Observation of charge density wave order in 1D mirror twin boundaries of single-layer MoSe2

TL;DR

This study provides direct evidence of charge density wave order at mirror twin boundaries in single-layer MoSe2, revealing a bandgap opening and periodic density modulations, advancing understanding of defect-induced electronic properties in 2D materials.

Contribution

It is the first to directly observe charge density waves at mirror twin boundaries in single-layer MoSe2 using combined microscopy, spectroscopy, and theoretical calculations.

Findings

Charge density waves observed at twin boundaries.

Bandgap of 60-140 meV opens at the Fermi level.

Density functional theory supports experimental results.

Abstract

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.