Optical extinction spectra of silicon nanocrystals: size dependence upon the lowest direct transition.

We investigate the size-dependent optical extinction properties of colloidal silicon nanocrystals (Si NCs) from the near infrared (NIR) to the ultraviolet (UV). Experimental results are compared to the Mie solution to Maxwell's equations using the same refractive index as bulk Si to evaluate the deviation from bulk properties. We find that the energy for the lowest direct transition (E(1)) continuously blueshifts from near bulk-like at ~3.4 eV in large NCs (16 nm) to ~3.6 eV for small NCs (3.9 nm), contrary to the Mie solution. The extinction cross-section of NCs on a per atom basis was found to be independent of the NC size, within our experimental resolution. The results suggest that quantum confinement effects strongly influence excitons associated with the E(1) transition.