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Decanethiols on Gold: The Structure of Self-Assembled Monolayers Unraveled with Computer Simulations Dominik Fischer, Alessandro Curioni, and Wanda Andreoni* IBM Research, Zurich Research Laboratory, 8803 Ru¨ schlikon, Switzerland Received January 6, 2003. In Final Form: February 24, 2003 Self-assembled monolayers generated by alkanethiols on gold substrates are prototypical systems in today’s nanotechnology, but their structure still constitutes an unresolved puzzle. An important step forward is made here by focusing on the long-debated case of decanethiols and using unprecedented largescale simulations that accurately account for the interplay between the chemistry at the interface and the interchain interactions. It is established that thiolates are thermodynamically favored relative to sulfurpaired chains and that the energy difference decreases linearly with chain length. The nature of the chemical bonding at the interface is understood for both adsorbates. It is also shown that experimental data always reveal the presence of thiolates but that disulfides may coexist in some cases. These results open a new perspective for the molecular-scale characterization of complex self-assembled monolayers on metal supports and for simulations-aided nanotechnology.
Introduction Almost 20 years have elapsed since the discovery1 that organosulfur chain compounds can assemble spontaneously in an organized manner on gold surfaces. By now self-assembled monolayers (SAMs),2,3 generated from either alkanethiols (RSH) or disulfides (RSSR) (R ) CnH2n+1), have become a standard practical tool for the functionalization of metal surfaces (e.g., gold, silver, copper, platinum) and for the fabrication of novel nanostructured materials. Various current technological applications depend on the intelligent use and manipulation of SAMs, ranging from microcontact printing to corrosion inhibition, from the synthesis of organic semiconductors to that of metal quantum dots. Yet the science underlying the self-assembling process is poorly understood. Here the debate about the structure of the chains/metal interface, which has filled the scientific literature for the entire past decade, is particularly outstanding. Indeed, despite intense experimental investigation, it is still unclear whether at full coverage the long organic-sulfur chains (n ) 8-18) are adsorbed on gold in the form of thiolates, having either one or four chains in the periodically repeated cell, or in that of disulfides. Moreover, computer simulations have not solved this structural puzzle. So far, only classical molecular dynamics and molecular mechanics calculations could be performed; see, e.g., refs 4-6. These relied on the use of empirical potentials to represent all interatomic interactions, namely, the metal-metal, metal-sulfur, and chain-chain interactions. Classical potentials are indeed well established only for the organic building blocks of the assembly. Otherwise, in view of the strong interplay between the reorganization of the electronic structure and the structural relaxation induced by adsorption, classical interatomic potentials fail to provide the level of accuracy that is necessary to render calculations predictive. The situ* To whom correspondence may be addressed: phone, +41-17248344; e-mail,
[email protected]. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-257. (4) Mar, W.; Klein, M. L. Langmuir 1993, 10, 188-196. (5) Gerdy, J. J.; Goddard, W. A., III J. Am. Chem. Soc. 1996, 118, 3233-3236. (6) Bhatia, R.; Garrison, B. J. Langmuir 1997, 13, 4038-4043.
ation calls for a correct description of the adsorption process, for which a quantitative and thus parameterfree approach to the chemistry of the interface is crucial. Here we report on computer simulations that are indeed able to provide the desired quantitative description, for the interfacial configurations as well as for the energetics of the model structures that have emerged from experiment so far. Thus they are capable of solving the structural puzzle. We focus on the prototype system, i.e., on SAMs of decanethiols adsorbed on Au(111), which is the subject of the long-standing worldwide debate, and also determine the influence of the chain length on the relative stability of the various configurations. Our calculations are based on an accurate approach that combines the quantummechanical description of the metal surface and its interaction with the chain headgroups with an empirical description of the weak interchain interactions. For the former, we have adopted a density functional theory (DFT)7 with gradient-corrected exchange-correlation functionals.8 This choice relies on a previous study by some of us9,10 of the adsorption of methanethiol on gold at low coverage: This established the correct modeling of the gold surface and critically evaluated the validity of several DFT exchange-correlation functionals for the description of the headgroup/metal interaction. Indeed methanethiol at low coverage is the only case that can be studied accurately within DFT present schemes, because of the failure of any current approximation to the DFT exchange-correlation functionals to describe the tail of dispersion forces. Given that their contribution to the energy scales with the length of the chains and with the packing density, the error in the approximations becomes increasingly less controllable. Therefore, in the present study of the monolayer regime, we have used well-established classical potentials11,12 for the expression of intra- and interchain (7) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (8) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868. (9) Gro¨nbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839-3842. (10) Andreoni, W.; Curioni, A.; Gro¨nbeck, H. Int. J. Quantum Chem. 2001, 80, 598-608. (11) Karasawa, N.; Dasgupta, S.; Goddard, W. A., III J. Phys. Chem. 1991, 95, 2260-2272. (12) Majo, S. L.; Olafson, B. D.; Goddard, W. A., III J. Phys. Chem. 1990, 94, 8897-8909.
10.1021/la034013c CCC: $25.00 © 2003 American Chemical Society Published on Web 03/27/2003
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Figure 1. SAMs on gold: (A) thiolate and (B) disulfide models. In each case, the model illustrated here corresponds to the unit cell of the periodically repeated system used in our simulations: 16 C10H21S chains and 4 layers of gold atoms. This orthorhombic supercell contains 16 (x3 × x3)R30 and 4 c(4×2) unit cells.
interactions and the link-atom scheme13 for the coupling of the quantum and classical subsystems. In this way, one is also able to clarify the role of the diverse interactions in stabilizing one configuration relative to the other. Method Section For the quantum subsystem, we have applied the method established in refs 9 and 10: the PBE-DFT scheme,8 plane waves as basis set for the valence electron wave functions (cutoff 50 Ry) and norm-conserving angular-momentum pseudopotentials (with the core 5d atomic shell included in the valence space of gold). The surface model consisted of a slab of four layers, with the bottom layer kept in the structure of the bulk. We used the interatomic potential introduced in ref 11 for the interchain interactions and the DREIDING potential from ref 12 for the interaction of sulfur with the rest of the chains. For the coupling between the QM and the MM systems, we use the scaled position link atom method (SPLAM) introduced in ref 13. Hydrogen was taken as the link atom and constrained on the line joining the carbon atoms. Its distance from the QM carbon was scaled so as to simulate the effect of the stretching force along the bond between QM and MM carbon. This accounts for the coupling between the MM subsystem and the updating of the electronic structure of the QM part at each step of the geometry optimization. Our implementation of the QM/MM method uses the Car-Parrinello molecular dynamics (CPMD) code14 for the QM subsystem. All atomic coordinates were optimized within the full QM/ MM scheme. To obtain unbiased results, for each of the three models proposed, several optimization runs were performed using different initial geometries for the headgroups. The final steps of the geometry optimization were obtained with an efficient linear scaling method newly introduced15 and implemented in the CPMD code. (13) Eichinger, M.; Tavan, P.; Hutter, J.; Parrinello, M. J. Chem. Phys. 1999, 110, 10452-10467. (14) CPMD, copyright 1990, 1997, and 2000 by IBM Corporation and 1997 by Max-Planck-Institut fu¨r Festko¨rperforschung, Stuttgart, Germany. See http://www.cpmd.org. (15) Billeter, S.; Curioni, A.; Andreoni, W. Comput. Mater. Sci., in press.
Results and Discussion For a long time, evidence has been brought forth in support of either the thiolate (or “standard”) or the disulfide (or “sulfur pairing”) models (see Figure 1). In particular, the former has been supported by a range of spectroscopies and for different chain lengths, e.g., X-ray photoemission spectroscopy (XPS),16,17 also at high-resolution,18 infrared absorption,17,19 low-energy electron diffraction (LEED),19 transmission electron diffraction,20,21 helium diffraction,22 scanning-tunnelingmicroscopy (STM),23,24 and high-resolution electron energy loss spectroscopy (HREELS).25 The periodicity of the crystalline structure has been identified either as (x3 × x3)R30 with one chain per unit cell20,21 or as its c(4×2) superstructure with four chains per unit cell.22-24 (We shall refer to these two models as single-chain and four-chain thiolate, respectively.) The validity of the thiolate picture was first questioned by the results of a grazing incidence X-ray diffraction study26 and replaced by a picture in which the molecular units of the adsorbate are disulfides distributed on a c(4×2) superlattice. Data from the X-ray standing waves technique27 and HREELS28 have later offered further support of this view. Still, more recent X-ray photoemission spectra29 show that the disulfide component may only be the spurious result of radiation-induced effects and accompany surface degra(16) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (17) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (18) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058-4061. (19) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678-688. (20) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-558 (21) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-692. (22) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503-3511. (23) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (24) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, Ch. Adv. Mater. 1996, 8, 719-729. (25) Kato, H. S.; Noh, J.; Hara, M.; Kawai, M. J. Phys. Chem. B 2002, 106, 9655-9658. (26) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218.
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Figure 2. Top views of the three models: A, single-chain thiolate; A′, four-chain thiolate; B, disulfide. In the upper row: the sulfur-gold interface is shown, with two metal layers. Note the positions of the sulfur atoms: in A, they occupy all equivalent slightly off-centered bridge positions (0.57 Å toward the 3-fold hollow site); in A′ two nonequivalent sites, of which one is an almost ideal bridge position and the other is off-centered (∼0.4 Å toward the 3-fold hollow site); in B, there are four nonequivalent sulfur positions, of which two are 0.3-0.5 Å away from the ontop site, whereas the other two are at larger distances (3.0-3.1 Å) from the closest surface atoms. The vertical distances are 2.84 ( 0.04 and 3.03 ( 0.03 Å, to be compared with the refinement in ref 27: 2.21 ( 0.05 and 2.97 ( 0.05 Å. The S-S distance is 2.1 Å. The structure of the metal surface changes in a different way for the three configurations of the adsorbate: the root-mean-square displacements for the first, second, and third layer are, respectively, 0.19, 0.12, and 0.07 Å in the single-chain thiolate; 0.29, 0.18, and 0.09 Å in the four-chain model; and 0.12, 0.09, and 0.04 Å in the disulfide. In the lower row, the chains are shown with an orientation selected so as to clarify the different types of crystal packing.
dation. Recent measurements of conductivity through an octanedithiol molecule inserted into an octanethiol monolayer yield the same intensity over thousands of molecules, which tends to contradict the idea of having two chemically different sulfur species in the system.30 Within our hybrid quantum-classical scheme, we optimized the structure of the full periodically repeated system for both single- and four-chain decanethiolates (Figure 2A,A′) and for the corresponding disulfides (Figure 2B) in the same orthorhombic supercell. Interestingly, our results predict that the two thiolate models are energetically degenerate within ∼1 kcal/mol, namely, within the accuracy of the calculations. This is in agreement with the fact that the two periodicities are observed to coexist.31 By contrast, the sulfur-pairing model is clearly disfavored over the thiolates (Figure 3). The energy difference of 8 kcal/mol is indeed significant within the (27) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 412/413, 213-235. (28) Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. Rev. B 1999, 59, R10449-R10452. (29) Zerulla, D.; Chasse, T. Langmuir 1999, 15, 5285-5294. (30) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571-574. (31) Pflaum, J.; Bracco, G.; Schreiber, F.; Colorado, R., Jr.; Shmakova, O. E.; Lee, T. R.; Scoles, G.; Kahn, A. Surf. Sci. 2001, 498, 89-104.
accuracy of the calculations and when compared to the thermal energy at room temperature (∼0.6 kcal/mol). Given the high degree of order and the structural similarity of the two systems, entropy effects are not expected to alter this result. The validity of this reasoning has been checked by estimating the vibrational entropy contribution to the free energy, which differs by less than 1 kcal/mol in the two systems. It is possible to approximately decompose the energy into contributions due to interactions treated quantum mechanically (QM) and those due to classically treated (MM) ones, as done in Figure 3. Both the chemical bonding with the metal substrate ands although to lesser extentsthe chain packing favor the thiolate forms for the adsorbates. The same analysis, on the other hand, shows the essential degeneracy of both energy contributions for the two thiolate models. The basis for the understanding of these results is provided by the structural analysis of the three models as they result from our optimization (Figure 2). Indeed, the two configurations of thiolate adsorbates differ in a subtle way. The ordering in the c(4×2) superstructure clearly implies a different organization of the chains relative to each other (see Figure 2), which however does not seem to affect the binding energy. We observe that, on passing from the single- to the four-chain structure, the chains
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Figure 3. Energy difference (∆E) between the disulfide and the thiolate model (A) as a function of the chain length. For n ) 10 and n ) 4, we report the results (squares) of the full quantum-classical calculations described here and of the decomposition ∆E ) ∆E(QM) + ∆E(MM), where MM includes the coupling of the two subsystems. For n ) 6 and n ) 8, we report an estimate (diamond) derived from the calculations of ∆E(MM), assuming substrate structures identical to those of the n ) 10 systems and corrected for geometry relaxation by using the relaxation energy difference calculated explicitly for n ) 4 (
