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2007, 111, 3325-3327 Published on Web 03/10/2007
Polymerization at the Alkylthiolate-Au(111) Interface Henrik Gro1 nbeck*,† and Hannu Ha1 kkinen‡ Department of Applied Physics and Competence Centre for Catalysis, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and Departments of Physics and Chemistry, Nanoscience Center, UniVersity of JyVa¨skyla¨, FI-40014 JyVa¨skyla¨, Finland ReceiVed: January 2, 2007; In Final Form: February 22, 2007
The density functional theory is used to explore alkylthiolates (RS) bonded to Au(111) and gold adatoms on Au(111). Adsorption of RS to adatoms in a structure characterized as an (RSAu)x polymer is strongly preferred over terrace adsorption. The energetic gain upon polymerization is large enough to drive the required Au(111) reconstruction. The results are discussed in relation to thiolate protected gold nanoparticles and homoleptic gold-thiolate complexes.
Alkylthiolates on Au(111) is a prototype system of organic self-assembled monolayers (SAMS) on metals.1,2 Their potential applications include nanofabrication, corrosion prevention, and biocompatible materials.3 SAMs are formed from thiols (RSH, where R is an alkyl chain) or disulfides (RSSR) adsorbed on the metal surface. After dissociation, the adsorbates are generally assumed to be bonded as thiolates (RS). Despite considerable efforts, however, the interfacial RS-Au structure is still an open issue.3 This is due to both the apparent flexibility of the systems, with several observed surface structures at submonolayer coverage,3 and the buried interface at saturation. It was early suggested that RS should occupy threefold hollow positions, forming a (x3 × x3R30°) overlayer at full coverage.4 Such a structure was later confirmed,5 and moreover, a c(4 × 2) superstructure was revealed.6 The thiolate model was challenged by X-ray standing wave (XSW) experiments suggesting RS-SR pairing.7 More recently, scanned-energy and scanned-angle photodiffraction (PD)8 and XSW9 measurements have been interpreted with a structure where RS is bonded atop a terrace atom. To this point, all structural models assumed an unreconstructed Au(111) surface. Very recently, however, a new kind of interface was proposed.10 On the basis of XSW measurements, it was suggested that thiolates are anchored to the surface via atop adsorption on Au adatoms.10 Moreover, RS-Au-SR complexes on Au(111) have been observed with scanning tunneling microscopy (STM) at lower coverage.11 The long-standing experimental uncertainty concerning the RS bonding to Au(111) in SAMs has stimulated several theoretical density functional theory (DFT) studies.12-17 These reports predict that RSSR dissociate12 and that RS is adsorbed in a bridge-fcc site (bridge with S slightly bent over fcc).13-17 In addition, it has been shown that the RS-Au bond can be enhanced by adsorption on defects18,19 and adatoms.20 The recent experimental observations of gold-thiolate complexes on the Au(111) surface10,11 have made it imperative to * Corresponding author. E-mail:
[email protected]. † Chalmers University of Technology. ‡ University of Jyva ¨ skyla¨.
10.1021/jp0700128 CCC: $37.00
establish the potential energy surface for such systems. In this Letter, density functional theory (DFT) calculations are used to explore the structure of alkylthiolates on Au(111) and adatoms on Au(111). Key issues are the characterization of stable structures and whether RS bonding to adatoms is favorable enough to drive the required surface reconstruction. DFT is employed in the implementation with plane waves and pseudopotentials.21 In particular, the spin polarized PerdewBurke-Ernzerhof (PBE) approximation to the exchange and correlation (xc) functional22 and ultrasoft scalar-relativistic pseudopotentials are used to describe the interaction between the valence electrons and the core.23 A plane-wave kinetic energy cutoff of 25 Ry is used to expand the Kohn-Sham orbitals. As we focus on the RS-Au interaction, methyl (CH3) is chosen as R. However, for the stable polymeric structure, also hexylthiolate (C6H13S) was considered. The calculated lattice constant for bulk Au is 4.20 Å. The 3% expansion with respect to the experimental value can be attributed to the applied xc functional. Adsorption was investigated in a 3 × x3 unit cell of the Au(111) surface. The surface was represented by four atomic layers. Except the bottommost Au layer, all atoms in the cell were relaxed. To study saturation coverage, two RS units are considered in each cell. Reciprocal space integration over the Brillouin zone was approximated with finite sampling of eight special k-points.24,25 Gas-phase species were calculated in large cells assuming the lowest possible spin states: doublet for RS and Au and singlet for RSAu. As a reference, adsorption of methylthiolate (MeS) was reinvestigated on the Au(111) terrace. The results are collected in Table 1, and the structures are shown in the Supporting Information. Here, only atop and bridge-fcc sites are reported. In agreement with previous reports,13-17 bridge-fcc represents the stable configuration. In particular, this site is preferred over the atop site by 0.34 eV per MeS radical. Structural models for MeS adsorption on Au adatoms are reported in Figure 1. Here, the situation is considered with two adatoms and two MeS. Key adsorption properties are collected in Table 1. Adsorption atop the adatoms (left panel in Figure 1) is found to enhance the © 2007 American Chemical Society
3326 J. Phys. Chem. B, Vol. 111, No. 13, 2007
Letters
TABLE 1: Adsorption Properties of MeSa MeS MeS MeSAu (MeSAu)x
site
Ea
dAu-S
R
atop bridge-fcc atop bridge
1.29 1.63 2.05 2.43
2.38 2.49 2.30 2.44
64 56 72 46
a Ea is the adsorption energy in electronvolts, dAu-S is the average Au-S bond distance in angstroms, and R is the average angle (deg) between the surface normal and the S-C axis.
Figure 2. Relative energies with respect to (a) in electronvolts.
Figure 1. Optimized structures for MeSAu on Au(111) (left) and (MeSAu)x (right). The surface cell is indicated. Color code: orange (Au), yellow (S), gray (C), and white (H).
MeS-Au bond strength by 0.4 eV with respect to the terrace case. The MeSAu units are bonded to the surface in an fcc site which is preferred over the hcp site by 0.04 eV. The site preference is the same as that for the bare adatom. The relative orientation of the S-C axis of the two thiolates is found to have a minor effect on the stability. In fact, the geometry with the axis oriented in a parallel fashion is energetically degenerated with the orientation in Figure 1. The enhanced stability as compared to the terrace is reflected in the Au-S bond length. This distance is 0.08 Å shorter than the corresponding distance for atop adsorption on the terrace. Allowing for MeS bridge adsorption, the structure on the right in Figure 1 is obtained. Given the structure of the surface cell, this geometry is best described as a polymer, (MeSAu)x. In this geometry, one Au atom is bonded to the surface in an fcc position, whereas the other occupies an hcp position. Interestingly, this configuration stabilizes the (2Au + 2MeS)/Au(111) system by as much as 0.75 eV. The Au-S distance is in this case 2.44 Å, thus slightly shorter than the corresponding distance for bridge-fcc bonded MeS on the terrace. In similarity with the MeSAu case, the relative orientation of the two S-C axes has minor effects on the stability. To unravel the reason for the enhanced stability of the (MeSAu)x system, we note that the total energy of the system is a balance between the MeS-Au bond strength in the complexes and the interaction of the complexes with the surface. The MeSAu monomers are bonded to the terrace by 1.80 eV, whereas the corresponding interaction for (MeSAu)x is 0.66 eV per MeSAu unit. The difference in bond strength is reflected in the distance between the Au atoms in the complexes and the surface. The bond length is 2.80 Å for MeSAu and 2.91 Å for (MeSAu)x. To get a measure of the MeS-Au bond strength in the surface complexes, we evaluate the gas-phase processes: MeSAu f MeS + Au and (MeSAu)x f xMeS + xAu. These processes are endothermic by 2.52 and 4.03 eV (per MeSAu unit), respectively. Thus, the energetic penalty in the interaction between the Au-thiolate complex and Au(111) upon formation
of a (MeSAu)x polymer is compensated by a substantial gain in the MeS-Au interaction. Our results indicate that MeS-Au polymerization at the interface strongly enhances the binding energy. The importance of this structure depends on whether it may drive the required surface reconstruction (creating adatoms). To investigate this, we use Au bulk as an atom reservoir and construct the potential energy surface diagram in Figure 2. The starting point (a) is a perfect terrace (in our case, five surface layers) with two MeS adsorbed in the bridge-fcc configuration. An energy penalty is required to form two (separated) adatoms (b). If the MeS adsorption site is shifted atop the low-coordinated adatoms, the system is stabilized (c). Allowing for polymerization (d) makes the overall reaction exothermic by 0.28 eV. Hence, MeS induced reconstruction of Au(111) to form polymeric (MeSAu)x structures should be feasible. In ref 11, a MeS-Au-SMe complex was observed on Au(111) by STM and investigated by DFT calculations. For completeness, the complex was also considered here. The relative stability is included in Figure 2e. Displacing one adatom to the Au bulk slightly destabilizes the system with respect to the polymeric structure. The small energy difference suggests coexistence for MeS, whereas chain-chain interactions for longer R should stabilize the (MeSAu)x structure. On the basis of XSW experiments, it was recently suggested that MeS is adsorbed atop adatoms.10 In ref 10, the MeS-Au distance was determined to be 2.48 Å, which is considerably longer than what is found here for the corresponding case (2.38 Å). The chosen xc functional in this work is expected to provide a reasonable description of the Au-S bond length. We note that, in the crystal structure of (AuSC(SiMe3))4, the mean Au-S distance is measured to be 2.30 Å.26 For the gas-phase analog (MeSAu)4, we calculate 2.34 Å. We also note that the PD experiments in ref 8 indicated an angle of 50° between the Au(111) surface normal and the S-C bond, which agrees well with the one (46°) found here for the (MeSAu)x model. The proposed (RSAu)x structure is an attractive model also for longer carbon chains. We have considered the case with hexylthiolates. The adsorption energy of these ligands is sizeable, 2.31 eV per RS. DFT in the form used here does not include van der Waals interaction. A proper description of the chain-chain interaction would probably lead to further stabilization. The observation that (MeSAu)x complexes are relevant in the case of MeS adsorption on Au(111) is important for the general understanding of thiolate-gold interactions. Several experimental investigations of homoleptic complexes have shown that RS preferably adsorb in bridge configurations, forming zigzag rings or strands.27 Within DFT, we recently demonstrated the polymeric nature of cyclic (MeSAu)x structures with x g 4.28 (In fact, this property of gold-thiolate complexes indicates the possibility of several energetically near degenerate RS-Au(111) configurations based on such motifs.29) Moreover, for gold
Letters nanoparticles, a “divide and protect” scenario has been proposed, where (MeSAu)4 units were suggested to protect a metal core.30 Augmented by the results for Au(111) reported here, it appears that common RS-Au bonding motifs apply for Au systems of different size. Acknowledgment. The calculations were performed at PDC (Stockholm) and C3SE (Go¨teborg). Support from the Swedish Research Council (H.G.) and the Academy of Finland (H.H.) is acknowledged. Supporting Information Available: Structural models for all systems discussed. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Ulman, A. Chem. ReV. 1996, 96, 1533. (3) Vericat, C.; Vela, M. E.; Salvareza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (4) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (5) Chidsey, C. E. D.; Liu, G. Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (6) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1992, 98, 3503. (7) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 412/413, 213. (8) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokoyama, T.; Ohta, T.; Shimomura, M.; Kono, S. Phys. ReV. Lett. 2003, 90, 066102. (9) Roper, M. G.; Skegg, M. P.; Fisher, C. J.; Lee, J. J.; Dhanak, V. R.; Woodruff, D. P.; Jones, R. G. Chem. Phys. Lett. 2004, 389, 87. (10) Yu, M.; Bovet, N.; Satterley, C. J.; Bengio, S.; Lovelock, K. R. J.; Kevin, R. J.; Milligan, P. K.; Jones, R. G.; Woodruff, D. P.; Dhanak, V. Phys. ReV. Lett. 2006, 97, 166102.
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3327 (11) Maksymovych, P.; Sorescu, D. C.; Yates, J. T. Phys. ReV. Lett. 2006, 97, 146103. (12) Gro¨nbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839. (13) Akinaga, Y.; Nakajima, T.; Hirao, K. J. Chem. Phys. 2001, 114, 8555. (14) Vargas, M. C.; Giannozzi, P.; Selloni, A.; Scoles, G. J. Phys. Chem. B 2001, 105, 9509. (15) Yourdshahyan, Y.; Rappe, A. M. J. Chem. Phys. 2002, 117, 825. (16) Gottschalck, J.; Hammer, B. J. Chem. Phys. 2002, 116, 784. (17) Morikawa, Y.; Hayashi, T.; Liew, C. C.; Nozoye, H. Surf. Sci. 2002, 507, 46. (18) Molina, L.; Hammer, B. Chem. Phys. Lett. 2002, 360, 264. (19) Zhou, J. G.; Hagelberg, F. Phys. ReV. Lett. 2006, 97, 045505. (20) Cometto, F. P.; Paredes-Olivera, P.; Macagno, V. A.; Patrito, E. M. J. Phys. Chem. B 2005, 109, 21737. (21) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. ReV. Mod. Phys. 1992, 64, 1045. (22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (23) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (24) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (25) Pack, J. D.; Monkhorst, H. J. Phys. ReV. B 1977, 16, 1748. (26) Bonasia, P. J.; Gindelberger, D. E.; Arnold, J. Inorg. Chem. 1993, 32, 5126. (27) Dance, I. G. Polyhedron 1986, 5, 1037. (28) Gro¨nbeck, H.; Walter, M.; Ha¨kkinen, H. J. Am. Chem. Soc. 2006, 128, 10268. (29) Explicit calculations for a thiolate-surface interface based on (MeSAu)4 support this assumption. The difference in adsorption energy per MeS radical in a tetramer configuration and the polymer in Figure 1 is within 0.03 eV. These calculations were performed in a 3 × 2x3 structure with three special k-points. (30) Ha¨kkinen, H.; Walter, M.; Gro¨nbeck, H. J. Phys. Chem. B 2006, 110, 9927.
