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Fundamentals and Applications of Nano-Carbon Electron Emitters

Final Report Summary - FANCEE (Fundamentals and Applications of Nano-Carbon Electron Emitters)

Development of efficient electron sources is crucial for a wide range of applications including integrated vacuum microelectronics, MEMS, bright flat panel displays, energy conversion devices, and compact microwave amplifiers. The key element of these devices is a cathode that should be capable to produce an intense electron beam at low energy consumption. In order to create new cathodes with enhanced functionality nanocarbon materials are of special interest. The reason is three-fold. First, they are stable and capable to survive at high temperatures and currents. Second, carbon cathodes based on carbon nanotubes or nanographite flakes with high aspect ratio are capable providing high electron yield at relatively low applied voltage. Finally, nanocarbon materials are technologically attractive because they can be manufactured using cheap, well-established and ecologically safe techniques. In the framework of the FANCEE project we develop nanocarbon materials best suited for field emission and thermionic cathodes and demonstrate practical devices based on these cathodes.

In accordance with the objectives, we report the following main scientific results:

By using original Plasma Enhanced Chemical Vapor Deposition (PECVD) technique we fabricated a wide range carbon nanomaterials well suited for field emission cathodes of enhanced functionality. In particular we fabricated single crystal diamond needles that possess unique morphology and can be employed in advanced electronic and optoelectronic devices. These pyramidal single diamond crystallites with apex radius of several nanometers and length of tens of microns have atomically smooth rectangular basis that coincides with the diamond (100) plane, whereas the lateral faces are oriented in the {110} planes. We demonstrated that nanometric curvature of the diamond needles apex makes it possible to design new class of electron emitters of enhanced performance. The PECVD technique can be used also for fabrication of large area graphene that can be employed for various optoelectronic devices. Systematic measurements and in-depth study of the physical-chemical processes governing the nanocarbon films growth have allowed us to determine CVD process parameters that enable scalable fabrication of the diamond, nano-graphite and amorphous carbon films with pre-described electronic and morphological properties.
The CVD method has been employed to fabricate patterned single and few layer graphene on dielectric substrates. By using the electron lithography technique composite nanostructured carbon cathodes composed of dielectric nanogratings covered by ultrathin carbon films were produced. Low-temperature PECVD synthesis enables fabrication of the high-quality, metal-free, forest of multi-wall carbon nanotubes, which grow due to formation of pores in the Si substrate in the process of the hydrogen plasma etching. We have also introduced ultrathin PyC films that can compete with graphene in a number of applications. In contrast to graphene they can be deposited on both metal and dielectric surfaces of arbitrary size and shape being prospective for a wide range of applications that spans from electrochemistry to aerospace and automotive industries.
Comprehensive characterization of obtained carbon materials by using Raman spectroscopy, scanning electron microscopy and transmission electron microscopy revealed correlations between CVD process parameters and characteristics of the sythesized material. In particular, we found that carbon condensation from vapor phase may provide formation of nano-graphite crystallites with very high aspect ratio, while the termination of the deposition process with certain protocol can result in bridging of two adjacent graphene layers. This bends graphene sheets and results in the surface film heterogeneity and reduction of the work function thus improving electron emissivity. Such an improvement is expected in other materials containing bended graphene sheets, e.g. carbon nano-scrolls or diamond particles with graphene inclusions that can be produced in the same PECVD facility.
We have fabricated field emission (FE) cathodes from nanographite films consisting of few layer graphene crystallites and single-wall carbon nanotubes (SWNT). The manufactured cathodes are capable to produce the total current density of about 1 A/cm2. For the SWCNT cathodes, the field enhancement factor increases with the film thickness and varies over the film surface due to the stronger overlapping of the nanotubes in the thicker film and the stronger screening effect of the neighboring tubes. By using a scanning anode field emission microscope we demonstrated that difference in the mechanical properties of the nanographite and SWCNT films may differ field emission performance of the cathodes.
By measuring the I-V curves and distribution of the emission sites in the fabricated nanocarbon cathodes we revealed the transformation of field emission to thermionic emission. We demonstrated that thermionic and field mechanisms of the electron emission coexist under normal conditions in both conventional metal and nanocarbon cathodes. However, we discovered that the temperature dependence of the emission current in carbon cathodes is not always follows the predictions of both Richardson–Dushman and Fowler–Nordheim theories. This occurs due to dependence of the free electron density in graphite on temperature and/or change of the work function by sorption processes. We have developed a comprehensive phenomenological and numerical model the thermionic and field emission from nanocarbons that enables designing carbon-based cathodes for different applications.
It has been demonstrated, that irradiation with intense laser pulses can be used to control electron emission from nanocarbon materials. We showed, in particular, that in the nanographite cathodes, the laser-assisted emission has a pronounced threshold, which reflects the switching of the emission mechanism from multiphoton to thermionic one with increase of the pulse power. In diamond cathodes, laser irradiation increases the FE current, which depends linearly on the light power. By changing the applied voltage we observed transformation of the FE regime from the Fowler-Nordheim emission to depletion zone limited emission, and then to the impact ionization.
We have demonstrated also, that irradiation of the nanocarbon film with an intense femtosecond pulse results in the electron emission with charge density of as high and 13 nC/cm2 under laser pulse energy of 0.75 mJ and pulse duration of 80 fs. The measured dependence of the collected charge on the applied voltage indicates that the electron emission is suppressed by the accumulation of spatial charge, while the dependence of emission efficiency on the photon energy indicates the importance of multiphoton processes for the laser-assisted emission. The developed two-temperature model of the laser-assisted emission shows that the electron temperature can be as high as several thousand degrees at a moderate energy of the femtosecond laser pulse.
The prototypes of the cathodoluminescence lamps employing cold nano-graphite cathodes composed of bended graphene layers have been presented. The quantum tunneling character of the electron field emission process is responsible for the extremely low energy consumption of the designed lamps. Together with an appropriate design, the cold electron emission has provided the power efficiency of as high as 10% for green light. Another advantage of this technology is the potential ability to provide light sources with any colors by mixing phosphorous materials.
Additionally to multiemitter nano-graphite cathodes the point electron emission source was proposed on base of single crystal diamond needles with apex radius of 2–10 nm and length of tens of microns. We have shown that despite the large bang gap (~5.5eV) the diamond needle can produce stable field emission current up to several microamperes. The trap-assisted electron transport in the needle is associated with the doping of the needles during CVD synthesis from both substrate material and gaseous environment. Excellent stability of the emission current and its sensitivity to temperature and laser irradiation make diamond needles attractive for fabrication of high brightness electron point sources and sensors.
We have manufactured highly efficient field emission carbon cathode for electron gun in E-sail space thruster. The obtained characteristics of the cathode fully satisfy the requirements of the E-gun in terms of efficiency and stability, however, the E-gun performance can be further improved by changing the anode electrode design. The test mission satellite ETSCube-1 with a nanocarbon E-gun onboard has been launched in 2013. The detailed analysis of the nano-graphite cathodes has shown that they can provide electron beams of current density up to 1 A/cm2 from surface area of up to several square centimeters. These parameters can not be reached with conventional cathodes based on metals or semiconductors. This opens avenues towards application the nano-graphite cold cathodes in industry and commercialization of the FANCEE results.