| AVS 58th Annual International Symposium and Exhibition | |
| Nanometer-scale Science and Technology Division | Tuesday Sessions |
| Session NS+AS-TuA |
| Session: | Frontiers in Nanoscale Imaging and Characterization |
| Presenter: | Peter Maksymovych, Oak Ridge National Laboratory |
| Authors: | P. Maksymovych, Oak Ridge National Laboratory M.H. Pan, Oak Ridge National Laboratory Q. Li, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
A small fraction of electrons tunneling across a vacuum junction will undergo inelastic scattering, exciting surface phonons, molecular vibrations, magnons and plasmons in the contact leads. Although the study of surface phonons is a most straightforward inelastic electron tunneling spectroscopy (IETS) experiment, relatively few systematic studies have been done to date. One of the most surprising observations was that of atomic resolution in the IETS of Au(111) [1], and an equally intriguing variation phonon energy from 20 meV to 9 meV depending on the vertical stacking across the 22x√3 reconstructed surface.
To investigate the origin of these effects on Au(111), we have carried out systematic IETS measurements using a home-built scanning tunneling microscope operating in the temperature range from 20K to 77 K. Particular emphasis was put on understanding of the role of the surface state in the electron-phonon coupling, judged from the intensity of the IETS signal and the energy of the observed vibrational modes.
We have found that largely independent of the measurement temperature, IETS spectra feature a broad peak centered around 18 meV, representing a weighted average of the phonon density of states within the Brillouin zone. Lower lying surface phonon bands (with energies from 7 to 9 meV) were resolved, but the majority of spectra are dominated by the peak at 18 meV. corresponding to the bulk phonon modes at the zone boundary. Although the spectrum remained largely unchanged across the surface, in contrast to earlier observations, the IETS intensity markedly dropped at the step edges. Furthermore, the IETS intensity exhibited long-range oscillations, the wavelength of which coincided with the Friedel oscillations of the surface state in the vicinity of the defects. Combined, the observations attest to the important role of the surface state in electron-phonon coupling [2], likely emphasizing the critical enhancement in the lifetime of hot electrons that tunnel into surface states relative to bulk states. We will also discuss the important role of tip effects in the IETS measurement. Tip-sensitivity may produce significant variations in the IETS spectrum across the herringbone reconstruction because of selectivity toward a particular phonon mode due to the convolution of the tip-surface interactions and heterogeneity of the surface state across the surface.
Research was conducted at the Center for Nanophase Materials Sciences and sponsored by the Division of Scientific User Facilities, U.S. Department of Energy.
[1] H. Gawronski, M. Mehlhorn, K. Morgenstern, Science 319 (2008) 930-933.
[2] Q. Li, P. Maksymovych, M. Pan et al, to be submitted (2011)