Quantum chemistry of the minimal CdSe clusters

Ping Yang, Sergei Tretiak, Artëm E. Masunov, Sergei Ivanov
2008 Journal of Chemical Physics  
Colloidal quantum dots are semiconductor nanocrystals ͑NCs͒ which have stimulated a great deal of research and have attracted technical interest in recent years due to their chemical stability and the tunability of photophysical properties. While internal structure of large quantum dots is similar to bulk, their surface structure and passivating role of capping ligands ͑surfactants͒ are not fully understood to date. We apply ab initio wavefunction methods, density functional theory, and
more » ... theory, and semiempirical approaches to study the passivation effects of substituted phosphine and amine ligands on the minimal cluster Cd 2 Se 2 , which is also used to benchmark different computational methods versus high level ab initio techniques. Full geometry optimization of Cd 2 Se 2 at different theory levels and ligand coverage is used to understand the affinities of various ligands and the impact of ligands on cluster structure. Most possible bonding patterns between ligands and surface Cd/ Se atoms are considered, including a ligand coordinated to Se atoms. The degree of passivation of Cd and Se atoms ͑one or two ligands attached to one atom͒ is also studied. The results suggest that B3LYP/LANL2DZ level of theory is appropriate for the system modeling, whereas frequently used semiempirical methods ͑such as AM1 and PM3͒ produce unphysical results. The use of hydrogen atom for modeling of the cluster passivating ligands is found to yield unphysical results as well. Hence, the surface termination of II-VI semiconductor NCs with hydrogen atoms often used in computational models should probably be avoided. Basis set superposition error, zero-point energy, and thermal corrections, as well as solvent effects simulated with polarized continuum model are found to produce minor variations on the ligand binding energies. The effects of Cd-Se complex structure on both the electronic band gap ͑highest occupied molecular orbital-lowest unoccupied molecular orbital energy difference͒ and ligand binding energies are systematically examined. The role played by positive charges on ligand binding is also explored. The calculated binding energies for various ligands L are found to decrease in the order OPMe 3 Ͼ OPH 3 Ͼ NH 2 Meജ NH 3 ജ NMe 3 Ͼ PMe 3 Ͼ PH 3 for neutral clusters and OPMe 3 Ͼ OPH 3 Ͼ PMe 3 ജ NMe 3 ജ NH 2 Meജ NH 3 Ͼ PH 3 and OPMe 3 Ͼ OPH 3 Ͼ NH 2 Meജ NMe 3 ജ PMe 3 ജ NH 3 Ͼ PH 3 for single and double ligations of positively charged Cd 2 Se 2 2+ cluster, respectively.
doi:10.1063/1.2965532 pmid:19044794 fatcat:cosxavybczau5amgwviqqukjqu