| AVS 62nd International Symposium & Exhibition | |
| 2D Materials Focus Topic | Wednesday Sessions |
| Session 2D+MN+NS+SP+SS+TF-WeM |
| Session: | Mechanical and Thermal Properties of 2D Materials |
| Presenter: | Fabian Natterer, NIST/CNST |
| Authors: | F.D. Natterer, NIST/CNST Y. Zhao, NIST/CNST J. Wyrick, NIST/CNST W.Y. Ruan, Georgia Institute of Technology Y.-H.C. Chan, Georgia Institute of Technology M.-Y.C. Chou, Georgia Institute of Technology N.B. Zhitenev, NIST/CNST J.A. Stroscio, NIST/CNST |
| Correspondent: | Click to Email |
Phonon spectroscopy of graphene by inelastic electron tunneling spectroscopy with the STM has been elusive in previous measurements [1–3]. The difficulty lies within the weak phonon signatures that are buried by other dominant spectral features that inhibit a clear distinction between phonons and miscellaneous excitations. Utilizing a back gated graphene device that permits continuous adjustment of the global charge carrier density, we employ an averaging method where individual tunneling spectra at varying charge carrier density are condensed into one representative spectrum [4]. This method improves the signal for inelastic transitions that appear at constant threshold, while it broadens and thereby suppresses dispersive spectral features. We use this method to demonstrate the mapping of the total graphene phonon density of states, in good agreement with density functional calculations. Using the knowledge about the phonons thusly obtained, we closely examine our gate resolved spectra and observe a surprising and abrupt change in the phonon intensity 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 carrier type, depending on the sign of the tunneling bias. We invoke a resonance mediated tunneling process that relies on the presence of tip-induced quasi-bound state resonances in graphene, resembling whispering gallery modes for electrons and holes [5]. Our tip-sample system thereby mimics a giant molecular state and shares analogies with resonant enhanced excitations of molecular vibrational or rotational modes [6–9].
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