Physicists are producing new materials that can be used to develop the next generation of nanotechnology-based semiconductors and sensors, writes Anna Nolan
An unusual new nanomaterial produced at the University of Limerick could be used to make a new generation of nanotechnology-based sensors and other electronic devices and microsystems.
Noel Buckley, who holds the chair of physics at UL, leads a team of researchers and postgraduates who have etched the substance, indium phosphide to produce a new nanomaterial.
Indium phosphide is a so-called III-V compound, meaning that it is made by combining an element from group III of the Periodic Table with one from group V. These compounds are semiconductors, because their electrical behaviour is intermediate between being a good insulator and a good conductor of electrons. Semiconductors are intrinsic components of many of today's sophisticated electronic devices.
Prof Buckley described how they took a small piece of indium phosphide and poked tiny holes into it using a specialised process. "The etching produces nanopores, tiny holes with diameters of 10 to 50 nanometres."
Since a nanometre is one-thousandth millionth of a metre, these are very small holes indeed. The holes propagate below the surface in very specific directions arising from the arrangement of the atoms in the indium phosphide crystal, and grow over time into a network of tunnels.
"The interesting thing is that apart from the small holes through it, the layer at the top remains unaffected to a depth of about 25 nanometres, and that we can control the growth of the tunnels by stopping the process at any time," he explained.
"Inside the tunnels there is a specific geometry and a high surface area, so it could be used for very sensitive sensors for biochemicals and chemicals, including gases."
He suggested that another application would be as a photonic crystal for use in processing light signals. And it's not just the tunnels that could be useful.
"The area between the tunnels is also very small, around 10 to 50 nanometres wide. In general when the size of any semiconductor structure reaches this tiny scale quantum effects are observed, and the properties of the material change," says Prof Buckley.
"We can see quite a difference between the properties of nanoporous indium phosphide and those of the bulk material, and we hope that the fact that the material has different electronic properties would also make it useful for sensors. The fundamental principle is that we can use electrochemical nanotechnology techniques to tailor these kinds of structures," he adds.
Prof Buckley and his colleagues are able to produce and study these effects because they have adapted an atomic force/scanning tunnelling microscope (AFM/STM). This and two other specialist microscopes are used for studying the nanotunnels.
The UL team has adapted their AFM/STM system to provide a small electrochemical chamber in which electrodeposition can take place and be tracked. For example, copper sulphate, a material widely used in the electronics industry, can be studied minutely in this way.
"There is a problem in that the substrate onto which the deposition is occurring can be corroded by oxygen, and we have one of the first systems in which we can work in the complete absence of oxygen," says Prof Buckley.
During manufacturing, the use of certain organic additives in the electrodeposition of copper is essential to produce high-quality interconnections. Quite small amounts can have strong effects on properties. "We have the unique capability to flow in additives while studying the surface," he said.
Prof Buckley spent 17 years at Bell Laboratories in New Jersey, US, and is a vice-president and fellow of the Electrochemical Society.