Interlayer Interactions in Low-Dimensional Layered Hetero-structures: Modeling and Applications

The field of nanoscale material science has experienced a true revolution over the past 30 years with the discovery of the quasi-zero- and one-dimensional cagelike structures of fullerenes and nanotubes. The successful isolation of graphene about a century ago has further triggered an avalanche of studies unraveling its unique physical and chemical properties. This, in turn, has led to numerous breakthroughs of basic science nature as well as diverse potential technological applications in the fields of nanoscale electronics, flexible displays, solar cells, DNA sequencing, chemical sensing, composite materials, solid lubrication, and many others. Following extensive studies of the properties of graphene, much attention was recently paid to other members of the two-dimensional (2D) hexagonal layered materials family including hexagonal boron nitride (h-BN) and several transitionmetal dichalcogenides. Interestingly, stacking individual layers of these materials to form homogeneous and heterogeneous structures results in unique physical characteristics that depend on the specific chemical composition of the system and can be tuned via the application of external perturbations. This opens numerous opportunities for combinatorial materials design with versatile structure-function relations. Understanding what determines the properties of such complex structures and how to control them remains a challenge yet to be met in order for these materials to fulfill their full potential. Theory and computation may offer a valuable microscopic perspective toward achieving this goal. The reduced-dimensions of materials at the nanoscale allow for a unique interplay between theory, computation, and experiment. Here, accurate fully atomistic simulations can complement experiments both in analyzing and rationalizing experimental results and in providing reliable predictions that can minimize the need for demanding trial and error experimental efforts. Such theoretical treatments of 2D materials require special attention to their anisotropic nature characterized by a strong in-plane covalent bonding network and weaker interlayer interactions. While state-of-the-art quantum mechanical approaches can simultaneously describe these interactions with high accuracy, their computational demand often limits their applicability to relatively small systems. An efficient alternative can be provided by carefully tailored classical force-fields. When appropriately parameterized against experimental results or high accuracy calculations of small model systems, these can provide a reliable description of the structural, mechanical, tribological, and heat transport properties of realistic nanoscale systems with atomic scale resolution. Such force-fields that provide a proper description of intralayer interactions in a variety of 2D materials have been developed over the years and are accessible via standard molecular dynamics simulation codes. Surprisingly, despite the great scientific interest in 2D layered materials, complementary interlayer force-fields that can accurately capture both their binding and sliding energy landscapes are currently available for a very limited set of systems. In the present chapter, I provide a brief review of recent developments of reliable, efficient, and transferable anisotropic interlayer force-fields for homogeneous and heterogeneous low-dimensional hexagonal layered materials. To demonstrate the performance of these methods, a few applications to the study of the structural and tribological properties of quasi-one- and quasi-two dimensional layered structures will be presented.