• Electrical India
  • Apr 5, 2015

Single Molecule Light Bulb

In general concept, a light bulb, or electric light or electric lamp is a device that produces light from electricity.  In addition to lighting a dark space, they can be used to show an electronic device is on, to direct traffic, for heat, and many other purposes. Improved vacuum pumps and better materials made them shine longer and brighter late in the century.

- Dr. S.S. Verma

 Electric generator stations brought electricity to urban and later rural areas to power them. Later gas discharge lights including fluorescent lights use less electricity to make more light. Energy demand with its rising cost and environmental consciousness have motivated inventors and users to look for green yet environmental friendly lighting products and many people have already changed their house lighting system, from the compact fluorescent lights to the energy-efficient LEDs lights. The LEDs, especially the organic LEDs, are becoming increasingly popular in these days. For information, the organic LEDs are mostly thin films made from organic polymers. It can be easily coated onto large areas at a very low cost. Energy-efficient LEDs are widely tipped to become the predominant lighting source of the next decade and beyond, replacing the fast-disappearing incandescent bulb, as well as the compact fluorescent lights that are replacing them.

  It's like there isn't any engineering problem that can't be solved with carbon nanotubes. Now, it turns out it can solve problems we didn’t even anticipate, like making the world’s smallest light bulb. It’s so small, it’s only a few molecules in size. With the going on pace of development in this direction, future light bulb will be one glowing molecule. A single molecule that reliably emits white light and could speed the development of low-energy LEDs has been developed. A single molecule that reliably emits white light could speed the development of low-energy LEDs for the next generation of light sources and displays, say researchers. The nano-bulb consists of a special molecule trapped in the microscopic gap in a carbon nanotube. When current is applied to this molecular circuit, we get light. Scientists have built a molecule, which is able to behave like two separate light-producing molecules. It produces orange and blue light, when it is stimulated with a voltage. The mix of orange and blue light will turn into white. This means, with this technology, it allows the manufacturers to create white emission in much the same way, as creating white light from independent lights. In this case, we’re not only can save much money but it greatly reduces the carbon offset too.

  In recent years, many countries have begun looking to switch from incandescent lighting to compact fluorescent bulbs because the latter are so much more energy efficient. There has also been a lot of interest in using light-emitting diodes (LEDs) for displays and general lighting, again because of the potential energy savings they offer. But with both fluorescent and LED lighting, the quality of white light produced has always left something to be desired. Fluorescent lighting can make people appear unhealthy because less red light is emitted, while most white LEDs on the market today have a bluish quality, making them appear cold. In contrast, OLEDs can be made from a wide range of materials, so achieving good-quality white light is less challenging. It has not been the quality of light that has let OLEDs down but rather their efficiencies. Fluorescent lighting typically operates at around 60 to 70 lumens per watt, while incandescent bulbs operate at about 10 to 17 lumens per watt. In contrast, the best reported power efficiency of an OLED until now was 44 lumens per watt. OLEDs have the potential to grow into a really very energy-efficient light source. One involves reducing its operating voltage by doping the organic material that connects the light-emitting material to its metallic contacts. The efficiency of the device is highly reduced if it is near a metal contact because of a phenomenon called quenching. Another trick was to make the outer surfaces of the device from types of glass that have optical properties that more closely match those of the device substrate.

  Otherwise, much of the emitted light is reflected and either reabsorbed or lost through heat. The most novel aspect of this new OLED, however, is the organization of different light-emitting materials within the device. Three materials are used–one each for emitting blue, green, and red light–along with a host matrix material in between. Indeed, a major drawback of OLEDs is their longevity. Although companies like Philips are able to make devices with life spans equivalent to fluorescent bulbs–in excess of 10,000 hours–materials that yield higher efficiencies tend not to last so long.

  Previous attempts using the same basic concept involved linking together two separate molecules into one. But, because energy is able to flow between the two molecular sub-units, one unit typically emits more light than the other, resulting in an unwanted tint. The new molecule does not suffer that problem, and only contains one light-emitting chemical group. When connected to a voltage, this group switches to a high-energy form that emits blue light as it reverts to its original state. Roughly half the time, though, the high-energy form picks up extra oxygen and hydrogen atoms, becoming a short-lived form that produces orange light before reverting to the original state. A large population of the molecules reliably produces equal quantities of orange and blue light that mix to produce an even white. This allows creating white emission in much the same way as creating white light from independent lights.

  Light emitting diodes are components that emit light when an electric current passes through them and only let light through in one direction. LEDs play an important role in everyday life, as light indicators. They also have a promising future in the field of lighting, where they are progressively taking over the market. A major advantage of LEDs is that it is possible to make them very small, so point light sources can be obtained. With this in mind, one final miniaturization hurdle has recently been overcome by researchers as they have produced the first ever single-molecule LED. A single molecule, in contrast, is better measured in nanometers, a unit just a thousandth of the size. Shrinking the light-emitting element of a pixel by the order of hundreds could, then, make for insane, molecular-scale resolution.

  The device is formed from a single polythiophene wire placed between the tip of a scanning tunneling microscope and a gold surface. It emits light only when the current passes in a certain direction. They observed that the thiophene wire acts as a light emitting diode: light was only emitted when electrons went from the tip of the microscope towards the gold surface. When the polarity was reversed, light emission was negligible. The researchers showed that this light was emitted when a negative charge (an electron) combined with a positive charge (a hole) in the nanowire and transmitted most of its energy to a photon. For every 100,000 electrons injected into the thiophene wire, a photon was emitted. Its wavelength was in the red range. Therefore, the ultimate challenge in the race to miniaturize light emitting diodes (LED) has now been met. From a fundamental viewpoint, this device gives researchers a new tool to probe phenomena that are produced when an electrical conductor emits light and it does so at a scale where quantum physics takes precedence over classical physics. Scientists will also be able to optimize substances to produce more powerful light emissions. Finally, this work is a first step towards making molecule-sized components that combine electronic and optical properties. Similar components could form the basis of a molecular computer. Single molecule light bulb needs an efficiency boost before it can be used in commercial lighting and displays. Currently, the molecule converts electrons into photons at least 30 times less efficiently than commercial LEDs.

The Author is a Professor at Sant Longowal Institute of Engineering and Technology.

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