Scanning Tunneling Microscopy Study of Graphene Electronic Nanostructures
- Wang, Yang
- Advisor(s): Crommie, Michael F
Abstract
This dissertation focuses on the study of the electronic properties of graphene using scanning tunneling microscopy (STM). A particular focus is the behavior of charge carriers in monolayer graphene around a single Coulomb impurity at the microscopic scale, and the interactions between two vertically stacked graphene layers.
In order to probe the intrinsic electronic properties of graphene, hexagonal boron nitride (BN) was employed as a new substrate for supporting graphene due to its ultra-flatness and high purity. The surface roughness and charge inhomogeneity of graphene on BN were measured to be significantly smaller than graphene on SiO$_\text{2}$ using STM. The high quality of graphene on BN enables us to investigate the fundamental physics question of how Dirac fermions in graphene respond to a single Coulomb impurity. This problem is divided into two separate regimes (subcritical and supercritical) depending on the strength of the Coulomb potential. The subcritical regime was investigated by using charge-tunable cobalt trimers on graphene, and the graphene LDOS around these charged impurity showed electron-hole asymmetry but no bound states. The supercritical regime was achieved by manipulating calcium dimers into clusters in order to surpass the supercritical charge threshold, and atomic-collapse quasi-bound states were demonstrated. The screening properties of graphene under various doping conditions were also measured using charge-stable calcium monomer adatoms, and the characteristic screening length of graphene was observed to scale inversely with its Fermi wavevector.
Besides depositing charged impurities, the electronic structure of graphene can also be modified via vertical layer stacking. The lattice orientation difference between two contacting layers of graphene induces a series of Moir\'{e} bands which manifest themselves as Van Hove singularities and other higher-order spectroscopic features that can be measured in twisted bilayer graphene.
The results presented in this dissertation contribute to our understanding of the microscopic electronic structure of graphene under the influence of charged impurities and layer-layer interactions, and shed new insight on how we can tailor graphene properties for use in real world applications.