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Interplay between Hydrogen Bonding, Epitaxy, and Charge Transfer in the Self-Assembly of Croconic Acid on Au(111) and Ag(111) J. Hooper,*,† D. A. Kunkel,‡ E. Zurek,§ and A. Enders*,‡ †

Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland Department of Physics and Astronomy, University of NebraskaLincoln, 855 N. 16th Street, Lincoln, Nebraska 68588, United States § Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, United States ‡

S Supporting Information *

ABSTRACT: The structure and ordering of two-dimensional sheets of croconic acid (CA) on Au(111) were studied with scanning tunneling microscopy and first-principles calculations. Extended porous honeycomb networks are formed that differ from those that were recently reported to form on Ag(111) and density functional theory (DFT) calculations are used to help rationalize how the two substrates, Au(111) and Ag(111), influence the CA networks. The CA network that is synthesized on Au(111) corresponds to the 2D structure that has the lowest ground-state DFT energy when modeled without the substrate and uses a different isomer of CA than the one that is found within the Ag(111) network and the bulk three-dimensional crystal; this is attributed to the weaker CA−CA and CA− substrate interactions on Au(111). The Au(111) network has no net dipole moment, but a hydrogen transfer mechanism which forms a metastable polar network seems plausible if the latter remains dynamically stable. The formation of the Ag(111) network is attributed to the larger, and varied, influence of the binding site on the electronic structure of CA.

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INTRODUCTION Croconic acid (CA), a small organic molecule with a fivemembered carbon ring, has recently been shown to be ferroelectric in its bulk hydrogen-bonded 3D crystal structure.1,2 The particularly enticing finding was that the crystal’s electric dipole is effectively reversed after an applied electric field induces a cooperative proton tautomerism that swaps the role of hydrogen bond donor and hydrogen bond acceptor for each hydrogen bond.1,3 This development has inspired further work to seek out similar organic crystal structures for their potential uses as topological organic ferroelectric materials.1,4,5 Inspired by the finding of ferroelectric behavior in crystalline CA, we recently reported the formation of two-dimensional (2D) chiral and porous CA networks on an Ag(111) substrate,6 in the hopes that the 2D networks would themselves be polar. Such CA networks have been proposed, for example, in interesting theoretical models of functionalized molecule/ transition-metal materials.7 It was deduced from computations and chemical intuition that the simplest models of the CA networks which form on Ag(111) were built from an elementary CA dimer building block that had no net dipole moment, and likewise, the periodic network model that most closely matched the experimental STM images also had no net dipole moment. It was noted, however, that the substrate could stabilize tautomeric CA models and decrease the relative energies of CA motifs that are polar.6 A recent computational © 2015 American Chemical Society

study confirmed that a CA/Ag(111) network can indeed be polarized and tautomerized under the influence of an electric field.8 The manner in which molecules self-assemble on metal surfaces and the underlying balance between the molecule− molecule interactions and the molecule−substrate interactions is itself a highly studied and diverse field.9 For polar molecules, for example, the sum of the molecular dipoles that the molecules would assume in the gas phase can reinforce each other in extended one-dimensional chains 10 and local domains,11 or the dipole moments can cancel each other out.12 They can self-assemble into porous 2D networks where the dipole moments are arranged in more complex manners.10,13 The substrate itself has been found to influence many networks, like with methanol deposited on Au(111) and Cu(111),14,15 or as in our recent findings for a simple zwitterionic molecule on Cu(111) and Au(111).12 Here, we build on the characterization of substrate-mediated organic monolayers by reporting the 2D networks that form after CA is deposited on Au(111). Porous CA networks are found once more, like on Ag(111), but with a distinctly different network architecture; they represent an example of a Received: July 9, 2015 Revised: October 28, 2015 Published: October 28, 2015 26429

DOI: 10.1021/acs.jpcc.5b06589 J. Phys. Chem. C 2015, 119, 26429−26437

Article

The Journal of Physical Chemistry C

The PBE-D3 functional corresponds to the gradient-corrected exchange and correlation functional of Perdew−Burke− Ernzerhof (PBE)25 coupled with the third-generation postSCF dispersion corrections proposed by the Grimme research group,26,30 as implemented with the IVDW=12 tag available in VASP. The Γ-centered Monkhorst−Pack scheme was used to generate k-point grids whose meshes ranged from 3 × 3 × 1 to 7 × 7 × 1 depending on the size of the simulation cell. The geometries of all the structures were converged such that the magnitude of the largest force acting on the constituent atoms was less than, at most, 0.03 eV/Å2, and a dipole correction was applied perpendicular to the metal slab using the LDIPOL tag available in VASP. Unless explained otherwise in the main text, the slabs were built such that distances between metal atoms in the slabs were close to their computed lattice constants, the experimental lattice constant was used for the PBE-D3 calculations, a 4.16 Å lattice constant was used for the optB88 simulations with Au(111), and a 4.14 Å lattice constant was used for optB88 with Ag(111). The Bader charges31 were computed by using a freely available script32 to parse the topology of the computed valence charge density; i.e., the charge density from the core electrons was not considered. Because of this, the electrostatic potential around the CA molecules was also studied in order to complement the Bader charges and ensure the validity of the observed trends. The computed electrostatic potentials are shown in the Supporting Information. The binding site preference study of CA monomers was carried out in the cell that is shown in the main text; the CA molecules were systematically rotated after the center of CA was positioned over one of three surface sites: (1) a top (T) site, directly over a surface metal atom, (2) a bridge (B) site, directly over the midpoint of a nearest-neighbor metal−metal contact, and (3) a hollow (H) site, directly over the point that is equidistant from the three nearest metal atoms. The notation used in the main text indicates which site the molecule was placed at and how it is oriented with respect to the [110̅ ] direction. For example, T0 means that the molecule is on a T site and its axis of symmetry lies parallel to the [11̅0 ] axis, and T30 means that it has been rotated 30°. The binding energies referred to in the main text correspond to the energy difference between the system and the sum of its isolated parts; for example, the binding energy, ΔEbind, of a molecular network adsorbed on a metal surface is computed as

synthetic 2D Kagome lattice. Kagome lattices have wide scientific relevance ranging from being theoretically linked to a variety of novel magnetic properties, including high temperature superconductivity, to being perfect templates for studying host guest chemistry due to the presence of two differently sized regular pores.16−18 The lowest energy model that was found for the Au(111) Kagome lattice is built from fundamentally different building blocks than what we characterized on Ag(111), and both experiment and computation suggest the Au(111) networks are primarily a consequence of the weaker influence that Au(111) has on the CA/CA interactions. We comment on a proton transfer mechanism that can polarize models of the most stable Au(111) network; the small energy barrier and small energy difference between the nonpolar and polar states suggest there may be opportunity to polarize such a sheet if the polarized sheet is dynamically stable. We also find from computations that there is a clear relationship between the preferred CA binding site on Ag(111), which involves Ag···O interactions that have been known to influence other hydrogen-bonded organic networks,19 and the lowest-energy models of the network that was characterized on Ag(111).

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METHODS Experimental Section. An Omicron low-temperature scanning tunneling microscope (STM) run at ultrahigh vacuum (UHV) with an electrochemically etched W tip was used for all measurements. An Au(111) single crystal, purchased from Princeton Scientific, was cleaned using repeated cycles of Ar+ sputtering and subsequent annealing. Cleanliness of the crystal was checked by scanning with the STM prior to molecular deposition. Croconic acid, 98% purity, was purchased from Sigma-Aldrich and evaporated in a home-built Knudsen cell evaporator onto the Au crystal held at room temperature. All STM measurements were taken with the sample cooled to liquid nitrogen temperature. Computations. First-principles density functional theory (DFT) calculations20 were run on CA monomers and 2D networks of CA over Ag(111) and Au(111) using asymmetric slab models, i.e., where CA is adsorbed to only one of the metal−vacuum interfaces. The slabs were either two layers deep (for the CA networks) or four layers deep (for the CA monomers and the hydrogen-transfer models). For the fourlayer slabs, the metal atoms in the top two layers (the two layers directly underneath the adsorbate layer) were allowed to relax. The separation between periodic images across the vacuum was set to 15 Å for the isolated planar CA networks and was tested with 20 Å models to ensure the binding energies were converged to less than 0.01 eV per CA. The separation between periodic images was set to 23 Å for the two-layer models (and checked with respect to 20 Å separations) and to 25 Å for the four-layer models (and checked with respect to 30 Å separations). The calculations were performed with the Vienna ab initio Simulation Package (VASP),21,22 version 5.3.5. The projector augmented wave (PAW) method23,24 was used to treat the core states along with a plane-wave basis set with an energy cutoff of either 500 eV, for geometry optimizations, or 600 eV, for computing binding energies. The C 2s/2p, O 2s/2p, H 1s, Ag 5s/4d, and Au 6s/5d electrons were treated explicitly in all of the calculations. The dispersion-corrected PBE-D3 functional25,26 and the dispersion-including optB88 functional27−29 were used to model both the Au(111) and Ag(111) systems.

ΔE bind = Esystem − Eslab − nECA

where Esystem is the DFT electronic energy of the total system, Eslab is the energy of the metal slab, and nECA is the energy of an isolated CA molecule multiplied by the number (n) of CA molecules in the system.

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RESULTS AND DISCUSSION Experimental Results. When deposited at room temperature on Au(111), croconic acid self-assembles into extended porous networks as shown in Figure 1a,c. Each CA molecule has four nearest neighbors and presumably forms hydrogen bonds with each of them. The networks are large, often over 100 nm across, and singly domained. A proposed model for these networks is shown in Figure 1d. The networks are notably composed of molecules whose hydrogen atoms are pointed symmetrically outward (see I1 in Figure 2a), leading to a symmetric and achiral network. These networks differ greatly 26430


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Figure 1. (a) STM image of CA deposited at room temperature on Au(111) taken at −0.1 V, 350 pA; green triangles highlight the Kagome geometry. (b) STM image of CA on Ag(111). (c) STM image of CA on Au(111) showing the uniformity of large domains, taken at 0.7 V and 350 pA. (d) Proposed model of the porous CA network on Au(111) with a trimer-based building block; see also Figure 2a.

Figure 2. (a) Atomistic structures of the CA building blocks and networks: the CA monomers, I1 and II1, come together in (left) trimers to form the I3n network or (right) in dimers to form the II2n network. (b) Relative energies of plausible isolated 2D (flat) CA networks built from either a CA trimer, X3n, or a CA dimer, X2n. X = I when monomer I1 is used to build the network, X = II when monomer II1 is used, and X = [I−II] when both monomers are used. The vertical gold and silver bars correspond to the coverages that are obtained with the slab models that are used in the surface-adsorbed networks.

from the ones we previously reported on Ag(111); for reference, an example of the CA networks observed on Ag(111) is shown in Figure 1b. The most notable difference is that a CA dimer is clearly the basic building block of the Ag(111) network,6 but the dimer motifs are absent in the Au(111) network. The networks and their building blocks are discussed in more detail in the computational section. The Au(111) surface reconstructs into a “herringbone” pattern which can be seen rippling underneath the CA networks, especially in Figure 1c. These herringbone ridges lie along the ⟨21̅2⟩ directions and thus are aligned 30° to the densely packed directions of the underlying Au lattice.33,34 We analyzed the direction of molecular rows with respect to the Au substrate and found that different orientations exist. It appears that there are three distinctive orientations, but we also observed exceptions. This implies that there is some site preference of the molecules but that the corrugation of the potential energy surface across the Au(111) surface is small. The average molecular separation along a molecular row is 7.2 ± 0.2 Å, which points to incommensurate alignment to the substrate. It is thus reasonable to conclude that the diffusion barrier for molecules is low and also that the molecules are not strictly pinned to particular adsorption sites on the surface, which would be consistent with earlier studies of other molecules on Au(111), such as p-benzoquinonemonoimine zwitterions and benzene.12,35 It is noted that at low coverages of CA, approximately less than 0.5 ML, the networks are highly unstable when scanned, and a considerable number of molecules are present in an uncondensed, gaseous phase, seen as characteristic streaks in the STM images. This further suggests that the influence of the CA−surface interactions is weak. However, when approaching one monolayer of coverage the honeycomb networks become much more stable. Finally,

our observation of few nucleation events followed by the growth of very large islands is also consistent with high molecule mobility and low diffusion barriers on the surface. Closer inspection of the geometry reveals that CA resembles a 2D Kagome lattice on Au(111), highlighted in green in Figure 1a. Despite scientific interest in Kagome lattices, there are few natural examples and synthetic Kagome lattices have proved difficult to create. Until recently, the only Kagome lattices were complex, inorganic structures that were difficult to make defect free.16 The first reported nanoscale Kagome lattice was synthesized in 2002,36 and since then a spate of new nanosized organic Kagome lattices have been synthesized with the help of modern surface science techniques. These lattices exhibit a variety of chemical bonds, including hydrogen bonds,37−40 metal coordination bonds,41 CN−π bonds,42 and van der Waals forces.43−46 More broadly, Kagome lattices have been recently synthesized in a liquid crystal form,47 using DNA,48 and even on the micrometer scale using colloidal spheres.49 Therefore, our newly created CA networks join a rapidly expanding group of novel Kagome lattices. Computations of Model Networks without the Substrate: Rationalizing the Au(111) Network. Since the CA network that was imaged on Au(111) was found to be influenced weakly by the substrate, we first use density functional theory (DFT) calculations to explore model networks of CA which are constrained to lie flat and are modeled in a vacuum (i.e., without a metal substrate). The lowest energy models we found to represent the dominant networks that were imaged on Au(111) and Ag(111) are 26431


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the fact that substrate interactions have often been used to rationalize the formation of different organic networks on different substrates, for example in trimesic acid50 or benzene1,3,5-triyl-tribenzoic acid,51 we studied the binding energy of a single I1 monomer on both Au(111) and Ag(111) to get a sense for whether or not the influence of the substrate on the binding energy encompasses a similar energy range. Computations on the Interaction of CA with Ag(111) and Au(111): Rationalizing the Ag(111) Network. The lowest-energy (most stable) and highest-energy (least stable) binding sites we found at the PBE-D3 level of theory for a single CA molecule, in the I1 conformer geometry, on Au(111) and Ag(111) are shown in Figure 3a. The lowest-energy sites

shown in Figure 2a. The biggest difference between them starts from the geometry of the monomer that is used to build them. The Au(111) network incorporates the lowest-energy isomer of CA, which we label I1 in Figure 2a, and the Ag(111) network uses the II1 monomer, like the bulk (three-dimensional) crystal structure does. This illustrates how CA is in itself quite interesting from the perspective of crystal packing since the free energy corrections and/or intermolecular interactions and/or molecule−substrate interactions are evidentally enough to stabilize the otherwise metastable II1 isomer in the bulk crystal and on Ag(111), but not on Au(111). The energy difference between I1 and II1 is small, roughly 0.2 eV, but is comparable with the strength of a weak hydrogen bond. On Au(111), the I1 monomers come together to form trimer-like building blocks which are merged together to form the extended Kagome lattice, which is pictured as I3n on the left of Figure 2a; this model is the same as the one which is shown in Figure 1d. On Ag(111), the II1 monomers form the distinct CA dimer motifs within an extended chiral lattice, II2n, which is shown to the right of Figure 2a. STM images actually revealed two phases on Ag(111):6 one chiral and one achiral. The achiral network, imaged in Figure 1b, occurred less frequently than the chiral one (II2n), and the biggest difference between the two Ag(111) networks is that the achiral phase is made up of the I1 conformer instead of II1. This again suggests there is close competition between the intramolecular and supramolecular interactions in the CA networks. We note that as the dimensionality is reduced from 3D in the bulk crystal to 2D in the surface-adsorbed networks, the absolute strength of the CA−CA interactions is of course also reduced; the binding energy of CA to form the bulk crystal structure is computed to be −1.61 eV per molecule, but it decreases to only −0.90 eV per molecule in the flat I3n network. From this, we speculate that the weaker influence of Au(111) results in a stabilization of I1 on Au(111), due to there being no packing of the II1 monomer which allows the CA−CA interactions to overcome the stability of the I1 monomer in two dimensions, and a competition between I1 and II1 on Ag(111). In fact, both model networks, the trimer-based network (X3n) and the dimer-based network (X2n), can easily be built from either monomer, I1 or II1. The structures of the four plausible networks I3n, II3n, I2n, and II2n are shown in the Supporting Information, and scans of the networks’ potential energy surface with respect to the “coverage” that they would have on a surface are shown in Figure 2b. The energy of a third variant of the X3n network, which uses both the I1 and II1 monomers and is labeled [I−II]3n, is also shown. It can readily be seen that the I3n network is indeed the lowest energy network; this agrees with the experimental assertion that the CA molecules are influenced weakly by the Au(111) surface, and they would therefore form the lowest-energy network available to them in a vacuum. The II2n network is 0.16 eV/CA higher in energy than the I3n network. Since II2n is observed in experiments on Ag(111) instead of I3n, it is also clear that the Ag(111) substrate affects the manner in which CA assembles. Since the energy difference between the I1 and II1 building blocks is 25% larger than the energy difference between I3n and II2n, this suggests that the CA−CA interactions in the networks are already beginning to somewhat offset the stability of I1. Furthermore, the 0.06 eV/ CA energy difference between II2n and I2n suggests that the packing effects in the X2n networks offset over half of the difference in energy between I1 and II1. Considering this and

Figure 3. (a) Lowest- and highest-energy binding sites found for I1 on Au(111) and Ag(111). (b) Lowest- and highest-energy binding sites found for II1 on Ag(111). (c) Low-energy binding site for a CA II2 dimer over Ag(111); the site is a slight distortion of the T30 site. (d) Proposed binding site of the II2n network on Ag(111); the enlarged purple Ag atoms represent the metal atoms that lie under the CA carbonyl oxygen atoms. The alternate binding site of II2n that is considered in this study is overlaid on the figure as a transparent stick model.

were found to be similar on both substrates in the sense that the center of the CA molecule is positioned directly over a metal atom (i.e., over a T site) and the highest-energy binding site places the CA center directly between two adjacent metal atoms (i.e., over a B site). The closest contacts between CA and the substrate in the lowest energy binding sites are the carbonyl oxygen atoms which, incidentally, are also positioned roughly 2.3−2.6 Å over T surface sites on Ag(111), depending upon the level of theory that is used. A T site preference for oxygen atoms in small organic molecules was also found, for example, for a water molecule adsorbed over noble metal surfaces;52 the site preference was used to propose a general rule that the most favorable binding site of a molecule over noble metal surfaces is the one which promotes the most overlap between the surface and adsorbate wave functions.52 The T site preference of the CA oxygen atoms is loosely suggestive of a Ag···O interaction. We point this out because the Ag···O interaction has already been singled out as an important factor in determining the 26432


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favorable binding site maximizes the overlap between adsorbate and surface wave functions.52 Futhermore, the computed charges suggest clearly that there is less charge transfer on Au(111), which is consistent with the CA−CA interactions being only weakly influenced on Au(111). Importantly, the computed charges also suggest that the site over which CA sits on Ag(111) can induce significant differences in the degree of the charge transfer and, thus, on the nature of the electron density around CA. This could certainly influence the stability of a network of molecules if the molecules adsorb to different binding sites. This seems particularly relevant concerning our prior finding that a tautomerized form of a particular II2n model on Ag(111) becomes kinetically accessible during a short molecular dynamics simulation.6 A more recent molecular dynamics study of that model network under the influence of an external electric field confirmed the accessibility of the tautomer, and the authors speculated that the stabilization of the tautomer corresponds with the underlying symmetry of the binding site.7 This is consistent with our finding that each binding site induces significant changes in the electronic structure of an adsorbed CA molecule and inspired us to look for a more symmetric II2n model. When the PBE-D3 optimized II2n or I3n networks are positioned over a Ag(111) surface, we were unable to find a model which situates every CA molecule over a T0 site. After looking for alternative binding sites for II2n, it was found that the II2n network can be placed over Ag(111) such that each CA molecule is skewed slightly off a T30 site, as shown in Figure 3c; this site positions two of the carbonyl groups, including the carbonyl oxygen atom that does not engage in a hydrogen bond, and both hydroxyl groups near T sites. The monomer binding site that corresponds to this dimer placement, labeled T30* in Figure 3c, is less stable than the T0 site but only by 0.02−0.03 eV/CA. Furthermore, the surface slab shown in Figure 3d can be built wherein all the II1 molecules in the optimized II2n network lie over a T30* site, creating a cell that naturally optimizes both the molecule−molecule and molecule−substrate interactions and promotes the chirality of the network (since a network with opposite handedness cannot be placed on the same slab to recover the molecular binding sites). The optimized periodic II2n network model is found to lie further above the surface than the CA monomers, therefore suggesting that the hydrogen-bonded networks bind more weakly to Ag(111) than the isolated II1 monomers would (see the Supporting Information for a summary of key CA−surface interatomic distances); the shortest Ag···O contact is, for example, 2.97 Å in our II2n/Ag(111) model and 2.52 Å in our II1/Ag(111) model with the PBE-D3 functional, and the computed Bader charge per CA drops to −0.20e in the II2n/ Ag(111) system. As a test of the II2n/Ag(111) network’s binding site preference, it was found that the binding energy weakens by 0.04−0.05 eV/CA when the network is shifted off the T30* sites by 2.0 Å and reoptimized; the site of this shifted network is shown in Figure 3d by the transparently drawn stick models. The high-symmetry II2n model and the increase in energy when it is shifted away from its binding site are certainly strong indications that the carbonyl site preference, i.e., the T30* site preference for each of the CA molecules in II2n, helps drive the formation of II2n over I3n. The I3n network cannot be placed over Ag(111) such that each CA is positioned over a T site, and the intermolecular hydrogen bonds are left unchanged from the

structure of other hydrogen-bonded networks like, for example, formamide on Ag(111).19 We will address the electronic structure differences between the CA binding sites in more detail in a forthcoming paper, but here, we give the difference in electronic energy between the lowest- and highest-energy sites as a measure of the PES “corrugation” energy. This corrugation energy is roughly 3 times as large on Ag(111) versus what it was found to be on Au(111), measuring 0.11 eV/CA vs 0.04 eV/CA, and the relative energies are shown explicitly in Table 1. Table 1. Relative Energies (eV/CA) of the Systems Illustrated in Figure 3 binding site

ΔEPBE‑D3

I1 (isolated) II1 (isolated) T30-I1/Au B60-I1/Au T0-I1/Ag B60-I1/Ag T0-II1/Ag B30-II1/Ag T30*-II1/Ag

0.00 0.20 0.00 0.04 0.00 0.11 0.15 0.27 0.18

ΔEoptB88 exp

ΔEoptB88

QPBE‑D3 BADER

QoptB88 BADER

0.00

0.00 0.19 0.00 0.04 0.00 0.20 0.11 0.26 0.13

−0.08 −0.09 −0.47 −0.35 −0.43 −0.26 −0.41

−0.07 −0.08 −0.54 −0.46 −0.53 −0.30 −0.47

0.13 0.30 0.16

a

PBE-D3 and optB88 computations were carried out on four-layer surface slabs built with a metal−metal spacing that is consistent with the computed lattice constant, and optB88 was additionally used with slabs built from the experimental lattice constant (exp). The estimated sum of atomic Bader charges in the CA molecule (QBADER) from the PBE-D3 and optB88 calculations are also provided.

The lowest- and highest-energy binding sites of II1 on Ag(111) are further shown in Figure 3b, and their relative energies are shown in Table 1. The binding site preference was found, for the most part, to mirror what was found for the I1 monomer. The corrugation energy was likewise determined to be roughly 0.12 eV/CA, and since this is comparable to the difference in energy between the I3n and II2n networks, it certainly suggests that the larger corrugation energy of Ag(111) somehow influences or relates to the stabilization of II2n. The energy difference between the I1/Ag(111) and II1/Ag(111) models is 0.15 eV/CA. Although this value could certainly be influenced by the interaction of the CA monomer with its own periodic images, the 0.05 eV/CA decrease from the 0.20 eV/ CA energy difference in the gas phase is signficant and is comparable to the relative energy differences between the isolated molecular networks. Further calculations with the optB88 functional suggest that the energy difference between I1 and II1 on Ag(111) is even narrower, approaching 0.1 eV/CA (see Table 1). Another relevant trend that was observed from the binding site study was that the most favorable site maximizes the amount of electronic charge transferred to CA, as supported by the computed Bader charges using both the PBE-D3 and optB88 functionals (see Table 1). As shown in the Supporting Information, the electronic states which correspond to the two lowest unoccupied molecular orbitals in CA (i.e., the LUMO/ LUMO+1 bands) lie near the Fermi energy on Ag(111), which is consistent with their partial occupancy in surface-adsorbed geometries. The apparent increase of charge transfer into CA at the T0 sites, versus the B30 and B60 sites, is synonymous with increased charge transfer into the CA LUMO/LUMO+1 states, and that is consistent with models that suggest the most 26433


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The strength of the CA−CA binding in the II2n/Ag(111) system, ∼0.74 eV/CA, is comparable to the strength of the CA−substrate interactions, ∼0.74 eV/CA, and kinetic factors are known from experiment to guide the structures of molecular networks on metal surfaces.53 We recently speculated, for example, that the role of entropy may be important in determining the structures of another hydrogenbonded assembly on Ag(111).54 Polarizability of the I3n Network. In this section, we comment briefly on the polarizability of the stable I3n/Au(111) network. The I3n network can be polarized quite easily, within DFT at least, through a concerted hydrogen transfer that forms the [I−II]3n network that was introduced in Figure 2b. Unlike the polar or tautomeric modifications of the X2n network which can be built on Ag(111), the process results in a polar network that is composed solely of monomers I1 and II1 (see Figure 4a), and the energy difference between the isolated [I−II]3n and I3n networks is only 0.05 eV/CA. Potential energy scans of the hydrogen transfer mechanism which forms [I−II]3n from I3n on Ag(111), on Au(111), and on no substrate are shown in Figure 4b; two different binding sites are shown for each surface model, and one of the models of [I−II]3n is pictured in Figure 4c. The PBE-D3 scan shows, once more, the low sensitivity of the potential energy surface to Au(111) and the high sensitivity to Ag(111); the polar and nonpolar networks become almost energy-neutral, for example, when modeled over a higherenergy binding site on Ag(111). The PBE-D3 energy barriers are quite low, but this is an artifact of compressing the networks beyond their preferred gas-phase coverage of ∼1.89 to 1.98 molecules/nm2 (as indicated by the PBE-D3 bar shown in Figure 2b); the isolated planar [I−II]3n network model is unstable when compressed past coverages of 2.0 molecules/ nm2 and spontaneously tranforms into I3n during geometry optimizations.

optimal gas-phase values. However, if we accept the binding energies computed with DFT using two-layer slabs (see Table 2), then the site preference of our model II2n network on Ag(111) is still not enough to offset the binding energy of the I3n network on Ag(111). Table 2. Binding Energies, ΔEbind, in eV/CA of the I3n and II2n Networks Optimized over Two-Layer Surface Slabsa network

ΔEPBE‑D3 bind

ΔEoptB88 bind

EPBE‑D3 bind,corr

ΔEoptB88 bind,corr

I3n/Au II2n/Au I3n/Ag II2n/Ag

1.62 1.47 1.55 1.48

1.66 1.49 1.60 1.48

1.65 1.48 1.58 1.48

1.66 1.50 1.61 1.49

ΔEbind is computed as ΔEbind = E(system) − E(slab) − nE(I1), where n is the number of CA molecules in the simulation cell. The PBE-D3 and optB88 computations were carried out on surface slabs built with metal−metal spacings that are consistent with the computed lattice constants. The ΔEbind,corr term accounts for the difference in energy between the freely optimized isolated CA network and the optimized CA network that is obtained when constrained to the slab simulation cell. a

The computed difference in energy between I3n and II2n is smaller on Ag, however (0.10 eV/CA with PBE-D3 and 0.12 eV/CA with optB88), which represents a substantial decrease from the 0.16−0.17 eV/CA energy difference of the Au(111) models. The binding energy of CA to form the isolated II2n geometry as it lies on the surface is only 0.02 eV/CA weaker than the optimized gas-phase network. Since this suggests that the weak charge transfer between the network and the surface does not greatly affect the nature of the static CA−CA interactions, i.e., the hydrogen-bonding and dipole−dipole interactions, it is likely that the binding site preference narrows the energy difference between I3n and II2n to the extent that other contributions, like perhaps the vibrational corrections to the systems’ free energies or surface strain, become important.

Figure 4. (a) Schematic illustration of a simple proton transfer mechanism that can be modeled on the I3n network that yields a network which aligns all of the CA molecular dipole moments. (b) Potential energy scans of the hydrogen transfer shown in part (a). (c) Picture of the “1 T site” model, which places one CA molecule over a top site. 26434



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A slab model built from the computed lattice constant of Au(111) with the optB88 functional gives a coverage that is closer to the energy minimum in Figure 2b, and the potential energy scan of the hydrogen transfer with optB88 is also given in Figure 4b. Like PBE-D3, the energy profile is almost unaffected by the Au(111) substrate. Furthermore, the energy barrier is only 0.11 eV/CA, rougly half of what is required in the 3D crystal.1 The lower barrier corresponds, in part, with the fewer number of hydrogen atoms being shifted; only a third of the hydrogen atoms are moved in this mechanism versus all of them moving in concert in the 3D crystal. Despite the lowenergy barrier to forming [I−II]3n, we do note that the lowestenergy reaction mechanism that is needed in order to reverse the directions of its dipole moment would require passing through the nonpolar and more stable “intermediate” form of I3n. And of course the polar structures would also need to be dynamically stable.

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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.jpcc.5b06589. Computed electrostatic potentials and summed projected densities of states are provided for monomers/networks of CA adsorbed to Ag(111) and Au(111); optimized coordinates are also provided for the networks (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.H.). *E-mail: [email protected] (A.E.). Notes

The authors declare no competing financial interest.

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CONCLUSIONS It has been demonstrated that two-dimensional sheets of croconic acid molecules can also be stabilized on Au(111), building on the prior report of 2D CA networks on Ag(111).6 The Au(111) substrate stabilizes a fascinating synthetic Kagome lattice that differs from the chiral honeycomb network that was observed on Ag(111); the Au(111) network is driven primarily by optimizing the strength of the molecule−molecule interactions and is less influenced by the interactions of CA with the substrate than Ag(111). We have also shown through first-principles calculations that the lowest-energy model of the CA Au(111) networks is not polar due to the nonpolar trimer building blocks that construct them. But because polar networks are comparable in energy (∼0.05 eV/CA less stable) and the potential energy barrier that must be overcome in order to form them is comparable with what was computed for crystalline CA, a hydrogen transfer mechanism which polarizes the Au(111) models seems plausible, provided the resulting networks are dynamically stable. A clear epitaxial relationship with the CA network that forms on Ag(111) was also found, which helps rationalize the differences between the Au(111)- and Ag(111)-mediated CA networks. With the computational models, the epitaxial relationship matches naturally with the optimal CA−CA spacings that would occur in the absence of substrate− molecule interactions. The epitaxy is best characterized by the apparent energetic preference to place four of the five oxygen CA atoms near Ag surface sites, particularly the carbonyl oxygen atom that is not participating in a hydrogen bond. Such Ag···O interactions, which have been previously discussed for other hydrogen-bonded networks on Ag(111), are consistent with the notion of maximizing the overlap between the adsorbate and surface wave functions over noble metal surfaces. The computed binding energies with the models described in this work would not predict that the network which was identified experimentally on Ag(111) has the lowest groundstate DFT energy. But the interactions between the CA network and the substrate are stronger, and the Ag(111) model allows each CA molecule to be placed in a chemically equivalent environment. Since our computations also suggest that different binding sites of CA on Ag(111) can have different effects on the nature of the electron density around the molecule, we suggest that the high-symmetry network could be influenced, and ultimately stabilized, by entropy and/or finite temperature effects.

ACKNOWLEDGMENTS The authors acknowledge the Center of Computational Research at SUNY Buffalo for computational support. They also acknowledge support from the National Science Foundation through the Materials Research Science and Engineering Center (Grant DMR-1420645) and through NSF Grant EPS-1004094. E.Z. thanks the Alfred P. Sloan Foundation for a Research Fellowship (2013−2015). J.H. acknowledges financial support from the Homing Plus Program (HOMING PLUS/2012-6/4) granted by the Foundation for the Polish Science and cofinanced by the European Regional Development Fund.

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REFERENCES

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