Theoretical Modeling of the Formation and Functionality of Low-Dimensional Materials

This dissertation presents a series of work under the topic of designing and modeling novel low-dimensional materials and structures with desired and coherent structural, electronic, and magnetic properties, using a variety of theoretical tools, including first-principles density functional theory (DFT) method, numerical Monte Carlo (MC) method, and analytical phenomenological approaches, etc. The contents are divided into three major topics:

(1) Magnetic properties of n-p codoped materials. The noncompensated n-p codoping method is proposed to increase the density of magnetic dopants in diluted magnetic semiconductors (DMS) while keeping the magnetic coupling strength, which may lead to a high Curie temperature, comparable to room temperature. A two-step approach, combining first-principles and Monte Carlo methods, is developed to estimate the Curie temperatures of these systems.

(2) Growth and functionality of graphene. We focus on the nucleation processes of carbon adatoms in the early stages of graphene epitaxial growth on transition metal surfaces, and revealed that the strong interaction between C adatoms can lead to very different nucleation behaviors compared to conventional epitaxial growth. Our other works along this line include the prediction of stable carbon "nanoarches" in the early stages of graphene growth on Cu surface, the realization of graphene p-n superlattices using wedged Pb islands, and the proposal to suppress grain boundaries of epitaxial graphene by using superstructured Mn-Cu(111) surface alloy as substrates and a two-step kinetic pathway. In an ongoing work, we also found the compensated n-p codoping method to be promising to establish long-range ferromagnetic order in graphene.

(3) Electronic and chemical properties of topological insulator (TI) heterostructures. By studying the structure of the TI Bi₂Se₃ covered by an ultrathin gold film, we found that the unconventionally robust topological surface states (TSS) of the Bi₂Se₃ can make the gold film a better platform for the adsorption and reaction of both CO and O₂ molecules, by acting as a charge reservoir. Following this work, we investigated the behavior of TSS in the heterostructures of TI and thin films of conventional insulators (CI), and discovered dual-proximity effects, implied by the spatial relocation of the TSS in the vertical direction.