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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.

Since two-dimensional materials with their unique physical properties have become accessible to experimental physics, the scientific interest in this material class has been very high as they promise a multitude of possible applications. In particular, it has been shown that ionizing particle irradiation allows structuring of these 2d materials. In addition to graphene, so-called carbon nanomembranes (CNMs) also belong to the class of the new two-dimensional materials. They offer the advantage that they can be produced and transferred by standard chemical and physical methods instead of mechanical exfoliation. For this experiment we have prepared single layer graphene as well as CNMs samples. The latter were prepared by electron induced crosslinking of aromatic selfassembled monolayers like biphenylthiol which was evaporated on copper. Our samples have been irradiated with swift heavy ions under glancing angles of incidence. The samples were analysed by atomic force microscopy in ambient conditions and in ultrahigh vacuum. The latter measurements served as test measurements for our newly built setup at the M branch of the UNILAC beamline in GSI (Darmstadt, Germany) and were performed insitu directly after irradiation. The CNMs show modifications in form of extended pores with shapes different from those which we typically observe in graphene. Finally, experiments performed at different accelerator facilities (GSI, GANIL, and RBI) with different beams energies were used to determine the threshold for pore formation.

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. © 2014 Ochedowski et al.

Duisburg, Essen, Univ., Diss., 2014 Since the isolation of graphene in 2004, two-dimensional crystals have attracted a lot of attention in a variety of scientific fields. Thus, Andre Geim and Kostya Novoselov were honoured with the Nobel Prize in 2010 for their findings. Ten years later, the interest in materials like graphene and single layer MoS2 is still growing. In order to actually implement these new materials in the plentitude of applications that are envisaged, new tools are needed. The purpose of this work is to investigate whether swift heavy ions (highly charged ions with typical energies ranging from 100 MeV to GeV) can be used as such a new tool for the modification of the morphology and doping of these materials. To obtain two-dimensional crystals of the highest quality, the mechanical exfoliation technique is used to prepare graphene and MoS2 single layers on various substrates. However, as the samples are prepared in ambient the resulting single layers are usually contaminated with adsorbates and water is intercalated between the crystal and the substrate. Furthermore, the presence of the substrate alone results in a significant change of the properties of two-dimensional crystals. Therefore graphene and MoS2 prepared on different substrates are investigated using non-contact atomic force microscopy and Kelvin probe force microscopy in situ to probe the contribution of adsorbates, intercalated water and the substrate itself under defined conditions. It has already been shown that graphene is folded upon swift heavy ion impact when irradiated under grazing incidence. In this work, the mechanism of the folding formation is studied and it will be shown that the size of the foldings can be controlled precisely by the energy of the ion, the incidence angle of the ion with respect to the sample surface and the choice of the substrate. In case of single layer MoS2 an additional modification to the morphology can be observed in form of nanoscale slit pores. The length of these slit pores can be controlled by the incidence angle again and the aspect ratio up to 1:600 (width:length). A mechanism for the formation of the slit pores is formulated: a thermal spike generated by the intense electronic excitation of the swift heavy ion is heating up the substrate on which MoS2 is attached. The temperature at the surface is exceeding the melting temperature of MoS2 and results in the formation of the slit pore. The last part of this thesis deals with the possibility of doping graphene using swift heavy ions. By preparing graphene samples without an intercalated water layer, the folding formation can be prevented. Instead, a surface track is created into the atomically thin carbon layer. This surface track significantly increases the hole charge carrier concentration in graphene and it will be shown that the surface tracks are created by implantation of Si atoms from the substrate due to the swift heavy ion irradiation.

Since its discovery in 2004, graphene research attracts great attention and its numerous applications are envisaged in the near future [1]. In this contribution, we show how swift heavy ions can be used as a tool for controled modification of graphene on the nanoscale. Very recently, it was demonstrated that passage of swift heavy ions delivered from cyclotron (GANIL, France) at grazing incidence with respect to the graphene results in unzipping and folding of exfoliated graphene [2]. These modifications can be produced in controlled manner since number of pores is determined by ion fluence. Here we show how less energetic swift heavy ions delivered from 6 MV Tandem Van de Graaff accelerator (Ruđer Bošković Institute, Zagreb) can be used for graphene modifications as well. In addition, we present first results on swift heavy ion induced modifications on other 2D materials. [1] K.S. Novoselov et al., A roadmap for graphene, Nature 490 (2012) 192 [2] S. Akcöltekin et al., Unzipping and folding of graphene by swift heavy ions, Appl. Phys. Lett. 98 (2011) 103103

Nakon otkrica 2004 godine, istraživanja grafena privlace veliku pažnju jer se očekuju mnogobrojne primjene u buducnosti [1]. U ovome radu, bit ce pokazano kako se snopovi brzih teških iona mogu koristiti kao oruđe za kontrolirane modikacije grafena na nanoskali. Nedavno je pokazano kako prolazak brzog teškog iona dobivenog iz ciklotrona (GANIL, France) koji prolazi kroz grafen pod malim kutem u odnosu na njegovu površinu, rezultira cijepanjem grafena koji je pripremljen eksfolijacijom [2]. Ovakve modikacije se mogu proizvoditi kontrolirano jer broj pora odgovara broju ozracenih iona, a njihov oblik je određen kutem ozracavanja. U ovom radu ćemo pokazati kako i brzi teški ioni manjih energija dobivenih iz 6 MV Tandem Van de Graašff akceleratora (Institut Ruđer Bošković, Zagreb) mogu također biti iskorišteni za modikacije grafena. Također, prikazat ćemo prve rezultate ozračavanja drugih 2D materijala poput MoS2 i h- BN. [1] K.S. Novoselov et al., A roadmap for graphene, Nature 490 (2012) 192 [2] S. Akcöltekin et al., Unzipping and folding of graphene by swiŸ heavy ions, Appl. Phys. Lett. É98 (2011) 103103