Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation.

Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.

Fabrication of nanoporous graphene/polymer composite membranes.

Graphene is currently investigated as a promising membrane material in which selective pores can be created depending on the requirements of the application. However, to handle large-area nanoporous graphene a stable support material is needed. Here, we report on composite membranes consisting of large-area single layer nanoporous graphene supported by a porous polymer. The fabrication is based on ion-track nanotechnology with swift heavy ions directly creating atomic pores in the graphene lattice and damaged tracks in the polymer support. Subsequent chemical etching converts the latent ion tracks in the supporting polymer foil, here polyethylene terephthalate (PET), into open microchannels while the perfectly aligned pores in the graphene top layer remain unaffected. To avoid unintentional damage creation and delamination of the graphene layer from the substrate, the graphene is encapsulated by a protecting poly(methyl methacrylate) (PMMA) layer. By this procedure a stable composite membrane is obtained consisting of nanoporous graphene (coverage close to 100%) suspended across selfaligned track-etched microchannels in a polymer support film. Our method presents a facile way to create high quality suspended graphene of tunable pore size supported on a flexible porous polymeric support, thus enabling the development of membranes for fast and selective ultrafiltration separation processes.

Graphitic nanostripes in silicon carbide surfaces created by swift heavy ion irradiation.

The controlled creation of defects in silicon carbide represents a major challenge. A well-known and efficient tool for defect creation in dielectric materials is the irradiation with swift (E(kin) ≥ 500 keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.

Effect of contaminations and surface preparation on the work function of single layer MoS2.

Thinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance.

Doping of graphene exfoliated on SrTiO3.

We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO(3) substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be Φ(SLG) = 4.409 ± 0.039 eV and Φ(BLG) = 4.516 ± 0.035 eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons.