Chemistry Reference
In-Depth Information
of r
2
(Figure 10.1b). Assuming a 'two-step field enhancement
(TSFE)'
approach, the electric field on the primary tip is
E
1
¼
b
1
(V/d)
¼
(h
1
/r
1
)(V/d),
(10.5)
d
n
3
r
4
n
g
|
0
while that on the very end of the secondary protrusions is
E
2
¼
b
2
E
1
¼
(h
2
/r
2
)E
1
¼
b
1
b
2
(V/d).
(10.6)
Later on, Solntsev et al.
7
considered FE cathodes consisting of N distinct
stages or substructures, which is defined as a multistage field enhancement
effect; the field enhancement factor for such structures has been given as
E
2
¼
Y
N
b
i
E
o
(10
:
7)
i
¼
0
Later, Zhirnov pointed out that in most cases this analytical model
overestimates the field enhancement factor,
8
indicating that the local
surface structure and thus the dimensional interaction between the various
secondary geometric protrusions of the hierarchical emitter is an important
electrostatic factor that significantly affects the surface field and potential
barrier. Such nano and mesoscale electrostatic perturbations are complex
phenomena that are extremely dicult to treat analytically and without the
use of sophisticated numerical methods that account for the multiscale
nature of the secondary structures.
To date, a wide variety of hierarchical field emitters (HFE) have been
demonstrated including, carbon-based, metallic, metal oxide, metal-organic
and semiconducting, as summarized in Table 10.1. This chapter demon-
strates the FE characteristics of those exhibiting the best performance and
stability under device operation. It is concluded that multiscale emitters are
superior compared with single-length scale ones, demonstrating the great
potential of hierarchical FE technology for future electron device applications.
.
10.2.1 Carbon-based Field HFE Cathodes
Carbon-based nanostructures, including nanotubes, nanowalls and
graphene, exhibit a unique combination of properties for superior field
emission performance. Sato et al. were the first to indicate that the field
emission of single-wall carbon nanotubes (SWCNTs) can be enhanced via
depositing them on a micro-rough substrate.
9
For this purpose, using
chemical vapor deposition (CVD), they had selectively grown SWCNTs on
lithographically etched silicon microprotrusions. FE experiments showed
that this approach was favourable towards the improvement of field
emission characteristics including the FE threshold and enhancement
factor. Improved FE properties had also been measured for SWCNTs grown,
using an electrophoretic method, on Ag microparticles.
10
According to a
TSFE process, the field enhancement of each emission site can be under-
stood by a coupling of field enhancement of Ag particles and intrinsic field
Search WWH ::
Custom Search