Rotational superstructure in van der Waals heterostructure of self-assembled C₆₀ monolayer on the WSe₂ surface

Abstract

Hybrid van der Waals (vdW) heterostructures composed of two-dimensional (2D) layered materials and self-assembled organic molecules are promising systems for electronic and optoelectronic applications with enhanced properties and performance. Control of molecular assembly is therefore paramount to fundamentally understand the nucleation, ordering, alignment, and electronic interaction of organic molecules with 2D materials. Here, we report the formation and detailed study of highly ordered, crystalline monolayers of C molecules self-assembled on the surface of WSe in well-ordered arrays with large grain sizes (∼5 μm). Using high-resolution scanning tunneling microscopy (STM), we observe a periodic 2 × 2 superstructure in the C monolayer and identify four distinct molecular appearances. Using vdW-corrected ab initio density functional theory (DFT) simulations, we determine that the interplay between vdW and Coulomb interactions as well as adsorbate-adsorbate and adsorbate-substrate interactions results in specific rotational arrangements of the molecules forming the superstructure. The orbital ordering through the relative positions of bonds in adjacent molecules creates a charge redistribution that links the molecule units in a long-range network. This rotational superstructure extends throughout the self-assembled monolayer and opens a pathway towards engineering aligned hybrid organic/inorganic vdW heterostructures with 2D layered materials in a precise and controlled way.

Visualizing molecular orientational ordering and electronic structure in CsnC60 fulleride films

“…We thus arrive at our first major finding: the molecular orientations of C n− 60 anions become ordered solely for stoichiometric Cs n C 60 compounds with integer n, including zero. Such finding accords quite nicely with the local orientational ordering in K 3 C 60 [31,32], as well as the preference of molec-ular orientational ordering in pristine C 60 multilayers [10,11,36,42] or thin films supported by chemically inert substrates [12,43]. In the latter situations, the C 60 assemblies on question are only limitedly affected by the substrates and conserve their charge neutrality.…”

Hybrids of Fullerenes and 2D Nanomaterials

“…Sci. [131] The deposited C 60 molecules formed a monolayer that extended uniformly over WSe 2 with large grain sizes (≈5 µm). [130] To thoroughly understand the basic physical and chemical phenomena that rule highly crystalline architectures comprised of crystals of organic molecules and 2D materials, Wang and coworkers fulfilled the growth of high-quality self-assembled monolayers of C 60 on WSe 2 , which served as a weakly interacting organic/2D vdW heterostructure system.…”

2D–Organic Hybrid Heterostructures for Optoelectronic Applications

“…Following the synthesis of vdW 2D materials, organic molecules can be deposited via thermal evaporation, spin coating, dip coating, or physical vapor deposition (PVD) . Among these methods, PVD is advantageous for controlling the film thickness and/or morphology.…”

“…In particular, the PVD conditions (e.g., evaporation speed, temperature, and pressure) can be easily modulated to alter the structure of organic thin films from randomly oriented to highly ordered crystalline. This structural tunability would be beneficial for optimizing the structure–property–performance relationships . For example, for exciton transport in organic thin films, it is preferable to have a structure that is oriented normal to the π–π stacking direction to ensure optimized charge injection and/or extraction .…”

“…Additionally, another combination based on 2D TMDCs was reported previously. Santos et al reported the epitaxial growth of monolayer C 60 on 2D tungsten diselenide (WSe 2 ) (Figure d) . An STM image showed that the C 60 layer was assembled into a close‐packed hexagonal structure with a domain size of up to ≈5 µm.…”