Date on Master's Thesis/Doctoral Dissertation

8-2010

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Committee Chair

Cohn, Robert W.

Author's Keywords

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

Subject

Nanostructured materials; Electrons--Emission

Abstract

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 ratio (greater than 1000), the ideal density where the inter-emitter distance is comparable to the emitter height, the cathodes' uniformity, the treatment of the emitting surface, and integrating triode arrangement. Initially, the thermionic Emission (TE) investigations of these nanostructures produce emission at an onset temperature of 500° C, current densities of 160 mA/cm² at temperatures of 700-1200°c and predict the work function of the emitting materials. In addition, nanostructures can enhance the local electric field and increase the packing density to produce better EE properties. Then, FE investigations from different nanostructures showed that the small tip's diameter, high aspect ratio and tapered structures enhance emission through low turn-on fields (0.8 V/µm), low threshold fields (3 V/µm) and high current densities (520 mA/cm²). CCNTs having inter-emitters distance comparable to their average height contribute to the reduction of the field screening effect through large field enhancement factor ß (7000) and enhancement of the EE properties. EE experimental data along with its analysis demonstrate that CBNs have lower turn-on electric field, lower threshold fields, higher current density and higher field enhancement factor than those of microscopic metallic cathodes and oxide nanowires. Therefore, nanomaterial based emitters with their superior intrinsic properties based on the achieved EE results can be turned into potential EE point sources.

Download

Share

COinS