Strain Lattice Imprinting in Graphene by C60 Intercalation at the Graphene/Cu Interface

Anisotropic strain in epitaxial single-layer molybdenum disulfide on Ag(110)

In this work we prove that ordered single-layer MoS₂ can be grown epitaxially on Ag(110), despite the different crystalline geometry of adsorbate and substrate. A comprehensive investigation of electronic and structural features of this interface is carried out by combining several techniques. Photoelectron diffraction experiments show that only two mirror crystalline domains coexist in equal amount in the grown layer. Angle-resolved valence band photoelectron spectroscopy shows that MoS₂ undergoes a semiconductor-to-metal transition. Low-energy electron diffraction and scanning-tunneling microscopy experiments reveal the formation of a commensurate moiré superlattice at the interface, which implies an anisotropic uniaxial strain of the MoS₂ crystalline lattice of ca. 3% in the [11̄0] direction of the Ag(110) surface. These outcomes suggest that the epitaxial growth on anisotropic substrates might be an effective and scalable method to generate a controlled and homogeneous strain in MoS₂ and possibly other transition-metal dichalcogenides.

Orientation Ordering and Chiral Superstructures in Fullerene Monolayer on Cd (0001)

The structure of C₆₀ thin films grown on Cd (0001) surface has been investigated from submonolayer to second monolayer regimes with a low-temperature scanning tunneling microscopy (STM). There are different C₆₀ domains with various misorientation angles relative to the lattice directions of Cd (0001). In the (2√3 × 2√3) R30° domain, orientational disorder of the individual C₆₀ molecules with either pentagon, hexagon, or 6:6 bond facing up has been observed. However, orientation ordering appeared in the R26° domain such that all the C₆₀ molecules adopt the same orientation with the 6:6 bond facing up. In particular, complex chiral motifs composed of seven C₆₀ molecules with clockwise or anticlockwise handedness have been observed in the R4° and R8° domains, respectively. Scanning tunneling spectroscopy (STS) measurements reveal a reduced HOMO–LOMO gap of 2.1 eV for the C₆₀ molecules adsorbed on Cd (0001) due to the substrate screening and charge transfer from Cd to C₆₀ molecules.

The structure of graphene on graphene/C60/Cu interfaces: a molecular dynamics study

Two experimental studies reported the spontaneous formation of amorphous and crystalline structures of C₆₀ molecules intercalated between graphene and a surface. The findings observed included interesting phenomena ranging from reaction between fullerene C₆₀s ('C₆₀s' stands for plural of C₆₀) under graphene to graphene sheets sagging between C₆₀s and control of strain in these sheets. Motivated by this work, we performed fully atomistic reactive molecular dynamics simulations to investigate the formation and thermal stability of graphene sheet wrinkles as well as graphene attachment to and detachment from a surface when the sheet is laid over a previously distributed array of C₆₀ molecules on a copper surface at different temperatures. As graphene compresses the C₆₀s against the surface, and graphene attachment to the surface in between C₆₀s depends on the height of the wrinkles in the graphene sheet, configurations with both frozen and non-frozen fullerenes were investigated in the simulations in order to examine the experimental result of stable, sagged graphene sheets when the distance between C₆₀s is about 4 nm and the height of the wrinkles in the sheet is about 0.8 nm. Below a distance of 4 nm between fullerenes, the graphene is predicted to become locally suspended and less strained. The simulations predict that this happens when the fullerenes can deform under the compressive action of the graphene sheet. If the fullerenes are kept frozen, spontaneous 'blanketing' of graphene is predicted only when the distance between neighbouring C₆₀s is equal to or great than about 7 nm. These predictions agree with a mechanical model relating the rigidity of a graphene sheet to the energy of graphene-surface adhesion. This work further reveals the structure of intercalated molecules and the role of stability and sheet wrinkling on the preferred configuration of graphene. This study thus might assist in the development of two-dimensional confined nanoreactors for chemical reactions.

Hybrids of Fullerenes and 2D Nanomaterials

Fullerene has a definite 0D closed-cage molecular structure composed of merely sp2-hybridized carbon atoms, enabling it to serve as an important building block that is useful for constructing supramolecular assemblies and micro/nanofunctional materials. Conversely, graphene has a 2D layered structure, possessing an exceptionally large specific surface area and high carrier mobility. Likewise, other emerging graphene-analogous 2D nanomaterials, such as graphitic carbon nitride (g-C3N4), transition-metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), and black phosphorus (BP), show unique electronic, physical, and chemical properties, which, however, exist only in the form of a monolayer and are typically anisotropic, limiting their applications. Upon hybridization with fullerenes, noncovalently or covalently, the physical/chemical properties of 2D nanomaterials can be tailored and, in most cases, improved, significantly extending their functionalities and applications. Here, an exhaustive review of all types of hybrids of fullerenes and 2D nanomaterials, such as graphene, g-C3N4, TMDs, h-BN, and BP, including their preparations, structures, properties, and applications, is presented. Finally, the prospects of fullerene-2D nanomaterial hybrids, especially the opportunity of creating unknown functional materials by means of hybridization, are envisioned.

Graphene as a Mechanically Active, Deformable Two-Dimensional Surfactant

In crystal growth, surfactants are additive molecules used in dilute amount or as dense, permeable layers to control surface morphologies. We investigate the properties of a strikingly different surfactant: a 2D and covalent layer with close atomic packing, graphene. Using in situ, real-time electron microscopy, scanning tunneling microscopy, kinetic Monte Carlo simulations, and continuum mechanics calculations, we reveal why metallic atomic layers can grow in a 2D manner below an impermeable graphene membrane. Upon metal growth, graphene dynamically opens nanochannels called wrinkles, facilitating mass transport while at the same time storing and releasing elastic energy via lattice distortions. Graphene thus behaves as a mechanically active, deformable surfactant. The wrinkle-driven mass transport of the metallic layer intercalated between graphene and the substrate is observed for two graphene-based systems, characterized by different physicochemical interactions, between graphene and the substrate and between the intercalated material and graphene. The deformable surfactant character of graphene that we unveil should then apply to a broad variety of species, opening new avenues for using graphene as a 2D surfactant forcing the growth of flat films, nanostructures, and unconventional crystalline phases.

Layered Insulator/Molecule/Metal Heterostructures with Molecular Functionality through Porphyrin Intercalation

Intercalation of molecules into layered materials is actively researched in materials science, chemistry, and nanotechnology, holding promise for the synthesis of van der Waals heterostructures and encapsulated nanoreactors. However, the intercalation of organic molecules that exhibit physical or chemical functionality remains a key challenge to date. In this work, we present the synthesis of heterostructures consisting of porphines sandwiched between a Cu(111) substrate and an insulating hexagonal boron nitride ( h-BN) monolayer. We investigated the energetics of the intercalation, as well as the influence of the capping h-BN layer on the behavior of the intercalated molecules using scanning probe microscopy and density functional theory calculations. While the self-assembly of the molecules is altered upon intercalation, we show that the intrinsic functionalities, such as switching between different porphine tautomers, are preserved. Such insulator/molecule/metal structures provide opportunities to protect organic materials from deleterious effects of atmospheric environment, can be used to control chemical reactions through spatial confinement, and give access to layered materials based on the ample availability of synthesis protocols provided by organic chemistry.

Surface chemistry and catalysis confined under two-dimensional materials

Interfaces between 2D material overlayers and solid surfaces provide confined spaces for chemical processes, which have stimulated new chemistry under a 2D cover.