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November 2003

Feature

 


Nanotechnology - the next big thing 
is very, very small


Nanotechnologists at work in the lab. 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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