pubs.acs.org/Langmuir © 2009 American Chemical Society
Description of Ferrocenylalkylthiol SAMs on Gold by Molecular Dynamics Simulations F. Goujon,† C. Bonal,*,† B. Limoges,‡ and P. Malfreyt† †
Laboratoire de Thermodynamique et Interactions Mol eculaires, FRE 3099 CNRS, Universit e Blaise Pascal, 63177 Aubi ere Cedex, France, and ‡Laboratoire d’Electrochimie Mol eculaire, UMR CNRS 7591, Universit e Paris Diderot, 15 rue Jean-Antoine de Baıf, ¨ 75205 Paris Cedex 13, France Received February 27, 2009. Revised Manuscript Received April 23, 2009
Molecular dynamics simulations of mixed monolayers consisting of Fc(CH2)12S-/C10S-Au SAMs are carried out to calculate structural (density profiles, angular distributions, positions of atoms) and energetic properties. The purpose of this paper is to explore the possible inhomogeneity of the neutral ferrocene moieties within the monolayer. Five systems have been studied using different grafting densities for the ferrocenylalkylthiolates. The angular distributions are described in terms of the relative contributions from isolated and clustered ferrocene moieties in the binary SAMs. It is shown that the energetic contributions strongly depend on the state of the ferrocene. The ability of molecular dynamics simulations to enable better understanding the SAM structure is illustrated in this work.
Introduction On account of the broad range of applications of selfassembled monolayers of alkanethiolates on gold, there has been continuing interest in a comprehensive study of their structure and properties.1-4 Among the most promising technologies developed with these monolayers, there are sensors for analytes of chemical and/or biological importance5-8 and molecular electronic devices.9-11 SAMs also offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. Due to stable Au-S chemisorption, functionalized self-assembled alkylthiolate has been widely used to anchor electroactive species onto electrodes, providing a model system for studying interfacial electron transfer processes and interactions between immobilized functional moieties.3,12-18 For most of applications, SAMs are constituted of two or more different molecules.19,20 Although several methods of making *E-mail:
[email protected]. (1) Ulman, A. An Introduction of Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly Academic; Academic Press: San Diego, 1991. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Lee, L. Y. S.; Lennox, R. B. Phys. Chem. Chem. Phys. 2007, 9, 1013. (5) Izumi, R.; Hayama, K.; Hayashi, K.; Toko, K. Chem. Sens. 2004, 20, 50. (6) Kishimoto, M.; Ikenaga, Y.; Siigi, H.; Nagaoka, T. Chem. Sens. 2002, 18, 31. (7) Kim, D.-S.; Park, H.-J.; Park, J.-E.; Shin, J.-K.; Kang, S.-W.; Seo, H.-I.; Lim, G. Sens. Mater. 2005, 17, 259. (8) Campuzano, S.; Pedrero, M.; Pingarron, J. Talanta 2005, 66, 1310. (9) Kitagawa, K.; Morita, T.; Kimura, S. Langmuir 2005, 21, 10624. (10) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (11) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (12) Chidsey, C. E. D. Science 1991, 251, 919. (13) Chidsey, C. E. D.; Bertozzi, C. R.; Putwinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (14) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (15) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (16) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991, 246, 233. (17) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186. (18) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203. (19) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383. (20) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Caroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (21) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (22) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662.
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these binary or tertiary SAMs have been reported,21-25 some uncertainties persist in terms of their preparation and stability. It appears that the preparation of binary SAMs is highly dependent on specific experimental parameters. Thus, the limited understanding of what is important in SAM formation leads to control problems in terms of both coverage and homogeneity. Electrochemistry provides a straightforward way to determine the number of electroactive centers adsorbed on a gold electrode and thus allows an indirect evaluation of the layer composition. Among electrochemical techniques, cyclic voltammetry is often employed to characterize electroactive self-assembled monolayers. Electroactive monolayers comprising ferrocene-terminated thiols are probably the most widely employed and the best characterized of all mixed self-assembled systems to date.12,13,16,26-28 This system presents a quasi ideal electrochemistry signature when the surface coverage of ferrocenylalkylthiolates is low and diluted by nonelectroactive alkanethiolates. However, at relatively high surface coverage of ferrocenylalkylthiolates, cyclic voltammetric studies have reported nonideal electrochemical behavior,13,14,17,29-39 where two oxidation (23) Ma, F.; Lennox, R. B. Langmuir 2000, 16, 6188. (24) Shon, Y. S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (25) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502. (26) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141. (27) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286. (28) Sumner, J. J.; Weber, K. S.; Hockett, L. A.; Creager, S. E. J. Phys. Chem. B 2000, 104, 7449. (29) Chambers, R. C.; Inman, C. E.; Hutchison, J. E. Langmuir 2005, 21, 4615. (30) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (31) Voicu, R.; Ellis, T. H.; Ju, H.; Leech, D. Langmuir 1999, 15, 8170. (32) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203. (33) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (34) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E. J.; Hussey, C. L. Langmuir 1998, 14, 124. (35) Yao, X.; Wang, J.; Zhou, F.; Wang, J.; Tao, N. J. Phys. Chem.B 2004, 108, 7206. (36) Quist, F.; Tabard-Cossa, V.; Badia, A. J. Phys. Chem.B 2003, 107, 10691. (37) Seo, K.; Jeon, I. C.; Yoo, D. J. Langmuir 2004, 20, 4147. (38) Kawaguchi, T.; Tada, K.; Shimazu, K. J. Electroanal. Chem. 2003, 543, 41. (39) Auletta, T.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1996, 18, 1288.
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peaks whose shapes, positions, and relative sizes are dependent on ferrocene mole fraction in solution were frequently observed.17,29,31,37-39 Several assumptions were proposed to explain these phenomena such as structural disorder in the SAM due to steric crowding of Fc groups,13,15,30 double-layer effects,15,17,18,40,41 different structural orders of the monolayer,14 and differences due to the Au polycrystalline structure.42 In fact, it is generally assumed that the kinetics and thermodynamics of electron transfer of each ferrocene center are identical, but this is probably rarely so. Recently, Lee et al.43 studied the oft-cited complexity of tethered ferrocene electrochemistry by voltammetry experiments. They obtained cyclic voltammograms from the SAMs formed from different Fc(CH2)12S-/C10S solutions where the Fc mole fraction in solution ranges from 0.1 to 1.0. When the mole fraction of Fc is small, they observed an ideal voltammetric behavior of the Fc. From a Fc mole fraction in solution of 0.4, they obtained the presence of an additional peak in the voltamogramm. The two voltammetric peaks are assigned to Fc groups that are isolated from other ferrocene molecules (by alkylthiolates, for example) and to Fc groups that are proximal to one other (in “clustered” state).43 At this stage, computer simulation is now a vital adjunct to experimental studies of SAM to obtain microscopic information about these heterogeneous system.44 We have previously used a methodology for the molecular simulation of such systems with a finite length in the third dimension by molecular simulation.45 From a computational viewpoint, the presence of the surface creates a nonuniformity of the local density along the direction normal to the surface and gives rise to important issues concerning the truncation procedures and the methodology used to take into account the Coulombic interactions. We have applied our methodology to systems that concern monolayers of metalchelating ligands grafted onto graphite surfaces in water.46 However, few experimental studies on the details of molecularscale surfaces or on the energetics of these systems were reported. Consequently, the quantitative and qualitative comparisons with experimental data were difficult in this case. In this paper, we propose to use our previous methodology concerning the molecular dynamics simulations of heterogeneous systems to the study of the ferrocenes in binary SAM, a well-known electrochemically active system. On the basis of the experimental work of Lee et al.,43 we apply our methodology to obtain a detailed study of Fc(CH2)12S-/C10S-Au monolayers using different grafting densities for the neutral ferrocene. We focus on the molecular description and energetic properties of these monolayers. The outline of this work is as follows. In the section concerning the experimental methods, we give the details of the computational procedures and we present the potential model. In this part, we also describe the five systems of Fc(CH2)12S-/C10S-Au SAMs studied using different grafting densities for ferrocenylalkylthiolate molecules. In the next section, we discuss the results as a function of the possible non-homogeneity of the ferrocene moieties within the monolayer in the SAM. Finally, in the last section we draw the main conclusions from this work. (40) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (41) Fawcett, W. R. J. Electroanal. Chem. 1994, 378, 117. (42) Brett, D. J. L.; Williams, R.; Wilde, C. P. J. Electroanal. Chem. 2002, 538, 65. (43) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438. (44) Leach, A. R. Molecular modelling principles and applications; Pearson Education: Upper Saddle River, NJ, 2001. (45) Goujon, F.; Bonal, C.; Limoges, B.; Malfreyt, P. Mol. Phys. 2008, 106, 1397. (46) Goujon, F.; Bonal, C.; Limoges, B.; Malfreyt, P. J. Phys. Chem. B 2008, 112, 14221.
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Computational Procedures Model. The system consists of five layers of a Au(111) surface grafted with n-C10 alkylthiolate (C10H21S) and n-C12 ferrocenyalkylthiolate (C22H33FeS) molecules. We used the all-atom (AA) version of the Cornell force field AMBER47 for grafted molecules. The Au parameters48 were taken from the work of Ayappa and co-workers. The ferrocene part was modeled using the parameters described by Canongia et al.49 The general potential function is of the form X X U ¼ kb ðr -ro Þ2 þ kθ ðθ -θo Þ2 þ bonds
X
angles
kφ ½1 þ cosðlφ þ δÞ þ
dihedrals
N -1 X i ¼1
8 2 9 !12 !6 3 = N < ¥ X X σ σ q q ij ij i j 5þ 4εij 4 : rij rij jrij þ nLj; j ¼i þ1 l
ð1Þ
where kb, kθ, and kφ are the force constants for deformation of bonds, angles, and dihedrals, respectively. The equilibrium values of bond distances and valence angles correspond to ro and θo, respectively. In the dihedral angle term, l is the periodicity and δ is the phase factor. The C-H covalent bonds were kept of fixed length by using of the SHAKE algorithm.50 The intermolecular and intramolecular interactions consist of a van der Waals repulsion-dispersion term calculated using the Lennard-Jones (6-12) potential, represented by the penultimate term in eq 1. In the AMBER force field, the nonbonded interactions between atoms separated by exactly three bonds (1-4 van der Waals interactions) are reduced by a factor of 0.5.47 The Lennard-Jones potential parameters for the interactions between unlike atoms were calculated by using the Lorentz-Berthelot mixing rules (quadratic and arithmetic rules for εij and σij parameters, respectively). The water molecules were represented with the TIP4P/ 2005 model.51 As the system is nonperiodic in the direction normal to the surface (z-axis), the simulation cell is closed by an additional gold layer. The distance between the two inner surfaces was chosen to be 80 A˚, which is large enough for the water molecules in the middle of the cell to recover a bulk behavior.45 The simulation cell was then extended both ways along the z-axis with vacuum in order to prevent electrostatic interactions between neighboring cells evaluated using the Ewald summation. Long-Range Coulombic Interaction. The last term in eq 1 corresponds to the electrostatic energy (UELEC) of the system. Considering N molecules in a volume V = LxLyLz with center of mass ri, each molecule contains ni charges qia at position ria. The electrostatic interactions handled with the Ewald sum method52,53 in a box with orthogonal axis are given by the following contributions 1 X QðhÞSðhÞSð -hÞ þ 2εo V k6¼0 1 XXX X qia qjb erfcðRriajb Þ=riajb 8πεo i a j6¼i b R XX 2 1 X X X qia qib 1 M2 q erfðRriaib Þ þ 2εo V z 4π3=2 εo i a ia 8πεo i a b6¼a riaib
UELEC ¼
ð2Þ (47) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. J.; Ferguson, D. M.; Spellmeyer, D. M.; Fox, T.; Caldwell, J. W.; Kollman, P. J. Am. Chem. Soc. 1995, 117, 5179. (48) Rai, B.; Sathish, P.; Malhotra, C. P.; Pradip; Ayappa, K. G. Langmuir 2004, 20, 3138. (49) Canongia-Lopes, J. N.; Cabrol-do-Couto, P.; Minas-da-Piedade, M. J. Phys. Chem. A 2006, 110, 13850. (50) Ryckaert, J. P.; Cicotti, G.; Berendsen, H. J. C. J. Comput. Phys. 1977, 23, 327. (51) Abascal, J. L. F.; Vega, C. J. Chem. Phys. 2005, 123, 234505. (52) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford University Press: Clarendon, 1981. (53) Smith, E. R. Proc. R. Soc. London, Ser. A 1981, 375, 475.
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where erfc(x) is the complementary error function and erf(x) is the error function. R is chosen so that only pair interactions in the central cell need to be considered in evaluating the second term in eq 2. Note that the overall charge of the simulation cell must be zero. In the last term of eq 2, Mz is the net dipole moment of the simulation cell given by ΣN i = 1qi ri. This contribution is the correction term from Yeh and Berkowitz,54 which results from the planewise summation method proposed by Smith.53 Adding this term to the total energy amount implies the use of a z-component force for each atom given by Fi , z ¼ -
qi Mz εo V
ð3Þ
The EW3DC method differs from the standard EW3D method only by the presence of this dipole correction. This issue has been discussed in a previous article45 to model electrostatics efficiently in a 2D periodic system with the standard 3D Ewald method. The functions S(h) and Q (h) are defined using the eqs 4 and 5, respectively SðhÞ ¼
XX i
a
qia expðih 3 ria Þ
1 h2 QðhÞ ¼ 2 exp - 2 h 4R
ð4Þ
! ð5Þ
where the reciprocal lattice vector h is defined as h = 2π(l/Lxu, m/Lyv, n/Lzw) where u, v, and w are the reciprocal space basis vectors and l, m, and n take values of 0, (1, (2, 3 3 3 ( ¥. The reciprocal space sum is truncated at an ellipsoidal boundary at the vector |hmax|. Simulations were carried out in the constant-NVT statistical ensemble using a Hoover thermostat55 with a coupling constant of 0.5 ps. Keeping the cell volume constant is useful for preserving a vacuum zone between periodic images along the z-axis. However, as the inner distance between the closing surfaces cannot change, care has to be taken when choosing the number of water molecules. For each system, the number of water molecules is adjusted so that the bulk density is recovered not very far from the surfaces. Methodology. The equations of motion were integrated using the Verlet Leapfrog algorithm scheme at T = 298 K with a time step equal to 2 fs. The cutoff radius for the Lennard-Jones contribution was fixed to 12 A˚, whereas it was equal to 18 A˚ for the real space of the electrostatic interactions. We also used a reciprocal space cutoff radius of 1.14 A˚-1. The configurations were generated using the parallel version of the modified DL_POLY_MD package56 by using up to 8 processors at a time. The parameters of the long-range electrostatics interactions evaluated with the Ewald summation technique were calculated to satisfy a relative error of 10-6. We simulated five systems using different grafting densities or surface organizations for ferrocenylalkylthiolate molecules (Figure 1). The following notations are used through this paper: ALK is a classical 100% alkylthiolate SAM surface, used as a reference system; FERRO is a surface entirely covered with ferrocenylalkylthiolate at maximal grafting density; CLUS20 corresponds to Fc molecules that are proximal to one another and form clustered Fc moieties in the binary SAMs (surface density F = 0.20). The surface density of ferrocenylalkylthiolate is expressed depending on the maximal experimental surface coverage for Fc.43 (54) Yeh, I.-C.; Berkowitz, M. L. J. Chem. Phys. 1999, 111, 3155. (55) Hoover, W. G. Phys. Rev. A 1985, 31, 1695. (56) DL - POLY - MD is a parallel molecular dynamics simulation package developed at the Daresbury Laboratory Project for Computer Simulations under the auspices of the EPSRC for the Collaborative Computational Project for Computer Simulation of Condensed phases (CCP5) and the Advanced Research Computing Group (ARCG) at the Daresbury Laboratory.
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The RAND20 system also contains grafted ferrocenealkylthiolate at F = 0.20, with a random surface distribution. This system corresponds to the ideal behavior of tethered ferrocenes on a gold surface with Fc molecules that are isolated from other Fc molecules; RAND40 contains grafted ferrocenylalkylthiolate at F = 0.40, with a random surface distribution. As the ferrocene headgroup is bigger than the methyl group of an alkylthiolate grafted chain, the SAM grafting density of 1/3* in terms of Au atoms is not possible to reach when grafting ferrocenylalkylthiolates. The ferrocenylalkylthiolate grafting density uses as a reference (F = 1.0) the maximal experimental grafting density, which corresponds to 1/5 in terms of grafted Au atoms. As a regular 1/5 grafting is not possible to be reached with hexagonal Au(111) surface, the experimental maximum grafting density must correspond to a mix of holes and 1/4 grafting local values on the surface. The theoretical maximum grafting density is then 1/4, which is used for the FERRO ideal cluster system (the ferrocenylalkylthiolate grafting density for this system is then F = 1.25 according to the previous definition). The construction algorithm proceeds as follows: the ferrocenylalkylthiolate molecules are grafted on Au atoms using distance criteria defined by 1/4 grafting density. The choice of those positions may be random or not, depending on the system. Alkanethiolate molecules are then grafted on the surface, using 1/3 grafting density distance criteria (the ferrocene headgroup does not affect the smaller alkyl chains). As the 1/4 and 1/3 hexagonal subnetworks are not fully compatible, it is not possible to fill the surface in an orderd way when containing randomly grafted ferrocenylalkylthiolate. This issue is illustrated by the grafting points in Figure 1. Consequently, the RAND20 system contains fewer alkylthiolate molecules than the well-ordered CLUS20 system. One simulation consisted of an equilibrium period of 500 ps and an acquisition period of 1 ns. The simulation details are listed in Table 1, including the number of water molecules used for an accurate description of the bulk behavior.
Results and Discussion First, we focus on the two limit ALK and FERRO systems. In the ALK system, the surface is covered by entirely100% alkylthiolate. Figure 2a shows the molecular density profiles of the carbon groups in the alkylthiolate chains along the z-axis, normal to the gold suface. The first peak corresponds to S-C bond, whereas the remaining peaks represent the C-C bonds. The presence of a doublet pattern is an indication of all-trans structure in the backbone place. The C-C bond takes an alternative sequence of two orientations, nearly parallel and normal, with respect to the surface normal. This result is thoroughly consistent with previous simulation.57 The density profile of the water molecules along the z-direction in the ALK system is also reported in Figure 2a. As expected, for the alkylthiolate SAM, the position of the first peak is shifted to a larger distance of 16 A˚. The gap between water molecules and carbon groups shows that water molecules could not penetrate into the chains. Thus, the resulting difference (approximately 3 A˚) corresponds to the respective equilibrium van der Waals separation distance between a water molecule and a carbon atom. Strong oscillations of the water density are then obtained. Analogous behavior has already been observed in simulations of water in interaction with hydrophobic surfaces.58,59 This result highlights that the dense packing geometry of alkylthiolates
(57) Vemparala, S.; Karki, B. B.; Kalia, R. K.; Nakano, A.; Vashista, P. J. Chem. Phys. 2004, 121, 4323. (58) Marti, J.; Nagy, G.; Gordillo, M. C.; Guardia, E. J. Chem. Phys. 2006, 124, 094703. (59) Pertsin, A. J.; Grunze, M. Langmuir 2000, 16, 8829.
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Figure 1. Snapshots of the initial configurations for the CLUS20 (a), RAND20 (c), and RAND40 (e) systems. For each system, the nature of the surface gold atoms is displayed on the next picture ((b), (d), and (f ), respectively): dots are the surface atoms, circles are the alkyl grafting points, and numbered circles are the ferrocenylalkylthiolate grafting points. Table 1. Summary of the Different Parameters for the Simulated Systemsa system
surface size (x y)
ALK CLUS20 RAND20 RAND40 FERRO
15 16 (43.2 39.92) 15 16 (43.2 39.92) 15 16 (43.2 39.92) 15 16 (43.2 39.92) 12 16 (34.6 39.92)
(1/4) nAu n(1/3) graft ngraft nalk nalkFe nwater natoms
240 240 240 240 192
80 80 80 80 64
60 60 60 60 48
80 63 52 44 0
0 10 10 19 48
3700 3650 3650 3600 2750
18800 18646 18294 18369 14984
a (1/4) The surface size is given in unit cells and A˚2. n(1/3) graft and ngraft are the maximum number of grafting points for a grafting density of 1/3 and 1/4, respectively. nalk and nalkFe are the number of effectively alkylthiolate and ferrocenylalkylthiolate molecules, respectively. The number of water molecules nwater is adjusted so that the bulk density is preserved.
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chains creates a hydrophobic wall that induces layering in the water molecules. It is also interesting to note in Figure 2a that the water density reaches its bulk density at approximately 26 A˚. Indeed, it is essential from a computational viewpoint to verify whether our box size is sufficiently large to have a significant region of bulk water. In Figure 2b, we have studied the structure of water by calculating the z-component of the total dipole moment across the z-direction. A quasi-zero profile of the z-component of the total dipole moment is obtained from z = 26 A˚ in agreement with the fact that from this position the bulk-like behavior is recovered. To deeply investigate the orientation of water molecules, we have also recorded the distribution of the cosine of the corresponding angle DOI: 10.1021/la9007087
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Figure 2. (a) Density profiles of the carbon groups and water molecules along the z-direction in ALK system. (b) z component of the
total dipole moment of water molecules of the slab centered at z along the z-direction. (c) Angular density profile of probability n (cos β as a function of cos θ for three different zones A (16 A˚ < z < 18 A˚), B (24 A˚ < z < 28 A˚), and C (40 A˚ < z < 80 A˚). β is formed by the dipole moment of water molecules and the z-axis direction, represented by a unit vector perpendicular to the gold surface. (d) Density profiles of the carbon groups and water molecules along the z-direction in FERRO system. (e) z component of the total dipole moment of water molecules of the slab centered at z along the z-direction. (f ) Angular density profile of probability n (cos β as a function of cos θ for three different zones A (22 A˚ < z < 24 A˚), B (30 A˚ < z < 34 A˚), and C (40 A˚ < z < 80 A˚). β is formed by the dipole moment of water molecules and the z-axis direction, represented by a unit vector perpendicular to the Au surface.
β between the water dipole vector and the surface normal in three different zones A (16 A˚ < z < 18 A˚), B (24 A˚ < z < 28 A˚), and C (40 A˚ < z < 80 A˚) near the SAM surface (Figure 2c). Obviously, a uniform distribution of cos β is found for distributions in the bulk-like regions (zones B and C). In zone A, the molecular dipole moments have a preferred orientation away from the surface, as expected for water in interaction with a hard wall.59 We now focus on a system consisting of a surface entirely covered with ferrocenylalkylthiolate at maximal grafting density (FERRO system). The molecular density profiles of the carbon groups (Figure 2d) show that the doublet pattern of the chains is less pronouced compared to that of the ALK system. Morever, the peaks are broader, indicating a lower degree of positional order of the backbone atoms. This feature could probably be explained by the large size of the ferrocene headgroup and by a smaller value of alkyl covering compared to that of 9168 DOI: 10.1021/la9007087
the ALK system. As a result, the surface packing density is lower, which induces more mobility of the ferrocenylalkylthiolate chains. This is also confirmed by the molecular density profile of the water molecules (Figure 2d), which are less layered by comparison to that of the ALK system. One can also note that the water reaches its bulk density at a larger distance of z = 32 A˚ instead of z = 26 A˚ in the case of the ALK system. In fact, this variation can be easily attributed to the difference in the alkylthiolate chain length: FERRO and ALK are constituted by n-C12 ferrocenylalkylthiolate and n-C10 alkylthiolate, respectively. From z = 32 A˚, we obtain a quasi-zero profile of the local Mz(z) across the z-direction (Figure 2e). On the basis of this result, we confirmed that the z-dimension is developed enough to yield a bulk-like region . In Figure 2f, the distribution of the cosine of the corresponding angle β between the water dipole vector and the surface normal for the three different zones A (22 A˚ < z < 24 A˚), Langmuir 2009, 25(16), 9164–9172
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B (30 A˚ < z < 34 A˚), and C (40 A˚ < z < 80 A˚) is reported. In zone B, the distribution of cos β has no preferred orientation, in agreement with a bulk-like region. Now that the two limit systems are known from a structural viewpoint, we focus on the others systems simulated (CLUS20, RAND20, and RAND40 systems). These systems have been selected on the basis on the experimental results of Lee et al.43 The cyclic voltammogramms of binary SAMs formed from solutions containing various ratios of Fc(CH2)12SH and C10SH show a ideal volammetric behavior with a single symmetric reversible wave when the mole fraction of Fc is small with the remaining portion of the SAM being made up of nonelectroactive alkylthiolates.43 This ideal behavior of ferrocenes on a gold surface is simulated here by a system with low grafting density (F = 0.20) and a random surface distribution (RAND20). In order to compare, a model system with the same grafting density (F = 0.20) and only clustered Fc moieties is also designed (CLUS20). At relatively high surface coverages of ferrocenylakylthiolate, an additional reversible wave located at more positive potential is observed in the voltammogramm.43 We have then undertaken to simulate a system with grafted ferrocenylalkylthiolate at F = 0.40 and a random surface distribution to evidence possible inhomogeneity of the ferrocene moieties within the layer with Fc moieties both in “isolated” and “clustered” states (RAND40). Concerning the conformations of ferrocenylakylthiolate and alkylthiolate chains, which constitute the binary SAM, we have calculated different angles for the orientation of the molecule with respect to the surface. The tilt angle θ is defined as the angle between the final backbone atom and the surface normal direction while the tilt angle R is the angle of the ferrocene unit with the surface normal direction as represented in Figure 3. The angular distributions of θ angle of the alkylthiolate chains for the ALK, CLUS20, RAND20, and RAND40 systems are presented in Figure 4a. With ALK constituted by a classical 100% alkylthiolate SAM surface, a well-defined peak with a value of the tilt angle close to 32 in excellent agreement with the literature values of 30 57,60,61 is observed. With regard to CLUS20, we observe a slightly lower and broader peak at approximately 32. This also shows a rather ordered conformation of the alkylthiolate chains. By contrast, the angle distributions become practically uniform for RAND20 and RAND40. This implies no privileged orientation of chains, indicating that the chain conformations can substantially change in these systems. This feature is in perfect agreement with the fact that RAND20 contains fewer alkylthiolate chains than CLUS20 because of the distance criteria between the grafting sites (Table 1). The angular distributions of θ and R of the ferrocenylalkylthiolate chains have also been determined. All the results are summarized in Figure 4b,c. A distribution of θ with only a unique broad peak centered on 31 is observed for CLUS20. This can be explained by considering that the ferrocene moieties forms a cluster, adopting a vertical position in the SAM. This behavior is mainly arising from the steric crowding of the relatively large ferrocene groups. We observe in the angular distribution of R for CLUS20 (and also in the one obtained for FERRO) narrow peaks located at small values of R. On the basis of these results, we can conclude that the corresponding conformations to Fc moieties in “clustered” state rather lead to small values of both θ and R, reflecting thus a higher ordering within the cluster structure. (60) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (61) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
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Figure 3. Schematic representations of the tilted alkylthiolate molecule and of the tilted ferrocenylalkylthiolate molecule: θ and R are the tilt angles of the molecule and of the ferrocene headgroup, respectively, with the surface normal.
Figure 4. (a) Probability distribution of the tilt angle θ for the alkylthiolate chains in the ALK, CLUS20, RAND20, and RAND40 systems. (b) Probability distribution of the tilt angle θ for the ferrocenylalkylthiolate chains in the CLUS20, RAND20, and RAND40 systems. (c) Probability distribution of the tilt angle R for the FERRO, CLUS20, RAND20, and RAND40 systems. DOI: 10.1021/la9007087
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The angular distribution of θ in RAND20 shows one broad peak centered at 27 and one other at approximately 45 (Figure 4b). On the other hand, a broad peak around 90 is obtained in the angular distribution of R (Figure 4c) highlighting a higher flexibility of the ferrocene headgroup. The ferrocene
Figure 5. Schematic representations of the tilted ferrocenylalkylthiolate molecule.
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groups would prefer to interact with the methyl alkyl chains rather than with the hydrophilic interfacial region. For this purpose, two limit behaviors could be considered (Figure 5). The first one (Figure 5a) is related only to the tilting of the ferrocene headgroup, whereas the second one (Figure 5b) concerns rather the tilting of the ferrocenylalkylthiolate chain. The angles θ and R previously described (Figure 3) provide a quantitative basis to determine the molecular structure and then to deduce the ordering tendency of chains. It is worth noting that the lower angles θ associated with the larger angles R correspond to the inclination of the ferrocene headgroup whereas the larger θ values with the lower R values imply an inclination of the chain. This θ-R anticorrelation can be studied by plotting θ as a function of R. Figure 6a shows the θ-R scatter distribution, calculated for each Fc-terminated chain. One dot represents
Figure 6. (a) 2D scatter plot of the two angles θ and R for the CLUS20 and RAND20 systems. (b) Calculated tilt angles θ distribution
plotted as a function of tilt angle θ for two peculiar chains (chain 1 and chain 3) for the RAND20 system. Shown in the inset is the calculated tilt angle R distribution plotted as a function of tilt angle R for the same chains.
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Figure 7. z position of all atoms of the ferrocenylalkylthiolate chains along the z-direction for the CLUS20, RAND20, RAND40 and, FERRO systems.
a time average of 5 ps so that one molecule is represented by 200 dots in the distribution. Both systems CLUS20 and RAND20 are displayed for comparison. As expected from the previous analysis, the cluster structure is represented by the zone defined by low θ and low R values. There is very little overlap between the two systems, which shows that θ and R are anticorrelated in the case of a random distribution. We also note that both limit behaviours described in Figure 4 are not equally probable. A large proportion of RAND20 chains adopt rather a large R configuration, which can be explained by the fact that it involves less movement of the neighbor chains. We have considered two peculiar chains (chain 1 and chain 3) in RAND20 system (Figure 1). These molecules have been chosen because they do not change their average orientational behavior (in terms of Figure 4). Thus, all along the simulation, in chain 1 the ferrocene headgroup tilts, whereas in chain 3, the tilting of the ferrocenylalkylthiolate chain is involved. The results of the angular distributions for the two chains are plotted in Figure 6b. The value of R is noticeably larger in chain 1 than in chain 3, whereas the value of θ is smaller in chain1 than in chain 3. Thus, the analysis of the distribution of both θ and R provides details of the molecular-scale architecture. To analyze the possible inhomogeneity of the binary SAM, we now focus on the RAND40 system. To this end, we examine both the angular distributions of θ (Figure 4b) and R (Figure 4c). The distributions exhibit two broad and small peaks. They are compared to the ones obtained in the case of RAND20 in order to understand the origins of the two peak distributions. The peaks at higher values of θ and R are roughly the same. It is then reasonable to assign these peaks to isolated ferrocene. We have previously demonstrated that the ferrocene moieties in the clustered state adopt a vertical position in the SAM and that the corresponding conformations should correspond to small values of θ and R due to the steric crowding of the relatively large ferrocene groups in the clusters. Obviously, the remaining peaks located at θ and R equal to 28 and 48, respectively, are consistent with the vertical position for ferrocene in the clustered state. These interesting results show that it is possible to evidence the degree of heterogeneity in a SAM from the angular distributions. In Figure 7, the positions of all atoms of ferrocenylakylthiolate chains along the z-direction are reported and confirm the features Langmuir 2009, 25(16), 9164–9172
Table 2. Energy Contributions in kJ mol-1 for RAND20, CLUS20, and FERRO Systems contribution EFe-Fe EFe-Alk EFe-Water
RAND20
CLUS20
FERRO
-8.6 -39.1 -43.3
-33.3 -26.4 -37.2
-64.5 -27.4 -22.0
discussed above. The ferrocene headgroup rather adopts a vertical position in FERRO and CLUS20 systems. The agreement also extends to the RAND20 where the tilting of chains are a consequence of the hydrophobic character of the ferrocene headgroup. The RAND40 system exhibits an intermediate behavior. This can be interpreted by the coexistence between Fc moieties in “isolated” and “clustered” states in good agreement with the previous discussion. A further, quite interesting aspect of the results shown in Figure 7 is that the average distance of the ferrocene heads with respect to the surface normal is significantly increased (i.e., by a few tenths of nanometer) as the surface coverage of ferrocene moieties is increased. This tiny change of SAM thickness might be at the origin of a significant change in long-range electron transfer kinetics. The formation of self-assembled monolayer depends on the competition among several forces, such as the intrachain, interchain, chain-surface, or chain-bulk interactions, which ultimately determine the molecular orientation near the surface. It is important to compare the energy of the interactions mainly involving the ferrocene group as a function of the system. The total interaction energy of the ferrocene groups has been split into three distinct parts: EFc-Fc, the contributions between two ferrocene headgroups; EFc-Alk the contributions between a ferrocene group and the alkyl groups of both ferrocenylalkylthiolate and alkylthiolate chains; and EFc-Water between a ferrocene headgroup and the water molecules. Each energy contribution sums the Lennard-Jones and electrostatic energy parts. The average contributions obtained for RAND20, CLUS20, and FERRO systems are reported in Table 2. Figure 8a shows the contribution of each interaction in the total energy. In RAND20, the contribution of Fc-Fc interaction in the total energy is small as expected, since the separation is large between the ferrocene units. On the other hand, the magnitude of EFc-Fc strongly increases in the FERRO system due to the clustered state DOI: 10.1021/la9007087
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ones observed in the FERRO system. Interestingly, we observe that the state of ferrocene is governed by the different energetic contributions.
Conclusion
Figure 8. (a) Calculated ferrocene-ferrocene, ferrocene-alkyl, and ferrocene-water energies contributions for the RAND20, CLUS20, and FERRO systems. (b) Calculated corresponding energies contributions for chains located in the center of the cluster and at the boundary of the cluster in RAND20 system.
of the ferrocene moieties. The energy EFc-Alk in RAND20 is strongly higher than for the other systems. Obviously, the tilting of chains then makes the Fc-Alk interactions more predominant. Concerning the Fc-Water interaction, its energy is smaller in the FERRO system. Once again, this result is consistent with the fact that a vertical orientation adopted by the chains favors the Fc-Fc contributions. For CLUS20, it is interesting to note that the contributions depend on the position of the chains in the cluster, that is to say in the center or in the region which borders the cluster (Figure 8b). As expected, the relative contributions of each interaction type in the center of the cluster are of the same order of magnitude as the
9172 DOI: 10.1021/la9007087
We have reported molecular dynamics simulation of mixed monolayers constituted by Fc(CH2)12S-/C10S-Au SAMs. By varying the ferrocene mole fraction in solution, we have explored the possible inhomogeneity of the ferrocene moieties within the monolayer. Five reference systems were simulated using different grafting densities for ferrocenealkylthiolates. These systems were designed to model dilute or clusterized Fc systems to study the structuration of the Fc moieties. The computed quantities (density profiles, angular distributions, positions of atoms, and energetic description) improve the understanding of the ordering tendency of chains. A dense packing geometry of alkylthiols chains in the ALK system was found and it induces layering in the water molecules in perfect agreement with the literature values. For the ideal cluster system (FERRO), a molecular mobility is observed in the ferrocenylalkylthiolate chain due to the fact that the ferrocene moieties are too large to adopt the tight packing geometry characteristic of alkylthiols monolayers. With regard to RAND20, the two-peak distribution obtained in θ and R angles was explained by the flexibility of isolated chains and the hydrophobicity of the ferrocene units. Thus, the ferrocene groups prefer to interact with the alkyl layer rather than with the hydrophilic interfacial region. The combination of θ and R angle distributions can precisely quantify the conformation of ferrocenylalkylthiolate chains. We established that a moderately grafted surface RAND40 exhibits distinct behavior for Fc headgroups. These results are confirmed by an energetic study of the Fc group in various environments. Our results showed a perfect agreement with the conformations of chains deduced from the angular distributions. We have also shown that the contributions at each interaction (Fc-Fc, Fc-Alk, Fc-Water) strongly depends on the state of the ferrocene, isolated or clustered. We have then demonstrated that it is possible to describe in terms of the relative contributions from isolated and clustered Fc moieties by connecting the structural features and the energy contributions deduced from molecular simulations. This study represents a prerequisite work before further investigating these systems. In a forthcoming paper, we plan to study the factors which influence the interfacial electron transfer process. For this purpose, we will use a perturbation method for changing the ferrocenyl groups to the ferrocenium ions.
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