Surface color centers on epitaxial NaCl films

Zielasek, Volkmar; Hildebrandt, T.; Henzler, M.

doi:10.1103/physrevb.62.2912

Cited by 43 publications

(50 citation statements)

References 26 publications

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“…If we retract the tip by more than 10 Å from the surface and apply a positive sample voltage .7 V we can desorb single Cl ions from the surface. Electron-stimulated halogen desorption is well known from experiments where electron bombardment creates F centers [9,10]. After extended periods of electron bombardment also metallic Na was observed in the present case.…”

Section: (Received 7 September 2000)supporting

confidence: 60%

Ionic Films on Vicinal Metal Surfaces: Enhanced Binding due to Charge Modulation

et al. 2001

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NaCl films on Cu(311) exhibit a remarkably strong and localized binding between adlayer and substrate. The binding sites of the ions in the NaCl film with respect to the Cu surface are determined from atomically resolved scanning tunneling microscopy images. A new model is proposed in which the binding mechanism is controlled by the charge modulation of a regularly stepped surface due to the Smoluchowski effect. This model can be extended to explain the growth of ionic adlayers on regularly stepped and kinked metal surfaces in general. DOI: 10.1103/PhysRevLett.86.252 PACS numbers: 68.55. -a There is a long standing interest in the growth of ultrathin insulating films on conductive substrates due to their importance in catalysis and device applications. Additionally, the use of thin films circumvents the problem of surface charging and thereby allows one to investigate insulator surfaces by standard surface analysis techniques. To grow thin insulating films different techniques have been developed, depending on the selected substrate and deposit. In the important field of metal oxides, for example, both components of the film are introduced separately and the film can be grown either by metal deposition and subsequent oxidation, or by simultaneous oxidation during growth [1,2]. In the case of alkali halides the film is grown by direct deposition of the complete alkali halide molecule. While the chemical reactivity of insulators varies, a common feature is their polar nature. In the case of nonreactive alkali halides surface binding is dominated by Coulomb interactions between ionic and/or image charges [3]. Therefore, substrates with polar character will have a strong influence on the growth characteristics. In this respect vicinal surfaces are of utmost interest [4] since charge smoothing at step edges due to the Smoluchowski effect and the resulting electrostatic dipole at steps produces a charge-modulated surface [5]. Since regularly stepped or regularly kinked surfaces can be prepared in multiple orientations, the lateral periodicity of this kind of charge modulation can be varied with great flexibility.In this Letter, we show that the concept of the chargemodulated substrate surface is a powerful tool to control epitaxial 2D growth of ionic insulator films with high perfection. As an exemplary case, we have studied the epitaxial growth of NaCl on Cu(311) by low temperature scanning tunneling microscopy (LT-STM). From a series of detailed STM experiments we determine the location of the chlorine ions to be on top of the intrinsic step edges of the Cu(311) surface. This observation can be explained by a very general model, which takes into account the charge modulation of a stepped surface due to the Smoluchowski effect. The model can be also extended to the growth behavior observed on regularly kinked surfaces as shown in the case of Cu(531).Our experiments were performed with a LT-STM [6] operated at 13 K. We used a chemomechanically polished Cu(311) single crystal which was cleaned by Ne 1 sputtering an...

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Section: (Received 7 September 2000)supporting

confidence: 60%

Ionic Films on Vicinal Metal Surfaces: Enhanced Binding due to Charge Modulation

et al. 2001

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“…This new electron level lies 2.4 eV below the bottom of the conduction band, which is comparable to earlier EELS results. 10,49 B. Adsorption at the F s center…”

Section: A Nacl(100) F S Centermentioning

confidence: 99%

Tailoring band gaps of insulators by adsorption at surface defects: Benzoic acids on NaCl surfaces

et al. 2009

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The adsorption of benzoic acid and its OH-substituted derivatives, namely, salicylic acid ͑SA͒ and parasalicylic acid on various NaCl surfaces has been investigated by density-functional theory with hybrid exchange-correlation functional. The ideal NaCl͑100͒ surface is chemically inert as evidenced by the low binding energies. The molecular adsorption can be enhanced by both an anion vacancy and a surface step site. The bonding between the surface Na and the carboxylic O atom is of covalent character for all adsorption geometries. Our calculations show that the adsorption of SA has the largest binding energy of all three acids due to the additional interaction between Na and the phenolic O atom. Charge transfer between the molecule and the surface is generally very small, except in the presence of an anion vacancy where the unpaired electron is mostly transferred to the adsorbate. Surface defects generally have a strong influence on the electronic structure of the adsorbed molecules. Specifically, the adsorption of SA at ͓011͔-oriented steps can significantly reduce the effective band gap to 1.6 eV due to the up shift of the Cl 3p levels at the undercoordinated step edge. Implications of these results to the contact charging effect between wide-band-gap insulators will be discussed.

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“…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.…”

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

et al. 2005

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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...

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Surface color centers on epitaxial NaCl films

Cited by 43 publications

References 26 publications

Ionic Films on Vicinal Metal Surfaces: Enhanced Binding due to Charge Modulation

Ionic Films on Vicinal Metal Surfaces: Enhanced Binding due to Charge Modulation

Tailoring band gaps of insulators by adsorption at surface defects: Benzoic acids on NaCl surfaces

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

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