Harvesting solar energy as a clean and renewable source of energy can be improved by using semiconductor Quantum Dots (QDs). However, photophysics of QDs is extremely sensitive to their surface chemistry.   Using density functional theory we study the effect of common passivating ligands, such as carboxylate, phosphine, phosphine oxides, and amine groups, on the electronic and optical properties of CdSe and PbSe QDs. We specifically investigate how ligand attachment to different QD's facets with different stoichiometry affects the QD-ligand interactions and optical response of ligated QDs. The most stable conformations are associated with the ligated QDs having an excess of Cd or Pb ions on the surface � of stoichiometric surfaces. The strength of QD-ligand interactions depends on the polarity of a solvent and the facet of the QD to which the ligand is attached. Full or partial passivation of QD surfaces with the stoichiometry 1:1 by various ligands, eliminates any surface states from the gap of the QDs. Independent on the number of passivating agents, ligands do not introduce any electronic states to the band gap of stoichiometric QDs, but appear deep inside in the valance and conduction bands of QDs. However, absorption spectra of the bare or partially ligated stoichiometric QDs demonstrate the lowest energy transitions with weak intensities, whereas fully passivated QDs have brighter and slightly blue-shifted transitions. Off stoichiometry in QDs with an excess of surface metals interacting with ligands, results in surface states appearing nearer to the edge of the conduction band and red shifts in absorption spectra.

Effect of surface stoichiometry and passivating ligands on optical spectra of quantum dots

247th National Meeting (2014)

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