understanding the 
 structural and compositional origins of midgap 
 states in semiconductor nanocrystals is a longstanding challenge in 
 nanoscience . here , we report a broad variety of reagents useful for 
 photochemical reduction of colloidal cdse quantum dots , and we establish 
 that these reactions proceed via a dark surface prereduction step 
 prior to photoexcitation . mechanistic studies relying on the specific 
 properties of various reductants lead to the proposal that this surface 
 prereduction occurs at oxidized surface selenium sites . 
 these results 
 demonstrate the use of small - molecule inorganic chemistries to control 
 the physical properties of colloidal qds and provide microscopic insights 
 into the identities and reactivities of their localized surface species .