DFT Calculations of InP Quantum Dots: Model Chemistries, Surface Passivation, and Open-Shell Singlet Ground States

Density functional theory (DFT) calculations on large clusters of indium phosphide are presented. Several quantum dot-sized models, (NH3)(64)(InP)(117), (COOH2)(45)(InP)(117), (In-Cl-3)(29)(InP)(147), and (ZnCl2)(29)(InP)(147), were passivated with organic or inorganic ligands; in some systems, both types were used. Initial results with the PBE1PBE functional proved puzzling as many clusters were initially found to have open-shell paramagnetic ground states, which is not sensible for nanoparticles of a direct band-gap semiconductor. In the case of QDs passivated with organic ligands, implementation of a robust geometry optimization procedure demonstrated that these findings were due to localization to metastable states and that the ground states are in fact diamagnetic singlets. However, the "nonstoichiometric" inorganic-passivated clusters (InCl3)(29)(InP)(147) and (ZnCl2)(29)(InP)(147) have ground nonet and septet states, respectively. Examination of the molecular orbitals revealed non-Aufbau state filling, suggesting the potential for open-shell singlet ground states, which is supported by calculations at the more robust M06-2x level of theory. Experimental evidence for paramagnetic or open-shell singlet ground states was not realized, which may be due to a mixture of inorganic and organic passivations.