Hierarchical Field Emission Devices - Hierarchical Nanostructures for Energy Devices - page 241

d n 3 r 4 n g | 0

Figure 10.4 Semiconducting and metal-organic HFE cathodes: AlN hierarchical

nanoarchitectures (a) - reproduced with permission from ref. 30; free-
standing WS 2 nanotubes on Si microspikes - reproduced with permis-
sion from ref. 32; metal-organic charge transfer complex nanowires on
carbon fabric fibres - reproduced with permission from ref. 34.

such materials can be freely emitted into a vacuum. The nanostructures
consisted of stems growing along the o 00014 direction decorated with
comb-like nanotips growing perpendicularly out from both sides of the
stems along the o 01104 direction (Figure 10.4a). Such a hierarchical
architecture exhibited remarkable FE properties. The intrinsically low
electron anity of AlN and the enhanced emission from sharp nanotips and
thin edges are assumed to contribute to the high emission currents
observed. Later, the growth of well-aligned AlN nanorods with hairy surfaces
via a vapor-solid process was reported. 31 The overall hierarchical cathode
had an aligned columnar structure, while each nanorod exhibited a hairy
surface.
Hierarchical WS 2 nanotube (NT)-based cathodes have been fabricated via
drop casting of WS 2 NTs/P3HT solutions onto low and high-aspect ratio
Si microspike arrays fabricated by ultrafast laser processing. 32 Following
drop-casting, the microspikes were partially covered by the nanocomposite,
while some NTs protruded out of the P3HT matrix and decorated the top of
the microspikes, forming a random array of free-standing NTs
(Figure 10.4b). It was demonstrated that free-standing WS 2 NT-based field
emitters exhibit comparable threshold field but superior stability compared
to optimized CNT mats.
Zhong et al. reported 33 on the hydrothermal synthesis of SnS 2 and In-
doped SnS 2 exhibiting a flower-like hierarchical structure. It was found that
both materials showed excellent FE properties, while the emitters fabricated
by doped structures exhibited superior FE behavior.

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10.2.4 Metal-organic-based HFE Cathodes
Maiti et al. have fabricated metal-based hierarchical FE arrays via the
chemical growth of metal-organic charge transfer complex nanowires on
conducting carbon fabric fibres. 34 Control over the growth parameters
during the organic solid phase reaction gave rise to a modification in
nanowire morphological properties, as well as their distribution atop the