Strong Asymmetric Charge Carrier Dependence in Inelastic Electron Tunneling Spectroscopy of Graphene Phonons

Abstract

†These authors contributed equally to this work 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 carrier type, depending on the sign of the tunneling bias. The ability to detect phonons has been very important in many graphene applications.This has led, for example, to the routine application of Raman spectroscopy to graphene physics and devices [9]. IETS detects vibrational excitations without the stringent selection rules active in Raman spectroscopy [17], and it's employment in a scanning tunneling microscope (STM) brings about spatial mapping of inelastic processes [18]. Inelastic excitations appear in scanning tunneling spectroscopy as a set of steps in the differential tunneling conductance at the excitation energy ћ , and as a peak and dip pair at positive and negative sample biases, , in the second derivative [18]. Previously, IETS has had limited success in measuring phonons in graphene because of weak signals and spectral overlap with other miscellaneous excitations. To date only a few reports of phonons in graphene have been published [13][14][15][16]. Reference [13] characterized an excitation at 63 meV and assigned it to a K-point phonon, while reference [14] revealed an additional feature at 150 meV and also assigned it to a K-point phonon. Reference [15] detected an excitation at 360 meV, which was tentatively assigned to a breathing mode. followed by e-beam lithography to deposit Au contacts [19]. Measurements were performed with a custom-built low temperature STM, operating at 4.3 K and a base pressure better than 5 10 hPa [26,27]. All phonons were observed in repeated measurements, independent of sample location, and were unperturbed by the exposure of graphene to molecular hydrogen and deuterium. The latter gases were used to rule out false assignment of the observed features to background gas or contamination from device fabrication processes [24,28,29]. The graphene

Klein tunnelling and electron trapping in nanometre-scale graphene quantum dots

“…These resonances are observed only below the Fermi energy, and are not seen outside the GQD (green curve). Furthermore, on the GQD we observe a graphene inelastic tunnelling channel34,35 at E ≈ ±67 mV. b, Spectroscopic images at the Fermi energy and at the location of the first three peaks in the dI/dV spectra observed on the GQD.…”

Understanding and Engineering Phonon-Mediated Tunneling into Graphene on Metal Surfaces

“…and SiO 2 covered with hexagonal boron-nitride. 9,10 Recently, IET signals of graphene phonons have been reported from delaminated graphene nanostructures on Pt(111) 11 and Ir(111) 12 as well as from bilayer graphene on Ir(111). 13 At present, the occurrence of graphene phonon signals in IETS is far from being understood.…”

Inelastic vibrational signals in electron transport across graphene nanoconstrictions

“…Recently, several papers have examined inelastic signals due to vibrational excitations in the second derivative of the current-voltage characteristics, so-called inelastic electron transport spectroscopy (IETS), of gated pristine graphene [22][23][24][25][26][27], and heterostructures of graphene and hexagonal boron nitride [28,29]. With the rapid development in fabrication and electronic characterization of nanostructured graphene [19,20,30] it is interesting to investigate the presence of inelastic vibrational signals for GNCs.…”