Growth and characterization of two-dimensional carbon nanostructures

Abstract

This dissertation focuses on the electronic transport properties of carbon nanowalls and graphene flakes. The former has been carried out by using both normal metal (Ti) and superconductor (Nb) electrodes. Bottom electrodes are employed in the experiments. Comparing to top-electrode configuration, this configuration could help to narrow the electrode spacing of devices down below 1 µm. In the Ti/CNW/Ti junctions, the experimental results show the presence of a narrow band gap and conductance fluctuations within a certain temperature range. Excess conductance fluctuations observed between 4 and 300 K are attributed to the quantum interference effect under the influence of thermally induced carrier excitation across a narrow bandgap. The sharp suppression of conductance fluctuation below 2.1 K is accounted for by the formation of a layer of He 4 superfluid on the nanowalls. The results obtained here have important implications for potential application of CNWs in electronic devices. A giant gap-like behavior of dI/dV is also observed in some samples. The gap indicates that some phase transition may happen in those CNWs at low temperature. For Nb/CNW/Nb junctions, superconducting proximity effect was observed in two samples with short electrode spacing. Their temperature dependence of critical current is in good agreement with both Josephson coupling in long diffusive model and Ginzburg-Landau relationship. The above-gap feature and Andrev reflection were observed in the two samples. Their magnetic field dependence was also discussed. However, in other Nb/CNWs/Nb devices, results of proximity effect with respect to the electrode spacing are not consistent. This may be due to many reasons, such as the orientation of CNWs, quality of CNW sheet, the transparency of Nb/CNWs interface. In the second part of this thesis, we discuss the electric transport properties of graphene on SiO2 substrate with different number of layer under ambient condition. By examining carrier mobility, minimal conductivity and conductance hysteresis in graphene devices, it is found that the substrate interface and surface impurity may greatly affect the transport properties of graphene on SiO2 substrate. Our experimental results indicate that magneto transport and conductance fluctuation in graphene devices are greatly affected by the charged impurities at the substrate/graphene interface.