(433f) 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.