Electron emission from nanostructured materials.

Field emission;Thermionic emission;Carbon nanotubes;Nanowires;Energy conversion;Electron emission

Abdelilah Safir

University:University of Louisville

Department:Electrical and Computer Engineering

关键词: Field emission;    Thermionic emission;    Carbon nanotubes;    Nanowires;    Energy conversion;    Electron emission;   

Others  :  https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=2245&context=etd

美国|英语

来源: The Universite of Louisville's Institutional Repository

【 摘 要 】

In this dissertation, standardized methods for measuring electron emission (EE) from nanostructured materials are established. Design of an emitter array platform, synthesis and nanomanipulation of different types of are successfully conducted. Preexisting as well as novel nanostructures are examined for possible use as electron point sources. Three main categories of emitters are under evaluation: oxide nanowires, metallic nanowires and carbon based nanomaterials (CBNs). Tungsten oxides nanowires have low work function, then metallic nanowires have high electrical conductivity and abundant number of free electrons at and below their Fermi level and lastly, CBNs have superior electrical, mechanical, chemical and thermal properties. This evaluation is designed to compare and choose among the nanoemitters that are suitable for EE. Simulation through theoretical modeling is provided to optimize the parameters directly or indirectly affecting EE properties. The models are to enhance the emitter's performance through increase the packing density, reduce the field screening effect, lower the turn-on and the threshold electric fields and increase the emission current densities. The current estimations and the modeling of the validity regions where EE types theoretically exist, help to select and fabricate optimum emitters. An assembly consisting of sample substrate, electrical feedthroughs, electrodes, nano/micro-manipulator and insulators are mounted within a vacuum chamber. An ion vacuum pump and a turbo pump are used to reach a vacuum pressure of 10?7 Torr. Two systems are used for EE characterization of nanostructures: bulk and In-situ configurations. The bulk investigation is realized by designing a vacuum chamber and different sample holders that can resist harsh environment as well as high temperature for both FE and TE experiments. In-situ experiments are conducted in the chamber of the scanning electron microscope (SEM), it consists of designing special sample holders plus modifications of the SEM chamber for the ease of EE characterization. Samples with different materials, densities, radii of curvatures, and lengths ranging respectively from 107-10? emitter/cm², 5-300 nm, and 3*10³-107 nm, are produced. The CBNs used are characterized by different structures and shapes that are defined by the monolayer carbon sheet takes. Cylindrical sheets are equivalent to nanotubes while graphene are flat sheets. Emitter's structures are varied by altering the critical growth parameters such as temperature, pressure and constituent materials. Enhancement of the FE properties, the design of an optimum emitter density ad reduction of the field screening effect is possible by selecting appropriate materials, synthesizing nanostructures with small radius (10 nm), high aspect