Scanning Tunneling Microscopy of Graphene Quantum Dots
Quantum confinement of Dirac fermions is an important frontier in graphene research. In this dissertation, we report on our use of a scanning tunneling microscope (STM) to investigate electrostatically confined Dirac fermions in graphene quantum dots. We first describe a technique for patterning embedded gates in backgated graphene/hexagonal boron nitride (hBN) heterostructures by STM manipulation of defect charges within the hBN substrate. In conjunction with a tunable backgate, this allows us to engineer p-n junctions in monolayer and bilayer graphene whose geometries can be flexibly designed with nanoscale precision. Using scanning tunneling spectroscopy (STS), we image and spatially characterize the behavior of Dirac fermions in the vicinity of p-n junctions and show that circular p-n junctions in monolayer and bilayer graphene act as gate-tunable quantum dots with unique energy spectra. For monolayer graphene quantum dots, comparison with theoretical simulations of the massless Dirac equation enables us to identify each experimentally observed spectroscopic peak as a quantum dot eigenstate with a unique set of quantum numbers. In bilayer graphene, we demonstrate a gate-tunable evolution of locally gated graphene from classical dots to quantum dots and achieve control over the number of massive Dirac fermions contained in a quantum dot by using the STM tip as a top gate. Furthermore, we explore the electronic properties of quantum double dots and non-circular monolayer graphene p-n junctions using spatially resolved STS. Our work yields insight into the spatial behavior of Dirac fermions under the influence of local electrostatic potentials and provides a platform for further experimental investigation of physics related to p-n junctions in graphene.