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The nanowire gurus of the National Institute of Standardization and Technology (NIST) have manufactured light-emitting diodes (LEDs) which, thanks to a special shell type, produce a light intensity five times that of diodes comparable based on a simpler hull.
Ultraviolet LEDs are used in a growing number of applications such as polymer polymerization, water purification and medical disinfection. Micro-LEDs are also interesting for visual displays. NIST staff are experimenting with nanowire-based light emitting diodes for scanning probe tips for electronic and biological applications.
The new, brighter LEDs are the result of NIST's expertise in producing high quality gallium nitride (GaN) nanowires. Recently, researchers have experimented with silicon-doped GaN nanowires, which contain additional electrons, surrounded by magnesium-doped GaN layers and have excess "holes" for the missing electrons. When an electron and a hole combine, energy is released in the form of light, a process called electroluminescence.
The NIST group had previously demonstrated that LEDs producing light attributed to electrons injected into the shell layer recombined with holes. The new LEDs have a tiny bit of aluminum added to the shell layer, which reduces the losses due to the overflow of electrons and the reabsorption of light.
As described in the newspaper nanotechnology, the brightest LEDs are made from nanowires with a so-called "p-i-n" structure, a three-layer design that injects electrons and holes into the nanowire. The addition of aluminum to the hull helps to confine the electrons to the heart of the nanowire, multiplying by five the electroluminescence rate.
"The role of aluminum is to introduce an asymmetry in the electric current that prevents electrons from penetrating the shell layer, which would reduce efficiency and confine electrons and holes to the core nanowire, "said the first author, Matt Brubaker.
The nanowire test structures had a length of about 440 nanometers (nm) with a shell thickness of about 40 nm. The final LEDs, including shells, were almost 10 times larger. The researchers discovered that the amount of aluminum incorporated into the fabricated structures depends on the diameter of the nanowire.
The group leader, Kris Bertness, said that at least two companies were developing micro-LEDs based on nanowires, and NIST signed a cooperation agreement for research and development with one of the largest companies in the world. Among them in order to develop methods for characterizing dopants and structures. The researchers had preliminary discussions with scanning probe companies on the use of NIST LEDs in their probe tips, and NIST plans to introduce prototype LED tools soon.
The NIST team holds US Patent 8,484,756 on an instrument combining microwave scanning probe microscopy and an LED for non-destructive, non-contact testing of material quality for large semi-structured nanostructures. -conductors such as channels of transistors and individual grains in solar cells. The probe could also be used for biological research on protein deployment and cell structure.
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Material provided by National Institute of Standards and Technology (NIST). Note: Content can be changed for style and length.
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