Ordering and Dynamics for the Formation of Two-Dimensional Molecular Crystals on Black Phosphorene

Abstract

Phosphorene, a monolayer of elemental phosphorus, has emerged as the most important 2D atomic crystal post graphene. As a consequence of its suitable direct band gap, it has high carrier mobility, thereby making it suitable for potential applications in nanoelectronic devices. In this article, we have performed molecular dynamics simulations for the first time to unveil the potential of black phosphorene (BP) as a platform to fabricate two-dimensional crystals through van der Waals epitaxy of organic molecules. Eight molecules (benzene, 1,3,5-trimethylbenzene, 1,3,5-trifluorobenzene, 1,3,5-trihydroxybenzene, 7,7′,8,8′-tetracyanoquinodimethane (TCNQ), pentacene, coronene, and fullerene) of various size, shape, and polarity are shown to form highly periodic 2D monolayers on BP. We show that self-assembly proceeds through a hierarchy of steps, beginning with adsorption on the surface followed by interplay between the kinetic energy and intermolecular interactions. Crystalline 2D layers are obtained through the minimization of van der Waals interactions for nonpolar and electrostatic interactions for polar molecules. Long-range attractive electrostatic interactions contribute significantly toward the stabilities of the superlattices, thereby making the melting points of polar crystals much higher compared to their nonpolar analogs (Tm (polar) > Tm (nonpolar)). Interestingly, although nonpolar fullerene is shown to form highly stable superlattices due to extensive intermolecular van der Waals interactions, the underlying BP monolayer helps to orient the molecules and, hence, indirectly assists molecular ordering. It also contributes to the overall stability of the molecular crystals through interfacial adsorption. A comparison of self-assembly patterns over BP vis-a-vis graphene and hexagonal boron nitride shows that the nonaromatic nature and undulation of black phosphorene do not affect the intrinsic stability of the assemblies. These findings offer new avenues for generating new molecular doped crystalline superlattices for semiconductor industry.