Two-or three-dimensional metals are usually well described by weakly interacting, fermionic quasiparticles. This concept breaks down in one dimension due to strong Coulomb interactions. There, low-energy electronic excitations are expected to be bosonic collective modes, which fractionalize into independent spin and charge density waves. Experimental research on one-dimensional metals is still hampered by their difficult realization, their limited accessibility to measurements, and by competing or obscuring effects such as Peierls distortions or zero bias anomalies. Here we overcome these difficulties by constructing a well-isolated, one-dimensional metal of finite length present in MoS2 mirror twin boundaries. Using scanning tunneling spectroscopy we measure the single-particle density of the interacting electron system as a function of energy and position in the 1D box. Comparison to theoretical modeling provides unambiguous evidence that we are observing spin-charge separation in real space.

Based on an ultra-high vacuum compatible two-step molecular beam epitaxy synthesis with elemental sulphur, we grow clean, well-oriented, and almost defect-free monolayer islands and layers of the transition metal disulphides MoS2, TaS2 and WS2. Using scanning tunneling microscopy and low energy electron diffraction we investigate systematically how to optimise the growth process, and provide insight into the growth and annealing mechanisms. A large band gap of 2.55 eV and the ability to move flakes with the scanning tunneling microscope tip both document the weak interaction of MoS2 with its substrate consisting of graphene grown on Ir(1 1 1). As the method works for the synthesis of a variety of transition metal disulphides on different substrates, we speculate that it could be of great use for providing hitherto unattainable high quality monolayers of transition metal disulphides for fundamental spectroscopic investigations.

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.

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 prepared monolayers of tantalum sulfide on Au(111) by evaporation of Ta in a reactive background of H2S. Under sulfur-rich conditions, monolayers of 2H-TaS2 formed, whereas under sulfur-poor conditions TaS2–x with 0 ≤ x ≤ 1 were found. We identified this phase as TaS, a structure that can be derived from 2H-TaS2 by removal of the bottom S layer.