Odd–Even Effects on the Structure, Stability, and ... - ACS Publications

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Odd Even Effects on the Structure, Stability, and Phase Transition of Alkanethiol Self-Assembled Monolayers Leyla Ramin and Ahmad Jabbarzadeh* School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia ABSTRACT: Molecular dynamics simulations were conducted to predict the structural properties and phase transition temperatures of n-alkanethiols CH3(CH2)n-1SH (Cn, 4 e n e 22) self-assembled monolayers (SAMs) on Au (111) surfaces. We studied the effects of chain length on the structural properties, including tilt and orientation angles, and on phase transition temperature. We found clear dependence of the structural properties, on both the number of carbon atoms, n; and on n being odd or even. Alkanethiols with n e 7 show liquid-like behavior and large rotational mobility, whereas those with n g 12 are well-ordered and stable. For 12 e n e 15, odd even effects are observed, where for n = odd, larger tilt angles, oriented in the direction of their next next nearest neighbor (NNNN), and for n = even, lower tilt angles, mostly tilted toward next nearest neighbor (NNN), were observed. For 15 e n e 19, we find tilt angle and orientation to be independent of n. For all alkanethiols, a gradual decrease of the tilt angle occurred by increasing the temperature from 300 to 420 K. Order disorder phase transitions occurred at a certain temperature. This was signified by abrupt instabilities in the tilt orientation angle. This transition temperature showed an enhancement of ∼67 100 °C over the melting point of the corresponding n-alkane bulk system. This enhancement depended on n, and was larger for n = odd. Overall, we found that odd alkanethiols show better structural and thermal stability, and smaller gauche defects.

1. INTRODUCTION Self-assembled monolayers (SAMs)1 16 can be grown from solutions, gas and vapor deposition. They form covalent bonds with various types of metallic substrates such as Au,1 4 Ag,5 Cu, Pt, Pd,6 Hg,7 and Si.8 SAMs have wide range of applications such as in lubrication, surface engineering, adhesion modification, micro/nanofabrication,7 biosensors,9 and microelectronics.10 13 A detailed understanding of SAM structural properties and thermal stability is crucial for their efficient application in these areas of science and technology. SAMs are formed from selfassembly of molecules consisting of a headgroup, a tailgroup, and a functional group. Alkanethiols, [CH3(CH2)n-1SH], are the simplest thiol-based SAMs consisting of a head, a thiol (SH) group with strong adsorption to the substrate; and a tail, an alkyl chain with varying number of carbon atoms where the methyl group acts as the functional terminal group. Here, we will denote them by Cn for brevity, n being the number of carbon atoms. Due to their ease of preparation and well-ordered structure, these monolayers have been widely studied to get insight into their many rich properties. In these studies, gold has often been used as a substrate due to its inertness, ease of patterning, and strong interaction energy with sulfur.17 Monolayers with sufficient packing density and molecular length form ordered structures where the backbones of the molecules are tilted in a certain direction.18 These structural features depend on many factors, such as the type and size of the individual molecules, substrate, packing density, and temperature. These structural properties have important effects on their many properties, such as wetting r 2011 American Chemical Society

and friction. Our studies presented here stem from our interest in their tribological properties. A variety of experimental methods, such as Raman scattering, low energy helium diffraction,19 X-ray diffraction (XRD), infrared spectroscopy, atomic force microscopy (AFM), scanning tunneling microscopy (STM),20 infrared spectroscopy,21 sum frequency generation spectroscopy,22 electron diffraction, and electrochemistry,23,24 have been used to study structure and interfacial properties of thiol-based SAMs. In a pioneering work, using infrared spectroscopy, Nuzzo and Korenic25 suggested a very similar structure for n-alkanethiol monolayers on gold, and crystalline bulk phase of n-alkanes. The tilt angle that is reported for alkanethiols of various lengths by the experiments varies between 25° and 33°.17,26 Despite the wealth of information obtained by diverse experimental methods, many structural features of alkanethiols still remain inconclusive and sometimes contradictory. Porter et al.1 found that the structure was dependent on the length, with a disordered structure for short chains (n < 12) and highly ordered long chains (n > 12). In contrast, using sum frequency generation spectroscopy, Nishi et al.22 reported, regardless of the length of molecules(4 e n e 18), an all trans configuration of molecules. Using XRD, Fenter et al.26 reported that the tilt angle for “short” alkanethiols (10 e n e 14) was larger compared with “long” ones (n > 14). They also Received: April 21, 2011 Revised: July 8, 2011 Published: July 12, 2011 9748

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Table 1. Parameters for Intramolecular and Intermolecular Interaction Potentials type of potential harmonic

bending

potential ϕ(r) = (1/2)K(rij

potential parameters

r0)2

ϕ(θ) = (1/2)Kθ(cos θ

interacting sites

b

cos θ0)2

r0 = 0.153a nm, K = 312 kcal/molÅ2

CH2 CH2,CH2 CH3

r0 = 0.182a nm

HS-CH2

r0 = 0.244c nm

HS-Au

θ0 = 114°d

CH2 CH2 CH2

Kθ = 520a kJ/mol

CH3 CH2 CH2 HS-CH2 CH2

dihedral

ϕ(R) = ∑5i Ci(cos R)i

C0 = 9.2789 (kJ/mol), C1 = 12.1557 (kJ/mol), C2 = 13.1201 (kJ/mol), C3 = C5 =

Lennard-Jones Potential ϕLJ(r) = 4ε[(σ/r)12 ϕshift, ϕshift = 4ε[(σ/rc)

12

(σ/r)6]

6

(σ/rc) ]

3.0597 (kJ/mol), C4 = 26.2403 (kJ/mol), 31.4950 (kJ/mol)

mSH = 33.073, σSH = 0.425 (nm), εSH = 114 K

HSa

mCH2 = 14.027, σCH2 = 0.393 (nm), εCH2 = 47.0 K

CH2d

mCH3 = 15.034, σCH3 = 0.393 (nm), εCH3 = 114.0 K

CH3d

σAu = 0.2655(nm), εAu = 990.0 K a

b

CH2 CH2 CH2 CH2 CH3 CH2 CH2 CH2 HS-CH2 CH2 CH2

c

d

Aue e

Adopted from refs. 8 and 42. Adopted from ref. 37. Adopted from ref. 44. Adopted from ref. 36. Adopted from ref. 43. Dihedral potential parameters are adopted from refs. 36 and 37.

reported that “short” molecules were tilted closer toward their nearest neighbor (NN), whereas “long” ones were closer toward their next nearest neighbor (NNN). Despite these studies on the dependence of the structure on the length, the odd even effects on the equilibrium structure and on the sensitivity to temperature and phase transition events have not been systematically explored. Having an odd or even number of carbon atoms is known to result in different crystallographic structure and properties for bulk alkane systems. Similar odd even effects on structure of SAMs have been reported.22,21 The odd even effects for various SAMs have been discussed in a recent review by Tao and Bernasek.21 There have been many experimental studies27,28 to relate their properties to odd even effects. Experiments by Wong et al.29 and molecular simulations by Mikulski et al.30 have reported lower friction coefficients for even alkanethiols compared to those of their odd counterparts. Odd even effects on surface wettability were also reported where higher wettability was observed for odd alkanethiols.27,31 Since SAMs are used in a variety of situations with varying temperatures, an understanding of the effect of temperature is essential. The effects of the chain length have been studied by many simulation and experimental works.4,32,33 Simulations have shown that, for C1542,33 and C12,4 the tilt angle (with respect to surface normal) decreases when the temperature is increased. XRD34 and electrochemistry35 experiments have been used to study phase transition of alkanethiol-Au SAMs. The nature of the transition is suggested to be gradual with initial reduction in the tilt angle, followed by melt transition. Solid solid transitions at intermediate temperatures are suggested to exist, before SAMs go through order disordered melt transition. Such order disorder transition temperature is suggested to be linearly dependent on n.35 For C12, the reported melt transition temperature is 323 K, although it is suggested that solid liquid phases coexist at 323 343 K.34 Since the original work of Hautman and Klein,8 molecular simulation methods have been widely used, as an alternative tool, to study SAMs at the molecular level. In this paper, we present our comprehensive molecular dynamics simulation results on structural characterization of alkanethiols on Au (111), where short and long, and even and odd alkanethiols have been

simulated to obtain insight on the effect of the chain length, carbon parity, and temperature on the tilt and tilt orientation angles. We have used the instability seen in the tilt orientation angle as a marker to detect the phase transition temperature for various alkanethiols in good agreement to those reported by experiments.34 Here, we will show that structure and thermal stability, and phase transition temperature, not only depend on the length of the molecule, but also on the number of carbon atoms being odd or even. We show the melt transition events are often preceded by sudden untilting of 5 6°. This is followed by further gradual untilting and appearance of full disorder in the system.

2. MODEL AND METHODOLOGY 2.1. Modeling the Alkanethiols and Gold Surfaces. Alkanethiol molecules are simulated using a united atom model in which groups of CH2, CH3, and SH are treated as single interaction sites. Intramolecular architecture including bond stretching, angle bending, and torsional potentials36 are included in the model. Some of the parameters for the intramolecular and intermolecular potentials, involving the alkyl part of the molecule, are from Siepmann et al.36,37 and are given in Table 1. These parameters have been shown to produce results in good agreement with experiments for liquid vapor coexistence curves for linear and branched alkanes.36 We have shown the capability of this united atom model in the studies of n-alkane systems crystallization,38,39 confinement induced phase transitions,40 and lubrication with gold surfaces.41 All the interactions between the atoms of different molecules, between the atoms which belong to the same molecule and are not interacting by any of the bond potentials, and between alkyl chain atoms and gold surface are governed by a shifted 6 12 Lennard-Jones potential which is truncated at a distance of rc = 1 nm. For the interaction of unlike groups, Lorentz Berthelot’s combining rules are used so εij = (εi εj)1/2 and σij = (σi +σj)/2. In our system, as shown in Figure 1a, a single SAM is attached to a gold substrate. Both implicit and explicit models of gold surface are often used in molecular dynamics simulations. Here, we have used an explicit model, which is suggested,42 to be the preferred method when preferential tilt direction is explored. 9749

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Figure 1. (a) SAM system set up for octadecanethiol (C18) monolayer attached on the gold substrate, (b) a dodecanethiol (C12) chain, (c) snapshot of the model Au(111) substrate used for simulation of docosanethiol (C22), and (d) hexagonal arrangement of sulfur atoms on the Au surface, which produces packing density of 21.6 Å2/chain. The arrows show three main tilt orientational direction of molecules where NN (solid lines), NNN (dotted lines), and NNNN (dashed line), are the nearest neighbor, next nearest neighbor, and next next nearest neighbor of a sulfur group.

The Au(111) substrate, shown in Figure 1c, is modeled by a close-packed FCC atomic structure. The surface shown here is used in the simulation of C22, and its dimensions are 8.655  8.955 nm2 in the lateral (X Y) directions, with 4 layers of fixed gold atoms in the Z direction (surface normal). Surface dimensions for other systems are listed in Table 2. The lattice constant of gold is 0.408 nm, and the nearest-neighbor distance is 0.288 nm. The initial configuration of the molecules started as upright, all trans, with each sulfur group attached to a threefold lattice site on the gold surface via a harmonic potential with an equilibrium distance of r0 = 0.244 nm. The simulations have shown the choice of the location for the sulfur atom, being above a Au atom or on the threefold site, has only a little effect on the results, and this is only apparent for T < 250 K.33 The monolayer molecules on the gold substrate √ had√ their sulfur head groups arranged in a hexagonal ( 3  3)R30° structure relative to the underlying Au(111) lattice. This produces a packing density of 21.6 Å2/ chain and a distance of ∼0.5 nm between the grafting positions. This structure corresponds to a fully saturated coverage and maximum packing density, and has been reported by using several different experimental methods.25,19 Following the original work of Mar and Klein,42 in our model Au-SH CH2 was not

subjected to the bending potential. Simulations33 have shown that using an explicit atomic model for the gold surface mitigates the effect of using a flexible or rigid Au-SH CH2 interaction. Periodic boundary conditions were applied to the system in the X and Y directions. A vacuum space, 1.2 times the extended length of the molecule in height, was allowed on top of the monolayer. The interaction parameters of Au are chosen by fitting the calculated and experimental desorption data of alkanes from metal surfaces.43 These values are (εw/kB = 990 K) and σw = 0.2655 nm, which yield energy and length parameters for the interaction of CH2 and Au of εwf = 1.795 kJ/mol = 4.59εCH2 and σwf = 0.328 nm. Parameters for the potential model used here are given in Table 1 and are adopted from the literature.8,36,37,42 44 Initial random velocities for individual atoms were assigned to give a Maxwell Boltzmann distribution corresponding to the target temperature which was kept constant, by rescaling the velocities, at 300 K unless it was stated otherwise. The equations of motion were integrated using the velocity Verlet algorithm with a time step of 2.35 fs. A domain decomposition, parallel algorithm,45 which included link cell and neighbor list methods, was used in order to reduce the computational time. We studied the behavior of thiol based self-assembled monolayers on gold surfaces for different chain lengths varying from 9750

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Table 2. System size, Packing Density, Calculated Tilt (θ) and Tilt Orientation (u) Angles for Various Simulated Alkanethiol Systems on Au(111) Surface at T = 300 K orientation angle, j (deg) number of

number of

X-dimension of

Y-dimension of

total number of

system

sam molecules

gold atoms

the substrate (nm)

the substrate (nm)

C4

120

1440

5.193

C7

168

2016

6.059

C12

224

2688

C13

240

C14

270

C15

tilt angle

with respect to the next

united atoms

packing density
2/chain Å

θ (deg)

nearest neighbor (NNN)

4.997

2040

21.624

19.91 ( 0.69

-

5.996

3360

21.624

21.83 ( 0.71

-

6.9239

6.996

5600

21.624

26.35 ( 0.32

3.22 ( 2.02

2880

6.924

7.495

6000

21.624

30.09 ( 0.24

9.52 ( 2.20

3240

7.790

7.495

7020

21.624

26.98 ( 0.25

2.59 ( 1.78

270

3240

7.790

7.495

7560

21.624

30.06 ( 0.27

9.98 ( 0.59

C16

288

3456

7.790

7.995

8352

21.624

30.52 ( 0. 24

9.71 ( 0.63

C17 C18

288 288

3456 3456

7.790 7.790

7.995 7.995

8640 8928

21.624 21.624

30.64 ( 0.16 30.81 ( 0.22

12.52 ( 0.77 9.79 ( 0.68

C19

306

3672

7.790

8.495

9792

21.624

30.86 ( 0.15

9.23 ( 0.78

C22

360

4320

8.655

8.995

12600

21.624

30.49 ( 0.17

24.10 ( 0.81

C4 to C22. Depending on the simulated alkanethiol-Au system, the total number of united atoms ranged from 2040 to 12 600 (see Table 2). 2.2. Structural Properties. The structure of a well-ordered SAM is often characterized by two parameters, tilt angle, which is the angle of the molecular backbone with respect to the substrate normal, and the tilt orientation angle, which is the angle between the projection of the molecular backbone onto the substrate and an arbitrary direction. For molecules which are in all trans configuration, a simple end-to-end vector can be used to describe the molecular backbone. When the molecules show gauche defects or are disordered, as would be the case for some systems particularly at higher temperatures, an inertially equivalent ellipsoid would be more appropriate to obtain the structural orientation of individual molecules. We have used the unit vector along the longest axis of an inertially equivalent ellipsoid45,46 to calculate the orientation of each molecule. The schematic in Figure 2 shows the inertially equivalent ellipsoid and its semiaxes a, b, and c, tilt angle (θ), and tilt orientation angle (j). As shown in Figure 2, the tilt angle is defined as the angle between this unit vector and the Z-axis, normal to the substrate. The tilt orientation angle (j) is defined as the angle between the projection of this unit vector on the XY plane and an arbitrary direction. Here, we have calculated j with respect to the [100] (X) direction. Since the Au(111) surface has sixfold symmetry (60° symmetry), the possible orientation of the molecules, relative to the underlying gold lattice, can be represented with respect to the neighboring sulfur groups. This is depicted in Figure 1d where the possible orientations of the molecules are shown by NN ((30°, (90°, (150°), NNN (0°, (60°, (120°, and (180°), and NNNN (15°, 75°, 135°, 195°, 255°, 315°) directions, which, respectively are the nearest neighbor, next nearest neighbor, and next next nearest neighbor of a sulfur group. In the experiments using X-ray diffraction, the angles which are shown in italic face are not detectable, and this makes obtaining full crystallographic data impossible.26 However, our simulations can obtain full configurational data for the tilt orientation angle. The tilt orientation angle, j, often is measured with respect to NNN or NN directions. With NNN direction taken as the reference, j should be between 0 (NNN) and 30° (NN direction). Such measurement essentially shows the structure with respect to the underlying Au(111) surface,

Figure 2. Schematic representation of a tilted alkanethiol chain where θ is the tilt angle and j is the orientation angle; a, b, and c are the principal semiaxes of an inertially equivalent ellipsoid.

taking into account its sixfold symmetry. The measurements with respect to X [100] direction calculated in the simulations are useful to reveal the structural stability and transitional events to which experiments could be oblivious due to inherent symmetry of the gold surface. Here, we will present our results for tilt orientation angle, j, using both measurement schemes. 2.3. Preparing the SAM Layer and Equilibrium Simulations. The initial configuration of the molecules at the startup was all trans with their backbone normal to the surface. For all alkanethiol systems, the packing density was the same (21.6 Å2 per chain) and temperature was kept constant at 300 K. Each simulation was carried out for 2 ns to yield equilibrium. The equilibrated configuration was then used as the starting point and sampling was performed for another 1.9 ns. Snapshots of the equilibrated structures of C4, C12, and C19 on Au (111) obtained at a temperature of 300 K (Figure 3) show longer alkanethiols, such as C12 and C19, attain a more ordered configuration compared to that of shorter chains, such as C4. This chain length dependence behavior of the monolayers shows the importance of van der Waals interactions between the carbon atoms in the chain. Experiments47 have shown that alkanethiol SAMs with n > 6 are polycrystalline and C4 has a disordered and liquid structure. Such observations are consistent with the results of our simulations. As shown in Figure 3, systems with sufficiently long molecules display tilting of the chains in a certain direction. We will discuss the detailed structural differences that arise from the molecular size of alkanethiol SAMs in the following sections. 9751

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Figure 3. Snapshots of alkanethiol self-assembled monolayers attached on Au(111) surface and equilibrated at 300 K, shown for (a) C4, (b) C12, and (c) C19.

Figure 4. Average tilt angle (θ) versus the number of carbon atoms in the chain, n, for alkanethiol SAMs on Au (111) at T = 300 K. The line is only a guide for the eye. The open symbols are the results of our simulations and the filled symbols are the experimental results reported by Fenter et al.26 for some even alkanethiols.

3. RESULTS AND DISCUSSION 3.1. Effect of Chain Length on Tilt Angle. The average tilt angle (θ) calculated during equilibrium stage versus the number of carbon atoms is plotted in Figure 4. We have shown the experimental results by Fenter et al.26 for some of the even systems on the same plot. The calculated average tilt angles for C4 and C7 are 19.91° and 21.83°. We calculated tilt angles of 26.35°, 30.09°, 26.98°, 30.06°, 30.52°, 30.64°, 30.81°, 30.86°, and 30.49°, respectively, for C12, C13, C14, C15, C16, C17, C18, C19, and C22. We find three regimes here, short chains (n e 7), with significantly lower average tilt angle, midlength chains (12 e n e 15), with the tilt angle varying by up to ∼3° depending on the chain being odd or even, and finally long chains (15 e n e 22), where tilt angle shows no significant dependence on n, or it being odd or even. Chain length dependent structural variation arises from different contribution of van der Waals interactions in the monolayer due to different numbers of carbon atoms. For longer chains, the contributions from interactions among different chains are larger than those due to interactions between the headgroup and the substrate, which remains constant regardless of the chain size. On the other hand, for short chains (here, n e 7), intermolecular interactions between chains are less dominant than the interactions between the headgroup and the substrate.15 For C4, experiments have reported a 2D liquid structure.47 This is consistent with our simulation results here. At room temperature, the midweight n-alkanes (n < 17) are

Figure 5. Probability distribution of the tilt angle (θ) for various chain lengths from C4 to C22 for equilibrated monolayers on Au (111) at T = 300 K.

liquid whereas higher molecular weight alkanes are in a solid state. As the chain length (number of carbon atoms) increases, the melting point of the alkanes gradually increases. The melting point of dodecane, for example, is 264 K, meaning that the system is in a liquid state at room temperature, whereas the melting point for C19H40 is much higher at ∼305 K. The alkanethiols simulated here are expected to have a much higher melting point compared to that of their bulk alkane counterpart.18 This is due to anchoring of the head groups to the substrate that stabilizes a crystalline structure. However, we expect them to follow the same trend, where the melting point is increased with the length of molecules. We will explore this in more detail in section 3.4. For C12, our calculated tilt angle is in agreement with some simulation and experimental studies which found the tilt angle to be in the range of 24 ( 4°.3,32,47 Although for C12 our calculated tilt angle is lower than 33.7°, which is reported by Fenter et al.,26 for n g 16, our results show excellent agreement with their experimental results (see Figure 4). It is interesting to see a similar transitional regime at n = 15, in apparent agreement with “short” and “long” chain regimes reported in experiments by Fenter et al.26 To investigate the uniformity of structural and conformational properties, distribution of the tilt angle was calculated for various systems. This information is useful to determine whether the molecules form a single domain, with uniform tilt angle, or there are multiple domains present in the structure. In Figure 5, we have plotted the normalized probability distribution of the tilt angle (θ) for all alkanethiols in the range of C4 to C22. Broader distribution for C4 (between 0 and 49°), and C7 (8 34°) are evidence of more disordered state. As the chain length increases, the distribution become narrower reaching 26 40° for C22, and 9752

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Figure 6. Odd even effect on the tilt angle of an idealized all trans single alkanethiol molecule in a monolayer on Au (111) due to change in the orientation of the terminal group. Here, θ1 and θ2 are tilt angles for, respectively, the even and odd alkanethiol, and θ2 > θ1.

the peak probability also increases, all indicating a more uniform structure for longer molecules. Such differences cannot be immediately inferred from the average tilt angle shown in Figure 4. For system sizes examined here and for chains which show a strong order (C12 and longer), we do not see any evidence of multiple domains. Here, we can also see the markers of odd even effects for 12 e n e 15, where the distribution for even systems is shifted slightly toward lower angles. 3.1.1. Odd even Effect on the Tilt Angle. The odd even effect seen in Figure 4, in the range of C12 to C15, is consistent with odd even effects reported by the experiments.21 We can explain this effect from a geometrical point of view. As shown in Figure 6, for an ideal all trans configuration, and S Au bond remaining almost perpendicular, the end-to-end line for even number of carbons (n) produces a smaller tilt angle compared with a chain with odd number of carbons (n+1). According to this simplified picture, the structural difference seen for chains with odd and even number of carbon atoms is a result of variation in the orientation of the terminal C C bond in the molecules due to the steric constraints and end groups interactions.18,21 The terminal C C bond in the alkyl chain, with an even number of carbons, is almost perpendicular to the gold substrate, whereas for an alkanethiol with odd number of carbons, it is considerably tilted away from the surface normal. As a consequence, odd alkanethiols have slightly larger tilt angle than that of their even counterpart of comparable length. Although the difference seen in the tilt angle is small, ∼ ( 3°, such subtle differences in the tilt angle and odd even effect have been shown to result in significant differences in the wettability27,31 and tribology.29 The extra exposure of the terminal methyl group is suggested to be the reason for higher wettability27,48 observed for odd alkanethiols, seen with some liquids. The friction force is also reported to be larger for odd alkanethiols.29 Such odd even effects are analogues to the differences reported for crystallographic structure of alkanes with odd and even number of carbons, 2D structures of chiral molecules, and polymers such as nylon.22,49 51 In our results, the odd even effect disappears for n > 15; this we speculate is due to the fact that the chain is not in an ideal all trans configuration, and gauche defects and chain twisting distort this simple picture. As the chain size increases, this becomes more exacerbated. We will investigate these in section 3.3.

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Figure 7. Calculated (open symbols) average tilt orientation angle (j) as a function n, shown for various chain lengths in the range of C12 to C22; the orientation angle is shown in the range of 0° (NNN direction) and 30° (NN direction); the filled symbols are the experimental results reported by Fenter et al.26

3.2. Effect of Chain Length on Tilt Orientation Angle (u). We calculated average orientation angle as a function of time for alkanethiols C4 C22. To obtain information about the stability, the tilt orientation angle was monitored in the range of (180° with respect to the X axis. We found a fluid behavior for the shorter alkanethiols (n e 7) compared to the longer ones. For C4 and C7, we found that the orientation angle fluctuates by large values indicating a more fluid state for the monolayer. For C12 and longer chains, the orientation angle was very steady during the simulation time indicating a more rigid state than seen for shorter alkanethiols. In general, for short alkanethiols (C7 and shorter), the structural analysis shows a smaller degree of uniformity in the orientation angle and significant rotational mobility. Experimental studies on the effect of chain length on the orientation angle (j) are scarce. However, Fenter and co-workers26 studied this for a few even alkanethiols (C10 C30) and found tilt orientation variable from NN to NNN direction, depending on the size of the molecule being “short” or “long”. To compare our results with theirs, in Figure 7 the time averaged orientation angle is plotted as a function of n, the number of carbon atoms. Here, we have used a 0 30° scale, with NNN and NN directions (see Figure 1d) forming the limits of our measurement, as explained in section 2.2. On the same plot, we have also shown the experimental results of Fenter et al.,26 for a few even alkanethiol-Au systems. As discussed before, C4 and C7 show large fluctuations in the orientation angle. For the shorter molecules C4 and C7, the orientation angle distribution (Figure 9) indicates a flat distribution, as another indication of the rotational mobility of these shorter alkanethiols. Thus, C4 and C7 do not show any preferential tilt direction, and have not been included in Figure 7. For 12 e n e 15, interestingly we can see distinct evidence of odd even effect, where odd alkanethiols show preferential tilt closer to NNNN direction, and even ones tilted closer to NNN direction. In Figure 7, we find good agreement with the experimental results for even alkanethiols C16 and C18. However, for C12, C14, and C22 there is a clear difference. For C22, the 9753

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Figure 8. Snapshots of equilibrated C16 and C15 systems are shown from the top view. The value shown for j is the average value for the snapshot shown here.

calculated tilt orientation angle of ∼24.1° does not agree with experimental result of Fenter et al.26 The ensemble averages of the tilt orientation angle for all alkanethiols are listed in Table 2. The snapshots in Figure 8 show the configuration of the monolayer from the top for two alkanethiol systems. Figure 9 shows the distribution of the tilt orientation angle (j) obtained from our MD simulations for alkanethiols C4 C22. The orientation angle is shown in a 0 30° range. Here, a flat distribution for C4 and C7 is a signature of their fluid behavior. For n g 12, however, a clear preferential direction can be seen. In the range C12 C15, we see the signature of odd even effect where for even systems the peak is shifted closer to NNN direction. For C13 and other systems in the range of 15 e n e 19, the peak is away from the NNN direction by about ∼10°. 3.3. Odd Even Effects on Gauche Defects and Trans Percentage. Figure 10 shows the probability distribution of the torsional (dihedral) angle, obtained from our MD simulation for monolayers with different chain lengths in the range C4 C19. Torsional angle represents the rotation of the bonds from trans position. C4 and C7 clearly show larger probability of gauche defects as another indication of their mobile and fluid nature. In Figure 10, the distribution around the gauche angle, 120°, is magnified for C12 and longer alkanethiols. Two even alkanethiols C12 and C14 can be seen to show larger gauche probability. The overall gauche and trans percentage of the torsional angles versus the number of carbon atoms are presented in Figure 11. We define trans and gauche as the torsion angle being within (60° of, respectively, 0° and (120°. We can see from Figure 11 that, by increasing the chain length from C4 to C7, the gauche percentage drops from ∼24% to ∼12%. It drops further to ∼4% when the number of carbon atoms is increased to 12. From here, for 12 e n e 15, we can see a zigzag behavior where the odd alkanethiols show slightly lower percentage of gauche defects compared to the even alkanethiols. We can see, for n > 15, there is no odd even effect or substantial dependence on the length. Figure 12 gives us a deeper insight into the structural defects within the individual molecules, correlations with the molecular length, and odd even effects. Here, the percentage of gauche defects in conformation of torsional angles along the molecule is given for C12 C19. We can deduce a striking pattern of odd even effects from these results: (1) The gauche defects along the molecule generally decreases with increasing the length. (2) For all systems, defects are larger near the head

Figure 9. Probability distribution of the tilt orientation angle (j) for various chain lengths from C4 to C22 for equilibrated systems attached on Au (111). The orientation angle is shown in the range of 0° (NNN direction) and 30° (NN direction). The flat distributions are for C4 and C7.

and end groups, with most being concentrated near the free end of the molecules. (3) The last dihedral angle for the even alkanethiols has significantly larger defects, almost twice as much as those observed for the odd systems. For odd systems, almost the same amount of defects is shared by the last two dihedrals. On the other hand for even systems, defects in the second last dihedral are much smaller than the odd ones. This explains why the global gauche defects (Figure 11) for n > 15 show no indication of odd even effects. In terms of geometrical packing, odd n-alkanethiol systems here are equivalent to even n-alkane systems. So, our observations here are consistent with experimental findings,51 where higher order was reported for even n-alkane water interfaces. 3.4. Effect of Temperature on Thiol Based Monolayers Attached on Au (111). 3.4.1. Effect of Temperature on the Structure. To study the effects of the temperature on the structure and phase transition of the monolayers, the equilibrated configuration at 300 K for each alkanethiol-Au system was used as the 9754

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Figure 10. Probability distribution of torsional angle for various alkanethiols on Au(111) at T = 300 K. The inset shows the magnified gauche (120°) region for odd (open symbols) and even (filled symbols) alkanethiols, C12 C19, where noticeably larger gauche probability can be seen for even alkanethiols C12 and C14.

Figure 11. Gauche and trans percentage of the torsional angles versus the number of carbon atoms, for alkanethiols of different chain length attached on Au(111). Note the odd even effect for 12 e n e 15 with larger trans percentage seen for odd alkanethiols.

starting point, and the temperature was gradually increased to 420 K. MD simulations were carried out for 8.7 ns as the temperature was raised at a rate of 1 K per 47 ps from 300 to 420 K. The experimental works suggest that desorption of the chains from the surface takes place at T > 423 K,18 which is above the range we examined here. So, this will be consistent with our model, which does not allow for desorption of the molecules from the surface. The structural properties of interest including the tilt orientation and tilt angles were calculated during the heating period and after temperature reached 420 K. 3.4.2. Phase Transitions and Tilt Orientation Angle (j). To gain insight into the structural variations of the SAMs, in response to the heating, the tilt and orientation angles during heating time for some of the alkanethiol systems are plotted in Figure 13. The temperature profile, showing the ramp-up, during the simulation time is also shown on the same graph for each case. We can see, as the temperature rises, the tilt orientation

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Figure 12. Percentage of gauche defects in conformation of torsional angles along the molecule. The last carbon atom involved in each dihedral angle is represented by Natom, the position of the carbon atoms along the backbone. So the first dihedral angle involves the sulfur group at one end and the third carbon group at the other end. The results are shown for C12 C19.

angle j initially remains almost stable. However, at a certain temperature there is an abrupt variation in the tilt orientation angle followed by large fluctuations. We can detect several interesting features from these graphs: (1) For most systems, the tilt angle generally decreases with increasing the temperature to 420 K. (2) For all systems, at a certain temperature, tilt orientation angle exhibits sudden instability. (3) For most systems, the sudden instability in the tilt orientation angle j coincides with a sharp reduction in the tilt angle, by as much as ∼5 6°. The sharp changes seen in tilt orientation angle are either due to solid solid transition events or due to a melt phase transition. In most cases, the solid solid transition events stay stable for only a brief period, and then at higher temperatures are followed by large instabilities seen in j, as an indication of a melt transition. For C14, we see a solid solid transition with a jump in the tilt angle at T ∼307 K. To verify that these instabilities seen for j are true melt transition events and to accurately detect melt transition temperature (Tt), we have calculated the normalized distribution of tilt orientation angle at 2.5 K intervals, for all systems. An ordered solid phase would show as a peak in the distribution of the tilt orientation angle (similar to Figure 9), whereas a disordered phase will have a flat distribution signature, similar to those we observed for C4 and C7 (Figure 9). We have displayed these for C14 and C15 in Figure 14, for three typical temperatures. For C14, these are shown for T = 305 K, where the system is still solid and the distributions show that the tilt orientation angle has preserved NNN preferential direction. At T = 330 K, due to solid solid transitions the preferred tilt direction is closer to the NNNN direction. The beginning of this solid solid transition is at a temperature of ∼307 K. Note the sharp increase, of about ∼3° (Figure 13), in the tilt angle at this solid solid transition point. At T = 357.5 K, the distribution is flat indicating a disordered structure. For C15, at T = 305 and 330 K, the system is still solid, and although distribution is slightly broader at 330 K, 9755

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Figure 13. Tilt (θ) and tilt orientation angle (j) with respect to X axis, during the heating time, as the temperature is ramped up from 300 to 420 K, the results here are shown for various alkanethiols on Au (111). The onset of instabilities in j, and sudden drop in θ, marks the melt transition point.

they both show almost the same orientational directions close to NNNN. Using distribution plots and the onset of the fluctuations in the tilt orientation angle, we have estimated the melt transition temperature for all alkanethiol systems. This transition temperature is usually only a few degrees Celsius from

the point of sharp instability seen for the tilt orientation angle. For C15, our estimated melt transition temperature is Tt ∼370 K. This is very close to the solid solid transition at ∼370 K and melt transition point at ∼390 K, that was reported by earlier simulations of Mar and Klein,42 using an all atom SAM model. 9756

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Figure 15. Melt transition temperatures (Tt) for alkanethiol monolayers on gold, and the experimental52 melting point (Tm) of bulk n-alkanes versus n, the number of carbon atoms. The inset shows the enhancement in the melt transition points, ΔT = Tm Tt, against the number of carbon atoms, with apparent odd even effects.

Figure 14. Normalized probability distribution of the tilt orientation angle, j, for (a) C14 and (b) C15 systems, at three different temperatures. For C14, the shift in the peaks shows NNN to NNNN solid solid phase transitions (see also Figure 13). The flat distribution is an indication of melt transition.

In Figure15, the transition temperature Tt for alkanethiols of different length has been plotted versus the number of carbon atoms. It is clear that Tt increases with the chain length. This is analogous to the chain length dependence of the melting point for bulk n-alkanes.52 The melting point of bulk alkanes, with the same number of carbon atoms as of those alkanethiols examined here, is between 264 and 308 K. In Figure 15, we have also included the melting point of these bulk alkanes,52 as a function of the number of carbon atoms. We can clearly see, as expected, that the SAMs phase transition point is substantially higher than that of their alkane counterpart. Experimental work by Fenter et al.34 suggests that for alkanethiols on Au(111) the melting transition temperature is ∼60 K above that of their n-alkane counterpart. They report for dodecanethiol (C12) a melt

transition temperature of ∼323 K, which is in a good agreement with our simulation result of ∼330 K. This is within ∼7 K of their result, and is consistent with our model accuracy in prediction of the melting point of n-alkanes.38 In the inset of Figure 15, ΔT, the difference between the melt transition temperature of the alkanethiols and that of their n-alkane counterpart, is plotted against the number of carbon atoms. Clearly, ΔT is not a constant value and depends on the length of the alkanethiol molecule. C12 and C14 show the smallest enhancement. Depending on the length of the molecule, the enhancement in the melting temperature varies from ∼67 to ∼100 K. It is striking to also see clear evidence of odd even effects where odd alkanethiols show larger enhancement, by up to ∼16 K, in their melt transition temperature. For example, for C16 the enhancement ΔT is ∼84 K, whereas for C17, this is ∼100 K. We calculate the largest enhancement of ∼100 K for C17 and C19, both odd alkanethiols. The length dependence behavior of Tt for alkanethiols has striking similarity to that of n-alkane systems. Both Tt and Tm mostly increase with n; and in both cases, sharper increases take place at every other n. 3.4.3. Critical Tilt Angle and Phase Transition. For all systems shown in Figure 13, the molecules responded to heating by taking up a more upright position with respect to the surface. The sharp variations in the tilt angle often coincide with similar sharp transitions in the orientation angle. Such observations indicate that these two structural parameters are not independent, and geometrical constraints and packing requirements create interdependency pathways. To quantify how the variation in tilt angle is linked to the melt transition temperature, we define θc as the critical tilt angle at melt transition temperature. In Figure 16, we have plotted θc against n, the number of carbon atoms. We have also shown the reduction in the tilt angle (Δθc = θ300K θc) due to the rise of temperature from 300 K to Tt, melt transition temperature. Two interesting results emerge from these data in Figure 16: (1) The critical tilt angle, θc, at which the melt transition take place generally decreases with the length of molecules. That is, larger molecules can preserve their collective order even 9757

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Figure 16. The critical tilt angle, θc, (filled symbols) at which order disorder phase transition takes place, and the decrease in the tilt angle, Δθc = θ300 θc (open symbols), due to temperature increase from 300 to Tt, versus n, the number of carbon atoms.

at smaller tilt angles. (2) The variation in the tilt angle, Δθc, depends on the molecule being “short” or “long”, with larger Δθc observed for “long” molecules. Odd even effect shows itself with mostly larger Δθc, for odd alkanethiols, within each regime. This implies that most odd alkanethiols can vary their tilt angle over a larger range, without losing their ordered structure. In the range C12 C19, while higher molecular weight alkanethiols display better thermal stability compared with those of lower molecular weight, there is clear evidence that odd alkanethiols show more resilience to temperature induced melt transition.

4. CONCLUSIONS Molecular dynamics simulations of thiol based SAMs were carried out to obtain a better understanding about their structural properties as a function of molecular length and temperature. The simulation results showed clear correlations between the number of carbon atoms and the tilt angle. For shorter alkanethiols (n e 7), there were large fluctuations in the collective tilt and orientation angles; and on average, the tilt angle was smaller. A broad distribution of the tilt angle seen for C4 and C7 combined with their lack of tilt preferential direction and large gauche defects were all evidence of their mobile and fluid nature. With increasing length of the molecules, the chains adopted a well-ordered structure. There was also clear evidence of odd even effects on many properties of the monolayer. For alkanethiols with 15 g n g 12, there were small variations (∼2 3°) due to odd even effects, where slightly larger tilt angles were observed for odd alkanethiols. Odd alkanethiols showed a preferential tilt orientation angle, closer to NNNN direction at constant temperature of 300 K, whereas even alkanethiols showed a preferential tilt toward closer to NNN direction. In the same range of 15 g n g 12, odd even effects were also observed for gauche defects, which were lower for odd alkanethiols. In the range of 15 g n g 19, tilt and orientation angles were independent of n. We found that structural gauche defects for all even alkanethiols were concentrated at the first dihedral on the free end of the molecules, whereas for odd

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systems, these were shared by the first two dihedrals and were much smaller. Consistent with earlier simulations, our study showed that, by increasing the temperature, the molecules adopt a lower tilt angle. The nature of tilt reduction was a combination of almost linear reduction with temperature followed by a sharp drop. This sharp change in the tilt angle often led to a different structural order, or complete disorder. Odd alkanethiols displayed lower sensitivity to temperature in comparison to even ones. The instabilities and distribution of the tilt orientation angle were used as markers for detecting the phase transition temperature, which was shown to be in a very good agreement with the experimental results for C12. This transitional temperature, which was increased with the length of the SAMs molecules, was ∼67 100 K higher than that of the melting point of the corresponding bulk n-alkane system. Such an enhancement, however, was larger for odd alkanethiols, by up to ∼16 K, compared to even ones. This behavior was consistent with the lower gauche defects observed for odd alkanethiols. The findings presented here have important potential applications, where an ordered stable structure under varying thermal conditions is required. For such applications, our results show that odd alkanethiols offer better thermal and structural stability.

’ AUTHOR INFORMATION Corresponding Author

*E-mail address: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the support of this study by an Australian Research Council Discovery Project grant and an Australian Postgraduate Award (APA) scholarship for the first author. We also thank the Australian Centre of Advanced Computing and Communications and also Australian National Computational Infrastructure Facility for the generous time allocated for computing. ’ REFERENCES (1) Porter, M. D.; Bright, T. B; Allara, D. L; Chidsey, C. E. D J. Am. Chem. Soc. 1987, 109, 3559–3573. (2) Jiang, S.-Y. Mol. Phys. 2002, 100, 2261–2275. (3) Zhou, J.-H.; Zhu, R.-X.; Shi, L.-W.; Zhang, T.; Chen, M.-B. Chin. J. Chem. 2007, 25, 1474–1479. (4) Zhang, L.; Goddard, W. A., III J. Chem. Phys. 2002, 117, 7342. (5) Bhushan, B., Ed. (2005) Nanotribology and Nanomechanics, Springer, Berlin. (6) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498–1511. (7) Witt, D.; Klajn, R. Curr. Org. Chem. 2004, 8, 1763–1797. (8) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91 (8), 4994–5001. (9) Lingyan, L.; Shengfu, C.; Shaoyi, J. Langmuir 2003, 19, 666–671. (10) Curry, A. J. E. and Sungsoo, K. (2004) Encyclopedia of Nanoscience and Nanotechnology, Dekker. (11) Srivastava, P.; Chapman, W. G.; Laibinis, P. E. J. Phys. Chem. B 2009, 113, 456–464. (12) Birdi, K. S. Self-Assembly Monolayer Structures of Lipids and Macromolecules at Interfaces; Kluwer Academic/Plenum Publishers: New York, 1999. (13) Archer, A. J. Phys. Chem. B 2003, 107, 13123–13132. (14) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (15) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1608–1171. (16) Schreiber, F. J. Phys.: Condens. Matter 2004, 6, 881–900. 9758

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