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Scanning Tunneling Microscopy of Atomic Scale Phonon Standing Waves in Quasi-freestanding WSe2 Monolayers

Published online by Cambridge University Press:  26 February 2016

Igor Altfeder*
Affiliation:
Nanoelectronic Materials Branch, Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA
Sarah M. Eichfeld
Affiliation:
Department of Materials Science and Engineering and The Center for Two-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA
Rachel D. Naguy
Affiliation:
Nanoelectronic Materials Branch, Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA
Joshua A. Robinson
Affiliation:
Department of Materials Science and Engineering and The Center for Two-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA
Andrey A. Voevodin
Affiliation:
Nanoelectronic Materials Branch, Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA

Abstract

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Using scanning tunneling microscopy (STM) we observed atomic scale interference patterns on quasi-freestanding WSe2 islands grown on top of graphene. The bias-independent double atomic size periodicity of these patterns and the sharp Brillouin zone edge revealed by 2D STM Fourier analysis indicate formation of optical phonon standing waves due to scattering on intercalating defects supporting these islands. Standing wave patterns of both synchronized and non-synchronized optical phonons, corresponding to resonant and non-resonant phonon scattering regimes, were experimentally observed. We also found the symmetry breaking effect for individual phonon wave packets, one of the unique features distinguishing phonon standing waves. We show that vibrational and electronic anharmonicities are responsible for STM detection of these patterns. A significant contribution to the interference contrast arises from quantum zero-point oscillations.

Type
Articles
Copyright
Copyright © Materials Research Society 2016 

References

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