Hydrogen plasma defined graphene edges

Abstract

In this thesis, anisotropic etching of graphite and graphene in a hydrogen (H) plasma is investigated. The exposure of graphite flakes at different plasma pressures and sample-plasma distances reveals the existence of two different plasma regimes: the \\textit{direct} and the \\textit{remote} regime. In the direct regime, high energetic H-ions continuously induce new defects into the graphite surface during the etching process, thus leading to a perforated surface. In the remote plasma regime, on the other hand, well-defined hexagonal etch pits evolve, which grow in size, while their number remains constant. This indicates anisotropic etching, which takes place only at pre-existing defects and edges and leaves the graphite basal plane pristine. \nIn a second step of the experiment, the substrate dependence of single layer graphene etching in the remote plasma regime is investigated. Interestingly, the etching is only anisotropic for hexagonal boron nitride substrates but isotropic if graphene is placed on Si/SiO$_2$. It was previously found that the edges of H plasma defined hexagons on graphite run along the zigzag (ZZ) direction of the crystal lattice. Hence, by inducing artificial defects into a graphene flake, one can tailor diverse graphene nano-structures with presumably well-defined ZZ edges, such as e.g. graphene nano ribbons. \nHowever, it is not exactly known how good the quality of as-fabricated graphene edges really is. This open question is addressed in the second work, where the quality of H plasma defined graphene edges is investigated by means of atomic resolution atomic force microscopy (AFM), Raman spectroscopy and low-temperature electronic transport experiments. AFM measurements on hexagons created on graphite surfaces reveal that the edges are aligned to the ZZ direction and the absence of the Raman D-peak suggests that these edges are high quality ZZ edges. In contrast, hexagons created in single layer graphene on hexagonal boron nitride exhibit a relatively large D-peak, pointing towards the presence of edge disorder or armchair segments. Polarization-dependent Raman experiments indicate that the edges consist of a mixture of armchair and ZZ segments. Furthermore, electronic transport measurements, combined with quantum transport simulations, support the findings from the Raman experiments. Hence, H plasma defined edges still suffer from edge disorder and the etching process needs to be further optimized in order to get high quality crystallographic graphene edges. \nIn addition to the graphene experiments, investigations on Ge/Si core/shell nano wires are conducted. In particular, single, double, and triple quantum dots (QDs) of various sizes and with low occupation numbers are formed. In the single QD regime, indications for the last hole state are found. Moreover, Pauli spin blockade is observed in the double QD regime. These results open the door for exploring Ge/Si core/shell nano wires as a potential platform for hole spin-qubit experiments. \n