title: Strong Asymmetric Charge Carrier Dependence in Inelastic Electron Tunneling Spectroscopy of Graphene Phonons.
abstract: The observation of phonons in graphene by inelastic electron tunneling spectroscopy has been met with limited success in previous measurements arising from weak signals and other spectral features which inhibit a clear distinction between phonons and miscellaneous excitations. Utilizing a back-gated graphene device that allows adjusting the global charge carrier density, we introduce an averaging method where individual tunneling spectra at varying charge carrier density are combined into one representative spectrum. This method improves the signal for inelastic transitions while it suppresses dispersive spectral features. We thereby map the total graphene phonon density of states, in good agreement with density functional calculations. Unexpectedly, an abrupt change in the phonon intensity is observed when the graphene charge carrier type is switched through a variation of the back-gate electrode potential. This sudden variation in phonon intensity is asymmetric in the carrier type, depending on the sign of the tunneling bias.
authors.affiliation: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: Fabian D
authors.initials: FD
authors.lastname: Natterer
authors.affiliation: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.|Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Yue
authors.initials: Y
authors.lastname: Zhao
authors.affiliation: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Wyrick
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
authors.forename: Yang-Hao
authors.initials: YH
authors.lastname: Chan
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
authors.forename: Wen-Ying
authors.initials: WY
authors.lastname: Ruan
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.|School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
authors.forename: Mei-Yin
authors.initials: MY
authors.lastname: Chou
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
authors.forename: Kenji
authors.initials: K
authors.lastname: Watanabe
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
authors.forename: Takashi
authors.initials: T
authors.lastname: Taniguchi
authors.affiliation: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.|Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044 Japan.
authors.forename: Nikolai B
authors.initials: NB
authors.lastname: Zhitenev
authors.affiliation: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Stroscio