Since the discovery of graphene, 2D materials have attracted significant attention for their unique properties. Monoelemental 2D materials (ME2DMs) are of particular interest because of their ...superiority in synthetic exploration, excellent mobility, and wide range of band gaps over other 2D compounds. Substantial efforts are devoted to fabricate high‐quality materials through various substrates and reveal the growth mechanism at the atomic scale. In this review, the recent progress in the preparation of these ME2DMs is explored, the roles of different substrates is highlighted, and the challenges of and perspectives on their heterostructures is discussed.
Recent progress in molecular beam epitaxy of the main group monoelemental 2D materials is reviewed, with an emphasis on the scanning tunneling microscopy characterization of their electronic and atomic structures. A range of substrate effects on phase engineering and growth mode are explored. By identifying current challenges and opportunities, a roadmap for heterostructures and binary 2D systems is provided.
Abstract
Antiferromagnets have recently moved into the focus of application-related research, with the perspective to use them in future spintronics devices. At the same time the experimental ...determination of the detailed spin texture remains challenging. Here we use spin-polarized scanning tunneling microscopy to investigate the spin structure of antiferromagnetic domain walls. Comparison with spin dynamics simulations allows the identification of a new type of domain wall, which is a superposition state of the adjacent domains. We determine the relevant magnetic interactions and derive analytical formulas. Our experiments show a pathway to control the number of domain walls by boundary effects, and demonstrate the possibility to change the position of domain walls by interaction with movable adsorbed atoms. The knowledge about the exact spin structure of the domain walls is crucial for an understanding and theoretical modelling of their properties regarding, for instance, dynamics, response in transport experiments, and manipulation.
On-surface synthesis has emerged as a powerful tool for the construction of large, planar, π-conjugated structures that are not accessible through standard solution chemistry. Among such ...solid-supported architectures, graphene nanoribbons (GNRs) hold a prime position for their implementation in nanoelectronics due to their manifold outstanding properties. Moreover, using appropriately designed molecular precursors, this approach allows the synthesis of functionalized GNRs, leading to nanostructured hybrids with superior physicochemical properties. Among the potential "partners" for GNRs, porphyrins (Pors) outstand due to their rich chemistry, robustness, and electronic richness, among others. However, the use of such π-conjugated macrocycles for the construction of GNR hybrids is challenging and examples are scarce. Herein, singly and doubly Por-capped GNR segments presenting a commensurate and triply-fused GNR-Por heterojunction are reported. The study of the electronic properties of such hybrid structures by high-resolution scanning tunneling microscopy, scanning tunneling spectroscopy, and DFT calculations reveals a weak hybridization of the electronic states of the GNR segment and the Por moieties despite their high degree of conjugation.
Singly and doubly porphyrin-capped graphene nanoribbon segments are reported and their electronic properties are studied by high-resolution scanning tunneling microscopy and spectroscopy, and DFT calculations.
It has recently been proposed that combining chirality with topological band theory results in a totally new class of fermions. Understanding how these unconventional quasiparticles propagate and ...interact remains largely unexplored so far. Here, we use scanning tunneling microscopy to visualize the electronic properties of the prototypical chiral topological semimetal PdGa. We reveal chiral quantum interference patterns of opposite spiraling directions for the two PdGa enantiomers, a direct manifestation of the change of sign of their Chern number. Additionally, we demonstrate that PdGa remains topologically non-trivial over a large energy range, experimentally detecting Fermi arcs in an energy window of more than 1.6 eV that is symmetrically centered around the Fermi level. These results are a consequence of the deep connection between chirality in real and reciprocal space in this class of materials, and, thereby, establish PdGa as an ideal topological chiral semimetal.
N‐Heterocyclic carbenes (NHCs) have superior properties as building blocks of self‐assembled monolayers (SAMs). Understanding the influence of the substrate in the molecular arrangement is a ...fundamental step before employing these ligands in technological applications. Herein, we study the molecular arrangement of a model NHC on Cu(100) and Cu(111). While mostly disordered phases appear on Cu(100), on Cu(111) well‐defined structures are formed, evolving from magic‐number islands to molecular ribbons with coverage. This work presents the first example of magic‐number islands formed by NHC assemblies on flat surfaces. Diffusion and commensurability are key factors explaining the observed arrangements. These results shed light on the molecule‐substrate interaction and open the possibility of tuning nanopatterned structures based on NHC assemblies.
A model N‐heterocyclic carbene adsorbs on Cu(100) and Cu(111) leading to different nanostructures. On Cu(111), the combination of intermolecular interactions and commensurability promotes the formation of magic‐number islands. The understanding of this self‐assembly process represents important progress for the design of tailormade N‐heterocyclic carbene (NHC) assemblies.
Photo-assisted tunneling in scanning tunneling microscopy has attracted considerable interest to combine sub-picosecond and sub-nanometer resolutions. The illumination of a junction with visible or ...infrared light, however, induces thermal expansion of the tip and the sample, which strongly affects the measurements. Employing free-space THz pulses instead of visible light has been proposed to solve these thermal issues while providing photo-induced currents of similar magnitude. Here we compared the impact of illuminating the same tunneling junction, reaching comparable response in the tunneling current, with red light and with THz radiations. Our data provide a clear and direct evidence of thermal expansion with red light-illumination, while such thermal effects are negligible with THz radiations.
•Photo-assisted tunneling with visible light is associated with thermal expansion.•Thermal effects are negligible with terahertz illumination.•Sub-chopping-cycle resolution of the terahertz-induced current is achieved.
Research on in‐plane and vertically‐stacked heterostructures of graphene and hexagonal boron nitride (h‐BN) have attracted intense attentions for energy band engineering and device performance ...optimization of graphene. In this review article, recent advances in the controlled syntheses, interfacial structures, and electronic properties, as well as novel device constructions of h‐BN and graphene heterostructures are highlighted. Firstly, diverse synthesis approaches for in‐plane h‐BN and graphene (h‐BN‐G) heterostructures are reviewed, and their applications in nanoelectronics are briefly introduced. Moreover, the interfacial structures and electronic properties of h‐BN‐G heterojunctions are discussed, and a zigzag type interface is found to preferentially evolve at the linking edge of the two structural analogues. Secondly, several synthetic routes for the vertically‐stacked graphene/h‐BN (G/h‐BN) heterostructures are also reviewed. The role of h‐BN as perfect dielectric layers in promoting the device performance of graphene is presented. Finally, future research directions in the synthesis and application of such heterostructures are discussed.
In‐plane and vertically stacked heterostructures of graphene and hexagonal boron nitride have attracted intense attention for energy band engineering and device performance optimization of graphene. The synthesis and interfacial properties of both in‐plane (h‐BN‐G) and vertically stacked (G/h‐BN) heterostructures, which should evoke further investigations of the related novel properties, as well as their versatile applications in various aspects, are discussed.
The oxides of copper have attracted the attention of scientists already for more than hundred years. This fascination is fueled by many outstanding properties of the material, for example, a ...semiconducting behavior that led to the first diode fabricated in electronics, a pronounced excitonic response that stimulated an intense search for Bose-Einstein condensation, and a pivotal role in unconventional superconductivity. Despite this central position in past and present research activities, many aspects of copper oxides are not sufficiently understood to date. This applies in particular to their surface characteristics, where even fundamental questions, such as the energetically favored termination of low-index Cu2O and CuO planes, are still subject of debates. This review aims at addressing these deficiencies by compiling state-of-the-art knowledge of the surface science of copper oxides, and especially of cuprous oxide.
A first focus of the article lies in the oxidation characteristic of copper as a means to prepare well-defined oxide surfaces. It demonstrates that low-pressure oxidation only results in the formation of ultrathin precursor oxides, with properties deviating substantially from those of the bulk material. Consequently, reliable pathways to produce high-quality and bulk-compatible surfaces, either of Cu2O thin films or bulk crystals, are presented. The following chapter provides a comprehensive introduction into the atomic structure of the most relevant Cu2O surfaces, i.e., the (111), (100) and (110) planes. It gives an overview of important diffraction and microscopy experiments on the most accessible Cu2O terminations, and complements this with state-of-the-art theoretical studies to develop corresponding atomistic models. The chapter closes by presenting the atomic configurations of the most relevant Cu2O surfaces at given thermodynamic conditions.
Chapter four develops a surface-science view onto the unique optical response of cuprous oxide. After introducing the well-known bulk behavior, it highlights how optical properties can be probed on surfaces with high spectral and spatial resolution. The chapter discusses how optical near-field techniques are employed to analyze oxide excitons and their trapping at lattice defects in real-space experiments. The last chapter summarizes efforts to alter intrinsic Cu2O properties, e.g., the p-type conductivity, the width of the band gap and the exciton trapping and recombination behavior, via doping. It illuminates this topic from an experimental and theoretical viewpoint and highlights several unsolved questions related to the topic.
Despite considerable efforts, this review can only present the current state of knowledge on Cu2O surfaces, a subject that continuously advances due to new scientific findings and innovations. We nonetheless hope that it provides a comprehensive and topical overview of the unusual properties of this fascinating oxide system.
Cyano‐functionalized tetraphenyl porphyrins fused together by cyano‐Cu‐cyano interactions with Cu adatoms form a snowflake‐like structure with a triangular central pore on Cu(111). More information ...can be found in the Full Paper by B. Meyer, H.‐P. Steinrück, et al. on page 13408.
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