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

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.

• The paper demonstrates 1D CDW order at mirror twin boundaries in MoSe₂ with a measured bandgap of 60–140 meV.
• Researchers used low-temperature STM/STS, nc-AFM, and DFT calculations to explore both the electronic and atomic structures.
• The study highlights how defect-induced Peierls distortions modulate electronic energy, informing the design of advanced 2D optoelectronic devices.

Observation of Charge Density Wave Order in 1D Mirror Twin Boundaries of Single-Layer MoSe$_2$

The study at hand explores the observation and characterization of charge density wave (CDW) order at mirror twin boundaries (MTBs) in monolayer molybdenum diselenide (MoSe$_2$), a 2D transition metal dichalcogenide (TMD). The paper represents a noteworthy investigation of CDWs in a semiconducting TMD, a phenomenon typically associated with metallic systems. Researchers employed a combination of low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), high-resolution non-contact atomic force microscopy (nc-AFM), and density functional theory (DFT) calculations to probe both the electronic and atomic structure of the MTBs.

The findings reveal the existence of isolated, one-dimensional CDWs along MTBs, evidenced by a bandgap of 60-140 meV at the Fermi level in the one-dimensional metallic structure within the MoSe$_2$. This is a manifestation of a reduction in electronic energy achieved through bandgap opening and charge density modulation with a periodicity of approximately three lattice constants. Interestingly, the charge distribution modulations above and below the Fermi level were noted to be spatially out of phase, a hallmark of CDW order.

A substantial part of the research discusses the atomic structure of the MTBs, elucidated through nc-AFM. The defect line, comprised of a continuous hexagonal lattice of selenium atoms and mirrored positions of molybdenum atoms, indicates a mirror symmetry across the MTB. Coupling this structural insight with DFT calculations, the study demonstrates that the MTBs induce a metallic band crossing at the Fermi level, which is absent in a pristine MoSe$_2$ monolayer. The Peierls distortion, induced via calculated lattice distortions commensurate with the MTB periodicity, effectively described the gap opening at the zone edge, aligning well with experimental observations.

This work is pivotal in illustrating the intrinsic electronic behavior of MTBs in 2D TMDs and the role of such defects in modulating electronic properties. The implications of these findings are substantial, offering a deeper understanding of defect-induced CDWs and paving the way for engineered defect structures in 2D materials to optimize electronic properties for applications in optoelectronic devices.

Theoretical implications of the study include refined modeling of 1D correlated systems within 2D materials frameworks and exploration of phenomena like Fröhlich conduction and pinning effects due to defects or adatoms. Future developments could explore the integration of controlled defects into multifunctional devices, advancing diverse applications from nanoscale energy transport systems to quantum computing components.

In summary, the paper contributes a detailed exploration into the interface of structural defects and electronic modulation, showcasing the utility of sophisticated microscopy and theoretical techniques to uncover nuanced behaviors in atomic-scale systems, and indicating potential avenues for technological exploitation of CDWs in 2D materials.