Based on an ultra-high vacuum compatible two-step molecular beam epitaxy synthesis with elemental sulphur, we grow clean, well-oriented, and almost defect-free monolayer islands and layers of the transition metal disulphides MoS2, TaS2 and WS2. Using scanning tunneling microscopy and low energy electron diffraction we investigate systematically how to optimise the growth process, and provide insight into the growth and annealing mechanisms. A large band gap of 2.55 eV and the ability to move flakes with the scanning tunneling microscope tip both document the weak interaction of MoS2 with its substrate consisting of graphene grown on Ir(1 1 1). As the method works for the synthesis of a variety of transition metal disulphides on different substrates, we speculate that it could be of great use for providing hitherto unattainable high quality monolayers of transition metal disulphides for fundamental spectroscopic investigations.

MoS monolayer samples were synthesized on a SiO/Si wafer and transferred to Ir(111) for nano-scale characterization. The samples were extensively characterized during every step of the transfer process, and MoS on the final substrate was examined down to the atomic level by scanning tunneling microscopy (STM). The procedures conducted yielded high-quality monolayer MoS of milimeter-scale size with an average defect density of 2 × 10 cm. The lift-off from the growth substrate was followed by a release of the tensile strain, visible in a widening of the optical band gap measured by photoluminescence. Subsequent transfer to the Ir(111) surface led to a strong drop of this optical signal but without further shifts of characteristic peaks. The electronic band gap was measured by scanning tunneling spectroscopy (STS), revealing n-doping and lateral nano-scale variations. The combined use of STM imaging and density functional theory (DFT) calculations allows us to identify the most recurring point-like defects as S vacancies.

Growth of 2D materials under ultrahigh-vacuum (UHV) conditions allows for an in situ characterization of samples with direct spectroscopic insight. Heteroepitaxy of transition-metal dichalcogenides (TMDs) in UHV remains a challenge for integration of several different monolayers into new functional systems. In this work, we epitaxially grow lateral WS 2 −MoS 2 and vertical WS 2 /MoS 2 heterostructures on graphene. By means of scanning tunneling spectroscopy (STS), we first examined the electronic structure of monolayer MoS 2 , WS 2 , and WS 2 /MoS 2 vertical heterostructure. Moreover, we investigate a band bending in the vicinity of the narrow one-dimensional (1D) interface of the WS 2 −MoS 2 lateral heterostructure and mirror twin boundary (MTB) in the WS 2 /MoS 2 vertical heterostructure. Density functional theory (DFT) is used for the calculation of the band structures, as well as for the density of states (DOS) maps at the interfaces. For the WS 2 −MoS 2 lateral heterostructure, we confirm type-II band alignment and determine the corresponding depletion regions, charge densities, and the electric field at the interface. For the MTB, we observe a symmetric upward bend bending and relate it to the dielectric screening of graphene affecting dominantly the MoS 2 layer. Quasi-freestanding heterostructures with sharp interfaces, large built-in electric field, and narrow depletion region widths are proper candidates for future designing of electronic and optoelectronic devices.

We present a study of sulfur adsorption on bare Ir(111). Two well-defined superstructures are found: a and a c(4 × 2) S-adlayer. Moreover, we also investigate sulfur intercalation of graphene on Ir(111). For adsorption, sulfur is provided either in the form of the precursor molecule H2S or as elemental sulfur through sublimation from FeS2 heated in a Knudsen cell. On the basis of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), as well as density functional theory calculations (DFT), we present a model for the c(4 × 2) superstructure consistent with surface relaxations. We show that above a graphene coverage threshold, when islands of the two-dimensional (2D) material start to coalesce, the sulfur superstructure intercalated below graphene depends on the form in which sulfur is provided: c(4 × 2) forms in the case of exposure to elemental sulfur, while the superstructure forms in the case of H2S exposure. The two intercalation structures influence the graphene moiré corrugation in different ways. We have used DFT calculations to determine sulfur adsorption energies, surface relaxations, and the influence of sulfur intercalation on the density of electronic states of graphene on Ir(111).

Both silicon and graphite are radiation hard materials with respect to swift heavy ions like fission fragments and cosmic rays. Recrystallisation is considered to be the main mechanism of prompt damage anneal in these two materials, resulting in negligible amounts of damage produced, even when exposed to high ion fluences. In this work we present evidence that these two materials could be susceptible to swift heavy ion irradiation effects even at low energies. In the case of silicon, ion channeling and electron microscopy measurements reveal significant recovery of pre-existing defects when exposed to a swift heavy ion beam. In the case of graphite, by using ion channeling, Raman spectroscopy and atomic force microscopy, we found that the surface of the material is more prone to irradiation damage than the bulk.

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Environmental contextSolubility and dissolution rates of mineral surfaces depend on both the surface properties of the mineral and the composition of the aqueous solution. We investigated the link between the interfacial reactions and dissolution of a fluorite crystal. The study provides a detailed microscopic picture of the dissolution phenomena at the fluorite surface, and the results have wider application to general mineral dissolution processes taking place in the environment. AbstractDissolutions of the fluorite (111) crystallographic plane and fluorite (CaF2) colloidal particles were studied as a function of pH. The process was examined by measuring the concentration of released fluoride and calcium ions by ion-selective electrodes. Additionally, electrokinetic and inner surface potentials were measured by means of electrophoresis and a fluorite single crystal electrode respectively. The rate of fluorite dissolution was analysed assuming a reaction mechanism with a series of elementary steps, which included the reaction of surface groups with H+ ions, the formation of F− vacancies, the dissociation of surface groups and the release of calcium and fluoride ions into the interfacial region as well as the diffusion of ions from the interfacial region. The proposed reaction mechanism indicates that H+ ions play a necessary role in allowing the dissolution to take place, a concept not possible to confirm by looking at the overall equation of fluorite dissolution. The order of the total reaction with respect to H+ ions was found to be 0.37, which is in good accordance with the value derived from the reaction mechanism (1/3). The experimentally determined rate coefficient of fluorite dissolution was found to be kdis=9×10−6mol2/3dmm−2s−1.

We show growth of heterostructures on Ir(111) crystal and subsequent transfer to a Si wafer. This remedies substrate constraints imposed by MBE and allows to harness its advantages for applications in (opto)electronics and quantum technology.