Influence of Substrate Surface-Induced Defects on the Interface State

Article pubs.acs.org/JPCC

Influence of Substrate Surface-Induced Defects on the Interface State between NaCl(100) and Ag(111) S. Heidorn,† C. Bertram,†,‡ J. Koch,† K. Boom,† F. Matthaei,†,‡ A. Safiei,† J. Henzl,† and K. Morgenstern*,†,‡ †

Leibniz Universität Hannover, Institut für Festkörperphysik, Abteilung für atomare und molekulare Strukturen (ATOMS), Appelstr. 2, D-30167 Hannover, Germany ‡ Ruhr-Universität Bochum, Lehrstuhl für physikalische Chemie I, NC 5/72 D-44801 Bochum, Germany ABSTRACT: NaCl islands on Ag(111) are investigated by low temperature scanning tunneling microscopy and spectroscopy. The thermodynamically stable growth mode consists of bilayer-high rectangular-shaped islands that are (100) terminated with a large band gap. Deviations from this bulk-like (100) growth are induced by surface defects as intrinsic step edges and point defects in the supporting Ag(111) surface. The interface between NaCl(100) and Ag(111) induces an interface state that is completely depopulated with its onset at (92 ± 4) meV. The influence of the Ag surface-induced defects on the interface state is discussed.

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INTRODUCTION Heteroepitaxy of an insulator on a metal is interesting from a fundamental point of view for enhancing our knowledge about elementary interactions between ionic and metallic material. Furthermore, research on the preparation of thin insulating films is triggered by their potential applications in the field of nanoelectronics. Nanostructures supported by these films have their electronic states decoupled from the substrate. At the same time, no large charging problems are expected during the application of classical methods to characterize their geometric and their electronic structure. Thin NaCl layers are model systems for insulating films. The full crystal band gap is already established in rather thin films.1,2 The layers were utilized for imaging individual molecular orbitals of adsorbed molecules3 and to investigate their functionalities.4−6 The growth of NaCl layers was first investigated on Ge(100) surfaces by low-energy electron diffraction (LEED).7 The small lattice mismatch of 0.5% enables a defect free epitaxial growth. Scanning tunneling microscopy (STM) measurements showed that the first layer grows as a NaCl double layer at 150 K.8 Furthermore, only one of the ions is imaged in atomic resolution. This result was confirmed for NaCl films grown on Al(111) and Al(100).9 Theory attributed the protrusions to the anions because of the higher density of occupied states above them.9 On Cu(111), the lattice mismatch of NaCl(100) is with more than 10% substantial, while it fits almost perfectly on the Cu(311) face. It was demonstrated that the growth of NaCl on vicinal Cu(211) and Cu(532) surfaces leads to a massive © 2013 American Chemical Society

restructuring of the surface even at 300 K, such that the Cu(311) face is formed and wetted.10,11 Silver surfaces, as investigated here, have a smaller misfit of 2.7% with the NaCl crystal than copper surfaces. On Ag(100), an influence of the step density on the growth was demonstrated by LEED and EELS (electron energy loss spectroscopy).12 An incommensurate growth between 300 and 500 K was concluded in a STM study for the same system.13 However, a SPA-LEED analysis revealed higher order commensurate NaCl superstructures on Ag(100) with an azimuthal mosaicity of approximately 14° driven by a small contraction of about 0.9% of the NaCl layer.14 On Ag(111), we recently investigated the growth of NaCl islands across step edges.15 High-resolution images provided evidence that the interaction of the islands with the step dipole leads to a strong bending of the layer and a restructuring of the Ag step. Furthermore, NaCl layers on Ag(111) were used as support in a self-assembling study16 and in an isomerization study.17 In most of the studies mentioned above, the perfectly grown islands are discussed and it is assumed that the electronic structure is bulk-like. However, the substrate surface is in close proximity and might thus influence the electronic structure either directly by states leaking into the layer or via defects that provide a rich variety of electronic structures and are thus often of pivotal interest, especially if one is interested in the Received: May 29, 2013 Revised: July 10, 2013 Published: July 17, 2013 16095

dx.doi.org/10.1021/jp405297h | J. Phys. Chem. C 2013, 117, 16095−16103

The Journal of Physical Chemistry C

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interaction of a substrate with molecules18 as, for example, in catalytic reactions. In this article, we provide details about the growth of (100) terminated islands of NaCl on Ag(111) in the submonolayer regime at around room temperature gathered by high resolution STM imaging. We determine the electronic structure of the islands in both apparent height spectroscopy and dI/dV spectroscopy. The latter reveals an unpopulated interface state that is measurable across the island. The substrate surface induces deviations from bulk-like growth, in particular a Moiré pattern, line defects, and point defects. For point and line defects, we investigate their effect onto the interface state. The article is organized as follows: To set the stage, we first describe the geometric and electronic structure of defect free islands on terraces. Note that the lattice constant and the apparent height have been published before15 and are repeated here for completeness. The determination of commensurability via a Moiré pattern and determination of the real heigth via apparent height spectroscopy have not yet been published. Then, we report substrate surface-induced deviations from perfect growth, both one-dimensional and two-dimensional ones. Finally, we describe the influence of these defects onto the interface state.

occupied side of the spectrum is probed. Note that a bilayer of NaCl leads to a double junction such that the voltages given differ from the energies at the islands by up to 10% as estimated from I(z) spectroscopy.33 The experiments are performed on two Ag(111) surfaces with azimuthal orientations that differ by 23°. Furthermore, the scanning direction is sometimes rotated to align structures with the x-scanning direction. The surface orientation is always known from atomic resolution images. We mark the ⟨110⟩ direction of the Ag(111) surface by a 6-fold cross in those images, for which the surface orientation is of interest.

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RESULTS AND DISCUSSION The described procedure leads to rectangular islands as shown in Figure 1. Most islands nucleate at the intrinsic Ag(111) step

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EXPERIMENTAL METHODS STM measurements are performed with low-temperature STMs under ultrahigh vacuum conditions with a base pressure below 4 × 10−10 mbar.19 A clean Ag(111) substrate is obtained by repeated cycles of Ne+ sputtering and annealing. The sputtering is performed for 30 min at a partial neon pressure of 3 × 10−5 mbar. The acceleration of the Ne+ ions with 1.3 keV results in a sputtering current of 2 μA. The sample is then annealed to 900 K for 30 min. NaCl is evaporated with a rate of 0.02 ML/min on the surface from a Mo crucible heated by electron bombardment. During deposition the sample is held at specific temperatures in between 292 and 305 K. At higher temperature the island edges are rounder and the island density is smaller. There is no qualitative difference in the island growth mode discussed here within this temperature range. Next layer nucleation starts at (293 ± 1) K after ∼5 min at the chosen deposition rate. Some of the islands shown in this article are grown with a two step procedure to increase their size without next layer nucleation at the same nucleation density. For this aim, the growth temperature is increased from (294 ± 1) K to (303 ± 1) K after 5 min of deposition. STM images are taken in constant current mode. They are displayed such that a brighter contrast reflects a retraction of the tip from the sample. The investigated islands are easily disturbed by the scanning process. Even complete islands are moved across terraces during scanning at elevated currents presumably due to direct tip-island interaction. Low currents are utilized to avoid this interaction. Scanning tunneling spectroscopy (STS) is used to measure the first derivative of the tunneling current (dI/dV), which yields information about the electronic structure of the substrate. For a spectrum, the tip is positioned above an island and the feed-back loop is switched off. The bias voltage is then swept over the energy range of interest with a sinusoidal modulation of 1 to 4 meV (peak-to-peak; typical frequency in the range of a few 100 Hz). This leads to a modulation of the tunneling current with the same frequency, which is then detected by a lock-in amplifier. Voltages are applied to the sample with respect to the tip. Thus, for a negative voltage the

Figure 1. Overview images of NaCl growth on Ag(111); tunneling parameters: V = 200 mV, I = 32 pA; scale bars = 100 nm.

edges (Figure 1a). Only for large terraces of several 100 nm, homogeneously nucleated islands are observed (Figure 1b). Island sizes from different preparations range from ∼20 to ∼2000 nm2. Geometric Structure of Islands on Terraces. To set the stage, we first characterize the geometry of the homogeneously nucleated islands on the terraces. In the submonolayer regime, NaCl grows on a multitude of metal surfaces at room temperature as rectangular nonpolar (100) terminated islands on Cu(111),20 Ag(100),13 Au(111),21 and Cu(100).22 In all cases, the lattice constant was found to be slightly smaller than the bulk value. Our study shows that Ag(111) is no exception. The NaCl islands on the Ag(111) terrace are indeed rectangular indicative of nonpolar (100) growth (Figure 2). These islands show in atomic resolution a quadratic lattice with a distance between the protrusions of (395 ± 6) pm and (390 ± 8) pm in the two perpendicular directions in atomic resolution images (Figure 2c). As only chlorine ions are imaged by STM,8,9 this indicates at most a very small contraction from the bulk chlorine distance of 399 pm. Also with respect to the island’s height, the islands are similar to those grown on other metallic surfaces. On several metal surfaces, submonolayer growth of NaCl(100) at room temperature leads to the growth of islands of bilayer height, as shown for Cu(111),20 Ag(100),13 and Au(111).21 Such a growth is energetically favorable, because each ionic charge in the first layer is compensated by an ionic charge of opposite polarity in the second layer. 16096



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Figure 2. NaCl islands on terraces: (a) at negative voltage, −300 mV, 17 pA; scale bar = 10 nm (b) at positive voltage, 200 mV, 39 pA; scale bar =10 nm (c) atomic resolution on island, 60 mV, 47 pA with schematic view of NaCl(100) surface; scale bar =2 nm.

Figure 3. Moiré pattern; Ag(111) lattice is indicated by a 6-fold cross; NaCl(100) unit cell is marked in the images with atomic resolution; scale bars = 2 nm: (a−c) different orientation with respect to the Ag(111) lattice. (d,e) different orientation with respect to the NaCl(100) lattice tunneling parameters: (a) 502 mV, 20 pA, (b) −503 mV, 25 pA, (c) 2093 mV, 240 pA, (d,e) 37 mV, 440 pA.

Figure 4. Spectroscopy. (a,b) Band gap determination: (a) I−V curves measured on NaCl island; set-points are at −4.15 and 4 eV; (b) corresponding dI/dV curves. (c,d) Surface and interface state: (c) I−V curve, red/gray, on Ag(111); black, on NaCl island; arrows point to changes of slopes; set points are at −200 mV; (d) dI/dV curve, red/gray, on Ag(111); black, on NaCl island; modulation parameters for (b) 3 mV, 337.1 Hz, and for (d) 4 mV, 667 Hz.

with (353 ± 45) pm between −2 V and +2 V likewise indicative of a bilayer. Commensurability of NaCl islands has been under debate for several surfaces in the literature, in particular for islands on Ag(100) (cf. introductory section). Due to the intrinsic uncertainty of the determination of lateral distances by STM, we here analyze Moiré patterns to solve this issue for NaCl on Ag(111). For NaCl on Ag(111), we observe a one-dimensional Moiré pattern with a varying distance between 2.0 and 2.6 nm and a corrugation between 2 and 6 pm at most tunneling voltages (Figure 3). Only at 2.1 V a corrugation of up to 22 pm is observed (Figure 3c). The wavelength is not voltage depend-

On Cu(111) and Cu(100), the bilayers are imaged within the NaCl band gap with apparent heights of 3206 and 280 pm,22 respectively. On Ag(100) the bilayer and the third layer are imaged at low bias with 310 and 450 pm, respectively.13 The apparent heights determined for NaCl layers on Al(111) are 280 and 400 pm at −2.2 V and 0.12 nA.9 On Cu(100), the apparent height of the bilayer is 280 pm. Thus, an apparent height of around (300 ± 20) pm at low bias is indicative of the first bilayer. This measured height is considerably less than the distance of 564 nm between next nearest (100) layers of NaCl. Indeed, theory calculated that the apparent height is approximately halved within the band gap.23 The apparent height of the NaCl islands on Ag(111), investigated here, is 16097



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Figure 5. Determination of layer thickness; scale bars = 10 nm. (a,b) STM images of island edge from a series of images recorded between −0.25 V and −5.0 V, shown at indicated voltages, 190 pA. (c−e) STM images of island corner from a series of images recorded between 0.5 and 5.0 V shown at the indicated voltages, 21 pA. (f) Line scans at different voltages between −0.25 V and −3.0 V across island edge shown in (a,b). (g) Apparent height of defect-free region of NaCl layer above Ag(111) in dependence of voltage for several different islands below 3 V and only one island above 3 V; horizontal lines are geometric values for a single and for a bilayer, respectively. (h) Line scans at different voltages between 0.5 and 5 V across island edge shown in (c−e). (i) Line scans across a different NaCl step edge at indicated voltage; four sets of line scans are shifted horizontally for clarity; on each set the voltage range is indicated.

first Stark-shifted image potential state26 (called field state or field emission resonance in literature). This is unlikely here, because a band gap even larger than the bulk value is not expected for a thin film that is not strained as shown above. So far, the islands are thus electronically equivalent to bulk NaCl. However, an additional feature is observed close to the Fermi energy, which is only visible in STS measured for a smaller voltage range (Figure 4c,d). Ag(111) supports an occupied surface state, which is clearly observed already in the I/V spectra as a change in slope (Figure 4c). A similar change in slope is situated in the region of unoccupied states for NaCl/ Ag(111). This increased conductivity results from the onset of the two-dimensional interface state at the NaCl/Ag(111) interface. The position of this interface state between NaCl and Ag(111) is determined from dI/dV spectra (Figure 4d) to be at Eonset = (92 ± 4) meV and the onset of the surface state on Ag(111) at −(67 ± 3) meV. The latter value is in good agreement with earlier studies.27 The interface state is situated at a (159 ± 5) meV higher value than the surface state, because the NaCl layer changes the potential in front of the surface due to its larger dielectricity as compared to the one of the vacuum region alone. Although the potential induced by the overlayer is attractive, the ionic film acts as a repulsive potential for the electronic state. This implies that the interface state is less bound than the surface state. This phenomenon was observed for other dielectric layers before.28−32 In particular, for NaCl on Cu(111) (Au(111)), the interface state is situated with −(225 ± 10) meV (−220 meV) at higher energy than the surface state at −430 meV33 (−480 meV34). On Au(111), the shift was found to depend on the type of underlying surface stacking, i.e. the shift is to −220 meV on fcc stacked regions of the surface and to −270 meV on hcp stacked regions.34 In contrast to the Cu(111) and Au(111) cases, the interface state of NaCl on Ag(111) is unpopulated.

ent. The voltage dependence of the corrugation indicates an electronic Moiré pattern caused by a modulation of the electrostatic potential, i.e. the charge density is perturbed differently for atoms adsorbed in on-top sites or in hollow sites (cf.13). A similar effect was also observed on Cu(100).22 The angle of the pattern both with respect to the Ag(111) surface lattice (Figure 3a,b,c) and with respect to the NaCl lattice (Figure 3d,e) varies for islands nucleated at step edges. The unspecific angles are consistent with the large variety of island orientations observed with respect to the substrate lattice.15 The derived lattice constants are consistent with the one obtained directly in atomic resolution images. In general, the existence of the Moiré pattern indicates higher order commensurability, here in one dimension. Summarizing the geometrical characterization, the NaCl islands are (100) terminated with (almost) the bulk lattice constant and of bilayer height. With respect to the surface, they exhibit a long-range commensurability in one dimension. Electronic Structure of NaCl Islands. STS. We first characterize the island’s electronic structure by STS. The first rise in conductivity is determined both in the occupied and the unoccupied region in I/V and in dI/dV spectroscopy (Figure 4a,b). The insulating character of the NaCl within the band gap is obvious. The current raises strongly at approximately −5 V and +3 V (Figure 4a). The onsets are determined from a multitude of dI/dV spectra similar to the one shown in Figure 4b as the voltage value at the middle of the rise. The first rise in the unoccupied region is situated at (3.3 ± 0.1) V and the first rise in the occupied region at −(5.4 ± 0.1) V. This gives an apparent band gap of (8.7 ± 0.2) V. Comparison to literature values for bulk NaCl, which range from 8.5 V24 to 8.97 V,25 shows that the full band gap is already reached in the bilayer. Theoretical studies for NaCl/Cu(111) imply that the first rise in the unoccupied region is not the conduction band but the 16098



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Figure 6. NaCl islands at step edges; scale bars = 10 nm. (a) Large scale image with seven surface steps, 104 mV, 19 pA. (b) Island across six steps, 193 mV, 16 pA. (c) Medium scale image showing different types of islands at two step edges, shown in more detail in (d,e); 200 mV, 39 pA. (d) Island with ⟨100⟩ edge at bottom side of step, 200 mV, 47 pA. (e) Island with ⟨110⟩ edge across step, 200 mV, 0.1 nA.

Apparent Height Spectroscopy. Apparent height spectroscopy provides additional electronic features. In addition, it facilitates to determine the height of an insulating structure. This spectroscopy records the apparent height of an insulating structure in dependence of voltage.35 Though measured heights depend on the exact difference in local density-of-states between the different layers, tunneling into the valence band or from the conduction band leads to an apparent height that resembles the real height of an insulating layer on a metal surface much closer than tunneling at lower voltages. We use apparent height spectroscopy to corroborate the indirectly concluded thickness of the NaCl islands. STM images at different voltages are shown in Figure 5a−e. At negative voltage down to −5 V, there is only a slight decrease observed on the island’s terrace from approximately 250 to 110 pm (Figure 5f,g). The edge of the NaCl island does not change in apparent height at all. As shown above, the valence band is not reached within the accessible range, which is limited by the island’s and the tip’s stability. The decrease of the apparent height below −2.5 V is attributed to an increased density of states of Ag(111) because of its d-bands. This effect is seemingly compensated at the edge by the edge effect. At positive voltage, the apparent height changes more dramatically. First, some defects on the layer and the polar step edge increase in apparent height at 2 V and above (Figure 5d). Above approximately 3 V, the whole island is imaged at a larger apparent height. The values as measured on defect-free regions of the islands are compared to bulk layer distances of NaCl in Figure 5g. The height raises above the next nearest layer height of NaCl(100) planes and starts to oscillate above this value. Such oscillations are shown in more detail in Figure 5i. The fact that the apparent height oscillates is attributed to the Starkshifted image potential states (often called field states or field emission resonances) above the NaCl layer,36,37 which were calculated for NaCl/Cu(111).26 In the case shown in Figure 5g, the first Stark-shifted image potential state is situated just above the band edge. The exact position of the states varies (cf. Figure 5g−i), because it depends on the field between tip and sample38 and thus on the current set-point and on the tip’s geometry on the nanometer scale. The observed variation in energy thus corroborates the assignment of the oscillations as Stark-shifted image potential states. Such states always increase the conductivity. The value of the apparent height in between the states is thus indicative of the

real height. In this case, the measured height of about 550 pm is close to the value that is expected for the geometric height of a bilayer. Apparent height spectroscopy therefore proofs that the NaCl grows on Ag(111) at room temperature initially in bilayers. Summarizing the electronic structure investigation, the metallic surface influences the electronic structure of the bulk-like NaCl islands in two respects, an interface state exists within the band gap and Stark-shifted image potential states exists beyond the conduction band. Both states arise from the states of the metal, but penetrate through the insulating layer and will thus interact with any adsorbed molecule. We now turn to substrate induced deviations from bulk-like growth to these islands and investigate their influence on electronic structure. Surface Induced Defects. Line Defects. Under usual preparation conditions, the deposited NaCl molecules are so mobile that they easily reach the intrinsic surface steps and preferentially attach to them for island nucleation (Figure 6a− c). The rectangular shape of the islands is usually preserved, even if the islands grow across a large number of step edges (Figure 6b). This growth has been recently discussed in detail.15 Mostly, two types of islands are observed at step edges. Either the island grows with its nonpolar step edge at the lower part of the metallic surface step (Figure 6d) or it grows across the step with its polar edge in parallel to the step edge (Figure 6e). For the former, no difference to terrace islands are detectable. Islands that overgrow the step edge maximize interaction with the step dipole by bringing their polar edge in parallel to it,15 Moreover, the step edge is reconstructed during the growth in order to maximize the step dipole.15 The steps within the islands induced by the intrinsic surface steps are thus polar line defects within the island. They are expected to influence their electronic structure. Point Defects. Defects in ionic crystals are usually Schottky defects for charge neutrality reasons and even these are rare. The creation of a Schottky defect in NaCl bulk costs approximately 2 eV. This implies that the defect density at 300 K is only approximately 10−9 per site. Consequently, less than one defect is expected for a typical bilayer high island of 20 nm × 20 nm, even if the creation energy is expected to be reduced for a thin layer. As the natural density of point defects in the NaCl island is virtually zero, point defects in NaCl as 16099



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Figure 7. Point defects; scale bars = 2 nm. (a) Large scale image of complete island. (b) Atomically resolved medium scale image. (c) line scan as indicated in (b). (d) Atomic resolution image of a protruding defect. (e) Image with several defects. (f) Two representative line scans across a depression and a protrusion as indicated in (e); tunneling parameters: (a,b) 40 pA, 97 mV; (d) 440 pA, 37 mV; (e) 25 pA, 500 mV.

Figure 8. Interface state close to defects. (a) STM image of island 16 nm × 11 nm, 18 pA, −0.3 V; scale bar = 5 nm. (b) Onset energy Eonset along line marked in (a), position of island is sketched on bottom. (c) Relative intensity I. (d) Intrinsic width of onset Δint, Vmod = 2.1 mV, νmod = 532.6 Hz.

gas at reduced temperature.39 Such an island is shown in Figure 7a. From the increase in NaCl defect density with Ag surface defect density, we infer that these defects are primarily induced

observed, for example, in Figure 5d are most probably surface induced. Indeed, their density is increased through additional point defects on the surface provided by implanting the sputter 16100



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Figure 9. Interface state close to point defects as marked by arrows. (a) STM image of island 20 nm × 19 nm, 200 pA, −0.2 V; scale bar = 5 nm. (b) Onset energy Eonset along line marked in (a), arrows mark positions of point defects, position of island is sketched on the bottom. (c) Intrinsic width Δint; Vmod = 1.4 mV, νmod = 707.8 Hz.

states in the band gap of of the NaCl layer as seen in the apparent height spectroscopy (Figure 5d). Interface State near Defects. The influence of the defects onto the electronic structure of the NaCl layer is further investigated by analyzing the interface state in dependence of the position on NaCl islands close to the island edge and close to defects. We extract values for the onset of the interface state Eonset from dI/dV spectra at the middle of the rise, the intensity of the interface state I and the width of the rise Δexp from fitting a line to the slope as depicted in Figure 4d from such dI/dV spectra. The intrinsic width is determined via Δint = (Δexp − (π/2 × eVmod)2 − (6kT)2)1/241−43 to reduce the value from broadening due to the modulation voltage (Vmod, peak-to-peak) and the broadening by temperature T with k being the Boltzmann constant. As examplified in Figure 8b, the energetic position of the interface state varies close to the line defects, that is, NaCl step edges at ∼5 and ∼20 nm and the overgrown Ag(111) edge at ∼15 nm. The shift is with up to 60 meV remarkable. It is largest at the lower side of the polar edge induced by the Ag step edge. In fact, the overgrown step edge divides the island into two parts that independently confine the interface state, as obvious from the intensity distribution of the onset (Figure 8c). Moreover, the width of the onset differs on the two terraces (Figure 8d). In addition, a broadening is observed close to the step edges. As the width of the onset is inversely proportional to the lifetime of the electrons in the state, this broadening indicates a shortening of the lifetime. Exact values for the lifetime cannot be extracted from STS due to a tip induced Stark shift of the state that reduces the intrinsic lifetime.44 Furthermore, the closeness to the step edges itself influences the width of the onset even at infinite lifetime.45 Although a quantitive determination is not feasible, it is obvious that the

by surface defects. The same types of defects (described in detail below) are also observed for islands grown on nominally “perfect” surfaces. Several types of point defects are observed: protrusions of different sizes, depressions of different sizes, and combinations. The laterally smallest defect consists of a depression of one Cl− ion in size surrounded by four chlorine atoms of increased apparent height. Three of these defects are shown in Figure 7b. This type of defect is characteristic for a chlorine defect in the top layer as observed before for NaCl on Cu(111).40 The line scan across the one on top of Figure 7b reveals that this one is imaged with an increased apparent height of 50 pm (Figure 7c, left protrusion). An increased apparent height by up to 60 pm is observed. A laterally larger protrusion (three are shown on the right-hand side of Figure 7b) consists of a square or a rectangle of around 20−30 chlorine atoms (Figure 7d and e) imaged at an up to 80 pm increased height (Figure 7f, right panel). Note that this type of defect is only visible with a so-called modified tip, that is, a tip with a molecule at its apex. Finally, two to four chlorine ions are imaged as a depression of 20−60 pm (depending on tip condition) below the NaCl plane. These depressions are surrounded by a protruding ring of 10−20 pm in height. The ring’s diameter is somewhat larger than the one of the protrusion-only point defects. On the basis of the larger extension as compared to the toplayer defect, we interpret the latter two defects as defects within the contact layer to the Ag(111) and the Ag(111) itself, respectively. Possible defects within the contact layer are missing Na+ ions, missing Cl− ions or missing NaCl pairs. Due to the similarities to a missing Cl− ion in the top layer, we interpret the depression surrounded by the bright rim as a missing Cl− in the contact layer. The broad protrusions are tentatively attributed to defects within the Ag(111) layer that bent the NaCl layer locally. The point defects create defect 16101



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lifetime reduction is substantial and of the order of several ten percent. Again, it is largest at the lower side of the polar edge. Variations in Eonset and Δexp are likewise observed close to point defects (Figure 9). The enhanced contrast of the STM image (Figure 9a) allows to identify two point defects on the line, along which dI/dV spectra are recorded. Both the energy onset and the width of the onset change discontinuously at these point defects (Figure 9b and c). The change in lifetime is substantial (Figure 9c). Here, the change is clearly beyond the edge effect discussed above and thus a clear effect of the point defect onto the interface state properties is revealed. Both line and point defects thus alter the electronic properties of the NaCl layer, which in turn is expected to influence the interaction of the islands with adsorbates.

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CONCLUSION We investigate NaCl grown on Ag(111) in submonolayer coverage from the bulk-like island growth on defect free Ag(111) terraces to islands influenced by several defects on this terrace, the lattice mismatch, line defects, and point defects. The lattice mismatch leads to a minute electronic structure variation within the islands, only. Point defects in both layers and line defects lead to a more substantial change, in particular of the interface state’s energy. Furthermore, the interface electron’s lifetime is reduced. All deviations from bulk-like layers influence the electronic structure on the layer and are thus expected to influence the interaction of NaCl with molecules. An influence on the interaction of molecules with the NaCl layer is expected from the electronic Moiré pattern, from the step edge induced polar edge, and from point defects in the contact layer. As our islands are shown to be equivalent to all NaCl islands grown so far on a multitude of metallic surfaces, these surfaceinduced changes to the electronic structure are expected to influence all studies that use thin NaCl layers for molecule decoupling. We furthermore expect other insulating layers to show similar effects.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +492343225529. Fax: +492343214182. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Israeli Foundation for financial support. REFERENCES

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Influence of Substrate Surface-Induced Defects on the Interface State

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