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

Quasiparticle band alignment and stacking-independent exciton in MA2_2Z4_4 (M = Mo, W, Ti; A= Si, Ge; Z = N, P, As)

Motivated by the recently synthesized two-dimensional semiconducting MoSi$_2$N$_4$, we systematically investigate the quasiparticle band alignment and exciton in monolayer MA$_2$Z$_4$ (M = Mo, W, Ti; A= Si, Ge; Z = N, P, As) using ab initio GW and Bethe-Salpeter equation calculations. Compared with the results from density functional theory (DFT), our GW calculations reveal substantially more significant band gaps and different absolute quasiparticle energy but predict the same types of band alignments

Data-driven multi-mode adaptive operation of soft open point with measuring bad data

The high penetration of distributed generators (DGs) deteriorates the uncertainty of active distribution networks (ADNs). Soft open points (SOPs) can effectively improve flexibility and deal with operational issues in ADNs. However, the formulation of SOP control strategies depends on the accurate mechanism model. Data-driven method can utilize only measuring data to conduct operation and becomes a promising way. In practical conditions, the measuring data may suffer from bad data and measuring errors, which poses a challenge to meet the diverse operational requirements. This paper proposes a data-driven multi-mode adaptive control method for SOP with measuring bad data. First, considering the inaccurate network parameters and quality of measuring data, a robust data-driven framework for SOP operation is proposed based on robust hierarchical-optimization recursive least squares (HO-RLS). Then, a multi-mode control strategy for SOP is proposed to adapt to the diverse operational requirements. A dynamic triggering mechanism is designed to achieve adaptive mode switching. The case studies on practical distribution networks show that the proposed method can fully explore the benefits of SOP to improve the operational performance of ADNs. The potential limitations are discussed to enhance practicality