Computational Study of Charge conducting Spacer molecules in Lead Chalcogenide Quantum dots

Q uantum dots (nanocrystals) have found applications in transistors, medical imaging,and solar cells, since they have band gaps that can be tuned into the far infrared region. This tuning is typically difficult to achieve with traditional semiconductor materials. The band gap tuning and self-assembly into a variety of large two-dimensional & three-dimensional superlattices makes them ideally suited for use in photovoltaic devices and other optoelectronic applications. Colloidal quantum dots of low band gap materials, like lead chalcogenide nanocrystals, have been the focus of considerable research efforts due to their potential ability for multiple exciton generation that theoretically may lead to efficiencies exceeding the Shockley-Queisser limit. However, quantum yield is greatly affected by the presence of capping ligands. The alkyl chain ligands presently used to prepare colloidal quantum dots are insulating in nature and, hence, impede charge transfer. Recent studies on ligand length have shown that charge transport decays as a function of the inter-quantum dot distance. In this study, we have introduced a new class of planar charge conducting spacer molecules. We have used Molecular Dynamics simulations to understand the behavior of such spacer-capped lead chalcogenide quantum dots. We have determined force fields to describe the interaction between these planar spacers and the nanocrystals. We have investigated the behavior of aggregation of these planar molecules surrounding arrays of quantum dots. Specifically, we have calculated the energy preference of stacking these planar molecules on each other (aggregation) versus interaction of the spacers with the surface of the quantum dots. Our results for the radial distribution function of the spacer molecules and a study of their displacement indicate that these planar spacer molecules help avoid the sintering of quantum dots and reduce the distance between quantum dots to a significant extent compared to the previously used functionalized alkyl chain ligands.