[Show abstract][Hide abstract] ABSTRACT: We report new results from field emission microscopy studies of multiwall carbon nanotubes and from energy spectrum measurements of beams from diamond field emitters. In both systems, we find that resonant tunneling through adsorbed species on the emitter surface is an important and sometimes dominant effect. For diamond emitters our observations include order-of-magnitude emission enhancement without spectral broadening, complex spectral structure, and sensitivity of that structure to the applied electric field. For carbon nanotubes we have observed electron beams from individual adsorbates which are estimated to approach the maximum beam brightness allowed by Pauli exclusion.
Full-text · Article · Nov 2010 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: This article reports the fabrication and characterization of a CNT field emission cell with a built-in electron beam source for electron excited amplified field emission. A monolithic lateral field emission cell (FEC) with integrated metallic anode was fabricated. Then the field emission behaviors with and without activation of the built-in electron beam were characterized in diode configuration. A high voltage of 1.8kV was applied to generate the bombarding electron beam on the FEC. The emission current of the FEC increases markedly with the activation of the electron beam source due to impact ionization and direct interaction with the FEC CNT cathode. The emission behaviors were confirmed by F–N plots. It was found that almost 10 times current amplification was achieved. These results demonstrate the feasibility of an electron beam amplified field emission using carbon nanotube emitters.
No preview · Article · Feb 2010 · Diamond and Related Materials
[Show abstract][Hide abstract] ABSTRACT: The authors present recent advances in the uniformity conditioning of diamond field emitter arrays (DFEAs). Postfabrication conditioning procedures consisting of thermal annealing and high field/current operation have been examined. Nonuniformity due to varying contamination states of the emitters can be mitigated by moderate temperature (∼150–300 °C) operation. Operating the emitters at elevated current levels was found to enhance the spatial uniformity in a self-limiting manner. The conditioning mechanism is most likely thermal-assisted field evaporation of the diamond nanotips, however, the nature of the dc tests does not definitively exclude back bombardment as a possible contributor. Pulsed testing is underway to remove this ambiguity, provide conditioning for high-density arrays, and demonstrate the operational current density limits of DFEAs.
Full-text · Article · Sep 2009 · Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society
[Show abstract][Hide abstract] ABSTRACT: The electron field emission properties of nano-crystalline diamond (ND) thin-film can be enhanced through nitrogen incorporation, resulting in increased sp2 carbon concentration leading to low binding energy beneficial for electron field emission. Three distinct ND films exhibiting differentiable grain structures and carbon chemical bonding were grown by microwave plasma chemical vapor deposition (MPCVD) using different growth conditions. The ND films were grown on highly conductive silicon substrates using MPCVD-facilitated reaction of CH4/H2/N2 precursor gases. The nitrogenated ND film deposited under relatively low-pressure low-power growth condition yielded a turn-on field of 3.5 V/µm, which is the lowest value of the three films. XPS results reveal that the lowest turn-on field corresponded with the highest sp2 intensity. The same film also exhibited the shallowest Fowler–Nordheim slope, indicating a stronger field enhancement factor and lower effective work function.
No preview · Article · Mar 2009 · Diamond and Related Materials
[Show abstract][Hide abstract] ABSTRACT: Traditional vacuum-based thermal energy conversion (VTEC) devices with “flat” metal emitters have been compromised by high working temperature mainly due to high work function and electron space charge effect. However, a VTEC system is inherently efficient due to unimpeded flow of electrons through vacuum and reduction of heat conduction by the vacuum medium. Consequently, VTEC devices are attractive in terms of space, weight and energy efficiency for waste heat recovery in a TEC system. Carbon nanostructured emitters including multiwalled carbon nanotubes (CNTs) and “ridge” nanodiamond thin films have shown relatively low threshold fields for electron field emission mainly due to their nanoscaled, high aspect ratio, emitting surfaces. In this study, the field-enhancing features of carbon nanostructures were investigated as thermal-field emitters for waste heat recovery. The turn-on field of CNTs was found to decrease from ~ 1.9 V/μm at room temperature to ~ 0.9 V/μm at 400 °C. A high emission current of ~ 26 μA was achieved at relatively low field of ~ 1.5 V/μm and low temperature of 400 °C. The maximum efficiency of the VTEC device is estimated to be ~ 15% at 300 °C with a current density of 54 mA/cm2.
No preview · Article · Feb 2009 · Diamond and Related Materials
[Show abstract][Hide abstract] ABSTRACT: We present new results in the measured electron energy spectrum from diamond field emitters. The energy spectrum from a clean diamond surface has been measured and is comparable in shape and width to that of metal emitters. The results suggest that the emitted spectrum is sensitive to the presence of adsorbed species on the emitter surface. Electrons significantly below the cathode's Fermi level are emitted by resonant tunneling. Furthermore, these resonant surface states can increase the total emitted current by more than an order of magnitude while maintaining a narrow spectral width (∼0.5 eV). Experiments are also being performed with individual multiwall carbon nanotubes (MWCNTs). We have observed beams emitted from individual residual gas molecules that approach the quantum-degenerate limit of electron-beam brightness. This limit has profound consequences for the behavior of an electron. Tightly bound designer adsorbates may greatly enhance the emission properties and improve performance in electron injector systems.
[Show abstract][Hide abstract] ABSTRACT: We present recent advances in the uniformity conditioning of diamond field-emitter arrays (DFEAs), and new results from emittance measurements of their emitted electron beams. DFEAs have shown considerable promise as potential cathodes for free-electron lasers. They have demonstrated their rugged nature by providing high per-tip currents, excellent temporal stability, and significant resistance to back-bombardment damage during poor vacuum, close-diode DC operation. Until now, the successful conditioning of high-density arrays has been precluded by thermal damage to the anode. We report successful uniformity conditioning of densely packed DFEAs using microsecond-pulsed high-current conditioning (HCC). A high degree of spatial uniformity was confirmed in low-current DC testing following these HCC procedures. The conditioned arrays will be used to refine previous measurements of the normalized transverse emittance of the emitted electron beams.
[Show abstract][Hide abstract] ABSTRACT: Diamond field-emitter arrays (DFEAs) possess several advantages over photocathodes: ruggedness, no drive laser requirement, and minimal heating. A gated DFEA with micron-scale cathode-gate spacing has the added benefits of direct e-beam modulation and low operating voltages <100 V. A second gate can be integrated, creating built-in focusing capability. We have developed two types of self-aligning gate fabrication methods. First, pyramidal molds are formed on a SOI (silicon on insulator) substrate then coated with CVD nanodiamond. The bulk layer of silicon is thinned, followed by oxide etching and opening the diamond tip isolating the gate electrode and insulating layer from the cathode. The second method uses additive physical evaporation depositions of insulating and gate electrode layers on top of the DFEAs. Chemical etching of the insulating layer separates and opens cathode tip due to "lift off" type step coverage of the evaporation technique. A 2-mask fabrication process has been used to pattern the gate to optimize active gate area and increase yield. Fabrication techniques and electrical behavior of the gated DFEAs will be discussed.
[Show abstract][Hide abstract] ABSTRACT: In this work, nanocarbon-derived vacuum electronic devices, viz., the nanodiamond lateral triodes and transistors and the CNT integrated differential amplifiers are examined. The material properties, device structure and fabrication process, and the electrical performance of these devices are presented.
[Show abstract][Hide abstract] ABSTRACT: High-quantum-efficiency (QE) photocathodes used for free-electron lasers tend to be fragile and demand complex drive lasers. Field-emitter arrays eliminate both these problems, but introduce other problems along with interesting new physics. Diamond field-emitter arrays (DFEAs) are rugged and forgiving of poor vacuum. They are easily conditioned to give uniform emission, current density on the order of 100 A/cm2 before phase compression, and emittance smaller than 10 μm-radians for a 1 cm cathode. In gated versions the emission can be phased to the rf drive and the emittance can be reduced by the focusing effect of the gate. Experimental evidence from diamond pyramids and carbon nanotubes suggests that field emission is enhanced by resonant tunneling through molecules adsorbed on the surface. The emission from individual molecules appears to reach the fundamental limits imposed by the Heisenberg uncertainty principle and by the Pauli exclusion principle.
[Show abstract][Hide abstract] ABSTRACT: Field emitter arrays (FEAs) need improved novel cold cathode materials for better and more reliable performance. The presence of negative electron affinity (NEA) on hydrogen-terminated diamond surfaces, coupled with practical chemical vapor deposition (CVD) processing of deposited diamond as a thin film on a variety of substrates, has promoted further interest in the use of diamond and diamond-like carbon materials as field emitters. Experimentally, diamond and carbon nanotube emitters have been observed to emit electrons at relatively low electric fields and generate useful current densities. In this work, nanocarbon-derived vacuum devices, viz., the nanodiamond lateral diode and the gated carbon nanotube triode are examined. The material properties, device structure and fabrication process, and the electrical performance of the two configurations are presented. Nanocarbon-derived electron emission devices, specifically, nanodiamond lateral field emission diodes and gated lateral field emission diamond and carbon nanotube triodes are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. For example, nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to utilize the material as an electron field emitter. Arrays of laterally arranged "finger-like" nanodiamond emitters constitute the cathode in a versatile diode configuration with small interelectrode separation. A low diode turn-on voltage of 7 V and a high emission current of 90 muA at an anode voltage of 70 V (electric field of ~ 7 V/mum) is reported for the nanodiamond lateral device. Also, a field emission triode amplifier based on aligned carbon nanotubes (CNTs) with low turn-on voltage and small gate leakage current has been developed.
[Show abstract][Hide abstract] ABSTRACT: A novel integrated vacuum field emission (VFE) differential amplifier (diff-amp) utilizing carbon nanotube (CNT) emitters has been developed. A dual-mask microfabrication process was employed to achieve a VFE diff-amp by integrating identical CNT VFE transistors with built-in split gates and integrated anodes. The identical pair of triode amplifiers was well-matched in their device characteristics. The measured ac small-signal characteristics of the diff-amp showed a common-mode-rejection ratio (CMRR) of ~ 320 (~ 50 dB). The proposed analytical model of the CMRR was verified to be in good agreement with the experimental data. The successful implementation of the CNT diff-amp demonstrates a new way to achieve temperature and radiation tolerant VFE integrated microelectronics.
No preview · Article · Apr 2008 · Diamond and Related Materials
[Show abstract][Hide abstract] ABSTRACT: Two configurations of boron-doped diamond ultramicroelectrode arrays (D-UMEAs) with microband (2 μm wide×80 μm long) geometry and different interelement spacings (20 μm and 100 μm) were fabricated using steps of silicon processing technology combined with selective area diamond deposition (SAD) to yield ultramicro-voltammetric electrodes. Cyclic voltammetry measurements showed that with sufficient interelement separation, the D-UMEAs displayed sigmoidal pseudo-steady-state cyclic voltammograms characteristic of microband electrodes. The D-UMEAs displayed higher faradaic current per unit area, than either conventional planar diamond electrodes or glassy carbon electrodes. This work also reports on the sensing of DA in a chemically controlled solution at physiological pH where the use of patterned D-UMEAs provided better voltammetric resolution without further surface modification or surface functionalization of the working electrode.
No preview · Article · Apr 2008 · Diamond and Related Materials
[Show abstract][Hide abstract] ABSTRACT: Boron-doped diamond ultramicroband arrays with different array densities and interelement spacings were fabricated using silicon technology and selective diamond deposition (SAD) technique to yield microvoltammetric electrodes. The electroactive ultramicroband elements were designed with one microscopic critical dimension to impart microelectrode behavior while the other dimension was made larger to yield an increase in signal current. Cyclic voltammetry studies in this work showed that with sufficient interelement separation, the ultramicroband arrays display sigmoidal pseudo-steady-state cyclic voltammograms characteristic of microband electrodes. The ultramicroband arrays yielded higher faradaic current per unit area, than either square ultramicroelectrode array or conventional planar diamond electrode from earlier reported work. This is due to enhanced mass transport to the ultramicroband elements at slow scan rates. Larger current density and higher signal-to-noise (S/N) ratio leads to better limits of detection, making it possible to fabricate a more sensitive electrode for applications such as electroanalysis, electrocatalysis, trace element analysis, mechanistic and fast transfer kinetics studies, electrochemistry in highly resistive media, as well as sensors in flow and biological system.
No preview · Article · Mar 2008 · Diamond and Related Materials
[Show abstract][Hide abstract] ABSTRACT: Nanocarbon-derived electron emission devices, specifically nanodiamond lateral field emission (FE) diodes and gated carbon nanotube (CNT) triodes, are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. For example, nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to use the material as an electron field emitter. Arrays of laterally arranged 'finger-like' nanodiamond emitters constitute the cathode in a versatile diode configuration with a small interelectrode separation. A low diode turn-on voltage of 7V and a high emission current of 90 microA at an anode voltage of 70V (electric field of approx. 7V microm(-1)) are reported for the nanodiamond lateral device. Also, a FE triode amplifier based on aligned CNTs with a low turn-on voltage and a small gate leakage current has been developed.
Preview · Article · Feb 2008 · Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences
[Show abstract][Hide abstract] ABSTRACT: A high-resolution retardation energy analyzer has been developed to examine the energy spread of electron beams from diamond field-emitter arrays. Analyzer design was guided by previous work at UMER , and simulations performed in SIMION. The analyzer incorporates a cylindrical focusing electrode which, when properly tuned, gives millivolt resolution for multi-kilovolt beams. The analyzer is integrated into a low-energy cathode test stand that allows arbitrary adjustment of the anode-cathode spacing and planarity during operation.
[Show abstract][Hide abstract] ABSTRACT: Field-emitter arrays (FEAs) have several advantages as cathodes for free-electron lasers (FELs): they are rugged, require no laser driver, and generate little heat. We have developed two methods to fabricate diamond FEAs for FEL applications. In the first method, pyramids are formed on a Si substrate and sharpened by microlithography and then coated with CVD nanodiamond . The advantages of this approach are a rigid, planar Si substrate, and microelectronic type fabrication. Typically, tip radii on the order of hundreds of nanometers are formed on 20 μm pyramids. In the second method, all-diamond pyramids are formed by a mold-transfer process in which they are sharpened by an oxide layer in the mold process. The diamond array is then brazed to a Mo substrate and the Si mold removed. The advantage of this process is that the tips are sharper, with tip radii smaller than 10 nm formed on 10 μm pyramids. The fabrication techniques and the performance of these cathodes will be discussed and compared.