F. Piazza

Numerical study of the electrostatic field gradients present in various planar emitter field emission configurations relevant to experimental research

The generally assumed validity of the V∕dCA approximation for the cathode surface electric field under commonly employed electron field emission configurations was studied. Using appropriate typical dimensions for each configuration, the magnitude of the electric field over the cathode area under the probe was obtained, and especially near critical (i.e., sharp) regions that could lead to residual gas ionization, dielectric breakdown and emission of electrons from unintended areas. The results indicate that the V∕dCA approximation is far from being universally applicable to all the field emission measuring configurations. In particular, the cylindrical probe anode with flat tip gives the most uniform ES, which nearly equals V∕dCA over most of the cathode area under the probe. Spherical and hemispherical probes, on the other hand, result in ES close to V∕dCA only locally near the center, and much lower anywhere around the center. Moreover, the parallel-plate configurations lead to significantly detrimental...

Effects of a nanocomposite carbon buffer layer on the field emission properties of multiwall carbon nanotubes and nanofibers grown by hot filament chemical vapor deposition

The electron field emission properties of multiwall carbon nanotubes (MWCNTs) grown on sulfur-incorporated nanocomposite carbon (n-C:S) buffer layer were investigated. Both the MWCNTs and the n-C:S films were synthesized in a hot filament chemical vapor deposition system at relatively low methane concentrations. The n-C:S buffer layer provides good contact and adhesion to the Mo substrate and good contact and interface to the MWCNTs. The presence of this buffer layer was shown to improve the reproducibility and stability of the field emission behavior of MWCNTs. The turn-on field (EC) varies as much as 1.1V∕μm after high current density operation when there is no buffer layer, but variations up to only 0.3V∕μm are observed when the buffer layer is present. These results are interpreted in terms of the n-C:S buffer layer role, providing good adhesion and contact to the substrate side and to the MWCNTs, hence ensuring a high density of continuous paths for electrons from the substrate to the MWCNTs.

High-yield synthesis of stoichiometric boron nitride nanostructures

Boron nitride (BN) nanostructures are structural analogues of carbon nanostructures but have completely different bonding character and structural defects. They are chemically inert, electrically insulating, and potentially important in mechanical applications that include the strengthening of light structural materials. These applications require the reliable production of bulk amounts of pure BN nanostructures in order to be able to reinforce large quantities of structural materials, hence the need for the development of high-yield synthesis methods of pure BN nanostructures. Using borazine (B3N3H6) as chemical precursor and the hot-filament chemical vapor deposition (HFCVD) technique, pure BN nanostructures with cross-sectional sizes ranging between 20 and 50 nm were obtained, including nanoparticles and nanofibers. Their crystalline structure was characterized by (XRD), their morphology and nanostructure was examined by (SEM) and (TEM), while their chemical composition was studied by (EDS), (FTIR), (EELS), and (XPS). Taken altogether, the results indicate that all the material obtained is stoichiometric nanostructured BN with hexagonal and rhombohedral crystalline structure.

Nonlinear effects in collision cascades and high energy shock waves during ta-C:H growth

The surface topography of hydrogenated tetrahedral amorphous carbon (ta-C:H) is critical for various applications such as microelectromechanical devices, magnetic and optical storage devices, and medical implants. The surface topography of ta-C:H films deposited by distributed electron cyclotron resonance plasma from C2H2 gas precursor was investigated. The effects of pressure, together with ion flux and energy, are studied by atomic force microscopy in relation to the structural evolution of the films. The results are compared with the predictions of the Edward-Wilkinson model [Proc. R. Soc. London, Ser. A 44, 1039 (1966)] recently proposed to account for ta-C:H growth and with previous interpretations based on hypersonic shock waves. The random hillocks observed on the smooth surfaces of ta-C:H films deposited at high pressure are thought to result from the interference of high energy shock waves triggered by C4Hx+ ions that produce overlapping collision cascades and induce nonlinear effects.