In this report, size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) are studied 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. The exciton formation time increases 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. See the full article here.