Hierarchical Field Emission Devices - Hierarchical Nanostructures for Energy Devices - page 240

the base of individual CNTs through the low-temperature wet-chemical
batch deposition technique, while the size and interspacing of the nanowires
can be controlled by precursor concentration, growth temperature and time
duration. 23 The special double-stage structure of the achieved nano-
structures in addition to the high density of ZnO's tiny tips caused an
enhancement in their field emission behaviour, in comparison with ZnO-
NW and CNT arrays.
HFEs have also been fabricated out of materials other than ZnO
metal oxide nanostructures. Using a hydrothermal process, Sarkar et al.
fabricated 24 different rutile TiO 2 -based hierarchical nanostructures
(Figure 10.3d). In particular, cauliflowers, 3D microspheres, densely-packed
nanorod arrays, step edge faceted nanorods and branched structures have
been demonstrated. The FE characteristics of these architectures were
studied showing that the multilevel branched cathodes exhibited the best
electron emission properties. Besides this, hierarchical TiO 2 nanostructures
over conducting carbon fabric was realized via hydrothermal route. 25 The
resulting FE cathodes showed remarkable FE performance, even in extreme
bending conditions. The FE properties of MoO 2 nanowires decorated with
nanoprotrusions (Figure 10.3e), grown on a silicon substrate have been
studied too. 26 Such HFEs exhibited superior emission with a turn-on field of
0.2 V mm 1 and an enhancement factor of 42991. It was found that FE
measurements depend on cathode-anode distance, d, while the relationship
between 1/b and 1/d was linear, suggesting that the FE behaviour followed a
two-step FE model. Self-organized, sallow-like, hierarchical SnO 2 nano-
structures have been successfully prepared by a vapor phase transport
method. 27 Field emission measurements of these nanostructures showed a
high emission current density and low turn-on field of 1.88 V mm 1
(Figure 10.3h). Li et al. studied the FE properties of single-crystalline WO 3
nanowires on carbon paper synthesized by a chemical vapor deposition
process. 28 SEM imaging showed that the WO 3 nanowires were uniformly
grown onto the carbon fibers (Figure 10.3f). The FE cathodes tested showed
remarkable FE performance and current stability. The successful fabrication
of well-ordered and aligned hematite (Fe 2 O 3 ) hierarchical columnar arrays
via pulsed laser deposition was demonstrated. 29 The hematite comprised an
array of microcolumns decorated by nanoplates or nanoparticles that can
be controlled by oxygen pressure during PLD. It was found that the FE
properties of such SFE cathodes depend on cathode-anode distance, d, while
the relationship between 1/b and 1/d was linear, suggesting that the FE
behaviour followed a two-step FE model.

d n 3 r 4 n g | 0

.

10.2.3 Semiconducting HFE Cathodes
Single-crystalline wurtzite AlN hierarchical nanoarchitectures were self-
assembled through a chemical vapor transport and compensation process. 30
FE applications of AlN materials are of interest, considering their negative
electron anity; therefore electrons excited into the conduction band of