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On the Role of Long Range Interactions for the Structure and Energetics of Olympicene Radical Adsorbed on Au(111) and Pt(111) Surfaces Handan Yildirim, Jeronimo Matos, and Abdelkader Kara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08191 • Publication Date (Web): 13 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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The Journal of Physical Chemistry

On the Role of Long Range Interactions for the Structure and Energetics of Olympicene Radical Adsorbed on Au(111) and Pt(111) Surfaces Handan Yildirim,* †Physics

,†,‡, §

Jeronimo Matos,

†,§

and Abdelkader Kara*,

†

Department, University of Central Florida, Orlando, FL 32816, United States §

Authors contributed equally to the work

Abstract We report on the results of the van der Waals (vdW) inclusive density functional theory (DFT) calculations for the adsorption characteristics of Olympicene radical (C19H11) on Au(111) and Pt(111) surfaces. The nature of bonding between the Olympicene radical and Au and Pt(111) surfaces are evaluated along with the effects of vdW interactions as well as the surface properties on the adsorption. Our results show a significant increase in the adsorption energies with the inclusion of vdW interactions with the largest enhancement is obtained using the optimized exchange functionals, while the smaller enhancement is found using the rPW86-vdW2 functional, in agreement with the trends obtained in our earlier observations for other molecular adsorption on similar surfaces. The adsorption of Olympicene radical on Au(111) leads to negligible change in the structures of Olympicene and the surface layer, while adsorption on Pt(111) indicates significant structural changes. This observation suggests strong interaction with the Pt(111) surface, while the interaction with the Au(111) surface is rather weak. While the bonding of Olympicene on Pt(111) is governed by a combination of the covalent bonding and the vdW forces, on Au(111), on the other hand, the bonding is mostly governed by the vdW forces. Upon adsorption on both surfaces, no interface state is formed, while a charge transfer ~ 0.6 e- is obtained. We also find the surface work function to be reduced on both surfaces upon adsorption with the largest change is observed on Pt(111) surface, by over 1 eV. Based on the results obtained using the exchange functionals, we conclude that on Au(111), there is a transition in the nature of bonding from weak physisorption to strong physisorption, while the nature of bonding on Pt(111), which is chemisorption, is not affected by the inclusion of vdWs interactions. This in contrast with the case of adsorption on Cu(111) where we have found earlier a transition from weak physisorption to strong chemisorption upon inclusion of vdW.

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I. Introduction A detailed understanding of the nature of the adsorption of organic molecules on metal surfaces has attracted increasing attention over the years.1-8 This is due, in part, to an interest in exploiting the properties of organic molecules in many technologically relevant applications such as catalysis, molecular switches, photovoltaics, and sensors.9-12 Furthermore, the electronic and transport properties are susceptible to the adsorbate structure as well as the interface properties. Thus, the accurate description of the nature of bonding between molecular adsorbates and metal substrates is the key for gaining control over their functionality. Achieving an atomistic and electronic level understanding of the structure and characteristics of these interfaces is challenging with the experimental techniques, consequently, computational modeling is the most promising route to resolve such critical atomistic level details. Throughout the years, we have witnessed many exciting developments for predictive modeling and simulations based on density functional theory (DFT), and at present, it is the most valuable technique for studying solid-state systems and beyond. DFT has been shown to provide valuable information on the atomic and electronic structure of many systems using such standard exchange correlation functionals as the localized gradient approximation (LDA)13 and the generalized gradient approximated (GGA),14 which properly treat the covalent, ionic, and strong hydrogen bonds, however, these functionals fail to account for the long-range vdW interactions. Such interactions are ubiquitous, and their inclusion is critical for accurately determining the nature of organic molecule adsorption on metal surfaces. Thus, vdW-inclusive DFT approaches to account for these interactions have become an active field of research over the past decade. There have been several methods developed over the years including DFT-D2,15 DFT-D3,16 vdW-DF2,17 vdW-DF type functionals with modified exchange,18-20 BEEF-vdW functionals,21 the XDM method,22-23 and the DFT+vdW method.24 For recent reviews and specific applications, we suggest the reader to refer to the review articles.1-8, 25 The development of vdW inclusive DFT approaches has made it possible to incorporate and assess the effect of vdW interactions in many relevant systems, including adsorption and reaction processes on extended systems.26-34 Before these developments, the adsorption of organic molecules on metal substrates was treated within the standard DFT functionals, and the first studies, which used the vdW inclusive approach, were mostly concerned with the weakly bound, physisorbed systems.35-42 The most often attempted benchmark system is benzene adsorption on metal surfaces.30,

34, 38, 43

These

studies reported somewhat universal observation regarding the effect of vdW interactions, meaning that the inclusion of these interactions often leads to an increase in binding energy, and results in better 2


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agreement with experimental adsorption configurations, energetics, and adsorption heights to the underlying substrate. These studies revealed that vdW forces are sometimes the key for changing the binding from physisorption to weak chemisorption.26, 30-31, 33-34, 38, 42 In the last few years, the vdWinclusive DFT-based methods have demonstrated promising accuracy in a broad class of organic molecules (benzene,43 anthracene,38 thiophene,44 NTCDA,45 and PTCDA26) and C60,46 adsorbed on surfaces. With these recent developments in vdW inclusive DFT approaches, we explored in details the performance of these vdW functionals, and applied them in our studies for the adsorption of benzene, olympicene, sexithiophene, and cyano-functionalized triarylamine derivatives on close-packed and open surfaces.30-34 The focus of these studies was to assess the effect of vdW interactions on the adsorption characteristics for different types of systems with varying molecular size and functional groups as well as bonding types. We showed that the inclusion of vdW interactions is critical for predicting the adsorption geometry observed experimentally, for the adsorption of benzene on transition metal substrates,30, 34 and the adsorption of olympicene radical on Cu(111).31 These studies have also revealed an unexpected observation that for strongly bound molecules on reactive transition metal substrates, the vdW interactions contribute more to the binding than on the coinage metal substrates.26, 30-34, 38, 43 This occurs in spite of the notion that for physisorbed systems, where there is weak overlap of orbitals between the adsorbate and the substrate, the vdW interactions are expected to play an important role in the binding, while for chemisorbed systems the covalent or ionic bond is the most dominant player, and the role of vdW interactions was assumed to be weak. In this study, we perform vdW-inclusive DFT calculations to assess the effect of vdW interactions on the adsorption of olympicene radical on Au(111) and Pt(111) surfaces and to further reveal the difference in the vdW contribution between the chemisorbed and physisorbed systems. A few years ago, the olympicene molecule (C19H12) and its radical (C19H11) were synthesized at the University of Warwick, and the adsorption configuration was atomically resolved using Atomic Force Microscopy (AFM) at IBM-Zurich.47-48 The AFM measurements of the olympicene radical adsorbed on Cu(111) appeared brighter at the two rings region and the edges,48 which suggests bending and tilting upon adsorption on Cu(111). Subsequently, we reported an extensive vdW-inclusive DFT study on the adsorption of the olympicene radical on Cu(111)31 and confirmed the AFM results, showing that the experimental observations could be reproduced when the vdW interactions are taken into account. We reported a detailed comparison of the structural and electronic structure changes upon adsorption, and provided comparisons for the performance of different vdW functionals for an accurate description of the interface characteristics.31 In that study, we also illustrated the transition from physisorption to chemisorption with the inclusion of vdW interactions, using the optimized exchange (optB88-vdW, 3



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optB86b-vdW, optPBE-vdW)2, 19-20 and rPW86-vdW217, 49 functionals, and outlined the minimal list of characteristics associated with this transition. In this present work, we study the nature of bonding for the adsorption of the olympicene radical on the close-packed Au(111) and Pt(111) surfaces by examining the geometric, energetic, and electronic structure changes upon adsorption. The adsorption is studied using the non-local exchangecorrelation functionals, the vdW- DF2 of Murray et al.,49 and Lee et. al.,17 and two of the optimized versions of the exchange functionals introduced by Klimes et al.,18-20 as well as the PBE functional14 for assessing the role of vdW interactions on the overall adsorption characteristics. To our best knowledge, there are no experimental reports for these systems; our interest here is to compare the contribution of the vdW interactions to the overall adsorption for differently bound systems. For the equilibrium adsorption geometries, we explore the structural changes upon adsorption such as bending and tilting, and the adsorption height from the surface. These structural changes help in classifying the nature of bonding on these metal surfaces. For instance, weak physisorption is often accompanied by a large adsorption height (i.e. > 3 Å) from the surface, while strong chemisorption is associated with a much smaller adsorption height, ~ 2 Å. Both bending and tilting often indicate strong molecular interaction with substrate. The electronic structure analyses include the change in the work function upon adsorption, net charge transfer using the Bader charge analysis, and charge density distribution upon adsorption of the radical on the surfaces. Our calculations demonstrate that both covalent bonding and vdW interactions play an important role in the adsorption. We predict larger contribution from the vdW interactions to the binding, in particular for strongly adsorbed olympicene radical on Pt(111) as compared to the weakly physisorbed system, adsorption on Au(111). We show how recently developed methods for incorporating vdW interactions treat both the weakly and strongly adsorbed olympicene radical on metal surfaces. After a brief summary of the computational details, we first report on the equilibrium adsorption geometries, adsorption energies, non-local contribution to the adsorption energies, and the electronic structure analysis - the changes in the work function and charge transfer calculated using PBE, and all vdW functionals considered in this study. A comparison with our previous study of the adsorption on Cu(111) is also provided. Finally, we summarize our findings in the conclusion. II. Computational Details The vdW-inclusive first principles DFT calculations are performed within the periodic DFT framework using the Vienna Ab Initio Simulation Package (VASP)50-52 version 5.2.12 to study the nature of olympicene radical adsorption on Au(111) and Pt(111), and to evaluate the role of vdW 4


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interactions on the adsorption characteristics. The equilibrium adsorption geometries, adsorption energies, charge transfer, and the interface electronic structure between the olympicene radical and the substrates are calculated using vdW functionals, and the GGA-PBE14 functional. The non-local exchange-correlation functionals, which are used to evaluate the effect of vdW interactions, are the vdW- DF2 functional of Murray et al.49 and Lee et. al.,17 and two of the optimized versions of the exchange functional introduced by Klimes. et. al.18-19, 53 For two optimized versions of by Klimes, et. al.19 the original GGA functional was replaced by the optimized Becke88 (optB88) or optimized Becke86b (optB86b) to improve the accuracy of the vdW-DF scheme, and referred throughout the manuscript as optB88-vdW and optB86b-vdW, respectively. The choice of vdW functionals is based on the results of our earlier studies for different types of molecule/metal substrate systems.30-34 In our earlier study for benzene adsorption on (111) metal surfaces,30 the adsorption energy of benzene on Cu, Ag, and Au(111) surfaces is found to follow the trend as EoptB86b-vdW > EoptB88-vdW > EoptPBE-vdW > ErevPBEvdW >

ErPW86-vdW2 > EPBE, while for Pd, Pt, Rh, and Ni(111), the trend is found as EoptB86b-vdW > EoptB88-

vdW >

EoptPBE-vdW > EPBE > ErevPBE-vdW > ErPW86-vdW2. Due to the similarities observed in our earlier study

for the changes introduced in the adsorption energies using the class of the opt-type functionals, and between the results of the revPBE-vdW and rPW86-vdW2 functionals,30, 34 in this study, we perform the calculations using only the optB86b-vdW, optB88-vdW, and rPW86-vdW2 functionals to capture the differences introduced by these functionals on the adsorption characteristics. The interaction between the valence electrons and ionic cores is described by the projector augmented wave (PAW) method.50,

52

A plane-wave kinetic-energy cutoff of 400 eV is used, and

Brillouin zone is sampled using 4×4×1 k-point mesh for structural relaxations. The total energy optimization is performed using the conjugate gradient (CG) method54-55 with the force criterion on each atom set for the convergence to be 0.01 eV/Å. The calculated bulk lattice constants for Au and Pt for different exchange-correlation functionals are shown in table S1 of the supporting information.30, 56 The calculations are performed using a supercell containing a six layers slab with 30 atoms per layer, and 20 Å of vacuum between the slabs. The x and y dimensions of the supercell in matrix form are: ቀ

5 0 ቁ , where a1 and a2 for the 1x1 surface are shown in Figure 1b. The molecule-molecule 6 −3

interactions are assessed in the gas phase, using optB88-vdW and rPW86-vdW2, to determine the role of intermolecular interactions on the adsorption of the olympicene radical on the Au and Pt surfaces. After increasing the dimensions of the supercell up to 6 Å in each direction (x,y), our results suggest that molecule-molecule interactions are negligible, with a total energy difference of ~1 meV obtained by increasing the size of the computational cell. 5



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For the calculation of the adsorption energy, the structural optimizations of olympicene radical alone, metal substrates alone, and the olympicene adsorbed on the substrates are performed. The olympicene radical in the gas phase is found to have a flat geometry. During the optimization of the molecule/substrate system, the bottom two layers of the substrate are fixed at their bulk-truncated positions, while all other atoms in the system are allowed to relax. Using the total energies calculated for each case, we then calculate the adsorption energy with the following description: Eads = - (EOL/metal – Emetal – EOL)

(1)

where EOL/metal is the total energy of the olympicene radical/substrate system, Emetal is the total energy of the adsorbate-free substrate, and EOL is the total energy of the olympicene radical in the gas phase. The adsorption energies are evaluated for all vdW functionals in addition to the GGA-PBE functional. We further extended our analysis to evaluate the non-local correlation part of the total exchange correlation energy to determine the corresponding non-local correlation contribution to the calculated adsorption energies, Enlcads defined by the following description: Enlcads = - (EnlcOL/metal – Enlcmetal – EnlcOL)

(2)

where EnlcOL/metal, Enlcmetal, and EnlcOL are the non-local contributions to the total energy of the olympicene radical/substrate system, adsorbate-free substrate, and olympicene radical in the gas phase, respectively. II.1. Initial Adsorption Geometries of Olympicene Radical on Au(111) and Pt(111) The potential energy surface experienced by the olympicene radical upon adsorption on different metal substrates varies significantly from one substrate to another, and it depends on the interaction strength with the substrate and the vdW functional used. In our earlier publication on the adsorption characteristics of an olympicene radical on Cu(111),31 we evaluated the adsorption characteristics using seven adsorption configurations, which are obtained by successive rotations and translations of the molecule on the surface. On Cu(111), the PBE calculations predicted very similar adsorption energies for all adsorption configurations, and the inclusion of vdW interactions introduced a difference of about 200 meV in the adsorption energies between different adsorption configurations. In the current study, our interest is to reveal the difference in the adsorption characteristics of olympicene radial on Au and Pt(111) surfaces, and to reveal both the role of substrate electronic structure, and vdW interactions on the nature of molecular adsorption. Using four (c1 to c4) out of seven adsorption configurations (see Figure 1.a-d) evaluated on Cu(111), we compare the results of the 6


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adsorption characteristics on Au and Pt(111) with those on Cu(111).31 We also evaluate a few additional configurations (c5, c6, and c7) on Pt(111) for selected cases as the interaction of olympicene radical on this surface is stronger yielding noticeable changes in the adsorption from one configuration to another. This is not observed for the adsorption on Au(111) surface as we shall see in the following sections.

Figure 1. Top views of the initial adsorption configurations of olympicene radical on (111). The blue and gray spheres represent the substrate atoms, while the red and white spheres are C and H atoms, respectively.

For both Au and Pt(111) surfaces, the adsorption characteristics of the olympicene radical are evaluated for the four adsorption configurations, named c1 to c4, and shown in Figure 1a-d. For the c1 configuration, the carbon atom labeled ‘T’ (see Fig. 1a) is positioned over a top site of the first layer substrate atom (gray spheres). The c2 configuration (see Fig. 1b), on the other hand, is obtained by a translation of the c1 configuration such that the carbon atom labeled ‘T’ is above the HCP site. The c3 and c4 configurations are obtained by rotating the c1 configuration by 30° and 50°, respectively, about the carbon atom labeled ‘R’ in Fig 1a. Based on the results obtained in our previous study for the adsorption of olympicene radical on Cu(111),31 we found that these adsorption configurations are sufficient to sample the adsorption energies on Au(111) surface. As we mentioned above, we have explored three additional adsorption configurations on Pt(111), using only PBE and optB88-vdW, to further assess the differences introduced to the adsorption with varying adsorption configurations. The three additional adsorption configurations (c5, c6, and c7) are shown in Figure 1e-g. For the c5 and c7 configurations, the carbon atom labeled ‘T’ (see Fig. 1a) is positioned between a top and HCP site and over an FCC site, respectively. The configuration c6 is obtained by rotating the c1 configuration about 7



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the carbon atom labeled ‘R’ in Fig. 1a, by 12˚. III. Results and Discussions The discussion on the results is divided into five sections. In these sections, we focus our discussion on the role of substrate chemistry, and the choice of the exchange correlation functionals on the nature of the adsorption on Au and Pt(111) surfaces. In the first three sections, the equilibrium adsorption geometries, adsorption energies, and adsorption heights on the Au(111) and Pt(111) surfaces calculated using PBE, rPW86-vdW2, optB88-vdW, and optB86b-vdW functionals are presented. Next, we discuss the trends in the adsorption energy and the non-local contribution to the adsorption energy for different vdW functionals and surface chemistries. Following these sections, we report on the electronic structure changes characterized by the changes in the work function (∆Φ), charge transfer upon adsorption, and charge density distribution at the interface. The observations we made form the framework for our understanding of the interaction strength, and the nature of bonding with Au(111) and Pt(111) surfaces. Finally, the last section is used to provide a comparison with the results of our earlier study for the adsorption on Cu(111) surface,31 and to reveal the effect of substrate electronic structure on the adsorption characteristics. III.1. Equilibrium Adsorption Geometries on Au(111) and Pt(111) In this section, we discuss the adsorption geometry of an olympicene radical on Au and Pt(111) surfaces. The optimized, gas phase geometry of the olympicene radical is flat. Upon adsorption on the Au(111) and Pt(111) surfaces, olympicene radical tilts (α) and bends (∆z) to varying degrees, depending on the surface and the adsorption configurations. The adsorption also results in the buckling of the top layer surface atoms. The changes in the geometry for each adsorption configuration are summarized in Table 1. The description of the tilting and bending is shown in Figure 2. The bending of olympicene is calculated as the maximum perpendicular distance of the carbon atom in the center of the molecule to a line segment connecting carbon atoms on the opposite edges of the molecule. The tilting of olympicene is calculated as the maximum angle made by a line segment connecting two carbon atoms on the opposite edges of the molecule and a plane of reference parallel to the substrate. In Table 1, we also summarize the adsorption energies and heights for each of the adsorption configurations, and they are discussed in detail in the following two sections. Let us start with the changes in the equilibrium adsorption geometry of olympicene radical on Au(111) and Pt(111). When the olympicene radical is adsorbed on Au(111), for all the adsorption configurations considered, we find a small tilting (0.4°–2.5°) and almost no bending (0.01Å - 0.06Å). 8


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These structural changes are much less than those previously found for the adsorption on Cu(111).31 The comparison of the results obtained using the optB88-vdW and optB86b-vdW functionals, with those obtained using the PBE and rPW86-vdW2 functionals suggests that for the optimized exchange functionals, the interaction with the substrate is increased, leading to slight increase in the tilting. This can be attributed to the lower adsorption height achieved using the optimized exchange functionals. The degree of buckling of the first layer surface atoms provides evidence for the small structural changes in the support, at most 0.2 Å, upon adsorption. These small structural changes observed for both olympicene and the substrate atoms suggest that the interaction of olympicene with Au(111) is mostly governed by the vdW interactions.

Figure 2. Schematic representations of bending (a) and tilting (b) of olympicene radical upon adsorption on metal substrate. The gray spheres represent the first layer substrate atoms, while the red and white spheres are C and H atoms, respectively.

The changes in the geometry of both olympicene radical and the substrate is found to be much more significant when it is absorbed on Pt(111). The optimized adsorption configurations on Pt(111) reveals the tilting to be in the range of 2°-18°, which is considerable higher than that of the adsorption on Au(111), suggesting that olympicene’s interaction with Pt(111) is strong as compared to that on Au(111). Additionally, it experiences bending upon adsorption on Pt(111) that vary between 0.11 Å and 0.54 Å, which is substantially different from that observed on Au(111). It is worth mentioning that even with the PBE functional, these non-negligible structural changes are observed for the adsorption on Pt(111). This suggests that the nature of adsorption on Pt(111) is governed by strong covalency in addition to the vdW interactions, leading to enhanced molecule substrate interaction as compared to that on Au(111). In the following sections, the details on the adsorption heights and energies on Au(111) and Pt(111) will provide further evidence for the difference in the nature of adsorption. 9



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Figure 3. Equilibrium adsorption configurations of olympicene radical on Au (left) and Pt(111) (right).

To further illustrate the difference in the adsorption strength and the changes in the geometry of olympicene radical on Au(111) and Pt(111) surfaces, in Figure 3, we present the side views of the equilibrium adsorption configurations obtained using PBE and the vdW functionals considered. The adsorption configurations of olympicene radical on Au(111) show very small changes in the structure upon adsorption. For each adsorption configuration on Au(111), olympicene radical undergoes slight tilting. As opposed to these negligible structural changes, the optimized structures on Pt(111), shown in Fig. 3 (right) are characterized by significant alterations in the structure of olympicene radical, with respect to the flat gas phase geometry, as well as tilting of the molecule. The figure indicates that the resulting optimized structures not only depend on the adsorption configurations but also are influenced by the vdW functionals used. Since these structural changes are also observed using the PBE functional, we suggest that the interaction of olympicene radical with Pt(111) is likely governed by a combination of covalent and vdWs bonding. III.2. Adsorption Energies on Au(111) and Pt(111) The results of the adsorption energies of olympicene radical on Au(111) and Pt(111) are summarized in Table 1 along with the adsorption heights for all configurations considered. Let us first examine the results obtained using the PBE functional to set the reference for evaluating the effect of vdW interactions on the adsorption characteristics. The adsorption energies for all configurations on Au(111), calculated using the PBE functional, are found to be very similar, and ranging from 0.33 eV to 0.36 eV. Similarly to the PBE results for the adsorption on Cu(111),31 we find the adsorption on 10


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Au(111) to be weak. Thus, we expect the vdW interactions to govern the nature of bonding for olympicene radical on Au(111) surface. On the other hand, we find that the adsorption energy of an olympicene radical on Pt(111), calculated using the PBE functional, is much higher than that on both Cu(111)31 and Au(111), with the adsorption energy varying from 0.93 eV to 2.28 eV, depending on the adsorption configuration. Such a large deviation in the adsorption energies results from different adsorption geometries obtained after the structural optimization, see Fig. 3. The high adsorption energy obtained using the PBE functional suggests that the nature of adsorption on Pt(111) is mostly governed by covalency, as we shall see later. We note that the strong adsorption on Pt(111) is also accompanied by a significantly lower adsorption height on this surface as compared to that of the adsorption on Au(111); see Table 1. We now report on the effect of vdW interactions on the adsorption characteristics of an olympicene radical on both Au and Pt(111) surfaces. The results of the adsorption characteristics summarized in Table 1 show that on both surfaces, the adsorption energies increase with the inclusion of vdW interactions. The adsorption energies calculated on both surfaces follow the trend in the increasing order as: EPBE < ErPW86-vdW2 < EoptB88-vdW < EoptB86b-vdW. Such enhancement obtained in the adsorption energies with the inclusion of vdW interactions was also reported in our earlier studies30-34 and by others in the literature43 for many other organic molecules adsorption on the similar metal surfaces. Comparison of the adsorption energies of the olympicene adsorption on Au(111) and Pt(111) shows that the vdW functionals yield greater enhancements in the adsorption energy on Pt(111) than that on Au(111). The analysis of the adsorption heights on both Au and Pt(111) indicate that olympicene radical adsorbs on Pt(111) with a rather small adsorption height as compared to the large adsorption height on Au(111). This is also true when the PBE functional is used, and hence suggesting that the nature of interaction on Pt(111), is governed by both covalent and vdWs bonding. For the adsorption on Au(111), we find that the rPW86-vdW2 functional enhances the adsorption energy by about 1.3 eV from that calculated using PBE functional. This is a fairly similar enhancement (~ 1.1 eV) obtained for the adsorption on Cu(111).31 On the other hand, the adsorption energies calculated using the optimized exchange functionals by Klimes, et. al.19 (optB88-vdW and optB86b-vdW) are above 2.0 eV. The difference between the enhancement introduced by the optimized exchange functionals and the rPW86-vdW2 functional can be tied to the strong repulsive nature2 of the rPW86-vdW2 at short separations leading to large adsorption heights away from the surface. These observations are in agreement with those reported in our recent publications,30-34 and those reported by others.38, 42

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Table1. Adsorption energies (Eads), C-metal surface adsorption heights (Hads), tilting (α), bending (∆z), and the buckling of the top-most surface layer. The C-metal adsorption height is calculated as: Hads = Czmin – Metalavez. Method Configuration Eads (eV) Hads (Å) α (deg) ∆z (Å) Buckling (Å) Surface Au(111) PBE

rPW86-vdW2

optB88-vdW

optB86b-vdW

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

0.36 0.35 0.33 0.33 1.69 1.67 1.66 1.66 2.30 2.23 2.22 2.22 2.38 2.29 2.28 2.28

3.78 3.74 3.70 3.70 3.49 3.38 3.39 3.39 3.09 3.14 3.11 3.12 2.96 3.06 3.03 3.04

0.4 0.9 2.1 1.5 0.8 0.5 1.0 0.8 1.9 0.6 1.5 1.4 2.5 0.7 1.8 1.7

0.03 0.06 0.05 0.06 0.02 0.06 0.04 0.03 0.01 0.05 0.04 0.06 0.03 0.04 0.06 0.06

0.03 0.03 0.04 0.04 0.14 0.18 0.17 0.20 0.12 0.10 0.11 0.12 0.11 0.09 0.10 0.10

1 2 3 4 5 6 7 1 2 3 4 1 2 3 4 5 6 7 1 2 3 4

1.47 2.28 0.93 2.04 2.27 1.67 1.42 2.41 2.07 2.27 2.29 4.11 4.03 3.42 3.88 4.42 3.42 3.97 5.64 5.64 4.38 5.34

2.31 1.88 2.03 1.92 1.88 2.01 2.20 3.17 2.27 3.25 3.20 2.21 1.93 2.03 2.00 1.89 2.03 2.06 1.89 1.90 2.00 1.93

18.4 2.2 9.0 14.5 2.3 5.6 17.6 3.5 13.0 3.4 3.1 8.1 8.2 8.6 13.0 2.23 12.55 12.44 2.0 2.0 8.1 11.0

0.11 0.36 0.26 0.54 0.34 0.33 0.18 0.03 0.14 0.06 0.10 0.14 0.58 0.12 0.33 0.33 0.20 0.25 0.31 0.31 0.13 0.45

0.22 0.48 0.54 0.32 0.49 0.27 0.21 0.10 0.40 0.09 0.11 0.24 0.47 0.52 0.35 0.52 0.33 0.26 0.46 0.45 0.53 0.30

Pt(111) PBE

rPW86-vdW2

optB88-vdW

optB86b-vdW

Similarly, on Pt(111), the adsorption energies calculated using the rPW86-vdW2 functional are 12


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similar or slightly greater than those of the PBE functional, while the optimized exchange functionals enhance the adsorption energies by up to ~ 3 eV. While both the adsorption energies and the enhancements in the adsorption energies obtained with the inclusion of the vdW interactions are very similar on Au(111) and Cu(111),31 the adsorption energies on Pt(111) are significantly higher. Furthermore, the adsorption energies calculated using the PBE functional on Pt(111) are relatively high, indicating that the nature of adsorption is strong chemisorption. The difference in the interaction strength of olympicene with Pt(111) surface is also evident from the optimized structures (see Fig. 3), which show that the olympicene radical undergoes noticeable structural change upon adsorption, while on Au(111), the structural changes are small. This observation is attributed to the differences in the governing interactions between the olympicene radical and the underlying substrate. The adsorption energies summarized in Table 1 suggest that for both surfaces, the configuration 1 (c1, see Fig. 1a), in general, has the highest adsorption energy. We should note that on Pt(111), among the additional adsorption configurations tested (c5-c7), the configuration c5 has higher energy (~ 300 meV) than that of the configuration c1. The adsorption energy differences between the configurations for Au(111) is rather small (~ 100 meV), while for Pt(111), the differences in the adsorption energies between the configurations can go beyond 1 eV. III.3. Adsorption Heights on Au(111) and Pt(111) The differences in the equilibrium adsorption geometries (side views) of olympicene radical on Au(111) and Pt(111) along with the adsorption heights between (C-metal) are shown in Figure 4 for the most stable adsorption configuration. The figure shows that upon adsorption on Au(111) surface, the structure of the olympicene radical is similar to its gas phase geometry (flat). On the other hand, the adsorption on Pt(111) surface, as discussed above, leads to both tilting and bending of the olympicene radical, and for most of the adsorption configurations on Pt(111), hydrogen atoms of the olympicene radical lift away from the surface, similar to that is observed in our earlier study for the adsorption of benzene on reactive metal surfaces, including Pt(111).30, 34 The figure also shows that the adsorption heights on Pt(111) is much shorter than that on Au(111). Overall, these observations indicate that the interaction of an olympicene radical with the reactive substrate, Pt, is much stronger than that on Au. The adsorption heights, which are calculated using the difference in the average z position of all of the first layer surface atoms and the lowest z position of the C atoms, obtained using PBE and vdW functionals are presented in Table 1. Note that there is no unique way for calculating the adsorption heights, and the description of the adsorption height is rather challenging for olympicene radical on Pt(111) on which the structure of the molecule varies significantly depending on the adapted adsorption 13



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configurations after the structural optimization. The buckling of the surface layer is calculated using the maximum difference in z positions of the first layer atoms, and the large buckling suggests strong interaction between the molecule and the underlying support.

Figure 4. Side views of the optimized geometry for the highest adsorption energy configuration, c1, of olympicene radical (a) on Au(111) and (b) on Pt(111). The optimized geometries and heights (Å) are obtained using the optB86b-vdW functional. Gray, red, and white spheres represent substrate atoms (Pt or Au), C, and H atoms, respectively.

The adsorption heights on Au(111) calculated using PBE functional are found to be large, in the range of 3.69Å -3.77Å. The interaction between olympicene radical and Au(111) surface is governed by the vdW interactions, hence the large adsorption heights obtained using PBE is expected to become smaller with the inclusion of vdW interactions. This is similar to the findings in our earlier calculation for the adsorption on Cu(111) surface,31 for which the PBE functional predicted the adsorption heights in the range of 3.11Å-3.34Å. It is also similar to the adsorption heights obtained for benzene on Cu, Ag, and Au(111) varying between 3.42Å and 3.55Å.30 We expect more accurate description of the adsorption geometry to be achieved with the inclusion of vdW interactions as shown in our earlier study for benzene adsorption.30 Let us start with the results obtained using the rPW86-vdW2 functional. The adsorption heights on Au(111), obtained with this functional range between 3.33Å-3.49 Å. Although the adsorption heights are reduced from the PBE predicted values, they are still large because of the strong repulsive nature of this vdW functional.2 On the other hand, the adsorption heights calculated using the optimized exchange functionals are further reduced, and vary between 2.93Å-3.11 Å. Among the vdW functionals, the lowest adsorption height is found using the optB86b-vdW functional, in agreement with the earlier reports for other organic molecules on the similar metal 14


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surfaces.30-34 These trends are also very similar to those obtained in our earlier publication for olympicene radical on Cu(111).31 The adsorption heights are overall shorter on Cu(111) surface. The adsorption heights on Pt(111) calculated using the PBE functional are significantly lower as compared to those on Au(111), and range between 1.88Å -2.31Å depending on the adsorption configuration. The covalent interaction, for the case of Pt(111), is clearly evident from the adsorption heights calculated using the PBE functional. Similarly to the adsorption on Au(111), the inclusion of vdW interactions using the optimized exchange functionals lowers the adsorption heights, and the lowest adsorption heights are obtained using the optB86b-vdW and the optB88-vdW functionals. The changes in the adsorption heights with the inclusion of vdW interactions is found to be very similar to that obtained in our earlier calculation for the adsorption of benzene on Pt(111).30 We also found that the adsorption heights calculated using the rPW86-vdW2 functional are larger than those calculated using the PBE functional. Similar observation is encountered for the adsorption of benzene on Pt(111)30. The effect of the vdW interactions on the adsorption heights introduced by the rPW86-vdW2 functional is different for Au(111) than that of Pt(111). The interaction in the latter is governed by both covalency and the vdW interactions, while for the former; the vdW forces mostly govern the interaction. In summary, we observe that in addition to the significant enhancement introduced to the adsorption energies with the inclusion of vdW interactions, the adsorption heights are also lowered, suggesting that the molecule–surface interaction is enhanced with the inclusion of vdW interactions. The degree of enhancement, on the other hand, is strongly dependent on the functional, and the characteristics of the substrate. III.4 Non-Local Contribution to the Adsorption Energy on Au(111) and Pt(111) In this section, we evaluate the nonlocal contribution to the adsorption energy (Enlcads) for all the vdW functionals considered. In Figure 5, the adsorption energies and the nonlocal contributions to the adsorption energies are plotted as a function of the corresponding vdW functionals for the most stable adsorption configurations. The adsorption energies and the nonlocal contributions are calculated using the Eqs.1-2, and the adsorption energies and Enlcads are summarized in Table 2. As shown in the Fig. 5 and the Table 2, the nonlocal contribution to the adsorption energy is higher than the adsorption energy, regardless of the adsorption configuration and vdW functional used. This suggests that the enhanced adsorption energy mainly results from the nonlocal correlations. The smallest nonlocal contribution to the adsorption energy, Enlcads, is found for the rPW86-vdW2 functional, in particular for the adsorption 15



on Pt(111). This is in agreement with the observation that the adsorption energies are overall much less enhanced when this functional is used. For the same vdW functional, we find that Enlcads is less on Au(111) surface than on Pt(111). The larger Enlcads for the Pt case is the result of the large covalent bonding character for this surface as compared to Au(111). 4.5 4.0

(a)

OL/Au(111)

8 (b)

3.5

7

3.0

6

Energy (eV)

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5 2.0 1.5 1.0 0.0

W rP

8

nlc ads Eads E

5 4 3 2 1

0.5

d 6- v

OL/Pt(111)

0 W2

2 dW dW dW v 8- v b- v 8 6 6 8 tB tB 8 W op rP op

dW dW b-v 8-v 6 8 tB tB8 op op

Figure 5. Nonlocal contribution to the adsorption energy for the highest binding energy configuration (c1) for olympicene radical adsorption (a) on Au(111), and (b) Pt(111).

When we compare the magnitude of the nonlocal contributions and the adsorption energies summarized in the Table 2, we find that, for the adsorption on Au(111), the difference in the Enlcads using the optB88-vdW and the optB86b-vdW functionals is at most 0.37 eV, and the difference in adsorption energies calculated with these functionals is also small. On the other hand, the difference in Enlcads for Pt(111) is larger, and this is also reflected in the difference obtained for the computed adsorption energies. This suggests that for the adsorption of olympicene radical on Au and Pt(111), the vdW interactions are critical even for the adsorption on Pt(111) where the covalent bonding is expected to be the main contributor. This is a different observation from the results reported by Carrasco et. al.,57 on the adsorption of benzene on (111) coinage and transition metal surfaces. The authors reported the covalent bonding of benzene on transition metals substrates resulted in large changes in the adsorption energy between the optimized exchange functionals, while the Enlcads were mostly similar (with the variation of ~ 100 meV) for these functionals. Note that the larger variation in the Enlcads obtained in our study suggests that the nonlocal contribution depends on the adsorption geometry. For the case of benzene adsorption on (111) surfaces, the benzene molecule adsorbs at a particular site, and remains flat on the surface. In our earlier study for the adsorption of benzene on the same set of (111) transition metal substrates,30 we find the same trend reported by Carrasco, et. al.57 On the other hand, for olympicene radical, we find large differences in the final configurations depending on the vdW 16


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functional used even when the initial adsorption configuration is the same. The differences in the atomic positions of the molecule and/or the substrate atoms introduced by different vdW functionals can lead to changes in the charge density, and may affect the nonlocal energies. These differences are more dramatic on Pt(111) surface due to the strong interaction of the olympicene radical with this substrate leading to significant changes in the structure. Our result supports the observation we made for benzene adsorption on (110) surfaces34 that the difference in the adsorption configurations captured using different vdW functional affect the nonlocal contribution. Table 2. The adsorption energies (Eads) and the nonlocal correlation contribution to adsorption energy (Enlcads) for all configurations. Method Configuration Eads (eV) Enlcads (eV) Eads (eV) Enlcads (eV) OL/Au(111) OL/Pt(111) rPW86-vdW2 c1 1.69 1.81 2.41 2.54 c2 1.67 2.06 2.07 2.86 c3 1.66 2.05 2.27 2.41 c4 1.66 2.05 2.29 2.44 optB88-vdW c1 2.30 3.41 4.11 6.14 c2 2.23 3.44 4.03 6.59 c3 2.22 3.44 3.42 6.45 c4 2.22 3.40 3.88 6.22 c5 4.42 7.16 c6 3.42 5.37 c7 3.97 5.45 optB86b-vdW c1 2.38 3.77 5.64 7.44 c2 2.29 3.77 5.64 7.44 c3 2.28 3.72 4.38 6.82 c4 2.28 3.65 5.34 6.91

III.5. Change in Work Function and Charge Transfer upon Adsorption on Au(111) and Pt(111) The surface work function change upon molecular adsorption is often utilized to reveal the formation of an interface dipole. Experimental studies often report the interfacial electronic structures for many organic molecule adsorptions on metal surfaces. For instance, using UV experiments, an interface dipole of 0.7 eV was reported for sexithiophene adsorbed on polycrystalline Ag determined from the reduction of surface work function with the adsorption.58 In a recent publication for the adsorption of sexithiophene on Ag(110),33 using vdW inclusive DFT study, we find the reduction of work function upon adsorption of sexithiophene on Ag(110). The largest change in the work function is found to be ~ 0.6 eV using the optB86-vdW functional. The results were in good agreement with the available experimental observations only when the vdW interactions were considered. The reduction in the work function was attributed to the formation of an interface dipole as well as to the chemical interaction between the molecule and the substrate. 17



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Table 3. The work function (Φ) and the change in the work function (∆Φ) calculated for the most stable adsorption configuration on Au(111) and Pt(111) using the optB86b-vdW, rPW86-vdW2, and PBE functionals. Negative sign indicates the reduction in the work function from the adsorbate free surface. Method System Φ (eV) ∆Φ (eV) System Φ (eV) ∆Φ (eV) PBE 5.15 5.73 Au(111) Pt(111) 4.64 -0.51 4.72 -1.01 OL/Au(111) OL/Pt(111) rPW86-vdW2 5.63 6.10 Au(111) Pt(111) -0.56 5.45 -0.65 5.07 OL/Pt(111) OL/Au(111) optB86b-vdW 5.38 5.95 Au(111) Pt(111) 4.88 -0.50 4.76 -1.19 OL/Au(111) OL/Pt(111)

For olympicene radical on Au and Pt(111) surfaces, we repeated the similar analysis to evaluate the change in the surface work function upon adsorption of olympicene. The work function is calculated for the highest adsorption energy configuration of olympicene radical on both surfaces using all the vdW functionals and PBE. Our results on the work function of the molecule/substrate system, of the clean surface, and the work function change (∆Φ) are summarized in Table 3. The results indicate that for both surfaces, upon adsorption of olympicene, there is a reduction of the surface work function with respect to the adsorbate-free surfaces. While the decrease in the surface work function is similar (~ 0.5-0.6 eV) on Au(111) surface regardless of the functional used, we find noticeable variation for the case of Pt(111). For Pt(111), both PBE and optB86b-vdW functionals predict similar reduction in the work function, with that obtained using the optB86b-vdW functional is higher ~ 200 meV. Again for this surface, the rPW86-vdW2 functional predicts the lowest reduction. The changes in the work function for Pt(111) are found to be 0.65 eV for rPW86b-vdW2, 1.01 eV for PBE, and 1.19 eV for optB86b-vdW.

Figure 6. Side views of the charge density difference of olympicene radical on Au(111), left, and Pt(111), right, using the PBE (top), rPW86-vdW2 (middle), and optB86b-vdW (bottom) functionals. The charge accumulation is presented by red and the depletion is in blue. The isosurface values for Au(111) are set to 2.6x10-3 eÅ-3 and 3.0x10-3 eÅ-3 for Pt(111).

18


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We also performed Bader charge analysis to evaluate the charge transfer between the olympicene radical and Au(111) and Pt(111) surfaces upon adsorption for all adsorption configurations using the vdW functionals and PBE. The net charge transfer between the olympicene and the substrates is found to vary between ~ 0.2–0.6 electrons. The charge transfer to the substrate is found to be in the order of ~ 0.3-0.5 electrons for Au(111), while for Pt(111), it is in the order of ~ 0.2-0.6 electrons depending on the adsorption configurations and the vdW functional used. We also note that the analyses of the total and partial electronic densities of states, (dz2) of the surface atoms before and after the adsorption, show no interface state formation. Finally, the charge density difference upon adsorption of olympicene on the substrates are calculated using the PBE, optB86b-vdW, and rPW86-vdW2 functionals by subtracting the charge density of the molecule/substrate system from the individual molecule and substrate charge densities, without further relaxation of the ions. The results are presented in Figure 6 for the adsorption configuration c1 to reveal the differences in the charge accumulation and depletion features at the interface between olympicene radical and the metal surfaces introduced by different functionals. From the figure, one can see the difference between the charge distribution at the interface with Au and Pt(111) surfaces. On Au(111), no charge accumulation is present regardless of the functional used, while on Pt(111), charge accumulation is clearly identifiable with PBE and optB86b-vdW functionals. There is no identifiable charge accumulation at the interface using the rPW86-vdW2 functional. These results are in agreement with the adsorption strengths of olympicene with these substrates and confirm the changes in the adsorption heights with the use of different functionals. Note also that similar observation is made in our earlier study for the adsorption of olympicene on Cu(111).31 IV. Comparison of the Adsorption Characteristics of Olympicene Radical on Cu, Au, and Pt(111) In an attempt to evaluate the role of substrate on the chemical interaction with the olympicene radical, in Figure 7, we compare the adsorption energies and heights on Cu, Au, and Pt(111) surfaces for the adsorption configurations, c1-c4. For both on Cu(111) and Au(111), the adsorption energies calculated using PBE functional range between 0.2-0.4 eV suggesting the nature of bonding is weak physisorption - the bonding on these surfaces mostly governed by the vdW interactions. When the vdW interactions are taken into account, for both surfaces, a systematic enhancement in the adsorption energies is observed with the highest enhancement obtained using the optimized exchange functionals. For both surfaces, the rPW86-vdW2 functional leads to the smallest enhancement in agreement with the results of our earlier studies.30-34 The adsorption heights on these surfaces also follow the trends 19



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observed for the adsorption energies. The adsorption heights on Au(111) are found to be as high as (> 3.3 Å) for the PBE and rPW86-vdW2 functionals, while they are slightly lower for the optimized exchange functionals (~ 3 Å). For the adsorption on Cu(111), the adsorption heights are ~ 3.3 Å for the PBE and rPW86-vdW2 functionals, while the optimized exchange functionals lead to reduced adsorption heights reaching as low as 2.3 Å. Finally, the inclusion of vdW interactions for the olympicene adsorption on Au(111) surface changes the nature of bonding from weak physisorption to strong physisorption, with adsorption energies in the range of 2-2.5 eV. On the other hand, on Cu(111), the nature of bonding changes from weak physisorption to strong chemisorption as a result of significant charge transfer and the formation of an interface state, which is lacking in the case of Au(111).

Figure 7. Comparison of the adsorption energies and heights of olympicene radical adsorbed on Cu, Au, and Pt(111) calculated using PBE and vdW functionals.

The adsorption characteristics on Pt(111) is different than Cu and Au(111) surfaces. On Pt(111), the adsorption energies calculated using PBE are in the range of 0.9-2.28 eV suggesting chemisorption. We find that the inclusion of vdW interactions enhances the adsorption energies and further leads to significant structural changes to the olympicene radical. We shall note, such significant structural change is not observed for the adsorption on Cu and Au(111) surfaces. The largest enhancement in the adsorption energies on Pt(111) surface is obtained for the optimized exchange functionals, while the rPW86-vdW2 functional leads to the smallest as similar to that observed for both Cu and Au(111). For this surface, in contrast to Cu and Au(111), the adsorption is mostly dominated by covalency as evidenced from the result of the PBE calculations. The adsorption heights calculated using PBE functional for all configurations range between 1.88 – 2.31 Å. This is clearly the smallest adsorption height obtained among the surfaces using the PBE functional and again suggests that the adsorption on this surface is mostly dominated by covalency. As a result of the short distance to Pt(111) surface, and 20


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enhanced interaction, we observe significant structural changes in both the substrate and the molecule upon adsorption. The adsorption heights obtained using the rPW86-vdW2 functional, on the other hand, are found to be much higher than those obtained using PBE and the optimized exchange functionals on this surface. This is in agreement with our earlier observations30-34 that the strong repulsive nature of this vdW functional leads to larger adsorption heights on reactive transition metal surfaces. The overall comparison between the adsorption characteristics of olympicene on Cu, Au, and Pt(111) surface reveals that the bonding for both Cu and Au is mostly dominated by the vdW interactions, while for Pt(111), the strong covalency nature dominates the bonding on this surface along with the vdW interactions. V. Conclusions The role of vdWs interactions on the adsorption characteristics of olympicene radical on Au(111) and Pt(111) surfaces are evaluated using the self-consistent vdW-inclusive DFT approach. Our calculations evidenced that the bonding of olympicene radical on Au(111) is mostly dominated by the vdW interactions, while on Pt(111), the molecule-metal bonding is dominated by the combination of covalency and vdW interactions. The results obtained for Au(111) surface suggest that the optimized exchange functionals substantially change the adsorption heights and energies in comparison with those obtained using PBE functional, leading to a change in the nature of adsorption from weak physisorption to strong physisorption. The olympicene radical adsorbs relatively flat - as similar to its gas phase geometry - on Au(111) without any significant change in its structure. On the other hand, on Pt(111) surface, upon adsorption, olympicene radical undergoes significant structural change in the form of bending, and adsorbs mostly in a tilted configuration. These structural changes were also observed using the PBE functional suggesting that the nature of bonding is mostly governed by covalency. For both surfaces, the change in the adsorption energies is the least when using the rPW86-vdW2 functional, due to the strong repulsive nature of this vdW functional. The largest enhancement in the adsorption energies is obtained using the optimized exchange functionals. Additionally, olympicene radical is found to adsorb much closer on Pt(111) than on the Au(111) surface, and the optB86b-vdW functional predicts the smallest adsorption heights. In summary, we demonstrated that both covalency and vdW bonding play an important role for the interaction of olympicene radical with the Pt(111) surface using the vdW- inclusive DFT methods. The contribution of the vdW interactions for the adsorption of olympicene radical on Pt(111), a typical strongly adsorbed system, is greater than that on the adsorption on the Au(111) surface, a typical physisorbed system. Our findings indicate that vdW-inclusive DFT calculations are essential even for 21



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the case of strongly bound molecules on surfaces where the interaction is mostly dominated by covalent bonding. Finally, the effects of the inclusion of vdW interactions into the binding between an olympicene radical on Au and Pt surfaces differ from those on Cu(111) where the nature of the bonding changes from weak physisorption to strong chemisorption.

AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected] (H.Y.); [email protected] (A.K.) ‡

Present Address: School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, United States. ACKNOWLEDGEMENTS A.K. acknowledges the support from the U.S. Department of Energy Basic Energy Science under contract no. DE-FG02- 11ER16243. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under contract no. DE-FG02-11ER16243. REFERENCES 1. Becke, A. D., Perspective: Fifty Years of Density-Functional Theory in Chemical Physics. J. Chem. Phys. 2014, 2014 140, 18A301. 2. Klimeš, J.; Michaelides, A., Perspective: Advances and Challenges in Treating Van Der Waals Dispersion Forces in Density Functional Theory. J. Chem. Phys. 2012, 2012 137, 120901. 3. Gräfenstein, J.; Cremer, D., An Efficient Algorithm for the Density-Functional Theory Treatment of Dispersion Interactions. J. Chem. Phys. 2009, 2009 130, 124105. 4. Burns, L. A.; Vázquez-Mayagoitia, Á.; Sumpter, B. G.; Sherrill, C. D., Density-Functional Approaches to Noncovalent Interactions: A Comparison of Dispersion Corrections (Dft-D), Exchange-Hole Dipole Moment (Xdm) Theory, and Specialized Functionals. J. Chem. Phys. 2011, 2011 134, 084107. 5. Johnson, E. R.; Mackie, I. D.; DiLabio, G. A., Dispersion Interactions in Density‐Functional Theory. J. Phys. Org. Chem. 2009, 2009 22, 1127-1135. 6. Tkatchenko, A.; Romaner, L.; Hofmann, O. T.; Zojer, E.; Ambrosch-Draxl, C.; Scheffler, M., Van Der Waals Interactions between Organic Adsorbates and at Organic/Inorganic Interfaces. MRS Bull. 2010, 2010 35, 435-442. 7. Grimme, S., Density Functional Theory with London Dispersion Corrections. Wiley Interdiscip. Rev. Comput. Mol. Sci 2011, 2011 1, 211-228. 8. Berland, K.; Cooper, V. R.; Lee, K.; Schröder, E.; Thonhauser, T.; Hyldgaard, P.; Lundqvist, B. I., Van Der Waals Forces in Density Functional Theory: A Review of the Vdw-Df Method. Rep. Prog. Phys. 2015, 2015 78, 066501. 9. Horowitz, G., Organic Thin Film Transistors: From Theory to Real Devices. J. Mater. Res. 2004, 2004 22


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TOC Figure: Top view of the olympicene radical adsorption on (111) surfaces of Au and Pt and the change in the adsorption energies with the functionals used.

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Role of Long-Range Interactions for the Structure ... - ACS Publications

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