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Appl. Phys. Lett., Vol. 84, No. 14, 5 April 2004
Zhang, Jiang, and Wang
tance. According to the Fowler–Nordheim theory,21 we cal-
culated the field enhanced factor  at different gaps ͓Fig.
4͑d͔͒. That the  increases with increasing d can be attrib-
uted to the increasing emission area. A field emission current
density of 1 mA/cm2 is achieved at ϳ1700 V and the current
density of 10 mA/cm2 can be achieved at ϳ2100 V when the
gap distance is set to be 2 mm. The well performance of the
field-emission properties to these materials may be attributed
to the large number of emission sites formed by the tips and
edges of the CNHs.
In summary, we successfully fabricated the aligned car-
bon nanohelices ‘‘film’’ on iron needle and find that the
alignment is directly related to the bias voltage added on the
substrate. The microstructure analysis shows that they have
graphite layer-like structure with amounts of layer bending
coupled with lattice stress. The field emission properties are
very good and comparable to that of carbon nanotubes. The
work for characterizing the mechanical, electrical, and
chemical properties are on the way.
FIG. 4. ͑a͒ A field emission I–V curve of an iron wire ͑cutting off the tip
without CNHs͒ at different gaps. ͑b͒, ͑c͒ The optimized I–E curves and FN
plots corresponding to ͑a͒, ͑d͒ d– curve.
The authors thank W. J. Zhang for the assistance with the
Raman measurements. The work was partly supported by the
Ministry of Education and Research, Germany ͑BMBF͒
within a Germany–China bilateral cooperation program, the
NSF of China ͑10134030, 60021403͒, the National Key
Project for Basic Research ͑G2000067103͒, and the National
Key Project for High-Tech of China ͑2002AA311150͒.
smaller catalyst particles are difficult to transform into drop-
let shape, which is essential to grow helical structures. The
alignment of the CNHs is supposed to be caused by the high
bias voltage added as has been reported for carbon
nanotubes/fibers or micro coils ͑such as Refs. 14 and 15͒.
The field-emission characteristics are studied in home-
made instrument, which used a phosphorus glass plate coated
with indium tin oxide as anode. The iron needle coated with
aligned CNHs acts as cathode. In order to eliminate the in-
fluence of the iron tip when emitting, we cut off the iron tip
͑where it is not grown with CNHs͒ and aligned the iron wire
axis normal to the transparent anode. The I–V curve shown
in Fig. 4͑a͒ is measured at different gaps between the anode
and cathode. Due to the rather small size of the cathode and
large gap between the anode and cathode, the electrical field
on the near sample surface is not uniform and can no longer
be calculated as in the works done for large-area samples
before. Two extreme conditions should be considered, one is
that the gap distance is very small and another is the gap
distance is very large. In the first case, the average electrical
field is proportional to V/d, where V is the applied electrical
field and d is the gap distance. In the second case, the elec-
trical field ͑near the sample surface͒ is roughly proportional
to V(1/dϩ1/r) when adopting a sphere approximation for
the cathode tip, where r is the radius of the tip. At present the
experimental parameters are between these two extreme con-
ditions, and we assume that the average electrical field ͑E͒ is
roughly equal to V(1/dϩ1/␣), where ␣ is a parameter.
Given that the turn-on electrical fields (Et) at different gaps
are roughly the same, we set the optimized ␣ value of 6
based on the I–V data ͓Fig. 4͑a͔͒. And we can get the Et of
ϳ0.6 V/m, which is similar or smaller compared to that of
carbon fibers or nanotubes.16–20 The optimized I–E curve is
shown in Fig. 4͑b͒, and the corresponding Fowler–Nordheim
͑FN͒ plots ͓Fig. 4͑c͔͒ are straight lines for different gap dis-
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