Observation of charge density wave order in 1D mirror twin boundaries of single-layer MoSe 2

the of the crystal structure. detailed of defect structure to of properties through “defect engineering”. Here we provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries in single-layer MoSe 2 . Our low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a substantial bandgap of 60 140 meV opening at the Fermi level in the otherwise one dimensional metallic structure. We find an energy-dependent periodic modulation in the density of states along the mirror twin boundary, with a wavelength of approximately three lattice constants. The modulations in the density of states above and below the Fermi level are spatially out of phase, consistent with charge density wave order . In addition to the electronic characterization, we determine the atomic structure and bonding configuration of the one-dimensional mirror twin boundary by means of high-resolution non-contact atomic force microscopy. Density functional theory calculations reproduce both the gap opening and the modulations of the density of states. Here we report the direct observation of one-dimensional charge density waves (CDWs) at mirror twin boundaries in monolayer MoSe 2 , a 2D-TMD semiconductor. A 1D CDW is a macroscopic quantum state, where atoms in a 1D metallic system relax and break translational symmetry to reduce electronic energy by opening a small bandgap at the Fermi energy and modulating the charge density at the periodicity of the lattice distortion 24,25 . While CDW order has been observed in 2D-TMD metals such as NbSe 2 and TiSe 2 at low temperature 26,27 , CDWs have not previously been associated with 2D-TMD semiconductors. High-resolution STM images acquired on monolayer MoSe 2 grown on bilayer graphene (BLG) show a periodic superstructure (moiré pattern) on top of the atomic lattice periodicity. This moiré pattern is due to the different lattice parameters of MoSe 2 and graphene unit cells, as well as the relative stacking orientation between them (angle θ). The most commonly observed moiré pattern for single-layer MoSe 2 grown on BLG by molecular beam epitaxy arises from the overlay of the MoSe 2 lattice aligned (θ=0°) respect to the BLG lattice. A

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