Critical Radius for Forming Oriented Few-Layer MoS2 on High-Curvature Nanostructures

Abstract

The high strength and flexibility of two-dimensional materials enable their use in flexible devices and structures with a high curvature. However, thin films adhered to curved structures can wrinkle, fold, buckle, delaminate, or crack, depending on the curvature, film thickness, film mechanical properties, and film-substrate adhesion. How adhesion relates to 2D film properties such as crystallinity, defects, and substrate interactions is not understood due to the multiphysical, complex interplay between chemistry, structure, and adhesion. Moreover, it is intrinsically challenging to observe 2D materials and their interfaces with substrates laterally with atomic resolution in complex geometries. Here, we address these challenges using customized transmission electron microscopy (TEM) methodology to examine the atomically resolved structures of nanocrystalline few-layer molybdenum disulfide (MoS2) films deposited onto silicon (Si) tips with nanoscale curvature. We observe a transition from conformal film growth to film delamination with a substantial degree of structural defects below a critical tip radius of curvature (Rc) of approximately 5 nm. Continuum mechanics approximations based on membrane bending and stretching energies overestimate Rc and fail to explain the nature of the observed transition. We thus conducted molecular dynamics (MD) simulations of nanocrystalline MoS2 films on nanoscale Si hemispheres. The predicted Rc from MD is in excellent agreement with the experimental findings when the grain size is accounted for. This suggests an interdependence among film mechanical properties, grain size, adhesion, and curvature. In particular, defects, in this case, structural disorder at grain boundaries, enable curvature exceeding that predicted by continuum mechanics-based membrane models.