The quantum anomalous Hall (QAH) effect in two-dimensional (2D) topological materials has attracted widespread attention due to its potential for dissipationless chiral edge transport without an external magnetic field, which is highly promising for low-power electronic applications. However, identifying materials that exhibit these properties remains particularly challenging, as only a limited number of such materials are known, raising the intriguing question of whether it is possible to induce the QAH effect in materials with ordinary properties through structural modifications. In this work, we grow an unreported 2D titanium selenide (Ti₃Se₄) on a Cu(111) substrate using molecular beam epitaxy. Low-energy electron diffraction and scanning tunneling microscopy characterizations reveal a  brick-like structure. First-principles calculations and X-ray photoelectron spectroscopy measurements confirm its composition to be Ti₃Se₄. Our calculations further demonstrate that monolayer Ti₃Se₄, in its grown form on Cu(111), has the potential to host the QAH effect. Interestingly, when we examine its freestanding form, the monolayer transitions from a QAH insulator candidate into a conventional semiconductor, despite only minor differences in their atomic structures. This transition enlightens us that subtle lattice adjustments can induce a transition from semiconductor to QAH properties in freestanding Ti₃Se₄. This discovery provides a potential route to engineering practical materials that may exhibit the QAH effect.

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.

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.

Two-dimensional (2D) materials with defects are desired for catalysis after the adsorption of monodispersed noble metal atoms. High-performance catalysts with the absolute value of Gibbs free energy (|ΔG_H|) close to zero, is one of the ultimate goals in the catalytic field. Here, we report the formation of monolayer titanium selenide (TiSe₂) with line defects. The low-temperature scanning tunneling microscopy/spectroscopy (STM/S) measurements revealed the structure and electronic states of the line defect. Density functional theory (DFT) calculation results confirmed that the line defects were induced by selenium vacancies and the STM simulation was in good agreement with the experimental results. Further, DFT calculations show that monolayer TiSe₂ with line defects have good catalytic activity for hydrogen evolution reaction (HER). If the defects are decorated with single Pt atom, the HER catalytic activity will be enhanced dramatically (|ΔG_H| = 0.006 eV), which is much better than Pt metal (|ΔG_H| = 0.09 eV). Line defects in monolayer TiSe₂/Au(111) provide a wonderful platform for the design of high-performance catalysts.

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.