Tunable Electronic and Optical Properties of Low-Dimensional Materials

Two-dimensional (2D) materials with single or a few atomic layers, such as graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDCs), and the heterostructures or one-dimensional (1D) nanostructures they form, have attracted much attention recently as unique platforms for studying many condensed-matter phenomena and holds great potentials for nanoelectronics and optoelectronic applications. Apart from their unique intrinsic properties which has been intensively studied for over a decade by now, they also allow external control of many degrees of freedom, such as electrical gating, doping and layer stacking. In this thesis, I present a theoretical study of the electronic and optical properties of many different 2D materials and nanostructures using first-principles density functional theory and many-body perturbation theory. I will show what we learn from these theoretical calculations about the relation between the partially extended, partially confined structure and the tunability of their electronic and optical properties with free-carrier doping and electrical gating. First, we investigate the effect of free-carrier doping on the quasiparticle and exciton properties of 2D material. On one hand, we discuss the origin of the doping-induced band gap renormalization in 2D materials and demonstrate the simplifications that can be made to the theory to allow more efficient calculation. On the other hand, using MoS2 as an example, we study the effect of dynamical screening on the electron-hole interaction and excitonic properties in doped 2D material using the Bethe-Salpeter Equation. Combining them, we show that the quasiparticle band gap of 2D material drops as a non-linear function of doping density by several hundred meV due to the free-carrier screening, but this is offset by the drop in the exciton binding energy and makes the exciton energy remain nearly constant. Then, we switch gear to study the effect of electrical gating on excitons in bilayer TMDC heterostructures. We reveal the important role of interlayer coupling in deciding the band alignment and excitonic properties. We show that due to the interlayer coupling of valence states, the excitons are superpositions of intralayer and interlayer electron-hole pairs which can be described by a simple tight-binding model. As a result, their dipole oscillator strength and radiative lifetime can be tuned by over an order of magnitude with a practical external gate field of a few V/nm. Finally, we study the effect of quantum confinement on the formation of magnetism in confined nanostructures. In two one-dimensional structures, graphene nanoribbon and tellurium chain, we find doped free-carriers can have half-metallic ferromagnetic ground state due to the Stoner mechanism. This comes from the quantum-confinement of the electronic state which enhances the density of state and Stoner parameter at the same time. For graphene nanoribbons, we find magnetism in general edge types with large spin polarization energy up to 17 meV/carrier. It can bypass the requirement of specific zigzag edge in previous proposals of graphene nanoribbon magnetism. For tellurium chain, we find magnetic ground state with a significant 6 meV/carrier spin-polarization energy. Due to the strong spin-orbit interaction of tellurium and its unique helical chain structure with chirality, the spins of the magnetic carriers are pinned along a specific direction with an enhanced magnetic anisotropy energy that is larger than the spin-polarization energy, making it of broad interest for spintronics applications

Radiative Properties and Excitons of Candidate Defect Emitters in Hexagonal Boron Nitride

Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structure and radiative properties remains incomplete. Here we employ first-principles calculations to compute the structure, excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN, generating a large data set of light emission energy, polarization and lifetime. Bayesian analysis of our results combined with existing experimental data allows us to identify the native V_NN_B defect as the most likely single-photon emitter in hBN. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to systematically investigate defect emitters in 2D materials

Distributed state estimation in digital distribution networks based on proximal atomic coordination

With the emerging digitalization technologies represented by edge computing, distribution networks are gradually transforming into digital distribution networks (DDNs). The realization of edge computing drives the distributed operation of DDNs, where multiple areas exchange boundary information through edge computing devices. Benefitting from the data acquisition and computing capacity of edge computing devices, it is feasible to perform accurate and real-time state estimation on the edge side. Aiming at the state perception with edge computing devices in DDNs, this article proposes a distributed state estimation (DSE) method based on the proximal atomic coordination (PAC) algorithm. First, based on convex relaxation optimization, the state estimation model is converted into a positive semidefinite programming (SDP) model to solve the nonconvexity caused by nonlinear measurements, which ensures the accuracy and convergence of state estimation. Then, a DSE method based on the PAC algorithm is proposed to exchange information of each area, which reduces the computation time and realizes the efficient state estimation on the edge side. The model and the effectiveness of the proposed method are numerically demonstrated on the modified PG&E 69-node system and the test case from a practical pilot in Guangzhou, China

MiR-550a-3p restores damaged vascular smooth muscle cells by inhibiting thrombomodulin in an <em>in vitro</em> atherosclerosis model

Thrombomodulin (TM) is involved in the pathological process of atherosclerosis; however, the underlying mechanism remains unclear. Oxidised low-density lipoprotein (Ox-LDL; 100 μg/mL) was used to induce human vascular smooth muscle cells (HVSMCs) into a stable atherosclerotic cell model. The expression levels of miR-550a-3p and TM were detected by real-time reverse transcription-polymerase chain reaction. Cell proliferation was estimated using CCK8 and EDU assays. Wound scratch and transwell assays were used to measure the ability of cells to invade and migrate. Propidium iodide fluorescence-activated cell sorting was used to detect apoptosis and cell cycle changes. A dual-luciferase reporter assay was performed to determine the binding of miR-550a-3p to TM. Our results suggested the successful development of a cellular atherosclerosis model. Our data revealed that TM overexpression significantly promoted the proliferation, invasion, migration, and apoptosis of HVSMCs as well as cell cycle changes. Upregulation of miR-550a-3p inhibited the growth and metastasis of HVSMCs. Furthermore, miR-550a-3p was confirmed to be a direct target of TM. Restoration of miR-550a-3p expression rescued the effects of TM overexpression. Thus, miR-550a-3p might play a role in atherosclerosis and, for the first time, normalised the function of injured vascular endothelial cells by simultaneous transfection of TM and miR-550a-3p. These results suggest that the miR-550a-3p/TM axis is a potential therapeutic target for atherosclerosis