ARTICLE pubs.acs.org/JPCC
Molecular Dynamics Study of the Formation of a Self-Assembled Monolayer on Gold Yoonho Ahn,† Joyanta K. Saha,‡ George C. Schatz,§ and Joonkyung Jang*,‡ †
Supercomputing Center and ‡Department of Nanomaterials Engineering, Pusan National University, Busan 609-735 Republic of Korea § Department of Chemistry, Northwestern University, Evanston Illinois 60208-3113, United States ABSTRACT: Molecular dynamics simulations have been used to study the formation of nanoscale islands of self-assembled monolayers (SAMs) starting from alkanethiol molecules initially lying down in a disordered physisorbed layer on gold. These islands form when tens of alkane thiols stand up together within tens of ns after chemisorption begins. The alkane chains in these islands are found to be tilted, and the tilt direction precesses around the center of the island. This precession, together with the packing of the sulfur atoms, signals the formation of a SAM island, occurring prior to the tilting and orientation ordering of the chains.
1. INTRODUCTION Self-assembled monolayers (SAMs) are a dominant structural theme in the formation of an organized, stable, and versatile monolayer on a surface.16 SAMs formed from alkanethiols on gold (Au) are well-known in both the vapor deposition2 and solution phase growth.3 With the help of various diffraction, scanning probe microscopy, and spectroscopic experiments,2 some of the molecular details of SAMs have been √ determined, √ such as that the sulfur (S) atoms form a compact 3 3 R30 structure on Au (111), and that the alkyl chains are tilted 2030 away from the surface normal. Secondary ordering of the chains √ gives rise to a c(4 2) super lattice,7,8 and a transient (2 3) 9,10 At a low rect structure with a 50 tilt angle is also found. surface coverage, the thiol molecules are physisorbed with their chains lying flat on the surface.11 These lying down molecules can be packed with their chains parallel to each other or disordered (above 15 C12). With increasing time13 or increasing surface coverage,14 they stand up and form a compact SAM. Unfortunately, there are still many gaps in our understanding of SAMs at the molecular level. In particular, we do not understand the molecular-level mechanism for the transition from the lying down to standing up states or phases. Molecular simulations1524 should be able to give fine details of the SAM formation process, and in fact there have been numerous molecular dynamics (MD)16,17,1921,2329 and Monte Carlo (MC)15,18,30,31 simulations on SAMs. The orientation and conformation of alkyl chains in SAMs were simulated and compared favorably with experiments.15,16,26,30 MC simulations18,31 reproduced the experimental observation of the phase separation for a mixed SAM composed of long and short alkanethiols.32,33 Several studies19,20,24 proposed a herringbone structure in the CCC planes of alkyl chains as the c(4 2) superlattice. Temperature effects on the orientational order of chains were investigated via the MD method, and the so-called rotator phase r 2011 American Chemical Society
was predicted at high temperature.26 The SAM structures formed on spherical gold nanoparticles were studied.23,34 Nearly all the previous simulations considered bulklike SAMs and focused on the equilibrium properties, viz. the tilt angle and conformation of the alkyl chain and the overlayer structure of sulfur atoms. The investigation on the dynamics of SAM is rare. Morgner studied the gas-phase adsorption of methanethiol on gold and found the sticking probability of thiol decreases with increasing coverage.21 This thiol is too short to be related to typical SAM experiments using thiols with 8 to 30 C atoms. Zhao et. al17 simulated the adsorption of benzendithiolate and found that the presence of solvent does not influence the ordering and adsorption pattern of the thiol. Gannon et. al studied the diffusion of excess thiol molecules on top of a preexisting SAM and the role of defects in SAM in the context of microcontact printing.29 Ryu and Schatz35 performed MC simulation for the growth of binary SAM in the nanografting experiment of Liu and co-workers,36,37 but their approach was phenomenological. Herein, by using molecular dynamics (MD) simulations based on a united atom (UA) model for alkanethiols,16,20 we have studied growth of the SAM from molecules initially lying down on a surface. The simulations show that the first step in growth involves the formation of nanoscale SAM islands in which tens of the alkane thiols stand up together within tens of nanoseconds. As with the bulk SAM, these islands involve close packing of the S atoms and the tilting of the alkyl chains. The tilt direction of the chains, however, precesses clockwise or counterclockwise around the center of the island. This precession, along with the close packing of the S atoms, signals the formation of SAM islands, and precedes the tilting and orientational ordering of the chains. The Received: January 15, 2011 Revised: April 18, 2011 Published: May 05, 2011 10668
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Figure 1. (a) Schematic representation of the UA model for ODT. For each molecule i, we defined the tilt direction vector Bui and its surface projection Bvi, the backbone plane orientation vector Bbi, and the tilt angle θi from the surface normal. Initial (b) and final (c) snapshots of the MD simulation. Both top and side views are drawn. The final snapshot is taken at 30 ns, at which point eleven islands have formed from 365 molecules initially lying flat on Au (111).
formation of SAM islands is energetically favorable, because stabilization due to interchain packing dominates over destabilization from the reduced chainsurface interactions.
2. SIMULATION METHOD The thiol molecules are taken to be 1-octadecanethiol, SH(CH2)17CH3, (ODT), which has been the prototypical molecule in the SAM fabrication using soft nanolithography.3843 The CH3, CH2, and SH groups of ODT are treated as UAs (part a of Figure 1).16,23 We point out that UA models for thiols16,20,27 have successfully reproduced the tilt angle (TA) and tilt direction of the chain measured in experiments and found in all-atom simulations.19,44 For each molecule, we picked nine of the UAs of CH3 or CH2 (drawn as filled circles in part a of Figure 1) corresponding to odd (117) numbers of intervening CH2 groups between them and the S atom for the purpose of defining the backbone direction. Then, the tilt vector B u i (i is the molecular index throughout this work) was defined as the average of the direction vectors from the S atom to these nine UAs. The TA, θi, is defined as the polar angle of B u i from the surface normal u i on the surface plane (XY plane). (Z axis).Bv i is the projection of B We normalized both B u i and Bv i as unit vectors. The backbone plane orientation of ODT was defined by B bi ¼
15
15
∑ ð 1Þj Br j =j j∑¼ 1 ð 1Þj Br jj j¼1
ð1Þ
where Br j is the jth CC bond vector starting from the S atom. Note, B b i is the average direction of the CCC and CCS planes, and the three UAs at the tail of the chain (j = 17, 18, 19) are excluded in this definition because they have many gauche defects. We included the bond stretching, bending angle, torsion, and nonbonded potentials between the UAs in the force field. Both the bond stretching and bending angle interactions were taken to be harmonic.45 The four-atom torsion potential takes a triple cosine function of the dihedral angle φ where φ = ( 180 and φ = ( 60 correspond to the trans and gauche conformations, respectively.46 The SAu chemisorption
Table 1. LennardJones (LJ) and Morse Potential Parameters of the United Atoms. a LJ
a
σ (Å)
ε (kcal/mol)
SH
4.250
0.397
CH2
3.905
0.118
CH3
3.905
0.175
Au Morse
2.935 De(kcal/mol)
0.039 re(Å)
R
AuS
8.763
2.65
1.47
See eqs 2 and 3 for the definition.
potential is modeled by a Morse potential,17 VAuS ðrÞ ¼ De exp ½ Rðr re Þfexp½ Rðr re Þ 2g ð2Þ where r is the distance of the SAu atom pair. De and re are the well depth and the equilibrium bond distance of the potential (8.763 kcal/mol and 2.65 Å), respectively. All of the nonbonded interactions, including the SAu physisorption potential, are taken to be LennardJones (LJ) potentials, " 6 # σ 12 σ VLJ ðrÞ ¼ 4ε ð3Þ r r where ε and σ are the LJ energy and length parameters, respectively.16 The LJ and Morse parameters for each atom are listed in Table 1. We used the LorentzBerthelot (LB) combination rules for the LJ interactions of hetero atomic pairs.47 The well depth and the distance at the minimum of the LJ potential for the SAu pair are 0.1244 kcal/mol and 4.03 Å, respectively. This LJ potential is used to model the physisorption of S onto Au. The simulation uses the chemisorption (Morse) potential for most calculations, but we also ran a simulation which considers both chemisorption and physisorption. To accomplish this, we constructed a SAu pair potential, which consists of the Morse potential for distances below 2.7 Å, the LJ potential for distances above 3.5 Å, and a cubic polynomial for distances in between these values. This 10669
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At the end of the simulation, the ODT molecules formed eleven SAM islands (part c of Figure 1). The growth dynamics of each island is analyzed by selecting out molecules that comprise the island. For a SAM of N molecules, the precession in the tilt direction P is defined as P ¼
1 N ^r i B vi N i¼1
∑
ð4Þ
where ^r i is a unit vector pointing to the S atom of the ith molecule from the center of all S atoms (all the S atoms essentially lie in the XY plane). P is þ1 (1) if all Bv i s precess counterclockwise (clockwise) with respect to the center of the island. Ordering in the molecular orientation of a SAM island was checked by evaluating the order parameter,51 Xj Þ2 1æi6¼ j OX ¼ Æ0:5½ð B Xi 3 B
ð5Þ
Figure 2. The potential for the SAu pair, including the barrier for the transition from the physisorbed to chemisorbed states of S. See the Simulation Method for details. The potential curves with barrier heights of kBT and 0.16 kBT are plotted.
Bi is where Ææi6¼j represents an average over intermolecular pairs. X b i). The the orientation vector of the ith molecule (u Bi or B reorientation dynamics of a SAM island was investigated by calculating the time correlation function (TCF),47
polynomial has negative curvature and therefore introduces a barrier between the minima of the Morse and LJ potentials. This barrier can be chosen as the barrier for the physisorption to chemisorption transition estimated previously (6.9 kcal/mol).2 However, this choice of barrier leads to overestimation of its value because the chemisorption barrier refers to the transformation of an entire molecule, not the SAu distance only. Therefore, we tried smaller values for the barrier comparable to the thermal energy, kBT (0.6 kcal/mol) and 0.16 kBT (the potential curves are shown in Figure 2). The present Au (111) surface is made up of two layers and 12 800 atoms. We used a parallelepiped simulation box with lateral lattice vectors of (195.8 Å, 115.4 Å, 0) and (0, 230.7 Å, 0). Part b of Figure 1 shows the initial configuration where 364 ODT molecules lie flat and are physisorbed on the surface. This configuration, similar to the 2D liquid phase of lying-down molecules,12 was prepared in stages. We first placed a drop of ODT molecules on top of Au (111). We then ran a 3 ns long MD simulation using a LJ potential for Au reported by Heinz et al.48 The LB combination rule gave an LJ potential for the SAu pair of ε = 1.15 kcal/mol and distance at the minima of 4.032 Å. Compared to the SAu LJ potential described above, this LJ potential has nearly the same distance at the minimum, but a well depth which is about 10 times larger. This should provide a more realistic description of ODT physisorption, but chemisorption is not described. With the Heinz potential, the drop of ODT spreads and covers the surface, as shown in part b of Figure 1. (Additionally we removed those molecules above the monolayer of molecules shown in part b of Figure 1). Starting from the configuration in part b of Figure 1, the SAu potential was switched from the Heinz potential to the Morse potential and a MD simulation was run for 30 ns. Periodic boundary conditions with the minimum image convention47 were applied in the X and Y directions (using the lattice vectors mentioned above). The Au atoms were fixed in position throughout the simulation. The equation of motion was integrated via the velocity Verlet algorithm with a time step of 1.0 fs. The temperature was fixed at 300 K using a Berendsen thermostat.49 We used the DL POLY package50 to implement the MD methods described above.
CXX ðtÞ ¼ Æ B Xi ð0Þ 3 B Xi ðtÞæi
ð6Þ
Bi, B u i, where B Xi i(t)is the ith molecular orientation vector at time t(b or Bv i), and Ææi represents an average over the molecules in the island.
3. RESULTS AND DISCUSSION Starting from the initial configuration shown in part b of Figure 1, we ran a simulation by turning on the chemisorption potential and switching off the physisorption potential for the SAu pair. Eleven SAM islands formed within 30 ns, as shown in part c of Figure 1. The number of molecules belonging to an island, N, varied from 19 to 52. At the start of the simulation, the S atoms were immediately chemisorbed and the SAu distance decreased to 2.45 Å. The backbone of each molecule precessed and underwent repeated folding and unfolding with its S atom attached to the surface. Later, the S atoms slowly diffused on the surface, and the diffusion coefficient of a single ODT molecule was calculated to be 3.58 106 cm2/s, which is 10 times smaller than that of butanethiol calculated previously.52 Through the thermal motion described above, the molecules are entangled to form islands on the surface. As time goes by, the molecules within each island gradually become aligned and stand upright on the surface to achieve close interchain packing. Some of the islands divide into islands at this stage. Finally, the molecules √ two smaller √ form a ( 3 3) R 30 packing of S atoms with their chains tilted away from the surface normal, as shown in part c of Figure 1. An interesting feature of the SAM islands is that the tilt direction of the chain varies with time and precesses around the center of the island. Parts ad of Figure 3 show snapshots of the Bv i s at different times for the SAM island consisting of 52 molecules. At a given time, the Bv i s rotate clockwise (a and c) or counterclockwise (b and c) relative to the center of the island. The value of P defined in eq 4 is 0.895 (a), 0.915 (b), 0.885 (c), and 0.882 (d). The precession in the tilt direction is also manifested in the oscillatory behavior of Cvv (t)(dot-dashed line in part e of Figure 3) and Cuu (t) (broken line in part e of Figure 3). The period of oscillation is roughly 4.5 ns for both Cvv (t) and Cuu (t). In contrast, the TCF for the backbone orientation 10670
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Figure 3. Precession in the tilt direction of the alkyl chain. For the SAM island of 52 molecules, theBvi s (drawn as arrows) are displayed at times of 22.63 (a), 24.89 (b), 29.19 (c), and 27.50 ns (d). Panel (e) shows the reorientation TCFs, Cuu (t) (broken line), Cvv (t) (dot-dashed line), and Cbb (t) (solid line).
Cbb (t) (solid line) does not show any coherent motion, but decays to zero within 10 ps. An exponential fit to Cbb (t) gave a relaxation time of 5.9 ps, which is close to that reported in the UA simulation of the SAM formed from 1-hexadecanethiol (5 ps).16 Figure 4 shows various dynamical properties associated with growth of a SAM island with N = 52. TA (averaged over molecules) versus time is shown in part a of Figure 4. TA decreases from its initial value (90) and converges to nearly 15 after 10 ns or so. The equilibrated TA for this island is smaller than that of the SAM in the bulk (20303,53,54). Plotted in part b of Figure 4 is the average distance of the nearest neighbor (NN) pairs of S atoms, dSS. dSS decays to a value near 5 Å within 5 ns, which is notably faster than the relaxation of TA to its equilibrium value. In parts c and d of Figure 4, we plot the order parameters of the tilt direction and backbone plane orientation, Ou and Ob, respectively. As time goes by, Ou gradually increases from 0 and approaches 0.8, indicating the nearly complete alignment of the u i s (1 if the orientation order is complete). Ob increases from B zero and converges to a value near 0.2. Therefore, the orientational ordering in the backbone plane is not as complete as that in the tilt direction. We did not find any herringbone structure for the backbone plane orientation, which was speculated to be the c(4 2) superlattice of the SAM in the bulk.19,20,24 Both Ob and Ou level off after 10 ns, similarly to the behavior of TA. Part e of Figure 4 plots the time dependent tilt direction P defined in eq 4. We see that P is close to 0 for times up until 5 ns and then jumps to a value near 1. Later, P alternates between 1 and 1, corresponding to the transition from counterclockwise (þ1) to clockwise (1) precession or vice versa. The duration of the precession in one direction has the values 2.50, 0.24, 1.17, 3.86, 2.00, 1.39, 2.92, and 1.93 ns (averaging to about 2 ns).
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Figure 4. Growth dynamics for the SAM island that contains 52 molecules. Plotted are the time dependences of the tilt angle TA (a), the nearest SS distance dSS (b), the orientation order parameters for the tilt direction Ou (c), the backbone plane orientation Ob (d), the precession P (e), and the van der Waals energy per molecule VvdW (in units of kcal/mol) (f). TA, dSS, and VvdW are averaged over the molecules, and Ou and Ob are averaged over intermolecular pairs.
Unlike the other quantities in Figure 4, P is almost discrete in value (1, 0, or 1). Notice that a nonzero P occurs at nearly the same time as dSS reaches its equilibrium value. Therefore, the close packing of the S atoms and the precession in the tilt direction signal the formation of a SAM prior to the tilting and orientation ordering of the chains. Part f of Figure 4 plots the van der Waals (vdW) energy per molecule, VvdW, versus time. With time, VvdW decreases and levels off, indicating that growth of the SAM is energetically favorable. We found that the ODT-Au interaction energy actually increases as the SAM island forms (not shown). The decrease in VvdW is therefore due to the dominance of the intermolecular (ODTODT) energy. For smaller and smaller islands, the intermolecular energy decreases more rapidly than the molecule surface interaction energy, so growth of the SAM island becomes energetically unfavorable. This suggests that there is a lower limit to the size of a SAM island that can be formed. A systematic study of this size dependence of the SAM island will be published elsewhere. We examined the probability distribution for the orientation parameters of SAM islands made of 19, 37, and 52 molecules. A histogram distribution of the tilt angle H(θi) is plotted in the top panel of Figure 5. H(θi) for N = 19 has major and minor peaks at θi = 30 and 45, respectively. As N increases to 37 and 52, H(θi) evolves to have a distinct peak at 15 with a small minor peak at 0. The intermolecular cohesion should be weaker for the smallest island with N = 19 and, therefore, the molecules are more tilted toward the surface, thereby increasing the tilt angle. X-ray diffraction experiments have shown54 that ODT molecules in a bulk SAM are tilted toward their next NNs (NNNs). We checked the tilt direction for the SAM as follows. We drew a straight line originating from the S atom of each molecule i toward the direction Bv i, and selected out other S atoms lying close to this line. We then calculated the distance rtilt i from the origin to these selected S atoms. The histogram distribution for 10671
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Figure 5. Probability distribution for various orientation parameters. Plotted are histograms of the tilt angle θi[top], the tilt distance rtilt i [middle], and the torsion angle φ [bottom]. The histograms are drawn for the SAM islands of 19, 37, and 52 molecules. All of the Y axis labels are omitted for visual clarity. tilt rtilt i , H(ri ), is shown in the middle of Figure 5. In the case where N = 19, the maximum probability is found for the NN direction (rtilt i = 5 Å). However, for N = 37, the highest probability is found = 10 Å), which is in the next NN (NNN) direction (rtilt i consistent with the behavior of the SAM in the bulk. A similar peak located at 13 Å represents the direction between the NNN and the next NNN direction. For N = 52, the chains are tilted toward the direction between the NNN and the next NNN direction. The NN direction is equally probable as well. Shown in the bottom of Figure 5 is a histogram distribution of the CCCC torsion angle of ODT H(φ). The symmetric maxima located at 180 and þ180 represent the trans conformations. The distribution is broadened in that the peaks at 165 and 165 are only slightly smaller than the maximum peaks. The gauche defects located near 60 and 60 are negligible. The dynamic features of the SAM island with 52 molecules described above were valid for other SAM islands as well (Figure 6): for example, growth of the SAM is indicated by close packing of the S atoms and precession, followed by the tilting and ordering in the alkyl chains and decrease in the intermolecular potential energy. There are some quantitative differences however. As the size of the SAM decreases, the precession changes its direction more frequently (see the second panel from the bottom in Figure 6) and the TA increases (top panel of Figure 6). We ran an additional simulation in which the bond distance between neighboring UAs of ODT was fixed, instead of using a harmonic stretching potential. Such a simulation makes the molecules more rigid, and we obtained transient structure √ resembling the 50 tilted phase with (2 3) rect packing found previously (Figure 7).10 Presumably, molecular flexibility makes this transient structure unstable and its lifetime insufficient for it to be observed for the present small SAM islands. If we decrease the chain length of thiol, this transient structure will be more stable and can be observed in a simulation using flexible CC and CS bonds. In our simulation, the physisorbed molecules become immediately chemisorbed as the SAu interaction is switched from LJ to the Morse potential. In reality, this transition from physisorbed to chemisorbed states will not be instantaneous because of the finite barrier for the transition (estimated to be 6.9 kcal/mol2). Inclusion of this chemisorption barrier of course slows down the
Figure 6. Growth dynamics for SAM islands with different numbers of molecules, viz. 19, 37, and 52. Plots include the TA [top], the nearest SS distance dSS [second from top], the precession P [second from bottom], and the van der Waals energy per molecule VvdW (in units of kcal/mol) [bottom]. Here, VvdW includes the intermolecular potential energy only.
√ Figure 7. Transient structure similar to the (2 3) rect structure reported for the SAM in the bulk (inside the right circle). The neighboring CC bond distances of the ODT molecule are held rigid in this simulation. Drawn is a snapshot taken at an intermediate √ √time before the 74 molecules finally form a SAM island with ( 3 3)R 30 structure. Only atoms are shown for visual clarity. Shown in the √ the S√ left circle is the ( 3 3)R 30packing of S atoms.
growth of the SAM significantly. Indeed when we used a barrier equal to kBT (= 0.60 kcal/mol) in the potential, no SAM islands formed within 30 ns. Instead, a single pile of physisorbed molecules formed, making a drop on the surface. Only if the barrier was reduced to 16% of kBT or less did SAM islands form within several tens of ns. The growth of the SAM islands in this case is the same as that found in the simulation without the chemisorption barrier, except that some molecules stand upside down (Figure 8). As molecules get entangled and gradually align during growth of the island, some molecules pack with others such that their S atoms are upside down. Overall, although the 10672
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Figure 8. SAM islands formed in the MD simulation using the SAu potential shown in Figure 2. The barrier height is 0.16 kBT. SAM islands are similar to those obtained from the MD simulation without the chemisorption barrier of S onto Au, but some molecules stand upside down. The snapshot is taken at 30 ns. Both top and side views are drawn.
growth of the SAM is accelerated in our simulation due to the sudden switch in the potential, the qualitative behavior of the growth dynamics found here is generally valid for potentials that have two minima. What should happen for times beyond the window of the present simulation is that the SAM islands will diffuse and merge, giving rise to ripening of the SAM islands. This behavior is known experimentally for hexadecane thiol, where it takes place over several days.9 Simulation of the ripening of nanometer sized SAMs will be prohibitively slow for an atomistic simulation. A phenomenological model (e.g., diffusion model) or an accelerated MD simulation21 will be necessary. We are also interested in simulating the SAM growth with increasing the surface coverage (which is the normal method to prepare a SAM). This simulation also requires a modeling or a special MD technique to overcome the small time scale of a conventional MD simulation. Although SAMs are commonly prepared in solution, many fundamental studies on SAM were performed in gas phase where various spectroscopic and diffraction methods are applicable without the complication due to solvent.2 Moreover, SAM patterns in soft nanolithography (tip-based nanolithography and microcontact printing)3843 are normally fabricated without any solvent. Therefore, the gas-phase growth of SAM studied here is related to a wide range of experiments on SAM. In the presence of solvent, the growth dynamics of SAM will be more complex, but the resulting SAM structures will be equivalent to those from a vapor deposition (as is well-known experimentally). The previous MD simulation17 has also shown that the presence of solvent does not affect the ordering and adsorption pattern of
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SAM. An explicit consideration of solvent in the present simulation is computationally challenging: the presence of solvent retards the motion of the alkanethiol molecule and therefore slows down the formation of SAM in simulation. It turns out that inclusion of solvent make simulation too time consuming for our computational resources. In this initial study, we content ourselves with focusing on the gas-phase growth of SAM. We have selected a rather long alkanethiol molecule because ODT is an archetypal molecule in the SAM fabrication using nanolithography. The growth mechanism found for the SAM of ODT is expected to be valid for alkanethiols with different chain lengths (typically varying from 8 to 30 in the number of C atoms). That is, the sulfur atoms will become packed and the precession in the tilt direction of chains will appear, followed by the order in the orientation and conformation of chains. There will be some quantitative differences however. The alkanethiol molecules shorter than ODT should diffuse and aggregate faster than ODT molecules do. The SAM formation will be accelerated in this respect. However, the energetic stabilization of interchain packing will be weaker for a shorter alkanethiol, requiring more molecules to form a SAM. Therefore, the size of SAM will increase, and it will take more time to form a bigger SAM. The consideration of interchain packing also suggests that there should be a minimum chain length required for a standing up SAM. For an alkanethiol longer than ODT, the growth of SAM will be slower because of an increased molecular mass. With an enhanced stabilization of interchain packing however, a SAM can be formed with a lesser number of molecules that is needed for ODT. This will reduce the size of SAM for a longer alkanethiol. The present UA model has been proven to reproduce the equilibrium SAM structures, such as the tilt angle of the chain and the packing structure of sulfur atoms.16,25,26 Unfortunately, we are unaware of any time-resolved experiment that can be compared with the present dynamics. Available kinetic studies are the time-dependent coverages55,56 and in situ AFM images.57,58 These experiments however dealt with macroscopic time and length scales (more than seconds and micrometers), unable to provide any ns dynamics on a nanometer-sized SAM. It has been known that grafting density also affects SAM ordering and orientation, especially for SAMs made of a mixture of thiol molecules.5961 For the present nanoscale SAM islands, we found SAMs consisting of more than 20 molecules are close to a bulklike SAM in that S atoms and the alkyl chains are closely packed and the chains are slightly tilted from the surface normal. With a number of molecules less than 20, the chains lie down and S atoms are not compactly packed, and therefore a stable SAM could not be formed. We also found that the precession occurs less frequently as the size of SAM increases. Specifically, the precession changes direction on a time scale that increases from picoseconds to nanoseconds as SAM size varies from 2 to 3 nm. A detailed account of the size dependence of the SAM will be presented elsewhere.
4. CONCLUSIONS Using an MD simulation, we investigated the molecular details of the evolution of thiol molecules initially lying flat and physisorbed on a gold surface. With the onset of chemisorption of the S atoms, small SAM islands form within several tens of nanoseconds. The alkyl chains in the SAM islands are tilted from the surface normal and the S atoms are close packed. Unlike the SAM in the bulk, however, the tilt direction of the chain precesses 10673
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The Journal of Physical Chemistry C around the center of the island. This precession, along with close packing of the S groups, precedes tilting and ordering in the chain orientation. Growth of the SAM is energetically favorable in these simulations because of the dominance of energetic stabilization arising from interchain packing over the increasing chainsurface interaction energy due to the standing up of the molecules. While there are important uncertainties in the potential functions used in this study, as well as with the UA model, qualitative features of the SAM formation process compare sensibly with available experiments. This is the first molecular dynamics study that reveals the detailed time dependence of SAM formation and growth.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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