Terahertz (THz) Spintronic emitters based on ferromagnetic/metal junctions have become an important technology for the THz range, offering powerful and ultra-large spectral bandwidths. These developments have driven recent investigations of two-dimensional (2D) materials for new THz spintronic concepts. 2D materials, such as transition metal dichalcogenides (TMDs), are ideal platforms for SCC as they possess strong spin-orbit coupling (SOC) and reduced crystal symmetries. Moreover, SCC and the resulting THz emission can be tuned with the number of layers, electric field or strain. Here, epitaxially grown 1T-PtSe$_2$ and sputtered Ferromagnet (FM) heterostructures are presented as a novel THz emitter where the 1T crystal symmetry and strong SOC favor SCC. High quality of as-grown PtSe$_2$ layers is demonstrated and further FM deposition leaves the PtSe$_2$ unaffected, as evidenced with extensive characterization. Through this atomic growth control, the unique thickness dependent electronic structure of PtSe$_2$ allows the control of the THz emission by SCC. Indeed, we demonstrate the transition from the inverse Rashba-Edelstein effect in one monolayer to the inverse spin Hall effect in multilayers. This band structure flexibility makes PtSe$_2$ an ideal candidate as a THz spintronic 2D material and to explore the underlying mechanisms and engineering of the SCC for THz emission.

Spintronic emitters have become an important THz source with gapless broadband THz emission and the ability to magnetically control the emitted polarization through ultrafast spin-to-charge conversion (SCC). This work has recently driven investigations of two-dimensional (2D) materials for new types of spintronic THz sources. Indeed 2D materials are ideal platforms for spin-to-charge conversion (SCC) as a result of their strong spin-orbit coupling and low crystal symmetries. One of such 2D materials is the transition-metal dichalcogenide PtSe 2 . In this work we present THz spintronic sources based on high quality epitaxially grown CoFeB/PtSe 2 /graphene heterostructures, with PtSe 2 thicknesses ranging from 1 to 15 monolayers. The unique thickness dependent electronic structure of PtSe 2 permits to demonstrate the different origins of the THz emission - from the inverse Rashba-Edelstein effect in monolayer PtSe 2 to the inverse spin Hall effect for multilayers through the strength of the THz emission. This unique bandstructure flexibility makes PtSe 2 an ideal candidate as a THz spintronic 2D material and to further study and explore the underlying mechanisms and engineering of the SCC for THz emission.

We present a study by Scanning Tunneling Microscopy, supported by ab initio calculations, of the interaction between graphene and monolayer (semiconducting) PtSe$_2$ as a function of the twist angle ${\theta}$ between the two layers. We analyze the PtSe$_2$ contribution to the hybrid interface states that develop within the bandgap of the semiconductor to probe the interaction. The experimental data indicate that the interlayer coupling increases markedly with the value of ${\theta}$, which is confirmed by ab initio calculations. The moir\'e patterns observed within the gap are consistent with a momentum conservation rule between hybridized states, and the strength of the hybridization can be qualitatively described by a perturbative model.

We have analyzed by means of scanning tunneling microscopy and spectroscopy the atomic and electronic structure of monolayers of 1T-VxPt1-xSe2 alloys grown by molecular beam epitaxy on epitaxial graphene substrates. We have focused on the composition range (0.1<x<0.35) where ferromagnetic behaviour has recently been demonstrated. For low Pt concentration, (x=0.07 and x=0.21), small domains (a few nanometres in diameter) exhibiting the characteristic superstructure of the charge density wave (CDW) state of pristine VSe2 monolayer remain visible on most of the sample surface. Thus alloying preserves the short range order of the CDW phase, although it destroys its long range order. For higher Pt concentration (x=0.35) a disordered alloy forms. It presents a fully developped gap (a few tens meV in width) at the Fermi level and is thus a disordered insulator. This gap exhibits strong variations at the nanometer scale, reflecting the local fluctuations in the composition. An unexpectedly large interaction of the TMD layer with the graphene substrate sets in for this composition range.

We report on a novel material, namely two-dimensional (2D) V$_{1-x}$Pt$_x$Se$_2$ alloy, exhibiting simultaneously ferromagnetic order and Rashba spin-orbit coupling. While ferromagnetism is absent in 1T-VSe$_2$ due to the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe$_2$ (10-50 %) to form an homogeneous 2D alloy restores ferromagnetic order with Curie temperatures of 6 K for 5 monolayers and 25 K for one monolayer of V$_{0.65}$Pt$_{0.35}$Se$_2$. Moreover, the presence of platinum atoms gives rise to Rashba spin-orbit coupling in (V,Pt)Se$_2$ providing an original platform to study the interplay between ferromagnetism and spin-orbit coupling in the 2D limit.

We study the adsorption of atomic hydrogen on graphene by combining scanning tunneling microscopy experiments and first principle calculations. Our results reveal the existence of a physisorption channel over the graphene layer, dominated by van der Waals forces and thus homogeneous over the whole atomic lattice, where atomic hydrogen can move freely. Such physisorption channel is essential to understand the final configuration of hydrogen atoms chemisorbed on graphene. We find that similar to 95% of chemisorbed H atoms form non-magnetic dimers even for very dilute concentrations (<0.1%) deposited at low temperatures (140 K). Our data shows that this scenario holds from mono to multilayers graphene on SiC(0 0 0 - 1), SiC(0 0 0 1) and graphite.

By means of scanning tunneling microscopy and spectroscopy, we investigate the electronic properties of lead islands (width 5–100 nm, thickness 5–25 monolayers) deposited by molecular beam epitaxy on twisted graphene layers grown on SiC(000-1). We find that elastic scattering processes govern the local density of states probed at the surface of the Pb islands, inducing (i) the well-known quantum well states due to electron confinement in the direction perpendicular to the surface and (ii) spatial in-plane periodic modulations related to quasiparticle interferences off the island edges. Through a quantitative analysis of these effects, compared with ab initio calculations for a two-dimensional Pb slab, we conclude that the lead islands grown on the surface of graphene can be considered as freestanding from the point of view of their electronic structure, leaving the surrounding graphene layer unperturbed. Accordingly, low bias tunneling spectra show evidence of a sizable interface resistance. Nevertheless, we suggest that the transparency of the interface, which can be estimated from its resistance, is good enough to induce superconductivity within the underlying graphene layer by proximity effect with the Pb islands.

We report scanning tunneling microscopy/spectroscopy (STM/STS) investigations of the band-bending in the vicinity of charged point defects and edges of monolayer MoSe2 and mono-and trilayer WSe2 films deposited on graphitized silicon carbide substrates. By tracing the spatial evolution of the structures of the STS spectra, we evaluate the magnitude and the extent of the band-bending to be equal to few hundreds milielectronvolts and several nanometres, respectively. With the aid of a simple electrostatic model, we show that the spatial variation of the Coulomb potential close to the film edges can be well reproduced by taking into account the metallic screening by graphene. Additionally, the analysis of our data for trilayer WSe2 provides reasonable estimations of its dielectric constant (epsilon(WSe2) = 20) and of the magnitude of the charge trapped at the defect site (Q = +e).

We have investigated the electronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs), namely trilayer WSe2 and monolayer MoSe2, deposited on epitaxial graphene on silicon carbide, by using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum. Depending on the number of graphene layers below the TMD flakes, we identified variations in the electronic dI/dV(V) spectra measured by the STM tip: the most salient feature is a rigid shift of the TMD spectra (i.e. of the different band onset positions) towards occupied states by about 120 mV when passing from bilayer to monolayer underlying graphene. Since both graphene phases are metallic and present a work function difference in the same energy range, our measurements point towards the absence of Fermi-level pinning for such van der Waals 2D TMD/Metal heterojunctions, following the prediction of the Schottky-Mott model.

Twisted bilayers (tBL) form a class of graphene based material whose low energy electronic structure can be controlled by a geometric parameter, namely by the rotation angle  between the graphene layers. For undoped (neutral) systems, this property has been established by a number of theoretical [1,2] and experimental studies [3,4]. For large angles (>10°), the layers are electronically decoupled and the low energy band structure looks like a simple superposition of the Dirac cones of the individual graphene planes. For smaller angles, a pair of logarithmic divergences in the density of states (DOS) called van Hove singularities (vHs), related to a saddle point in the band structure, develop within 1 eV from the Fermi level [2-4]. Their energies are almost symmetric with respect to the Dirac point and decrease with . For even smaller angles (<1-2°), flat bands appear at low energy [2], the vHs tend to localize in AA stacked areas [2,3] and additional low energy DOS features related to confinement appear [2]. The twist induced changes in the band structure should be reflected in the physical properties of the bilayers. A rich physics is anticipated in magnetotransport experiments provided the Fermi level EF can be brought in the vicinity or above the vHs [5], although it has not yet been revealed by the experiments reported so far. Structures in the optical conductivity induced by the presence of the vHs have been reported in a wide energy range depending on the value of  [6]. Calculations moreover predict that doping could markedly influence the optical properties when EF reaches the vHs [7]. Owing to the variety of original properties expected upon doping, it is important to determine in a direct way the influence of this parameter on the low energy electronic structure of the twisted graphene layers. This is especially interesting for the doping levels which are accessible using a backgate, typically a few 10 12 cm -2 , to determine for instance at which angle one vHs crosses EF for a given charge. This would additionally allow the experimental investigation of the many body instabilities expected in this configuration [8].

Crystal structure imperfections in solids often act as efficient carrier trapping centers which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are wellwidth or composition fluctuations in semiconductor heterostructures (resulting in a formation of quantum dots) and coloured centers in wide bandgap (e. g., diamond) materials. In the case of recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials, exfoliated on alien substrates. Here, we report on comprehensive optical microspectroscopy studies of thin layers of tungsten diselenide, WSe2, a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of WSe2 flakes, transferred onto Si/SiO2 substrates, we discover centers which, at low temperatures, give rise to sharp emission lines (0.1 meV linewidth). These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to two-dimensional properties of the WSe2 monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities, relevant for the domain of quantum optoelectronics.

The electronic configurations of Fe, Co, Ni and Cu adatoms on graphene and graphite have been studied by x-ray magnetic circular dichroism and charge transfer multiplet theory. A delicate interplay between long-range interactions and local chemical bonding is found to influence the adatom equilibrium distance and magnetic moment. The results for Fe and Co are consistent with purely physisorbed species having, however, different 3d-shell occupations on graphene and graphite (d(n+1) 1 and d(n), respectively). On the other hand, for the late 3d metals Ni and Cu a trend towards chemisorption is found, which strongly quenches the magnetic moment on both substrates.

We show how the weak field magneto-conductance can be used as a tool to characterize epitaxial graphene samples grown from the C or the Si face of Silicon Carbide, with mobilities ranging from 120 to 12000 cm^2/(V.s). Depending on the growth conditions, we observe anti-localization and/or localization which can be understood in term of weak-localization related to quantum interferences. The inferred characteristic diffusion lengths are in agreement with the scanning tunneling microscopy and the theoretical model which describe the "pure" mono-layer and bilayer of graphene [MacCann et al,. Phys. Rev. Lett. 97, 146805 (2006)].

Pseudospin, an additional degree of freedom emerging in graphene as a direct consequence of its honeycomb atomic structure, is responsible for many of the exceptional electronic properties found in this material. This paper is devoted to providing a clear understanding of how graphene's pseudospin impacts the quasiparticle interferences of monolayer (ML) and bilayer (BL) graphene measured by low-temperature scanning tunneling microscopy and spectroscopy. We have used this technique to map, with very high energy and space resolution, the spatial modulations of the local density of states of ML and BL graphene epitaxially grown on SiC(0001), in presence of native disorder. We perform a Fourier transform analysis of such modulations including wave vectors up to unit vectors of the reciprocal lattice. Our data demonstrate that the quasiparticle interferences associated to some particular scattering processes are suppressed in ML graphene, but not in BL graphene. Most importantly, interferences with 2(qF) wave vector associated to intravalley backscattering are not measured in ML graphene, even on the images with highest resolution where the graphene honeycomb pattern is clearly resolved. In order to clarify the role of the pseudospin on the quasiparticle interferences, we use a simple model which nicely captures the main features observed in our data. The model unambiguously shows that graphene's pseudospin is responsible for such suppression of quasiparticle interference features in ML graphene, in particular for those with 2qF wave vector. It also confirms scanning tunneling microscopy as a unique technique to probe the pseudospin in graphene samples in real space with nanometer precision. Finally, we show that such observations are robust with energy and obtain with great accuracy the dispersion of the p bands for both ML and BL graphene in the vicinity of the Fermi level, extracting their main tight-binding parameters.

We investigate the electronic structure of terraces of single layer graphene (SLG) by scanning tunnelling microscopy (STM) on samples grown by thermal decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the perturbations of the local density of states (LDOS) in the vicinity of edges of SLG terraces. Armchair edges are found to favour intervalley quasiparticle scattering, leading to the (√3 x √3)R30° LDOS superstructure already reported for graphite edges and more recently for SLG on SiC(0001). Using the Fourier transform of LDOS images, we demonstrate that the intrinsic doping of SLG is responsible for a LDOS pattern at the Fermi energy which is more complex than for neutral graphene or graphite, since it combines local (√3 x √3)R30° superstructure and long range beating modulation. Although these features have already been reported by Yang et al (2010 Nano Lett. 10 943-7) we propose here an alternative interpretation based on simple arguments classically used to describe standing wave patterns in standard two-dimensional systems. Finally, we discuss the absence of intervalley scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene junctions.

We investigate the graphene-impurity interaction problem by combining experimental - scanning tunneling microscopy (STM) and spectroscopy (STS) - and theoretical - Anderson impurity model and density functional theory (DFT) calculations - techniques. We use graphene on the SiC(000-1)(2x2)_C reconstruction as a model system. The SiC substrate reconstruction is based on silicon adatoms. Graphene mainly interacts with the dangling bonds of these adatoms which act as impurities. Graphene grown on SiC(000-1)(2x2)_C shows domains with various orientations relative to the substrate so that very different local graphene/Si adatom stacking configurations can be probed on a given grain. The position and width of the adatom (impurity) state can be analyzed by STM/STS and related to its local environment owing to the high bias electronic transparency of graphene. The experimental results are compared to Anderson's model predictions and complemented by DFT calculations for some specific local environments. We conclude that the adatom resonance shows a smaller width and a larger shift toward the Dirac point for an adatom at the center of a graphene hexagon than for an adatom just on top of a C graphene atom.

Received 9 November 2012DOI:https://doi.org/10.1103/PhysRevLett.109.209905© 2012 American Physical Society

Extensive scanning tunneling microscopy and spectroscopy experiments complemented by first-principles 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.

We examine in detail the structure and evolution upon annealing of the SiC(3 x 3) reconstruction which is known to be present at the interface between the SiC-C face substrate and the graphene layer for samples prepared in high vacuum. We use ab initio calculations to test the validity of proposed or classical structural models in comparison with scanning tunnelling microscopy (STM) images. We analyse the electronic structure of the bare surface and detect interface states which can pin the surface Fermi level. From a comparison of the signal coming from the bare and graphene-covered SiC(3 x 3) reconstruction we propose that the transparency of the graphene in high-bias STM images results from an enhancement of the local density of states of the interface plane by the graphene layer. We discuss the thermal stability of the SiC(3 x 3) surface, and show that it transforms more easily into the SiC(2 x 2) C reconstruction in the graphene-covered region than for the bare surface. This evolution generates both structural and electronic heterogeneities at the interface.

It has been shown that the first C layer on the SiC(0001)(2{\times}2)C surface already exhibits graphene-like electronic structure, with linear pi bands near the Dirac point. Indeed, the (2{\times}2)C reconstruction, with a Si adatom and C restatom structure, efficiently passivates the SiC(0001) surface thanks to an adatom/restatom charge transfer mechanism. Here, we study the effects of interface modifications on the graphene layer using density functional theory calculations. The modifications we consider are inspired from native interface defects observed by scanning tunneling microscopy. One H atom per 4 {\times} 4 SiC cell (5 {\times} 5 graphene cell) is introduced in order to saturate a restatom dangling bond and hinder the adatom/restatom charge transfer. As a consequence, the graphene layer is doped with electrons from the substrate and the interaction with the adatom states slightly increases. Native interface defects are therefore likely to play an important role in the doping mechanism on the C terminated SiC substrates. We also conclude that an efficient passivation of the C face of SiC by H requires a complete removal of the reconstruction. Otherwise, at variance with the Si terminated SiC substrates, the presence of H at the interface would increase the graphene/substrate interaction.