Silicon is the most ubiquitous, least toxic, widely studied and broadly applicable of all the known semiconductors, yet the physical size at which it begins to show strong quantum size effects (QSE) in three-dimensional silicon nanocrystals remains an open question experimentally. It is claimed that around 5 nm, the size of the Bohr exciton in bulk silicon, significant QSE emerge; however, no direct measurement of the transition from bulk to strongly quantum confined silicon has been reported.

Now, a new publication reports that the size dependence of the absolute luminescence quantum yield of size-separated organic capped silicon nanocrystals (ncSi:R) reveals a “volcano” behavior, which switches on around 5 nm, peaks near 3.7-3.9 nm and monotonically decreases thereafter. These three regions of the volcano plot respectively define the (i) transition from bulk non-emissive silicon to strongly quantum confined emissive silicon, (ii) increasing spatial confinement of electron-hole pairs enhancing radiative recombination to yield the silicon nanocrystals brighter, and (iii) increasing contributions of organic-capping group vibrations and defects favoring nonradiative recombination which dampens the luminescence.