Novel two Dimensional Materials: Graphene and Beyond

Two-dimensional (2D) materials, with only a single-atom or a single-polyhedral thickness, are an emerging class of systems with exotic electrical, optical, and mechanical properties. Graphene (GA) is the prime example of this class that has created a lot of excitement in materials science since its discovery in its free-standing form in 2004. Other 2D materials of interest include BN and transition-metal dichalcogenides (TMDs) MX2 where M = metal and X = S, Se or Te. The electronic nature of these 2D systems varies where GA has a conical Dirac spectrum of energy states without a bandgap and a linear dispersion, whereas BN is an insulator, and the TMDs are semiconductors in their ground state structure. In these layered materials, the properties at the single layer are generally distinct from the bulk due to quantum confinement effects. For instance, MoS2 in its bulk form is an indirect band insulator (~1.3 eV) while as a monolayer, it has a direct bandgap of ~1.8 eV. This indirect-to-direct bandgap change has a dramatic effect on the optical properties, where MoS2 monolayer emits light strongly, and exhibits more than 104 increase in luminescence quantum efficiency compared to the bulk material.

Advances in materials synthesis have reached a stage where 2D materials can be used as building blocks that can be restacked layer-by-layer in a precisely chosen sequences yielding composites with unusual properties.

Relevant Publications

Contacts between metal surfaces and MoS2 are crucial for the utilization of MoS2 in different technologies. Here we systematically investigate using first-principles density functional theory the adsorption and diffusion on MoS2(001) of a wide range of metals from Groups I–IV in addition to all of the 3d transition metals (TMs) and selected 4d and 5d TMs. The binding mechanisms as well as trends in the binding energies are elucidated by examining the electronic structure of the system, and in particular the interplay between Coulomb interactions, Pauli repulsion, and ndm(n + 1)sxndm+1(n + 1)sx–1 (x = 1, 2; n = 3, 4, 5) promotion energies. We show that the metal-induced workfunction reduction is correlated with the ionization potential of the isolated atom and is furthermore linearly dependent on the interfacial dipole moment with an offset term. Additionally, the growth morphologies of the metal nanoparticles on MoS2 are predicted by analyzing the monomer adhesion energy and its mobility on the substrate. Our results are in line with recent experiments showing that Ag and Au follow a Volmer–Weber growth mode on MoS2(001).
The dispersion of Pt metallic nanoparticles on different supports is of high relevance for designing more efficient and less expensive catalysts. In order to understand the nucleation and epitaxial growth of Pt nanoparticles and thin films on MoS2 monolayers, we have systematically analyzed, by first-principles density functional calculations, the evolution of morphology and atomic structure of supported (Pt)n nanoparticles (NPs) on MoS2(001) for n ≤ 12. We find that n = 5 is the cluster size where the growth of the NPs transforms from two- to three-dimensional (2D to 3D). Owing to the topography of MoS2(001), the 2D NPs mostly attach to the support via direct bonding with Mo atoms that sit in the troughs of the surface, while the 3D NPs are bonded to the sulfur atoms that are more extended in the vacuum region. Furthermore, we find that Pt is sufficiently mobile on the surface where the number of hopping events per second is ≈103 s–1 along [101̅] and ≈10 s–1 along [11̅0] at room temperature. The somewhat large mobility suggests that monomer diffusion is not likely to be the rate-limiting step for Oswald ripening and that Pt sputtering on MoS2(001) will result in relatively large particles rather than a fine dispersion. The existence of a fast diffusion channel along [101̅] suggests that the morphology of the NPs is anisotropic.
The stability and the electronic structure of layered heterostructures MX2 (M = Mo or W and X = S or Se) and graphene (GA) are systematically investigated using first-principles methods. The calculations cover pristine and defected GA systems with up to 12% nitrogen substitutional defects. It is found that the van der Waals (vdW) epitaxy of MX2 on undoped GA substrate, whether pristine or defected, follows a Volmer–Weber growth-mode resulting in thick MX2 films. On the other hand, nitrogen doping of pristine GA (N-GA) and also of GA with Stone–Wales (SW) defects increases the MX2/GA heterostructure adhesion energy favoring the growth of ultrathin MX2 layers. This growth-mode change in MoS2 due to nitrogen doping is in agreement with recent experiments. Furthermore, our study demonstrates that the yield of ultrathin MX2 films can be increased if the N-GA samples have a larger concentration of SW defects or nitrogen. The underpinnings of the extra stability of these N-GA substrates are due to charge-transfer effects that decrease the Pauli repulsion between the two layered systems.