Nanophotonics: painting the lighting of the future
First came fire. Cavemen rubbed sticks together and there it was, a source of ‘artificial’ heat and light, giving us freedom from the night and control over our days. Torches, candles, lanterns and kerosene lamps have each played their part in the evolution of human-controlled light over the last two millennia. In some respects though, the problems with the ways in which the world gets its light, haven’t changed since ancient times.
Hanging in the majority of homes in the developed world today is the unobtrusive incandescent light bulb. Invented in the late nineteenth century, Thomas Edison built on the work of others to successfully run electricity through a thin filament, encased within a bulb. This produced light at the flick of a switch.
These incandescent bulbs work by using electrical current to heat up the filament to such a high temperature that light is produced almost as an after-thought, a by-product. The problem with this process, which goes on in billions of homes and offices every day, is that it is grossly inefficient: only five to 10 per cent is used to make what we want - visible light. Most of the electricity that we pay for to power these light bulbs is lost as useless heat.
While this is not good news for the person who pays the electricity bill, on a global scale these inefficient light bulbs are damaging the environment. In 2007 the world’s electricity consumption was estimated to be over 16000000Gwh according to the International
Energy Agency. More than 17 per cent of that, the IEA estimates, is used to give us lighting. This sizeable global hunger for electricity is mostly met by burning fossil fuels and results in billions of tonnes of carbon dioxide emissions each year.
Harder to see the light
Look closely at Britain’s supermarket shelves though, and it may be surprising to see that there are now few of these traditional energy-greedy bulbs on sale.
Governments across the globe have begun to institute bans on these inefficient bulbs. Europe’s phase out started over a year ago, when bulbs of one hundred watts were banned on 1 September 2009. Over the next few years, fewer and fewer bulbs of this type will be available, with the European Union committed to removing all incandescent bulbs from the market by 2012.
Changing the light bulb
But what are the alternatives? Many European households are now using Compact Fluorescent Lamps (CFLs), or energy saving bulbs, as they are commonly known in England. These bulbs are not only designed to save energy but also to last longer than incandescent ones. This switch over however, has been the subject of heated debate. Opponents argue that they are unsatisfactory for a number of reasons, including their bulky appearance and the time taken for them to work at full brightness. CFLs have improved in the last decade, but many consumers are still in search of a more perfect light.
The blinking answer
The answer may be blinking at us from the corner of our television screens. Light emitting diodes (LEDs) have snuck into our everyday lives, arguably without fanfare. They are now
commonplace in car indicators, traffic lights and computer display screens. These LEDs have been shown to use less energy than both incandescent and CFL bulbs, and last considerably longer.
“LEDs last more than five times longer than their CFL alternatives – with a rated lifespan of 50,000 hours compared with 8,000 for traditional tubes,” says a representative of Greenland Light, one of the UK’s sustainable lighting retailers. But to make a significant impact on the domestic lighting industry, LED technology must overcome more than a few barriers.
Early LEDs emitted low intensity red light. Over the years LEDs of different colours and greater efficiency have been engineered by manipulating the materials they are made of. The biggest problem encountered by the LED industry is how to make efficient, high quality white light, the type of light that we now take for granted in homes and offices.
Traditionally, in order to make white LEDs, blue light emitting diodes have been coated with materials known as phosphor (substances capable of emitting light after the absorption of some form of radiation). By combining the blue emissions of the LEDs with the yellow luminance of the phosphor, white light can be produced. The LED-phosphor combination, however, produces a cold light, akin to that in operating theatres and harsh clinical conditions. The crusade for warm, white light that is more attractive to consumers has been underway for many years.
Crystal coats bring warmth to light
Invisible crystals can help in the generation of comfortable white light and simultaneously reduce the world’s carbon footprint, says a scientist, whose middle name means
Volcano. Professor Hilmi Volkan Demir’s craft is known as nanotechnology, the design, manipulation and engineering of substances on the nanoscale. This impressive alchemy takes place on a scale that is hard to comprehend: A nanometer is 1 millionth of a metre. “When you start working with materials at this level,” says Evren Mutlugun, one of Demir’s team, “things become unpredictable. They do not act like the same material in bulk form.”
Giving the example of gold, Tuncay Ozel, a masters student at Bilkent University, Turkey, which is home to Demir’s group, explained that unlike the shiny yellow gold that is used to make jewelry, gold nanoparticles, are actually red in colour. Not only that, but they can also be highly reactive, in comparison to the rather safe, larger blocks of gold we hang around our necks.
What is equally as exciting about nanotechnology is that it provides scientists with the ability to engineer the properties of materials on a scale that was never possible before. By making nanoscale materials from scratch, small alterations can dramatically affect the properties of the structure, making it possible to fine-tune a material to perfection.
Nanocrystals are small ordered groups of atoms making particles on the nanoscale. By making almost imperceptible changes to their size, the wavelength of light they emit, and thus their colour, also changes. Researchers at Bilkent have shown that nanocrystals offer the world a palette of light that can be artfully mixed to make near perfect light.
In 2005 they were first in the world to use these highly tunable crystals to make warm white light from LEDs. Demir’s research (published in Applied Physics Letters in 2008) involved combining two different sized nanocrystals specifically made to emit wavelengths of green and red light. Coating blue LEDs with these specially engineered nanocrystals, they were able to mix the blue, green and red light, to give out a warm white glow. The crystals work by absorbing a proportion of the LED's blue output, whilst emitting their own red and green light. The blue, red and green lights combine to produce the much desired soft, white light.
While this soft white light is arguably one of the most important discoveries in LED technology in recent years, LED and nanocrystal combinations still remain challenging to mass-produce for domestic lighting. As Sedat Nizamoglu, of Bilkent says, “The LED plus nanocrystal combination is commercially available today. But according to our knowledge, the high-quality, efficient white LEDS with nanocrystals that we have demonstrated in our labs are currently lacking in industry.”
Bilkent’s researchers continue to work on the magical nanoscale to try and increase the possibilities of LEDs being a feasible option with which to light our homes. They are currently combining nanocrystals with nanometal particles, and have found that if placed close enough to each other, these materials can give off much more light, than when used alone. The beauty of engineering at the nanoscale is that invisible coats of nanocrystals and nanometal particles can be layered together, in a film thin enough that it can be used to cover LEDs.
Crystals for climate change
These highly tunable crystal coats could be used to cover and improve a wide range of existing applications, such as painting coats of these crystals on solar panels to make them more efficient. The Bilkent team has shown that these painted panels can trap a larger spectrum of the sun’s energy, making them spectrally more efficient.
Governments around the world have acknowledged the potential for nanotechnology to tackle one of the world’s largest problems, climate change. Demir’s research group has recently joined forces with nine other institutions across Europe, to work on a project funded by the European Union, Nanophotonics for Energy Efficiency. The members of this international collaboration are physicists, biologists, chemists, electrical engineers and some of the leading names in the lighting industry. Over the next two years they aim to use the invisible tools of nanotechnology to make light in a greener, cleaner and brighter way.
This report was compiled while on an European Union funded trip to Bilkent University, Turkey. Smitha wishes to thank RELATE (Research Labs for Teaching Journalists) and the researchers at Bilkent.