Photonically and thermally excited electron emission from modified graphitic nanopetals

Scott J Vander Laan, Purdue University

Abstract

Efficient electron emission requires a low work function and a stable emitter material. Carbon nanomaterials have shown promise as electron emitters, and recently graphite, carbon nanofibers, carbon nanotubes, and graphene have been examined. The extraordinary properties of carbon nanotubes and graphene in particular have attracted extensive investigation of their electrical, mechanical, and chemical properties. The work function of these crystalline carbon materials can be significantly reduced by the intercalation of alkali metals, thus increasing their emission current. In this work the emission from potassium-intercalated carbon nanosheet extensions grown on electrode graphite is investigated. These petal-like structures, composed of 5–25 layers of graphene, are synthesized using microwave plasma chemical vapor deposition. Samples are intercalated with potassium, and a hemispherical energy analyzer is used to measure the emission intensity due to thermal and photo-excitation. Results indicate that these structures could potentially be useful in direct thermal and solar energy conversion devices. The emission from the potassium-intercalated structures is compared to unaltered samples, and intercalation is consistently found to decrease the work function by 2.4–2.8eV. The thermal stability of the intercalated petals is investigated, and it is found that after an initial heating and cooling cycle the samples are relatively stable at low temperatures. However, electron energy distributions undergo significant changes at higher temperatures suggesting the samples are not stable at elevated temperatures. X-ray diffraction analysis performed before and after low temperature testing indicates stage-1 intercalation. Palladium electrodeposition is also investigated as a means to inhibit potassium uptake during the intercalation process. Moreover, illumination with a solar simulator has resulted in emission intensities increasing up to two orders of magnitude. Emission intensity is also shown to scale linearly with the intensity of incident radiation. A simple combined thermionic and photoemission model is developed that provides good fits to measured electron energy distributions by accounting for multiple work functions on the samples. This model suggests that high energy electrons tend to emit in a thermionic-like distribution when subject to solar radiation, even at temperatures where no thermionic emission is measured.

Degree

M.S.E.C.E.

Advisors

Janes, Purdue University.

Subject Area

Electrical engineering|Nanotechnology

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