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.