Adsorption of Benzene on Cu (100) and on Cu (100) Covered with an

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Adsorption of Benzene on Cu(100) and on Cu(100) Covered with an Ultrathin NaCl Film: Molecule−Substrate Interaction and Decoupling Maitreyi Robledo,† Gianfranco Pacchioni,‡ Fernando Martín,†,§,∥ Manuel Alcamí,†,§ and Sergio Díaz-Tendero*,† †

Departamento de Química, Módulo 13 and ∥Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain ‡ Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 53, I-20125 Milano, Italy § Instituto Madrileño de Estudios Avanzados en Nanociencias (IMDEA-Nanociencia), Cantoblanco 28049 Madrid, Spain ABSTRACT: We present a theoretical study of the adsorption of benzene C6H6 on the Cu(100) metal surface. The insulating effect of ionic films on this system has also been investigated by adsorbing C6H6 on the same surface covered with 1, 2, and 3 monolayers of NaCl. For this purpose, we employed density functional theory (DFT) including the van der Waals dispersion forces via a DFT-D2 scheme. For all the studied systems we analyzed the adsorption energies and geometries as well as the density of states in order to get a complete description of the type of binding, the charge transfer between the molecule and the surface, and the electronic level alignment after adsorption. We show that the molecule−substrate interaction is weak and mainly governed by dispersion forces, with an almost insignificant charge transfer between the substrate and the adsorbate. We found a progressive decoupling of the molecule from the metal surface when the size of the ultrathin insulating NaCl film increases.

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INTRODUCTION Organic molecules are considered as the key building blocks of the new emerging field known as molecular electronics.1−5 In particular, the adsorption of organic molecules on metal surfaces is a matter of current interest due to its important role in organic-based electronic nanodevices.6 For instance, understanding the interaction between n-conjugated molecules with metal surfaces has been crucial for the development of electroluminescence organic materials such as organic lightemitting diodes (OLEDs), light-emitting electrochemical cells (LECs), and electro-generated chemiluminescence (ECL) devices.7 The structure of the most employed molecules combines π-delocalized orbitals (aromatic compounds) with other functional groups (alcohol, carboxyl, cyano, etc.), as, e.g., in TCNQ,8−10 PTCDA,11−16 or azobenzene derivatives17−20 (see also the reviews21,22). All of these molecules contain benzene rings in their structure. Therefore, investigation of the adsorption of benzene itself on a metal surface is of prime importance to understand the interaction between organic molecules with π-delocalized electrons and metal substrates. Thus, in this work we first focus on the adsorption of benzene on Cu(100). Many different metals are used as metallic substrates for studying the properties of adsorbed organic molecules. However, the interest in copper has grown due to the crucial role that this noble metal plays in industrial applications such as hydrogenation catalysis and cracking reactions. An example is © XXXX American Chemical Society

the dissociation of individual benzene molecules adsorbed on Cu(100) by tunneling electrons observed in a scanning tunneling microscopy experiment.23 Nevertheless, the full d band of copper makes this metal quite inert, thus leading to a weak interaction when benzene adsorbs on it.24,25 Experimental studies have shown as well the weak interaction between benzene and Cu(111),26 Ag(111),27 and Au(111).28 Different spectroscopic techniques have confirmed weak binding energies in aromatic cyclic hydrocarbons adsorbed on metal surfaces.29 From a theoretical point of view, the importance of including weak dispersion forces for a proper description of organic molecule−metal surface adsorption has been recently demonstrated.15,19,20,30−33 Indeed, the description of the interaction between benzene and the copper surface cannot be appropriately described by using standard density functional theory (DFT). In fact, DFT calculations in similar systems show large discrepancies with experimental results, mostly concerning adsorption energies.34 Therefore, in order to take into account the contributions of the van der Waals (vdW) interactions between benzene and the metallic substrate, one has to account for dispersion forces. In this work we do so via the semiempirical Grimme correction.35 This method has been Received: October 23, 2014 Revised: January 26, 2015

A

DOI: 10.1021/jp5106604 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

may provide different results.53,54 Indeed, these forces are missing in most of the functionals within GGA. In order to solve this drawback we used an approach that considers dispersion forces contributions to the total energy of the system. DFT-D235 includes vdW contributions using a semiempirical potential added to the total DFT energy. However, this functional is known to overestimate adsorption energies due to the neglect of the screening effect in the metal substrate. We therefore evaluated the performance of this method in the first system of our study (C6H6/Cu(100)) as follows (see Figure 1a and 1b).

successfully applied in previous studies of the interaction between organic molecules on noble metal surfaces.36−42 Not only the structural rearrangement upon adsorption and the characterization of the interaction are of interest but also the electronic structure of the benzene−metal interface. In particular, the mismatch of the electronic levels at the interface limits the number of electrons that can be transferred from the metal to the organic molecule,43 i.e., the charge transfer between the adsorbed molecule and the metal surface. In this context, thin insulating ionic NaCl films have been recently employed to decouple electronically individual pentacene molecules from a metallic substrate,44 allowing for the study of the isolated adsorbate electronic properties and a direct imaging of its molecular orbitals. In this paper we report as well on the effects of including an ultrathin insulating ionic film (1, 2, and 3 atomic layers of NaCl) on the metal surface prior to benzene adsorption. We evaluate if a benzene molecule placed on top of an insulating film interacts with the copper surface through the ionic overlayer, i.e., if the insulating layer provides an effective decoupling of the benzene orbitals from the metal bands or if, on the contrary, electrons can be transferred from the metallic surface to the organic adsorbate through the ionic film.

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COMPUTATIONAL DETAILS Density functional theory (DFT) calculations including periodic boundary conditions (PBC) were performed using the Vienna Ab initio Simulation Package (VASP).45−48 The exchange and correlation effects have been described using the generalized gradient approximation, in particular employing the Perdew−Wang 91 functional (GGA-PW91).49,50 The interaction of the electrons with the atoms has been taken into account within the projector augmented wave method (PAW pseudopotentials).51,52 The electron density has been described employing a plane wave basis set expanded up to a kinetic energy cutoff of 400 eV. The metal surfaces were modeled by a slab consisting of four atomic layers, separated by a vacuum space of 10 Å in the coordinate perpendicular to the surface, z. The adsorption of benzene has been considered in all cases, in a low coverage regime, within a unit cell of (4 × 4) Cu atoms in the direction parallel to the surface (xy coordinates). Geometries have been optimized sampling the Brillouin zone with a Γ-centered Monkhorst−Pack of 1 × 1 × 1 K-mesh. Final electronic energies have been computed employing the previously optimized geometries by single-point energy calculations using a Γ-centered Monkhorst−Pack of 4 × 4 × 1 K-mesh. Both samplings of the Brillouin zone include a Methfessel− Paxton smearing of 0.2 eV. The geometries of the studied systems were optimized by relaxing all atoms of the adsorbate, C6H6, in the three spatial directions (xyz), all atoms of the insulating films, NaCl, in the z direction, and the first two layers of the metal slab, Cu, in the z direction as well. The layer spacing of the two lower Cu layers was taken from the optimized lattice constant parameter (a0 = 3.365 Å). All these coordinates were optimized until all forces were smaller than 0.001 eV Å−1. The electronic self-consistent convergence was set in an energy difference of 10−5 eV with respect to the previous cycle. An accurate description of the adsorption of aromatic molecules on metal surfaces requires accounting for the dispersion forces. This is a theoretical challenge since different approaches implemented to account for these contributions

Figure 1. (a) Three different approaches have been evaluated to include van der Waals forces in the C6H6/Cu(100) system: (top-left) with vdW, (top-center) partial vdW, and (top-right) without vdW. Atoms in blue represent those accounting for vdW dispersion forces. (b) Scan in the coordinate normal to the surface with the molecule interacting on the Hollow adsorption site employing the three approaches.

(a) With vdW, we included the contribution of the dispersion forces in the whole system, i.e., all the atoms of the adsorbate and the substrate. (b) Partial vdW, dispersion forces have been only considered in the atoms of the adsorbate, the insulating film, and the first layer of the Cu slab. (c) Without vdW, dispersion forces have been excluded completely from the total energy. B


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The Journal of Physical Chemistry C We finally chose partial vdW for all calculations, i.e., we include the pairwise Grimme’s potential35 between atoms of the molecule, the ionic layer, and the top layer of the metal surface (see Discussion below). The results of this analysis can be seen in Table 1, where the three approaches were considered for evaluating the interaction Table 1. Interaction Energy and Distance Obtained Afer Geometry Optimization of the C6H6/Cu(100) (Hollow site) at the Three Different Implementation Proceadures Employed To Apply vdW Forces (without vdW, partial vdW, and with vdW)a procedure

Eads (eV)

ZC6H6−Cu (Å)

with vdW partial vdW without vdW

−1.38 −1.02 −0.23

2.16 2.26 3.12

a

Initial geometries have been taken from the most favorable distance previously found in the scan calculations.

between the benzene molecule and the metal substrate on a hollow site. Adsorption energies and interaction distances shown in the table are quite different and show the importance of a correct use of the Grimme potential. Finally, charge transfer after the adsorption of the molecule on the surfaces considered [Cu(100) and nML-NaCl/Cu(100)] has been analyzed employing the quantum theory of atoms in molecules (QTAIM) by R. Bader55,56 as implemented by Henkelman et al.57−59

Figure 2. Benzene adsorption sites on the Cu(100) surface: (a) hollow, (b) bridge, and (c) top. Schematic overview of the distances in the system (d).

stronger molecule−surface binding. Figure 1 shows Eads as a function of the molecule−surface distance for the three different implementations of the disperse forces: without vdW, partial vdW, and with vdW, as explained in the previous section, employing a 1 × 1 × 1 K-mesh. The results obtained in a further optimization of the geometry with the three options are given in Table 2. Adsorption energies are underestimated

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RESULTS In this section we present in detail the optimized geometries, the computed adsorption energies, and the charge transfer for the different studied systems in the following order: C6H6/ Cu(100), nML-NaCl/Cu(100), C6H6/nML-NaCl/Cu(100), and C6H6/NaCl. C6H6/Cu(100). The interaction between benzene and Cu(100) is mainly dominated by dispersion forces. Previous works have already shown how not including vdW forces will lead to an underestimation of the benzene−metal surface adsorption energies:60 they found more than 0.5 eV of difference in the adsorption energies computed at the DFTGGA level of theory in comparison with experimental values. Furthermore, recent works19,20 have shown that inclusion of dispersion forces in the whole slab does not consider screening effects on the metal and does not allow one to accurately compute adsorption energies. Therefore, in a first step of our study we evaluated the importance of including van der Waals forces in our calculations and how to implement it. To this we computed the adsorption energies as a function of the benzene−surface distance in a frozen scan: according previous work on C6H6/Cu(100),25 we assumed a configuration of the molecule parallel to the surface with adsorption on the hollow site and performed the scan in the z axis (normal to the surface) without any optimization (see Figures 1 and 2a−d). The adsorption energy is defined as Eads = EC6H6 /Cu(100) − [EC6H6 + ECu(100)]

Table 2. Optimization Results of C6H6/Cu(100) at the Hollow, Bridge, and Top Sites with a 1 × 1 × 1 K-Mesh, Adsorption Energies with a 1 × 1 × 1 K-Mesh, and with 4 × 4 × 1 K-Mesh over the Geometries Previously Obtaineda 1 × 1 × 1 K-mesh

4 × 4 × 1 K-mesh

adsorption site

dC6H6−Cu (Å)

xy angle (deg)

Eads (eV)

Eads (eV)

hollow bridge top

2.26 2.57 2.90

0 25 0

−1.14 −0.90 −0.96

−1.02 −0.85 −0.75

a

In all calculations the partial vdW procedure was employed to include dispersion forces. Initial geometries in the optimization have been taken from the most favorable adsorption distances and angles previously obtained in both scans.

when vdW forces are not included in the calculation,61 and applying vdW forces in the whole system leads to an overestimation of the chemisorption energy and a misinterpretation of the screening effect inside the metal as has been previously shown.15,19,62−64 We thus chose partial vdW as the best procedure to obtain accurate benzene−Cu adsorption geometries and energies. We performed a second scan rotating the molecule in the xy angle, parallel to the surface, in the three possible adsorption sites: the center of the molecule lying on the top of a hollow, a bridge, and a top site. These single-point energy calculations have been performed employing the partial vdW procedure with a Γ-centered 1 × 1 × 1 K-mesh and keeping the molecule−surface distance frozen at dC6H6−Cu = 2.16 Å.

(1)

where EC6H6/Cu(100) is the energy of the complete system, EC6H6 is the energy of the adsorbate, and ECu(100) is the energy of the clean metal surface. Thus, negative values of Eads implies bounded systems; the smaller the value the energetically C


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The Journal of Physical Chemistry C Starting from the most stable distances and orientations obtained in the previous scans (z distance and xy angle), we then optimized the geometry with a 1 × 1 × 1 K-mesh and computed the final adsorption energies with a 4 × 4 × 4 Kmesh in single-point calculations over the previously optimized geometry. The results of such optimizations are shown in Table 2. The adsorption of benzene on the metallic substrate induces an expansion of the C−C bond in the ring: from 1.39 Å obtained in the optimization of benzene in the gas phase to 1.41/1.42 Å for the molecule on the hollow site and 1.39/1.40 Å on the top and bridge sites. The different values in each site depend on the C−C bond orientation with respect to the surface since the D6h symmetry is broken upon adsorption. The greater elongation of the C−C bond obtained in the hollow position is accordingly accompanied by the greatest adsorption energy and the shortest molecule−surface equilibrium distance. The hollow site is thus the most favorable site, showing the greatest interaction between C6H6 and the copper surface, which in essence leads to a greater disruption of the π−π C6H6 bonding. The slight geometrical distortions that benzene adsorption induces on the metal surface is also remarkable. Changes in the interlayer distances obtained in the optimization have been summarized in Table 3. The Cu atoms lying Table 3. Optimized Distances between the First and Second Layers, d1st−2ndCu layer, and the Second and Third Layers, d2nd−3rdCu layer in the Hollow, Bridge, and Top C6H6/Cu(100) Systems and in the Clean Cu(100) Metal Slab Obtained with a 1 × 1 × 1 K-Mesha adsorption site

d1st−secondCu layer(Å)

d2nd−thirdCu layer(Å)

hollow bridge top Cu(100)

1.846 1.834 1.831 1.829

1.740 1.770 1.767 1.765

Figure 3. Density of states projected on the Cu, NaCl, and C6H6 atoms for the studied systems: (right) surface, (left) molecule adsorbed on the surface. Bottom graph corresponds to the isolated molecule (for reference). Energy referred to the Fermi level. Work function is also given.

located in the gap that appears in the projected bulk band structure.65−67 The molecule−metal interaction is reflected in the shift of the orbitals with respect to the gas phase and in their broadening. The charge transfer is seen in the mixing of the LUMO with metal states. nML-NaCl/Cu(100). In this section we present the results obtained for the optimized geometries and the electronic structure of 1, 2, and 3 monolayers of NaCl adsorbed on the Cu(100) surface. The lattice constant of an isolated NaCl monolayer is 5.55 Å68 with the Na cations and the Cl anions in a flat configuration. A lattice matching between the NaCl layers and the Cu(100) surface is reached with a biaxial compression of 1.77%, i.e., with a final lattice parameter of of 5.45 Å. This compression represents a NaCl:Cu matching of 2:3 along the x and y directions (parallel to the surface plane), as shown in Figure 4. Therefore, when a NaCl overlayer is deposited on the Cu(100) surface, the unit cell contains 2 NaCl pairs. This structure is built up by placing one Cl atom on a top site of the copper surface, thus leading to three different Cl positions and one type of Na position. The second NaCl layer was added to keep the assemblage as in the bulk, i.e., Na on top of Cl and vice versa, leading thus to three positions of Na and one for Cl. Finally, the third layer is placed on the top of the second one, recovering the orientation of the first one. Table 4 summarizes the distances obtained in the geometry optimization. From this table we can conclude that the most noteworthy difference between the NaCl monolayers is the decreased rumpling of the interface, i.e., Na+ and Cl− ions positions get flatter as more overlayers are added to the structure, i.e., the corrugation becomes smaller with the number of monolayers. Furthermore, we also observe that, upon deposition of the NaCl overlayers, the copper atoms in the

a The third and fourth layers have been fixed, and their positions are thus not allowed to be relaxed.

immediately beneath the adsorbate are raised up toward the benzene molecule 0.072 (hollow), 0.016 (top), and 0.037 Å (bridge), accordingly with the obtained binding energies: the hollow site shows the highest adsorption energy and the largest distortion of the Cu layers, while the top site presents the weakest bonding energy and the smallest distortion of the atoms in the surface. Consequently, the molecule−Cu distance is smaller for the hollow configuration due to the higher interaction energy. We evaluated as well the charge transfer taking place from the surface to the molecule performing a Bader analysis: the final charge on the molecule is −0.223 (hollow), −0.013 (bridge), and −0.002 (top). These small values confirm that the interaction between benzene and Cu(100) is predominantly due to the dispersion forces, and therefore, it will not be correctly addressed without taking them in consideration. The small charge transfer from the metal to the molecule also explains the minor geometrical distortions of both the substrate and the adsorbate upon adsorption. On the other hand, we studied the electronic structure by evaluating the projected density of states (PDOS). Figure 3 depicts the PDOS for the most favorable position C6H6/ Cu(100)−hollow. The computed work function for the clean Cu(100) surface is 4.57 eV, and for C6H6/Cu(100)−hollow it is 3.74 eV. We observe in the figure that the Fermi level is D


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Table 5. Adsorption Energies (Eads) and Equilibrium Distances (dC6H6−X) for All Inequivalent Sites between Benzene and the Ion Immediately Beneath (X may be either Cl− or Na+ depending on the system) C6H6/1ML-NaCl/Cu(100)

C6H6/2ML-NaCl/Cu(100)

C6H6/3ML-NaCl/Cu(100)

Figure 4. Structure of one NaCl monolayer on Cu(100). Large purple and small green open circles represent Cl− and Na+ ions, respectively. On the Cu(100) surface there are three inequivalent sites for Cl− ions: top, bridge, and hollow sites, depending on the Cu atom lying beneath, while Na+ ions are all placed in equivalent positions. Dashed line shows the unit cell with the 2:3 mismatching in NaCl:Cu. Solid line shows the unit cell employed for studying adsorption of benzene molecules.

C6H6/NaCl

first layer

second layer

third layer

atom

1 ML

2 ML

3 ML

Cltop Clbridge Clhollow Na Natop Nabridge Nahollow Cl Cltop Clbridge Clhollow Na

2.609 2.953 2.692 2.588

2.666 2.748 2.702 2.707 5.492 5.532 5.478 5.608

2.676 2.769 2.721 2.652 5.586 5.617 5.569 5.514 8.449 8.475 8.438 8.312

Eads (eV)

dC6H6−X (Å)

Cltop Clbridge Clhollow Na Natop Nabridge Nahollow Cl Cltop Clbridge Clhollow Na Cl Na

−0.54 −0.75 −0.49 −0.39 −0.38 −0.37 −0.35 −0.47 −0.29 −0.35 −0.17 −0.26 −0.19 −0.12

3.221 3.212 3.312 2.531 2.583 2.661 2.686 3.217 3.146 3.223 3.146 2.626 3.192 2.913

geometries, and charge transfer in comparison with the main adsorption sites here presented. Adsorption energies and distances on the different studied sites are shown in Figure 5. We observe the following. (i) The distance between C6H6 and the ion of the insulating film, immediately below the adsorbate, is always shorter

Table 4. Distances (Angstroms) between Each Inequivalent Atom of the First, Second, and Third NaCl Layers and the Cu Surface n

ads site

interface get slightly compressed (∼0.01−0.04 Å). The geometrical distortion of the Cu surface is smaller when the number of NaCl monolayers increases. We also studied the electronic structure of the three systems 1ML-NaCl/Cu(100), 2ML-NaCl/Cu(100), and 3ML-NaCl/Cu(100) by analyzing the PDOS (see Figure 3). We see that the position of the metal-projected band gap is not affected with the number of NaCl and that the work function is reduced with respect to the metal surface (ϕ = 4.57, 4.00, 3.62, and 3.84 eV for 0, 1, 2, and 3 ML of NaCl, respectively). C6H6/nML-NaCl/Cu(100). Table 5 summarizes the results obtained in geometry optimization of C6H6/nML-NaCl/ Cu(100) for n = 1, 2, and 3 monolayers. There are four inequivalent adsorption sites on the NaCl surface: for the first and third layer Cltop, Clhollow, Clbridge, and Na; for the second layer Natop, Nahollow, Nabridge, and Cl. The notation corresponds to the Cu atom placed below each ion. In the case of one monolayer of NaCl we also checked intermediate positions in between Clbridge and Cltop sites (HBT) and in between Clhollow and Clbridge sites (HHB). For these two last positions we found almost no substantial differences in adsorption energies,

Figure 5. Adsorption energies and distances for the most stable geometries in C6H6/nML-NaCl/Cu(100). E


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The Journal of Physical Chemistry C when the benzene ring is placed on top of Na+ than when it is on top of Cl−, regardless of the number of NaCl layers in the system. The reason is that Cl− presents a larger radius than Na+. It is however remarkable that when the interaction takes place with Cl− a greater distortion is observed (see Figure 6). The Cl− ion interacting with the molecule is 0.73/0.40/0.42 Å below the layer for 1ML/2ML/3ML of NaCl.

the limit when a large number of NaCl layers are placed on the metal surface. We first computed the bare NaCl surface. We included 4 atomic layers and allowed relaxation of the first two layers. In the geometry optimization we observed that the Cl− ions in the first layer are 0.134 Å above the Na+ ions, while in the second layer Cl− ions are found 0.053 Å below the Na+ ions. After this we explored the two main adsorption sites on the NaCl surface: benzene on top of a Cl− ion and on top of a Na+ ion. Adsorption energies and distances for both are given in Table 5. The interaction strength is lower than in the 1 and 2 ML cases but of the same order than in the 3 ML case. Distances in the bulk are of the same order as in the previous nML-NaCl/Cu(100) cases: benzene is closer to a Na+ than to a Cl− ion. Also, in the same manner as for nML-NaCl/Cu(100), in the case of NaCl bulk, when the benzene molecule interacts with Cl−, the ion moves toward the bulk (0.312 Å); however, when the molecule is placed on the top of Na+, the ion is raised toward the molecule out the surface (0.266 Å). In the PDOS we observe a shift with respect to the cases with metal surface. This is due to the larger work function in the NaCl as insulator. The low interaction between the molecule and the substrate is reflected in the PDOS, which is quite similar to in the molecule on the gas phase.

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DISCUSSION Figure 7a, 7b, and 7c shows the adsorption energy, charge transfer, and molecule−surface distance for the most stable position as a function of the number of NaCl monolayers, respectively. The adsorption energy gets smaller on increasing the number of monolayers. For comparison, we represented in the same figure the adsorption energy for 1, 2, and 3 ML with and without metal substrate assuming a flat configuration for the NaCl ions (i.e., before geometry optimization). We can thus evaluate two effects: the presence of metal substrate under the NaCl monolayers and the distortion of the NaCl layers after geometry optimization; both contribute to a stabilization in ∼0.3−0.5 eV each. In the case of direct interaction with the metal surface a small charge transfer is observed, but with the presence of NaCl it disappears. Further analysis of the charge transfer can be carried out with visualization of the induced charge density, ρinduced

Figure 6. Top and lateral view of the most stable structure obtained in C6H6/nML-NaCl/Cu(100) and C6H6/NaCl.

ρinduced = ρ(Ads/Subs) − ρ(Ads) − ρ(Subs)

(2)

where ρ(Ads/Subs), ρ(Ads), and ρ(Subs) are the electron densities of the adsorbed system, the isolated adsorbate, and the isolated substrate, respectively. In Figure 8 we show top and lateral views of the induced charge density of the most stable structure of each studied system. In the case of C6H6/Cu(100) we observe changes in the electron density in between the adsorbate and the metal substrate. The density is reduced in the π orbitals of benzene and the Cu atoms lying beneath and is augmented in the molecule/surface interface. The rise in charge density is responsible for the interactions between the benzene and the Cu(100). On the other hand, for C6H6/nML-NaCl/ Cu(100) we observe a depletion of the electron density immediately beneath the benzene molecule and in between the former and the NaCl surface. Furthermore, the molecule− metal decoupling is clearly observed, since no remaining electron density between them is appreciated. Similar behavior is observed for C6H6/NaCl(Bulk): electron density depletion in the middle part between the adsorbate and the substrate. These observations represent a further confirmation of the decoupling

(ii) The adsorption energy decreases with the number of monolayers, i.e., we observe a larger decoupling between the benzene and the metal surface when increasing the number of layers. (iii) Adsorption on top of a Cl− is more effective than on top of a Na+, with the bridge one (ClB) being more stable when several Cl− positions are possible. Figure 3 shows the electronic structure of the C6H6/nMLNaCl/Cu(100) systems studied. As we previously observed in the nML-NaCl/Cu(100) systems, the workfunction decreases with respect to the clean metal surface: ϕ = 3.84, 3.48, and 3.49 for 1, 2, and 3 ML of NaCl, respectively. We observe weaker interaction of the molecular orbitals with the substrate: only a shift in the position (due to the change of the work function) is observed with respect to the gas phase. C6H6/NaCl. We also considered the interaction between benzene and the surface of sodium chloride bulk (i.e., without any metallic substrate underneath). This can be considered as F


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Figure 7. Adsorption energy, charge transfer, and adsorption distance in the favorable position of the C6H6/nML-NaCl/Cu(100) systems studied as a function of the number of monolayers. In the adsorption energy we considered three cases: flat NaCl geometry without metal substrate (triangles dashed red line), flat NaCl geometry with metal substrate (triangles with dotted lines), and geometry after optimization with metal substrate (circles with full lines); (∞) bulk NaCl surface, without metal.

When the molecule interacts with the metal surface, the distance is smaller than in cases with NaCl. The reason is the interaction between the π electronic cloud of the molecule with the Cl− and Na+ ions. At this point it is relevant to consider the interaction of single Cl− or Na+ ion with benzene in the gas phase. Interaction of these ions with aromatic π structures in the gas phase has been widely studied (see, e.g., ref 69−77). These studies reveal that interaction above the center of the benzene ring for Na+ is of the order of 25 kcal/mol69,72−76 and with a large electrostatic component,75,76 while for Cl− the interaction is repulsive (a minimum above the ring is located with an interaction energy of +0.9 kcal/mol77). Indeed, interaction with Cl− becomes attractive only when electronwithdrawing substituents are considered, as in perfluorbenzene (C6F6).70−72,77 These results strongly differ with our findings that the preferred interaction is in the ClB position. Deeper analysis reveals that when a flat NaCl layer without metal substrate is considered the interaction energy is practically zero or positive (red triangles in Figure 7a). It is the presence of the metal and the distortion of the NaCl layer upon adsorption that are responsible for a strong polarization of the Cl− ions and creation of local atomic dipoles (see below). When these atomic dipoles are created, the part of the Cl− ions facing the surface is electron deficient and attracts the electronic cloud of benzene. As clearly seen in Figure 8, the π electronic cloud of benzene is polarized toward the Cl− and an accumulation of electron density immediately beneath the molecule is observed. Figure 8 also shows that part of the charge recovered by Cl− is accumulated between the ion and the metal atom underneath (Cu or Na). This accumulation of charge is coherent with the displacement of the Cl− ion interacting with the molecule out of the NaCl plane. We also evaluated the importance of including the vdW forces to study the decoupling when NaCl layers are introduced. Table 6 shows the results after geometry optimization with and without including the Grimme correction for the most stable position for the systems C6H6/ Cu(100), C6H6/1NaCl/Cu(100), and C6H6/2NaCl/Cu(100).

Figure 8. Top and cross-sectional views of the induced charge density ρinduced. Red and blue correspond to accumulation and depletion, respectively. The isovalue for the surface is 5 × 10−5 au.

of the molecule from the metallic surface when an ultrathin ionic insulator (NaCl) covers the Cu(100) surface. G


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The Journal of Physical Chemistry C Table 6. Adsorption Energy, Eads, Equilibrium Distances, dC6H6−Surf, Work Function, Δϕ, and Bader Charge Analysis, qBader, after Geometry Optimization with and without the Grimme Correction for the C6H6/n−NaCl/Cu(100) Systems, n = 0, 1, and 2 ML system C6H6/Cu(100) with vdW C6H6/Cu(100) without vdW C6H6/1NaCl/Cu(100) with vdW C6H6/1NaCl/Cu(100) without vdW C6H6/2NaCl/Cu(100) with vdW C6H6/2NaCl/Cu(100) without vdW

Eads (eV)

dC6H6−Surf (Å)

Δϕ (eV)

qBader

−1.02 −0.23 −0.75

2.26 3.12 3.21

3.74 4.16 3.84

−0.22 0.00 −0.02

−0.02

3.43

3.93

0.00

−0.47

3.21

3.48

0.01

−0.01

3.46

3.70

0.00

We can see that the interaction energies and distances are underestimated when weak forces are missing in the simulation, and thus, even in the direct interaction between the molecule and the metal surface (without ionic layers) spurious decoupling is shown. Figure 9 represents the work function as a function of the number of NaCl monolayers for cases with and without

Figure 10. Schematic representation of the different effects that modify the local dipole on the surface and thus change the work function. Brown and blue rectangles represent the metal surface and the NaCl overlayer, respectively, black line in a shows the electron density of the metal surface. Arrows indicate the dipole (μ). Na+ and Cl− ions are represented by circles. Positive and negative symbols (+ and −) represent the charge.

(b) In the metal−molecule interface, a dipole is created due the charge transfer between the surface and the molecule. This dipole also modifies the work function with respect to the bare surface: if electrons are transferred to the molecule the work function is increased. (c) In the geometry optimization we observed that the Na+ and Cl− ions are placed at different heights. This results in a layer of positive charges and a layer of negative charges at different heights, i.e., a dipole layer. The potential of this dipole layer also changes the work function with respect to the metal surface.79 Depending on the relative position of the positive and negative ions the work function is increased or reduced. (d) The atoms in the ionic layer (Na+ and Cl−) are polarized due to the potential created by the dipole layer (c). In particular, Cl− is quite polarizable, and thus, a layer of atomic dipoles appears on the surface. The potential created by this atomic dipoles also modifies the work function.79 The computed surface dipole for each studied system is given in Table 7. Therefore, the combination of these factors influences in the changes overved in the work function upon adsorption of NaCl layers and benzene. Our simulations are

Figure 9. Work function as a function of the number of NaCl monolayers. Computed values for the substrate and the substrate with the adsorbed molecule are shown: (∞) bulk NaCl surface, without metal.

molecular adsorption. As a general trend we observe reduction with the presence of NaCl and with the adsorption of the molecule. Changes in the local dipole of the surface, upon adsorption of the molecule and due to the presence of NaCl layers, modifies the work function. Four different effects contribute to change the dipole (see schemes in Figure 10 a−d). (a) The electron density of the metal surface is pushed toward the bulk due to the presence of the molecule and the NaCl layer; the compressive electrostatic density modifies the local dipole, thus reducing the work function with respect to the bare metal surface.78

Table 7. Surface Dipole (in e− Å) for the Most Stable Configuration in the Different Studied Systemsa C6H6/Cu(100) C6H6/1ML-NaCl/Cu(100) C6H6/2ML-NaCl/Cu(100) C6H6/1ML-NaCl/Cu(100) C6H6/NaCl a

H

−0.475 −0.848 −1.211 −1.574 +0.492

−0.012 −0.672 −1.175 −1.517 +1.005

Cu(100) 1 ML-NaCl/Cu(100) 2 ML-NaCl/Cu(100) 3 ML-NaCl/Cu(100) NaCl

The corresponding bare surface dipole is given for comparison. DOI: 10.1021/jp5106604 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article


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thus in agreement with the experimental measurements of work function reduction in NaCl films on metal surfaces.80−82

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CONCLUSIONS We presented a theoretical study of the adsorption of benzene on Cu(100) and Cu(100) covered with one, two, and three monolayers of NaCl. We included in our simulations van der Waals dispersion forces via a DFT-D2 approach applied to the atoms in the molecule, the ionic film, and the first layer of Cu in the slab. We found that adsorption energies decrease as the number of NaCl layers lying beneath the Cu surface and the molecule increases. Charge transfer becomes negligible when including NaCl layers. The analysis of the electronic structure and the differences in charge density leads us to conclude that the adsorbate is electronically decoupled from the surface and, therefore, that the interaction is weak, mainly due to van der Waals dispersion forces. Overall, ultrathin films behave very different from bare surfaces, pointing out the important role of 2D ionic materials in modulating molecule/metal substrate interaction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the generous allocation of computer time at the Centro de Computación Cientfica at the Universidad Autónoma de Madrid (CCC-UAM) and the Red Española de Supercomputación (RES). Work was partially supported by the projects FIS2010-15127 and CTQ2010-17006 (MICINN), FIS2013-42002-R and CTQ2013-43698-P (MINECO), and S2009/MAT1726 (CAM). M.R. acknowledges the FPI grant associated with the project CTQ2010-17006 of the Spanish Ministerio de Economiá y Competitividad. S.D.-T. gratefully acknowledges the “Ramón y Cajal” program of the Spanish Ministerio de Educación y Ciencia.

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Adsorption of Benzene on Cu (100) and on Cu (100) Covered with an

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