Dennis M. Manos

Free-standing subnanometer graphite sheets

Free-standing graphite sheets with thickness less than 1nm, “carbon nanosheets,” were synthesized on a variety of substrates by radio-frequency plasma-enhanced chemical vapor deposition without any catalyst or special substrate treatment. The nanosheets consist of one to three graphene layers with a large smooth surface topography, standing roughly vertical to the substrate. Due to the atomic thickness and corrugated nature of nanosheets, low-energy vibrational modes are present in the Raman spectra. The low turn-on field of 4.7 V/μm for electron field emission suggests that the carbon nanosheets could be used as a potential edge emitter.

High field emission reproducibility and stability of carbon nanosheets and nanosheet-based backgated triode emission devices

The authors have characterized field emission properties of freestanding, 1nm thick graphene layers, called carbon nanosheets (CNSs), which were grown perpendicular to the growth surface using a radio-frequency plasma-enhanced chemical vapor deposition technique. The CNSs are metallic impurity-free and have uniform height distribution (standard deviation of 200h at 1.3mA emission current level. Over this time, no degradation has been observed, the variability of the individual I-V curves is small among 7216 voltage cycles, and the standard deviation at the maximum current was no more than 2.3%. A nanosheet-based backgated triode emission device has been developed to take advantage of the nanosheet field emission performance. Prototype devices have confirmed triode operation and stable electron emission.

Analysis of diamond-like carbon and Ti/MoS2 coatings on Ti–6Al–4V substrates for applicability to turbine engine applications

Abstract Ti–6Al–4V substrates have been coated by diamond-like carbon (DLC) films, with no surface pretreatment, and have been coated by Ti/MoS 2 films, with a simple surface pre-cleaning. The DLC films were deposited by planar coil r.f. inductively-coupled plasma-enhanced chemical vapor deposition (r.f. ICPECVD); the Ti/MoS 2 films were deposited by magnetron sputtering. Both the DLC and Ti/MoS 2 films were characterized by pull tests, hardness tests, scanning electron microscopy (SEM), and wear tests (pin-on-disk and block-on-ring) to compare their adhesion, hardness, surface topology, and wear properties to plasma-sprayed Cu–Ni–In coating currently used for turbine engine applications. The DLC films were easily characterized by their optical properties because they were highly transparent. We used variable-angle spectroscopic ellipsometry (VASE) to characterize thickness and to unequivocally extract real and complex index of refraction, providing a rapid assessment of film quality. Thicker coatings yielded the largest hardness values. The DLC coatings did not require abrasive pretreatment or the formation of bond-layers to ensure good adhesion to the substrate. Simple surface pre-cleaning was also adequate to form well-adhered Ti/MoS 2 on Ti–6Al–4V. The results show that the DLC and Ti/MoS 2 coatings are both much better fretting- and wear-resistant coatings than plasma-sprayed Cu–Ni–In. Both show excellent adhesion to the substrates, less surface roughness, harder surfaces, and more wear resistance than the Cu–Ni–In films.

Uniform and enhanced field emission from chromium oxide coated carbon nanosheets

Carbon nanosheets, a two-dimensional carbon nanostructure, are promising electron cathode materials for applications in vacuum microelectronic devices. This letter demonstrates a simple approach to improve the spatial emission uniformity of carbon nanosheets by coating them with a chromium oxide thin film. Photoelectron emission microscopy observations and in situ field emission tests revealed that chromium oxide coated carbon nanosheets not only have spatial uniformity but also have coating thickness dependent field emission properties. For example, a coating thickness of ∼1.5nm gave a substantially greater field emission than as-grown nanosheets or other thickness coatings.

CHARACTERIZATION AND MODELING OF A MICROWAVE DRIVEN XENON EXCIMER LAMP

We designed, constructed, and studied a novel probe-coupled 2.45 GHz microwave arrangement to drive a Xe excimer lamp. Electrical efficiency and output power in the 160–200 nm range (Xe second continuum) both increased with pressure and input power up to 1500 Torr and 600 W (42.5 W/cm3), respectively. The maximum in-range optical power was 60 W, more than 80% of the total. Cooling with liquid nitrogen boil off rather than room air more than doubled output power. Model calculation and experimental measurement of the angular distribution of emission find considerable intensity well away from the surface normal. Correcting experimental measurements of excimer lamp output power accordingly brings good agreement with the 20%–40% electrical efficiency predicted by theory.

Field emission from MO2C coated carbon nanosheets

Carbon nanosheets have recently evolved into useful edge emitters with high emission current densities, low threshold electric fields, and long lifetimes. In addition to further improvement in these characteristics, good stability and repeatability are also essential for these materials to be suitable for high vacuum applications such as microwave tubes and flat panel displays. Since the work function of graphite, carbon nanotubes, and amorphous carbon is relatively high, 4.6–4.8eV, selective thin film coatings may offer significant advantages. Carbides are a good film choice for their corrosive resistance, chemical stability, and substantially lower work function. Approximately 3 ML (monolayer) (∼1nm) of molybdenum were deposited on carbon nanosheets by physical vapor deposition and the carbide (Mo2C) formed by heating to >200°C at 1×10−8Torr. The carbide stoichiometry was confirmed in situ by the characteristic Auger triple peak at 272eV. A stoichiometric Mo2C calibration sample was used to acquire the ...

Time-resolved electrostatic probe studies of a pulsed inductively-coupled plasma

We report a systematic study of a pulsed 13.56 MHz transformer-coupled plasma over a range of frequencies from 200 Hz to 10 kHz using electrostatic probes. The time-resolved plasma density and electron temperature from the double-probe analysis were compared with single Langmuir probe results using both the orbital-motion-limited and radial-motion theories with sheath displacement corrections. The results were compared to a spatially-averaged model. Our experiments and analysis show that the double probes can be used reliably in pulsed plasma measurements. The good agreement between the equivalent resistance method and the nonlinear regression method indicates that the equivalent resistance method, which is faster and easier to automate, can be used effectively to analyse the double-probe data. The transient behaviours of the plasma density and electron temperature, although complicated, are in accord with the simple model of the discharge. The plasma density and the electron temperature can be decoupled in a pulsed plasma by adjusting the pulse parameters. This decoupling suggests that a pulsed plasma source can provide additional control for ion-assisted or neutral-assisted etching and deposition.

Hyperthermal atomic hydrogen and oxygen etching of vertically oriented graphene sheets

Carbon nanosheets have previously been shown to be promising high current field emission cathodes for a variety of potential applications. The vertically oriented planar sp2 carbon nanosheets grown by rf plasma-enhanced chemical vapor deposition terminate with one to seven graphene sheets and grow to ∼1u2002μm in height. High current field emission, Je∼0.15u2002mAu2009mm−2 (8u2002Vu2009μm−1), conducted within an ultrahigh vacuum system in a diode configuration in line-of-sight to a mass spectrometer, shows that CH4, CO2, and CO are generated as a result of cathode bombardment by hyperthermal oxygen and hydrogen neutrals and ions generated by electron stimulated desorption at the Cu anode. Confirmation of the mechanism was achieved by repeating the experiments using a Au anode. Simultaneous acquisition of I-V data and the partial pressures of reaction products in the mass spectrometer have shown repeatable, sustained CH4, CO2, and CO production. As these hyperthermal atomic hydrogen and oxygen species impinge on the sidewalls...

Process damage assessment of a low energy inductively coupled plasma-based neutral source

In this article, we report process damage studies using a low energy inductively coupled plasma-based neutral stream source. Low energy neutrals are generated by the surface reflection neutralization method. Quasistatic capacitance-voltage measurement results of this work demonstrate that this low energy neutral source, which provides controllable fast neutrals for cleaning applications, induces much less damage than a pulsed inductively coupled plasma source. Most of the neutral process damage is caused by ultraviolet photons escaping from the plasma source zone. The process-induced damage by fast neutrals increases as the reflector bias is lowered. The fast neutral process damage also increases with the rf power. Unlike the neutral process-induced damage, the damage induced by pulsed plasmas is found to be a function of both the gas composition and rf pulsed frequencies.

Simulation of a surface‐reflection neutral stream source

We have developed a computational model to optimize the design of a surface reflection neutralization source of hyperthermal neutrals for charge‐free processing. For the deployment of this technique to production (≥8 in. wafers) processing, a system design study has been completed. A Monte Carlo model is used to determine the energy and angular distributions of reflected hyperthermal neutrals at the surface of a wafer, as well as flux uniformity. A simple form for the plasma profile is chosen to allow simulation of various profiles reported in the literature. Neutrals are launched from the reflector consistent with the relative plasma density at each position on the reflector with angular and energy distributions consistent with experimental observations. These neutrals are then followed through interactions with thermal background atoms and the plasma. Charge exchange, ionization, and elastic scattering processes are considered for argon as the main feedstock gas. The results show the trade off between a...