INNOVATION PROFILE/Science Foundation Ireland:IMPROVED SOLAR cells, anti-bacterial surfaces for medical equipment, more efficient lasers, enhanced battery life for smartphones, and coatings to render objects invisible are just a few of the applications of the work being done by the Science Foundation Ireland-funded Advanced Materials Surfaces Group (AMSG) at the Tyndall Institute at UCC.
Led by Prof Martyn Pemble, the group has gained global recognition through numerous national and international collaborative research projects in the areas of advanced chemical vapour deposition (CVD) systems and atomic layer deposition (ALD) systems.
The group is made up of about 20 researchers and recently received additional SFI funding for one of its main strands of research which involves materials known as photonic crystals.
“These are materials which are analogues of natural materials, the most commonly cited of which are opal gemstones,” explains Pemble. “The colours of these stones change when you look at them. This happens because the structure of the material enables it to reflect and refract light in different ways. We call this opalescence.”
He points out that opals and photonic crystals are the optical equivalents of semiconductors. In the case of a semiconductor there are areas or energies where electrons can’t normally be placed – this is known as the electron band gap. In photonic crystals the equivalent is known as the photonic band gap and it is an energy region where light can’t exist.
“These are areas where light can’t pass through and it must be reflected,” says Pemple. “The colour of the reflected light varies as a function of the angle from which you view it. Nature makes this in opals or butterfly wings.”
Opals are made up of balls of silicon dioxide which were formed millions of years ago and during the formation process arranged themselves in an ordered structure. “The scale of this structure is comparable to the wavelength of light and this can have interesting refractive and diffractive effects,” he notes.
“We are trying to make these things in the lab and exploit the fact that they can control the way light is reflected, diffracted and refracted. For example, if we can trap light in a solar cell for longer we can get more energy from the cell. Also, we could make it travel in curves and go around corners and make optical circuits using light instead of electrons.”
In theory, this could mean having a computer operating at the speed of light if its transistors were connected by optical circuits. But this might not be as fast as it might appear.
“Everyone thinks the speed of light is a constant, but this is only when it’s passing through a vacuum. When it passes through a material it slows down, and the degree to which it slows down is known as the refractive index. But certainly optical circuits can speed up processes and make them more efficient.”
The characteristics of these crystals have other interesting applications. “We can use them to attract light into a structure or to exit it”, Pemple points out. “For example, by putting some of these materials on top of a laser or an LED we can actually make them brighter. We are using them as a natural amplifier.”
And it is not only the crystals themselves which have interesting and potentially very useful characteristics. The group is also doing important work in the area of metamaterials, which are materials that do not occur naturally and have properties which are based largely on their structure rather than what they are actually made of.
In this case the metamaterials are being made by the insertion of material inside the crystals by a process called atomic layer deposition. This is an extremely precise process which takes 10 cycles to deposit just a single nanometre layer on a surface.
“Atomic layer deposition will coat the most complex shape imaginably completely uniformly,” says Pemple. “That’s what allows us to fill all the holes in the crystals with different materials.”
The properties of these metamaterials when it comes to light are particularly exciting. “One thing we are looking at is materials with a negative refractive index. This offers the possibility of creating the perfect lens which would be able to magnify an object without any aberration whatsoever.”
But that’s the comparatively dull bit; the really exciting bit is the potential to create a “cloaking device” of the type normally associated with Star Trek. “We could make materials which would wrap around an object and when lights hits it would not be reflected, diffracted, refracted or absorbed in any way. It would make it appear as if the object was not there. People are already trying to make these materials.”
The atomic layer deposition process also plays a key role in the other main strand of the group’s work, which sees it collaborating with global leaders such as Intel on the development of the next generation of semiconductors. Complementary Metal Oxide Semiconductors or CMOS represent this next generation.
“We are looking at materials where electrons can travel faster,” says Pemble. “The object is not necessarily to make the semiconductor faster but to reduce its power consumption. There are huge savings to be made globally on energy for just a small reduction in the power consumption of semiconductors. This could also lead to longer battery life on smartphones.
“These materials are things we can already grow at Tyndall and we can take the process all the way through to making the transistor. Being able to take the material or concept through to the final device is very important when it comes to commercialisation.”
And commercialisation is very much top of mind with the Pemble and the group. “The photonic crystals will probably emerge into real applications within the next five to 10 years, but the stuff we are doing with Intel and others will be in device within the next year or two.
“This gives us the right balance between fundamental and applied research. Now is the time to put our money where our mouth is. We now have to bridge the credibility gap between the laboratory and the companies which make these devices.
While it may be a few years before we see photonic crystals in our daily lives the application for their use are already in sight. “We’ll be putting crystals into solar cells, LEDs and so on. One application is a concentrator where the crystals are put on top of the glass in a transparent window in a home.
“As we increase the number of layers to five or six, more light gets trapped in the window. We can then place a solar cell in the window frame to use that light. The solar cell never looks at the sun, but the window itself acts as a solar concentrator.”
Other applications include using the technology to deposit layers of zinc dioxide or titanium dioxide on glass or other materials to make them self-cleaning and anti-bacterial.
“These materials react to UV light in a certain way. Our aim is to make them more efficient under normal room lights to make them more effective in hospital and other settings.”
For the future he sees an increasing focus on working with industry to develop practical commercial applications for the technologies.
“We want to take what we do in the lab, scale it up and make it attractive from a commercial point of view. We want it to be seen as having commercial applications and not just interesting chemistry.”
