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Surface Passivation and Oxidation of PbSe and PbS Quantum Dots: Theoretical Insights

NST Nanoscience Colloquium
Svetlana Kilina, North Dakota State University
January 29, 2014 4:00PM to 5:00PM
Building 440, Room A105-106
Quantum Dots (QDs) show promise for many technological applications, including photocatalysis and photovoltaics. However, their photophysical properties are sensitive to surface reactions, resulting in uncontrollable luminescence quenching. Using density functional theory (DFT) and time dependent DDFT (TDDFT), we simulate the oxidation process on the surface of Pb16Se16 and Pb68S68 QDs and its effect on their electronic and optical properties. When oxygen is substituted for Se/S ions, the electronic properties of the QD are insignificantly perturbed. In contrast, if atomic oxygen is adsorbed on the QD surface and coordinated with two Pb ions, it introduces additional unoccupied states inside the QD’s band gap, so called mid-gap trap states. Such states are hybridized between the oxygen and the QD’s surface atoms and contribute to the lowest-energy optically dark or semi-dark transitions resulting in quenching of luminescence of QDs.

In contrast, if the oxygen is coordinated with Se/S and Pb ions on the surface, the mid-gap states are not present and the optical transitions are similar to those of the non-oxidized QDs. We have also observed similar trends when Cl radical is adsorbed to the QD surface: a few trap states originated from Cl appear at the band gap of the QD, when Cl is coordinated with Pb ions. However, when ionized, interaction of Cl- with the QD surface, leads to elimination of trap states from the band gap and a slight increase in the gap of the QD. Our calculations demonstrate different preferential binding of Cl in its radical and ionized forms to different QD surfaces. Thus, Cl ion has a stronger interaction with {111} PbSe surface, while Cl radical has similar binding energies to {100} and {111} surfaces.

Attachment of Cl in the form of salt PbCl2 favors adsorption to the {111} surface of the QD. However, its binding energy is twice smaller than adsorbed Cl ions and Cl radicals on the same surface, while it also eliminates trap states from the QD’s band gap. The obtained results is a first step in understanding physical properties of QDs found in the presence of defects, surface ligands, and interactions with environment. We will further demonstrate how these properties could be controlled to accelerate the completion of their proof-of-concept development stage and facilitate practical usage of hybrid nanomaterials to produce efficiently operating optoelectronic devices, solar cells, sensors, bio-labels, etc.