the emissive materials coating (barium strontium oxide) with
a large Schottky effect induced by the CNTs that further
reduces the overall potential barrier as illustrated in Fig.
1(b). Details of the growth and fabrication of such thin films
are published elsewhere.23,24 Here in short, CNTs were
grown on thin tungsten ribbon using plasma enhanced chemical
vapor deposition method. Its surface was further functionalized
with a low work function barium strontium oxide
coating using magnetron sputtering technique. Figs. 1(c) and
1(d) are scanning electron microcopy (SEM) graphs of the
bare CNTs and CNTs with barium strontium oxide coating
on it, respectively. The work function of the surface was
measured and determined using the Richardson line method.
A typical sample used in this study has a work function
measured at about 1.5 eV. The role of sharp CNT tips here is
to induce a large field enhancement effect that further
reduces the potential barrier for electron emission or lowers
the effective work function. CNTs are known to induce a
large field enhancement effect due to its unique geometric
feature.23,24 The impact of a large field enhancement effect
is clearly shown in the governing Richardson equation for
the emission,