Guest post by Jen Dougan
‘May it be a light to you, in dark places. When all other lights go out.’
J. R. R. Tolkien
Yesterday saw the opening ceremony to mark the start of the International Year of Light (IYL). Today scientists and policy makers will meet in Paris for day two of the celebrations. Designated by the United Nations, the IYL aims to increase awareness about the importance of light in our modern and developing world, such is the breadth of light–based technologies – from biological sensing to next generation light emitting diodes (LEDs). Undoubtedly, our world is enriched by harnessing the energy of light, and one of the core aims of IYL is to focus on the plight of 1.5 billion of the world’s inhabitants for whom sunset means darkness.
Blue light emitting diodes (Blue LED). Image by Gussisaurio at wikipedia (CC-BY-SA)
With little or no access to electrical lighting, many rural communities in the developing world have limited ability to read after sundown, have restricted working hours, and hospitals have to power down the lights in the evening – limiting healthcare options. Many families rely on the use of paraffin or kerosene lamps. This isn’t without problems, kerosene is a flammable hydrocarbon producing toxic fumes when burned and is a significant fire-safety hazard. Attempting to address this, the IYL ‘study after sunset’ campaign seeks to promote the use of solar powered LED lights in the communities that need them most.
Anyone who has handled a traditional incandescent lightbulb can attest to its inefficiency. Producing significant amounts of heat (capable of burning fingers!), incandescent bulbs are economically and environmentally wasteful. But alternatives do exist. LEDs generate far more light, measured in lumens per Watt (lm/W), than standard incandescent or fluorescent lighting (Figure 1). Of course, the use of LEDs helps to reduce bills and energy consumption and, considering that lighting accounts for ~25% of electricity usage in developed countries, that presents a significant reduction. It is their efficiency and bulb lifetime of 100,000 hours (an order of magnitude greater than incandescent bulbs) that may enable LEDs to illuminate lives the world over.
Figure 1: Comparative brightness of lighting devices.
Image: © The Royal Swedish Academy of Sciences
With this potential impact, it’s clear why researchers Isamu Akasaki, Hiroshi Amano and Shuji Nakamura won the 2014 Nobel Prize in Physics for the development of blue LEDs, which enable the production of bright white light sources.
How science LED the way
To emit white light, additive colour mixing is employed, which involves combining red, green and blue light (Figure 2). Although red and green LEDs had been developed in the 1950s and 60s, it wasn’t until 1992 that a blue LED was produced, allowing white light to be created from LEDs. Using semiconductor technology, LEDs are much more efficient that traditional lighting – which relies on an electrical current heating a wire (typically tungsten in a white lightbulb) until it glows – because they convert electrical energy directly into light.
Figure 2: Additive colour mixing to produce white light.
Image: Mike Horvath on Wikipedia
Semiconductors can be p-type or n-type, indicating whether they have insufficient electrons (considered as a surplus of ‘holes’, so p for positive) or a surplus of electrons (n for negative). This characteristic of a semiconductor is tuned by increasing the level of doping – that is, the controlled addition of impurity atoms. From the interface between the p-type and n-type materials, in the active layer – where the electrons meet the holes, light is emitted (Figure 3). The energy (or wavelength/colour) of the light produced (or whether it is produced at all) is dictated by the materials used to create the semiconductor. The energy gap, or band gap, between the two materials must be such that light of the desired wavelength is produced. Blue LEDs are principally composed of gallium nitride (GaN) as the semiconductor material (Figure 3). Once GaN crystals of sufficient quality could be grown, and p-type GaN produced by elimination of hydrogen from the surface, LEDs were developed that emitted blue light.
Figure 3: Inside a blue LED (click for full size)
Image: © The Royal Swedish Academy of Sciences
With blue LEDs in hand, white light could be produced. This was achieved either by situating blue, red and green LEDs in close proximity, which appear white to the eye, or by applying a phosphor coating to a blue LED. The phosphor is a compound which, when irradiated, causes a shift in wavelength (colour) of light to yellow – this combines with the blue light to appear white. Research into the development of novel phosphors is underway to allow an increased tone palette to be achieved.
Blue LED technology was used for the development of Blu-ray discs and finds use in mobile phones and LCD screens. But it is for the potential to bring light to billions in night-time darkness that we should celebrate the beginning of this International Year of Light.
IYL is a great opportunity to celebrate light and its interaction with chemistry. I hope to focus on a broad range of topics over the coming months on this theme. I’d love to hear about any chemistry related activities going on during IYL and/or any topics you’d be interested in. Drop me a note below or contact me on twitter: @jendtweeting #IYLchemistry
