Kishore Uppireddi

Temporal field emission current stability and fluctuations from graphene films

Stable field emission currents and low fluctuations are important feasibility requirements for the application of materials in field emission devices and displays. The current stability and current fluctuations of field emitted electrons from graphene films are investigated for the periods of 24 and 100 h. The graphene films showed different percentage of variation from the initial current density for different films ranging from 6% to 46% and the standard deviation in the range of 2–6 μA/cm2. The short- and long-term stability and fluctuations of the graphene films are reported and the causes for degradation of the emission current are discussed.

Thermionic emission energy distribution from nanocrystalline diamond films for direct thermal-electrical energy conversion applications

In the ongoing quest for energy production by nonconventional methods, energy conversion by vacuum and solid-state thermionic emission devices is one of the potentially efficient pathways for converting thermal energy directly into electrical power. The realization of practical of thermionic energy conversion devices strongly depends on achieving low work function materials, which is thus far a limiting factor. In an attempt to develop a new low work function thermionic material, this work reports thermionic emission energy distributions (TEEDs) from nanocrystalline diamond (NCD) films in the temperature range from 700 to 900 °C that reveal a consistent effective work function of 3.3 eV. The NCD films also exhibit emission peaks corresponding to higher work functions as indicated by shifts in their energy position and relative intensity as a function of temperature. These shifts thus appear to be related to instabilities in the NCD’s surface chemistry. The analysis of these data yields information on the ...

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.

Study of the temporal current stability of field-emitted electrons from ultrananocrystalline diamond films

The temporal current stability and the current fluctuations of the field emission current obtained from ultrananocrystalline diamond (UNCD) films were investigated. The films were synthesized by Ar-rich dc plasma assisted hot filament chemical vapor deposition. The field emission properties were correlated to the films’ composition (sp2 and sp3 bonded carbon volume fractions) and nanostructure. The compositional analysis was done by modeling the films’ spectroscopic ellipsometry [Ψ (E), Δ (E)] data by using a two layer structure and the Bruggeman effective medium approximation. The results indicate that the local field enhancement in UNCD films is related to their intrinsic morphology at the diamond-grain boundary-vacuum interface, and the sp3∕sp2 carbon volume fraction plays a significant function on the short- and long-term current stabilities of field emitted electrons.

Study of temporal current stability and fluctuations of field emitted electrons from ZnO nanostructure films

Stable field emission currents and low fluctuations are important feasibility requirements for the application of materials in field emission devices and displays, more than the low turn on fields that are generally considered. The current stability and current fluctuations of field emitted electrons from ZnO nanostructures were investigated over the period of 2, 12, and 24 h. The films with nanoneedle structure having density around ten/μm2 showed better short and long-term (temporal) stability over a period of 24 h. The short- and long-term stability and the current fluctuations of the nanostructures are reported and the causes for the degradation of the emission current are discussed.

Iron Oxide Nanoparticles Employed as Seeds for the Induction of Microcrystalline Diamond Synthesis

Iron nanoparticles were employed to induce the synthesis of diamond on molybdenum, silicon, and quartz substrates. Diamond films were grown using conventional conditions for diamond synthesis by hot filament chemical vapor deposition, except that dispersed iron oxide nanoparticles replaced the seeding. X-ray diffraction, visible, and ultraviolet Raman Spectroscopy, energy-filtered transmission electron microscopy , electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to study the carbon bonding nature of the films and to analyze the carbon clustering around the seed nanoparticles leading to diamond synthesis. The results indicate that iron oxide nanoparticles lose the O atoms, becoming thus active C traps that induce the formation of a dense region of trigonally and tetrahedrally bonded carbon around them with the ensuing precipitation of diamond-type bonds that develop into microcrystalline diamond films under chemical vapor deposition conditions. This approach to diamond induction can be combined with dip pen nanolithography for the selective deposition of diamond and diamond patterning while avoiding surface damage associated to diamond-seeding methods.

Field emission stability and properties of simultaneously grown microcrystalline diamond and carbon nanostructure films

The temporal stability and electron field emission characteristics of simultaneously grown microcrystalline diamond and carbon nanostructures were investigated. The films were prepared by hot filament chemical vapor deposition using typical diamond deposition parameters, and iron oxide nanoparticles on the molybdenum substrate were employed as catalyst for their induction. The hybrid system presented better emission characteristics compared with microcrystalline diamond with turn-on fields as low as 2.45 V/μm (at 1 μA/cm2), and a current density of up to 0.24 mA/cm2 was achieved. The films showed relatively stable emission behavior for a period of 24 h.

8.4: A novel nanowire optical frequency rectifying diode: Application as an IR and optical sensor

We report the results of preliminary experiments on the laser irradiation of a metallic whisker antenna. Open circuit voltage measurements, with and without an applied DC bias, are reported. Voltages greater than 10mV have been measured.