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.

The correlation of surface impurity states with the antiferromagnetic ground states is crucial for understanding the formation of the topological surface state in the antiferromagnetic topological insulators MnBi₂Te₄. By using low-temperature scanning tunneling microscopy and spectroscopy, we observed a localized bound state around the Mn-Bi antisite defect at the Te-terminated surface of the antiferromagnetic topological insulator MnBi₂Te₄. When applying a magnetic field perpendicular to the surface (B_z) from –1.5 to 3.0 T, the bound state shifts linearly to a lower energy with increasing B_z, which is attributed to the Zeeman effect. Remarkably, when applying a large range of B_z from –8.0 to 8.0 T, the magnetic field induced reorientation of surface magnetic moments results in an abrupt jump in the local density of states (LDOS), which is characterized by LDOS-change-ratio $\text{d}\tilde{\sigma }\text{/d}B$ quantitatively. Interestingly, two asymmetric critical field, –2.0 and 4.0 T determined by the two peaks in $\text{d}\tilde{\sigma }\text{/d}B$ are observed, which is consistent with simulated results according to a Mills-model, describing a surface spin flop transition (SSF). Our results provide a new flatform for studying the interplay between magnetic order and topological phases in magnetic topological materials.

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.

The control of the Kondo effect is of great interest in single-molecule junction due to its potential applications in spin based electronics. Here, we demonstrate that the Kondo effect is reversibly switched on and off in an iron phthalocyanine (FePc) single-molecule junction by using a superconducting Nb tip. In a scanning tunneling microscope-based Nb-insulator-FePc-Au junction, we achieve a reversible switching between the Kondo dip and inelastic electronic tunneling spectra by simply adjusting the tip-sample distance to tune the tunnel coupling at low temperature. Further approaching the tip leads to the picking up of the molecule to the tip apex, which transfers the geometry of the single-molecule junction into a Nb-FePc-insulator-Au type. As the molecule forms an effective magnetic impurity embedded into the superconducting ground states of the Nb tip, the out-gap Kondo dip switched to an in-gap Yu–Shiba–Rusinov state. Our results open up a new route for manipulating the Kondo effect within a single-molecule junction.

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.

Platinum diselenide (PtSe₂) is a promising transition metal dichalcogenide (TMDC) material with unique properties. It is necessary to find a controllable fabrication method to bridge PtSe₂ with other two-dimensional (2D) materials for practical applications, which has rarely been reported so far. Here, we report that the selenization of Pt(111) can be suppressed to form a Se intercalated layer, instead of a PtSe₂ monolayer, by inducing confined conditions with a precoating of graphene. Experiments with graphene-island samples demonstrate that the monolayer PtSe₂ can be controllably fabricated only on the bare Pt surface, while the Se intercalated layer is formed underneath graphene, as verified by atomic-resolution observations with scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM). In addition, the orientation of the graphene island shows a negligible influence on the Se intercalated layer induced by the graphene coating. By extending the application of 2D confined reactions, this work provides a new method to control the fabrication and pattern 2D materials during the fabrication process.

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate.We used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

Two-dimensional (2D) materials have received significant attention due to their unique physical properties and potential applications in electronics and optoelectronics. Recent studies have demonstrated that exfoliated PdSe₂, a layered transition metal dichalcogenide (TMD), exhibits ambipolar field-effect transistor (FET) behavior with notable performance and good air stability, and thus serves as an emerging candidate for 2D electronics. Here, we report the growth of bilayer PdSe₂ on a graphene-SiC(0001) substrate by molecular beam epitaxy (MBE). A bandgap of 1.15 ± 0.07 eV was revealed by scanning tunneling spectroscopy (STS). Moreover, a bandgap shift of 0.2 eV was observed in PdSe₂ layers grown on monolayer graphene as compared to those grown on bilayer graphene. The realization of nanoscale electronic junctions with atomically sharp boundaries in 2D PdSe₂ implies the possibility of tuning its electronic or optoelectronic properties. In addition, on top of the PdSe₂ bilayers, PdSe₂ nanoribbons and stacks of nanoribbons with a fixed orientation have been fabricated. The bottom-up fabrication of low-dimensional PdSe₂ structures is expected to enable substantial exploration of its potential applications.

High-quality single-layered and bilayered graphene (SLG and BLG) was synthesized on copper foil surfaces by controllable chemical vapor deposition (CVD). Impurity nanoparticles formed on the copper foil surface by hightemperature annealing were found to play a crucial role in the growth of BLG. Analysis of energy-dispersive spectrometry (EDS) data indicated that these nanoparticles consisted of silicon and aluminum. According to the inverted wedding cake model, these nanoparticles served as nucleation centers for BLG growth and the free space between a nanoparticle and graphene served as the center of C injection for the continuous growth of the adlayer beneath the top layer. By combining phase-field theory simulations, we confirmed the mechanism of BLG growth and revealed more details about it in comparison with SLG growth. For the first time, this study led to a complete understanding of the BLG growth mechanism from nucleation to continuous growth in the CVD process, and it has opened a door to the thickness-controllable synthesis of graphene.