Synthesis of Few-Layered Transition-Metal Dichalcogenides by Ion Implantation of Chalcogen and Metal Species into Sapphire.

The growth of transition-metal dichalcogenides (TMDCs) has been performed so far using most established thin-film growth techniques (e.g., vapor phase transport, chemical vapor deposition, molecular beam epitaxy, etc.). However, because there exists no self-limiting mechanism for the growth of TMDCs, none of these techniques allows precise control of the number of TMDC layers over large substrate areas. Here, we explore the ion implantation of the parent TMDC atoms into a chemically neutral substrate for the synthesis of TMDC films. The idea is that once all of the ion-implanted species have reacted together, the synthesis reaction stops, thereby effectively stopping growth. In other words, even if there is no self-limiting mechanism, growth stops when the nutrients are exhausted. We have co-implanted Mo and S ions into c-oriented sapphire substrates using various doses corresponding to 1- to 5-layer atom counts. We find that the subsurface region of the sapphire substrates is amorphized by the ion implantation process, at least for implanted doses of 2-layer atom counts and over. For all doses, we have observed the formation of MoS2 material inside the sapphire after postimplantation annealing between 800 and 850 °C. We report that the order of implantation (i.e., whether S or Mo is implanted first) is an important parameter. More precisely, samples for which S is implanted first tend to yield thin crystals with a large lateral extension (more than 200 nm for 5-layer doses) and mainly located at the interface between the amorphized and crystalline sapphire. When Mo is first implanted, the MoS2 crystals still predominantly appear at the amorphous-crystalline interface (which is much rougher), but they are much thicker, suggesting a different nucleation mechanism.

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.

Breakdown of Universal Scaling for Nanometer-Sized Bubbles in Graphene.

We report the formation of nanobubbles on graphene with a radius of the order of 1 nm, using ultralow energy implantation of noble gas ions (He, Ne, Ar) into graphene grown on a Pt(111) surface. We show that the universal scaling of the aspect ratio, which has previously been established for larger bubbles, breaks down when the bubble radius approaches 1 nm, resulting in much larger aspect ratios. Moreover, we observe that the bubble stability and aspect ratio depend on the substrate onto which the graphene is grown (bubbles are stable for Pt but not for Cu) and trapped element. We interpret these dependencies in terms of the atomic compressibility of the noble gas as well as of the adhesion energies between graphene, the substrate, and trapped atoms.

Doping Graphene with Substitutional Mn.

We report the incorporation of substitutional Mn atoms in high-quality, epitaxial graphene on Cu(111), using ultralow-energy ion implantation. We characterize in detail the atomic structure of substitutional Mn in a single carbon vacancy and quantify its concentration. In particular, we are able to determine the position of substitutional Mn atoms with respect to the Moiré superstructure (i.e., local graphene-Cu stacking symmetry) and to the carbon sublattice; in the out-of-plane direction, substitutional Mn atoms are found to be slightly displaced toward the Cu surface, that is, effectively underneath the graphene layer. Regarding electronic properties, we show that graphene doped with substitutional Mn to a concentration of the order of 0.04%, with negligible structural disorder (other than the Mn substitution), retains the Dirac-like band structure of pristine graphene on Cu(111), making it an ideal system in which to study the interplay between local magnetic moments and Dirac electrons. Our work also establishes that ultralow-energy ion implantation is suited for substitutional magnetic doping of graphene. Given the flexibility, reproducibility, and scalability inherent to ion implantation, our work creates numerous opportunities for research on magnetic functionalization of graphene and other two-dimensional materials.

The adsorption of silicon on an iridium surface ruling out silicene growth.

The adsorption of Si atoms on a metal surface might proceed through complex surface processes, whose rate is determined differently by factors such as temperature, Si coverage, and metal cohesive energy. Among other transition metals, iridium is a special case since the Ir(111) surface was reported first, in addition to Ag(111), as being suitable for the epitaxy of silicene monolayers. In this study we followed the adsorption of Si on the Ir(111) surface via high resolution core level photoelectron spectroscopy, starting from the clean metal surface up to a coverage exceeding one monolayer, in a temperature range between 300 and 670 K. Density functional theory calculations were carried out in order to evaluate the stability of the different Si adsorption configurations as a function of the coverage. Results indicate that, at low coverage, the Si adatoms tend to occupy the hollow Ir sites, although a small fraction of them penetrates the first Ir layer. Si penetration of the Ir surface can take place if the energy gained upon Si adsorption is used to displace the Ir surface atoms, rather then being dissipated differently. At a Si coverage of ∼1 monolayer, the Ir 4f spectrum indicates that not only the metal surface but also the layers underneath are perturbed. Our results point out that the Si/Ir(111) interface is unstable towards Si-Ir intermixing, in agreement with the silicide phase formation reported in the literature for the reverted interface.