observed a significant reduction in bulk thermal conductivity in such materials.

Plasma Coating and Enhanced Dispersion of Carbon Nanotubes

Abstract: Ultrathin polymer films have been deposited on both multi-wall and aligned carbon nanotubes using a plasma polymerization treatment. TEM experimental results showed that a thin film of polystyrene layer (several nanometers) was uniformly deposited on the surfaces of the nanotubes including inner wall surfaces of the multi-wall nanotubes. The coated multi-wall nanotubes were mixed in polymer solutions for studying the effects of plasma coating on dispersion. It was found that the dispersion of multi-wall carbon nanotubes in polystyrene composite was significantly improved. The deposition mechanisms and the effects of plasma treatment parameters are discussed.

Muon spin rotation in GdSr2Cu2RuO8: implications

Abstract: Muon spin rotation measurements are reported for GdSr2Cu2RuO8, a material with an onset temperature for superconductivity of about 45 K (which is virtually the same for its superconducting sister compounds Sr2YRu1− u Cu u O6 and Gd2− z Ce z Sr2Cu2RuO10). The data indicate two magnetic ordering transitions, at about 15 K and about 130 K, in addition to the Gd ordering transition known to occur at about 2.6 K. We tentatively attribute the 130 K transition to Ru and the 15 K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15 K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset T c for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr2Cu2RuO8 and Gd2− z Ce z Sr2Cu2RuO10 (and not Sr2YRu1− u Cu u O6) contain cuprate planes imply that ...

Muon spin rotation in GdSr 2 Cu 2 RuO 8 : implications

Abstract: Muon spin rotation measurements are reported for GdSr2Cu2RuO8, a material with an onset temperature for superconductivity of about 45K (which is virtually the same for its superconducting sister compounds Sr2YRu1 uCuuO6 and Gd2 zCezSr2Cu2RuO10). The data indicate two magnetic ordering transitions, at about 15K and about 130K, in addition to the Gd ordering transition known to occur at about 2.6K. We tentatively attribute the 130K transition to Ru and the 15K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset Tc for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr2Cu2RuO8 and Gd2 zCezSr2Cu2RuO10 (and not Sr2YRu1 uCuuO6) contain cuprate planes imply that the superconducting layers of all three compounds must be the common SrO layers, and not the cuprate planes (which do not occur in Cu-doped Sr2YRuO6). Otherwise the coincidence of onset temperatures must be an accident, and there must be at least two mechanisms of high-Tc superconductivity: one for doped Sr2YRuO6 and another for cuprate-plane superconductivity. Philosophical Magazine ISSN 1478–6435 print/ISSN 1478–6443 online # US Government http://www.tandf.co.uk/journals DOI: 10.1080/1478643031000156399 kkEmail: drh@physikon.net. Permanent address. }

Making carbon nanotube probes for high aspect ratio scanning probe metrology

Abstract: Carbon nanotubes (CNT) have exceptional mechanical strength at small diameters needed for measuring high aspect ratio features. Manually attached carbon nanotube atomic force microscopy probes have demonstrated exceptional longevity. Unfortunately, due to the manual attachment process, and the usually arbitrary diameter and length of the used CNT, such probes are not suitable for high aspect ratio critical dimension metrology (CDM). For reproducible and accurate CDM measurements precisely defined CNT probes are necessary. We are reporting about the progress made growing carbon nanotubes (CNT) directly on top of standard Si probes. The goal is to produce well-defined long lasting probes for CDM measurements in the <100 nm pitch range. Our efforts currently focus on manufacturing precisely aligned CNT having defined locations, diameters and lengths. This is accomplished by using plasma assisted chemical vapor deposition in combination with focused ion beam (FIB) patterned catalyst films. Our results demonstrate that it is possible to manufacture 1:10 aspect ratio CNT probes at <100 nm diameters.

Novel ZnO nanostructures

Abstract: A variety of novel ZnO nanostructures such as nanowires, nanowalls, hierarchical nanostructures with 6-, 4-, and 2-fold symmetries, nanobridges, nanonails have been successfully grown by a vapor transport and condensation technique. Doping both In and Sn into ZnO hierarchical nanostructures can be created. The 2-fold eutectic ZnO structures can also be created without any doping in the source. It was found that the hierarchical nanostructures can be divided into two categories: homoepitaxial and heteroepitaxial where heteroepitaxy creates the multifold nanostructures. The novel ZnO nanowalls and aligned nanowires on a-plane of sapphire substrate have also been synthesized and the photoluminescence is studied. The ZnO nanowires also demonstrated very good field emission properties, comparable to carbon nanotubes. These nanostructures may find applications in a variety of fields such as field emission, photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength and multifunctional nanocomposites, etc. that require not only high surface area but also structural integrity.

Growth and Characterizations of Well-Aligned Carbon Nanotubes

Abstract: Large arrays of well-aligned carbon nanotubes are first made possible on substrates in 1998 by plasma enhanced chemical deposition [1, 2] in which the diameter and length of each carbon nanotube are under control, but not the growth angle, location, nor the spacing between them. Soon after, the titled growth has been achieved by controlling the plasma direction using the same growth technique [3], Almost at the same time, the control of location and spacing of the nanotubes have been accomplished using electron beam (e- beam) lithography to pattern the nickel dots first at where they are needed and then to grow the carbon nanotubes using the same growth technique [4, 5]. However, e-beam is not possible to be commercialized for large scale. Therefore, alternative cheap and scalable technique is sought. Fortunately, the catalytic dots have been fabricated by electrochemistry and excellent aligned carbon nanotubes arrays have been grown [6]. Due to the nature of electrochemistry, the control on location of each nanotube is lacking. For applications that do not require the pre-determined location of each nanotube such as regular electron source, the arrays grown using the dots by electrochemistry is good enough. However, for applications that do require the pre-determined location of each nanotube such as microscopic probing tips, nanophotonics, etc., the control of location of each nanotube is crucial. Recently, we have been successful to grow large arrays of carbon nanotubes with diameter, length, location, and spacing under control by a simple and scalable technique, nanosphere lithography [7]. Since the very first report on large arrays of well-aligned carbon nanotubes, numerous papers have used the same or a slightly modified technique to grow aligned carbon nanotube arrays by either DC or microvave plasma CVD [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18].

Optical properties of honeycomb arrays of multiwalled carbon nanotubes

Abstract: Carbon nanotubes (CNT) have been grown in a honeycomb configuration on silicon substrates using nanosphere self-assembly and plasma enhanced chemical vapor deposition. The optical properties of the arrays were also studied. Diffraction efficiency was found to be a function of the wavelength, angle of incidence and state of polarization of incident light. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicates that these materials may find many uses in the field of optoelectronics. In addition to their optical properties, periodic CNT arrays have a host of other unique electromagnetic and mechanical properties that may be exploited for numerous applications. Polarization measurements indicate that the intensity of both the diffracted light and diffusely scattered light is dependent on wavelength and angle of incidence. These arrays not only reflect and diffract light, but can also have a photonic band gap in, or around, the visible frequency range. The precise frequency location and size of this gap can be controlled by the structural and material parameters of the arrays.