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We present a detailed study on the growth process of cupric oxide (CuO) nanowires by thermal oxidation. The morphology of nanowires, obtained at different oxidation temperatures and times, was determined. The diameter of nanowires was found to depend linear on temperature whereas the time dependence of their length is modeled by a parabolic law. The results suggest that CuO nanowires are formed as a result of the competition between grain boundary and lattice diffusion of Cu atoms across a Cu2O layer. Electrical characterization of the nanowires was also performed. A field effect transistor was produced with an isolated nanowire showing p-type characteristics. The resistivity, mobility, and density of carriers were calculated. Nanowire growth by thermal oxidation is very simple and has great potential to be used for large scale production; this opens possibilities for various kinds of application. [ABSTRACT FROM AUTHOR]

We present a detailed study of the growth of multiwall and single-wall carbon nanotubes (SWCNTs) by chemical-vapor deposition using a thin-film triple metal (Al/Fe/Mo) catalyst. Using Nanoauger spectroscopy, a full map of the metals in the sample surface is constructed and their evolution followed at different deposition temperatures. During the formation of SWCNTs at high temperatures (∼1000 °C), the initial iron layer (∼1 nm) is transformed into nanosized particles at the surface. In addition, the Al layer also plays a critical role during the annealing process by being altered into AlxOy particles. These particles act as a suitable underlayer to stabilize the nanosized Fe catalyst for nanotube growth. We also show that it is possible to resolve SWCNTs by mapping the areal intensity of carbon KVV Auger electrons. [ABSTRACT FROM AUTHOR]

Hydrogenated carbon-nitride films (CN[sub x]:H) were deposited by evaporation of aza-adenine (8-aza-6-aminopurine, C[sub 4]N[sub 6]H[sub 4]) and irradiated during film growth by a nitrogen ion beam with energies of 25, 50, and 100 eV. The relationship between the deposition conditions and the chemical bonding structure was investigated by x-ray photoelectron and infrared spectroscopies. The mass density of the films was estimated from the C 1s plasmon energy and the optical properties were examined by ultraviolet-visible spectroscopy. The infrared spectra indicate that in the case of evaporated films, the molecular structure of aza-adenine is preserved without substantial nitrogen loss (N/Capprox. 1.3). With increasing energy of the assisting nitrogen ion beam, the structure is subsequently transformed into an amorphous phase. This transition is accompanied by an increasing nitrogen loss down to N/Capprox. 0.8, a decrease of the optical band gap, and an increase of the film density. The known molecular structure of aza-adenine permits the reconstruction of the C 1s and N 1s core-level spectra with deconvoluted spectral components representing the different bonding environments of C and N atoms present in the molecule. The spectral components are identified by comparing the experimental data with theoretical calculations of the corresponding binding energies. By analysis of the spectral changes induced by the nitrogen ion beam with increasing energies, it is possible to study the microstructural modifications of the material. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the coefficient of thermal expansion and biaxial modulus of some amorphous semiconductors and metallic thin films. Experimental procedure; Results and discussion; Conclusions.

Combining experiment and theory, we investigate how a naturally created heterojunction (pn junction) at a graphene and metallic contact interface is modulated via interaction with molecular hydrogen (H2). Due to an electrostatic interaction, metallic electrodes induce pn junctions in graphene, leading to an asymmetrical resistance in electronic transport for electrons and holes. We report that the asymmetry in the resistance can be tuned in a reversible manner by exposing graphene devices to H2. The interaction between the H2 and graphene occurs solely at the graphene-contact pn junction and induces a modification on the electrostatic interaction between graphene and metallic contacts. We explain the experimental data with theory providing information concerning the length of the heterojunction and how it changes as a function of H2 adsorption. Our results are valuable for understanding the nature of the metal-graphene interfaces and have potential application for selective sensors of molecular hydrogen. [ABSTRACT FROM AUTHOR]

We report on gate hysteresis of resistance in high quality graphene/hexagonal boron nitride (h-BN) devices. We observe a thermally activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375 K. In order to investigate the origin of the hysteretic phenomenon, we compare graphene/h-BN heterostructure devices with SiO2/Si back gate electrodes to devices with graphite back gate electrodes. The gate hysteretic behavior of the resistance is present only in devices with an h-BN/SiO2 interface and is dependent on the orientation of the applied gate electric field and sweep rate. We describe a phenomenological model which captures all of our findings based on charges trapped at the h-BN/SiO2 interface. Such hysteretic behavior in graphene resistance must be considered in high temperature applications for graphene devices and may open new routes for applications in digital electronics and memory devices. [ABSTRACT FROM AUTHOR]

The initial stages of graphene chemical vapor deposition at very low pressures (<10-5Torr) were investigated. The growth of large graphene domains (~up to 100 μm) at very high rates (up to 3 μm² s-1) has been achieved in a cold-wall reactor using a liquid carbon precursor. For high temperature growth (>900°C), graphene grain shape and symmetry were found to depend on the underlying symmetry of the Cu crystal, whereas for lower temperatures (<900°C), mostly rounded grains are observed. The temperature dependence of graphene nucleation density was determined, displaying two thermally activated regimes, with activation energy values of 6 ± 1 eV for temperatures ranging from 900°C to 960°C and 9 ± 1 eV for temperatures above 960±C. The comparison of such dependence with the temperature dependence of Cu surface self-diffusion suggests that graphene growth at high temperatures and low pressures is strongly influenced by copper surface rearrangement. We propose a model that incorporates Cu surface self-diffusion as an essential process to explain the orientation correlation between graphene and Cu crystals, and which can clarify the difference generally observed between graphene domain shapes in atmospheric-pressure and lowpressure chemical vapor deposition. [ABSTRACT FROM AUTHOR]

This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitaxial and demonstrates the crystallographic relation between the two using grazing incidence synchrotron x-ray diffraction (XRD). The in-plane lattice match between the sapphire and the nanowires was directly probed by using XRD at grazing angles of incidence, where the lattice match between the (0001) plane of the sapphire and the (11-20) plane of the ZnO were observed simultaneously. It will also be shown that gold acts as a catalyst to initiate ZnO nanowire growth, but it does not interfere with the epitaxial mechanism between the nanowires and the sapphire substrate. [ABSTRACT FROM AUTHOR]

Diamond-like carbon (DLC) films have always had high intrinsic stress due to their metastable structure and the fine balance between film density and bond stability. We show the effects of high intrinsic stress on the electron field emission performance, where a lower electric field for emission is recorded with increasing stress in the DLC films. In addition to examining “as deposited” films with different magnitudes of intrinsic stress, we subject the DLC films to external pressure by physically bending the a-C/silicon substrates. The result is a phenomenon where electrons are “squeezed” out of the films, and can be applied to the fabrication of stress sensors. [ABSTRACT FROM AUTHOR]

Neutral species and positive ions were extracted directly from a C2H2:NH3 plasma used to grow vertically aligned carbon nanotubes (CNTs) and analyzed by mass spectrometry. We observe that NH3 suppresses C2H2 decomposition and encourages CNT formation. We show that the removal of excess carbon, essential for obtaining nanotubes without amorphous carbon deposits, is achieved through gas phase reactions which form mainly HCN. We determine an optimum C2H2:NH3 gas ratio which is consistent with previous observations based upon postdeposition analysis. We find, in contrast to thin film growth by plasma-enhanced chemical vapor deposition, that the optimum condition does not correspond to the highest level of ionization. We also provide evidence that C2H2 is the dominant precursor for CNTs in our experiments. [ABSTRACT FROM AUTHOR]

The coefficient of thermal expansion (CTE) of hydrogenated amorphous carbon (a-C:H) was investigated as a function of the concentration of sp[sup 2] hybridization. The CTE, determined using the thermally induced bending technique, depends on the concentration of sp[sup 2] bonded carbon, increasing to the value of graphite as the sp[sup 2] concentration approaches 100%. By using a combination of the thermally induced bending technique and nanohardness measurements, we extract separately the Young’s modulus and Poisson’s ratio of the a-C:H films as function of the sp[sup 2] concentration. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Traditionally, the emission of electrons from materials have been explained using either the FowlerNordheim emission mechanism where high electric fields are used to extract electrons from surfaces or using conventional thermal emission where high currents are used to "boil" off electrons to vacuum. In this letter, we propose an alternative mechanism for electron emission from highly compressive thin films based on stress-induced "band structure" modification of nano-ordered sp² regions in the thin films. Experimental results are recorded which show that the localized compressive stress governs electron emission in the amorphous carbon thin films studied here rather than the surface nanostructures/features or the diamond-like sp³ hybridized bond component. This analysis is in agreement with the concept of an internal or nongeometric field enhancement from sp² nanostructures giving rise to high dielectric inhomogeneity within the carbon thin film. The results presented could be extended to explain the anomalous field emission behavior of carbon nanotubes. [ABSTRACT FROM AUTHOR]

Analysis of hard a-C:H films with low stress prepared by methane plasma decomposition is reported. Films with hardness as high as 14 GPa and stress as low as 0.5 GPa were obtained. These films have a high Raman I[sub d]/I[sub g] ratio (∼1.0), and small Tauc’s band gap (∼0.4 eV). This letter also supplies strong evidence that the subimplantation deposition model, used to explain the formation of ta-C and ta-C:H films, is also valid for a-C:H films deposited by methane plasma decomposition. It is proposed that the rigidity of the films is basically provided by a matrix of dispersed cross-linked sp[sup 2] sites, in addition to the contribution of the sp[sup 3] sites. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]