Graphene Quantum Dot States Near Defects

Smoothly confined graphene quantum dots (GQDs) localize Dirac electrons with conserved spin and valley degrees of freedom. Recent experimental realization of such structures using a combination of magnetic fields and a scanning tunneling microscope tip showcased their potential in locally probing and adjusting the valley degree of freedom. The present work models the influence of lattice defects on the level structure of GQDs. We study both the adiabatic level spacing "landscape"-orbital splitting and valley splitting-as well as transition dynamics between GQD states. The system is modeled using a tight-binding approach with on-site and hopping parameters in the vicinity of the defect region extracted from density functional theory via Wannier orbitals while time propagation is done using Magnus operators. Different defect types, such as double vacancy, Stone-Wales, flower, and Si substitution, are considered. We predict tunable valley splittings of the order of 2-20 meV. The level structure can thus be tailored at will by engineering appropriate defect geometries.