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Vacuum level
Thermionic
emission
e
e
Schottky
emission
Work function
Field-emission
Fermi level
e
Metal
Vacuum
Figure 2.13. Various electron emission mechanisms.
emission from the tip of a nanotube was triggered by a combination of an external
field and another electron beam hitting the nanotube with a very high multi-
plication factor (ratio of emitted electrons to the incoming ones) [45]. This could
have many applications in nanoscale vacuum transistors and electron detectors.
Other nanowires have also been investigated for electron emitter applications,
including silicon, zinc oxide, and tungsten nanowires [46-48]. Controllable
nanoscale electron emitters are in demand not only in electron beam litho-
graphy and microscopy, but also in vacuum nanoelectronics, flat panel display
technologies [49, 50], free-electron analog to digital conversion [51], time-resolved
electron holography, and synchrotrons.
2.4.3.2. Sensors and Actuators. In order to sense a molecule or particle,
one needs a device that shows a measurable change in characteristics as the result
of the proximity or adsorption of that particle. In nanostructures, due to the large
surface-to-volume ratio, even a single particle sitting on the nanostructure could
potentially have a large effect on the nanostructure properties such as electronic
transport characteristics. Sensor applications have, therefore, been investigated in
nanotubes and nanowires. The advantages of these sensors include very high
sensitivity, low-voltage operation, low power consumption, and portability.
Examples include gas sensors [52, 53], biosensors [54-56], and chemical sensors
e
e
e
e
Nanotube tip
Cathode structure
Figure 2.14. Schematic representation of a nanoelectron source for vacuum
nanoelectronic applications.
 
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