Mechanism of Ligand-Controlled Emission in Silicon Nanoparticles

Although bulk silicon (Si) is known to be a poor emitter, Si nanoparticles (NPs) exhibit size-dependent photoluminescence in the red or near-infrared due to quantum confinement. Recently, it has been shown that surface modification of Si NPs with nitrogen-capped ligands results in bluer emission wavelengths and quantum yields of up to 90%. However, the emission mechanism operating in these surface-modified Si NPs and the factors that determine their emission maxima are still unclear. Here, the emission in these species is shown to arise from a charge-transfer state between the Si surface and the ligand. The energy of this state is linearly correlated to the calculated ground-state dipole moment of the free ligand. This trend can be used in a predictive fashion for the design and synthesis of Si NPs with a broader range of emission wavelengths.