Visualizing Spatial Evolution of Electron-Correlated Interface in Two-Dimensional Heterostructures

Engineering Charge Density Waves by Stackingtronics in Tantalum Disulfide

In the realm of condensed matter physics and materials science, charge density waves (CDWs) have emerged as a captivating way to modulate correlated electronic phases and electron oscillations in quantum materials. However, collectively and efficiently tuning CDW order is a formidable challenge. Herein, we introduced a novel way to modulate the CDW order in 1T-TaS2 via stacking engineering. By introducing shear strain during the electrochemical exfoliation, the thermodynamically stable AA-stacked TaS2 consecutively transform into metastable ABC stacking, resulting in unique 3a × 1a CDW order. By decoupling atom coordinates, we atomically deciphered the 3D subtle structural variations in trilayer samples. As suggested by density functional theory (DFT) calculations, the origin of CDWs is presumably due to collective excitations and charge modulation. Therefore, our works shed light on a new avenue to collectively modulate the CDW order via stackingtronics and unveiled novel mechanisms for triggering CDW formation via charge modulation.

Ullmann-Like Covalent Bond Coupling without Participation of Metal Atoms

Ullmann-like on-surface synthesis is one of the most appropriate approaches for the bottom-up fabrication of covalent organic nanostructures and many successes have been achieved. The Ullmann reaction requires the oxidative addition of a catalyst (a metal atom in most cases): the metal atom will insert into a carbon-halogen bond, forming organometallic intermediates, which are then reductively eliminated and form C-C covalent bonds. As a result, traditional Ullmann coupling involves reactions of multiple steps, making it difficult to control the final product. Moreover, forming the organometallic intermediates will potentially poison the metal surface catalytic reactivity. In the study, we used the 2D hBN, an atomically thin sp²-hybridized sheet with a large band gap, to protect the Rh(111) metal surface. It is an ideal 2D platform to decouple the molecular precursor from the Rh(111) surface while maintaining the reactivity of Rh(111). We realize an Ullmann-like coupling of a planar biphenylene-based molecule, i.e., 1,8-dibromobiphenylene (BPBr₂), on an hBN/Rh(111) surface with an ultrahigh selectivity of the biphenylene dimer product, containing 4-, 6-, and 8-membered rings. The reaction mechanism, including electron wave penetration and the template effect of the hBN, is elucidated by combining low-temperature scanning tunneling microscopy and density functional theory calculations. Our findings are expected to play an essential role regarding the high-yield fabrication of functional nanostructures for future information devices.

Investigation of semiconductor properties of Co/Si(111)-7 × 7 by AFM/KPFS

Studies of the physics underlying carrier transport characteristics and band bending of semiconductors are critical for developing new types of devices. In this work, we investigated the physical properties of Co ring-like cluster (RC) reconstruction with a low Co coverage on a Si(111)-7 × 7 surface at atomic resolution by atomic force microscopy/Kelvin probe force microscopy at 78 K. We compared the applied bias dependence of frequency shift between two types of structure: Si(111)-7 × 7 and Co-RC reconstructions. As a result, the accumulation, depletion, and reversion layers were identified in the Co-RC reconstruction by bias spectroscopy. For the first time, we found that Co-RC reconstruction on the Si(111)-7 × 7 surface shows semiconductor properties by Kelvin probe force spectroscopy. The findings of this study are useful for developing new materials for semiconductor devices.

Impenetrable Barrier at the Metal-Mott Insulator Junction in Polymorphic 1H and 1T NbSe2 Lateral Heterostructure

When a metal makes contact with a band insulator, charge transfer occurs across the interface leading to band bending and a Schottky barrier with rectifying behavior. The nature of metal-Mott insulator junctions, however, is still debated due to challenges in experimental probes of such vertical heterojunctions with buried interfaces. Here, we grow lateral polymorphic heterostructures of single-layer metallic 1H and Mott insulating 1T NbSe₂ by molecular beam epitaxy. We find a one-dimensional metallic channel along the interface due to the appearance of quasiparticle states with an intensity decay following 1/x², indicating an impenetrable barrier. Near the interface, the Mott gap exhibits a strong spatial dependence arising from the difference in lattice constants between the two phases, consistent with our density functional theory calculations. These results provide clear experimental evidence for an impenetrable barrier at the metal-Mott insulator junction and the high tunability of a Mott insulator by strain.

Atomic-scale visualization of chiral charge density wave superlattices and their reversible switching

Chirality is essential for various phenomena in life and matter. However, chirality and its switching in electronic superlattices, such as charge density wave (CDW) superlattices, remain elusive. In this study, we characterize the chirality switching with atom-resolution imaging in a single-layer NbSe₂ CDW superlattice by the technique of scanning tunneling microscopy. The atomic arrangement of the CDW superlattice is found continuous and intact although its chirality is switched. Several intermediate states are tracked by time-resolved imaging, revealing the fast and dynamic chirality transition. Importantly, the switching is reversibly realized with an external electric field. Our findings unveil the delicate switching process of chiral CDW superlattice in a two-dimensional (2D) crystal down to the atomic scale.

Size Dependence of Charge-Density-Wave Orders in Single-Layer NbSe2 Hetero/Homophase Junctions

Controlling charge-density-wave (CDW) orders in two-dimensional (2D) crystals has attracted a great deal of interest because of their fundamental physics and their demand inse in miniaturized devices. In this work, we systematically studied the size-dependent CDW orders in single-layer hetero/homo-NbSe₂ stacking junctions. We found that the CDW orders in the top 1T-NbSe₂ layer of the junctions are highly dependent on its lateral size. For the 1T/2H-NbSe₂ heterojunction, the critical lateral size of 1T-NbSe₂ islands for the formation of well-defined CDW orders is ∼26 nm, whereas below 15 nm, the CDW orders melt. However, for the 1T/1T-NbSe₂ homojunction, the CDW orders in the islands can persist even with a lateral size of <11 nm. Our findings illuminate the fresh phenomenon of size-dependent CDW orders existing in 2D van der Waals hetero/homojunctions and provide useful information for the control of CDW orders.

Nanoscale Control of One-Dimensional Confined States in Strongly Correlated Homojunctions

Construction of lateral junctions is essential to generate one-dimensional (1D) confined potentials that can effectively trap quasiparticles. A series of remarkable electronic phases in one dimension, such as Wigner crystallization, are expected to be realized in such junctions. Here, we demonstrate that we can precisely tune the 1D-confined potential with an in situ manipulation technique, thus providing a dynamic way to modify the correlated electronic states at the junctions. We confirm the existence of 1D-confined potential at the homojunction of two single-layer 1T-NbSe₂ islands. Such potential is structurally sensitive and shows a nonmonotonic function of their interspacing. Moreover, there is a change of electronic properties from the correlated insulator to the generalized 1D Wigner crystallization while the confinement becomes strong. Our findings not only establish the capability to fabricate structures with dynamically tunable properties, but also pave the way toward more exotic correlated systems in low dimensions.

Visualization of Charge-Density-Wave Reconstruction and Electronic Superstructure at the Edge of Correlated Insulator 1T-NbSe2

Edges of low-dimensional quantum systems have profound effects on both fundamental research and device functionality. Real-space investigation of the microscopic edge structures and understanding the edge-modulated electronic properties are of great essence. Here we report the nanoscale structural reconstruction at the atomically sharp edge of a charge-density-wave (CDW) correlated insulator 1T-NbSe₂ and the induced electronic properties. We find the CDW unit cells at the edge of single layer (SL) 1T-NbSe₂ evolve from the well-defined CDW order in bulk and spontaneously reconstruct into the quartet along the edge. Moreover, we capture an anomalous electronic superstructure along the edge, the periodicity of which is four times that of ordinary CDW lattice. Our findings provide a way to design the one-dimensional electronic superstructure in 2D quantum materials.