Defect effects and electronic structure regulation in low-dimensional materials

Abstract

Low-dimensional materials have been drawing increasing interest due to the unique structures and fascinating properties which are quite promising for wide-range application ranging from new energy resources to information technology. The exotic electronic states of the low-dimensional materials which arise from the quantum-confinement effects of electrons render a potential strategy to overcome the size-limitation of traditional semiconducting materials and continue the Moores’ law which is difficult for the conventional bulk materials. The electronic structure modulation of low-dimensional materials to meet the relevant requirements serves as the key to reach this goal. Notably, in low-dimensional materials, the defect effects are more remarkable than those in bulk materials due to the high specific surface area and quantum confinement effects. Moreover, the compatibility of low-dimensional materials to nanoscaled devices is crucial for the next-generation device applications. Here, we focus on the role of defects in electronic structure modulation of one- (1D) and two-dimensional (2D) materials to summarize works of our group relevant to this topic in recent years. The point defects, such as vacancies and adatoms (or functional groups) are revealed to have significant contribution to the electron spin-polarization and excited states. The ferromagnetism of the metal-free materials is demonstrated in wide range of materials, such as graphite, hexagonal boron nitride and silicon carbide both theoretically and experimentally. The spin-polarization of p electrons in these materials enriches the family of ferromagnetic materials and offers a promising strategy for spintronics device applications. The hollow structure of carbon nanotubes provides an ideal platform for the study of atomic reaction, diffusion and condensation processes confined in nanospace. The electronic band structures of carbon nanotubes can be effectively regulated by encapsulating metal or hydrogen nanowires. Ultrahigh density of hydrogen nanowires can be confined inside carbon nanotubes, thanks to the excellent mechanical properties, which lead to metallization or superconductivity of the 1D hydrogen systems at relative low pressure and high temperature, due to the “physical compression” effect of carbon nanotubes. The superconducting hydrogen can be well explained in terms of the Eliashberg superconductivity theory for electron-phonon strong-coupling system. The metallization and superconducting phase of hydrogen nanowires encapsulated in carbon nanotubes open a promising platform for study of low-dimensional superconductivity at high pressure. Additionally, metal-organic frameworks (MOFs) which are composed of transition metal (TM) atoms and organic ligands emerged as a new family of 2D materials. Compared with traditional 2D materials, MOFs have the advantages of structural diversity, porous configurations, tunable electronic band structures. Some lattice models, such as Kagome, Lieb and Ruby lattice models can be achieved in 2D MOFs. The synergistic effect of TM atoms and organic ligands leads to exotic properties, such as Kane fermions, topological electronic states (quantum spin Hall effects and quantum anomalous Hall effect), superconductivity, high catalytic activity, etc. The well-distributed TM atoms in the porous structures of 2D MOFs facilitate the catalysis in hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which hold great promise in water splitting and fuel cells. In view of the experimental progresses on the synthesis of diverse 2D MOFs, the fascinating properties of 2D MOFs bring about new concepts for electronic devices and catalysis design.