Search

We report structure-property correlations in single-wall carbon nanotube (SWNT) fibers, among electrical, thermal, and chemical parameters with respect to stretch-induced preferential SWNT alignment along the fiber axis. Purified HiPco (high-pressure CO) conversion tubes are dispersed with the aid of an anionic surfactant and coagulated in the co-flowing stream of an adsorbing polymer. The fibers are then dried, rewetted under tensile load, and redried to improve the alignment. Complete removal of the polymer was assured by annealing in hydrogen at 1000°C. The degree of alignment was determined by x-ray scattering from individual fibers using a two-dimensional detector. The half width at half maximum describing the axially symmetric distribution of SWNT axes decreases linearly from 27.5° in the initial extruded fiber to 14.5° after stretching by 80%. The electrical resistivity ρ at 300K decreases overall by a factor ∼4 with stretching, for both as-spun composite and polymer-free annealed fibers. However, ...

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 T fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both ...

Wurtzite gallium nitride nanostructures were synthesized by catalyst-free thermal reaction of gallium oxide and ammonia. Depending on ammonia flow rate the resulting morphology varied. At 75 sccm only nanowires were obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. From scanning electron microscopy and transmission electron microscopy, both thin smooth and thick corrugated nanowires were observed. The growth orientations of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (), while the corrugated nanowires and the large polyhedra grew parallel to . We propose a model to explain these variations of morphology and growth orientation in terms of the Ga/N ratio and the different characteristic length of {0001} polar surface in each morphology.

For single-wall carbon nanotube (SWNT) films deposited from suspension onto filter membranes, or by drop casting or spin coating onto flat substrates, the tube axes lie preferentially in the film plane. Using x-ray scattering and a two-dimensional detector, we show that this out-of-plane mosaic spread can be easily and accurately quantified. It varies significantly with deposition conditions, and the aligning effects of deposition and external force in the film plane (e.g., magnetic field) are additive. Films from well-dispersed tubes show better alignment than from poor dispersions. The finite out-of-plane mosaic in C60@SWNT films enables quantitative separation of one-dimensional diffraction (chains of C60 peas) from the 2D rope lattice diffraction.

Laser ablation has been used to grow silicon nanowires with an average silicon crystal core diameter of 6.7 nm±2.9 nm surrounded by an amorphous SiOx sheath of 1–2 nm, the smallest silicon wires reported in the literature. Imaging, chemical, and structural analysis of these wires are reported. Due to the growth temperature and the presence of calcium impurities and trace oxygen, two distinct types of wires are found. They appear to grow by two different processes. One requires a metal catalyst, the other is catalyzed by oxygen. Suggestions for controlled synthesis based on these growth mechanisms are made.

We report measurements of electrical resistivity, thermopower, and thermal conductivity of highly C60-filled single-wall carbon nanotubes and unfilled controls, from 1.5 to 300 K. The data suggest that the C60 chains provide additional conductive paths for charge carriers, increase the rate of phonon scattering, and block interior sites from sorbing other gas molecules.

Ni/Nb and Nb/Ni bilayers on Si(100), Ni/NbSi2/Si(100) and Nb/Ni/Si/SiO2/Si(100) were produced by rf sputtering and annealed by rapid thermal processing. Thereafter, phase sequence and morphology were deduced by Auger electron spectroscopy and x‐ray diffraction. The first three cases lead to phase separation of the binary compounds NbSi2 and NiSi2. Conversely when SiO2 is used as a buffer to limit the consumption of Si, the ternary phase Nb4Ni4Si7 or Nb4Ni4Si7+NbSi2 can be produced depending on the amount of Ni available. These observations are consistent with the Nb‐Ni‐Si phase diagram, in which phase separation occurs in a Si‐rich environment whereas the ternary phase is stable in a Si‐poor environment.

The formation of NbSi2 from Nb/Si(100) is studied using rf sputtering and rapid thermal processing. For a long sputter‐etching time (10 min) of the Si substrate, NbSi2 is first formed at 720 °C. The lowest resistivity attained is 38 μΩ cm, one of the best for refractory metal silicides. The variation of the sheet resistance versus annealing temperature is correlated to grain growth. The grain size is deduced from the x‐ray coherence length of the NbSi2(111) peak which sharpens and increases in intensity for T≳720 °C. For shorter sputter etching time (5 min), NbSi2 only forms above 895 °C. This is attributed to the native oxide at the Si interface. An estimated ternary phase diagram of Nb‐Si‐O shows that Nb and SiO2 coexist at this temperature. Unlike other refractory metal silicides, oxygen in the Nb film is expelled as the silicide is formed.

We report measurements of thermal conductivity κ on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97to181nm grown by thermal chemical vapor deposition. We observed unexpectedly small κ values, in the range of 13–19W∕mK at 300K, with very weak diameter dependence. We also observe unusual power law κ∼Tn behavior with n=1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1–1 and 0.01–0.1at.%, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low κ and the T1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path.

High performance ambipolar silicon nanowire (SiNW) transistors were fabricated. SiNWs with uniform oxide sheath thicknesses of 6–7nm were synthesized via a gas-flow-controlled thermal evaporation method. Field effect transistors (FETs) were fabricated using as-grown SiNWs. A two step annealing process was used to control contacts between SiNW and metal source and drain in order to enhance device performance. Initially p-channel devices exhibited ambipolar behavior after contact annealing at 400°C. Significant increases in on/off ratio and channel mobility were also achieved by annealing.

High-quality silicon nanowires (SiNWs) were synthesized via a thermal evaporation method without the use of catalysts. Scanning electron microscopy and transmission electron microscopy showed that SiNWs were long and straight crystalline silicon with an oxide sheath. Field effect transistors were fabricated to investigate the electrical transport properties. Devices on as-grown material were p-channel with channel mobilities 1–10cm2V−1s−1. Postgrowth vapor doping with bismuth converted these to n-channel behavior.

Nanopatterned Pt by Ga+ focused ion beam (FIB) decomposition of an organometallic precursor forms low resistance ohmic contacts on 40–70nm diameter GaN nanowires (NWs) grown by thermal reaction of Ga2O3 and NH3. With no intentional doping, the wires are presumed to be n type. Thus, the linear I‐V behavior is surprising since evaporated Pt usually forms Schottky barriers on n GaN. Ohmic behavior was not obtained for 130–140 diameter wires, even with thicker Pt contacts. A second application of FIB Pt nanopatterning was demonstrated by position-selective growth of GaN NWs on Pt catalyst dots. NW locations and density are defined by the position, size, and thickness of the Pt deposit. Combining these techniques provides a versatile platform for nanostructure research and development.

Solid‐state physicists are turning more and more to complex synthetic materials in their search for novel phenomena and potentially useful properties. Many of these materials are highly anisotropic, so that interatomic interactions can for all practical purposes be neglected along one or two crystal axes. One of the oldest classes of prototype systems for exploring phenomena predicted to occur in two‐dimensional systems are graphite intercalation compounds. These consist of stacks of one or more layers of hexagonally arrayed carbon atoms, alternating with monolayers of guest atoms or molecules. Striking changes in the properties of both host and guest result from intercalation. In addition to their quasi‐two‐dimensional behavior, fundamental interest centers on their variable anisotropy, which results from the fact that the strength of the interlayer interactions depends on the nature of the intercalated species.

We describe a thin film technique of fabricating samples for surface barrier electroreflectance measurements. Aluminum oxide forms the dielectric spacer and semitransparent nickel is used for the field electrode. The resulting MOS capacitor is transparent from below 0.4 eV to at least 7.5 eV and has no low temperature limitation. Spectra have been obtained on a large class of semiconductor materials using this technique. Qualitative features of the optical response suggest that the surface potential is not clamped by slow surface states, even at room temperature. This method is therefore suitable for quantitative experiments in which the surface field is determined by an auxiliary measurement of surface conductance or capacitance. Application of these preparation techniques to other experiments is suggested.