The remarkable properties of atomically-thin semiconducting TMD layers include an indirect-to-direct bandgap crossover 1, 2, 9 , field-induced transport with high on-off ratios 16 , 3 valley selective circular dichroism [3][4][5][6] , and strong photovoltaic response 17,18 . Fundamental understanding of the electron/hole quasiparticle band structure and many-body interactions in 2D TMDs, however, is still lacking. Enhanced Coulomb interactions due to low-dimensional effects are expected to increase the quasiparticle bandgap as well as to cause electron-hole pairs to form more strongly bound excitons [10][11][12][13] . Untangling such many-body effects in single-layer TMDs requires measurement of both the electronic bandgap and the optical bandgap, the most fundamental parameters for transport and optoelectronics, respectively. The electronic bandgap (E g ) characterizes single-particle (or quasiparticle) excitations and is defined by the sum of the energies needed to separately tunnel an electron and a hole into monolayer MoSe 2 . The optical bandgap (E opt ), on the other hand, describes the energy required to create an exciton, a correlated two-particle electron-hole pair, via optical absorption. The difference in these energies (E g -E opt ) directly yields the exciton binding energy (E b ) (Fig. 2a). Here we provide evidence for Coulomb driven quasiparticle bandgap renormalization and unusually strong exciton stability in 2D TMD through direct determination of both E g and E opt via STS and PL spectroscopy, respectively. STS and PL measurements were carried out on the same high-quality sub-monolayer MoSe 2 films grown on epitaxial bilayer graphene (BLG) on a 6H-SiC(0001) substrate.Because the MoSe 2 surface coverage for our sample was ~ 0.8 ML, we were able to simultaneously image the MoSe 2 monolayer and the underlying graphene substrate using scanning tunneling microscopy (STM). We experimentally investigated both the electronic structure and the optical transitions in monolayer MoSe 2 /BLG by combining STS and PL spectroscopy. Fig. 2b shows a typical STM dI/dV spectrum acquired on monolayer MoSe 2 /BLG. The observed electronic structure is dominated by a large electronic bandgap surrounded by features labeled V 1-4 in the valence band (VB) and C 1 in the conduction band (CB). The MoSe 2 band edges are best determined by taking the logarithm of dI/dV, as shown in Fig. 2d.There the VB maximum (VBM) for monolayer MoSe 2 is seen to be located at -1.55 ± 0.03 V and the CB minimum (CBM) at 0.63 ± 0.02 V. The relative position of E F (V bias = 0 V) with respect to the band edges reveals n-type doping for our samples, although with 5 a very low carrier concentration. We tentatively attribute the n-doping of our MoSe 2 samples to intrinsic point defects such as vacancies and/or lattice antisites, which have been found to be responsible for n-doping in similar materials 20 . Our STS measurements yield a value for the single-particle electronic bandgap of E g = E CBM -E VBM = 2.18 eV ± 0.04 eV. The uncertainty ...

Atomic vacancies have a strong impact in the mechanical, electronic, and magnetic properties of graphenelike materials. By artificially generating isolated vacancies on a graphite surface and measuring their local density of states on the atomic scale, we have shown how single vacancies modify the electronic properties of this graphenelike system. Our scanning tunneling microscopy experiments, complemented by tight-binding calculations, reveal the presence of a sharp electronic resonance at the Fermi energy around each single graphite vacancy, which can be associated with the formation of local magnetic moments and implies a dramatic reduction of the charge carriers' mobility. While vacancies in single layer graphene lead to magnetic couplings of arbitrary sign, our results show the possibility of inducing a macroscopic ferrimagnetic state in multilayered graphene just by randomly removing single C atoms.

Layered transition metal dichalcogenides (TMDs) are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single 2D layer of NbSe2 by means of low temperature scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3x3 CDW order in NbSe2 remains intact in 2D. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of  = 4 meV at the Fermi energy, which is accessible via STS due to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing new insight into CDW formation and superconductivity in this model strongly-correlated system.

A quantum spin Hall (QSH) insulator is a novel twodimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling 1,2 . By investigating the electronic structure of epitaxially grown monolayer 1T'-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulk bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe 2 as a new class of QSH insulator with large bandgap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).A two-dimensional (2D) topological insulator (TI), or a quantum spin Hall insulator, is characterized by an insulating bulk and a conductive helical edge state, in which carriers with different spins counter-propagate to realize a geometry-independent edge conductance 2e 2 /h (refs 1,2). The only scattering channel for such helical edge current is back scattering, which is prohibited by time reversal symmetry, making QSH insulators a promising material candidate for spintronic and other applications.The prediction of the QSH effect in HgTe quantum wells sparked intense research efforts to realize the QSH state [3][4][5][6][7][8][9][10][11] . So far only a handful of QSH systems have been fabricated, mostly limited to quantum well structures of three-dimensional (3D) semiconductors such as HgTe/CdTe (ref.3) and InAs/GaSb (ref. 6). Edge conduction consistent with a QSH state has been observed 3,6,12 . However, the behaviour under a magnetic field, where time reversal symmetry is broken, cannot be explained within our current understanding of the QSH effect 13,14 . There have been continued efforts to predict and investigate other material systems to further advance the understanding of this novel quantum phenomenon 5,[7][8][9]15 . So far, it has been difficult to make a robust 2D material with a QSH state, a platform needed for widespread study and application. The small bandgaps exhibited by many candidate systems, as well as their vulnerability to strain, chemical adsorption, and element substitution, make them impractical for advanced spectroscopic studies or applications. For example, a QSH insulator candidate stanene, a monolayer analogue of graphene for tin, grown on Bi 2 Se 3 becomes topologically trivial due to the modification of its band structure by the underlying substrate 11,16 . Free-standing Bi film with 2D bonding on a cleaved surface has shown edge conduction 9 , but its topological nature is still debated 17 . It takes 3D out-of-plane bonding with the substrate and large stra...

Isolated hydrogen atoms absorbed on graphene are predicted to induce magnetic moments. Here we demonstrate that the adsorption of a single hydrogen atom on graphene induces a magnetic moment characterized by a ~20-millielectron volt spin-split state at the Fermi energy. Our scanning tunneling microscopy (STM) experiments, complemented by first-principles calculations, show that such a spin-polarized state is essentially localized on the carbon sublattice opposite to the one where the hydrogen atom is chemisorbed. This atomically modulated spin texture, which extends several nanometers away from the hydrogen atom, drives the direct coupling between the magnetic moments at unusually long distances. By using the STM tip to manipulate hydrogen atoms with atomic precision, it is possible to tailor the magnetism of selected graphene regions.

Extensive scanning tunneling microscopy and spectroscopy experiments complemented by firstprinciples and parametrized tight binding calculations provide a clear answer to the existence, origin, and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1 to 10 ) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and by calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the role of the layer topographic corrugation and of electronic effects in the apparent moiré contrast measured on the STM images. DOI: 10.1103/PhysRevLett.109.196802 PACS numbers: 73.22.Pr, 61.48.Gh, 68.37.Ef, 73.20.At Soon after the discovery of the unique electronic properties of graphene [1-3], suggestions were made for engineering the band structure of this material. It has been proposed that periodic potentials with wavelengths in the nanometer range could lead to anisotropic renormalization of the velocity of low energy charge carriers [4] or to the generation of new massless Dirac fermions [5]. Experimental works intended for verifying these theoretical predictions were recently reported [6][7][8], where the periodic perturbation was generated either by a lattice mismatch with the supporting material or by a self-organized array of clusters. An alternative route for modifying graphene's band structure would be to exploit a rotation between stacked graphene layers [9]. According to calculations, for large angles ( !15 ) the low energy band structure of graphene should be preserved [10][11][12]. For intermediate angles (1 15 ), it is predicted that, while the linear dispersion persists in the vicinity of the Dirac points of both layers, the band velocity is depressed and two saddle points appear in the band structure, giving rise to two logarithmic van Hove singularities (vHs) in the density of states (DOS) [9,[13][14][15][16][17][18]. For smaller angles ( 1 ) weakly dispersive bands appear at low energy [19,20] with sharp DOS peaks very close to the Dirac point [17,18].Twisted graphene layers are commonly found on different substrates, such as metals [13,21,22], the C face of SiC [23][24][25], or graphite surfaces [26,27]. Transfer techniques yielding large domains of twisted bilayers over a macroscopic sample [28] and quantitative, fast, Raman characterization tools [29,30] have recently been proposed. However, despite the fact that rotated graphene layers are readily available and a number of measurements have confirmed that large twist angles lead to an electronic decoupling of stacked graphene layers [...

We provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries (MTBs) of single-layer semiconducting MoSe 2 . Such MTBs have been previously observed by transmission electron microscopy and have been predicted to be metallic in MoSe 2 and MoS 2 1-7. Our low-temperature scanning tunnelling microscopy/spectroscopy measurements revealed a substantial bandgap of 100 meV opening at the Fermi energy in the otherwise metallic one-dimensional structures. We found a periodic modulation in the density of states along the MTB, with a wavelength of approximately three lattice constants. In addition to mapping the energy-dependent density of states, we determined the atomic structure and bonding of the MTB through simultaneous high-resolution non-contact atomic force microscopy. Density functional theory calculations based on the observed structure reproduced both the gap opening and the spatially resolved density of states.Properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) are highly sensitive to the presence of defects, and a detailed understanding of their structure may lead to tailoring of material properties through 'defect engineering' . Intrinsic defects have been studied extensively in graphene [8][9][10][11][12] . Defects in 2D semiconductors have been explored to a lesser extent, but are expected to substantially modify material properties. 2D TMD semiconductors are particularly interesting because they exhibit direct bandgaps in the visible range [13][14][15] , high charge-carrier mobility 16,17 , extraordinarily enhanced light-matter interactions [18][19][20][21] and potential applications in novel optoelectronic devices 22,23 . Individual atomic-scale defects in 2D TMDs are expected to modify charge transport 24 or introduce ferromagnetism 25 , whereas one-dimensional defects such as grain boundaries and edges may alter electronic 1 and optical properties 1,26 , and introduce magnetic 27 or catalytic 28,29 functionality. Here we report the direct observation of one-dimensional (1D) charge density waves (CDWs) intrinsic to the conducting MTBs of monolayer MoSe 2 . A 1D CDW is a macroscopic quantum state, where atoms in a 1D metallic system relax and break translational symmetry to reduce total energy by opening a small bandgap at the Fermi energy (E F ) and modulating the charge density at the periodicity of the lattice distortion 30,31 . Although CDW order has been observed in 2D TMD metals such as NbSe 2 and TiSe 2 at low temperature 32,33 , CDWs have not previously been associated with 2D TMD semiconductors.Most studies of 1D CDWs have been performed on ensembles of CDWs in conducting polymers, quasi-one-dimensional metals or self-assembled atomic chains adsorbed on semiconducting surfaces, where inter-CDW coupling can significantly impact CDW properties [34][35][36][37][38] . The CDWs observed here are electronically isolated from one another, and have truly one-dimensional character, forming an atomically precise model system t...

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe 2 and WS 2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.