Organic LEDs that use gold nanowires to boost light-emitting efficiency show promise for future lighting and display technology.
Nanowires brighten up organic LEDs
Light extraction
Nanowires can enhance the light-emitting efficiency of organic LEDs by up to 45%, say researchers in Taiwan. An added bonus is that the nanowires could replace expensive indium tin oxide as the anode in such devices. The technique might even be applied to liquid crystal displays to increase their brightness and possibly eliminate the need for colour filters (Applied Physics Letters 92 013303).
Organic light emitting devices (OLEDs) are promising for next-generation displays and lighting because they are simple to make. Although OLEDs are better than conventional inorganic LEDs in many ways, they do suffer from a low light-emitting efficiency of around 20%. This is because most of the light is trapped (due to total internal reflection) in the transparent substrate and the organic layers making up the devices.
Previous research extracted this trapped light by using dielectric microstructures or roughing the surface. Now, Pei-Kuen Wei of the Research Center for Applied Science in Taipei and colleagues have found that an array of 50 nm-thick gold nanowires placed on top of an indium tin oxide (ITO) anode can increase the light-emitting intensity of an OLED made from Alq3 by up to two times. The nanowires, which act as light scatterers, are placed 450 nm apart in the arrays. This period matches the peak emission wavelength of the emitted light, which enhances light extraction.
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Wei told nanotechweb.org that the gold nanowire arrays might be used to replace expensive ITO in OLEDS because they are conducting and have a high optical transmission. "The concept of using metallic nanowire arrays for efficiently extracting light from a substrate could also be applied to LCDs," he added. Modern liquid crystal backlights use a thin glass or PMMA plate to guide light that may also benefit from light scatterers. Moreover, since the wavelength of the light scattered depends on the periodicity of the nanowires in the arrays, it might be possible to produce red–green–blue colours directly from the guiding plate without using colour filters, he said.
The team is now trying to find the optimal nanostructures for light extraction. It will also test various 2D periodic nanostructures, such as square or hexagonal nanodots, and other metals like silver.
By Belle Dumé from optics.org
Thursday
Sea cucumbers inspire switchable material
A sudden stiffening of the skin can help the humble sea cucumber defend itself from predators. Now, scientists in America have designed a new composite material that mimics this feat. Christoph Weder, Stuart Rowan and colleagues at Case Western Reserve University in Cleveland, Ohio, say that their invention could be useful for biomedical applications.
The sea cucumber - a relative of the starfish that grazes the ocean floor for carrion - can become rigid in seconds, thanks to its ability to control the interaction between collagen fibrils in its tissue. The researchers tried to imitate this with a network of cellulose nanofibres, dubbed 'whiskers', and embedded them into a rubbery polymer substance.
In the absence of water the whiskers form a rigid network, giving the whole composite material a high rigidity. 'In the absence of water, the nanofibres are "glued" to each other, and the nanofibre network dominates the mechanical properties of the material,' explains Rowan. 'In this state the material is strong and rigid, much like a CD case.'
'But if the material is exposed to water, the water molecules "unglue" the nanofibers and the material becomes about 1000 times softer, so its properties resemble those of a soft rubber.'
'I think it is one of the most exciting recent opportunities in the design of new materials, and would open the door to applications in a number of different fields,' says Craig Hawker a materials scientist at the University of California, Santa Barbara, US. 'This could radically change the way scientists think about nanomaterials - it could be game-changing.'
The scientists think that the switchable material could be used in microelectrodes that are implanted into the brain to treat diseases such as Parkinson's, which could be made to go soft in the aqueous environment of the body and thus avoid scarring.
Experimenting with the composition of the material, the scientists also created a composite substance that can be switched by temperature changes, and they hope to find others where the same change can be induced by chemical or electrical signals.
'One can imagine protective clothing, for example, which is flexible and comfortable to wear but becomes rigid and protective when necessary,' adds Hawker. 'This is essentially what sea cucumbers use this process for.'
By Michael Gross from
RSC.org
The sea cucumber - a relative of the starfish that grazes the ocean floor for carrion - can become rigid in seconds, thanks to its ability to control the interaction between collagen fibrils in its tissue. The researchers tried to imitate this with a network of cellulose nanofibres, dubbed 'whiskers', and embedded them into a rubbery polymer substance.
In the absence of water the whiskers form a rigid network, giving the whole composite material a high rigidity. 'In the absence of water, the nanofibres are "glued" to each other, and the nanofibre network dominates the mechanical properties of the material,' explains Rowan. 'In this state the material is strong and rigid, much like a CD case.'
'But if the material is exposed to water, the water molecules "unglue" the nanofibers and the material becomes about 1000 times softer, so its properties resemble those of a soft rubber.'
'I think it is one of the most exciting recent opportunities in the design of new materials, and would open the door to applications in a number of different fields,' says Craig Hawker a materials scientist at the University of California, Santa Barbara, US. 'This could radically change the way scientists think about nanomaterials - it could be game-changing.'
The scientists think that the switchable material could be used in microelectrodes that are implanted into the brain to treat diseases such as Parkinson's, which could be made to go soft in the aqueous environment of the body and thus avoid scarring.
Experimenting with the composition of the material, the scientists also created a composite substance that can be switched by temperature changes, and they hope to find others where the same change can be induced by chemical or electrical signals.
'One can imagine protective clothing, for example, which is flexible and comfortable to wear but becomes rigid and protective when necessary,' adds Hawker. 'This is essentially what sea cucumbers use this process for.'
By Michael Gross from
RSC.org
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