SnSe, an environmental-friendly group-IV monochalcogenide semiconductor, demonstrates outstanding performance in various applications ranging from thermoelectric devices to solar energy harvesting. Its ultrathin films show promise in the fabrication of ferroelectric nonvolatile devices. However, the microscopic identification and manipulation of point defects in ultrathin SnSe single crystalline films, which significantly impact their electronic structure, have been inadequately studied. This study presents a comprehensive investigation of point defects in monolayer SnSe films grown via molecular beam epitaxy. By combining scanning tunneling microscopy (STM) characterization with first-principles calculations, we identified four types of atomic/molecular vacancies, four types of atomic substitutions, and three types of extrinsic defects. Notably, we have demonstrated the ability to convert a substitutional defect into a vacancy and to reposition an adsorbate by manipulating a single atom or molecule using an STM tip. We have also analyzed the local atomic displacement induced by the vacancies. This work provides a solid foundation for engineering the electronic structure of future SnSe-based nanodevices.

Single-element polarization in low dimensions is fascinating for constructing next-generation nanoelectronics with multiple functionalities, yet remains difficult to access with satisfactory performance. Here, spectroscopic evidences are presented for the spontaneous electronic polarization in tellurium (Te) films thinned down to bilayer, characterized by low-temperature scanning tunneling microscopy/spectroscopy. The unique chiral structure and centrosymmetry-breaking character in 2D Te gives rise to sizable in-plane polarization with accumulated charges, which is demonstrated by the reversed band-bending trends at opposite polarization edges in spatially resolved spectra and conductance mappings. The polarity of charges exhibits intriguing influence on imaging the moiré superlattice at the Te-graphene interface. Moreover, the plain spontaneous polarization robustly exists for various film thicknesses, and can universally preserve against different epitaxial substrates. The experimental validations of considerable electronic polarization in Te multilayers thus provide a realistic platform for promisingly facilitating reliable applications in microelectronic devices.

The design of novel low-dimensional carbon materials is at the forefront of modern chemistry. Recently, on-surface covalent synthesis has emerged as a powerful strategy to synthesize previously precluded compounds and polymers. Here, we report a scanning probe microscopy study, complemented by theoretical calculations, about the sequential skeletal rearrangement of sumanene-based precursors into a coronene-based organometallic network by stepwise intra- and inter-molecular reactions on Au(111). Interestingly, upon higher annealing, the formed organometallic networks evolve into two-dimensional coronene-based covalently-linked patches through intermolecular homocoupling reactions. A new reaction mechanism is proposed based on the role of C-Au-C motifs to promote two stepwise carbon-carbon couplings to form cyclobutadiene bridges. Our results pave avenues for the conversion of molecular precursors on surfaces, affording the design of unexplored two-dimensional organometallic and covalent materials.

Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice\'s morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology.

The ruby lattice is one of the tight-binding models which hosts a flat band in its electronic structure and has potential applications in future spintronics and quantum devices. However, the experimental realization of a ruby lattice in realistic materials remains elusive. Here, we have experimentally realized an atomic ruby lattice by fabricating monolayer CuCl1+x on a Au(111) substrate. Scanning tunneling microscopy/spectra (STM/STS) measurements combined with density-functional theory (DFT) calculations reveal that the Cu atoms are arranged in a ruby lattice in this monolayer. Moreover, a significant density of states (DOS) peak corresponding to the characteristic of a ruby system is observed, consistent with both the tight-binding model and first-principles calculations on the band structure. Our work provides a promising platform to explore the physics of the ruby model.

Controlling the surface diffusion of particles on 2D devices creates opportunities for advancing microscopic processes such as nanoassembly, thin-film growth, and catalysis. Here, we demonstrate the ability to control the diffusion of F4TCNQ molecules at the surface of clean graphene field-effect transistors (FETs) via electrostatic gating. Tuning the back-gate voltage (VG) of a graphene FET switches molecular adsorbates between negative and neutral charge states, leading to dramatic changes in their diffusion properties. Scanning tunneling microscopy measurements reveal that the diffusivity of neutral molecules decreases rapidly with a decreasing VG and involves rotational diffusion processes. The molecular diffusivity of negatively charged molecules, on the other hand, remains nearly constant over a wide range of applied VG values and is dominated by purely translational processes. First-principles density functional theory calculations confirm that the energy landscapes experienced by neutral vs charged molecules lead to diffusion behavior consistent with experiment. Gate-tunability of the diffusion barrier for F4TCNQ molecules on graphene enables graphene FETs to act as diffusion switches.

The successful synthesis of borophene beyond the monolayer limit has expanded the family of two-dimensional boron nanomaterials. While atomic-resolution topographic imaging has been previously reported, vibrational mapping has the potential to reveal deeper insight into the chemical bonding and electronic properties of bilayer borophene. Herein, inelastic electron tunneling spectroscopy (IETS) is used to resolve the low-energy vibrational and electronic properties of bilayer-α (BL-α) borophene on Ag(111) at the atomic scale. Using a carbon monoxide (CO)-functionalized scanning tunneling microscopy tip, the BL-α borophene IETS spectra reveal unique features compared to single-layer borophene and typical CO vibrations on metal surfaces. Distinct vibrational spectra are further observed for hollow and filled boron hexagons within the BL-α borophene unit cell, providing evidence for interlayer bonding between the constituent borophene layers. These experimental results are compared with density functional theory calculations to elucidate the interplay between the vibrational modes and electronic states in bilayer borophene.

Structural evolution of solid catalyst surfaces induced by direct exposure to reaction gas has been extensively studied and is well understood. However, whether and how subsurface atomic structures are affected by the reaction atmosphere require further exploration. In this work, our results confirm that Cu clusters supported on FeO/Pt(111) (Cun/FeO/Pt) transform into surface CuCO complexes (CuCO/FeO/Pt) with exposure to CO at 78 K. Surprisingly, Cu clusters on Pt(111) buried under monolayer FeO film (FeO/Cun/Pt) can also transform into surface CuCO complexes on FeO/Pt(111) upon CO adsorption at 150 K. The place exchange of surface and subsurface Cu atoms at the FeO/Pt(111) surface can be mediated by exposing to CO at 150 K and keeping in ultrahigh vacuum at 300 K, alternatively. Calculation results reveal that CO adsorption induces restructuring of the FeO film above the Cu clusters, generating a diffusion channel for Cu atoms to pass through the FeO film and form surface CuCO, while Cu atoms remaining at the FeO-Pt interface are more thermodynamically favored without CO. Our work suggests that buried subsurface atoms may be involved in strong restructuring processes driven by reaction gas, which could strongly influence the catalytic performance.

We increase the dynamical range of a scanning tunneling microscope (STM) by actively subtracting dominant current-harmonics generated by nonlinearities in the current-voltage characteristics that could saturate the current preamplifier at low junction impedances or high gains. The strict phase relationship between a cosinusoidal excitation voltage and the current-harmonics allows excellent cancellation using the displacement-current of a driven compensating capacitor placed at the input of the preamplifier. Removal of DC currents has no effect on, and removal of the first harmonic only leads to a rigid shift in differential conductance that can be numerically reversed by adding the known removal current. Our method requires no permanent change of the hardware but only two phase synchronized voltage sources and a multi-frequency lock-in amplifier to enable high dynamic range spectroscopy and imaging. • Active power filter • Dynamic range compression • High gain preamplifier.

The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems. We find that the ground (quasi-molecular) state of a pair of coupled phosphorous dopants often cannot be fully explained by the linear combination of single-dopant ground states. Although the contributions from excited single-dopant states are relatively small, they can lead to ambiguity in determining individual dopant positions from a multi-dopant STM image. To overcome that, we exploit knowledge about dopant-pair wave functions and propose a simple yet effective scheme for finding double-dopant positions based on STM images.