Monday, May 10, 2010

ELECTRONIC PROPERTIES OF CARBON NANOTUBES



Interest in carbon nanotubes has grown at a very rapid rate because of their many
exceptional properties, which span the spectrum from mechanical and chemical robustness to
novel electronic transport properties. Their physics, chemistry and perspectives for
applications are very challenging. Below we highlight the main results of the Lausanne group
and their collaborators on transport, electron spin resonance, elastic and field emission
properties of single wall (SWNT) and multi-wall (MWNT) carbon nanotubes.


SAMPLES

We use MWNTs produced by arc discharge or by thermal decomposition of
hydrocarbons, and SWNTs either prepared by the arc discharge method in the presence of
catalysts or commercially available (Carbolex, Rice University, MER, DEL). The first step in
the study of CNTs is technological: their purification. This is especially true for SWNTs,
which are severely contaminated with magnetic catalyst particles. The purity of the arc-
discharge fabricated MWNTs is much better, since magnetic materials are not used in their
production. Nevertheless they have to be separated from graphitic flakes, polyhedral particles
and amorphous carbon present in the raw soot. For MWNTs, we have developed a soft
purification method, which uses the properties of colloidal suspensions1. We started the
purification with a suspension prepared from 500 ml of distilled water, 2.5 g of SDS (sodium
dodecyl sulfate; a common surfactant) and 50 mg of MWNT arc powder sonicated for 15
minutes. Sedimentation and centrifugation (at 5000 rpm for 10 minutes) removed all graphitic
particles larger than 500 nm from the solution, as confirmed by low magnification SEM
observations (upper part of Fig. 1). We then added surfactant to the solution to reach 12 CMC
(critical micelle concentration). At these surfactant concentrations, micelles form and induce
flocculation, i.e. the formation of aggregates. These aggregates mostly contain large objects,
while smaller objects remain dispersed, and sediment after a certain time, typically a few
days. After decanting the suspension one week later, we repeat the procedure once or twice.
Fig. 1 (lower part) shows scanning electron microscopy images of a MWNT deposit after the
separation procedure. The untreated material contains a large proportion of nanoparticles
(typically 70 % in number and 40 % in weight). After the purification, the material remaining
in suspension consisted nearly exclusively of nanoparticles, while the sediment contained
nanotubes with a content of over 80 % in weight.





Figure 1. Scanning electron (SEM) micrographs of a MWNT deposit (top) and MWNTs after purification and as a Òside-productÊÓ, nanoparticles of carbon.


For SWNTs, the purification of the raw soot has been carried out by oxidative
dissolution of the carbon encapsulated metal particles with concentrated acid, which ensures
maximum efficacy of the process of metal elimination. A weighed amount of the raw soot
was sonicated in an ultrasonic bath at 25 ûC with concentrated nitric acid for a few minutes
and subsequently refluxed for 4-6 h. Thick brown fumes containing oxides of nitrogen were
seen, indicating the rapid oxidation of carbon to carbon dioxide. After cooling, water was
added so as to leave the samples in 6M HNO3 for the next 8-12h, after which, it was
centrifuged several times and the supernatant rejected until the pH of the solution was around
6.5. High resolution transmission electron microscopy of the material from this solution
showed the presence of long ropes of bundled nanotubes accompanied by small amounts of
carbon-coated metal particles. Parallel examination of the unpurified soot indicated that more
than 80% of the metal had been dissolved. The SWNTs in suspension were stabilized by
using a surfactant such as SDS and left undisturbed for 3-5 days until the slow aggregation of
the nanotubes allowed their separation from the nanoparticles in solution. The nanotube
suspension was filtered through a polycarbonate membrane (1 µm pore size) in order to
eliminate most of the particles. On drying, the sediment on the filter paper peeled away to
form a self supporting sheet of carbon nanotubes. Scanning electron micrographs of such a
sediment, like the one in Fig. 2 shows a network of SWNTs.


Figure 2. Scanning electron (SEM) micrographs of (a) a SWNT deposit and (b) a mat of SWNTs after purification. A few nanoparticles and embedded catalyst particles are still present.


MWNTs are also prepared by catalytic decomposition of acetylene (or other carbon-
containing materials) over supported transition metal catalysts in a temperature range of 700800oC.
This reaction can be carried out under relatively mild conditions in a fixed bed flow
reactor at atmospheric pressure. After optimization, the catalytic method can be suitable for
the production of either single and multiwall or spiral carbon nanotubes.



Figure 3. Scanning electron (SEM) micrographs of MWNTs prepared by catalytic decomposition of acetylene


A further advantage of this method is that it enables the deposition of carbon
nanotubes on pre-designed lithographic structures2, producing ordered arrays which can be
used in applications such as thin-screen technology, electron guns etc. The feasibility of the
deposition of carbon nanotubes on a ceramic membrane and its field emission properties was
demonstrated.


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