  Properties of two-dimensional transition metal dichalcogenides are highly
sensitive to the presence of defects in the crystal structure. A detailed
understanding of defect structure may lead to control of material 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 MoSe2. 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.
