Investigated by Density Functional Theory Calculations - American

Mar 15, 2010 - The density functional theory is used to explore structural properties of alkyl-thiolate (RS) monolayers on. Au(111) and ... Au(111) wa...
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J. Phys. Chem. C 2010, 114, 15973–15978

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Thiolate Induced Reconstruction of Au(111) and Cu(111) Investigated by Density Functional Theory Calculations† Henrik Gro¨nbeck* Department of Applied Physics and Competence Centre for Catalysis, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden ReceiVed: January 11, 2010; ReVised Manuscript ReceiVed: February 26, 2010

The density functional theory is used to explore structural properties of alkyl-thiolate (RS) monolayers on Au(111) and Cu(111). RS adsorption drives pronounced, albeit qualitatively different, reconstruction of both surfaces. On Au(111), the stable structure comprises RS-Au-RS complexes with RS in atop positions, whereas a pseudo-(100) phase is favored for Cu(111) with a majority of the RS radicals in 4-fold hollow positions. The difference is attributed to a more covalent character of the RS-metal interaction for Au, together with a lower energetic penalty to form surface adatoms. Introduction Adsorbed alkyl thiols on noble metal surfaces self-assemble into monolayer films with high order and stability. As the process is facile, self-assembled monolayers (SAMs) offer a convenient route to surfaces with tailored properties that have applications in surface-patterning, in corrosion prevention, and as biocompatible materials.1-3 SAMs can be grown either from disulfides (RSSR, where R is a carbon chain) or alkyl thiols (RSH) and experiments have demonstrated that the two precursors yield equivalent films.4,5 It is established that the RS-SR (RS-H) bond is broken upon adsorption of the disulfide (thiol), and the S-S bond cleavage has been confirmed by the loss of the S-S stretch vibration upon adsorption.6 The process of thiolate self-assembly on metal surfaces is governed by both the intermolecular interactions and the ancoring of the RS molecule to the surface. The interaction between the carbon chains is of weak van der Waals type (about 0.06 eV per methylene group7), whereas the anchoring is a chemical bond with a certain degree of ionic character. On Au(111), the RS-Au bond has been estimated to be about 2.1 eV.3 The self-organization follows a two step process8,9 where the phase at saturation is preceded by a low density striped phase. The phase transition involve a realignment of the molecular RS axes; it is oriented along the surface in the lowdensity phase and with an angle of about 30° to the surface normal at monolayer coverage. Common for the coinage (Cu, Ag, and Au) metal surfaces is that RS adsorption drives surface reconstruction.10 Among the three metals, the structural properties of SAMs grown on Au(111) have received the largest attention. This is connected with the low reactivity of Au toward, for example, O2 which render films on Au stable under atmospheric conditions and suitable for applications. On Au(111), it was early suggested that RS (thiolate) should occupy 3-fold hollow positions at full coverage and form a (3 × 3R30°) overlayer.6 Such a structure was later confirmed by helium diffraction11 and electron12,13 diffraction measurements. Subsequent helium dif† Part of the special issue “Protected Metallic Clusters, Quantum Wells and Metallic Nanocrystal Molecules”. * To whom correspondence should be addressed E-mail: ghj@ chalmers.se.

fraction measurements revealed a c(4 × 2) superstructure with respect to the hexagonal 3 × 3R30° lattice.14 The thiolate model was questioned when the first experiments that directly probed the RS-Au structure appeared; on the basis of X-ray standing wave (XSW) measurements, it was suggested that thiolates pair up into disulfides.15 Later scanned-energy and scanned angle photodiffraction (PD)16 as well as XSW17 measurements were rationalized with RS bonded atop a terrace atom. Up until a few years ago, all structural models assumed an unreconstructed (111) surface. In fact, adsorption of RS onto Au(111) was known to lift the large herringbone reconstruction of the bare gold surface. During the past few years, however, different structural models have been proposed that comprise Au adatoms. On the basis of XSW measurements, it was suggested that thiolates are anchored to the surface via atop adsorption on Au adatoms.17 To account for the c (4 × 2) superstructure, it has recently been put forth that the RSAu complexes might be paired.18 Another structural motif has been observed at low coverage by scanning tunneling microscopy (STM), namely RS-Au-RS complexes.19 The existence of RSAuSR complexes at the interface was also suggested on the basis of grazing incident X-ray diffraction (GIXRD).20 Recently, a combined GIXRD and ab initio molecular dynamics study proposed a model with different structural elements,21 namely one (RSAu)x polymer, and thiolates adsorbed at surface point defects. The existence of gold adatoms is consistent with observations of etch pits on Au(111) after SAMs formation.9 The fraction of gold adatoms on the SAMs has been estimated by STM measurements to be 0.14 ML.22 The value of 0.14 ML, indicates about 2 adatoms in each c (4 × 2) cell (one adatom for every two RS). A larger value of 0.22 ML (3 adatoms per c(4 × 2) cell) has also been reported on the basis of STM.23 The reason for the discrepancy is presently unclear. The nanoscale version of SAMs on Au(111) is thiolateprotected Au nanoparticles (AuNPs). Total structural determinations of two such systems were recently reported, namely Au102 (p-MBA)44 (where p-MBA is SC7O2H5)24 and Au25 (SCH2 CH2Ph)18 in anionic25,26 and neutral27 charge states. In both cases, compact metal cores are capped by gold-thiolate complexes: For Au102 (p-MBA)44, a core with 79 Au atoms is covered by 19 RSAuSR and 2 RS(AuSR)2 units,28 and for Au25 (SCH2

10.1021/jp100278p  2010 American Chemical Society Published on Web 03/15/2010

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CH2Ph)18, a 13 atom core is protected by 6 RS(AuSR)2 units. The possibility of a metal core capped by homoleptic goldthiolate complexes was first suggested for Au38 (RS)2429 and the structural motif observed for Au25 (SCH2 CH2Ph)18- was predicted with methyl thiolate (MeS) ligands.30 Turning to RS adsorption on Ag(111), low energy electron diffraction (LEED) measurements have revealed a (7 × 7)R 19°) surface cell31 which later also has been observed by STM.32 Moreover, on the basis of NIXSW measurements it has been suggested that the outermost Ag layer is reconstructed with an atomic density of only 0.43 of a monolayer and forms a hexagonal lattice with the thiolates adsorbed in 3-fold hollow positions.33 Early surface extended X-ray absorption fine structure (SEXAFS) and normal incident X-ray standing wave (NIXSW) measurements of methyl thiolate (MeS) or dimethyl disulfide on Cu(111) was rationalized with a model where MeS is adsorbed in 3-fold hollow hcp positions.34 However, STM measurements later demonstrated that the thiolates were occupying 4-fold hollow positions on a pseudo-(100) reconstructed surface in a c (2 × 2) mesh with respect to the reconstructed Cu layer.35 In this structure, the outermost atomic layer adopts an expanded (∼14%) Cu(100) lattice with only 0.66 of the atomic density in an unreconstructed surface. Although the reconstructed surface is the stable phase, thiolates may form a metastable ordered layer on an unreconstructed surface at low temperatures (below 173 K).36 The interaction of RS to the (111) surfaces of Cu, Ag and, in particular, Au have over the years received considerable theoretical attention. The theoretical efforts on Au(111) were pioneered by the work of Sellers where the extended Au(111) surface was modeled with a cluster model.37,38 At the Hartree-Fock level, with correlation treated in second order perturbation, MeSH was concluded to adsorb atop an Au atom and MeS in a 3-fold hollow hcp position. Later studies have almost exclusively relied on the density functional theory (DFT) for the description of the quantum mechanical effects and adsorption has been investigated on slab geometries by the use of supercells. These studies often use a pseudopotential (PP) to model the interaction between the valence electrons and core, and the electronic orbitals are expanded in plane-waves (PW). On the unreconstructed surface, it is now established that RSSR dissociate39,40 and that RS is adsorbed in a bridge-fcc site (bridge with S slightly bent over fcc).40,44 The calculated binding energy of RS to the terrace is severely underestimated with respect to the experimental value.45 The RS-Au bond can, however, be enhanced by RS adsorption on defects45,46 and adatoms.47 The experimental evidence for surface structures with gold-thiolate complexes has stimulated investigations of structures with goldthiolate complexes.48-51 Among the different structures, one that comprises two RS-Au-RS units in each c (4 × 2) supercell is predicted to be energetically favored.50 In the stable structure, the two methyl groups are arranged in a cis-configuration. It has been shown that the relative stability of the cis and trans configurations change with surface coverage,52 where the trans version becomes slightly preferred at low coverage. The barrier for cis-trans conversion is estimated to be 0.5 eV at low coverage which indicates frequent switches at room temperature.52 This result together with a low diffusion barrier (0.5 eV) for RS-Au-RS over the Au(111) surface51 indicates that the proposed structure50 has the flexibility often observed experimentally.53 The oligomeric (-RS-(AuRS)n-) motif is energetically robust54,55 and from energetic considerations, a range of surface conformations are possible.56

Gro¨nbeck For Ag(111), the bridge site has been calculated to be the preferred adsorption on the unreconstructed Ag(111) surface.41,47 Moreover, structures for the experimentally observed (7 × 7)R 19°) has been investigated and compared to adsorption on the terrace.57 The unreconstructed and reconstructed surfaces have been predicted to have similar surface free energies which suggests coexistence of the two phases.57 Thiolate adsorption on Cu(111) has been studied on the unreconstructed surface.41,47 In ref 41, DFT was used in an implementation with local basis functions and the 3-fold hollow fcc site was predicted to be the stable adsorption site. Based on periodic calculations within the PP-PW methodology, the bridge position was instead suggested to be the stable adsorption in ref 47. The origin of the conflicting conclusions in these two studies is probably linked to different choices of surface models. Because of the large recent advancement in the understanding of thiolate protected bulk and nanoscale gold surfaces, it becomes important to investigate more realistic surface models for Cu. In particular so, as similar structures are relevant for gold and copper in the limit of stochiometric metal-thiolate complexes (RSM)x,54 where M is either Cu or Au. In the present contribution, methyl thiolate adsorption on reconstructed Cu(111) is evaluated and compared with Au(111). It is concluded that the experimentally suggested pseudo-(100) reconstruction is the energetically preferred structure. The difference in stable structures for Cu and Au is traced to the more covalent character of the Au-S bond and a lower energetic penalty to form surface adatoms. Moreover, the recently proposed structure for MeS on Au(111) including a (RSAu)x polymer and surface vacancies21 is calculated to be unfavored with respect to the RS-Au-RS model.50 Computational Method and Systems The density functional theory (DFT)58,59 is employed in the implementation with plane-waves and pseudopotentials by use of the Quantum-ESPRESSO package.60 The spin polarized Perdew-Burke-Ernzerhof (PBE) formula is used as approximation to the exchange-correlation (xc) functional,61 and ultrasoft pseudo potentials are used to describe the interaction between the valence electrons and the core.62 The pseudo potentials are derived from scalar relativistic all-electron calculations and account for the relativistic effects that are crucial for Au, namely the contraction of the s orbital and the expansion of the d orbital. The pseudopotentials are consistent with the used xc-functional and the number of electrons treated variationally for each element are: Cu(11), Au(11), S(6), C(4), and H(1). A kinetic energy cutoff of 28 Ry (112 Ry) is used to expand the Kohn-Sham orbitals (electron density), yielding convergence in relative energies. It is well know that the choice of exchange-correlation functional may strongly influence molecular adsorption properties. For MeS adsorption on Au(111), PBE results and results based on the local density approximation (LDA) were recently compared and it was shown that both functionals provide consistent chemical pictures.50 That is an important result as PBE is known to underestimate the cohesion of Au, and consequently the energy required for adatom formation. The lattice constants of Cu and Au are calculated to be 3.63 and 4.17 Å, respectively. This is slightly larger than the experimental values of 3.61 and 4.08 Å63 but in agreement with other converged DFT-based calculations.64 Adsorption is investigated in a (3 × 2 3)rect unit cell of the (111) surface, which corresponds the substrate structure of the c (4 × 2) superstructure for Au(111). Each unreconstructed

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TABLE 1: Selected Bond Lengths (Å) and Binding Energies (eV) for MeSSMe and MeSHa MeS-H Theory Exp. a

MeS-SMe

dS-H

Eb

dS-S

Eb

1.35 1.34

3.82 3.79

2.04 2.03

2.86 2.83

The theoretical results are compared to experimental data.67

layer consists of 12 atoms. The surface is represented by four atomic layers. Repeated slabs are separated by at least 14 Å. To study saturation coverage, 4 RS units are considered in each cell. Reciprocal space integration over the Brillouin zone is approximated with finite sampling of 4 special k-points.65,66 For the pseudo-(100) reconstructed Cu(111) surface, the experimentally proposed structure35 is investigated. This structure is commensurate with the underlying (111) surface using a unit mesh of Figure 1. Optimized structures for MeS on Au(111). Atomic color codes: orange (Au), blue (Au adatoms), yellow (S), gray (C), and white (H). The surface cell is indicated for panel a.

( ) 5 0 1 3

TABLE 2: Adsorption Properties of MeS on Au(111)a

The model has 15 atoms in each layer and 4 special k-points are used for the Brillouin zone sampling. The stability of each structure is calculated according to

γ)-

(

NRS 1 ENRS/M(111) - EM(111) - NMEM E A 2 RSSR

)

Here, ENRS/M(111) is the total energy of NRS thiolates adsorbed on M(111) being either unreconstructed or reconstructed. EM(111) is the total energy of the unreconstructed (111) surface, NM is the difference between the number of metal atoms in the adsorbed system and bare surface, EM is the total energy of a metal atom in the bulk, ERSSR is the total energy of the disulfide in the gas-phase, and A is the surface area. This procedure offers a possibility to compare the stability of surface models with different numbers of metal atoms and surface areas. Gas phase species are calculated in a cubic (13.2 Å) cell, assuming the lowest possible spin states; doublets for H, M, MeS, and MeS-M-MeS and singlets for MeSSMe, MeSM, and (MeSM)4. The binding energies and bond lengths for MeSH and MeSSMe are collected and compared to experimental data in Table 1. The agreement between the theoretical results and available experimental data is very good; the bond lengths are slightly expanded and the binding energies are overestimated by only 0.03 eV. Results and Discussion Thiolate Adsorption on Au(111). Atomic models of the considered structures are shown in Figure 1 and the adsorption energies together with structural data are collected in Table 2. Panel a represents the traditional model with RS adsorbed on the unreconstructed (111) terrace in a bridge-fcc position. Panel b is a stochiometric gold-thiolate polymer adsorbed on (111) as originally proposed in ref 49. Panel c was first considered in ref 48 and comprises one adatom and one Au point defect. Panel d is a recently proposed structure based on ab initio molecular dynamics and GIXRD measurements.21 The structure has one stochiometric gold-thiolate polymer and two point-defects where RS is adsorbed. Panel e is RS adsorbed on a defect rich surface; one defect for every three Au atoms. This structure

γ a b c d e f

9 11 15 7 15 19

Surf dAu-S

Ad dAu-S

2.50 2.46/2.51 2.44 2.42 2.53

2.46 2.33 2.42 2.34

∆

dS-C

R

2.04 3.31 2.25 2.45 1.87 2.62

1.84 1.83 1.83 1.83 1.82 1.83

58 46 56 50 70 62

a

The labeling is the same as in Figure 1. γ is surface adsorption X energy in eV/Å2, dAu-S , (dS-C) is the average Au-S (S-C) bond distance. X denotes if S is bonded to a surface (Surf) or an adatom (Ad). ∆ is the average distance between the plane of sulfur atoms and the (111) surface plane. Distances are in Å. R is the average angle (°) between the surface normal and the S-C axis.

was first considered by Molina and Hammer.45 Panel f is a structure with RS-Au-RS complexes with the carbon chains arranged in cis configurations.50 The dissociative adsorption energy of RSSR on the unreconstructed surface is calculated to be 0.39 eV. This value is considerably lower than the experimental estimate of 1.3 eV based on temperature-programmed desorption (TPD).6 The discrepancy indicates that panel a does not represent the experimental structure. (Conclusions based solely on absolute comparisons between experimental and theoretical binding energies obtained with gradient corrected exchange-correlation functionals should, of course, be made with care.) With the exception of panel d, surface reconstruction is preferred with respect to the traditional model. In agreement with a previous report,50 panel f is the energetically favored structure. The preferred structure has the same structural motifs (RS-Au-RS complexes) as the protecting units for Au102 (pMBA)44.24 For the nanocluster, 8 units have a cis-configuration whereas 11 are in a trans structure. The complexes in panel f are all of cis-type. An isomeric structure with the complexes in trans configuration is 0.11 eV higher in energy per unit cell. At low coverage, both types have been observed in STM measurements.19 The reason for the preference of the cis-version is difficult to fully unravel. However, it seems to be related to the packing of RS-Au-RS on the surface; at saturation coverage (0.33), each cis complex is bonded to the surface by 1.74 eV. At half of this coverage (0.165), the binding energy is 1.81 eV. The corresponding values for the trans configuration are 1.68

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Gro¨nbeck TABLE 3: Adsorption Properties of MeS on Cu(111)a a b c d (100)

γ

Surf dAu-S

44 34 35 47 51

2.27 2.33 2.27/2.34 2.39 2.31

Ad dAu-S

2.18 2.16

∆

dS-C

R

1.86 2.43 2.03 1.12 1.38

1.85 1.84 1.84 1.85 1.85

42 46 43 4 0

a The labeling is the same as in Figure 2. (100) refer to adsorption on the Cu(100) surface. γ is the surface adsorption X energy in eV/Å2, dCu-S (dS-C) is the average Cu-S (S-C) bond distance. X indicates if S is bonded to a surface (Surf) or adatom (Ad). ∆ is the average distance between the plane of sulfur atoms and the (111) surface plane. Distances are in Å. R is the average angle (°) between the surface normal and the S-C axis.

Figure 2. Optimized structures for MeS on Cu(111). Atomic color codes: red (Cu), dark blue (Cu adatoms), yellow (S), gray (C), and white (H). The surface cells are indicated for panels a and d.

and 1.85 eV, respectively. Thus, the repulsive intercomplex interaction renders the cis configuration preferred at saturation coverage. The average S-Au(111) bond distance is only 0.01 Å shorter for the cis case as compared to that for the trans orientation. The S-Au-S angle in panel f is 169°, whereas it is 174° in the cis version. The average S-Au-S angle in the experimentally determined Au102 (p-MBA)44 is 172°.24 Note that the structure proposed by Cossaro et al.21 in panel d is higher in energy than the traditional thiolate model in panel a. The reason for the high energy is the strained configuration of the thiolates in the (RSAu)x polymer which yields a binding energy that does not compensate for the energetic penalty connected to the formation of the point defects. Selected structural data for panels a-f are collected in Table 2. The RS-Au bond in the unreconstructed surface is 2.50 Å, this is reduced to ∼2.44 Å, when RS is adsorbed at a vacancy. The S-Au bond length in a polymer is about 2.45 Å which is clearly longer than in an RS-Au-SR complex, where it is ∼2.33 Å. The complexes are bonded to Au(111) with a bond length of 2.53 Å. The distance between the adatoms in the complexes (which occupy bridge positions) and the surface atoms is about 2.97 Å. This is close to the Au-Au nearest neighbor distance in Au bulk (2.95 Å). For the investigated structures, the S-C bond distance shows small variations, with a moderate elongation with respect to the values in RSSR, namely 1.82 Å. This can be attributed to the fact that RS in all cases is bridge bonded. The average angle between the surface normal and the S-C axis is sensitive to the adsorption structure. Based on XSW experiments it has been suggested that MeS should be adsorbed atop an Au adatom, forming an MeSAu complex with an MeS-Au bond distance of 2.48 Å.68 Recent photoemission core level data (Au 4f7/2) has also been interpreted along such lines69 and the angle between the direction of the S-C bond and the surface normal has been measured in NIXSW experiments to be 61°.70 The MeSAu/Au(111) structure was investigated in ref 49 with a DFT method similar to the present one. The Au-S bond distance was calculated to be 2.30 Å and the angle between the S-C bond and the surface normal to be 72°. Thus, the calculated MeSAu/Au(111) structure is, besides being unstable with respect RS adsorption on the unreconstructed surface, not consistent with the structural data. Thiolate Adsorption on Cu(111). The considered structures for RS adsorption at saturation coverage (0.33) on Cu(111) are shown in Figure 2. Energetic and structural data are collected in Table 3. Panel a is the preferred structure on the unreconstructed surface; RS adsorbed in a bridge configuration. The

fcc position is found to be 0.2 eV higher in energy per RS molecule. Panel b is the stable configuration for Au(111), thus a structure based on RS-Cu-RS complexes. Panel c is a structure with one point defect and one RS-Cu-RS complex. Panel d corresponds to a structure relaxed from the experimentally proposed surface reconstruction.35 The initial configuration in panel d was a pseudo-(100) surfaces ontop a (111) facet, with RS adsorbed in 4-fold hollow positions. Upon relaxation, the (100) layer distorts in such a way that the copper atoms in the overlayer all occupy hollow positions. (Initially, some atoms are in bridge or even close to atop positions.) As mentioned in the Introduction, the proposed (100) surface has an extended lattice constant (14%) and the coverage is only 0.67 of the (111) substrate. Thus, it is not surprising to find marked relaxations from the ideal (100) registry. Despite large relaxations in the (100) overlayer, the thiolates preserve a square-like pattern. In the relaxed structure, three of the thiolates occupy 4-fold hollow positions, one bridge configuration and one 3-fold position. Energetically, panel d is calculated to be preferred with respect to the other investigated models. The models with add-atoms [panels b and c] are found to be unstable with respect to bridge adsorption on the terrace. Thus, we may conclude that RS drives surface reconstruction also on Cu(111), albeit distinctly different than on Au(111). The structure in panel d has a low order, and it is likely that other structures with similar motifs are energetically competitive. In this study, no attempt has been made to explore other structures. The important result is that the pseudo(100) surface is energetically preferred over adsorption on unreconstructed Cu(111) as well as conformations relevant for Au(111). The molecular configuration of MeS on Cu(111) has been experimentally investigated also for the unreconstructed surface.36 Adsorption on the Cu(111) facet represents a metastable situation that is experimentally accessible at low temperatures. On unreconstructed Cu(111), photoelectron diffraction measurements have determined the Cu-S distance to be 2.27 ( 0.03 Å, the height of the S atom over the (111) surface to be 1.87 ( 0.03 Å and the angle between the S-C bond and the surface normal to be 45 ( 12°. The data is in good agreement with the calculated values for (a), see Table 3. For the reconstructed surface, SEXAFS measurements indicate a S-Cu top layer spacing of 1.2 ( 0.1 Å. This result is in good agreement with the results obtained for panel d. The S-C bond is slightly longer than in the case of MeS adsorption on Au(111) which is consistent with a stronger RS-metal bond. Difference between MeS Adsorption on Au(111) and Cu(111). RS induces marked reconstruction of both Cu(111) and Au(111). In similarity with surface oxides, a new phase is formed on Au(111); the surface is covered RS-Au-RS units

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TABLE 4: Molecular Properties of MeSM and (MeSM)4a MeSCu (MeSCu)4 MeSAu (MeSAu)4

Eb

dM-S

dS-C

2.82 4.99 2.52 4.53

2.08 2.16 2.24 2.33

1.83 1.85 1.82 1.84

TABLE 5: Bulk and Surface Properties of Cu and Aua

RS-M-S 173 177

Cu Au

a

Ec

σ111

Ev (eV)

3.63 4.17

3.56 3.04

80 42

0.84 0.62

a a (Å) is lattice constant, Ec (eV) is cohesive energy, σ111 (eV/ Å2) is surface energy, and Ev (eV) vacancy formation energy.

a Eb is binding energy in eV. dM-S (dS-C) is the average M-S (S-C) bond distance in Å. RS-M-S is the S-M-S angle (°).

Figure 3. Optimized structures for metal-thiolate complexes. Atomic color codes as in Figures 1 and 2.

instead of RS radicals. On Cu(111), the reconstruction could be described as a pronounced surface relaxation. To investigate the difference in bond strength for RS adsorption on Cu and Au, it is convenient to compare the unreconstructed case. At 0.33 coverage, MeS is bonded by 2.18 eV to Cu(111). (Taking here MeS in the gas-phase as reference.) The corresponding value on Au(111) is 1.65 eV. Thus, RS is more strongly anchored to Cu than to Au. This effect is slightly enhanced on the (100) terrace. The binding energy of MeS to Cu(100) in a hollow position at 0.25 coverage is calculated to be 2.77 eV, whereas RS prefers a bridge site on Au(100) with an adsorption energy of 2.14 eV. The more open (100) facet enhances the RS bonding on both metals. The trend in bond strength obtained for the surface holds also for (RSM)x complexes. It is well-known that metal(I)-thiolate complexes adopt zigzag structures forming rings or strands.54,55 Here, the gas phase MeSM and (MeSM)4 have been explored. Energetic and structural data are collected in Table 4 and Figure 3. The binding energies per MeSM unit are evaluated with respect to separated metal atoms and MeS radicals. In line with the results for the surface, MeS is more strongly bonded to Cu than to Au. The difference in bond strength is manifested also in the nature of the S-M bond. The S-Cu bond is more ionic than is the S-Au bond. A Bader analysis71,72 shows that the average charge on the Cu and Au is 10.6 and 10.9, respectively, for (MeSM)4. The charge depleted from the metal atoms are calculated to be transferred to the S-atoms. The complexes provide, furthermore, an additional possibility to evaluate the performance of the theoretical method. The structure of Cu4 (SSiPh3)4 has been solved73 using X-ray crystallography and it adopts a square structure with Cu in close to linear coordination and S atoms in the corners. The mean Cu-S distance and S-Cu-S angle were reported to be 2.16 Å and 170°, respectively. A similar structure has been measured74 for Au4 [SC(SiMe3)]4 with a mean Au-S distance of 2.30 Å and an S-Au-S angle of 177°. The comparison with the experimental data, shows that the calculations predict the correct structural trend between RS adsorption on Cu and Au. The applied theoretical approach yields S-Cu bonds in close agreement with experiments, whereas the S-Au bonds are

slightly overestimated. This trend is in agreement with the results for MeS adsorption on M(111); the results for Cu are very close to the experimental results, whereas slightly elongated bond lengths are calculated for Au. Even if the MeS-metal bond strength is higher for Cu than for Au, the reconstruction is less pronounced. This indicates that the reason for the difference in structural motifs on Cu(111) and Au(111) is partly related to properties of the bare metals. Calculated key quantities are collected in Table 5. There is large differences in energetic properties between Cu and Au; cohesive energy, surface energy and vacancy formation energy are higher for Cu. It should be noted that the theoretical description is poorer for Au than it is for Cu. The experimental values for the lattice constant, cohesive energy, and surface energy are 3.61 (Cu) and 4.08 (Au) Å; 3.5 (Cu) and 3.8 (Au) eV; and 114 (Cu) and75 94 (Au)76 eV/Å2, respectively. The surface energy and vacancy formation energy are properties that influence the energetic penalty in the formation of gold-thiolate complexes. The formation of adatom structures is not as costly for Au as is the case for Cu and represents one reason for the difference in reconstruction patterns. Another reason is probably the difference in bond character for RS on Cu and Au. The RS-Cu bond is slightly ionic, whereas RS-Au is merely covalent. The more ionic RS-Cu bond favor higher coordination. According to a Bader charge analysis, RS is charged by 0.05 electrons in the RS-Au-RS complex on Au(111), whereas the corresponding charge is 0.43 electrons for RS adsorbed in a 4-fold hollow position on Cu(100). The preference of an ionic configuration is furthermore stabilized by the smaller lattice of Cu. Conclusions Over the past few years it has become clear that RS adsorption drives pronounced reconstruction of coinage metal surfaces. In the present work, the density functional theory has been used to compare the reconstruction of Au(111) and Cu(111). On Au(111), the stable structure comprises RS-Au-RS complexes. This is a structure that resembles the motifs determined for thiolate protected Au nanoparticles. In fact, the protecting ligands in these structures are the gold complexes rather than the RS radicals. The experimentally suggested pseudo-(100) phase with RS adsorbed in 4-fold hollow positions is found to be preferred for Cu(111). The difference in structural motifs between Au(111) and Cu(111) is traced to the larger degree of covalency in the case of Au, together with a low energetic penalty for adatom creation. Acknowledgment. Hannu Ha¨kkinen and Robert L. Whetten are acknowledged for many fruitful discussions. The calculations have been performed at NSC (Linko¨ping) and C3SE (Go¨teborg). Support from the Swedish Research Council is acknowledged. References and Notes (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1.

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