Lattice defects in edge-free graphene quantum dots

Abstract

Graphene, a single atomic layer of carbon first successfully fabricated in 2004 by A. Geim and K Novoselov [44], has since evoked great research interest. These efforts have lead to a good fundamental understanding of the electronic structure of the novel two-dimensional material [27,38,46]. Quantum dots (QDOT), popularly referred to as ”artificial atoms”, can be viewed as effectively zero dimensional structures in which confined electrons display sharp energy levels [47]. Their promising technological applications (quantum computation, quantum cryptography,...) recently increased re-search efforts in this field. The presence of Klein tunneling usually restricts graphene quantum dots to hold merely quasi-bound states. However, con-finement in single layer graphene via a combination of both electric (scan-ning tunneling microscopy tip) and magnetic fields (Landau regime) allows for so called edge-free quantum dots [1,3,16,17,18,43]. The smoothly con-fined quantum dots are regarded as possible alternatives for conventional semiconductor quantum dots (applications such as spin qubits, etc.). They generate growing research interest in recent years. Despite tremendous improvement in the synthesis of graphene nanostruc-tures [45], which enables very clean samples with low defect density and high mobility, understanding the influence of lattice defects in graphene [9,10,11,12] is often crucial to understanding the properties of a much larger system. The purpose of this thesis is to investigate the interplay of these edge-free quantum dots with various types of lattice defects in graphene [9,11,12,21,33]. We particularly focus on investigating the level spacing (orbital splitting O j and valley splitting k j ) of the QDOT states as a function of QDOT-defect distance and determine the ”character” of the respective states compared to an edge-free QDOT in pristine graphene. We describe the system on the tight-binding level of theory and extract the onsite and hopping terms for embedding different defect types from DFT su-percell calculations (VASP, [39,40,41,42]) via wannier90 [6,7,8]. Aside from the ”static” properties of such QDOT-defect systems we also investigate transition dynamics between QDOT states using time propagation by Mag-nus operators [32] and compare with analytical predictions (Landau Zener theory, [19,20]). Promising outlooks for the role of graphene in future spintronics applications (long spin life times and high spin mobility [5,10,48]) make the single vacancy defect (with its local magnetic moment) an interesting type of lattice defect in graphene. Our efforts to add the single vacancy defect to our ”portfolio of wannierized graphene lattice defects” resonate with recent scientific work [35,36,37] and opens the possibility for modeling spin scattering in defect afflicted graphene in future projects.