Yu Li Huang

This review presents several typical organic room-temperature phosphorescence materials with multiple excitation sources, focusing mainly on their internal mechanisms and structure–property relationships.

As promising candidates for two-dimensional (2D) magnetic semiconductors, layered transition metal halides (TMHs) have attracted increasing interest due to their diverse insulating bandgaps and magnetic properties.

The synthesis of atomically precise carbon nanostructures in ultra‐high vacuum has seen extensive progress on metal surfaces. However, this remains challenging on chemically inert surfaces. It is because the thermally activated C−C coupling encounters a severe “desorption problem” on weakly interacting substrates. In this study, we report an extraordinary [2+2]+[2+2] cycloaddition triggered by mild annealing (~210°C) in a highly ordered π‐conjugated molecular array on graphite using scanning tunneling microscopy. In contrast to irregular dendritic fragments typically obtained on metal substrates, large supramolecular islands are observed here with cycloaddition products and other polymers over 30 %, which are embedded as defective individuals or chains (grain boundaries). First‐principles calculations reveal that the energy barriers of the multiple dehydrohalogenation and cycloaddition reactions are reduced by catalytic Fe atoms but remain energetically unfavorable. A distinct driving mechanism is proposed for redirecting the reactions on graphite surface, where additional intermolecular coupling, steric hindrance, and interfacial interactions play significant roles. This study introduces a new paradigm for understanding on‐surface synthesis on non‐metal substrates.

The synthesis of atomically precise carbon nanostructures in ultra‐high vacuum has seen extensive progress on metal surfaces. However, this remains challenging on chemically inert surfaces. It is because the thermally activated C−C coupling encounters a severe “desorption problem” on weakly interacting substrates. In this study, we report an extraordinary [2+2]+[2+2] cycloaddition triggered by mild annealing (~210°C) in a highly ordered π‐conjugated molecular array on graphite using scanning tunneling microscopy. In contrast to irregular dendritic fragments typically obtained on metal substrates, large supramolecular islands are observed here with cycloaddition products and other polymers over 30 %, which are embedded as defective individuals or chains (grain boundaries). First‐principles calculations reveal that the energy barriers of the multiple dehydrohalogenation and cycloaddition reactions are reduced by catalytic Fe atoms but remain energetically unfavorable. A distinct driving mechanism is proposed for redirecting the reactions on graphite surface, where additional intermolecular coupling, steric hindrance, and interfacial interactions play significant roles. This study introduces a new paradigm for understanding on‐surface synthesis on non‐metal substrates.

The kagome lattice is a versatile platform for investigating correlated electronic states. However, its realization in two-dimensional (2D) semiconductors for tunable device applications is still challenging. An alternative strategy to create kagome-like bands is to realize a coloring-triangle (CT) lattice in semiconductors through a distortion of a modified triangular lattice. Here, we report the observation of low-energy kagome-like bands in a semiconducting 2D transition metal chalcogenide—Cr₈Se₁₂ with a thickness of 7 atomic layers—which exhibits a CT lattice and a bandgap of 0.8 eV. The Cr-deficient layer beneath the topmost Se-full layer is partially occupied with 2/3 occupancy, yielding a √3 × √3 Cr honeycomb network. Angle-resolved photoemission spectroscopy measurements and first-principles investigations reveal the surface kagome-like bands near the valence band maximum, which are attributed to topmost Se p_z orbitals modulated by the honeycomb Cr.

Structural topology and symmetry of a two‐dimensional (2D) network play pivotal roles in defining its electrical properties and functionalities. Here, a binary buckled honeycomb lattice with C_3v symmetry, which naturally hosts topological Dirac fermions and out‐of‐plane polarity, is proposed. It is successfully achieved in a group IV‐V compound, namely monolayer SiP epitaxially grown on Ag(111) surface. Combining first‐principles calculations with angle‐resolved photoemission spectroscopy, the degeneration of the Dirac nodal lines to points due to the broken horizonal mirror symmetry is elucidated. More interesting, the SiP monolayer manifests metallic nature, which is mutually exclusive with polarity in conventional materials. It is further found that the out‐of‐plane polarity is strongly suppressed by the metallic substrate. This study not only represents a breakthrough of realizing intrinsic polarity in 2D metallic material via ingenious design but also provides a comprehensive understanding of the intricate interplay of many exotic low‐dimensional quantum phenomena.

The kagome lattice has attracted intense interest with the promise of realizing topological phases built from strongly interacting electrons. However, fabricating two-dimensional (2D) kagome materials with nontrivial topology is still a key challenge. Here, we report the growth of single-layer iron germanide kagome nanoflakes by molecular beam epitaxy. Using scanning tunneling microscopy/spectroscopy, we unravel the real-space electronic localization of the kagome flat bands. First-principles calculations demonstrate the topological band inversion, suggesting the topological nature of the experimentally observed edge mode. Apart from the intrinsic topological states that potentially host chiral edge modes, the realization of kagome materials in the 2D limit also holds promise for future studies of geometric frustration.