Hierarchical Field Emission Devices - Hierarchical Nanostructures for Energy Devices - page 231

surfaces. 1 Later on, several technological applications were demonstrated. In
particular, field emission sources are routinely used in scanning electron
microscopy and e-beam lithography because of their small optical size.
Furthermore, a high density two-dimensional array of emitters can be
fabricated by a variety of techniques resulting in cathode elements suitable
for flat panel displays. 2 Fabrication of FEDs is considered to be the most
promising application of field electron emission. A FED is a thin, flat
cathode ray tube comprising two-dimensional arrays of electron cathodes
(the pixels) and gating elements that drive the emitting electrons towards a
patterned phospor screen (Figure 10.1a). Each pixel comprises thousands of
micro or nanotips (typical density of 10 6 cm 2 ). Among the advantages of
this technology, are the low power consumption and cost and the potential
to provide displays that are thin and have low weight. Although FEDs have
the optical quality of a cathode ray tube, they do not produce X-rays, nor are
they sensitive to magnetic fields or large temperature variations. For all of
these reasons, FEDs can find a wide range of applications in industrial
and automobile applications or as a monitor for scientific and medical
instruments (such as mass spectrometers and oscilloscopes).
Owing to the potential unique properties of FEDs, an intense research
effort has been devoted to the design and fabrication of cold cathode
electron emitters exhibiting low operation voltage, high current emissivity,
and increased durability under poor vacuum conditions. For this purpose, a
wide range of materials have been considered as cold cathode field emitters
including metallic, metal-insulator-vacuum structures, thin films, diamond
and diamond films, graphite and graphite pastes, polymers, carbon fibers,
nanowires and nanotubes. Research and technology in this area indicate
that the cathodes developed to date suffer from several factors that limit
the operating performance and stability such as degradation due to Joule
heating and chemical instability at high emission currents, as well as ion

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.

Figure 10.1 Schematic of: a flat panel FE display (a); a hierarchical field

emitting tip (b).