Success stories
Simon Brown: Nanoclusters
Simon Brown is a visionary. He possesses the rare ability to see practical uses for discoveries made deep in the realms of fundamental physics. His feet on the ground and his sights fixed on the stars in a marriage of unhindered exploration and sharp-witted practicality that is the key to his success. Seven years ago when Simon arrived at Canterbury University, news of nanotechnology had barely reached these shores. Simon helped build the nanotech programme from the ground up and now sits at the helm of New Zealand’s first nanotechnology start-up company.
Simon arrived with a background in semiconductor physics and a keen interest in nanotechnology. He spent a whole year scanning the literature, trudging through thick forests of theory, gathering snippets of inspiration here and there, his eyes constantly peeled for some connection with his own expertise. During this year he discovered clusters – nanoscopic structures that fall in size between isolated atoms and bulk material. Clusters form an important link between the microscopic and macroscopic realms. Their unusual symmetries and properties had provided intellectual fodder for theoretical physicists since early last century but no one had considered using them in practical devices. In 1998 a piece of equipment that could be used to make and analyse clusters mysteriously appeared at Massey University looking for a home. Simon saw the opening he was looking for. He had the idea for an easy and cheap way to make nanowires by depositing metallic clusters into trenches etched into insulating substrates. He had the equipment transported to his Canterbury University lab and the adventure began.
Simon worked closely with Richard Blaikie’s team in the electrical engineering department across campus. He used their lithography equipment to etch trenches in his substrates drawing on Richard’s expertise in lithography.
In 1998 the Physics and Electrical Engineering departments teamed up and formed NEST (Nanostructure Engineering Science and Technology) to nurture their fledgling nanotech projects.
After seven years Simon’s fledgling has now grown to full strength and Simon sits at the helm of New Zealand’s first nanotechnology start-up company – Nano Cluster Devices, which has been set up to commercialise the techniques his team have developed.
The Process
In the source chamber a small piece of metal is heated until it starts to evaporate. Cooling by an inert gas causes the plume of atoms to condense into millions of clusters as small as 10 billionths of a metre. The clusters are blown through a slim nozzle into a vacuum chamber, forming a beam of these clusters which is directed at the insulating substrate. The clusters bounce around the smooth surface and eventually settle in the grooves. The substrate is then heated to just below the metal’s melting temperature. The clusters fuse together and Voila! You are left with conducting nano-scale wires.
Percolation Theory
Films made out of nanoclusters have entirely different conduction properties from normal thin films. For a cluster-assembled wire to conduct, electrons need to find a path through the clusters from one end of the wire to the other. Percolation theory describes the probability of finding such a conduction path through an area splattered with clusters. It turns out that for each wire geometry there is a well-defined percentage of surface coverage above which the wire conducts and below which it doesn’t. For macroscopic areas this is usually around 60% but Simon has discovered that when the dimensions shrink to the nano-scale it shoots down to around 20%, and this opens up the possibility of forming nanowires at this critical coverage.
Applications
With the technique for making cluster-assembled nanowires down to a fine art Simon’s team turn their attention to applications. Nanowires could be fashioned as sensors for electromagnetic radiation, chemicals or biological molecules or as magnetic read heads for the hard drives in computers that would increase information density. The list of possibilities is endless! The team have fixed their sights on one particular application – hydrogen gas sensors. Hydrogen gas is widely used in many industrial processes, it is emitted by power transformers when they are about to fail and it is also very explosive. There is a great need for reliable and inexpensive hydrogen gas sensors to detect leaks. Simon’s team are making sensors that meet this need. They are made of clusters of palladium, which absorbs hydrogen gas and expands causing the cluster- assembled wire to conduct. Paladium has good selectivity (it is immune to other gases) and the wires are very reliable. It looks like they’re on to a winner! If the hydrogen economy takes off Simon’s sensors could be a worldwide big hit. The widespread applications of cluster-assembled wires in the semiconductor industry lead to Simon’s work appearing on the cover of Semiconductor International, a leading magazine serving that industry. Nano Cluster Devices has struck a deal with a US corporation, Nanodynamics Inc, which is marketing cluster assembled devices, including hydrogen sensors, around the world.
Snowflakes and Strange Symmetries
While one side of Simon’s team’s research focuses on technological problems – harnessing and utilising the power of nature, the other is allowed to dance freely into realms of discovery - observing and delighting in nature unhindered. Shelley Scott, one of Simon’s PhD students is growing nanoflowers. They are made in a similar way to nanowires but on a surface so smooth that clusters are free to diffuse over it, subject only to the subtle, little explored forces they exert on each other. The result is beautiful. They aggregate to form landscapes of structures that resemble the fractal patterns of snowflakes. Shelley’s nanoflower images have captured worldwide attention, winning the best AFM picture prize at the 2004 EIPBN conference and being selected for the prestigious Veeco annual SPM Calendar for 2005.
Left: Shelley Scott working on the UHV deposition apparatus. Right: Shelley Scott's image of self-assembled antimony nanostructures selected for the prestigious Veeco annual SPM Calendar for 2005.
Atomic Force Microscope (AFM) of a 'nanoflower' grown by deposition of antimony on a smooth carbon surface.
Because the proportion of surface atoms to interior ones is so much larger in clusters than in bulk material the surface tension can be overwhelming. It squeezes the clusters into strange new symmetries that are forbidden in the bulk. Simon’s group analyses cluster structure by colliding a beam of clusters with a beam of electrons fired from a converted scanning electron microscope. The resulting diffraction pattern tells them about the clusters’ structure.
The first electron diffraction pattern obtained in the atomic cluster research laboratory (observed on a phosphor screen). The sample is a thin film of polycrystalline gold and each diffraction ring is due to scattering from a different plane of atoms. Diffraction patterns for clusters are much weaker and must be detected with a sensitive electronic detector.
Small clusters can have five-fold symmetry and irregular atomic spacing that are never present in bulk material. Many fundamental questions remain unanswered: How do these structures nucleate and grow? All bulk material begins its growth as clusters so how do clusters transform from one structure to another as successive atoms are added during growth? At what size does the bulk structure prevail? The atomic structure is essential in determining all cluster properties- physical and electronic. Answering some of these fundamental questions will inform and inspire the technological side of research. Who knows how the unusual quantum properties of clusters could be harnessed for technology. At the moment billions of dollars are being poured into the technology for creating semiconductor quantum dots but the fabrication techniques can only be used for a limited number of materials. With clusters the control of material and size is far better. A little more financial input could make cluster quantum dots extremely successful.
Three typical structures that are observed for small particles but are forbidden in the bulk form of the material.
Simon insists that nothing his team is doing is new. “We’re just putting existing things together. People had made clusters and done lithography but never thought of using them together to make devices.” What’s new is the fact that Simon saw this connection and has been able to turn his idea into a moneymaking reality – a groundbreaking step for New Zealand Science. He has combined the worlds of business and nanoscopic physics – a successful and spirited partnership of vision and practicality, determination and flexibility, far sight and eye for detail. Nano Cluster Devices will paves the way for other New Zealand nano-tech ventures to follow.
