For quasi-freestanding 2H-TaS 2 in monolayer thickness grown by in situ molecular beam epitaxy on graphene on Ir(111), we find unambiguous evidence for a charge density wave close to a 3 × 3 periodicity. Using scanning tunneling spectroscopy, we determine the magnitude of the partial charge density wave gap. Angle-resolved photoemission spectroscopy, complemented by scanning tunneling spectroscopy for the unoccupied states, makes a tight-binding fit for the band structure of the TaS 2 monolayer possible. As hybridization with substrate bands is absent, the fit yields a precise value for the doping of the TaS 2 layer. Additional Li doping shifts the charge density wave to a 2 × 2 periodicity. Unexpectedly, the bilayer of TaS 2 also displays a disordered 2 × 2 charge density wave. Calculations of the phonon dispersions based on a combination of density-functional theory, density-functional perturbation theory, and many-body perturbation theory enable us to provide phase diagrams for the TaS 2 charge density wave as functions of doping, hybridization and interlayer potentials, and offer insight into how they affect lattice dynamics and stability. Our theoretical considerations are consistent with the experimental work presented and shed light on previous experimental and theoretical investigations of related systems.

In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal–insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS2 realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS2. At the Fermi level, the CDW induces a topological metal-metal (Lifshitz) transition. Non-linear coupling of transverse and longitudinal phonons is essential for the formation of the CDW and the full gap above the Fermi level. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to coexisting charge and spin density waves in the ground state.

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS 2 , grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at 5 K. Strong upwards bending of valence and conduction bands towards the line defects is found for the 4|4E mirror twin boundary and island edges, but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upwards band bending takes place. Focusing on the 1 arXiv:2007.06313v1 [cond-mat.mes-hall] 13 Jul 2020 common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS 2 mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS 2. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary. Keywords band bending, scanning tunnelling spectroscopy, MoS 2 , polarization charge, mirror twin boundary Coupled to the rise of MoS 2 and other transition metal dichalcogenide (TMDC) semiconductors as prospective two-dimensional (2D) device materials came the need to investigate their one-dimensional (1D) defect structures, e.g. grain boundaries (GBs). Depending on their structure, GBs impair device performance to differing degrees when positioned in the channel of a single layer MoS 2 field effect transistor. 1-4 It is thus evident that control of the type and concentration of GBs is of importance for device fabrication. Besides satisfying scientific curiosity, it therefore pays to understand their effect on band structure and charge carrier transport. The lowest energy GBs are those hardest to avoid during growth, as the energy penalty associated with their introduction is marginal. In the three-dimensional (3D) world, these low energy GBs are 2D stacking faults or twin planes. For the case of SiC devices such defects cause increased leakage current, reduced blocking voltage, and the degradation of bipolar devices. 5,6 In the world of 2D materials, the analog to twin planes is 1D mirror twin boundaries (MTBs). These structural defects have some surprising effects on the band structure of monolayer MoS 2 , to be investigated in this manuscript.

We apply scanning tunneling spectroscopy to determine the bandgaps of mono-, bi-and trilayer MoS2 grown on a graphene single crystal on Ir(111). Besides the typical scanning tunneling spectroscopy at constant height, we employ two additional spectroscopic methods giving extra sensitivity and qualitative insight into the k-vector of the tunneling electrons. Employing this comprehensive set of spectroscopic methods in tandem, we deduce a bandgap of 2.53 ± 0.08 eV for the monolayer. This is close to the predicted values for freestanding MoS2 and larger than is measured for MoS2 on other substrates. Through precise analysis of the 'comprehensive' tunneling spectroscopy we also identify critical point energies in the mono-and bilayer MoS2 band structures. These compare well with their calculated freestanding equivalents, evidencing the graphene/Ir(111) substrate as an excellent environment upon which to study the many feted electronic phenomena of monolayer MoS2 and similar materials. Additionally, this investigation serves to expand the fledgling field of the comprehensive tunneling spectroscopy technique itself. arXiv:1903.08601v1 [cond-mat.mes-hall]

Much effort has been made to modify the properties of transition metal dichalcogenide layers via their environment as a route to new functionalization. However, it remains a challenge to induce large electronic changes without chemically altering the layer or compromising its two-dimensionality. Here, a non-invasive technique is used to shift the chemical potential of monolayer MoS2 through p and n-type doping of graphene (Gr), which remains a well-decoupled 2D substrate. With the intercalation of oxygen (O) under Gr, a nearly rigid Fermi level shift of 0.45 eV in MoS2 is demonstrated, whereas the intercalation of europium (Eu) induces a metal-insulator transition in MoS2, accompanied by a giant band gap reduction of 0.67 eV. Additionally, the effect of the substrate charge on 1D states within MoS2 mirror-twin boundaries (MTBs) is explored. It is found that the 1D nature of the MTB states is not compromised, even when MoS2 is made metallic. Furthermore, with the periodicity of the 1D states dependent on substrate-induced charging and depletion, the boundaries serve as chemical potential sensors functional up to room temperature.