Kedong Wang

Automating scanning tunneling microscopy (STM) is essential for scaling atomically precise characterization and, ultimately, fabrication of next-generation materials and devices. A key challenge is to maintain atomic precision across extended surface regions, where local structures must be aligned, identified, and revisited in a common lattice frame. Here we use surface periodicity as an intrinsic reference frame to transform lattice-resolved STM images into lattice-registered unit-cell maps. Using Ag/Si(111)-(7×7) as a model system, the framework preserves lattice indices and neighborhood relationships while standardizing local structural units. This representation enables data-efficient supervised recognition of Ag adsorption configurations from one or a few reference images and supports environment-aware target-site selection. We further implement the framework in a continuous automated STM workflow, demonstrating autonomous scanning, target identification, and structure-specific characterization. Our results show that crystalline surface periodicity can provide a transferable basis for automated STM operations on lattice-resolved surfaces.

Kelvin Probe Force Microscopy (KPFM) is a crucial technique for characterizing nanoscale variations in local work functions, particularly at surface defects that significantly influence surface electronic properties. Although it is well established that surface defects perturb local work functions, their quantitative dependence on structural and doping parameters remains unclear. In this study, we use KPFM to systematically investigate the local work functions on Si(111) surfaces with steps. On both n-type and p-type Si surfaces, step edges exhibit higher local work functions compared to terraces. These deviations increase with step height and trend to saturate at high steps. We attribute the deviation and saturation phenomena to the local electronic states induced by dangling bonds, which dominate the local Fermi level via a density-dependent pinning mechanism. Our work establishes a clear correlation between defect morphology and electronic behavior, providing a universal framework for designing semiconductor surfaces and interfaces through defect engineering.

Chiral superconductors have long been theorized to break time-reversal symmetry and support exotic topological features such as Majorana modes and spontaneous edge currents, promising ingredients for quantum technologies. Although several unconventional superconductors may exhibit time-reversal symmetry breaking, clear microscopic evidence of chiral pairing has remained out of reach. In this work, we demonstrate direct real-space signatures of chiral superconductivity in a single atomic layer of tin on Si(111). Using quasiparticle interference imaging, we detected symmetry-locked nodal and antinodal points in the Bogoliubov quasiparticle wave function, tightly bound to atomic point defects in the tin lattice. These nodal features, along with their surrounding texture, form a distinct real-space pattern exhibiting a clear and exclusive hallmark of chiral superconductivity. Our findings, reinforced by analytical theory and numerical simulations, offer unambiguous evidence of chiral pairing in a two-dimensional material.

Chiral superconductors have long been theorized to break time-reversal symmetry and support exotic topological features such as Majorana modes and spontaneous edge currents, promising ingredients for quantum technologies. Although several unconventional superconductors may exhibit time-reversal symmetry breaking, clear microscopic evidence of chiral pairing has remained out of reach. In this work, we demonstrate direct real-space signatures of chiral superconductivity in a single atomic layer of tin on Si(111). Using quasiparticle interference imaging, we detected symmetry-locked nodal and antinodal points in the Bogoliubov quasiparticle wavefunction, tightly bound to atomic point defects in the tin lattice. These nodal features, along with their surrounding texture, form a distinct real-space pattern exhibiting a clear and exclusive hallmark of chiral superconductivity. Our findings, reinforced by analytical theory and numerical simulations, offer unambiguous evidence of chiral pairing in a two-dimensional material.

Defect engineering in the inherently inert basal planes of transition metal dichalcogenides (TMDs), involving the introduction of chalcogen vacancies, represents a pivotal approach to enhance catalytic activity by exposing high-density catalytic metal single-atom sites. However, achieving a single-atom limit spacing between chalcogen vacancies to form ordered superstructures remains challenging for creating uniformly distributed high-density metal single-atom sites on TMDs comparable to carbon-supported single-atom catalysts (SACs). Here we unveil an efficient TMD-based topological catalyst for hydrogen evolution reaction (HER), featuring high-density single-atom reactive centers on a few-layer (7 × 7)-PtTe_2-x superstructure. Compared with pristine Pt(111), PtTe₂, and (2 × 2)-PtTe_2-x, (7 × 7)-PtTe_2-x exhibits superior HER performance owing to its substantially increased density of undercoordinated Pt sites, alongside exceptional catalytic stability when operating at high current densities. First-principles calculations confirm that multiple types of undercoordinated Pt sites on (7 × 7)-PtTe_2-x exhibit favorable hydrogen adsorption Gibbs free energies, and remain active upon increasing hydrogen coverage. Furthermore, (7 × 7)-PtTe_2-x possesses nontrivial band topologies with robust edge states, suggesting potential enhancements for HER. Our findings are expected to advance TMD-based catalysts and exploration of topological materials in catalysis.

The century-long development of surface sciences has witnessed the discoveries of a variety of quantum states. In the recently proposed “obstructed atomic insulators”, symmetric charges are pinned at virtual sites where no real atoms reside. The cleavage through these sites could lead to a set of obstructed surface states with partial electronic occupation. Here, utilizing scanning tunneling microscopy, angle-resolved photoemission spectroscopy and first-principles calculations, we observe spectroscopic signature of obstructed surface states in SrIn₂P₂. We find that a pair of surface states that are originated from the pristine obstructed surface states split in energy by a unique surface reconstruction. The upper branch is marked with a striking differential conductance peak followed by negative differential conductance, signaling its localized nature, while the lower branch is found to be highly dispersive. This pair of surface states is in consistency with our calculational results. Our finding not only demonstrates a surface quantum state induced by a new type of bulk-boundary correspondence, but also provides a platform for exploring efficient catalysts and related surface engineering.

One-dimensional (1D) arsenene nanostructures are predicted to host a variety of interesting physical properties including antiferromagnetic, semiconductor-semimetal transition and quantum spin Hall effect, which thus holds great promise for next-generation electronic and spintronic devices. Herein, we devised a surface template strategy in a combination with surface-catalyzed decomposition of molecular As₄ cluster toward the synthesis of the superlattice of ultranarrow armchair arsenic nanochains in a large domain on Au(111). In the low annealing temperature window, zero-dimensional As₄ nanoclusters are assembled into continuous films through intermolecular van der Waals and molecule-substrate interactions. At the elevated temperature, the subsequent surface-assisted decomposition of molecular As₄ nanoclusters leads to the formation of a periodic array of 1D armchair arsenic nanochains that form a (2 × 3) superstructure on the Au(111) surface. These ultranarrow armchair arsenic nanochains are predicted to have a small bandgap of ∼0.50 eV, in contrast to metallic zigzag chains. In addition, the Au-supported arsenic nanochains can be flipped to form a bilayer structure through tip indentation and manipulation, suggesting the possible transfer of these nanochains from the substrate. The successful realization of arsenic nanostructures is expected to advance low-dimensional physics and infrared optoelectronic nanodevices.