Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation.

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.

Structural properties of Bi/Au(110).

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.

Electron beam analysis induces Cl vacancy defects in a NaCl thin film.

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-.

Blue phosphorene reactivity on the Au(111) surface.

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.

Epitaxial Synthesis of Blue Phosphorene.

Phosphorene is a new 2D material composed of a single or few atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable direct bandgap and high carrier mobility values, which make it suitable for a large variety of optical and electronic devices. However, the synthesis of single-layer phosphorene is a major challenge. The standard procedure to obtain phosphorene is by exfoliation. More recently, the epitaxial growth of single-layer phosphorene on Au(111) was investigated by molecular beam epitaxy and the obtained structure described as a blue phosphorene sheet. In the present study, large areas of high-quality monolayer phosphorene, with a bandgap value equal to at least 0.8 eV, are synthesized on Au(111). The experimental investigations, coupled with density functional theory calculations, give evidence of two distinct phases of blue phosphorene on Au(111), instead of one as previously reported, and their atomic structures are determined.

Sub-molecular spectroscopy and temporary molecular charging of Ni-phthalocyanine on graphene with STM.

In this study, the self-assembled molecular network and electronic properties of Ni-phthalocyanine (NiPc) molecules on monolayer graphene (MLG)/6H-SiC(0001) were studied by room temperature Scanning Tunnelling Microscopy (STM) and Density Functional Theory (DFT) calculations. In this study, a very weak electronic coupling between the graphene and the NiPc molecules is found. This is due to the very small charge transfer of only 0.035e- per molecule. The weak molecule-graphene interaction has two observable consequences: sub-molecular resolution was obtained in the STM spectroscopy at room-temperature with the molecules adsorbed directly on the graphene, and the occupied and unoccupied molecular resonance peaks were observed to shift their position in energy as a function of the tip-surface distance. This is due to the temporary local charging (either positive or negative) that is achieved by decreasing the surface voltage under the STM tip. This may have important consequences for future studies of the opto-electronic properties of such hybrid graphene-molecule systems.

Molecular Dissociation on the SiC(0001) 3×3 Surface.

In the framework of density functional theory, the adsorption of the halogenated polycyclic aromatic hydrocarbon 2,11-diiodohexabenzocoronene (HBC-I2 ) on the SiC(0001) 3×3 surface has been investigated. Nondissociative and dissociative molecular adsorption is considered, and simulated scanning tunneling microscopy (STM) images are compared with the corresponding experimental observations. Calculations show that dissociative adsorption is favorable and reveal the crucial importance of the extended flat carbon core on molecule-surface interactions in dissociative adsorption; the iodine atom-surface interaction is of minor importance. Indeed, removing iodine atoms does not significantly affect the STM images of the central part of the molecule. This study shows that the dissociation of large halogenated polycyclic aromatic hydrocarbon molecules can occur on the SiC surface. This opens up interesting perspectives in the chemical reactivity and functionalization of wide band gap semiconductors.

Formation of unconventional standing waves at graphene edges by valley mixing and pseudospin rotation.

We investigate the roles of the pseudospin and the valley degeneracy in electron scattering at graphene edges. It is found that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly decaying beat patterns in the electronic density profile. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from scanning tunneling microscopy. The armchair edge shows almost perfect intervalley scattering with pseudospin invariance regardless of the presence of the hydrogen atom at the edge, whereas the zigzag edge only allows for intravalley scattering with the change in the pseudospin orientation. The effect of structural defects at the graphene edges is also discussed.

Quantum interference channeling at graphene edges.

Electron scattering at graphene edges is expected to make a crucial contribution to the electron transport in graphene nanodevices by producing quantum interferences. Atomic-scale scanning tunneling microscopy (STM) topographies of different edge structures of monolayer graphene show that the localization of the electronic density of states along the C-C bonds, a property unique to monolayer graphene, results in quantum interference patterns along the graphene carbon bond network, whose shapes depend only on the edge structure and not on the electron energy.

Flat epitaxial quasi-1D phosphorene chains.

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.

Tip-induced oxidation of silicene nano-ribbons.

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.

SiC(0001) 3 x 3 heterochirality revealed by single-molecule STM imaging.

We report a description of the SiC(0001) 3 x 3 silicon carbide reconstruction based on single-molecule scanning tunneling microscopy (STM) observations and density functional theory calculations. We show that the SiC(0001) 3 x 3 reconstruction can be described as contiguous domains of right and left chirality distributed at the nanoscale, which breaks the to date supposed translational invariance of the surface. While this surface heterochirality remains invisible in STM topographies of clean surfaces, individual metal-free phthalocyanine molecules chemisorbed on the surface act as molecular lenses to reveal the surface chirality in the STM topographies. This original method exemplifies the ability of STM to probe atomic-scale structures in detail and provides a more complete vision of a frequently studied SiC reconstruction.

Compelling experimental evidence of a Dirac cone in the electronic structure of a 2D Silicon layer.

The remarkable properties of graphene stem from its two-dimensional (2D) structure, with a linear dispersion of the electronic states at the corners of the Brillouin zone (BZ) forming a Dirac cone. Since then, other 2D materials have been suggested based on boron, silicon, germanium, phosphorus, tin, and metal di-chalcogenides. Here, we present an experimental investigation of a single silicon layer on Au(111) using low energy electron diffraction (LEED), high resolution angle-resolved photoemission spectroscopy (HR-ARPES), and scanning tunneling microscopy (STM). The HR-ARPES data show compelling evidence that the silicon based 2D overlayer is responsible for the observed linear dispersed feature in the valence band, with a Fermi velocity of comparable to that of graphene. The STM images show extended and homogeneous domains, offering a viable route to the fabrication of silicene-based opto-electronic devices.

Silicon sheets by redox assisted chemical exfoliation.

In this paper, we report the direct chemical synthesis of silicon sheets in gram-scale quantities by chemical exfoliation of pre-processed calcium disilicide (CaSi2). We have used a combination of x-ray photoelectron spectroscopy, transmission electron microscopy and energy-dispersive x-ray spectroscopy to characterize the obtained silicon sheets. We found that the clean and crystalline silicon sheets show a two-dimensional hexagonal graphitic structure.

Selective internal manipulation of a single molecule by scanning tunneling microscopy.

We have studied the adsorption of the polyaromatic molecule 1,4"-paratriphenyldimethylacetone, which we have nicknamed Trima. The originality of this linear molecule is that it was designed and synthesized to have two functionalities. First, chemisorb itself to the surface by its two ends rather like a bridge. Second, the central part of the molecule could then be rotated by injecting electrons with the tip of the scanning tunneling microscope (STM). The length of the molecule corresponds exactly to the spacing between five dimers in a row on the Si(100)-2 x 1 surface. We found that the molecule adsorbs as expected on the clean silicon surface by using complementary STM and synchrotron radiation studies. Manipulation of individual molecules with the STM tip showed selective internal modifications that were highly voltage dependent. These manipulations were found to be compatible with an electronic excitation of the pi-pi* transition of the molecule.

Temperature control of electronic channels through a single atom.

The elastic electronic channels through a single hydrogen atom adsorbed on a Ge(111)-c(2 x 8) surface have been investigated by scanning tunneling microscopy and I(V) spectroscopy, whereas inelastic channels have been probed by the vertical and horizontal manipulation of individual hydrogen atoms. The substrate temperature, over the range 30-300 K, has proven to be a powerful parameter to freeze specific electronic channels, offering the possible control of elastic and inelastic channels through a single atom. This opens up very interesting perspectives for controlling the operation of nanodevices.