Lead Chalcogenide Quantum Dot Assembly and Attachment on Fluid Interfaces

The formation of tiles composed of quantum dots are thought to constitute a new class of self-assembled nanostructured material. Understanding the dynamic physicochemical processes that govern the assembly of quantum dot monomers at a functionalized fluid interface is crucial for material processing strategies. However, the mechanism of this assembly and attachment is still unclear. What is clear is that a functionalized liquid interface potentially provides more control in the diffusion, coupling, and orientation of the nanocrystals (NCs), which increases the complexity of the assembly process. To investigate this self-assembly process, we look to insight from the fundamental molecular-level interactions; we use Molecular Dynamics (MD) simulations to watch NCs assembly on a surfactant monolayer. The specific model system studied here was composed by lead chalcogenide nanocrystals, covered with lead oleate molecules and assembling on a monolayer composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) molecules. In this approach, a reactive force field is used to model the interactions between species in the system. For the PbS NCs, we use the existing Simple Molecular Reactive Force Field (SMRFF) as an effective force field. The ligands and DPPC molecules are represented by the OPLS force field. All the simulations aimed to reveal the role of the nature of the surface monolayer. Before testing the impact of monolayer parameters, we used density functional theory to confirm that the energy barrier for ligand dissociation was high and are unlikely to detach readily. We adjusted the density of the DPPC molecules in the monolayer from 0.5 to 2.5 molecules/nm² to determine the effect of ML density on assembly and attachment behavior of NCs on a DPPC monolayer. We studied the degree of NC in-plane and out-of-plane orientation during the process and the alignment and tendency to form interconnecting bridges. We found that the NCs prefer to diffuse and assemble rather than interacting with the monolayer. Our studies of the interaction energy and the potential of mean force (PMF) provide quantitative metrics for the interaction between the NCs and the monolayer. We show that there is a similar trend in the interaction energy between NCs and monolayer, which can help us to predict an optimal monolayer structure for NC assembly. We also studied the effect of hydrocarbon chain length in the monolayer molecule, varying the molecule from DPPC to DLPC and DMPC. This property indicates that NC assembly is facilitated by shorter lipid molecules.