Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, epitaxial graphene/single-crystal 2D gallium, indium, and tin heterostructures. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to graphene overlayer, i.e. they are "half van der Waals" metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals open compelling opportunities for superconducting devices, topological phenomena, and advanced optoelectronic properties. For example, the reported 2D-Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly-freeelectron Fermi surface that closely approaches the Dirac points of the graphene overlayer.Major advances in fundamental science have followed from the exfoliation, stacking, and encapsulation of atomically thin 2D layers 1 . The next step towards technological impact of 2D layers and heterostructures is to transition sophisticated "pick and place" devices to a wafer-scale platform. However, the sensitivity of 2D systems to interfacial reactions and environmental influences -especially for two-dimensional metals or small-gap semiconductors -poses challenges for large-scale integration. Very few metals resist degradation of their top few atomic layers upon environmental exposure, and for a 2D metal, these layers constitute the entire system. A general platform for producing environmentally stable and wafer-scale 2D metals that are not prone to interfacial interactions would represent a significant advance. Inspired by the success of wide-bandgap 2D gallium nitride 2 , we turn focus onto the metal alone and demonstrate a platform dubbed confinement heteroepitaxy (CHet), where the interface between epitaxial graphene (EG) and silicon carbide (SiC) stabilizes crystalline 2D forms of Group-III (Ga, In) and group-IV (Sn) elements. Defect engineering of the graphene overlayer enables uniform, large-area intercalation at the high-energy SiC/EG interface; this interface then templates intercalant crystallization at a thermodynamically defined number of atomic layers. The unreactive nature of as-grown EG on SiC (graphene plus buffer layer) performs multiple services: (1) it only partially passivates the SiC surface underneath, thereby sustaining the high-energy interface that drives intercalation; (2) it lowers the energy of the (otherwise exposed) upper surface of the metal, thus facilitating 2D morphologies; (3) it protects the newly formed 2D metal from environmental degradation after intercalation through in situ healing of the graphene defects. Stability of these 2D metals in air over months gr...

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS 2 ) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (Cr W ) and molybdenum (Mo W ) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (O S top/ bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. Cr W forms three deep unoccupied defect states, two of which arise from spin− orbit splitting. The formation of such localized trap states for Cr W differs from the Mo W case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions O S and Cr W are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between Cr W dimers. The important role of charge localization, spin−orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS 2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping. Here, we report the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS 2 by a combination of CO-tip noncontact atomic force microscopy and scanning tunneling microscopy. Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states followed by vibronic sidebands provide a unique fingerprint of this defect.Direct imaging of the defect orbitals reveals that the large splitting of 252 ± 4 meV between these defect states is induced by spin-orbit coupling.

The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by (i) the spontaneous polarization of the substrate, and (ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC (Ristein et al 2012 Phys. Rev. Lett. 108 246104).

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In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable bandgaps 1-3 and strongly bound excitons and trions emerge from strong many-body effects [4][5][6] , beyond the spin and valley degrees of freedom induced by spin-orbit coupling and by lattice symmetry 7 . Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions [8][9][10] . Here, we use micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping to manipulate the electronic structure of single-layer WS 2 on hexagonal boron nitride (WS 2 /h-BN). Upon electron doping, we observe an unexpected giant renormalization of the spin-orbit splitting of the single-layer WS 2 valence band, from 430 meV to 660 meV, together with a bandgap reduction of at least 325 meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials [11][12][13] . Coulomb interactions in 2D materials are several times stronger than in their 3D counterparts. In 2D TMDs, this is most directly evidenced by the presence of excitons with binding energies an order of magnitude higher than in the bulk 4 . Although the excitons in these 2D materials have been widely studied by optical techniques 13 , the impact of strong electron-electron interactions on the quasiparticle band structure remains unclear. Theory predicts that many-body effects will influence the spin-orbit splitting around the valenceband maximum (VBM) and conduction-band minimum (CBM) 14 . Although these should be observable by ARPES, a direct probe of many-body effects 15 , measurements so far have mainly focused on the layer-dependence of the single-particle spectrum and the direct bandgap transition in 2D TMD systems, including epitaxial single- . Unfortunately, the lateral size of mechanically assembled heterostructures is usually of the order of 10 μ m, much smaller than the beam spot of typical ARPES setups (≳ 100 μ m). Furthermore, sample charging on insulating bulk h-BN substrates would complicate ARPES experiments.We overcome these challenges as follows. We realize a high-quality 2D semiconductor-insulator interface by mechanically transferring a relatively large (~100 μ m) single-layer WS 2 crystal onto a thin flake of h-BN that has itself been transferred onto a degenerately doped TiO 2 substrate, as depicted in Fig. 1a. Sample charging is avoided because there is electrical contact from the continuous single-layer WS 2 flake to both the h-BN and the conductive TiO 2 . Figure 1b is an optical microscope image of the sample, including a flake of h-BN, approximately 100 μ m wide, surrounded by several transferred flakes of single-layer WS 2 on the Ti...

The doping of graphene to tune its electronic structure is essential for its further use in carbon based electronics. Adapting strategies from classical silicon based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni (111) calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller adsorption distance of boron compared to carbon leads to a bending of the graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective free-standing cases, we are able to distinguish between the effects that doping and adsorption have on the band structure of graphene. Both, doping and bonding to the surface, result in opposing shifts on the graphene bands.

We report on strong coupling of the charge carrier plasmon ωP L in graphene with the surface optical phonon ωSO of the underlying SiC(0001) substrate with low electron concentration (n = 1.2 × 10 15 cm −3 ) in the long wavelength limit (q → 0). Energy dependent energy-loss spectra give for the first time clear evidence of two coupled phonon-plasmon modes ω± separated by a gap between ωSO (q → 0) and ωT O (q >> 0), the transverse optical phonon mode, with a Fanotype shape, in particular for higher primary electron energies (E0 ≥ 20eV ). A simplified model based on dielectric theory is able to simulate our energy -loss spectra as well as the dispersion of the two coupled phonon-plasmon modes ω±. In contrast, Liu and Willis [1] postulate in their recent publication no gap and a discontinuous dispersion curve with a one-peak structure from their energy-loss data. PACS numbers:The graphene silicon carbide heterosystem is a promising system for the future application of graphene in micro-and nanoelectronics [2,3]. Silicon carbide as a substrate for microelectronics is already used industrially and the epitaxial growth of graphene on silicon carbide has already been investigated for several years now [2,4], and perfectionalized towards wafer scale homogeneous graphene [5]. Still, many of the interactions between the graphene and the silicon carbide substrate have yet to be understood. For example, the carrier dynamics may be strongly influenced by the long-range coupling to the polar modes of the substrate [6], which possibly results in a strong reduction of the graphene mobility, if compared to free standing graphene. This remote scattering can be important in future graphene devices.In this contribution we report about our experimental investigation of the carriers in the conduction channel with the long-range polarization field created at the conductor/dielectric interface. Emphasis is also given to the theoretical interpretation of the experimental inelastic electron scattering results by calculating the dielectric surface loss function. The coupling of collective electron (or hole) modes with optical phonons in semiconductors (e.g. InN, InP, GaAs and others) has already been a target of extensive investigations and helped to understand important interface characteristics [7][8][9]. Unlike these conventional two dimensional electron gas systems (2DEG), graphene exhibits a linear electron dispersion relation, but the plasmon dispersion remains [10]. Furthermore, the almost vanishing damping of the plasmon mode and the strong spatial confinement, in contrast to other sheet plasmons observed so far [11,12], makes it a showcase model for the investigation of the coupled phonon plasmon modes.Inelastic electron scattering utilizing special high resolution monochromators and analyzers, also known as high resolution electron energy loss spectroscopy (HREELS), is used in surface science to investigate intentional and unintentional adsorbates as well as surface phonon and plasmon modes on a wide variety of materials [13]. Di...