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A mirror twin-domain boundary (MTB) in monolayer MoSe2 represents a (quasi) one-dimensional metallic system. Its electronic properties, particularly the low-energy excitations in the so-called 4|4P-type MTB, have drawn considerable research attention. Reports of quantum well states, charge density waves, and the Tomonaga-Luttinger liquid (TLL) have all been made. Here, by controlling the lengths of the MTBs and employing different substrates, we reveal by low-temperature scanning tunneling microscopy/spectroscopy, Friedel oscillations and quantum confinement effects causing the charge density modulations along the defect. The results are inconsistent with charge density waves. Interestingly, for graphene-supported samples, TLL in the MTBs is suggested, whereas that grown on gold, an ordinary Fermi liquid, is indicated.

Confining interacting particles in one-dimension (1D) changes the electronic behavior of the system fundamentally, which has been studied extensively in the past. Examples of 1D metallic systems include carbon nanotubes, quasi-1D organic conductors, metal chains, and domain boundary defects in monolayer thick transition-metal dichalcogenides such as MoSe2. Here single and bundles of Mo6Se6 nanowires were fabricated through annealing a MoSe2 monolayer grown by molecular-beam epitaxy on graphene. Conversion from two-dimensional (2D) MoSe2 film to 1D Mo6Se6 nanowire is reversible. Mo6Se6 nanowires form preferentially at the Se-terminated zigzag edges of MoSe2 and stitch to it via two distinct atomic configurations. The Mo6Se6 wire is metallic and its length is tunable, which represents one of few 1D systems that exhibit properties pertinent to quantum confined Tomonaga-Luttinger liquid, as evidenced by scanning tunneling microscopic and spectroscopic studies.

Summary Two-dimensional (2D) non-covalent and covalent networks are interesting because their tailorable architectures offer fresh applications in sensing, catalysis, gas storage, and topological structures. So far, rational design of these networks has been explored exclusively involving organic molecules based on the concept of modular assembly. Here, we demonstrate that, by using molecular-beam epitaxy, we can isolate inorganic blocks to form a 2D porous gold-phosphorus network (AuPhoN), wherein blue phosphorene (blueP) subunits are linked by gold atoms on an Au surface. We provide evidence that such metal-phosphorus networks are tunable in their chemical functionalities and electronic properties by simply altering the linkers and subunits. Our work provides insights to tailor low-dimension architectures of inorganic networks that maintain good long-range order and are stable over a wide temperature range.

Monolayer (ML) transition-metal dichalcogenides (TMDs) continue to attract research attention, and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g.,...

High-index Bi 2 Se 3 (221) has been successfully grown on partially suspended Ga 2 Se 3 (001). The Ga 2 Se 3 layer was formed by selenation of GaSb(001) surface, which revealed a suspended structure supported only by some GaSb nano-pillars. Such a growth behavior may be beneficial for achieving heterostructures with large lattice misfits and suppressing the coupling between the substrate and deposit. Bi 2 Se 3 , a typical topological insulator, has been grown on Ga 2 Se 3 along the high-index [221] direction despite of the large lattice mismatch.

Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene, whereas for thicker films, the cell size is more consistent with that of the [101[combining macron]0] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.

Single layer (SL) phosphorus (phosphorene) has drawn considerable research attention recently as a two-dimensional (2D) material for application promises. It is a semiconductor showing superior transport and optical properties. Few-layer or SL black phosphorus has been successfully isolated by exfoliation from bulk crystals and extensively studied thereof for its electronic and optical properties. Blue phosphorus (blueP), an allotrope of black phosphorus where atoms are arranged in a more flat atomic configuration, has been recently suggested by theory to exist in the SL form on some substrates. In this work, we report the formation of a blueP-like epilayer on Au(111) by molecular-beam epitaxy. In particular, we uncover by scanning tunneling microscopy (STM) one-dimensional (1D) atomic chains at low coverage, which develop into more compact islands or patches of $(\sqrt{3}\times\sqrt{3})R30^\circ$ structure with increasing coverage before blueP-like islands nucleate and grow. We also note an interesting growth characteristic where the $(\sqrt{3}\times\sqrt{3})R30^\circ$ surface at intermediate coverage tends to phase-separate into locally low-coverage 1D chain and high-coverage blueP-like structures, respectively. This experiment thus not only lends a support of the recently proposed half-layer by half-layer (HLBHL) growth mechanism but also reveals the kinetic details of blueP growth processes.

It is generally known that dry etching induces surface damage, but the structure of etched surfaces has not been studied in detail. Nor has it been established how or whether a clean, ordered surface can be recovered after etching damage. Generally, etching damages the surface by introducing structural disorder and impurities to the surface region. Such damage may be so severe that a clean, ordered surface is not recoverable even after heating up to ∼1400 K in UHV. We subjected Si (1 1 1) surfaces to different reactive ion etching conditions and/or post-etch treatments and examined their effect on the surface. Low temperature STM revealed that a clean, ordered surface as evidenced by the appearance of large area 7 × 7 reconstructions can be obtained only under certain etching/post-etch recipes. On the other hand, LEED showing 7 × 7 diffraction spots but with high intensity background and Auger electron spectroscopy (AES) showing no impurity signal cannot be used as evidence that an ordered surface has been obtained.

Molecular -beam epitaxy of topological insulator Bi2Se3 on different substrates is carried out, including on clean and hydrogen-terminated Si(111), bismuth (Bi) induced Si(1 1 1)–( 3 × 3 ) and –(3×3) surfaces, GaN, and several selenide compounds. Epifilms surface and structural properties are characterized and the effects of surface structure and lattice misfit are indicated. It is found that substrates that are inert to chemical reaction with Bi and/or selenium (Se) and have small lattice misfits with Bi2Se3 appear advantageous to single-crystalline Bi2Se3 film growth. By using vicinal substrates, rotation or twin domains are effectively suppressed.

Many transition-metal dichalcogenides, such as MoSe2, are direct-gap semiconductors at monolayer thickness, which hold potentials in nano-electronics, optoelectronics, and some new concept spin- and valley-electronic applications. For device application, however, controllable doping of the materials is essential. Here we report hole doping of epitaxial MoSe2 by nitrogen (N) plasma treatment with the aim of understanding the defect structure and its electronic characteristics. Examinations by annular dark field scanning transmission electron microscopy clearly reveal substitutional doping of N by replacing Se atoms in MoSe2 monolayer upon N-plasma treatment, though creation of Se vacancies are also possible. Interestingly, we note an unexpectedly high concentration of 'dual defects', where both Se atoms in the top and bottom Se layers of MoSe2 at the same lattice site are substituted by N and/or become vacant, suggesting a catalytic effect of defect formation. X-ray photoelectron spectroscopy and electron energy loss spectroscopy confirm the presence of N–Mo bonds. Photoemission spectroscopy reveals an impurity band as well as the Fermi level shift, confirming the p-type doping effect in MoSe2 monolayer by N-plasma treatment. Consistent with the PES results, scanning tunneling spectroscopy measurement also reveal defect states peaked at 0.6–0.7 eV above the valance band maximum. The effectiveness of N-doping is discussed.

Bilayer (BL) MoSe2 films grown by molecular-beam epitaxy (MBE) are studied by scanning tunneling microscopy and spectroscopy (STM/S). Similar to monolayer (ML) films, networks of inversion domain boundary (DB) defects are observed both in the top and bottom layers of BL MoSe2, and often they are seen spatially correlated such that one is on top of the other. There are also isolated ones in the bottom layer without companion in the top-layer and are detected by STM/S through quantum tunneling of the defect states through the barrier of the MoSe2 ML. Comparing the DB states in BL MoSe2 with that of ML film reveals some common features as well as differences. Quantum confinement of the defect states is indicated. Point defects in BL MoSe2 are also observed by STM/S, where ionization of the donor defect by the tip-induced electric field is evidenced. These results are of great fundamental interests as well as practical relevance of devices made of MoSe2 ultrathin layers.

We report the observation of a dense triangular network of one-dimensional (1D) metallic modes in a continuous and uniform monolayer of MoSe(2) grown by molecular-beam epitaxy. High-resolution transmission electron microscopy and scanning tunneling microscopy and spectroscopy studies show that these 1D modes are midgap states at inversion domain boundaries. Scanning tunneling microscopy and spectroscopy measurements further reveal intensity undulations of the metallic modes, presumably arising from the superlattice potentials due to the moire pattern and the quantum confinement effect. A dense network of the metallic modes with a high density of states is of great potential for heterocatalysis applications. The interconnection of such midgap 1D conducting channels may also imply new transport behaviors distinct from the 2D bulk.

A high-index topological insulator thin film, Bi2 Se3 (221), is grown on a faceted InP(001) substrate by molecular-beam epitaxy (see model in figure (a)). Angle-resolved photoemission spectroscopy measurement reveals the Dirac cone structure of the surface states on such a surface (figure (b)). The Fermi surface is elliptical (figure (c)), suggesting an anisotropy along different crystallographic directions. Transport studies also reveal a strong anisotropy in Hall conductance.

Interests in two-dimensional transition-metal dichalcogenides have prompted some recent efforts to grow ultrathin layers of these materials epitaxially using molecular-beam epitaxy. However, growths of monolayer and bilayer WSe2, an important member of the transition-metal dichalcogenides family, by the molecular-beam epitaxy method remain uncharted probably because of the difficulty in generating tungsten fluxes from the elemental source. In this work, we present a scanning tunneling microscopy and spectroscopy study of molecular-beam epitaxy-grown WSe2 monolayer and bilayer, showing atomically flat epifilm with no domain boundary defect. This contrasts epitaxial MoSe2 films grown by the same method, where a dense network of the domain boudaries defects is present. The scanning tunneling spectroscopy measurements of monolayer and bilayer WSe2 domains of the same sample reveal not only the bandgap narrowing upon increasing the film thickness from monolayer to bilayer, but also a band-bending effect across the boundary between monolayer and bilayer domains. This band-bending appears to be dictated by the edge states at steps of the bilayer islands. Finally, comparison is made between the scanning tunneling spectroscopy-measured electronic bandgaps with the exciton emission energies measured by photoluminescence, and the exciton binding energies in monolayer and bilayer WSe2/MoSe2 are thus estimated.

We report the growth of single-domain epitaxial Bi2Se3 films on InP(111)A substrate by molecular-beam epitaxy. Nucleation of Bi2Se3 proceeds at steps, so the lattices of the substrate play the guiding role for a unidirectional crystalline film in the step-flow growth mode. There exists a strong chemical interaction between atoms at the heterointerface, so the growth does not follow the van der Waals epitaxy process. A mounded morphology of thick Bi2Se3 epilayers suggests a growth kinetics dictated by the Ehrlich-Schwoebel barrier. The Schubnikov de Haas oscillations observed in magnetoresistance measurements are attributed to Landau quantization of the bulk states of electrons.