Carbon Nanosheet Cathodes for Use in Milliamp Class Field Emission Devices

Abstract

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