Graphic illustrates a single row of nanowires (cylinders with red tops) with fin-shaped
nanowalls extending outward. Credit: NIST.

Speaking of ZnO nanowires, chemists at NIST who were perfecting new methods of creating these nanowire report that they have stumbled upon a way to create light-emitting nanowires that operate like very tiny LEDs.

TEM of four rows of the nanowires and nanowalls. Credit: NIST.

The chemists, Babak Nikoobakht and Andrew Herzing, had already found a way to grow ZnO nanowires horizontally across a substrate that was seeded with gold nanoparticles. These nanoparticles serve as growth sites and medium for the crystallization of zinc oxide molecules: As the zinc oxide nanocrystal grows, it pushes the gold nanoparticle along the surface of the substrate (“surface-directed” fabrication).

While working with a gallium nitride substrate, Nikoobakht and Herzing, unexpectedly found that when they increased the thickness of the gold nanoparticle from less than 8 nm to approximately 20 nm, the nanowires sprouted a secondary structure. Visually, the structure is said to look like a “dorsal fin.” Technically, it is a nanowall that forms where the ZnO portion is electron-rich and the gallium nitride portion is electron-poor.

At this location, the nanowall becomes a p-n heterojunction where electrons can flow across it when voltage is applied to the nanowire-nanowall combination. This flow of electrons produces light, which led the researchers to dub it a “nano LED.”

According to NIST

“Unlike previous techniques for producing heterojunctions, the NIST “surface-directed” fabrication method makes it easy to locate individual heterojunctions on the surface. This feature is especially useful when a large number of heterojunctions must be grouped in an array so that they can be electrically charged as a light-emitting unit.”

The duo says the simple design of the nanowires means that is scalable to literally any platform size. For example, the nano LEDs, with further improvements, could be used as light sources and detectors in photonic devices or lab-on-a-chip platforms.

TEM of nano LEDs emitting light. Credit: NIST.

TEM of nano LEDs emitting light. Credit: NIST.