Exploring linear defects in 2D semiconductors

May 18, 2016 // By Julien Happich
In a recent paper published in Nature Physics, "Charge density wave order in 1D mirror twin boundaries of single-layer MoSe2", scientists from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley highlight how linear defects in 3-atom thin layers of molybdenum diselenide affect the material's electronic behaviour.

Under the scrutiny of a modified scanning tunneling microscope, which the scientists “sharpened” by placing a single carbon monoxide molecule at the very end of the probing tip, the MoSe2 2D material revealed linear defects formed by lines of missing selenium atoms, creating one-atom thick metallic wires. Also described as 1D mirror twin boundaries across the 2D semiconductor sheets, the defects when cooled down to -269ºC (about 4ºK) caused the atoms along the metallic wires to rearrange themselves, giving place to a charge density wave (the atoms’ electrons no longer being uniformly distributed, but instead, modulated like a sinusoidal wave along the metallic wires).

According to the researchers, the presence of a charge density wave is especially intriguing because it indicates a strong coupling between the electrons, mediated by the atomic lattice, noting that similar strong coupling happens in superconducting states.

The left microscopy image shows linear defects that cross the 2-D semiconductor like veins. The defects are located between the parallel lines. The right image is a combination of the theoretical atomic structure on the bottom, and a microscopy image on top that shows individual selenium atoms in gold and the charge density wave in red. (Credit: Berkeley Lab)

Although they used liquid nitrogen to reach the 4ºK temperature of their experiments, the charge density wave was still observable at higher temperatures, which pushes the researchers to investigate other types of Transition Metal Dichalcogenides (TMDs) in search of new high temperature superconductors.

The research was conducted by Alexander Weber-Bargioni, D. Frank Ogletree, Sara Barja, Sebastian Wickenburg, Zhen-Fei Liu, and Jeff Neaton of Berkeley Lab’s Molecular Foundry. In addition, scientists from Berkeley Lab’s Advanced Light Source and Materials Sciences Division contributed to the research.

Visit the Lawrence Berkeley National Laboratory at  www.lbl.gov


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