Hierarchical Field Emission Devices - Hierarchical Nanostructures for Energy Devices - page 238

realized through plasma treatment of graphene, coaxially deposited over
individual carbon fibers by means of an aqueous phase electrophoretic
deposition technique. Upon variation of the duration of plasma etching, the
planar graphene structure sheathed around carbon fiber was turned into a
hierarchical geometry composed of vertical-wall or cone-like morphology
(Figure 10.2e). The cathodes developed exhibited outstanding electron
emission performance with a turn-on field as low as 0.41 V mm 1 , while the
performance was preserved upon extreme bending of the flexible cathode.
The same group has also demonstrated hierarchical FE cathodes based on
spin coating of a solution-processed ZnO nanowire array with graphene and
subsequent plasma modification (Figure 10.2f). 16 The superior field
emission properties were attributed in part to the multistage geometrical
field enhancement as well as to the facile electron transfer from nanowires
to graphene due to band bending at the ZnO-graphene interface.

d n 3 r 4 n g | 0

10.2.2 Metal Oxide HFE Cathodes
Metal oxides are proving to be the richest family of nanostructures de-
veloped to date, while a plethora of self-organized hierarchically structured
metal oxide nanomaterials have been synthesized. Among these, ZnO-based
HFEs are the most frequently tested as cathode field electron emitters.
Figure 10.3 summarizes the best performing ZnO HFE architectures to date.
In particular, Xu et al. fabricated, through thermal chemical vapor de-
position, four different hierarchical ZnO nanostructures 17 including nano-
sleeve-fishes, radial nanowire arrays, nanocombs and nanoflowers
(Figure 10.3a). The morphology can be controlled by both the growth tem-
perature and the gas flow rate. The FE measurements performed showed
that such structures exhibit high emission current densities and low turn-on
fields of 1.3, 1.9, 2.5 and 3.4 V mm 1 respectively (Figure 10.3g). Later, the FE
properties of well-aligned ZnO nanorod arrays with three kinds of tip
morphology—abruptly sharpened (Figure 10.3b), tapered and plane—have
been comparatively investigated. 18 It is found that the FE performance is
strongly affected by the tip morphology and relies not only on the nanorod's
radius of curvature and array density but also on the detailed tip morph-
ology. Among the three kinds of samples, the nanorods with abruptly
sharpened tips exhibit the best field emission property. This excellent per-
formance was attributed to the unique two-stage geometrical configuration
of the abruptly sharpened tip. Three different types of hierarchical ZnO
nanostructures, i.e. nanocombs, nanoscrewdrivers and nanonails, have been
through a simple thermal evaporation process and tested for their FE
properties. 19 The successful fabrication of HFE cathodes comprising aligned
arrays of single-crystalline, 6-fold-symmetrical, hierarchical ZnO nano-
structures was also reported. 20 Meanwhile, Wang et al. successfully syn-
thesized 21 by a chemical vapor deposition method hierarchical ZnO
hexagonal towers. Kuan et al. studied the FE properties of six-fold core-
shelled three-dimensional hierarchical Zn/ZnO micro/nano tip arrays,

.