Electronic structure and elemental composition of the lead sulfide colloidal quantum dots depending on the types of ligand and post-treatment

Thin films of lead sulfide colloidal quantum dots (CQDs) of 2.8 nm in diameter are fabricated and their surfaces are passivated by 3-mercaptopropionic acid (MPA) ligand or hybrid type (MPA+CdCl₂) ligand, respectively. The changes in valence band electronic structure and atomic composition of each PbS CQD film upon post-treatment such as air, N₂ annealing or UV/Ozone have been studied by photoelectron spectroscopy. The air annealing makes the CQD fermi level to move toward the valence band leading to "p-type doping" regardless of ligand type. The UV/Ozone post-treatment generates Pb(OH)₂, PbSO_x and PbO on both CQD surfaces. But the amount of the PbO has been reduced in hybrid type ligand case, especially. That is probably because the extra Pb cations in (111) surface are additionally passivated by Cl₂ ligand, which limits the reaction between the Pb cation and ozone.