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Electronic Transport in Few-layer Graphene


Graphene, a single atomic layer of carbon atoms, has been extensively studied in recent years. Its unique electronic property has attracted the attention of physicists in both theoretical and experimental areas. To experimentally study the electronic transport property of graphene devices, we acquire high mobility in suspended graphene devices fabricated by sophisticated techniques. My research focused on studying electronic transport property of few layer graphene, including bilayer, trilayer and tetralayer graphene devices, as well as heterojunctions that consist of regions with different number of layers.

In the first part of the dissertation, two types of techniques in fabrication are described -- the lithography free shadow mask method and the multi-level lithography technique developed from the fabrication protocols of suspended top gates. These free-standing devices eliminates substrates that are often the mobility bottleneck, and allow current annealing to remove contaminants, thus providing a platform for experimental realization of many fascinating phenomena in high mobility devices, such as fractional quantum Hall effect.

The second part of the dissertation focuses on experimental study of few layer graphene devices. Evidence for fractional quantum Hall effect is observed in bilayer and trilayer graphene. For heterojunction devices, both magnetic field and electrical field are applied to study the electrical transport properties in quantum Hall regime. Heterojunction trends to behave like the thinner layer graphene in quantum Hall regime. In one bilayer-trilayer heterojunction, we find an apparent transition from the nematic state to a gapped state that resembles the layer antiferromagnetic state induced by current annealing. This shed light on the debate of the ground state at the charge neutrality point in bilayer graphene. In tetralayer graphene, we observed an unusual re-entrant insulating state at the charge neutrality point, which is not predicted by the tight binding model calculations. This phenomenon could arise from Landau level crossing or Moire pattern due to a small relative twist between the layers.

Future work in this area include (1) using in-plane magnetic field effect to probe few layer graphene to determine their dependence on total spin, (2) top-gated heterojunction devices to locally control the carrier density and band structure of the two regions, (3) other layered materials, such as topological insulators and transition metal dichalcogenides. These studies could contribute to both our fundamental understanding of low-dimensional physics and technology applications.

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