Zigzag graphene nanoribbons (ZGNRs) with spin-polarized edge states have potential applications in carbon-based spintronics. The electronic structure of ZGNRs can be effectively tuned by different widths or dopants, which requires delicately designed monomers. Here, we report the successful synthesis of ZGNR with a width of eight carbon zigzag lines and nitrogen-boron-nitrogen (NBN) motifs decorated along the zigzag edges (NBN-8-ZGNR) on Au (111) surface, which starts from a specially designed U-shaped monomer with preinstalled NBN units at the zigzag edge. Chemical-bond-resolved non-contact atomic force microscopy (nc-AFM) imaging confirms the zigzag-terminated edges and the existence of NBN dopants. The electronic states distributed along the zigzag edges have been revealed after a silicon-layer intercalation at the interface of NBN-8-ZGNR and Au (111). Our work enriches the ZGNR family with a new dopant and larger width, which provides more candidates for future carbon-based nanoelectronic and spintronic applications.

Two-dimensional (2D) ferroelectric (FE) materials with relatively low switching barrier and large polarization are promising candidates for next-generation miniaturized nonvolatile memory devices. Herein, we screen out 39 new 2D ferroelectric materials, MX (M: Group III-V elements; X: Group V-VII elements), in three phosphorus-analogue phases including black phosphorene-like α-phase, blue phosphorus-like β-phase, and GeSe-like γ-phase using high-throughput calculations. Seven materials (α-SbP, γ-AsP, etc.) exhibit FE switching barriers lower than 0.3 eV/f.u., ferroelectric polarization larger than 2 × 10⁻¹⁰ C/m, and high thermodynamic stability with energy above hull smaller than 0.2 eV/atom. We find that the larger the electronegativity difference between M and X, the larger the ferroelectric polarization. Moreover, larger electronegativity differences result in lower in-plane piezoelectric stress tensor (e₁₁) for MX consisting of Group IV and VI elements and larger e₁₁ for those consisting of Group V elements. Further calculations predict a giant tunneling electroresistance in ferroelectric tunnel junction α-Sb(Sn)P/α-SbP/α-Sb(Te)P (1.26 × 10⁴%) and large piezoelectric strain coefficient in α-SnTe (396 pm/V), providing great opportunities to the design of non-volatile resistive memories, and high-performance piezoelectric devices.

Functionalized two-dimensional (2D) materials play an important role in both fundamental sciences and practical applications. The construction and precise control of patterns at the atomic-scale are necessary for selective and multiple functionalization. Here we report the fabrication of monolayer pentasilver diselenide (Ag₅Se₂), a new type of intrinsically patterned 2D material, by direct selenization of a Ag(111) surface. The atomic arrangement is determined by a combination of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density-functional-theory (DFT) calculations. Large-scale STM images exhibit a quasi-periodic pattern of stoichiometric triangular domains with a side length of ~ 15 nm and apical offsets. The boundaries between triangular domains are sub-stoichiometric. Deposition of different molecules on the patterned Ag₅Se₂ exhibits selective adsorption behavior. Pentacene molecules preferentially adsorb on the boundaries, while tetracyanoquinodimethane (TCNQ) molecules adsorb both on the boundaries and the triangular domains. By co-depositing pentacene and TCNQ molecules, we successfully construct molecular corrals with pentacene on the boundaries encircling TCNQ molecules on the triangular domains. The realization of epitaxial large-scale and high-quality, monolayer Ag₅Se₂ extends the family of intrinsically patterned 2D materials and provides a paradigm for dual functionalization of 2D materials.

Discovery of novel two-dimensional (2D) ferroelectric materials and understanding the mechanism are of vital importance for the design of nanoscale ferroelectric devices. Herein, we report the distinct geometric evolution mechanism of the newly reported M₂Ge₂Y₆ monolayers and find out a large group of 2D ferroelectric candidates based on this mechanism. The origination of the ferroelectricity of M₂Ge₂Y₆ is the vertical displacement of Ge-dimer in the same direction driven by a soft phonon mode of the centrosymmetric configuration. Interestingly, we find another centrosymmetric configuration which is dynamically stable but higher in energy comparing with the ferroelectric phase. The metastable centrosymmetric phase of M₂Ge₂Y₆ monolayers allows a new two-step ferroelectric switching path and may induce novel domain behaviors. Moreover, the ferroelectric M₂Ge₂Y₆ monolayers exhibit independently switchable dipoles and maintain their ferroelectricity after contacting with graphene electrodes, indicating their high application potentials in high-density storage. Furthermore, 16 ferroelectric (FE) M₂Ge₂Y₆ and 65 potential FE M₂Sn₂Y₆ monolayers are identified through high-throughput calculations. Our findings provide a new strategy for future discovery of novel 2D ferroelectric materials and also platforms for experimental design of related functional devices.

Two-dimensional semiconductors (2DSCs) with appropriate band gaps and high mobilities are highly desired for future-generation electronic and optoelectronic applications. Here, using first-principles calculations, we report a novel class of 2DSCs, group-11-chalcogenide monolayers (M₂X, M = Cu, Ag, Au; X = S, Se, Te), featuring with a broad range of energy band gaps and high carrier mobilities. Their energy band gaps extend from 0.49 to 3.76 eV at a hybrid density functional level, covering from ultraviolet-A, visible light to near-infrared region, which are crucial for broadband photoresponse. Significantly, the calculated room-temperature carrier mobilities of the M₂X monolayers are as high as thousands of cm²·V^-1·s^-1. Particularly, the carrier mobilities of η-Au₂Se and ε-Au₂Te are up to 10⁴ cm²·V^-1·s^-1, which is very attracitive for electronic devices. Benefitting from the broad range of energy band gaps and superior carrier mobilities, the group-11-chalcogenide M₂X monolayers are promising candidates for future-generation nanoelectronics and optoelectronics.

Cyclic-conjugated linkages between planar-macrocyclic molecules contribute to the robustness of the two-dimensional (2D) polymerization and extension of π-interactions. The fabrication of such linkages in 2D polymers remains challenging. Combining scanning tunneling microscope (STM) measurements and density functional theory (DFT) calculations, we demonstrate a linear polymerization of metal-free naphthalocyanine (NPc) molecules with [4]-radialene-like linkages on silver surfaces. Experimentally, by depositing NPc molecules on the Ag(110) surface and subsequent annealing up to 750 K, one-dimensional polymers are constructed along the [11(_)0] direction. High-resolution STM images show a stem-leaf-like feature. STM simulations based on a linear polymer of NPc molecules linked by four-membered carbon rings, [4]-radialene-like structure, agree well with the experimental observations. DFT calculations reveal that the polymerization process includes detaching two-terminal H atoms of NPc molecules along [11(_)0] direction, then bonding with a neighboring dehydrogenated NPc molecule by forming a four-membered ring. The dehydrogenation process can be promoted by on-surface impurities such as additional H atoms. Similar polymerizations have been achieved on Ag(111) surfaces in an amorphous way. Moreover, the energy gap of the NPc molecule decreases after linear polymerization, suggesting a red-shift for its optical absorption/scattering spectrum. Our study offers a new route to polymerize conjugated molecules with extended planar π-interactions.

We propose a novel class of two-dimensional (2D) Dirac materials in the MX family (M = Be, Mg, Zn and Cd, X = Cl, Br and I), which exhibit graphene-like band structures with linearly-dispersing Dirac-cone states over large energy scales (0.8-1.8 eV) and ultra-high Fermi velocities comparable to graphene. Spin-orbit coupling opens sizable topological band gaps so that these compounds can be effectively classified as quantum spin Hall insulators. The electronic and topological properties are found to be highly tunable and amenable to modulation via anion-layer substitution and vertical electric field. Electronic structures of several members of the family are shown to host a Van-Hove singularity (VHS) close to the energy of the Dirac node. The enhanced density-of-states associated with these VHSs could provide a mechanism for inducing topological superconductivity. The presence of sizable band gaps, ultra-high carrier mobilities, and small effective masses makes the MX family promising for electronics and spintronics applications.

As a new type of iron-based superconductor, CaKFe₄As₄ has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe₄As₄ plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe₄As₄ has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe₄As₄. Bias-dependent atomic resolution images show an evolvement from $\sqrt{2}$ × $\sqrt{2}$ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe₄As₄ as well as other iron-pnictide superconductors.

Controlling the atomic configurations of structural defects in graphene nanostructures is crucial for achieving desired functionalities. Here, we report the controlled fabrication of high-quality single-crystal and bicrystal graphene nanoislands (GNI) through a unique top-down etching and post-annealing procedure on a graphite surface. Low-temperature scanning tunneling microscopy (STM) combined with density functional theory calculations reveal that most of grain boundaries (GBs) formed on the bicrystal GNIs are 5-7-5-7 GBs. Two nanodomains separated by a 5-7-5-7 GB are AB stacking and twisted stacking with respect to the underlying graphite substrate and exhibit distinct electronic properties, forming a graphene homojunction. In addition, we construct homojunctions with alternative AB/twisted stacking nanodomains separated by parallel 5-7-5-7 GBs. Remarkably, the stacking orders of homojunctions are manipulated from AB/twist into twist/twist type through a STM tip. The controllable fabrication and manipulation of graphene homojunctions with 5-7-5-7 GBs and distinct stacking orders open an avenue for the construction of GBs-based devices in valleytronics and twistronics.

Magnetic two-dimensional (2D) topological insulators with spontaneous magnetization have been predicted to host quantum anomalous Hall effects (QAHEs). For organic topological insulators, the QAHE only exists in honeycomb or Kagome organometallic lattices based on theoretical calculations. Recently, coloring-triangle (CT) lattice has been found to be mathematically equivalent to a Kagome lattice, suggesting a potential 2D lattice to realize QAHE. Here, based on first-principles calculations, we predict an organometallic CT lattice, Cu-dicyanobenzene (DCB), to be a stable QAH insulator. It exhibits ferromagnetic (FM) properties as a result of the charge transfer from metal atoms to DCB molecules. Moreover, based on the Ising model, the Curie temperature of the FM ordering is calculated to be around 100 K. Both the Chern numbers and the chiral edge states of the semi-infinite Cu-DCB edge structure, which occur inside the spin-orbit coupling band gap, confirm its nontrivial topological properties. These make the Cu-DCB CT lattice an ideal candidate to enrich the family of QAH insulators.