Comparative Semiconductor Photophysics of II‐VI (CdS) and I‐VII (AgBr) Quantum Dots

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Abstract
When CdS and related II‐VI direct bandgap materials form semiconductor clusters in which the absorbance properties are dominated by quantum size effects, the emission properties are particularly sensitive to surface defects, which are associated with the unfilled valence of surface Cd2+ ions. These mid‐bandgap states result in large emission Stokes shifts. This shift can be overcome by binding ligand‐like amines to the surface. By contrast, the more ionic I‐VII materials (e.g., AgBr) show free exciton emission rather than surface defect emission. Since AgBr is an indirect bandgap material, the most obvious effect of size restriction is an increase in the free exciton probability which results from a breakdown in the momentum selection rules at small cluster size.