Liquid-Phase Exfoliation of Phosphorene: Design Rules from Molecular Dynamics Simulations

Liquid-Phase Exfoliation of Phosphorene: Design Rules from Molecular Dynamics Simulations

Vishnu Sresht¹, Agilio A. H. Pádua^1,2, Daniel Blankschtein¹

 

¹Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

²Institut de Chimie de Clermont-Ferrand, Université Blaise Pascal and CNRS, 63171 Aubière, France

Session: NANOSCALE SCIENCE AND ENGINEERING FORUM, 22002 Self and Directed Assembly at the Nanoscale

The liquid-phase exfoliation of phosphorene, the two-dimensional derivative of black phosphorus, in the solvents dimethylsulfoxide, dimethylformamide, isopropyl alcohol, N-methyl-2-pyrrolidone, and N-cyclohexyl-2-pyrrolidone is investigated using three molecular-scale “computer experiments”. We modeled solvent–phosphorene interactions using a new atomistic force field, based on ab-initio calculations and lattice dynamics that accurately reproduces experimental mechanical properties. We probed solvent molecule ordering at phosphorene/solvent interfaces and discovered that planar molecules such as N-methyl-2-pyrrolidone preferentially orient parallel to the interface. We subsequently employed a novel simulation technique to peel a single phosphorene monolayer from a stack of black phosphorus, and analyzed the role of “wedges” of solvent molecules intercalating between phosphorene sheets in initiating exfoliation. The exfoliation efficacy of a solvent is enhanced when either molecular planarity “sharpens” this molecular wedge, or strong phosphorene–solvent adhesion stabilizes the newly exposed phosphorene surfaces. Finally, we examined the colloidal stability of exfoliated flakes by simulating their aggregation, and showed that dispersion is favored when the cohesive energy between the molecules in the solvent monolayer confined between the phosphorene sheets is high (as with DMSO), and is hindered when the adhesion between these molecules and phosphorene is strong; the molecular planarity in solvents like DMF enhances the cohesive energy. Our results are consistent with, and provide a molecular context for, experimental exfoliation studies of phosphorene and other layered solids, and our molecular insights into the significant role of solvent molecular geometry and ordering should complement prevalent solubility-parameter-based approaches in establishing design rules for effective nanomaterial exfoliation media.