Showing papers by "Zhifeng Ren published in 2006"

Broadband ZnO Single-Nanowire Light-Emitting Diode

Abstract: We present a novel technique for reliable electrical injection into single semiconductor nanowires for light-emitting diodes and lasers. The method makes use of a high-resolution negative electron-beam resist and direct electron-beam patterning for the precise fabrication of a metallic top contact along the length of the nanowire, while a planar substrate is used as a bottom contact. It can be applied to any nanowire structure with an arbitrary cross section. We demonstrate this technique by constructing the first zinc oxide single-nanowire light-emitting diode. The device exhibits broad sub-bandgap emission at room temperature.

Superplastic carbon nanotubes

Abstract: Conditions have been discovered that allow extensive deformation of rigid single-walled nanotubes.

Aligned Ultralong ZnO Nanobelts and Their Enhanced Field Emission

Abstract: One-dimensional (1D) semiconducting nanoscale materials have attracted considerable attention because of their importance in understanding the fundamental properties of low dimensionality in materials as well as in nanodevice applications. Many methods, including vapor–liquid–solid (VLS), vapor–solid (VS), and solution-based, have been developed to synthesize 1D semiconducting nanoscale materials such as nanoscale wires, belts, rods, tubes, and needles. Usually, these methods require templates/catalysts and tedious operational procedures. Here, we demonstrate a new strategy for the growth of aligned ultralong ZnO nanobelts, yielding an average length of 3.3 mm and widths up to 6 lm, on metal substrates in a one-step process via molten-salt-assisted template-free thermal evaporation. These ultralong nanobelts show enhanced field emission. The electric field for an emission current density of 1 mA cm is 2.9 V lm, the lowest value ever reported for pure 1D ZnO nanostructures grown on flat surfaces, corresponding to a field-enhancement factor of about 1.4 × 10. This approach is simple, efficient, and inexpensive, which significantly facilitates device fabrication. By combining a general molten-salt process, which is usually used to prepare micrometer-scale ceramic powders (although it was also used for the synthesis of ZnO nanorods in a thermal evaporation process), we have designed a new approach, molten-salt-assisted thermal evaporation, and we demonstrate that this approach can produce aligned ultralong ZnO nanobelts over a large area. The key point of this new approach is the evaporation of Zn metal powder in a liquid environment of molten sodium chloride (NaCl) salt. A side-view camera photograph of the as-grown ZnO nanobelts on the Au substrate is shown in Figure 1a, indicating that the nanobelts can grow to several millimeters in length. Figure 1b shows a top-view optical microscopy photograph, demonstrating that the ZnO nanobelts are also transparent under an optical microscope. A higher-magnification optical microscopy image of the side-view is shown in Figure 1c, indicating nominal, though imperfect, alignment. Figure 2 shows field-emission scanning electron microscopy (SEM) images of the as-grown ZnO nanobelts under different magnifications. The low-magnification image shown in FigC O M M U N IC A IO N

Real-time observation of tubule formation from amorphous carbon nanowires under high-bias Joule heating.

Abstract: The tubule formation process from amorphous carbon nanowires under high-bias-caused Joule heating was observed in real time in a high-resolution transmission electron microscope. The crystallization of the amorphous carbon nanowires occurred in two distinct ways: the formation of tubular graphitic basal planes parallel to the nanowire axis on the surface and the formation of nano-onions in the interior of the nanowire. The tubule formation mechanism is a process of solid-state atom diffusion at high temperatures. Energetically, the tubule formation is caused by the exceptionally low surface energy of the (0002) plane of graphite. Higher input power to the amorphous nanowires generally leads to improved graphitization and, in turn, to increased conductance. The results suggest that nanotube formation in the arc-discharge growth process may involve the formation and crystallization of amorphous carbon.

Apparatus and methods for solar energy conversion using nanoscale cometal structures

Abstract: An apparatus and methods for solar conversion using nanoscale cometal structures are disclosed herein. The cometal structures may be coaxial and coplanar. A nanoscale optics apparatus for use as a solar cell comprises a plurality of nanoscale cometal structures each including a photovoltaic material located between a first electrical conductor and a second electrical conductor. A method of fabricating solar cells comprises preparing a plurality of nanoscale planar structures; coating a plurality of planar surfaces of the plurality of planar structures with a photovoltaic semiconductor while leaving space between the plurality of planar surfaces; and coating the photovoltaic semiconductor with an outer electrical conductor layer, wherein a portion of the outer electrical conductor layer is located between the planar structures to form coplanar structures.

Kink formation and motion in carbon nanotubes at high temperatures.

Abstract: We report that kink motion is a universal plastic deformation mode in all carbon nanotubes when being tensile loaded at high temperatures. The kink motion, observed inside a high-resolution transmission electron microscope, is reminiscent of dislocation motion in crystalline materials: namely, it dissociates and multiplies. The kinks are nucleated from vacancy creation and aggregation, and propagate in either a longitudinal or a spiral path along the nanotube walls. The kink motion is related to dislocation glide and climb influenced by external stress and high temperatures in carbon nanotubes.

Apparatus and methods for optical switching using nanoscale optics

Abstract: An apparatus and methods for optical switching using nanoscale optics are disclosed herein. A nano-optics apparatus for use as an optical switch includes a metallic film having a top surface, a bottom surface and a plurality of cylindrical channels containing a dielectric material, the metallic film acting as an outer electrode; and an array of non-linear optical components penetrating the metallic film through the plurality of cylindrical channels, the array acting as an array of inner electrodes.

Apparatus and methods for manipulating light using nanoscale cometal structures

Abstract: An apparatus and methods for manipulating light using nanoscale cometal structures are disclosed. A nanoscale optics apparatus (100) for manipulating light includes a plurality of nanoscale cometal structures each comprising a dielectric material (180) located between a first electrical conductor (120) and a second electrical conductor (160). A method for fabricating a nanoscale optics apparatus for manipulating light includes preparing a plurality of nanoscale planar structures; coating a plurality of planar surfaces of the plurality of planar structures with a dielectric while leaving space between the plurality of planar surfaces; and coating the dielectric with an outer electrical conductor layer, wherein the portion of the outer electrical conductor layer is located between the planar structures to form coplanar strucutres.

Field emission of silicon nanowires

Abstract: Field emission of single crystal silicon nanowires of 100nm in diameter grown at 480°C from silane using Au as catalyst has been investigated. An emission current density of 1mA∕cm2 over a 0.2cm2 area was obtained at an electric field of 3.4V∕μm with a turn-on field of 2V∕μm at 0.01mA∕cm2. The annealing of the as-grown samples at 550°C in vacuum has drastically improved the field emission performance. The low growth and annealing temperatures make the process applicable to glass substrates.

Apparatus and methods for solar energy conversion using nanocoax structures

Abstract: An apparatus and method for solar conversion using nanocoax structures are disclosed herein A nano-optics apparatus (100) for use as a solar cell comprises a plurality of nano-coaxial structures comprising an internal conductor (120) surrounded by a semiconducting material (180) coated with an outer conductor (160); a film (140) having the plurality of nano-coaxial structures; and a protruding portion (110) of the an internal conductor (120) extending beyond a surface of the film (140) A method of fabricating a solar cell comprises coating a substrate (190) with a catalytic material; growing a plurality of carbon nanotubes (120) as internal cores of nanocoax units on the substrate (190); oxidizing the substrate (190); coating with a semiconducting film (180); and filling with a metallic medium(160) that wets the semiconducting film (180) of the nanocoax units

Shape evolution of lead telluride and selenide nanostructures under different hydrothermal synthesis conditions.

Abstract: Face-open nanoboxes of lead telluride and selenide have been synthesized by a simple hydrothermal method. Nano- and microcrystals of various morphologies, including microflowers, semimicroflowers, cubic nanoparticles, etc., have also been observed at different synthesis conditions. Temperature, time, and concentrations of various reactants play a major role in controlling the morphology and shape evolution of the product. This simple synthesis technique for the growth of various nano- and microstructures opens a new route to prepare hierarchical structures of a variety of binary semiconducting materials in a large quantity. A possible growth mechanism of such nanoand microstructures has been proposed.

Enhancement of field emission of aligned carbon nanotubes by thermal oxidation

Abstract: To improve the field emission current density of aligned carbon nanotubes grown by thermal chemical vapor deposition, postgrowth thermal annealing was conducted at 850°C for 1h in vacuum plus at 465°C for 2h in air. It was found that the highest field emission current density significantly improved by a factor of 4 (from 19to79mA∕cm2), which is due to the substantial increase of the emitting area of the carbon nanotubes after annealing. This result is important for applications of using carbon nanotubes as high current electron sources, microwave devices, flat panel displays, etc.

Field Emission of Silicon Nanowires

Abstract: Field emission of single crystal silicon nanowires of 100nm in diameter grown at 480°C from silane using Au as catalyst has been investigated. An emission current density of 1mA∕cm2 over a 0.2cm2 area was obtained at an electric field of 3.4V∕μm with a turn-on field of 2V∕μm at 0.01mA∕cm2. The annealing of the as-grown samples at 550°C in vacuum has drastically improved the field emission performance. The low growth and annealing temperatures make the process applicable to glass substrates.

Visible light diffraction studies on periodically aligned arrays of carbon nanotubes: Experimental and theoretical comparison

Abstract: We have investigated visible light diffraction on honeycomb arrays of aligned carbon nanotubes grown on nickel nanoparticles prepared using the nanosphere lithography. A monolayer of 980nm polystyrene spheres was used as the mask for the deposition of nickel nanoparticles from which carbon nanotubes of 100nm in diameter and up to a couple of microns in length were grown. We show that a standard theory of diffraction from point scatterers explains all the observed diffraction features including Bragg’s law and the strong enhancement of the second and fifth order diffraction spots.

Nanoscale optical microscope

Abstract: Nanoscale optical probes for use with nanoscale optical microscopy are disclosed herein. A nanoscale optical probe (200) for use with a near-field scanning optical microscope includes an inner conductor (230) having a top end, a bottom end, and a body; a dielectric material (240) engaging the inner conductor (230); and an outer conductor (250) engaging the dielectric material (240), wherein the inner conductor (230) is longer at a tip surface (210) of the probe (200) than the dielectric material (240) and the outer conductor (250).

Apparatus and methods for nanolithography using nanoscale optics

Abstract: An apparatus and methods for nanolithography using nanoscale optics are disclosed herein. Submicron-scale structures may be obtained using standard photolithography systems (710) with a de-magnifying lens (600). A de-magnifying lens (600) for use in a standard photolithography system (710) includes a film (670) having a top surface (610), a bottom surface (620) and a plurality of cylindrical channels (160) containing a dielectric material (280); and an array of carbon nanotubes (640) penetrating the film (670) through the plurality of cylindrical channels (160), wherein an image on the top surface (610) of the film (670) is converted into a de-magnified image on the bottom surface (620) of the film (670) by the carbon nanotubes (640).

Varied morphology carbon nanotubes

Abstract: The present invention describes the preparation of carbon nanotubes of varied morphology, catalyst materials for their synthesis. The present invention also describes reactor apparatus and methods of optimizing and controlling process parameters for the manufacture carbon nanotubes with pre-determined morphologies in relatively high purity and in high yields. In particular, the present invention provides methods for the preparation of non-aligned carbon nanotubes with controllable morphologies, catalyst materials and methods for their manufacture.

High throughput growth of zinc oxide nanowires from zinc powder with the assistance of sodium chloride

Abstract: Sodium chloride (NaCl) was found to be very helpful in producing single crystal zinc oxide (ZnO) nanowires in gram quantities. The growth involves heating the mixture of zinc powder and NaCl to 600-700 degrees C in flowing gases of oxygen and argon. A conversion efficiency of 70-80% (Zn to ZnO) was achieved when NaCl was used, and 5-10% without NaCl. The NaCl was completely removed by soaking and rinsing the mixture in water a few times. Photoluminescence spectra using excitation of 325 nm showed a very strong emission only in the visible frequency range, indicating that the surface states dominate the emission.

Methods for growing carbon nanotubes on single crystal substrates

Abstract: Methods for growing carbon nanotubes on single crystal substrates are disclosed. A method of producing a nanostructure material comprises coating a single crystal substrate with a catalyst film to form a catalyst coated substrate; annealing the catalyst film by supplying a first promoter gas to the catalyst coated substrate at a first temperature and a first pressure; and supplying a second promoter gas and a carbon-source gas to the catalyst coated substrate in a substantially water-free atmosphere at a second pressure and a second temperature for a time period to cause growth of nanostructures on the catalyst coated substrate. The nanostructure material is used in various applications.

The complex optical response of arrays of aligned multiwalled carbon nanotubes

Abstract: The optical properties of periodic and nonperiodic arrays of aligned multiwalled carbon nanotubes are presented. Experimental analysis indicates a complex optical response that is attributed to both the individual carbon nanotube scatterers and also to the array ensembles. These studies indicate that by controlling the geometry and spacing of the arrays, it is possible to create structures that respond very strongly to specific wavelengths or bands of wavelengths. Structures such as these may form the basis for numerous applications in energy conversion. This would include highly efficient solar energy conversion as well as sensitive, finely tuned detectors that can respond to predetermined wavelength bands ranging from the ultraviolet to the infrared region. Experimental, theoretical and modeled results are discussed as they apply to these applications. Challenges and issues are discussed.

Broadband ZnO Single Nanowire Light Emitting Diode

Abstract: We present a novel technique for reliable electrical injection into single semiconductor nanowires for light-emitting diodes and lasers. The method makes use of a high-resolution negative electron-beam resist and direct electron-beam patterning for the precise fabrication of a metallic top contact along the length of the nanowire, while a planar substrate is used as a bottom contact. It can be applied to any nanowire structure with an arbitrary cross section. We demonstrate this technique by constructing the first zinc oxide single-nanowire light-emitting diode. The device exhibits broad sub-bandgap emission at room temperature.

Characterization and thermoelectric properties of Si-Ge nanocomposite

Abstract: Submitted for the MAR06 Meeting of The American Physical Society Characterization and thermoelectric properties of Si-Ge nanocomposite1 DEZHI WANG, WENZHONG WANG, SHUO CHEN, JIANYU HUANG, ZHIFENG REN, Boston College, Dept. of Physics, HOHYUN LEE, GANG CHEN, MIT, Dept. of mechanical Engineering, MING TANG, M. S. DRESSELHAUS, MIT, Dept. of Electrical Engineering, PAWAN GOGNA, JEAN-PIERRE FLEURIAL, Jet Propulsion Laboratory, BRADLEY KLOTZ, Dynamic Science, Inc. — Low dimension is one of the most promising directions to search for high-ZT thermoelectric materials. It has been predicted by theory and proved by experiments that structures such as quantum well and superlattice can increase ZT by several times vs the corresponding bulk materials. However, it is very difficult to manufacture those low dimensional structures in large scale for bulk applications. To realize those principles, we have designed and synthesized successfully a new structure so called Si-Ge nanocomposite—nano Silicon particles in SiGe alloy matrix. The Si-Ge nanocomposite was made of nano silicon and germanium particles by a unique hot-press procedure. The samples are being characterized by TEM, SEM, XRD and thermoelectric property measurements. The results will be reported in detail.