Hierarchical Field Emission Devices - Hierarchical Nanostructures for Energy Devices - page 237

aligned MWCNTs. 12 The resulting cathodes can be shown in Figure 10.2b.
The FE properties of the MWCNTs grown on pyramids with various ARs were
measured and compared with flat MWCNT mats grown on planar silicon
substrates. It is shown that the higher the AR of the Si pyramids, the lower
the threshold field of the respective cathodes. In particular, a threshold field
value as low as 1.95 V mm 1 was achieved for the hierarchical MWCNT
emitters with an AR value of 0.6, which was lower by more than 40% com-
pared to planar ones (Figure 10.2g). Analysis of the respective FN curves
revealed the existence of two distinct low-field (LF) and high-field FE re-
gimes. In both regimes, the estimated b values for the hierarchical cathodes
were found to be almost two times higher compared to those measured on
flat cathodes.
Stratakis et al. reported an ecient methodology for the fabrication of
large scale regular arrays of hierarchical carbon nanowall (CNW) field
emitters. 13 The respective cathodes had been produced by CVD of CNWs on
forests of micro-conical Si spikes (CNW/mSi) fabricated by ultrafast laser
structuring. Figure 10.2c depicts the corresponding SEM images of CNW/mSi
field emission cathodes after the CVD process. It is observed that CNWs
follow the surface and decorate the microspikes forming a flower-like coat-
ing. The FE properties of CNWs grown on spikes with low and high AR were
measured and compared with CNWs layers grown on planar silicon sub-
strates. It is found that the FE performance of hierarchical CNW structures is
far superior to that of planar CNW mats and comparable to that reported for
optimized CNT-based emitters. The improved field emission properties of
the fabricated arrays were attributed to the dual micro and nanomorphology
of the emitters, involving a TSFE (eqn 10.7).
Owing to its inherent 2D geometry, graphene should give rise to high
geometric field enhancement, allowing the extraction of electrons at low
threshold electric fields. However, typical deposition methods provide
graphene flakes that tend to lie parallel to or protruding at small angles from
the substrate, thus limiting the geometrical field enhancement. In this
respect, controlled deposition of free-standing graphene nanosheets on
microstructured substrates could provide state- of-the art hierarchical FE
cathodes. Stratakis et al. reported a simple and general solution-based
approach for deposition of free-standing few-layer graphene (FLG) sheets. 14
The method is based on drop-casting an FLG-polymer solution on forests of
conical micro-spikes engraved on Si (FLG/mSi) exhibiting different hydro-
philicities. It is shown that, depending on the deposition conditions, the
FLG flakes become free-standing with their edges anchored to the micro-
spikes (Figure 10.2d). As shown in Figure 10.2h, the hierarchical FLG/mSi
cathodes were found to exhibit excellent FE performance, with turn-on fields
as low as 2.3 V mm 1 and a field enhancement of a few thousand, and a
stability that was superior to that of thin-film-type FLG emitters prepared on
planar substrates.
Maiti et al. developed a hierarchical FE cathode comprising graphene
nanostructures over a flexible fabric substrate. 15 Nanostructuring was

d n 3 r 4 n g | 0

.