Kun Hou

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.

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.

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 ...

Transfer of carbon nanosheet films to nongrowth, zero thermal budget substrates

Carbon-based nanostructures and materials have become a popular subject of research due to their unique thermal, mechanical, electrical, and optical properties. For example, the strong C–C bonds of graphene-based systems allow for excellent thermal conduction at room temperature and the conjugation of the sp2 lattice enables extremely high electron mobility. However, the use of carbon nanostructures as a component in polymer composites, sensors, mirco-electro-mechanical systems, and both rigid and flexible electronics has been limited by several factors, including the incompatibility with standard photolithography techniques, the high temperatures required for the nanostructure growth, and the presence of—or complication—of removing noncarbon species. Here, the authors report on a novel method for the transfer of carbon nanosheets to a low or zero thermal budget substrate while maintaining their original morphology and electrical properties. Four-point probe measurements’ post-transfer shows the retention...

Field emission observation of carbon nanosheet thin film by photoelectron emission microscopy (PEEM)

In this study, the field emission characterization of carbon nanosheet thin film was conducted using a diode configuration with an anode-cathode distance of 254 mum. Photoelectron emission microscopy (PEEM) was used to investigate the field emission uniformity over the surfaces of carbon nanosheet thin films.

High brightness field emission from carbon nanosheets and back-gated devices

A 2D atomically thin nanostructure, carbon nanosheet, was synthesized via radio frequency plasma enhanced chemical vapor deposition. Carbon nanosheets are free-standing, and have flat surface morphologies, atomically thin edges, and defective graphitic structures. Field electron emission from carbon nanosheets was measured under diode mode. Carbon nanosheets samples have turn-on fields of 5-10 V/mum, and can yield a total emission current of 28 mA from an area of 8 times 8 mm2 and an emission current density of ~2 mA/mm2 from an area of 1 times 1 mm2. Back-gated triode devices using carbon nanosheets as cathode material were also designed, fabricated, and tested under triode mode. A total emission current of 1.3 mA was achieved from one single device.

Carbon Nanosheet Cathodes for Use in Milliamp Class Field Emission Devices

Summary form only given. Field emission sources have distinct advantages such as short turn-on time, high power efficiency, low thermal signature, modulation control and the ability to be a variable current source that are desirable for high-current applications. However, scale-up of current density, device lifetime and device robustness has been limited to date. In this talk we present recent results using carbon nanosheets (CNS) as the field emission source in a high-current, back-gated device. Carbon nanosheets consist of free-standing graphene layers <2 nm thick which are oriented perpendicular to the growth surface. As field emission sources, nanosheets offer several potential benefits as compared to carbon nanotubes or other similar nanostructures. Nanosheets do not require a catalyst for growth and can be patterned after deposition using standard photolithography techniques. This is a distinct advantage compared to the cumbersome process of nanotube placement via catalyst patterning or the inefficient use of printed pastes which do not allow for vertically oriented structures. Second, nanosheets have as low, or lower, turn on field compared to nanotubes; threshold fields <1.0 V/mum (10 nA threshold) have been achieved. Third, in contrast to nanotube results previously published in the literature, nanosheets tend to self-condition to lower turn-on thresholds and increased stability after high-current field emission operation; nanosheet samples have produced over 23 mA of unsealed DC current, have operated in a continuous DC mode for over 5 hours, without failure, and produced over 1 mA of current in a pulsed mode (14% duty cycle >100 microamps, 3% at max current; 100 sec/cycle) 200 hours, again without failure. Furthermore, the sweep-to-sweep repeatability was remarkably high over the entire 200 hours and the standard deviation of the maximum current was <2.3% for all 7216 pulses. A novel back-gated device for high-current applications has been developed with nanosheets as the emission source. The device inherently eliminates arcing between the gate and the cathode and creates a much more open cathode configuration for better vacuum conductance and getter pumping. Furthermore, exact positioning of the CNS is not necessary and the device inherently allows for emission site burn out and turn-on of secondary sites. Electrostatic and electron trajectory modeling indicate that the devices should be capable of operation at current densities of >10 mA/mm2 and internal modulation to GHz frequencies. Testing of prototype devices has produced upto 3.5 mA of current and lifetimes of over 20 hours. The primary device failure mode is dielectric breakdown due to Au diffusion. New Pt-based devices are under construction; testing results from these devices will also be presented