Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Atomically thin bismuth films (2D Bi) are becoming a promising research area due to their unique properties and their wide variety of applications in spintronics, electronic and optoelectronic devices. We report on the structural properties of Bi on Au(110), explored by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At a Bi coverage lower than one monolayer (1 ML) various reconstructions are observed, we focus on Bi/Au(110)-c(2 × 2) reconstruction (at 0.5 ML) and Bi/Au(110)-(3 × 3) structure (at 0.66 ML). We propose models for both structures based on STM measurements and further confirm by DFT calculations.

Silicene is a two-dimensional (2D) material with very promising electronic properties for applications in silicon modern technology. However, the first experimental synthesis of silicene on metallic surfaces shows strong interactions between the silicene and its substrate, which can alter its electronic properties. Here, we report on the first steps of silicene growth on an insulating surface (NaCl) using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angle-resolved photoemission spectroscopy (ARPES). We demonstrate the importance of temperature annealing in the growth of silicene on NaCl. Indeed, after deposition of silicon on the NaCl/Ag(110) surface, we observe the following stages: (i) at room temperature, the silicon atoms accumulate on top of the NaCl layer without any given order. (ii) At 60 °C, silicon dimers start to grow on the NaCl. (iii) At 140 °C, these dimers form a 2D silicon chains on the surface. (iv) After a post-annealing at 200 °C, evident 2D silicon nanoribbons with a honeycomb-like structure were observed. Our results of the first silicene growth stages on an insulating surface are a necessary step for exploring its growth mechanism further.

Journal Article Disentangling Exciton Linewidth Broadening Factors in Transition Metal Dichalcogenide Monolayer with Electron Energy Loss Spectroscopy Get access Fuhui Shao, Fuhui Shao Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Steffi Y Woo, Steffi Y Woo Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Nianjheng Wu, Nianjheng Wu Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, FranceUniversité Paris-Saclay, Institut des Sciences Moléculaires d'Orsay, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Robert Schneider, Robert Schneider Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Andrew J Mayne, Andrew J Mayne Université Paris-Saclay, Institut des Sciences Moléculaires d'Orsay, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Steffen Michaelis, Steffen Michaelis Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Ashish Arora, Ashish Arora Institute of Physics and Center for Nanotechnology, University of Münster, Münster, GermanyIndian Institute of Science Education and Research, Pune, India Search for other works by this author on: Oxford Academic Google Scholar Benjamin J Carey, Benjamin J Carey Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Johann A Preuß, Johann A Preuß Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Noémie Bonnet, Noémie Bonnet Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar ... Show more Cecilia Mattevi, Cecilia Mattevi Department of Materials, Imperial College London, London, UK Search for other works by this author on: Oxford Academic Google Scholar Kenji Watanabe, Kenji Watanabe Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan Search for other works by this author on: Oxford Academic Google Scholar Takashi Taniguchi, Takashi Taniguchi International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan Search for other works by this author on: Oxford Academic Google Scholar Rudolf Bratschitsch, Rudolf Bratschitsch Institute of Physics and Center for Nanotechnology, University of Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Luiz H G Tizei Luiz H G Tizei Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, Orsay, France Corresponding author: luiz.galvao-tizei@universite-paris-saclay.fr Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 1778–1779, https://doi.org/10.1017/S1431927622007036 Published: 01 August 2022

This work reports on the electron-induced modification of NaCl thin film grown on Ag(110). We show using low energy electron diffraction that electron beam bombardment leads to desorption and formation of Cl vacancy defects on NaCl surface. The topographic structure of these defects is studied using scanning tunneling microscopy (STM) showing the Cl defects as depressions on the NaCl surface. Most of the observed defects are mono-atomic vacancies and are located on flat NaCl terraces. Auger electron spectroscopy confirms the effect of electron exposure on NaCl thin films showing Cl atoms desorption from the surface. Using density functional theory taken into account the van der Waals dispersion interactions, we confirm the observed experimental STM measurements with STM simulation. Furthermore, comparing the adsorption of defect free NaCl and defective NaCl monolayer on Ag(110) surfaces, we found an increase of the adhesion energy and the charge transfer between the NaCl film and the substrate due to the Cl vacancy. In details, the adhesion energy increases between the NaCl film and the metallic Ag substrate from 30.4 meV Å-2for the NaCl film without Cl vacancy and from 39.5 meV Å-2for NaCl film with a single Cl vacancy. The charge transfer from the NaCl film to the Ag substrate is enhanced when the vacancy is created, from 0.63e-to 1.25e-.

Journal Article Strain Relaxation and Excitonic Absorption of Atomically-Reconstructed WSe2 Moiré Superlattices Get access Steffi Y Woo, Steffi Y Woo Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Fuhui Shao, Fuhui Shao Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Nianjheng Wu, Nianjheng Wu Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, FranceInstitut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Robert Schneider, Robert Schneider Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Ashish Arora, Ashish Arora Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, GermanyIndian Institute of Science Education and Research, Dr. Homi Bhabha Road, 411008 Pune, India Search for other works by this author on: Oxford Academic Google Scholar Johann A Preuß, Johann A Preuß Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Benjamin J Carey, Benjamin J Carey Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Steffen Michaelis de Vasconcellos, Steffen Michaelis de Vasconcellos Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar Andrew J Mayne, Andrew J Mayne Institut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, Orsay, France Search for other works by this author on: Oxford Academic Google Scholar Rudolf Bratschitsch, Rudolf Bratschitsch Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Münster, Germany Search for other works by this author on: Oxford Academic Google Scholar ... Show more Luiz HG Tizei Luiz HG Tizei Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France Corresponding author: luiz.galvao-tizei@universite-paris-saclay.fr Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2462–2463, https://doi.org/10.1017/S1431927622009424 Published: 01 August 2022

The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. \textit{h}-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which \textit{h}-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.

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Blue phosphorene (blue-P) has attracted considerable attention due to its potential applications in optical and electronic devices. However, its synthesis has remained a challenge. Here, we report an experimental investigation of the first steps of blue-P growth on Au(111) surface by molecular-beam epitaxy. The structure was characterized by in situ low temperature scanning tunneling microscopy, low-energy electron diffraction, combined with density functional theory calculations. We reveal two-dimensional (2D) phosphorus clusters (P-clusters) formed on surface at 150 degrees C, where the most prevalent structure of P-clusters is composed of triangles with six protrusions. We also demonstrate the transformation of these P-clusters into a single layer of blue-P after post-annealing at 260 degrees C. Our observation of the growth process is a necessary step for exploring the growth mechanisms further. (C) 2019 Elsevier Ltd. All rights reserved.

Silicene, a new 2D material has attracted intense research because of the ubiquitous use of silicon in modern technology. However, producing free-standing silicene has proved to be a huge challenge. Until now, silicene could be synthesized only on metal surfaces where it naturally forms strong interactions with the metal substrate that modify its electronic properties. Here, the authors report the first experimental evidence of silicene nanoribbons on an insulating NaCl thin film. This work represents a major breakthrough, for the study of the intrinsic properties of silicene, and by extension to other 2D materials that have so far only been grown on metal surfaces.

The emergence of peculiar phenomena in 1D phosphorene chains (P chains) has been proposed in theoretical studies, notably the Stark and Seebeck effects, room temperature magnetism, and topological phase transitions. Attempts so far to fabricate P chains, using the top-down approach starting from a few layers of bulk black phosphorus, have failed to produce reliably precise control of P chains. We show that molecular beam epitaxy gives a controllable bottom-up approach to grow atomically thin, crystalline 1D flat P chains on a Ag(111) substrate. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations reveal that the armchair-shaped chains are semiconducting with an intrinsic 1.80 ± 0.20 eV band gap. This could make these P chains an ideal material for opto-electronic devices.

The synthesis of blue phosphorene by molecular beam epitaxy (MBE) has recently come under the spotlight due to its potential applications in electronic and optoelectronic devices. However, this synthesis remains a significant challenge. The surface reactivity between the P atoms and the Au atoms should be considered for the P/Au(111) system. In the MBE process, the temperature of the substrate is a key parameter for the growth of blue phosphorene. During the initial growth stage, irregularly shaped Phosphorus clusters grow on top of Au(111) surface at room temperature. When the substrate temperature is increased, these clusters transform into a phosphorene-like structure with a honeycomb lattice. An atom exchange reaction is observed between the P and first layer Au atoms under thermal activation at higher temperature, where the P atoms replace Au atoms to form a blue phosphorene structure within the top Au layer and at the step edges.

We report on the oxidation of self-assembled silicene nanoribbons grown on the Ag (110) surface using scanning tunneling microscopy and high-resolution photoemission spectroscopy. The results show that silicene nanoribbons present a strong resistance towards oxidation using molecular oxygen. This can be overcome by increasing the electric field in the STM tunnel junction above a threshold of +2.6 V to induce oxygen dissociation and reaction. The higher reactivity of the silicene nanoribbons towards atomic oxygen is observed as expected. The HR-PES confirm these observations: even at high exposures of molecular oxygen, the Si 2p core-level peaks corresponding to pristine silicene remain dominant, reflecting a very low reactivity to molecular oxygen. Complete oxidation is obtained following exposure to high doses of atomic oxygen; the Si 2p core level peak corresponding to pristine silicene disappears.

We present an experimental investigation of a new polymorphic 2D single layer of phosphorus on Ag(111). The atomically-resolved scanning tunneling microscopy (STM) images show a new 2D material composed of freely-floating phosphorus pentamers organized into a 2D layer, where the pentamers are aligned in close-packed rows. The scanning tunneling spectroscopy (STS) measurements reveal a semiconducting character with a band gap of 1.20 eV. This work presents the formation at low temperature (LT) of a new polymorphic 2D phosphorus layer composed of a floating 2D pentamer structure. The smooth curved terrace edges and a lack of any clear crystallographic orientation with respect to the Ag(111) substrate at room temperature indicates a smooth potential energy surface that is reminiscent of a liquid-like growth phase. This is confirmed by density functional theory (DFT) calculations that find a small energy barrier of only 0.17 eV to surface diffusion of the pentamers (see Supplemental Material). The formation of extended, homogeneous domains is a key ingredient to opening a new avenue to integrate this new 2D material into electronic devices.

The oxidation of the Ni(111) surface under ultrahigh-vacuum conditions is studied experimentally with low-energy electron diffraction and high-resolution X-ray photoelectron spectroscopy. Exposure of the clean Ni(111) surface to molecular oxygen at room temperature followed by annealing at 400 K leads to the formation of two different structures (2x2) and (3 root 3 x 3 root 3)R30 degrees, prior to the formation of the NiO(111) monolayer. The O 1s core levels indicate that the obtained oxide is terminated by oxygen atoms while the valence band measurements clearly reveal the band gap of NiO. The energy difference between the Fermi level and the maximum of the valance band is extracted and is found to be 0.47 eV.

The long sought-after goal of locally and spectroscopically probing the excitons of two-dimensional (2D) semiconductors is attained using a scanning tunneling microscope (STM). Excitonic luminescence from monolayer molybdenum diselenide (MoSe2) on a transparent conducting substrate is electrically excited in the tunnel junction of an STM under ambient conditions. By comparing the results with photoluminescence measurements, the emission mechanism is identified as the radiative recombination of bright A excitons. STM-induced luminescence is observed at bias voltages as low as those that correspond to the energy of the optical band gap of MoSe2. The proposed excitation mechanism is resonance energy transfer from the tunneling current to the excitons in the semiconductor, i.e., through virtual photon coupling. Additional mechanisms (e.g., charge injection) may come into play at bias voltages that are higher than the electronic band gap. Photon emission quantum efficiencies of up to 10(-7) photons per electron are obtained, despite the lack of any participating plasmons. Our results demonstrate a new technique for investigating the excitonic and optoelectronic properties of 2D semiconductors and their heterostructures at the nanometer scale.

In this paper we report on the first steps of silicene growth on Ag(111) using scanning tunneling microscopy. We show that the topmost atomic layer is composed of both silicon and silver. The STM observations are consistent with an exchange process between the silicon and silver atoms preferentially taking place at the step edges of the Ag substrate. In addition, silicon stripes are observed as precursors of the formation of the silicene sheet.