First-principles study of the halide-passivation effects on the electronic structures of CdSe quantum dots

We studied the effects of chlorine passivation and iodine passivation on the electronic structures of Cd33Se33 quantum dots through partial chlorine replacement for surface pseudo-hydrogen atoms, taking full pseudo-hydrogen-terminated Cd33Se33 quantum dots as a reference. Our calculations demonstrate that the electrostatic interaction between surface Cd absorbates and the halide passivant removes the dangling-bond-derived states of surface Cd atoms. Due to the high electronegativity, the Cl passivants need to coordinate with three Cd atoms to saturate p orbitals and achieve complete saturation. The modulation of the Cl passivants to the electronic structures of quantum dots depends on the coordination number of the Cl passivants. With coordination numbers of up to three, Cl passivants contribute less to the HOMO state and the QDs are more energetically stable. As the electronegativity decreases from Cl to I, I passivants with a coordination number of 2 are energetically stable enough to passivate QDs, leading to a relatively inactive surface after passivation. The HOMO states of I-terminated QDs are composed of I 5p and Se 4p.