P-type contact in two-dimensional (2D) transition metal dichalcogenides (TMDs) faces more severe Fermi-level pinning (FLP) than their n-type counterparts due to the damage caused by high-work-function metal deposition. Here, we demonstrate a simple molecular beam epitaxy (MBE) contact strategy using a high-work-function (5.8 eV) Au1-xSex alloy to achieve Fermi-level depinning in monolayer WSe2 p-type transistors. The Au1-xSex alloy mitigates both defect-induced gap states (DIGS) and metal-induced gap states (MIGS) by gentle pre-deposition of selenium (Se) followed by the conversion to Au1-xSex alloy by deposition of Au at low-temperature, forming a van der Waals (vdW) interface between Au1-xSex and monolayer WSe2. Moreover, it imposes hole doping to the contacted monolayer WSe2, reducing the Schottky barrier height and enabling favorable p-type device performance. Combined experimental and theoretical analyses confirm quasi-ohmic contact behavior, yielding a contact resistance of 492 Ω·µm, an on-current of 385 µA/µm, and an on/off ratio greater than 108 for a p-FET with a 100 nm channel length. This work establishes Au1-xSex alloy contacts as a scalable solution for high-performance p-type 2D electronics.

Large-scale synthesis of van der Waals (vdW) layered 2D materials remain a major challenge for their integration into high-performance electronics and optoelectronics. Although the multi-grain coalescence method provides a promising route to large-area growth, its success critically depends on achieving uniform crystallographic orientation across all nucleated domains, a challenge that has not yet been fully resolved. Here, we present an effective strategy for realizing unidirectional epitaxial growth of 2D materials by controllably introducing single active sites on flat terraces. Using the heteroepitaxy of MoSe2 on Au(111) as a model system, we combine molecular beam epitaxy with first-principles calculations to validate this mechanism. We show that Se adsorption disrupts the intrinsic herringbone reconstruction of Au(111), releasing surface Au atoms that aggregate into stable dimers. These dimers act as symmetry-breaking nucleation centers, guiding the unidirectional alignment of MoSe2 domains. From the perspective of interfacial interactions, we further clarify how substrate surface activation enhances the epitaxial quality of 2D materials. This work establishes a scalable pathway toward large-scale single-crystal 2D films and provides a conceptual framework for advancing the epitaxial growth of 2D materials.

A topological phase transition (TPT) is realized in rhombohedral Sb2Se3 via interfacial proximity. Molecular beam epitaxy (MBE) enables the epitaxial growth of Sb2Se3 on Bi2Se3, a strong topological insulator (STI), and on In2Se3, an ordinary insulator (OI). Angle-resolved photoemission spectroscopy (ARPES) reveals a Dirac cone in Sb2Se3/Bi2Se3 up to 15 nm thickness, in dramatic contrast to the full bandgap observed in Sb2Se3/In2Se3. Structural characterization confirms strain-free interfaces and identical crystal phases. A k & sdot; p model has been developed to interpret the ultra-long range proximity effect. These results demonstrate ultra-long range topological order propagation driven by interfacial band hybridization, resolving longstanding debates on the energy band topology of Sb2Se3 and establishing heterostructuring as a route to engineer quantum phases.

Two-dimensional (2D) nanomaterials hold immense application potentials such as in high-performance nano-electronics, and asymmetric 2D structures with inherent electric dipoles will extend the application promises. Yet synthesizing asymmetric 2D structures remains challenging. Herein, we report the first synthesis of single-layer (SL) hexagonal (H-) phase polar Janus MoSeN via nitrogen-plasma-assisted molecular beam epitaxy. This is a significant achievement given the incommensurate valence between Mo, Se, and N, and the inherent strain from the Janus architecture. Using an array of compositional and structural characterization methods, we establish the atomic configurations of the synthesized MoSeN SL, confirming that they are 2D Janus transition-metal chalcogen-nitrides rather than alloys. By employing density functional theory calculations and transport measurements, we explore the structural feasibility and offer insights into its electronic properties, demonstrating its metallic behavior with ohmic contact characteristics. Piezoresponse force microscopy measurements reveal vertical piezoelectricity and ferroelectric potentials from the Janus MoSeN SL. Therefore, it exhibits great potential for applications in, e.g. piezoelectric and ferroelectric devices, sensing technologies, and optoelectronic devices. This work not only addresses existing challenges in 2D nanomaterial research but also opens new avenues for the development of advanced functional materials.

Feibelman d -parameter, a mesoscopic complement to the local bulk permittivity, describes quantum optical surface responses for interfaces, including nonlocality, spill-in and -out, and surface-enabled Landau damping. It has been incorporated into the macroscopic Maxwellian framework for convenient modeling and understanding of nanoscale electromagnetic phenomena, calling for the compilation of a d -parameter database for interfaces of interest in nano-optics. However, accurate first-principles calculations of d -parameters face computational challenges, whereas existing measurements of d -parameters are scarce and restricted to narrow spectral windows. We demonstrate a general broadband ellipsometric approach to measure d -parameters at a gold–air interface across the visible–ultraviolet regimes. Gold is found to spill-in and spill-out at different frequencies. We also observe gold’s Bennett mode, a surface-dipole resonance associated with a pole of the d -parameter, around 2.5 eV. Our measurements give rise to and are further validated by the passivity and Kramers–Kronig causality analysis of d -parameters. Our work advances the understanding of quantum surface response and may enable applications like enhanced electron field emission.

We introduce a general broadband ellipsometric method to measure the quantum optical surface response functions known as Feibelman d -parameters, and demonstrate it using a gold–air interface in the visible–ultraviolet regimes under ambient conditions.

Interplay between defects like mirror twin boundaries (MTBs) and dopants may provide additional opportunities for furthering the research on two-dimensional monolayer (ML) transition metal dichalcogenides. In this work, we successfully dope rhenium (Re) into molecular beam epitaxy grown ML MoSe 2 and confirm the formation of a new type of MTBs, named 4|4E-M (M represents metal, Mo/Re) according to the configuration. Data from statistic atomic resolution scanning transmission electron microscopy also reveals a preferable MTB enrichment of Re dopants, rather than intra-domain. In conjunction with density functional theory calculation results, we propose the possible routes for Re doping induced formation of 4|4E-M MTBs. Electronic structures of Re doped MTBs in ML MoSe 2 are also predicted theoretically and then preliminarily tested by scanning tunneling microscopy and spectroscopy.

Abstract Ultrathin two‐dimensional transition‐metal dichalcogenides (TMDs) have been pursued extensively in recent years for interesting physics and application potentials. For the latter, it is essential to synthesize crystalline TMD monolayers at wafer‐scale. Here, we report growth of single‐crystalline MSe2 (M = Mo, W) monolayers at wafer‐scale by molecular‐beam epitaxy at low temperatures (200–400°C) on nominally flat Au(1 1 1) substrates. The epifilms have low intrinsic defect densities of low 1012 cm−2. The grown films have then been exfoliated and transferred onto SiO2/Si by a wet chemical process, on which some optical measurements are performed, revealing high spatial uniformity of the samples. We also establish that MSe2 grows on Au via the van der Waals epitaxy mechanism, where a continuous film extends across the whole surface, overhangs atomic‐layer steps on substrate. We identify that the growth of highly crystalline MSe2 is promoted by an enhanced interaction between Au substrate and MSe2 islands rather than by the guidance of surface steps on substrate. The latter only arrests MSe2 lateral growth if they are multilayer high. Key points MBE growth of wafer‐scale highly crystalline TMD monolayers at low‐temperature is achieved. Island‐substrate interaction is found to play a critical role in vdW epitaxy of single‐crystalline TMDs on on‐axis substates. The TMD monolayers are of high uniformity and low intrinsic defect density.

Band bending near mirror twin domain boundaries (MTBs) in a MoSe2 monolayer grown on different substrates, i.e., highly oriented pyrolytic graphite (HOPG), graphene-on-SiC, and crystalline Au(110), is investigated by low temperature scanning tunneling microscopy/spectroscopy. Upshift bending of the valence band edge near MTB is observed on both graphene and HOPG substrates, whereas a downshift bending is found on Au(110). For the former, the magnitudes of bending are different. This is explained based on the static charge model, where an accumulative charge exists at MTBs due to both the electrical dipole discontinuity across the MTB as well as charge transfer between the substrate and MoSe2 epilayer. The relevance of the static electric model is further affirmed by noting a geometric effect on the band bending, where it is asymmetric across the vertex of an MTB loop.

Monolayer (ML) transition-metal dichalcogenides (TMDs) have attracted a lot of research interest in recent years due to their many interesting properties as well as their application promises. Depending on the specific combinations of metals (e.g., Mo and W) with chalcogen elements (e.g., S, Se, and Te), binary TMDs exhibit a wide spectrum of physical characteristics, e.g., from metal to semiconductor and/or superconductor. Extension from binary to ternary compounds and alloys may offer even wider variations of properties and are thus of interest from both fundamental and practical points of view. In this work, we substitute Mo for niobium (Nb) and rhenium (Re) in ML MoSe2 during molecular-beam epitaxy and probe their effects on structural and electrical properties. We find that low-concentration Nb and Re in ML-MoSe2 are both shallow dopants, with Re being an electron donor and Nb acceptor, respectively. By changing Nb(Re)/Mo flux ratios, we can effectively tune the Fermi level by varying electron or hole concentrations in MoSe2. On the other hand, both Nb and Re are found to cause mirror-twin domain boundary defects to proliferate in MoSe2.

Phosphorus (P) has been predicted to possess many two-dimensional (2D) allotropes, which have attracted intensive research attention due to their excellent properties and application promises. While only 2D black P films have been successfully obtained by the exfoliation method, fabrication of other phosphorus structures by epitaxial growth remains challenging. Recently, the metal-phosphorus network (MPhoN), a superstructure consisted of P and metal atoms has been realized, which offers a wider platform to study P and related compounds. In this paper, we report the observation of platinum-phosphorus networks obtained by P adsorption on Pt(111). Our findings not only enrich the diverse structures of the MPhoN family but also provide insights into the formation mechanism of epitaxial P and its interaction with the noble metal substrate.

Single-layer (SL) transition-metal chalcogenides (TMCs) represent an important family of two-dimensional (2D) materials that have attracted intensive research attention recently. It has been established that many TMCs are polymorphic that can exist in different crystal structures and correspondingly exhibit diverse physical properties. Discovery of new structural phases of a crystal is of great scientific and practical importance. In this work, we report a new polymorph of SL-TMC, i.e. SL-Mo5Te8, attained by molecular-beam epitaxy (MBE). Like the 1H-MoTe2, it possesses the hexagonal symmetry but a much larger unit cell with a basis containing as many as 39 atoms (15 Mo and 24 Te). We call it the variational hexagonal (v1H) phase. Coincidently, it may be viewed also as one containing the highest density possible of mirror-twin domain boundaries (MTBs) in an otherwise pristine 1H-MoTe2. Electronically, it is metallic and a comparison between theory and experiments of its density-of-states (DOS) at the Fermi level reveals features pointing to an importance of electron interactions that invites further investigations.

Tuning the conductivity and other electronic properties by doping in ultrathin layers of transition-metal dichalcogenides is of great scientific and practical interest. As with traditional semiconductors, controllable doping is essential for device applications of the materials. Here, hole doping in epitaxial MoSe 2 by phosphorus (P) are reported, where substitutional P at the Se sites acts as a shallow acceptor. P substituting Se in MoSe 2 is identified by annular dark field scanning transmission electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. Scanning tunneling spectroscopy and ultraviolet photoemission spectroscopy reveal in-gap defect states and Fermi-level shifts, suggesting the hole doping effect of substitutional P. Combining with density functional theory calculation and partial charge analysis, the binding energies of impurity levels of group V elements in a MoSe 2 monolayer are elucidated, where the dopant energy level becomes shallower with increasing atomic mass.

Phosphorous doping in a MoSe2 monolayer is achieved by co-deposition of P, Se, and Mo during molecular-beam epitaxy, as reported by Maohai Xie and co-workers in article number 1900830. P atoms substitute Se in MoSe2, acting as acceptors and causing a Fermi-level shift. The doping level can be tuned by changing the P/Se flux ratio. For dopants of the group-V elements, the binding energy becomes shallower with increasing atomic mass.

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.

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.

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.