Size-Dependent Exciton Formation Dynamics in Colloidal Silicon Quantum Dots

We report size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) using time-resolved terahertz (THz) spectroscopy measurements. THz photoconductivity measurements are used to distinguish the initially created hot carriers from excitons that form at later times. At early pump/probe delays, the exciton formation dynamics are revealed by the temporal evolution of the THz transmission. We find an increase in the exciton formation time, from ∼500 to ∼900 fs, as the Si QD diameter is reduced from 7.3 to 3.4 nm and all sizes exhibit slower hot-carrier relaxation times compared to bulk Si. In addition, we determine the THz absorption cross section at early delay times is proportional to the carrier mobility while at later delays is proportional to the exciton polarizability, αX. We extract a size-dependent αX and find an ∼r(4) dependence, consistent with previous reports for quantum-confined excitons in CdSe, InAs, and PbSe QDs. The observed slowing in exciton formation time for smaller Si QDs is attributed to decreased electron-phonon coupling due to increased quantum confinement. These results experimentally verify the modification of hot-carrier relaxation rates by quantum confinement in Si QDs, which likely plays a significant role in the high carrier multiplication efficiency observed in these nanomaterials.