Carbon Nantobes Production
Single wall carbon nanotubes (SWCNT) can be synthesized using thermal-CVD
in the gas phase process by catalytic disproportionation of CO
or by polymerization and hydrogenation of acetylene on iron particles.
Iron is suplied in the form of iron pentacarbonyl. The yield of material
from acetylene (aprox. 400mg/2h) is higher than from CO (approx.50mg).
For the acetylene case, the synthesis is performed at 900°C at
atmospheric pressure, while for the HIPCO case (CO disproportionation) the
temperature is 1100°C.
Figure is a typical TEM image showing that the CO material consists of
"ropes" of SWCNT, along with decoration by iron particles which are
encapsulated in graphitic carbon
Figure 1: TEM images of SWCNT
Single wall carbon nanotubes can be sysnthesized on the substrate as well when
the decomposition of methane at high temperature is used instead of acetylene.
The result of this type of growth can be found here.
Multi-wall carbon nanotubes (MWCNT) can be grown in both systems: thermal and
plasma-CVD. The catalyst (Fe) is deposited on a Si substrate via e-beam evaporation.
Typical films of MWCNT produced by thermal-CVD can be viewed in the figure 2. With
this method it is possible to grow large arrays of aligned and non-aligned MWNT
Fig.2. SEM images of aligned MWCNT grown on patterns.
The average diameter of such tubes is around 15nm. These tubes are produced
at atmospheric pressure at a temperature of 750°C using 10 time more hydrogen
than acetylene. Argon is also introducedas a carrier and diluted gas.
The homogeneity of the nanotube film on the substrate is
very poor for very short growth time in the thermal CVD set-up.
The density and diameter of the MWCNT can be changed as well if the amount of
the catalyst film is varied. By this method we can produce very clean and
aligned MWCNT film having a carpet-like aspect.
Using plasma enhanced-CVD as a production method, Fe catalyzed nanotubes
films are much thinner and theirs length can be better controlled by time.
The gaseous mixture and temperature conditions are similar for the
production in thermal-CVD. The only
difference is the pressure (7Torr) and the electric field used for
plasma ignition which seems to play an imporatant role in
their excellent alignment.
Figure 3 illustrates the CNT films whose their length was varied by time from
10 sec till 45 minutes.
Fig.3. SEM sequence of nanotubes alignment obtained in plasma-CVD set-up for different
In the first 5-8 minutes the nanotubes have a very
high growth rate of approx. 9um/min. Unlike the nanotubes produced by thermal-CVD
which will not suffer any changes in their structure and morphology if they
are grown for longer time (t.ex. 3hous), the nanotubes
synthesised by plasma-CVD are affected by the changing in the
chemical composition of the gaseous mixture in a plasma atmosphere. In the
case of tubes grown for time beyond 200s (tube lengths greater than ca. 30m),
was observed drastically modifications in their morphology. These changes are
not visible in the low resolution pictures, but can be clearly seen in the
TEM micrographs. Figure 4 shows a combination of SEM and TEM images.
The higher resolution TEM resolved very well the upper part of the tube.
The outer layer consists of graphitic flakes with an interlayer distance of
Fig.4. SEM and TEM sequence of the nanotubes grown for longer time in plasma-CVD
Upper images are SEM which depicts the graduate modification in the NT film from
the bottom till top. TEM micrographs reveals the thorn-like aspect of the
nanotubes longer than 200 um.
Individual vertically aligned MWCNT and carbon nanofiber are produced
in plasma-CVD when are catalysed by nickel (Ni). These nanostrucutures
can be produced when using ammonia and acetylene in plasma
atmosphere at 700°C. Figure 5 shows examples of the nanostructures when a 10nm thick Ni films was deposited
on silicon substrate with native oxide layer.
Fig.4.SEM images of individual nantubes/nanofibers grown in acetylene: ammonia
plasma atmosphere at 700°C.
The nanotubes diameters are in the range 10-100nm. TEM investigation show that
the growth is a via tip-growth mechanism and as the nanotubes diameter
increases tubes change theirs structures. The small diameter structure are
MWCNT, while the large diameter (>100 nm) show a bamaboo-like aspect of
the inner-walls. Figure 6 show the TEM image of a substrate cross-section where
Ni catalysed nanostructures were grown for 15 minutes.
Fig.6.TEM cross- section of the substrate where vertically aligned CNT/(CNF)
The individuality of such nanostructures gives the
possibility of a more controllable growth on patterned substrates where
the position of the catalyst is specified. It was prepared
two types of substrates: Ni patterned dots and Ni dots situated at the bottom
of the 250nm deep holes in Si. The results after 15 minutes growth are shown in
Fig.7.Growth of individual CNT/CNF on patterned dots.
One of the aim of these experiments is to test the ability of direct growth of
individual nanostructures for future NEMS devices and CNT's based devices.
Figure 8 show the first attemp where the NT was directly grown between the
electrodes, but couldn't be measured due to the electrodes modification during
the plasma growth. In the present, we are in the begining of understanding
the fabrication process of these types of devices, but in the nearest future
we will be able to produce them with high reproducibility.
Fig.8. First device attempt