Phonon and plasmon excitation in inelastic electron tunneling spectroscopy of graphite

Abstract

The inelastic electron tunneling spectrum ͑IETS͒ of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy ͑STS͒ at 6 K. The observed spectral features are in very good agreement with the vibrational density of states of graphite calculated from first principles. We discuss the enhancement of certain phonon modes by phonon-assisted tunneling in STS based on the restrictions imposed by the electronic structure of graphite. We also demonstrate the local excitation of surface plasmons in IETS, which are detected at an energy of 40 meV. DOI: 10.1103/PhysRevB.69.121414 PACS number͑s͒: 68.35.Ja, 63.20.Ϫe, 68.37.Ef Initially applied to measure inelastic tunneling currents across metal-insulator-adsorbate-metal junctions, 1 inelastic electron tunneling spectroscopy ͑IETS͒ has in the recent past been revived and its concept extended to scanning tunneling microscopy ͑STM-IETS͒. 2 Superimposed on the elastic tunneling current, the inelastic tunneling process opens up alternative tunneling channels increasing the total conductance at the onset of its excitation. Very recently, elegant experiments ͑e.g., Refs. 2-5͒ have been performed in detecting local inelastic electron energy-loss spectra of individual molecule adsorbed on metallic substrates. The observed features in the tunneling spectra are found to correspond to the vibrational modes of the adsorbed species. Vibrations of single molecules as large as C 60 have have been detected. 6 Despite the increasing number of publications of STM-IETS experiments on single molecules adsorbed on metals, to our knowledge, only two studies were reported on collective vibrational excitations of surfaces. 7,8 In both these studies graphite was used as a substrate and several spectral features remained unexplained or were attributed to vibrational excitation in the tungsten tip. However, these tip effects have not been detected in many other STM-IETS experiments. In order to resolve the pure inelastic response of a graphite surface, we have performed STM-IETS experiments on a highly oriented pyrolitic graphite crystal ͑HOPG͒ at cryogenic temperature. In our experiments the whole vibrational spectrum of HOPG from the rigid layer shear mode at Ϸ5 meV to the optical modes at Ϸ200 meV is clearly observed without being perturbed by inelastic effects from the tip. The phonon modes of graphite can be clearly identified by the direct comparison of experimental data with first-principles density-function theory ͑DFT͒ calculations. We report also the observation of surface-plasmon losses with STM-IETS.Freshly cleaved HOPG has been introduced into the vacuum system and shortly annealed at 500 K. The experiments were performed at 6 K with a home built UHV-STM using a tungsten tip. The inelastic tunneling spectrum of HOPG was measured over a topographically clean and flat terrace with a lock-in technique. In this technique a sinusoidal signal with amplitude ⌬V is superimposed to the applied sample bias. In IETS the second-harmonic component of the modulated sig...

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