Modification of 2D-Materials by Swift Heavy Ion Irradiation

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

Plasma-enhanced chemical vapor deposition of graphene on copper substrates

A plasma enhanced vapor deposition process is used to synthesize graphene from a hydrogen/methane gas mixture on copper samples. The graphene samples were transferred onto SiO2 substrates and characterized by Raman spectroscopic mapping and atomic force microscope topographical mapping. Analysis of the Raman bands shows that the deposited graphene is clearly SLG and that the sheets are deposited on large areas of several mm2. The defect density in the graphene sheets is calculated using Raman measurements and the influence of the process pressure on the defect density is measured. Furthermore the origin of these defects is discussed with respect to the process parameters and hence the plasma environment

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

Folding two dimensional crystals by swift heavy ion irradiation

International audienceIon irradiation of graphene, the showcase model of two dimensional crystals, has been successfully applied to induce various modifications in the graphene crystal. One of these modifications is the formation of origami like foldings in graphene which are created by swift heavy ion irradiation under glancing incidence angle. These foldings can be applied to locally alter the physical properties of graphene like mechanical strength or chemical reactivity. In this work we show that the formation of foldings in two dimensional crystals is not restricted to graphene but can be applied for other materials like MoS2 and hexagonal BN as well. Further we show that chemical vapour deposited graphene forms foldings after swift heavy ion irradiation while chemical vapour deposited MoS2 does not

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

International audienceThe controlled creation of defects in silicon carbide represents a major challenge. A wellknown and efficient tool for defect creation in dielectric materials is the irradiation with swift (EkinZ500 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

Defect engineering of single- and few-layer MoS 2 by swift heavy ion irradiation

International audienceWehave investigated the possibility to use swift heavy ion irradiation for nano-structuring supported and freestanding ultra-thin MoS2 samples. Our comprehensive study of the ion-induced morphological changes in various MoS2 samples shows that depending on the irradiation parameters a multitude of extended defects can be fabricated. These range from chains of nano-hillocks in bulk-like MoS2, and foldings in single and bilayer MoS2, to unique nano-incisions in supported and freestanding single layers of MoS2. Our data reveals that the primary mechanism responsible for the incisions in the ultrathin supported samples is the indirect heating by the SiO2 substrate.We thus conclude that an energy of less than 2 keV pernmtrack length is sufficient to fabricate nano-incisions in MoS2 which is compatible with the use of the smallest accelerators