Red States versus Blue States in Colloidal Silicon Nanocrystals: Exciton Sequestration into Low-Density Traps

The ultrafast exciton photodynamics of red-emitting and blue-emitting colloidal Si nanocrystals are contrasted under low (1.5 mJ/cm(2)) and high (9.1 mJ/cm(2)) excitation powers with broadband transient absorption spectroscopy. While the low-power initiated transient signals differ strongly for the two samples, the high-power signals exhibit similar nonmonotonic kinetics, resulting in a new population formed on a 10 to 30-ps time scale with a sample independent spectrum and decay kinetics. This phenomenon is ascribed to the saturation of low-density red-emitting and blue-emitting traps via a state-filling mechanism to populate new meta-stable states at higher excitation powers. The states responsible for blue emission and high-power populations are ascribed to traps from low-density nitrogen and oxygen impurities, respectively, and share similar charge-transfer character with the silicon nanocrystal core.