Mesoscopic transport through graphene and molybdenum disulfide constrictions

This thesis reports on the investigation of graphene and molybdenum disulfide (MoS2) nanostructures in low-temperature transport experiments. Two different techniques are employed to study mesoscopic transport and quantum confinement of charge carriers in these two different two-dimensional materials. All devices are fabricated from two-dimensional heterostructures by a state-of-the transfer process. In a first transport experiment, etched graphene nanoribbons with a width around 40 nm and length of 100 nm fully encapsulated in hexagonal boron nitride (hBN) are investigated. At zero magnetic field the devices show a qualitatively comparable behavior to graphene nanoribbons on SiO2 or hBN. The magnetotransport reveals a crossover from the Coulomb blockade regime to a regime of completely suppressed transport which is dominated by a much larger energy gap of up to 30 meV induced by electronic correlation effects completely. In a following set of experiments a novel device scheme to investigate mesoscopic transport through molybdenum disulfide (MoS2) constrictions using long-lasting photo-induced doping is explored. The doping process is homogeneous and efficient throughout the entire device and yields metallic behavior of the devices at cryogenic temperatures, while preserving the charge carrier mobility of up to µ =600 cm^2/(Vs) at low temperatures (2 K). Subsequently, the photodoping technique is used to define MoS2 constrictions by using metal shadow masks. This demonstrates a way to produce complex doping profiles on a scale close to the diffraction limit and allows to observe quantum transport and reproducible signatures of quantum confinement effects in MoS2 constrictions. Finally, the photodoping technique is used to fabricate nanoconstrictions by reducing the constriction size. Here, we observe a gate and bias voltage tunable multilevel negative differential conductance (NDC). The NDC shows a maximum peak-to-valley ratio of 1.2. Comparing our results to simulations of the ballistic current through a nanoconstriction based on the non-equilibrium Green's function formalism already published by another group shows a qualitative agreement which indicates that our constrictions might only have a length on order of a few tens of nanometer