More is better

Nano Lett. http://dx.doi.org/10.1021/nl203275n (2011)

The poor optical properties of silicon have restricted its use in photonics applications. However, computer simulations by Holger Vach of the Ecole Polytechnique now suggest a surprising way to improve the optical properties of hydrogenated silicon nanocrystals — add more silicon!

Vach first computed the properties of nanocrystals that contain three parallel hexagons of silicon atoms, with hydrogen atoms bonded to the silicon atoms in the top and bottom hexagons. These Si₁₈H₁₂ nanocrystals absorb light in the ultraviolet region of the spectrum but not at any other wavelengths. Vach then computed what would happen if an extra silicon atom was inserted into the middle of these nanostructures. He found that the resulting Si₁₉H₁₂ nanocrystals also absorbed light at certain visible and infrared wavelengths. Furthermore, he found that the Si₁₉H₁₂ nanocrystals should be more stable than any other known silicon nanocrystals.

Vach attributes the increased stability and improved optical properties of the Si₁₉H₁₂ nanocrystals to the increased electron delocalization caused by the addition of the extra silicon atom. This delocalization means that the silicon hexagons resemble benzene rings in many ways. Moreover, the stability of the nanocrystals, together with their compatibility with the existing infrastructure for making silicon chips, could lead to a variety of applications.