Grain boundaries in two-dimensional (2D) semiconductors generally induce distorted band alignment and interfacial charge, which impair their electronic properties for device applications. Here, we report the improvement of band alignment at the grain boundaries of PtSe2, a 2D semiconductor, with selective adsorption of a presentative organic acceptor, tetracyanoquinodimethane (TCNQ). TCNQ molecules show selective adsorption at the PtSe2 grain boundary with strong interfacial charge. The adsorption of TCNQ distinctly improves the band alignment at the PtSe2 grain boundaries. With the charge transfer between the grain boundary and TCNQ, the local charge is inhibited, and the band bending at the grain boundary is suppressed, as revealed by the scanning tunneling microscopy and spectroscopy (STM/S) results. Our finding provides an effective method for the advancement of the band alignment at the grain boundary by functional molecules, improving the electronic properties of 2D semiconductors for their future applications.
Abstract: Magnetic topological insulators (MTIs) have received considerable attention owing to the demonstration of various quantum phenomena, such as the quantum anomalous Hall effect and topological magnetoelectric effect. The intrinsic superlatticelike layered MTIs MnBi2Te4/(Bi2Te3)n have been extensively investigated mainly through transport measurements; however, a direct investigation of their superlattice-sensitive magnetic behaviors is relatively rare. In this paper, we report a microscopic real space investigation of the magnetic phase behaviors in MnBi4Te7 using cryogenic magnetic force microscopy. The intrinsic robust A type antiferromagnetic (AFM), surface spin-flip (SSF) + AFM, ferromagnetic (FM) + SSF + AFM, and forced FM phases are sequentially visualized via the increased external magnetic field, consistent with the magnetic behavior in the M-H curve. The temperature-dependent magnetic phase evolution behaviors are further investigated to obtain a complete H-T phase diagram of MnBi4Te7. Tentative local phase manipulation via the stray field of the magnetic tip is demonstrated by transforming the AFM into the FM phase in the surface layers of MnBi4Te7. Our study provides key real-space ingredients for understanding the complicated magnetic, electronic, and topological properties of such intrinsic MTIs and suggests new directions for manipulating spin textures and locally controlling their exotic properties.
Two-dimensional (2D) magnetic materials with magnetic anisotropy can form magnetic order at finitetemperature and monolayer limit. Their macroscopic magnetism is closely related to the number of layers andstacking forms, and their magnetic exchange coupling can be regulated by a variety of external fields. Thesenovel properties endow 2D magnetic materials with rich physical connotation and potential application value,thus having attracted extensive attention. In this paper, the recent advances in the experiments and theoreticalcalculations of 2D magnets are reviewed. Firstly, the common magnetic exchange mechanisms in several 2Dmagnetic materials are introduced. Then, the geometric and electronic structures of some 2D magnets and theirmagnetic coupling mechanisms are introduced in detail according to their components. Furthermore, we discusshow to regulate the electronic structure and magnetism of 2D magnets by external (field modulation andinterfacial effect) and internal (stacking and defect) methods. Then we discuss the potential applications ofthese materials in spintronics devices and magnetic storage. Finally, the encountered difficulties and challengesof 2D magnetic materials and the possible research directions in the future are summarized and prospected. DOI: 10.7498/aps.71.20220301
Zheng, Fangyuan; Guo, Deping; Huang, Lingli; Wong, Lok Wing; Chen, Xin; Wang, Cong; Cai, Yuan; Wang, Ning; Lee, Chun-Sing; Lau, Shu Ping; Ly, Thuc Hue; Ji, Wei and Zhao, Jiong
Abstract
Phase patterning in polymorphic two-dimensional (2D) materials offers diverse properties that extend beyond what their pristine structures can achieve. If precisely controllable, phase transitions can bring exciting new applications for nanometer-scale devices and ultra-large-scale integrations. Here, the focused electron beam is capable of triggering the phase transition from the semiconducting T” phase to metallic T’ and T phases in 2D rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) monolayers, rendering ultra-precise phase patterning technique even in sub-nanometer scale is found. Based on knock-on effects and strain analysis, the phase transition mechanism on the created atomic vacancies and the introduced substantial in-plane compressive strain in 2D layers are clarified. This in situ high-resolution scanning transmission electron microscopy (STEM) and in situ electrical characterizations agree well with the density functional theory (DFT) calculation results for the atomic structures, electronic properties, and phase transition mechanisms. Grain boundary engineering and electrical contact engineering in 2D are thus developed based on this patterning technique. The patterning method exhibits great potential in ultra-precise electron beam lithography as a scalable top-down manufacturing method for future atomic-scale devices. DOI:10.1002/advs.202200702
