Scanning Tunneling Spectroscopy of Cl Vacancies in NaCl Films: Strong Electron-Phonon Coupling in Double-Barrier Tunneling Junctions

Abstract

Broad Gaussian line shapes are observed in scanning tunneling spectroscopy of single, localized electronic states induced by Cl vacancies in ultrathin NaCl films on Cu surfaces. Using a simple inelastic resonance tunneling model, we show that the observed broad line shapes are caused by a strong coupling between the localized state and the optical phonons in the film. The parameters for the model are obtained from density functional calculations, in which the occupation of the vacancy state temporarily taking place in the experiment has also been accounted for. DOI: 10.1103/PhysRevLett.95.225503 PACS numbers: 63.20.Mt, 68.37.Ef, 71.15.Mb, 73.20.Hb The recent spectacular progress in carrying out scanning tunneling microscopy (STM) and spectroscopy (STS) of adsorbates on ultrathin, insulating films supported by metal surfaces has opened up a new fascinating field in atomicscale science [1][2][3][4]. This field is of key importance, for example, in the realization of any future atomic-scale electronics that will necessarily be based on nanostructures hosting confined electronic states, which are insulated from the environment by nanostructured, wide-band-gap materials. A fundamental issue is the degree of decoupling of localized electronic states by an insulating film. The well-known lifetime broadening -the dominant broadening mechanism for localized electronic states on metal surfaces -should be dramatically reduced by an insulating film. Therefore, it could be expected that the adsorbate and defect-induced states on insulators exhibit very small linewidths. However, recent STS experiments revealed the contrary [5,6]. This apparent contradiction triggered this study on the different broadening mechanisms and resulting line shapes and widths in STS.An ideal model system for the study of tunneling through a localized electronic state decoupled by a polar insulator from the metal substrate and the associated line shape is provided by a single Cl vacancy in NaCl films on Cu surfaces. This atomic-scale defect in an otherwise defect-free environment is fairly simple, well defined, and stable. The intrinsic defects in bulk and at surfaces of NaCl have been studied both experimentally and theoretically in detail [7][8][9]. These studies have shown that a Cl vacancy, which is commonly referred to as a color or F center, introduces a singly occupied defect state in the bulk band gap.In this Letter, we show from STM and STS measurements that a single Cl vacancy in a NaCl film on a Cu surface introduces an unoccupied vacancy state (VS) with a broad Gaussian line shape in the dI=dV spectra. Using a simple inelastic resonance tunneling model with parameters from density functional calculations, we show that the observed line shape is caused by a strong electron-phonon (e-ph) coupling of the VS to the ionic lattice of the insulating film. This broadening mechanism should be a general phenomenon for defect and adsorbate states on polar, insulating films. For example, it provides an explanation of the recently observed, Gauss...

Scanning Tunneling Spectroscopy of Individual PbSe Quantum Dots and Molecular Aggregates Stabilized in an Inert Nanocrystal Matrix

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Investigating atomic contrast in atomic force microscopy and Kelvin probe force microscopy on ionic systems using functionalized tips

“…Moreover, this vacancy is an atomically welldefined, highly symmetric defect and thus it can be used to verify atomic resolution and to identify tip asymmetries. Finally, the Cl vacancy is also useful to investigate the spatial resolution of KPFM as it provides an uncompensated positive charge within the ionic lattice [30]. Here we used Cl vacancies in a bilayer of NaCl(100) on Cu(111) as model systems to study the atomic contrast of AFM and KPFM on ionic systems employing four different tips terminated with individual Cu, Au, Cl, and Xe atoms.…”

“…A direct way to identify the lattice sites experimentally, without a priori knowledge of the tip termination and its imaging contrast, is the application of markers with known adsorption sites [29]. We used Cl vacancies in the top layer of NaCl(100), which have previously been studied using scanning tunneling microscopy (STM) by Repp et al [30], to provide unambiguous lattice site identification. Moreover, this vacancy is an atomically welldefined, highly symmetric defect and thus it can be used to verify atomic resolution and to identify tip asymmetries.…”

Gundlach oscillations and Coulomb blockade of Co nanoislands on MgO/Mo(100) investigated by scanning tunneling spectroscopy at 300 K

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