Dynamic Trap Formation and Elimination in Colloidal Quantum Dots

Using first-principles simulations
on PbS and CdSe colloidal quantum
dots, we find that surface defects form in response to electronic
doping and charging of the nanoparticles. We show that electronic
trap states in nanocrystals are dynamic entities, in contrast with
the conventional picture wherein traps are viewed as stable electronic
states that can be filled or emptied, but not created or destroyed.
These traps arise from the formation or breaking of atomic dimers
at the nanoparticle surface. The dimers’ energy levels can
reside within the bandgap, in which case a trap is formed. Fortunately,
we are also able to identify a number of shallow-electron-affinity
cations that stabilize the surface, working to counter dynamic trap
formation and allowing for trap-free doping.