Indium Nitride Nanowires Go Green

The "green gap" may be conquered by shaping InN nanowires for quantum confinement.

SPIE has a summary of research from a University of Michigan doctoral student that was published earlier this year in Nano Letters of the American Chemical Society. The student, Dylan Bayerl, and his advisor Emmanouil Kioupakis, investigated indium nitride nanowires as emission sources for the efficient generation of green light.

Calculated electronic and optical properties for hexagonal and triangular nanowires with and without including the effect of the electron-hole (e-h) interaction.

(Source: ACS)

The motivation for the work was to plug the "green gap." When nitrides of indium (InN) and gallium (GaN) are alloyed in varying ratios to produce light at a range of freq­uencies across the visible spectrum, a number of factors limit the efficiency with which the light can be produced, including "...fluctu­ations in the alloy composition, lattice mismatch with the substrate, and strain-induced polarization fields," according to the researchers. As it happens, these adverse factors all come together when the InN/GaN alloy ratio is tuned for light in the green range of the spectrum.

The researchers modeled the behavior of nanowires of pure indium nitride. Using a pure semiconductor instead of an alloy reduces the damping factors listed above; and configuring the semiconductor in nanowires results in quantum confinement of exitons (bound electron-hole pairs), raising the material's band gap and thus its optical emission.

They found that nanowires of 1-nm diameter will produce light in the green range. At that size, only a small number of molecules can fit in the nanowire's cross-section, and their physical arrangement -- the wire's shape -- affects the quantum confinement and thus the frequency of light produced.

With a triangular cross-section, the researchers found that the nanowires would generate light of a cyan color, with the InN material having a band gap of 3.9 eV (up from the 0.6 eV of bulk InN). When the nanowire's cross-seciton is hexagonal, the band gap drops to 3.7 eV and the light is green.

As far as I can determine, the researchers didn't actually fabricate such nanostructures out of InN; instead the study is the result of calculations and modeling, presumably on a supercomputer. Bayerl and Kioupakis write: "While such small nanowires are difficult to synthesize, InN nanocrystals just a few nanometers in size have already been synthesized, and nanorods 1nm in diameter have been synthesized from other semiconductors."

The condensed summary of the paper did not spell out the distinction among nanocrystals, nanorods, and nanowires.

— Keith Dawson , Editor-in-Chief, All LED Lighting