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Cu-Adatom-Mediated Bonding in Close-Packed Benzoate/ Cu(110)-Systems M. Christina Lennartz,† Nicolae Atodiresei,† Lars Mu¨ller-Meskamp,† Silvia Kartha¨user,*,† Rainer Waser,†,‡ and Stefan Blu¨gel† Institute for Solid State Research and JARA-FIT, Forschungszentrum Ju¨lich GmbH, 52428 Ju¨lich, Germany, and Institute for Materials in Electrical Engineering and Information Technology 2, RWTH Aachen, Sommerfeldstraβe 24, 52074 Aachen, Germany ReceiVed June 11, 2008. ReVised Manuscript ReceiVed September 5, 2008 Using UHV-STM investigations and density-functional theory calculations we prove the contribution of Cu-adatoms to the stabilization of a new high-density phase of benzoate molecules on a Cu(110) substrate. We show that two different chemical species, benzoate and benzoate Cu-adatoms molecules, build the new close-packed structure. Although both species bind strongly to the copper surface, we identify the benzoate Cu-adatoms molecules as the more mobile species on the surface due to their reduced dipole moment and their lower binding energy compared to benzoate molecules. Therefore, the self-assembly process is supposed to be mediated by benzoate Cu-adatom species, which is analogous to the gold-thiolate species on Au(111) surfaces.
Introduction In the course of cost minimization and product optimization, special attention is focused on the development of electronic devices based on thin films of organic molecules. In analogy to the currently used silicon-based integrated circuits, in which metals, semiconductors, and dielectrics built transistors, capacitors, or interconnectors, the molecular circuits should provide a similar set of components.1,2 In future technological applications, the metals should act as electrodes or interconnectors to the CMOS world, whereas the molecules with their functional groups and their effort to self-assemble in ordered layers are envisaged as the functional elements. Irrespective of the final circuit design, it is necessary to find suitable molecule/metal combinations that provide the desired functionality reliably. Furthermore, reproducible methods for contacting the molecules to the usually metallic electrodes have to be developed. One fundamental requirement to build up future molecular devices is to control the optical and electronic properties of the metal/molecule combinations and to find methods to tune them. Especially the electronic properties depend crucially on the molecular orientation and the crystalline structure. Therefore, a major point of interest is to control the lateral order of molecular arrangements while determining the corresponding molecular transport properties. To get this information, scanning tunneling microscopy (STM)3-6 is one powerful tool for the experimentalists, whereas for the theorists density functional theory (DFT) calculations provide the necessary means to enable the success of molecular electronics. Both tools can offer information on bonding, * To whom correspondence should be addressed. E-mail: s.karthaeuser@ fz-juelich.de. † Forschungszentrum Ju¨lich GmbH. ‡ RWTH Aachen. (1) Rawlett, A. M.; Hopson, T. J.; Amlani, I.; Zhang, R.; Tresek, J.; Nagahara, L. A.; Tsui, R. K.; Goronkin, H. Nanotechnology 2003, 14, 377–384. (2) Stadler, R.; Forshaw, M.; Joachim, C. Nanotechnology 2003, 14, 138–142. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (5) Lu¨ssem, B.; Mu¨ller-Meskamp, L.; Kartha¨user, S.; Homberger, M.; Simon, U.; Waser, R. J. Phys. Chem. C 2007, 111, 6392–6397. (6) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201–341.
interaction, charge distribution, orientation, and alignment of molecular assemblies on surfaces. Combining these two techniques represents a way toward understanding and designing novel electronic elements based on organic molecules adsorbed on surfaces. Because of its low resistance and its high heat capacity, copper combined with molecular materials is supposed to provide the prerequisites for developing integrated circuits with decreasing switching times, reduced heat dissipation, and higher reliability. Carboxylic acids are known to link with the carboxylate moiety to the copper surface, forming ordered self-assembled monolayers.7-9 Therefore, the carboxylate/copper system is a promisingalternative to the intensively studied thiol/gold system.3-5 The development of the understanding of carboxylate/copper systems with special functionalities is assisted by the study of model systems, such as the benzoate/copper system, described in this article. First characterizations of benzoic acid (C6H5COOH) on Cu(110), done by Richardson and co-workers, showed that benzoic acid forms large domains of several well-ordered structures, depending on temperature and coverage. Whereas at low coverages benzoic acid is flat-lying on the surface,7,10,11 the molecules are oriented perpendicular to the surface at high coverages and high temperatures.7-9 ESDIAD studies of the standing-up benzoate species done by J. T. Yates, Jr., and coworkers show the exact orientation of the molecule on the Cu(110) surface.12,13 In this article, we present a UHV-STM experimental study and DFT calculations on the adsorption of benzoic acid on a Cu(110) surface. We demonstrate the role of the copper adatoms in building a new high-coverage phase of benzoate molecules (7) Frederick, B. G.; Leibsle, F. M.; Haq, S.; Richardson, N. V. Surf. ReV. Lett. 1996, 3, 1523–1546. (8) Frederick, B. G.; Chen, Q.; Leibsle, F. M.; Lee, M. B.; Kitching, K. J.; Richardson, N. V. Surf. Sci. 1997, 394, 1–25. (9) Frederick, B. G.; Chen, Q.; Leibsle, F. M.; Dhesi, S. S.; Richardson, N. V. Surf. Sci. 1997, 394, 26–46. (10) Chen, Q.; Frederick, B. G.; Murray, P. W.; Haq, S.; Richardson, N. V. Surf. Sci. 2000, 446, 63–75. (11) Dougherty, D. B.; Masksymovych, P., Jr Surf. Sci. 2006, 600, 4484– 4491. (12) Lee, J.; Kuzmych, O.; Yates, J. T. Surf. Sci. 2005, 582, 117–124. (13) Lee, J.; Dougherty, D. B.; Yates, J. T. J. Phys. Chem. B 2006, 110, 9939–9946.
10.1021/la801822e CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
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Figure 1. STM topography scans of benzoate monolayers on Cu(110): (a) Disordered structures after deposition at 293 K for 5 min (additional copper islands covered by molecules are marked by black circles), and (b) c(8 × 2) ordered monolayer structure (back circles) after annealing the surface to 575 K for 90 min.
on Cu(110). At the same time, our studies enable a better understanding of the phenomena at the benzoate/copper interface. They can provide information on the bonding, charge distribution, orientation, and the alignment of molecular adsorbates on the surface.
Experimental Method STM Investigations. Cu(110) single crystals were polished mechanically and electrochemically ex situ and then transferred to the UHV system. Repeated cycles of Ar+-sputtering (1 keV) and annealing at 700-950 K were used to obtain a surface with the perfect reconstruction of the clean Cu(110) surface. XPS measurements were performed to ensure that no adsorbates like oxygen or carbon are present on the metallic surface. Benzoic acid was purchased from Sigma-Aldrich (99% sublimation cleaned quality) and used for evaporation after further purification in vacuum by freezing/thawing cycles. The molecules were vapor-deposited in a separated chamber. The substrate temperature during deposition was varied between room temperature and 500 K. The pressure was adjusted between 1 × 10-6 and 5 × 10-4 mbar, and the deposition times were varied between 1 and 10 min for different experiments. After the deposition of benzoic acid, the Cu(110) crystal was immediately transferred into the UHV-STM observation chamber, without breaking the vacuum. The STM studies of the benzoate/Cu(110) system were performed at room temperature under UHV conditions of 1 × 10-10 mbar. All images were obtained in constant-current mode with a JSPM-4500S STM head using homemade electrochemically etched tungsten tips. Special care was required for the selection of the set point because benzoate monolayers can be easily disordered by scanning with high voltages or currents.9 The samples were scanned with bias voltages of -1.7 to -0.5 V and currents of 150 to 400 pA.
Results and Discussion STM Investigations. Straightforward deposition to full covered monolayers of benzoic acid on Cu(110) surfaces, using a substrate temperature of 293 K, a deposition time of 5 min, and a deposition pressure of 4.7 × 10-4 mbar, leads to only poorly ordered monolayers. Such a thin film of self-assembled benzoates on a Cu(110) surface was imaged by STM under normal tunneling conditions (UT ) -1.69 V, IT ) 0.18 nA) and is shown in part a of Figure 1. The atomically flat copper terraces are decorated with chemisorbed benzoate molecules showing no clearly visible short-range order.
In addition, there are some remaining spots indicating adlayers of physisorbed molecules. The surface has no domain structure but it features a directed texture caused by the symmetry of the underlying Cu(110) surface. The close-packed Cu rows are spaced by 0.36 nm and indicate a preferential orientation of the chemisorbed molecules. In the high-resolution inset, the molecular arrangement is visible in detail, showing the molecules adsorbed with no visible short-range order. Heating the surface to 500 K does not change its characteristics considerably, whereas heating to around 575 K results in an altered surface structure with obvious annealing effects (part b of Figure 1). Within the disordered phase, observed after deposition, domains of ordered structures occur. These domains were identified as the close-packed c(8 × 2) phase, which was found earlier by Frederick et al.7 and contains typical molecular rows clearly visible in our enlarged STM scan (part a of Figure 2). The ordered structures first arise at certain step edges and then grow into the middle of the copper terraces, in agreement with observations reported.7 The benzoate molecules of this close-packed c(8 × 2) surface structure,6-9,13 are standing upright, and the unit cell can be described by the lattice vectors a0 ) 0.72 nm in the [001] direction and b0 ) 2.04 nm in the [110] direction. A surface model is shown in part b of Figure 2, where the benzoate molecules bind on top of two copper adatoms on the outermost surface layer rows, resulting in a surface coverage of 0.25. Upon heating up to 590 ( 3 K, the surface characteristics continue to change. Parts of clean copper arise due to desorption of molecules from the surface, and furthermore the high temperature elevates the mobility of the molecules on the surface, enabling the remaining molecules to form new structures. The desorption products for an initial coverage of 1 ML benzoic acid are benzene, CO2, and their mass spectrometer cracking products.13 Because of the fact that adsorption of the CO2 molecules can hardly occur on Cu(110) surfaces even at 110 K and of benzene molecules at 300 K, we conclude that the observed molecules after this temperature treatment are remaining benzoic acid molecules. Thus, we observed areas of lower molecular coverage together with an increasing size of ordered c(8 × 2) domains and a new, different benzoate structure evolved, as seen in part a of Figure 3. A region on one terrace with three different surface structures is visible here. The structure at the outside margin can be identified as c(8 × 2) structure, the parts in the middle region without spots are regions of a clear copper surface, and the structure in the middle is a new high-coverage structure. The latter has a smaller feature size of the spots displaying a row direction rotated by
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Figure 2. (a) High-resolution STM scan (UT ) - 0.63 V, IT ) 0.39 nA) of the c(8 × 2) phase. The unit cell and the surface vectors are drawn in. The phase can be described with the model shown in (b). For clarity, the benzoate species are represented just by the carboxylate group.
Figure 3. (a) New close-packed structure of benzoates on Cu(110) with a different texture and changed row direction is visible in the center, surrounded by c(8 × 2) domains. (b) High-resolution image (UT ) - 0.78 V, IT ) 0.36 nA) of the new structure with unit cell and substrate vectors. The linescans along the 〈1,1〉 and the 〈-4,2〉 direction of the surface allow us to determine the unit cell vectors a0 (c) and b0 (d).
an angle of about 35° with respect to the row direction of the known c(8 × 2) structure. Assuming commensurability and a standing-up configuration, the high-resolution STM image (part b of Figure 3) together with the linescans along and between the molecular rows (parts c and d of Figure 3) allow us to identify the unit cell of the new high-coverage phase as a (1 1; -4 2) structure with two molecules in the unit cell. The unit cell vectors are a0 ) 0.51 nm in the 〈1,1〉 direction, corresponding to the [112] direction of the bulk crystal and b0 ) 1.23 nm in the 〈-4,2〉 direction, corresponding to the [111] direction in bulk, respectively. To assign an arrangement of the molecules on the copper lattice, the observed molecular structure is matched with the Cu(110) surface lattice. Both the benzoates at the corners and the benzoates in the centers built rows in the [112] direction. The molecules at the corner of the unit cell can be identified as chemisorbed on top of the copper atom rows of the outermost substrate layer, whereas the benzoate molecule in the center of the unit cell could not be allocated to a lattice site of the copper atoms of the outermost substrate layer. This is remarkable regarding theoretical14-16 and experimental7-11 investigations of carboxylic acids on Cu(110), demonstrating that the carboxylate
moiety always binds at the short copper lattice sites on top of the copper first-layer atoms. One possibility to explain adsorption at the observed lattice site is the faceting of the surface like those observed in the presence of formate, acetate, or benzoate.7,17 The identified 〈1,1〉 surface direction, the direction of the unit cell vector a0, is precisely the direction of the bulk [112] direction favored for step-bunching. In this case, the 〈-4,2〉 surface vector of the new (1 1; -4 2) phase should show two steps downward to provide a short bridge site on the intermediate terrace for the adsorption of the center molecule. As a consequence, the linescan in the 〈-4,2〉 direction (part d of Figure 3) should pinpoint two steps, amounting to 0.26 nm, which is clearly not the case and rules out this possibility. An alternative to provide a short bridge site at the center of the unit cell of the new (1 1; -4 2) phase is the incorporation of copper adatoms. This assumption is supported by studies on flat-lying benzoate monolayers,7,10,11 which can be interpreted by the incorporation of copper adatoms, and also in other molecule/metal systems.18,19 Following this idea, the benzoate (14) Atodiresei, N. First Principles Theory of Organic Molecules on Metal Surfaces: Formate, 3-Thiophene-carboxylate and Glycinate on Cu(110). Ph.D. Thesis, RWTH-Aachen, 2004.
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Figure 4. (a) Schematic of the new adatom-stabilized (1 1; -4 2) structure of the benzoates on the Cu(110) surface. (b) Linescan over the molecular rows marked with a green arrow in (a).
Figure 5. All constituents used to calculate the benzoate/copper system are shown here: benzoic acid (a), benzoate (b), benzoate Cu-adatoms (c), the crystal lattice of copper (d), and the typical striped Cu(110) surface (e). The atoms are described by different colors: gray for C, red for O, dark gray for H, yellow/green for copper adatoms, brown for the first Cu layer, and yellow for the second Cu layer.
binds on top of two copper adatoms adsorbed on the copper surface. This interpretation is in accordance with the heights shown in the linescans in parts c and d of Figure 3 and in part b of Figure 4. Therefore, the new (1 1; -4 2) structure is described with a row of adatoms between the central benzoate molecule and the outermost copper layer so that all benzoate molecules of the unit cell bind on top of copper atoms and bridge the short lattice site. In part a of Figure 4, a schematic of the molecular arrangement of the new, adatom-stabilized structure is drawn. Comparing the size of the unit cell of the c(8 × 2) structure (Figure 2) with the size of the new (1 1; -4 2) structure (part b of Figure 3), we emphasize a difference in packing density. With two molecules per unit cell, the new structure has a packing density of one molecule per three outermost surface copper atoms (surface coverage of 0.33), whereas the c(8 × 2) structure has a packing density of one molecule per four surface atoms (surface coverage of 0.25). The existence of such a higher covered phase was already predicted by Frederick et al. in 19967 but their existence was not proved so far. A reason for this is the preparation conditions we used (T ) 500 K), which cause a high mobility of the benzoate molecules and lead to an increased supply of copper adatoms. To gain a deeper understanding of the STM measurements and to prove the close-packed adatom-stabilized structure, we employed first-principles calculations, which are described in the following section. Theoretical Method. The electronic structure calculations were performed using DFT within the generalized gradient approximation20 as implemented in the VASP code.21-23 The electron-ion interaction is described by the projector-augmented
wave scheme.24 To calculate accurate forces, the plane-wave basis set includes all plane waves up to a cutoff energy Ecut of 500 eV. To compensate the dipole of the asymmetric slabs (because the molecule is placed on one side of the slab), a dipole sheet is introduced in the middle of the vacuum slab.25 To explain the interaction between the copper surface and the benzoate molecule, with and without copper adatoms (parts b and c of Figure 5), we performed calculations for single molecules as well as for a molecular layer of benzoates adsorbed on the copper surface. The molecule/metal system was modeled by a 3D repeated slab consisting of nine atomic layers separated by a vacuum region of 15 Å. The Cu(110) surface (parts d and e of Figure 5) was generated using the theoretical lattice parameters calculated for bulk copper to 3.636 Å. In the calculations of the single molecule adsorbed on the Cu(110) surface, a (5 × 6) in-plane surface unit cell for the Cu(110) lattice was used to (15) Atodiresei, N.; Blu¨gel, S.; Schroeder, K. Phys. ReV. B 2007, 75(115407), 1–14. (16) Atodiresei, N.; Caciuc, V.; Schroeder, K.; Blu¨gel, S. Phys. ReV. B 2007, 76(115433), 1–8. (17) Leibsle, F. M.; Haq, S.; Frederick, B. G.; Bowker, M.; Richardson, N. V. Surf. Sci. 1995, 343, L1175–L1181. (18) Quek, S. Y.; Biener, M. M.; Biener, J.; Bhattacharjee, J; Friend, C. M.; Waghmare, U. V.; Kaxiras, E. J. Phys. Chem. B 2006, 110, 156631–15665. (19) Lundgren, E.; Kresse, G.; Klein, C.; Borg, M.; Andersen, J. N.; De Santis, M.; Gauthier, Y.; Konvicka, C.; Schmid, M.; Varga, P. Phys. ReV. Lett. 2002, 88(246103), 1–4. (20) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865– 3868. (21) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558–561. (22) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251–14269. (23) Kresse, G.; Hafner, J. Phys. ReV. B 1996, 54, 11169–11186. (24) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758–1775. (25) Makov, G.; Payne, M. C. Phys. ReV. B 1995, 51, 4014–4022.
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Figure 6. First starting configuration with one benzoate molecule on top of the first and one on top of the second copper layer (a). The system relaxes to a configuration where both molecules are on top of the first copper layer with a (1 1; -2 1) structure (b). The second configuration has a double row of copper adatoms and the benzoate molecules relax into a (1 1; -4 2) structure (c).
avoid the interactions between the molecule and its periodically repeated images. In these simulations, the Brillouin zone was sampled by the Γ point only because of the large size of the supercells. For the calculations of the molecular layer adsorbed on the Cu(110) surface, the Brillouin zone integrations were replaced by a sum over a (9 × 3 × 1) Monkhorst-Pack k-mesh. All atoms of the molecules and those of the two top copper layers have been relaxed. For the optimized geometries, the calculated Hellmann-Feynman forces were smaller than 0.001 eV/Å. Geometries. We found theoretically that benzoate molecules anchor, in the case of high coverage, perpendicular to the copper surface through the carboxylate group in bridge configuration, like suggested in the experiments. After the deprotonation of the benzoic acid, both oxygen atoms of the carboxylate moiety are chemisorbed on short bridge sites of the outermost copper layer, forming rows in [001] direction of the substrate (Figure 6). The phenyl rings of the molecules are arranged in the same plane as the carboxylate moiety, perpendicular to the surface. The specific aspect of this adsorption geometry is that each oxygen atom of the carboxylate moiety forms chemical bonds with two copper atoms. A similar bonding mechanism is described for high coverages of other molecules, which also use the carboxylate moiety to anchor to the copper surface, that is, formate,14,15 3-thiophene carboxylate,26 and terephthalate.16 The average Cu-O bond length is 1.940 Å, which agrees well with the measured length of M. Pascal27 (1.91 Å) and with the lengths of other calculated carboxylate systems, that is, formate systems14,15 (1.994 Å), 3-thiophene carboxylate,26 (1.925 Å) and terephthalate16 (1.950 Å). In comparison to formate systems, a shorter Cu-O bond length of the benzoate/copper system reveals a stronger attractive Cu-O interaction.14,16,28 As a consequence, the O-C-O bond angle is increased to 127° for the adsorbed molecule compared to 121° in a free molecule. The experiments have shown the existence of a new closepacked (1 1; -4 2) structure with two benzoate molecules per unit cell. Neglecting the copper adatoms, we calculate a first structure where one benzoate is placed on two copper first-layer atoms and another benzoate molecule is placed on top of two copper second-layer atoms (part a of Figure 6). During the geometry optimization, the second molecule relaxes to a configuration where it binds directly to copper atoms of the first layer due to a stronger covalent interaction, which can not be established with copper adatoms of the second layer.14-16,27 (26) Frederick, B. G.; Cole, R. J.; Power, J. R.; Perry, C. C.; Chen, Q.; Richardson, N. V.; Weightman, P.; Verdozzi, C.; Jennison, D. R.; Schultz, P. A. Phys. ReV. B 1998, 58, 10883–10889. (27) Pascal, M.; Lamont, C. L. A.; Kittel, M.; Hoeft, J. T.; Terborg, R.; Polcik, M.; Kang, J. H.; Toomes, R.; Woodruff, D. P. Surf. Sci. 2001, 492, 285–293. (28) Barbosa, L. A. M. M.; Sautet, P. J. Am. Chem. Soc. 2001, 123, 6639– 6648.
Analogous to the 3-thiophene carboxylate26 and terephatalate16 adsorbed on Cu(110), the molecules are slightly tilted due to the molecule-molecule interactions. This relaxed structure does not agree with the one observed in the experiment because it is a (1 1; -2 1) structure with only one molecule per unit cell (part b of Figure 6). In a second configuration, one benzoate molecule is placed on top of copper first-layer atoms as before, but the second benzoate is substituted by a benzoate copper-adatom molecule placed on top of two copper second layer atoms (i.e., the central benzoate sits on top of a double row of copper adatoms). This configuration relaxes toward the (1 1; -4 2) structure (part c of Figure 6), in which all benzoate molecules sit in bridge position.14-16,26 Energetics. Although structures with adatoms in benzoateCu(110) systems are known only for the low coverage limit, for example flat-lying monodentate or bidentate benzoate Cu-adatoms molecules,7,10,11,17,29 we found in our experiments a new highcoverage structure containing upright benzoate Cu-adatoms molecules. Therefore, we conclude that after adsorption and deprotonation of the benzoic acid on Cu(110) two chemical species are present on the Cu(110) surface, benzoate molecules (part b of Figure 5) and benzoate Cu-adatoms molecules (part c of Figure 5). It is important to know the stability of the benzoate and benzoate Cu-adatoms molecules on Cu(110). This can be assessed from the adsorption energies Eads and the adsorption enthalpies Hads (T ) 0 K) per molecule, which have been calculated as follows: (a) A single benzoate molecule binding to two copper atoms of the surface:
E1ads ) Esystem - (Ebenzoate + ECu(110)) ) -3.48 eV ∆H 1ads ) (Esystem + 0.5EH2) - (Ebenzoic acid + ECu(110)) ) -1.05 eV (1) (b) A single benzoate molecule binding to two copper adatoms adsorbed on the surface:
E 2ads ) Esystem - (Ebenzoate + Eadatoms+Cu(110)) ) -3.96 eV ∆H 2ads ) (Esystem + 0.5EH2) - (Ebenzoic acid + Eadatoms+Cu(110)) ) -1.53 eV (2) (c) A single benzoate Cu-adatoms molecule adsorbed on the surface:
E 3ads ) ∆H 3ads ) Esystem - (Ebenzoate Cu-adatoms + ECu(110)) ) -2.58 eV (3) (d) A molecular layer of benzoate molecules adsorbed on the surface containing adatom rows: (29) Perry, C. C.; Haq, S.; Frederick, B. G.; Richardson, N. V. Surf. Sci. 1998, 409, 512–520.
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1 E 4ads ) [Esystem - (Ebenzoate-layer + Eadatoms-rows+Cu(110))] ) 2 -3.73 eV 1 4 ∆H ads ) [(Esystem + EH2) - (2Ebenzoic acid + 2 Eadatoms-rows+Cu(110))] ) -1.30 eV (4) where Esystem is the total energy of the relaxed benzoatecopper system, Ebenzoate, Ebenzoate Cu-adatoms, Ebenzoic acid, EH2 are respectively the total energy of the isolated benzoate, benzoate Cu-adatoms, benzoic acid and hydrogen molecules, Ebenzonate-layer is the total energy of the benzoate molecular layer, and ECu(110), Eadatoms+Cu(110), Eadatoms-rows+Cu(110) are respectively the total energies of the clean Cu(110) surface, of the two copper adatoms on Cu(110), and of the double row of adatoms adsorbed on Cu(110). The large negative values of the adsorption energies imply a strong attractive interaction between the molecule and the Cu(110). The interaction is of similar strength as for other carboxylate systems, for example formate22 (Eads ) - 3.51 eV) and terephthalate23 (Eads ) -3.08 eV). The absolute value of the adsorption energy of the single benzoate molecule to two copper 2 ) is 0.48 eV larger than the one of adatoms on the surface (Eads a single benzoate molecule binding to two copper atoms on the 1 ). Therefore, the adsorption between the Cu(110) surface (Eads benzoate molecule and the surface mediated by copper adatoms is much stronger than the direct adsorption between the molecule and the surface. Moreover, the strength of this interaction is maintained in case of the (1 1; -4 2) benzoate layer adsorbed 4 is on Cu(110) because the adsorption energy per molecule Eads 1 1 2 almost half of the sum /2(Eads + Eads) ) - 3.72 eV. A second 4 value is that, although conclusion that can be drawn from the Eads the (1 1; -4 2) structure is a high-coverage structure, the molecules are still far away from each other and no significant interaction between the adsorbed benzoate molecules occurs. The absolute value of the adsorption energy of the benzoate Cu-adatoms molecule adsorbed on Cu(110) is 0.9 eV smaller than the one of the benzoate adsorbed directly on Cu(110). Therefore, the adsorption of the benzoate Cu-adatoms molecule is weaker, the bonds formed between the Cu-adatoms and the Cu(110) surface are easier to break, and this suggests that the benzoate Cu-adatoms molecules may have a higher surface mobility than the benzoate molecules. According to thermodynamic principles, the negative adsorption enthalpies for all calculated configurations indicate an exothermic reaction between benzoic acid and the Cu(110), which imply that the reactions require no external energy, therefore they are possible even at low temperatures. However, the adsorption enthalpy does not include any information about the reaction barrier. Because in the experiments the structures are obtained at elevated temperatures, we conclude that the reactions require a high activation energy (a high reaction barrier) to take place. Electronic Structure and STM Simulations. To understand the configuration and the characteristics of a chemical bond, it is adjuvant to analyze the electronic structures of the bonding participants. Whereas the surface of a metal is characterized by reactive electron energy levels that are corresponding to the surface states, the molecule is characterized by discrete energy levels in which the most reactive orbitals are the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). The bonding between the molecule and the surface involves the interaction of these reactive molecular orbitals with the surface states of the metal and results in new bonding molecular orbitals of the adsorbed molecule/surface system. There
Figure 7. Local density of states (LDOS) as a function of energy for the benzoate (upper panel) and the benzoate Cu-adatoms molecule (lower panel) together with the isosurface plots (charge density) of the relevant orbitals.
are three questions to deal with to obtain a clear picture of the adsorption process of a molecule on a surface: (i) Which molecular orbitals participate in the adsorption process - HOMO, LUMO, or maybe both? (ii) What is their position relative to the Fermi level or to the d-band of the metal? (iii) What is their effect on the work function? A basic aim of our study is to investigate in which way the electronic structure changes during the adsorption process due to the interaction between benzoate and copper surface. We present the angular-momentum resolved local density of states (LDOS) calculated for the isolated benzoate (Figure 7, upper panel) and for benzoate Cu-adatoms molecules (Figure 7, lower panel) to offer a picture consistent with LDOS obtained for adsorbed benzoate on Cu(110). The Cartesian coordinate system was chosen such that the atomic orbitals px and py are in the molecular plane and represent the σ bonds, whereas the pz orbital is oriented perpendicular to the molecular plane and gives rise to the π bonds in the molecule. With this specification, σ1 (HOMO) and σ2 are σ-bonding molecular orbitals, whereas the bonding orbitals π1, π2, π3, and the antibonding π2* (LUMO), π1* are π-molecular orbitals. Because the spatial distribution of the charge density, corresponding to these molecular orbitals and their nodal structure, is a key factor for the description of the interaction between benzoate and the metal surface, the charge-density plots for the relevant orbitals are presented in Figure 7. In the case of benzoate Cu-adatoms molecules, the HOMOs σ1, σ2, and σ*1 (located at the carboxylate functionality) are split σ orbitals, and they represent the antibonding combinations of the σ1,2 states of the benzoate molecule with the d-electrons of the copper adatoms. The π3,4 states are the antibonding combinations of the π3 states of the benzoate molecule with the copper d-electrons. The π1, π2, π*1, and π*2 orbitals, located predominantly at the benzene ring, are shifted down in energy compared to the benzene molecule. Note also the huge decrease in the HOMO-LUMO gap in case of the benzoate Cu-adatoms molecule.
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Figure 8. Plane-averaged electrostatic potential (eV per elementary charge qe) of the benzoate and the benzoate Cu-adatoms molecules. The vertical dashed lines represent the positions of the copper and oxygen atoms.
In Figure 8, we present the plane-averaged electrostatic potential of the benzoate and the benzoate Cu-adatoms molecule. The difference between the right and the left vacuum level shows the magnitude of the dipole moment and represents the change in the electrostatic potential along the molecule when going from the right to the left. On the basis of frontier molecular orbital interaction theory arguments, due to the huge decrease of the dipole moment and the HOMO-LUMO gap together with the smaller adsorption energy of the benzoate Cu-adatoms molecule as compared to benzoate, we conclude that the bonds between benzoate Cu-adatoms molecules and the Cu(110) are much easier to break than those between benzoate molecules and the copper surface. The feature indicates that the chemical species with higher surface mobility are the benzoate Cu-adatom molecules. This is not surprising because for the well-studied thiol/gold systems it has also been shown that gold-thiol complexes are more mobile on the metal surface than thiol molecules.30,31 Obviously, the chemical bonding between the benzoate and the copper surface is neither purely ionic (Coulombic) nor purely covalent. Both interactions play a role in the bonding of the benzoate molecule with the copper surface. As previously shown for similar systems,14,16,26 the stronger covalent interaction favors the adsorption of the molecule via the carboxylate group in the bridge position. Because no significant intermolecular interaction between the adsorbed molecules occurs in the (1 1; -4 2) structure, we present the LDOS of the single molecules (benzoate and benzoate Cu-adatoms) adsorbed on the Cu(110), as seen in Figure 9. For clarity, we resume our discussion of those systems because apart from a slight broadening of the energetic levels no changes appear in the LDOS characteristics when going from single molecules to a molecular layer. The analysis of the LDOS reveals that in the case of both molecules (benzoate and benzoate Cu-adatoms molecule) the bonding mechanism involves a strong hybridization of molecular orbitals (HOMOs), located at the carboxylate moiety or at the carboxylate Cu-adatoms moiety with the d-bands of the surface. This results in a significant broadening of these molecular orbitals. On the contrary, the LUMOs do not play a role in this bonding process because they remain unoccupied in the molecule/surface system. The bonding mechanism can be understood easily by applying (30) Yu, M.; Bovet, N.; Satterley, C. J.; Bengio, S.; Lovelock, K. R. J.; Milligan, P. K.; Jones, R. G.; Woodruff, D. P.; Dhanak, V. Phys. ReV. Lett. 2006, 97, 166102. (31) Maksymovych, P.; Sorescu, D. C. T., Jr Phys. ReV. Lett. 2006, 97, 146103.
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Figure 9. Local density of states (LDOS) plots for benzoate (upper panel) and benzoate Cu-adatoms (lower panel) molecules adsorbed on the Cu(110) surface. For the adatom system, the charge-density plots of the relevant states below and above Fermi level are shown below the plot.
Figure 10. Schematic of the hybridization of the molecular orbitals with the Cu d-bands. The hybridization leads to a splitting of the molecular orbitals into two sub-bands with bonding and antibonding character.
the Anderson-Newns model,32 which describes the interaction of a localized atomic orbital with the extended metallic states. In the case of copper with a completely occupied d-band, the model predicts that both bonding and antibonding states are formed, analogous to the bonding and antibonding molecular orbitals that are formed by the orbitals of two interacting atoms (Figure 10). This simple model is also qualitatively valid in the case of the adsorption of molecules that use anchoring groups to bind the metal surfaces.15,16,26,32,33 The features discussed above can be identified in the LDOS and in the charge-density plots of the states shown in Figure 9. Note that the σ and π orbitals located at the carboxylate group play the main role in binding the molecule to the surface. Analysis of the states around the Fermi level EF clearly shows the difference of the two systems: for the benzoate Cu-adatoms/Cu(110) system the σ1 states show no contribution above the Fermi level, whereas for the benzoate/Cu(110) system the σ1 states give also a contribution above the Fermi level. Because these states play a major role in determining the current-voltage characteristics in single-molecule transport experiments, it is expected that different I-V curves will be measured for benzoate and benzoate adsorbed on two copper (32) Hammer, B; Norskov, J. K. Chemisorption and ReactiVity of Supported Clusters and Thin Films; Kluwer Academic: Dordrecht, 1997. (33) Felice, R. D.; Selloni, A. J. Chem. Phys. 2003, 120, 4906–4914.
Cu-Adatom-Mediated Bonding
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Figure 12. Simulated constant-current STM image of the (1 1; -4 2) monolayer chemisorbed on Cu(110) for an applied bias voltage of UT ) - 0.78 eV. Figure 11. Plane-integrated charge-density redistributions for the calculated systems: benzoate-Cu on the left and benzoate Cu-adatoms-Cu on the right side. Vertical lines label the positions of the Cu and O atoms. The value y ) 0 corresponds to the highest surface layer; negative values indicate the bulk direction, whereas positive values indicate the direction of the molecule (vacuum side). In the 3D isosurface plots, the red color represents the accumulation of charge density, whereas the blue color represents the depletion of the charge density.
adatoms on Cu(110), each curve reflecting a different magnitude of electronic coupling between molecule and electrode. To understand the coulombic contribution to the binding, we focus on the analysis of the charge transfer and on the polarization effects induced by the adsorption of the molecules on the copper surface. As a result of the planarity of the molecule and the perpendicular adsorption on the surface, the polarization effects were calculated considering the plane-integrated charge-density difference δF ) ∫∆F dy dz. The charge-density difference ∆F was obtained by subtracting the sum of the ground-state electronic charge density of the free molecule Fmol and of the substrate Fsurf from that of the interacting adsorbate/surface system F, that is ∆F ) F - (Fsurf + Fmol). Figure 11 shows the plane-integrated charge-density redistribution for the calculated systems. The analysis of ∆F and δF leads to the following conclusions: • The charge difference density rearrangement occurs at the interstitial region, that is the charge is shifted from both, molecule and metal, toward the interface region. • No significant charge transfer from the metal into the molecule (or vice versa) occurs. • The charge rearrangement decreases rapidly in bulk (within the first two layers) and in the molecule (within the carboxylate functionality). Another key quantity to evaluate the magnitude of the charge transfer between an adsorbate and a substrate is the work function. It is defined as the difference between the Fermi energy EF and the potential energy in vacuum Evac: Φ ) EF - Evac. The work function of the (1 1; -4 2) structure increases with 0.47 eV compared to the clean Cu(110) and implies an electronic transfer toward outside. This corresponds to a small charge transfer from the surface to the organic/metallic interface,which is located mostly at the carboxylate moiety. To summarize, we found that the contribution of the coulombic interaction to the binding is negligible. Moreover, the wave-function analysis shows interface orbitals with a
clear-cut bonding and antibonding character (Figure 9). Because of the effective molecule-metal hybridization, the adsorption process can indeed be described as chemisorption, with the formation of covalent bonds at the carboxylate/copper interface and with a mixed molecule and metal character.15 We have also analyzed the real space topography of the (1 1; -4 2) structure by simulating STM images for an applied voltage of UT ) - 0.78 eV (Figure 12). Comparing the experimentally measured (part b of Figure 3), the surface schema (part c of Figure 6), and the theoretically simulated (Figure 12) STM images, we conclude that the bright spots seen in experiment correspond to the spacing between the molecules. These spots represent the tail of the σ and π bonding wave functions, which have a maxima located at the anchoring carboxylate moiety. Although less obvious in the experimental STM image, the different highs of the molecular rows clearly show up in the measured linescans (part b of Figure 4) and they are clearly visible in the simulated STM image. Consistent with the experiments and easily seen from the LDOS (Figure 9), the STM images simulated for the unoccupied states by applying a bias voltage up to +1.8 eV show no clear corrugation or specific features. In our experiments, above this bias voltage the molecular layers are destroyed.
Conclusions We combined experimental investigations with DFT calculations to study the adsorption of benzoic acid on a Cu(110) surface. We found a new high-coverage phase with two benzoate molecules per unit cell packed in a (1 1; -4 2) structure, corresponding to a surface coverage of 0.33. For the first time, we show that the high-coverage structure arises at high temperatures and is stabilized by the copper adatoms. The theoretical calculations prove that the strong covalent interaction between the oxygen atoms of the carboxylate moiety and copper surface atoms favors the adsorption of molecules in the so-called bridge position. The specificity of this adsorption geometry is that the oxygen atoms of the carboxylate moiety are on top and form single bonds with copper atoms. We demonstrate that two different chemical species, namely the benzoate and benzoate Cu-adatoms molecule, are strongly binding to the Cu(110) surface and build the experimentally found (1 1; -4 2) high-coverage structure. Moreover, we demonstrate that the interaction between the benzoate molecule and the surface, mediated by copper adatoms, is stronger than the direct interaction between the benzoate molecule and the surface.
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Characteristic for the benzoate/Cu(110) system, at low coverage, the flat-lying copper-benzoate organometallic complexes proved to have a high mobility at room temperature.14,17 We show that at high coverage the up-right benzoate Cu-adatoms molecule is the chemical species with higher surface mobility due to the smaller dipole moment and the smaller binding energy to the Cu(110) as compared to the benzoate molecule. This picture is consistent with the one observed in the thiol/gold system, where gold-thiol complexes are the species with higher mobility as compared to thiol molecules.30,31 Because the theoretical calculations allowed us to address some ambiguous assignments in experiments, our study emphasizes once more the importance of the experiment and
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theory combination to establish a better understanding of the phenomena at the organic/metal interface. Acknowledgment. We like to thank N.V. Richardson for valuable discussion. Further, we thank K. Szot and J. Szade for XPS measurements, C. Thomas and U. Linke for the copper single crystals. This work was supported by the DFG Priority Program “Quantum Transport at the Molecular Scale” SPP1243. The theoretical calculations have been performed on the IBM Regatta and Blue Gene/L supercomputers in Ju¨lich Supercomputing Centre (JSC). LA801822E
