Nanotechnology - the next big thing
is very, very small
Nanotechnology is the almost-invisible science of construction on scales of a
billionth of a metre. It involves making things using beams, girders, pumps and
wheels just one millionth of a millimetre long. Nanoelectronics has enormous
applications, particularly in computing. But there's more….
Although sub-Saharan Africa may be a late entrant in this new technological
race, an African Materials Forum due to be held in Johannesburg, South Africa,
at Wits University in December should provide a distinct kick-start. A recent
South African strategy document does outline two distinct opportunities in
nanotechnology for the southern region of this continent. Nanotechnology can add
enormous value to African minerals - gold, titanium, palladium, platinum and so
on - once simply exported abroad in their raw state to be transformed by others
into valuable commodities. The other focus is using nanotechnology to fight
poverty. In the arena of social development, nanotechnology could lead towards
low cost energy, low cost electronics, and more efficient drug delivery.
Making silicon light up
Nanotechnology uses the scale of the nanometre, equal to one millionth of a
millimetre. In this range, differences of size become important: when something
like silicon is deliberately carved down to its smallest possible level, it
behaves distinctly differently. Bulk silicon doesn't emit light. But if you make
silicon very small, it emits light. So one of the most interesting things about
small lumps of matter is that their properties change dramatically as the
samples shrink. Materials might become more stable or longer lasting, for
example, which can make paint coatings more durable or colour displays brighter
and more effective. In medicine, administering drugs that don't dissolve in
water is difficult, but nanoparticles could carry the drug molecules around the
body suspended in the blood.
Nanotech takes off in Africa
Africa has distinct opportunities for applications of South Africa's
University of Zululand's work, in areas such as energy storage and water
treatment. "Our synthesis and characterisation of nanomaterials for
possible industrial applications places our team in a leadership position in the
country in this rapidly growing field," says UniZul's Department of
Chemistry project leader, Professor Gabriel Kolawole. What is particularly gratifying about this research is that it has
come about after just a few years at an institution that was never meant to be
more than a glorified high school, not to educate scientists, not to conduct
research - and which has suffered decades of academic neglect.
The team led by Kolawole focuses on coordination chemistry, the science of
studying how well disparate chemicals dance together in various combinations.
Coordination chemistry is developing a variety of organometallic compounds, some
for potential use as inorganic antibiotics, and others for treating
chloroquine-resistant malaria. Compounds are also being investigated for
possible uses as microelectronic devices, and in treating industrial waste water
polluted with heavy metals.
Dr Neerish Revaprasadu is UniZul's deputy project leader and South Africa's
only formally trained nanomaterials chemist. Think of materials chemistry as
plain old chemistry with an added dimension, in which scientists make chemical
compounds, as in straightforward traditional chemistry, and then go further.
They break it down to its lowest common denominator, and that's the materials
side of the chemistry.
"On the one hand we are making a lot of compounds and that in itself is
pure chemistry and then we take the materials and deposit them. In other words
we use a technique, such as chemical vapour deposition, in which a material is
heated and volatised and it moves, depositing itself on a substrate such as
glass or a silicon wafer and binding itself to that substrate," he
explains.
Applications
Dr Revaprasadu notes that materials on the ultrasmall nanoscale exhibit
unique properties that - while it's very early days yet - are potentially useful
for various applications, in light-emitting devices such as billboards or solid state lasers such as used in medical applications.
Materials brought down to the nanoscale are also important in the process known
as catalysis, deliberately causing chemical reactions in order to create a new
combination or effect, and this is causing a great deal of interest in both the
mining industry and in pharmaceuticals.
The team has been making semiconductor nanoparticles using the so-called
"top-down" precursor approach in which, says Dr Revaprasadu, "you
take something big and break it down chemically to produce the nanoparticles
that you want". The top-down approach is important because it is an
environmentally friendly, simple route to high quality, high yield materials.
This method avoids the use of volatile and toxic compounds employed in other
organometallic routes, which means it is also safer in the laboratory.
The right precursors
The University of Zululand Department of Chemistry has been on the cutting
edge of this extremely interdisciplinary side of science, investigating the use
of various compounds as potential precursors for nanoparticle synthesis. A
precursor is a large molecule, not yet nano, which is then broken down by
heating to form nanosized materials.
"The trick is to keep materials at that nano scale," says Dr
Revaprasadu. "You don't want the individual particles to join back up
again. They need to be kept apart while at the same time being made to work in
unison. They need to communicate with each other, as a team, without changing
form."
The UniZulu team uses compounds containing both a metal, such as copper,
zinc, cadmium, platinum, nickel or palladium, with a chalcogenide (sulfur and
selenium) to make these single molecule precursors to create high quality
semiconductor nanoparticles. (More detail below). The nanoparticles are surface passivated (in other words, they need to
be coated by something, or passivated, to keep each nanoparticle separate from
its compatriots. A case of good fences make for good neighbours, perhaps.)
"We've concentrated on finding the best compounds to use as precursors
and how best to use them," explains Revaprasadu. "We've been
fine-tuning our method, and also investigating ways to achieve high yields while
maintaining high quality and low cost, because demand for quality will increase
worldwide in the next decade, and our project addresses the need for simple,
low-cost synthetic routes to nanosized materials."
This top-down approach creates semiconductors with a difference. The team can
take a material - such as cadmium sulfide, which is used in photovoltaic cells
which operate solar panels, or zinc sulfide, used in electroluminescence devices
such as big advertising billboards that light up at night. These chemicals are
classified as semiconductors but behave differently at nano level. The main
difference is band gap change, a kind of barrier to electrons that varies
between metals, which have a small band gap, and insulators, which have a large
band gap which makes it difficult for an electron to jump across. The band gap
properties of any particular material has huge implications for conducting
electricity. Nanotechnologists can engineer a band gap to a preferred size.
To date the team has synthesized close to twenty cadmium and zinc complexes
for use as precursors to Cadmium Sulfide and Zinc Sulfide nanoparticles. The
optical properties of the nanoparticles have been studied by ultraviolet light,
visible light and photoluminescence spectroscopy at Unizul in order to measure
the band gap. An increase in the band gap proves that they have succeeded in
making the material nano. The materials have been characterised by X-ray
diffraction and electron microscopy techniques at the University of Manchester,
which provides additional proof. Their work also involves studying the effect of
the precursor structure in relation to the quality and yield of the
nanoparticles.
"It's a very fundamental study," says Dr Revaprasadu. "We can
make any amount of nanoparticles but beyond that, the processing of it in
applications, is in the future. It can be put on a film, or on a polymer or
processed in other ways. It's about adding value to old technologies, not just
replacing them. Look at platinum and palladium in catalysis - if we increase its
value at nanoscale, we can use less of it to do more work. The same applies to
solar cells. Nanoparticles can improve the process."
The team is also the only group in South Africa to be using these new
precursors for the micron-sized thin films, which are potentially useful in
solar cells. Micron is not nano-scale but is still very small! (More
detail below).
Revaprasadu emphasises the importance of developing a global centre in South
Africa for the country to compete internationally: "We mustn't miss out at
the early stage in this emerging area, which, globally, has very few experts.
Our centre is rare because we offer expertise that is hard to find. Those we
train are marketable worldwide - so is the research done by our group."
More information:
Dr Neerish Revaprasadu can be contacted on nrevapra@pan.uzulu.ac.za
The South African Nanotechnology Initiative is on the web at www.sani.org.za
Footnotes:
1) Very briefly this method involves the dispersion of the single source precursor
in tri-n-octylphosphine (TOP), followed by injection into hot
tri-n-octylphosphine oxide (TOPO) at elevated temperatures. The formation of the
nanoparticles is consistent with the LaMer mechanism for colloids. The
decomposition of the precursor drives the formation of the nanoparticles with
termination of growth occurring when the precursor supply is depleted. After the
initial injection there is a rapid burst of nucleation, which is followed by
controlled growth of the nuclei by Ostwald ripening. The resultant nanoparticles
are passivated by TOPO, preventing agglomeration. Back
2) In addition to nanoparticle synthesis this group has been looking
at the deposition of thin films (> 100 nm) using chemical vapour deposition
(CVD). They are using solution methods such chemical bath deposition (CBD) to
deposit films of semiconductor materials. This work makes them very versatile in a
broad area of materials chemistry. Back
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