Pauli Repulsion Versus van der Waals: Interaction ... - ACS Publications

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Pauli Repulsion Versus van der Waals: Interaction of Indenocorannulene with a Cu(111) Surface Laura Zoppi,†,‡ Quirin Stöckl,† Anaïs Mairena,† Oliver Allemann,‡ Jay S. Siegel,§ Kim K. Baldridge,§ and Karl-Heinz Ernst*,†,‡ Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland § Health Science Platform, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, P. R. China † ‡

S Supporting Information *

ABSTRACT: Modification of metal electrode surfaces with functional organic molecules is an important step toward organic electronics. The interaction of the buckybowl indenocorannulene with a Cu(111) surface and the twodimensional self-assembly on the same surface was studied by means of scanning tunneling microscopy and dispersion-enabled density functional theory. Based on the conjecture of maximizing van der Waals interaction with the surface one would expect the indeno group to be aligned parallel to the surface. Theoretical investigations predict a nonparallel arrangement with the benzo ring of the indeno group located higher above the surface than the bowl rim connected to the indeno group. This adsorbate geometry is due to strong electronic interaction between molecule and surface, including substantial Pauli repulsion. The long-range ordered monolayer shows differences for two molecules of the unit cell in scanning tunneling microscopy contrast, suggesting either different polar alignments, and therefore a different tilt of the indeno group, or occupation of different adsorption sites.

1. INTRODUCTION The modification of metal electrode surfaces with functional organic molecules is a fundamental step for fabrication of organic electronics, such as organic light emitting devices (OLEDs), organic field effect transistors or organic solar cells. One aspect is the charge carrier injection and the alignment of the molecular orbitals with respect to the Fermi level of the electrode. Hence, the metal organic interface plays an important role,1 and tailoring the work function is one step of tuning the device properties.2 In order to minimize operational voltages, the cathode (electron injecting electrode) should have a low work function, while the hole injecting electrode (anode) should have a high work function. The work function, however, depends strongly on the interaction between substrate and organic adsorbate layer and the resulting interface dipole and not solely on the properties of the component materials.3 The bowl-shaped fragments of Buckminsterfullerene, socalled buckybowls,4,5 show a distinct difference in electron density between their concave and convex sides. Adsorbed on a metal surface the high electron density at the convex side causes Pauli repulsion in the substrate, which induces a large dipole moment of the entire adsorbate complex,6 and a strong electrostatic adsorptive bond of the molecule.7 When adsorbed on metal surfaces, buckybowls have been also found to show © XXXX American Chemical Society

very interesting phenomena, such as pentagonal molecular tiling,8−10 reversible phase transitions,7,11 chiral surface reconstructions,12 and bowl-in-bowl complexation.13,14 The buckybowl corannulene (C20H10, Figure 1a) has also large electron-acceptor ability, forming a 4-fold negative anion upon interaction with alkali metals.15,16 Corannulene packing on Cu(111) and Cu(110) is characterized by the convex surface of the bowl pointing toward the surface and a tilting of the molecular 5-fold axis.7,17 Due to the bowl depth, holding the 5-fold axis perpendicular to the surface requires the ten rim atoms of the bowl (and to some extent the five spoke atoms) to lift away from the surface, thereby weakening the van der Waals (vdW) interaction between substrate and molecule. At the same time, the electron density is largest above the center pentagonal ring at the convex side. Again, with the pentagonal axis perpendicular, the Pauli repulsion in the substrate would be substantial. The tilt for corannulene can be therefore explained as maximizing the vdW contact and minimizing the Pauli repulsion. For indenocorannulene (IC, C26H12, Figure 1b,c), the 5-fold symmetry is Special Issue: Miquel B. Salmeron Festschrift Received: July 15, 2017 Revised: September 9, 2017 Published: September 14, 2017 A

DOI: 10.1021/acs.jpcb.7b06967 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

surface of copper, comparing scanning tunneling microscopy (STM) with force field and dispersion-enabled density functional theory (DFT-D) calculations (using the optB86vdW functional).

2. METHODS IC had been synthesized as described previously.19 It was evaporated from a Knudsen cell at 423 K for 13 min in order to achieve a monolayer on the Cu(111) single crystal surface held at room temperature. The sample was then cooled to 60 K in order to have a better STM resolution. STM studies were performed in a stainless steel ultrahigh vacuum (UHV) chamber (p = 2 · 10−10 mbar) equipped with a variabletemperature instrument (Omicron Nanotechnology). The polished copper(111) single crystal surface (Matek, Jülich) could be liquid helium cooled to about 60 K and indirectly heated to 800 K. STM measurements were performed at room temperature (RT) in constant current mode. The given bias voltage always refers to the sample with respect to the tip; that is, a positive value indicates tunneling from the tip to the sample. The in vacuo Cu(111) surface preparation consisted of cycles of prolonged argon ion bombardment plus annealing. The adsorbate lattice periodicity with respect to the substrate lattice periodicity was determined by inverse Fourier transformation of STM images. Single adsorbates and dimers were computed with the Amber force field of Hyperchem 8.0. A four-layer Cu(111) slab with periodic boundary conditions was used as template, with the copper atoms fixed in space during the calculations. For the single molecule and the dimers 216 and 288 different initial configurations (x, y, z coordinates, z = axis normal to the copper surface, no bowl-opening-down configurations) were tested, respectively. The molecules were enabled to move freely during the optimization. DFT calculations were performed with a optB86-vdW functional,20,21 which accounts for dispersion effects, and ultrasoft pseusopotentials as implemented in the Quantum Espresso package (http://www.quantum-espresso.org/). The single-particle electronic wave functions and charge densities were expanded in a plane-wave basis set, up to an energy cutoff of 25 and 250 Ry, respectively. An isolated molecule on the surface was considered, where a periodic four-atom-layer slab consisting of 440 Cu atoms and a vacuum space of 23 Å perpendicular to the surface direction was used. Dipole corrections along the direction perpendicular to the metal surface area were applied. Structural relaxations have been performed at the GAMMA point with the bottom two layers fixed and all other atoms allowed to relax unconstrained until the forces on each atom are less than 0.013 eV/Å.

Figure 1. Top and side views of ball-and-stick molecular models of the buckybowl corannulene (a) and indenocorannulene (b). (c) Definition of bonds and labeling of four carbon atoms.

broken by a benzene tab added to the rim of corannulene, which also results in a substantially deeper bowl depth. Following the empirical observation from previous studies, one would predict that a packing conformation of the molecule with the five-membered hub ring parallel to the surface would be even worse for placing the spoke and rim atoms away from the surface; furthermore the benzene tab at the rim would essentially be out of contact with the substrate. Therefore, a clear prediction is that IC would tilt so as to bring the tab into contact with the substrate and thus also maximize vdW contact between substrate and coating molecule. This logic is a sound syllogism as long as the coating molecule-to-substrate interaction is the dominant component determining the preferred packing, however, it must be kept in mind that packing is a cooperative phenomenon of all the lateral interactions of coating molecules-to-coating molecules as well as their interactions with the substrate. This is well illustrated in the polymorphism seen in studying corannulene on Cu(111)7 with the epitaxial coating of C60 onto a monolayer of corannulene on copper, where evidence for a competition between C60 internal island interactions and C60-corannulene interactions influenced packing behavior at room and low temperatures,14 and for a C70-derived bowl, where partial bowlopening-down configurations due to lateral π−π interactions have been observed.18 Therefore, even when the energy of the system would be optimized by simply maximizing vdW surface area, the solution to that function may manifest greater complexity in packing patterns. Along with maximum vdW contacts, a surface-to-coating molecule positional preference is known for benzenoid rings to sit over 3-fold hollow sites on (111) surfaces. These secondary determinants of the molecular positioning can have substantial effects on the overall packing motif, even if the rough orientation of the coating molecule to the surface plane remains similar. This article presents a combined experimental and theoretical study of the binding and self-assembly of IC on the (111)

3. RESULTS AND DISCUSSION An ordered phase of IC on Cu(111) can be observed in STM at room temperature only at coverages close to the monolayer saturation coverage. It remains stable during cooling to 60 K (Figure 2a) and annealing of the sample does not change the molecular distribution on the surface nor the quality of the ordering. Long-range order STM images show the molecules apparently aligned in zigzag rows; rotational as well as mirror domains (with an angle of 22° between the two mirror domains) are present. The unit cell has a (5 1, 2 7)22 periodicity with respect to the Cu surface and contains two molecules covering 33 Cu(111) surface atoms. Hence, one B


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Figure 3. Top and side view of ball-and-stick models of IC on Cu(111) after optimization. (a) Result of the optB86-vdW DFT calculations. (b) Configuration as result of Amber force field calculations.

configuration, a similar final alignment to the one shown in Figure 3a was obtained (Figure S2). Just a different relative registration with respect to copper surface sites became adopted in that case. Other starting configurations (for example with the C6 ring opposite to the indeno group parallel to the surface) led to adsorbate geometries that were substantially higher in energy, i.e., the relaxations got trapped in local energy minima (Figure S3). The lowest DFT-derived energy configuration yields an adsorption energy of 3.4 eV, which is substantial for molecular adsorbates. Amber force field calculations yielded a molecule-to-surface alignment in which the benzene tab is almost parallel to the surface (Figure 3b) with carbon #4 above a hcp-3-fold hollow site, but with a substantially larger distance to the surface (3.48 Å to the two Cu atoms below the “1−4” spoke) than obtained by DFT. Molecular dynamics simulations, such as offered by Amber software, can be valuable when comparing different identical molecular configurations on surfaces,18,24 but it is not suitable for absolute adsorption energies, because adsorbate and the isolated components (surface and molecule) present different potential energy surfaces.25 The motivation to compare DFT with Amber force fields in this case came from previous results for polyaromatic pentahelicene adsorption on the same surface where both methods were shown to yield almost identical results with regard to the final molecular conformation on the surface.24 It is therefore of interest to compare the Amber performance with respect to the DFT-dispersion-enabled results for the buckybowl investigated in this work. Given this, it is wellknown that parametrized force fields cannot account for electronic polarization effects that, as will be shown further below, are important for the metal surface binding mechanism of buckybowls. STM images of single domains of the closed-packed monolayer under various tunneling conditions highlight the zigzag row structure (Figure 4). All molecules are more or less oriented identically, installing a polar alignment, i.e., all convex bowl sides point into the same direction in a single domain. However, it seems that the zigzag appearance is not necessarily due to different relative positions of the two molecules in the

Figure 2. STM images of IC self-assembled on Cu(111). (a) STM image (80 nm × 80 nm, U = −1.185 V, I = 129 pA, T = 63 K) showing a zigzag-like alignment of IC molecules. (b) STM image (1.36 nm × 1.26 nm, U = −1.162 V, I = 253 pA, T = 58 K) of a single molecule. (c) Top view of a superposition of a molecular ball-and-stick model with the STM image shown in (b).

molecule occupies an area equivalent to 16.5 surface Cu atoms. An STM image of a single molecule is shown in Figure 2b. Its appearance is dominated by a dark depression in the center and a bright protrusion marking the rim of the molecule. Such appearance is best explained by a substantial tilt of the molecule,23 with the benzene tab more or less parallel to the surface. The benzo moiety of the indeno group was resolved in STM as well (Figure 2c). In order to evaluate the exact geometry of the adsorbate complex, Amber force field as well as DFT calculations (for initial configurations see Supporting Information) were conducted. The lowest energy results are shown in Figure 3. The DFT calculations yielded basically two degenerate configurations (ΔE = 1.3 meV) in which carbon atom #4 (see Figure 1c) is closest to the surface and atoms #1, 2, 3 are located above Cu surface atoms at identical distances to them (Figures 3a, S1, and S2). The distances to the plane of surface atoms are 1.8 and 2.2 Å, respectively. For the relaxed configuration shown in Figure 3a, the starting configuration was chosen such that the indeno group was located already parallel to the surface with its benzo group located above a 3fold hollow site (Figure S1). Surprisingly, during relaxation the indeno group was somewhat lifted up away from its parallel alignment to the surface. Starting from another initial C


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Figure 4. STM images of the IC zigzag phase on Cu(111) and structure models. (a) STM image from a single domain (20 nm × 20 nm, U = −1.071 V, I = 179 pA, T = 85 K). (b) STM image from a different domain acquired under different conditions (12 nm × 12 nm, U = −1.115 V, I = 238 pA, T = 65 K). (c) Structure model of the (5 1, 2 7) phase, also highlighting the expected STM contrast by filled ellipses. The differences in STM contrast are indicated by different transparencies of the ellipses. The two molecules of the unit cell are located on different adsites. (d) Model structure of the (5 1, 2 7) phase with all molecules placed on identical adsites. In both models the molecular unit cell and the surface lattice vectors are indicated. In order to match the alignment of the model shown in (c), the original image of (b) has been reflected and turned by 180°.

unit cell, but rather due to a contrast difference between adjacent molecules in the zigzag row. For a (5 1, 2 7) lattice one molecule of the unit cell, if placed in its center, would be located above a different surface site than the four corner molecules of that cell. This alone may explain the zigzag appearance, but it could be as well due to different tilts with respect to the surface. That one molecule shows a different tilt under close-packing is not unlikely. Such detailed aspect, however, cannot be finally settled by STM here. The presented tentative model structure in Figure 4c has the molecule as much as possible in the center of the cell, but on different adsites (best seen by the different positions of the spoke pointing toward the indeno group). An alternate model (Figure 4d) has the two molecules basically on identical sites, but requires a somewhat more nonuniform distribution in the unit cell. Such nonuniformity could be explained by directed attractive forces in the molecular lattice. Indeed one could account for C−H···π bonds in this model (Figure S4). And although modeling of a dimer actually suggests such binding motif (Figure S4), one needs to be cautious drawing such conclusion. Short distances between functional groups of two molecules in molecular lattices should not be taken as attractive intermolecular bonding, because these short distances may emerge from packing constraints induced by the surface or surrounding molecules.26,27 In order to unravel the binding mechanism of IC to the surface, the charge rearrangement process at the organic-metal interface upon molecular adsorption needs to be considered. Figure 5 shows the charge redistribution in and around an IC adsorbate complex. Details of similar DFT calculations for the system corannulene/Cu(111) have been published previously.6,7 A pronounced charge displacement within the whole adsorbate is revealed for IC on Cu(111) (Figure 5). Underneath and on the side of the molecule a bowl-shaped accumulation of charge is observed (red cloud; blue color stands for depletion of charge). Electrons of the copper surface are repelled down and sideways. Note that on the bare surface electrons usually spill-out to some extent above the surface atoms. An overlap of occupied electronic wave functions of the molecule with those of the metal is strongly hindered due to the Pauli-exclusion-principle. As a consequence, deformation of the charge distribution in the topmost layers of the metal is induced. Together with this “push-back” effect in the substrate, a depletion of charge is caused in the molecular frame, leading to charge separation between the upper part of the molecule

Figure 5. Top (a) and side view (b) of the charge redistribution in the adsorbate of IC. A contour cutoff value of 8 · 10−4 e/Å3 was used. Red depicts accumulation of charge, blue depletion. (c) Plot of linear electron density (e/Å) perpendicular to the surface. The inset reveals clearly the accumulation of electron density between molecule and top surface layer and the depletion in the molecule.

and the Cu subsurface region. A charge density plot perpendicular to the surface (Figure 5c) highlights this charge rearrangement. From the top view (Figure 5a) it becomes clear that the Pauli repulsion is large around the indeno group, while negligible at the other end of the bowl, located substantially above the surface. Between surface and molecule, charge is accumulated, while in region of the molecule charge is depleted. This principle of “polarization-binding” has been described in detail previously.28−33 As consequence of the charge redisD



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CONCLUSIONS A better understanding of charge carrier transition at metal electrodes of molecular electronic devices should foster their targeted design. The adsorption, alignment, and self-assembly of the buckybowl indenocoranulene on Cu(111) has been studied with submolecular resolution with STM, suggesting the indeno group is oriented more or less parallel to the surface plane. As evaluated by dispersion-enabled DFT, the adsorption of IC leads to a strong surface binding of 3.4 eV. The interaction of IC with Cu(111) shows also pronounced electronic effects in the metal substrate, in particular Pauli repulsion around the molecule. At least 75% of this binding energy must be contributed to an electrostatic bond due to Pauli repulsion and subsequent push-back of the metal electrons away from the interface under the bowl. Due to the Pauli repulsion, the indeno group of IC was found not to align parallel to the surface, as would be expected for pure vdW contact, but somewhat lifted up. Because the Amber force field methodology is empirical in nature and does not account for polarization and charge rearrangement effects, the adsorbate configuration (molecule−surface distance and relative orientation) obtained via this force field approach was substantially different than the DFT-obtained configuration. The charge redistribution upon interaction of IC with the surface induces an interface dipole moment of 7.5 D, which is of the order of alkali metal adsorption on metal surfaces. In contrast to such alkali-metal surfaces systems, no charge transfer into the substrate is involved here. While previously described in great detail for planar adsorbates,28−33 the socalled “cushion effect” has only been observed to such large extent for buckybowls like corannulene, pentamethylcorannulene, and now here for indenocorannulene.6 In particular, the fact that a considerable work function decrease is already achieved at small coverages, makes buckybowl-modified metal surfaces interesting as cathodes in organic electronic devices. In combination with the intense photoluminescence properties of buckybowls,42 they may become very useful for OLED devices.

tribution a large interface dipole moment is created with a strong electrostatic contribution to the adsorption bond. Note that large interface dipoles are usually induced by charge transfer.34 Buckybowls are actually good electron acceptors,15 but it is important to point out that the charge reorganization described here should not be confused with charge transfer. The projected density of states (Figure S5) clearly shows no indication of charge transfer. For corannulene with an intrinsic dipole moment of 2.2 D, the interface dipole of corannulene on Cu(111), calculated through the present Quantum-Espresso dispersion-enabled methodology, corresponds to a value of 6.8 D. Due to the larger bowl depth, IC has an intrinsic dipole moment of 2.9 D in the gas phase and upon adsorption on Cu(111) the calculated interface dipole moment for the single adsorbate complex amounts to 7.5 D. Amazingly, the difference between the intrinsic dipole moments of both buckybowls of Δμ = 0.7 D is preserved in the induced interface dipoles as well. Hence, 4.6 D was induced beyond the intrinsic dipole moments in both cases. (Note that the “dipole moment” of a clean defect-free planar surface can be safely neglected in this evaluation.) The magnitude and direction of the interface dipole moments are similar to the ones observed after charge transfer effect upon alkali adsorption on metals,35−37 as for cesium on Pt(111),38,39 for example, creating a positively charged adsorbate. In both cases, that is, charge transfer to the metal or charge rearrangement by push-back, the induced interface dipole moment opposes the electron spill-out of the clean metal surface, leading to a substantial decrease of the work function.6,40 Interface dipole moments induced by charge rearrangement of such large magnitude have been previously determined experimentally and theoretically for corannulene and pentamethylcorannulene.6 With the inability of the Amber force field to include polarization and charge rearrangement effects, and therefore to account properly for a Pauli repulsion contribution, it becomes now clear, why the adsorption configuration obtained from Amber force field calculations yields−compared to the DFT calculations, a larger binding distance to the surface. When a π-conjugated molecular adsorbate is pulled toward a metal substrate via vdW forces, the net effect is a characteristic deformation of the surface metal charge, which is repelled down and sideways. Instead of maximizing the vdW contact between substrate and coating molecule, the Pauli repulsion is also responsible for the final molecular distance. In the present case, the indeno group stayed lifted up away from a parallel alignment to the surface such that the overlap of the electronic wave functions of the molecule with those of the metal is minimized. But it remains the question to which extent vdW and Pauli repulsion contribute to the adsorption bond. From the DFT calculations presented here, such differentiation is hardly possible. However, an upper limit of the vdW-effect can be estimated by comparing the ideno group of IC, which presents basically the maximum vdW contact, with a similar-sized molecule. Based on thermal desorption experiments of naphthalene on Cu(111) an adsorption energy of roughly 0.9 eV has been determined.41 Because this system is expected to show Pauli repulsion as well, the vdW binding must be somewhat lower. Compared to the overall binding energy of 3.4 eV, however, it becomes clear that the Pauli repulsion is the main contribution of the electrostatic surface binding of IC.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b06967. Sketches of DFT calculations, including initial and final geometries; details of molecular models of IC dimers; calculated density of states for the free molecule and the adsorbate on Cu(111) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kim K. Baldridge: 0000-0001-7171-3487 Karl-Heinz Ernst: 0000-0002-2077-4922 Notes

The authors declare no competing financial interest. Researcher IDs: Jay S. Siegel: E-6422-2011; Kim K. Baldridge: E-6415-2011; Karl-Heinz Ernst: O-6128-2015.

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ACKNOWLEDGMENTS The research described in this article has been supported by Schweizerischer Nationalfonds (Project Fundamental aspects of E


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The Journal of Physical Chemistry B molecular self-assemblyFUNDASA # 126776), the UZH Priority Program LightChEC, and the Competence Center for Materials Science and Technology (CCMX). K.K.B. and J.S.S. thank the National Basic Research Program of China (2015CB856500), the Qian Ren Scholar Program of China, and the Synergetic Innovation Center of Chemical Science and Engineering (Tianjin) for supporting this work.

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