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

Abstract

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...

Stabilization of bimolecular islands on ultrathin NaCl films by a vicinal substrate

“…The growth of ultrathin NaCl films and their properties have been characterized by STM on several metal and semiconductor surfaces such as Ge(1 0 0) [16], Al(1 1 1) and Al(1 0 0) [17], Cu(1 1 1) [18], Cu(2 1 1) [19], Cu(3 1 1) [20], Cu(5 3 2) [21], Ag(1 0 0) and Ag (1,1,19) [22], but to our knowledge no study of the growth of ultrathin NaCl films on Au(1 1 1) has been reported. Before studying the adsorption of molecules on the NaCl layers, we thus investigated the growth of NaCl on Au(1 1 1) and on Au (11 12 12) [for Au (11 12 12), see Section 3.3].…”

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Molecules on Insulating Films: Scanning-Tunneling Microscopy Imaging of Individual Molecular Orbitals

“…Consequently, a site determination is necessary when discussing STM images of adsorbates. On ultrathin NaCl(100) layers, atomically resolved images can be obtained [9,10]. However, a site determination by a simultaneous imaging of the atomic NaCl structure and the adsorbed pentacene molecules was not possible, because to image the molecules on the insulating layer requires scanning at a very low tunneling current, so as to prevent the tip from interacting with the molecules during scanning.…”