Kenneth A. Dean

Current saturation mechanisms in carbon nanotube field emitters

Recent studies have shown a current limiting effect in the field emission behavior of carbon nanotubes. In this letter, we demonstrate that an individual nanotube exhibits current saturation above 100 nA of emission current, and we show that this current saturation is a direct result of an adsorbate-enhanced field emission mechanism. Current saturation results from the displacement of adsorbates from configurations of tunneling enhancement as electric field and current are increased. Saturation is concurrent with rapid fluctuations in emission current and distinctive changes in the field emission patterns. At high fields, the adsorbate states are completely removed from the nanotube. A single, clean single-walled nanotube shows no evidence of current saturation for emission currents reaching 2 μA.

The environmental stability of field emission from single-walled carbon nanotubes

We demonstrate long-term field emission stability from single-walled carbon nanotubes. Unballasted nanotubes operate without degradation for over 350 h at 10−9 Torr. Nanotubes are shown to be significantly less sensitive to operating environments than metallic emitters. In 10−7 Torr of H2O, we demonstrate 100 h of continuous bias field emission with no current degradation. Protrusion growth and current runaway, typical problems for unballasted metal emitters, are not observed with nanotubes. Single-walled nanotubes do show susceptibility to damage by oxidation. We suggest that the exceptional environmental stability of carbon nanotubes is due to a combination of geometry, strong carbon bonding, and the lack of protrusion growth.

Field emission displays: a critical review

Abstract The goal of making attractive flat panel displays (FPDs) based on arrays of cold cathodes has now become a reality. Pixtech and Futaba have begun commercial production of low voltage, monochrome field emission displays (FEDs). Moreover, public response to the high voltage, full color, VGA FED prototypes shown by Candescent and Motorola at various technical meetings and exhibits has been extremely positive and encouraging. Yet, the future of the FED industry is uncertain. The tremendous improvements in visual quality and reduction in manufacturing cost of liquid crystal displays, as well as the formidable progress made in other FPD technologies has raised the standard for FEDs. In this article, we first review the status of FEDs based on the Spindt microtip emitter. We focus on the scalability of the Spindt process to large substrates, phosphor selection, high voltage stability, and display lifetime. Second, we discuss in detail the recent advances made in alternate cold cathode technology, including carbon nanotubes and composite materials, and their potential advantages for FPD. This new technology offers a tremendous opportunity to lower capital investment, to cut manufacturing costs and to challenge the existing flat panel industry.

Evaporation of carbon nanotubes during electron field emission

Extracting large field-emission currents from individual single-walled carbon nanotubes degrades their current–voltage behavior and changes the nanotube structure. Field-emission microscopy observations and field-emission current–voltage measurements revealed behavior characteristic of local heating, followed by thermally assisted field evaporation of the atoms on the end of the nanotube, and subsequent restructuring of the cap. Field evaporation of carbon was found to result in reduction of the length of the tube without catastrophic arcing events. This process defines the upper limit for the field-emission current that can be obtained from an individual single-walled nanotube.

Field emission microscopy of carbon nanotube caps

We report field emission microscopy images from single walled carbon nanotubes showing resolvable fine structure with five- and six-fold symmetry. We present evidence indicating that these images have atomic resolution and that they depict the electronic structure of individual nanotube caps under high-field conditions. Fine structure is observed only after removal of surface adsorbates. Prior to adsorbate removal, these images show symmetrical lobed patterns and several other behaviors characteristic of chemisorbed resonant tunneling states.

Three behavioral states observed in field emission from single-walled carbon nanotubes

We observe three distinct behavioral states in field emission from single-walled carbon nanotubes between temperatures of 300 and 1800 K. At room temperature, nanotubes emit through adsorbate states correlated to the presence of water. These states are removed above 900 K. After adsorbate removal, the apparently clean nanotube state shows lower emission current and substantially reduced emission noise. Current-temperature measurements identify a deviation from metallic tunneling behavior. Nanotube field emission undergoes a second, field-stabilized transition at high temperatures which reduces the current by as much as five orders of magnitude relative to the room temperature current. This current decrease is 100% recoverable. In addition, the stable behavior of clean nanotube states breakdown at extremely high currents and temperatures. Rings form around the field emission images, similar to those observed in metals at extreme current densities. Under these extreme conditions, we also find evidence for t...

Stability of carbon nanotubes under electric field studied by scanning electron microscopy

The influence of an applied electric field on carbon nanotubes protruding from a surface was investigated in situ using a high-resolution scanning electron microscopy. Under the applied electric field, the nanotubes flexed to orient themselves parallel to the electric field lines. For moderate field strengths below the electron field emission threshold, the flexed nanotubes relaxed back to their original shapes after the electric field was removed. However, when high electron field emission currents were extracted from the nanotubes, they were permanently deformed, leaving them aligned to the electric field direction after the electric field was removed. For high currents, the length of the carbon nanotubes were found to be shortened after field emission lasted for a period of time.

A full description of a simple and scalable fabrication process for electrowetting displays

Electrowetting displays provide a high white state reflectance of >50% and have attracted substantial world-wide interest, yet are primarily an industrially led effort with few details on preferred materials and fabrication processes. Reported herein is the first complete description of the electrowetting display fabrication process. The description includes materials selection, purification and all fabrication steps from substrate selection to sealing. Challenging materials and fabrication processes include dielectric optimization, fluoropolymer selection, hydrophilic grid patterning, liquid dosing, dye purification and liquid ionic content. The process described herein has produced pixel arrays that were switched at <15 V on active-matrix backplanes, and which have individual sub-pixel areas of <50 × 150 µm2. The majority of fabrication processes can conform to liquid-crystal style manufacturing equipment, and therefore can be readily adopted by many display practitioners. Also presented are additional tips and techniques, such as controlling the onset of oil film break-up in an electrowetting display. This paper should enable anyone skilled in displays or microfabrication to quickly and successfully set up research and fabrication of electrowetting displays.

Nanotube electronic states observed with thermal field emission electron spectroscopy

We observe nonmetallic electronic states above the Fermi level in single-walled carbon nanotubes by measuring the energy distribution of thermal-field-emitted electrons. This measurement method examines electronic states associated with the nanotube cap or end termination, and with it, we resolve electronic states greater than 3 eV above the Fermi level. The observed emitting states are broad at high temperatures (0.7–1.5 eV full width at half maximum), and the peak positions shift linearly with applied voltage. We present possible mechanisms responsible for these states.

Towards a low-cost high-quality carbon-nanotube field-emission display

We have developed field-emission-display technology driven by chemical-vapor-deposition-grown carbon-nanotube emitters incorporated in a simple, low-cost device structure. Here, we report on frit-sealed test displays with a brightness of 3000 cd/m 2 at 3 kV and a lifetime of 9000 hours with only 45% degradation. We also demonstrate the scalability of the technology with a uniform high-brightness 6-in. QVGA that displays video images with a switching voltage of 40 V.