Coexistence of Alkylated Sulfide Molecules along Two Orthogonal

Article pubs.acs.org/JPCC

Coexistence of Alkylated Sulfide Molecules along Two Orthogonal Directions of Graphite Lattice Masahiro Hibino*,† and Hiroshi Tsuchiya‡ †

Division of Applied Science, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan

‡

ABSTRACT: Two coexisting types of molecular arrangements in self-assembled alkylated sulfide monolayers adsorbed on a graphite surface have been studied by scanning tunneling microscopy (STM). Longer (hexadecyl and octadecyl) sulfide molecules showed two types of arrangements along the (1120̅ ) zigzag and (1010̅ ) armchair directions of the graphite lattice, while the shorter dodecyl sulfide molecules lay along the zigzag direction adopted by previously studied n-alkanes and their functionalized derivatives. STM observations showed that the molecules along the zigzag direction participate in more favorable interactions with the graphite surface when compared to those along the armchair direction. The coexistence of two types of arrangements enables us to roughly estimate the free energy for adsorption and the interaction energy between the different arrangements. In both types of arrangements, the free energy changes for molecular adsorption need to exceed a threshold value for monolayer formation on the graphite surface.

1. INTRODUCTION Controlling the bottom-up assembly of molecular arrays adsorbed on solid surfaces to form ordered nanoscale patterns is of great importance and considerable interest from the viewpoint of future advances within the fields of nanoscience and nanotechnology. Information on the precise configurations of molecular devices based on nanomaterials is essential for a fundamental understanding of the interactions governing selforganization in two-dimensional molecular systems. Scanning tunneling microscopy (STM) is an effective structural characterization tool for molecular self-assembled monolayers (SAMs) at a variety of surfaces and interfaces. Extensive studies on the self-assembly of n-alkanes and functionalized alkane derivatives adsorbed on graphite have already been conducted as ideal model systems.1−20 The domains in SAMs composed of n-alkanes and their derivatives are formed by well-ordered lamellae that consist of close-packed molecules oriented parallel to the graphite surface normal or at some angle to the lamellar boundary with their carbon skeleton extended linearly. The lamellae are formed by a combination of intermolecular and molecule−surface interactions resulting from the van der Waals forces associated with the alkyl chains. The alkyl chains in the lamellae are known to orient along the (112̅0) zigzag direction of the graphite lattice because the distance between methylene units in the alkyl chain and between the second nearest neighbors of graphite are nearly equal. The molecular orientation is further supported by theoretical studies: the monolayers with alkyl chains along the zigzag direction of the graphite lattice have much lower formation energies than those along the (101̅0) armchair direction.21,22 The free energy difference between molecular arrangements along the zigzag and armchair directions of the graphite lattice © 2013 American Chemical Society

is one of the major factors governing their arrangements on graphite. Considering that two types of arrangements in the monolayer appear on the same graphite surface, the orientational entropy and interaction energy between the arrangements will increase. In a situation in which the influence of the interaction energy cannot be ignored, it is possible that arrangements along the armchair direction may occur with molecules containing long alkyl chains. However, alkyl chains oriented along the armchair direction have not been observed in the monolayers of long-chain alkanes and their derivatives on graphite until now. In contrast to alkanes, the polymer backbones of poly(ε-caprolactam: nylon-6) in epitaxial layers on graphite have been observed to orient along the armchair direction of the graphite lattice.23 One reason for the lack of an armchair directional arrangement for long-chain alkanes/derivatives on graphite could be that it is a rare arrangement: the surface coverage of the arrangement is much smaller than that of the well-known zigzag arrangement (possibly less than 1%). For the detailed observation of molecular arrangements, a molecular marker indicating the direction of a lamellar structure is necessary to allow for easy detection. Sulfur atoms in alkane derivatives adsorbed on graphite have been reported to appear much brighter than the alkyl chains in STM images13−20 and are similar in size and conformation to the methylene units in the alkyl chain. Therefore, sulfur atoms serving as markers have allowed for an improved understanding of the characteristics of the assembled structure.18−20 Additionally, the direction of the lamellar structure formed by alkylated sulfide molecules on Received: July 10, 2013 Revised: October 8, 2013 Published: December 23, 2013 1484

dx.doi.org/10.1021/jp406796f | J. Phys. Chem. C 2014, 118, 1484−1491

The Journal of Physical Chemistry C

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graphite coincides with the direction of aligned bright spots, which corresponds to the sulfur atoms and allows for easy detection. The quantitative observation of a new predictable arrangement will allow for the free energy change of the adsorption process to be estimated on the corresponding selfassembled arrangement. In this paper, we study the molecular arrangements in the SAMs of dodecyl, hexadecyl, and octadecyl sulfides adsorbed at the interface between an organic solution and graphite surface using STM. We also present the coexistence of molecular arrangements along the zigzag and armchair directions of the graphite lattice in monolayers of hexadecyl and octadecyl sulfide molecules. The estimated free energy for adsorption provides a quantitative basis for the self-assembly process of molecules on graphite. These results can provide valuable information for the further design and control of selfassemblies.

2. EXPERIMENTAL METHOD Dodecyl sulfide ((CH 3 (CH 2 ) 11 ) 2 S), hexadecyl sulfide ((CH3(CH2)15)2S), and octadecyl sulfide ((CH3(CH2)17)2S) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used without further purification. Solutions of alkylated sulfide were prepared by dissolving it in phenyloctane (Sigma-Aldrich. Co., LLC, Milwaukee, WI) at concentrations near saturation. Monolayers were prepared by applying the solution to the basal plane of freshly cleaved, highly oriented pyrolytic graphite (HOPG, ZYB grade, Advanced Ceramics Co., Cleveland, OH). Mechanically sharpened Pt−Ir (80/20) wires were used as the STM tips. To image the monolayer, the tip was immersed in the solution and the liquid−solid interface was scanned using a Nanoscope II (Digital Instruments, Santa Barbara, CA) equipped with a 14.6 μm × 14.6 μm scan head. STM images were obtained in the constant-current mode under ambient conditions. The typical operating conditions were as follows: 0.70−0.85 nA current, 1.2−1.3 V bias voltage (sample positive), and 19.5 Hz scan rate. Images were obtained with different tips and samples to ensure that the images were reproducible and free from artifacts caused by the tip or sample. The dimensions of the images were measured using in-situ-obtained graphite at a low bias voltage. All the procedures were carried out at room temperature (24 ± 3 °C). The STM images presented here were flattened by calculating a second-order, least-squares fit for the selected segment and then subtracting it from the scan line using the Nanoscope III software in order to remove the vertical offset between the scan lines and the tilt and bow in each scan line. The unit-cell parameters were determined by examining at least 14 images. The surface coverage for each alkylated sulfide was estimated from more than 20 images with an area of 100 nm × 100 nm, which were obtained from different locations on the graphite surface using more than three STM tips. The operating conditions are indicated in the figure caption: tunneling current (Iset) and bias voltage (Vbias).

Figure 1. STM images of a hexadecyl sulfide monolayer adsorbed on HOPG. The sulfur-containing region in the middle of the molecule is brighter than the region containing methylene units. (A) Three domains are observed in the image. The area is 112 nm × 112 nm. (B) Two domains are observed in a high-resolution STM image of the upper left area of image (A). The white rectangle in image (B) indicates the approximate space occupied by a single molecule. The area is 17 nm × 17 nm. Imaging conditions: Iset = 0.83 nA, Vbias = 1.2 V.

upper right side in Figure 1A, lamellar axes of two large domains form an angle of 60° or 120°, which is characterized by the 6-fold symmetry of the graphite surface substrate. At the upper horizontal domain boundary in Figure 1A, two neighboring domains having the same direction of lamellae appear mismatched along half of their molecular length. Figure 1B shows a high-magnification image of the region depicted in Figure 1A and can be roughly classified into three parts: (1) a bright spot that corresponds to an increase in the local electrical conductance in the vicinity of the central sulfur atoms, (2) two relatively dim thin bands that correspond to the alkyl chains extending linearly from the sulfur atoms, and (3) dark regions that lie between two molecules or two lamellae consisting of two-dimensional molecular arrays. Thus, the molecules appear as a combination of bright spots and thin bands in the STM images.13−20 The white rectangle in Figure 1B indicates the approximate location of a single molecule. The

3. RESULTS AND DISCUSSION 3.1. Two Types of Molecular Arrangements on Graphite. Figure 1A shows a typical STM image of a SAM composed of hexadecyl sulfide molecules adsorbed on HOPG. A two-dimensional array of molecules with the long molecular axis parallel to the graphite surface is clearly evident. At the domain boundaries running from the lower left corner to the 1485



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Figure 2. Sequential STM images of a hexadecyl sulfide monolayer adsorbed on HOPG. (A) A small domain straddling two lamellae lies with its lamellar axis perpendicular to their axes. The area is 70 nm × 70 nm. (B−E) High-resolution STM images of the small domain in image (A). Molecular motion of the small domain in two lamellae is observed. Images B, C, D, and E were obtained 133, 300, 505, and 546 s after image A, respectively. The areas of the images are 22 nm × 22 nm. (F) A magnification of the small domain in image E. The domain straddling two lamellae in image E consists of 22 molecules. The area is 11 nm × 11 nm. Imaging conditions: Iset = 0.83 nA, Vbias = 1.2 V.

molecules were observed to align themselves with their molecular axes oriented approximately perpendicular to the

lamellae. The size of the bright spot in the vicinity of the sulfur atom is ∼0.5 nm at the center of the molecule. From the STM 1486



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images, it is apparent that each unit cell contains a molecule and is rectangular with a1 = 0.43 ± 0.01 nm and a2 = 4.42 ± 0.10 nm; these values correspond to the width and length of a hexadecyl sulfide molecule, respectively. The width of the molecules is equal to the corresponding distance between the carbon rows in the graphite lattice, i.e., 0.426 nm, indicating that the arrangement of the self-assembled hexadecyl sulfide molecules depends on the graphite lattice. In the monolayers, the alkyl chains were found to be oriented along the (112̅0) zigzag directions of the graphite lattice and form the basic lamellar structures, which is consistent with the extensively reported findings of SAMs based on alkanes and long-alkyl-chain compounds.1−20 As is seen in Figure 1, the well-ordered lamellae forming close-packed domains and oriented in the direction with 6-fold rotational symmetry will be referred to as the first lamellar type and designated hereafter as Lz (see Figure 6A). Figure 2 shows sequential STM images of hexadecyl sulfide molecules adsorbed on HOPG. The most interesting feature is the coexistence of two types of lamellae. The close-packed domains of Lz lamellae almost completely cover the graphite surface in Figure 2A. In the lower right region of Figure 2A, Lz lamellar axes of two large domains form an angle of 60° or 120° at the domain boundary. However, near the upper central area of Figure 2A, the small domain that straddles two Lz lamellae lies with its lamellar axis perpendicular to the axes of the surrounding Lz lamellae. The lamellae of the second type, which form right angles to the Lz lamellae and consist of molecules along the (101̅0) armchair direction of the graphite lattice, will be designated below as La. The La lamellae appeared oriented in the direction with 6-fold rotational symmetry (similar to Lz). Some remarkable features could be observed, such as the misalignment of the Lz and La lamellar directions and formation of a 30° or 90° angle between the Lz and La lamellae on the graphite surface. To our knowledge, La lamellae have not been observed in the graphite-adsorbed monolayers of simple n-alkanes, some of their functionalized derivatives, or even the same alkylated sulfides until now. These results indicate that molecular length is probably a major factor governing the orientations and lamellar structures of alkanes and their derivatives on graphite. Figures 2B−E show the high-magnification images of the La domain depicted in Figure 2A. Although the molecular dynamics in the organic monolayers consisting of the Lz lamellae have been observed by STM,18−20,24−26 this is the first report on the dynamics of the La lamellae. The replacement of the La domain in the images occurred within a time scale of minutes. The absence of defects on the graphite surface was verified by observing in-situ-obtained graphite at a low bias voltage. Because the La domain straddles two Lz lamellae, the sequential images of the molecular motion show that La and Lz lamellae alternately form on the same graphite surface. This directly indicates that two types of hexadecyl sulfide molecular arrangements orient along two orthogonal directions of the graphite surface, and the La lamella, the length and width of which are equivalent to a single molecule, is the minimum unit on the graphite surface. Figure 2F, a magnification of Figure 2E, shows that the La domain is formed by two La lamella units. Figure 3 shows an average scan line taken across the alkyl chains in the La domain shown in Figure 2F. Twenty-two peaks associated with the alkyl chains in Figure 3 indicate that the La domain consists of 22 hexadecyl sulfide molecules. In fact, it is evident in Figure 2F that the La

Figure 3. Average line scan across the alkyl chains in La domains of hexadecyl sulfide. The average line scan is taken of the alkyl chain portion with an area of 1 nm × 9 nm in the La lamella shown in Figure 2F. Each peak center position is determined by the Gaussian distribution function with 0.04 nm resolution; the distance between peaks is then estimated.

domain consists of 22 molecules and that each La domain formed by a La lamella in Figure 2D consists of 11 molecules. The width of the molecule in the La lamellae was estimated from five images to be 0.40 ± 0.04 nmequal to 8.84 nm length of two hexadecyl sulfide molecules divided by 22 moleculeswhich is less than the 0.43 nm width of the molecule in the surrounding Lz lamellae. The 0.04 nm standard deviation is caused by the 0.04 nm resolution of the peak position. Figure 4A shows an STM image of octadecyl sulfide molecules adsorbed on graphite. The domain of Lz lamellae almost completely covered the graphite surface; two small La domains straddling a single Lz lamella were also seen in the image. La domains with lengths equal to the integral multiple of their molecular length were often observed, but no La domains shorter than the molecular length were observed. These observations support the postulation that the minimum length of the domain consisting of La lamellae is equal to the length of a single molecule. Figure 4B shows the high-magnification image of the region depicted in Figure 4A. The white rectangle in the Lz domain indicates the approximate location of a single molecule. Each rectangular unit cell in the Lz domain contains a molecule, and its dimensions are a1 = 0.43 ± 0.03 nm and a2 = 4.95 ± 0.13 nm. Figure 5 shows an average line scan taken across the alkyl chains in the La domain shown in Figure 4B. Twelve peaks associated with the alkyl chains indicate that the La domain consists of 12 octadecyl sulfide molecules. The width of the molecule in the La lamellae was estimated from four images to be 0.41 ± 0.05 nmequal to 4.95 nm length of an octadecyl sulfide molecule divided by 12 moleculeswhich is shorter than that of the molecules in the surrounding Lz lamellae, and similar to or longer than the 0.40 nm width of the hexadecyl sulfide molecules in the La lamellae. 3.2. Molecular Models of Lamellar Structures. There are two observable orientations for the arrangements of molecules containing linear alkyl chains on the graphite surface and are as follows: (1) a lower energy orientation along the (112̅0) zigzag direction of the graphite lattice (Figure 6A) and (2) a higher energy orientation along the (1010̅ ) armchair direction of the graphite lattice (Figure 6B). The tendency for linear alkyl chains to adsorb along the zigzag direction of the graphite lattice has been demonstrated for alkanes and a variety of functionalized derivatives.1−20 The interaction between alkyl chains along the zigzag direction and the graphite surface is considered to have a significant effect on SAM formation. Figure 6A shows that the alkylated sulfide molecules forming 1487



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Figure 6. Proposed orientations of alkylated sulfides on HOPG. (A) Alkylated sulfide molecules lie parallel to the graphite surface and along the (112̅0) zigzag direction of the graphite lattice. (B) The molecules lie perpendicular to the graphite surface and along the (101̅0) armchair direction of the graphite lattice. For octadecyl sulfide, the repeating unit is the width of 6 molecules, which is consistent with 0.246 nm × 10. The short-length alkylated sulfide (CH3(CH2)9)2S), which do not form stable La lamellae on the graphite surface under these experimental conditions, is used as the model molecule.

Figure 4. STM images of an octadecyl sulfide monolayer adsorbed on HOPG. (A) The lamellar axes of two small domains are perpendicular to that of the surrounding region. The area is 60 nm × 60 nm. (B) High-resolution STM image of the left small domain in image (A). The white rectangle in the surrounding region indicates the approximate space occupied by a single molecule. The small domain straddling a single lamella consists of 12 molecules. The area is 17 nm × 17 nm. Imaging conditions: Iset = 0.68 nA, Vbias = 1.2 V.

the Lz lamella lie parallel to the graphite surface along the zigzag direction of the graphite lattice.20 The alkyl chains in the monolayer on graphite arrange themselves in a closely packed configuration to maximize their van der Waals interactions. The ideal packing arrangements of alkyl chains in the Lz lamellae have an intermolecular spacing (i.e., the distance between two molecules in a lamella or the width of a single molecule) equal to the corresponding distance between the carbon rows on the graphite surface, i.e., 0.426 nm. Indeed, the intermolecular spacing along the zigzag direction in this work was measured to be 0.43 ± 0.01 nm for hexadecyl sulfide and 0.43 ± 0.03 nm for octadecyl sulfide. These observed intermolecular spacings are smaller than the intermolecular spacing of 0.48 nm in the direction parallel to the alkyl carbon planes and larger than that of 0.42 nm in the direction perpendicular to the planes in bulk solid n-alkanes.27,28 In theory, molecular simulations have indicated the parallel configuration to be the thermodynamically preferred orientation for alkyl chains29 and related derivates21 in lamellae, both with and without graphite present. The parallel configuration on graphite yields an intermolecular spacing of 0.426−0.44 nm, whereas carbon backbones perpendicular to the graphite surface result in compressed packing with 0.35−0.38 nm between molecules. The difference between experimental and theoretical values for bulk solid nalkanes indicates that their intermolecular spacing on graphite is influenced by the graphite lattice.

Figure 5. Average line scans across the alkyl chains in La domains of octadecyl sulfide. The average line scan is taken of the alkyl chain portion with an area of 1 nm × 5.2 nm in the La lamella shown in Figure 4B. Each peak center position is determined by the Gaussian distribution function with 0.03 nm resolution; the distance between peaks is then estimated.

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Table 1. Surface Coverage of La Lamellae in the Alkylated Sulfide Monolayers on HOPG, Free Energy Difference between Lz and La Domains, Free Energies of Lz and La Molecular Arrangements, and Interaction Energies between Lz and La Arrangements name

formula

surface coverage (%)

ΔG (kJ/mol)

ΔGz (kJ/mol)

ΔGa (kJ/mol)

ΔGza (kJ/mol)

dodecyl sulfide hexadecyl sulfide octadecyl sulfide

CH3(CH2)11S(CH2)11CH3 CH3(CH2)15S(CH2)15CH3 CH3(CH2)17S(CH2)17CH3

0.00 0.96 2.26

−11.5 −9.3

−27.5 −36.3 −40.7

−14.3 −18.7 −20.9

6.1 10.5

direction to that of the arrangement along the armchair direction can be expressed as follows:

For the intermolecular spacing for molecules assembled along the armchair direction, the molecular widths in the La lamellae were measured to be 0.40 ± 0.04 and 0.41 ± 0.05 nm for hexadecyl and octadecyl sulfides, respectively. The La intermolecular spacing is smaller than both the Lz intermolecular spacing and theoretical intermolecular spacing of the parallel configuration and larger than that of the perpendicular configuration. These results indicate that a two-dimensional La lamella cannot form from a closely packed parallel configuration because some molecules would have an out-of-plane configuration: the widths of 11 and 12 molecules with a parallel configuration are theoretically calculated to be 4.686−4.84 and 5.112−5.28 nm, respectively, which are larger than the La lamellar widthsequal to the molecular length of 4.42 and 4.95 nm for hexadecyl and octadecyl sulfides, respectively. Therefore, we can infer that the molecules oriented along the armchair direction of the graphite lattice must lie perpendicular to the surface; however, we cannot rule out the possibility that the parallel and perpendicular configurations coexist in the La lamellae because the La spacing standard deviation is 0.04 nmapproximately equal to the difference between the intermolecular spacings of parallel and perpendicular configurations. As shown in Figure 6B, the molecules forming the La lamella lie perpendicular to the graphite surface along the armchair direction of the graphite lattice. In the perpendicular structure shown in Figure 6B, the carbon atoms of neighboring methylene units lie at varying distances from the graphite surface. For hexadecyl and octadecyl sulfides, five molecules of 0.40 nm width and six molecules of 0.41 nm width in the La lamellae correspond to an 8- and 10-fold increase in length of 0.246 nm distance between second-nearest neighbors of the graphite lattice, respectively. This indicates the molecular arrangement in the La lamellae is incommensurate with the graphite lattice, although the arrangement in the Lz lamellae is known to be commensurate with the graphite lattice. 3.3. Surface Coverages and Estimation of Energies. Table 1 shows the surface coverage of the La lamellae in the alkylated sulfide monolayers on HOPG. The surface coverage for each alkane was estimated from more than 20 images with an area of 100 nm × 100 nm. The La lamellae were observed in the monolayers of hexadecyl and octadecyl sulfide but were not observed in the monolayers of dodecyl sulfide. The surface coverage for octadecyl sulfide was larger than that for hexadecyl sulfide. These results show that the lengths of the alkylated sulfide molecules need to exceed a threshold length in order to form La lamellae on the graphite surface and that alkylated sulfide molecules with lengths exceeding this threshold form more stable La lamellar domains. It is evident from the STM experiments that molecular orientation along the HOPG (112̅0) zigzag direction results in more favorable interactions with the graphite lattice than orientation along the (101̅0) armchair direction. The surface coverage ratio of the arrangement along the HOPG zigzag

θz /θa = exp( −ΔG /RT )

(1)

ΔG = ΔGz − ΔGa + ΔGza

(2)

where θz and θa are the surface coverage values for the molecular arrangements along the zigzag and armchair directions of HOPG, respectively; ΔG is the free energy difference between the molecular arrangements along the zigzag and armchair directions of HOPG; ΔGz and ΔGa are the free energy changes of the molecular arrangements along the zigzag and armchair directions of HOPG, respectively; ΔGza is the interaction energy between the arrangements along the zigzag and armchair directions of HOPG; and R and T are the gas constant and thermodynamic temperature, respectively. For hexadecyl sulfide, the surface coverage ratio of Lz lamellae on the graphite surface to that of the La lamellae is expressed as follows:

θz 0.9904 = θa 0.0096

(3)

Thus, the resulting free energy difference at room temperature is ΔG = −11.5 kJ/mol

(4)

From the ratio in Table 1, ΔG for octadecyl sulfide is estimated at −9.3 kJ/mol. There is a difference of 2.2 kJ/mol between the ΔG for hexadecyl and octadecyl sulfide, which represents the quantified difference in surface coverage by the La lamellae of hexadecyl and octadecyl sulfide. Octadecyl sulfide has four more methylene units than hexadecyl sulfide. In the perpendicular alkyl chain structure shown in Figure 6B, the carbon atoms of neighboring methylene units lie at varying distances away from the graphite surface; thus, the minimum alkyl chain repeating unit along the armchair direction consists of two methylene units. Taking that into account, the free energy for two alkyl chain methylene units perpendicular to the graphite surface is considered to be 1.1 kJ/mol. The sulfur atoms of alkane derivatives adsorbed on HOPG have been reported to have a much brighter appearance than the alkyl chains in STM images,13−20 which indicates that the central sulfur atom of the symmetrical alkylated sulfide interacts more strongly with the graphite surface than any of the methylene units of the alkyl chain. Assuming that the free energy of the sulfur atom in alkylated sulfide falls within the same range as that of two methylene units (terminal methyl units can be counted as methylene units) and that the free energy of the molecule is proportional to its length, the free energy change of the molecules along the armchair direction of HOPG can be roughly estimated using the equation 1489



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precise formation of ordered long-chain alkane monolayers on graphite. The ΔGz of the observed alkyl sulfides are larger than the threshold value for the cohesion. There is sufficient free energy change for all observed alkyl sulfides to form SAMs consisting of Lz lamellae on the graphite surface. A flow microcalorimetry study of n-dotriacontane (C32H66) in low-weight hydrocarbon solvents reported the free energy change and heat of adsorption onto graphite in powdered form (graphon) to be approximately 27 and 60−120 kJ/mol, respectively,31 and also indicated that the energy was solvent-dependent (n-heptane and isooctane). The adsorption of alkanes and alcohols was also studied by surface excess quantification using a dilatometric technique, where the heat of adsorption of the methylene unit onto powdered graphite was estimated to be 3.8 kJ/mol.32 From a differential scanning calorimetry study of the adsorption of alkane/alkane mixtures on graphite, the free energy change of the methylene unit was reported to be 1.2−3.4 kJ/mol.33 These previous reports also support that the validity of the assumption that the free energy change of the methylene unit in the Lz lamellae is approximately 1.1 kJ/mol. The observed behavior of a short-chain alkane also corroborates the absorption tendency of long-chain alkane derivatives on HOPG. The STM image of n-heptadecane (C17H36) adsorbed on HOPG has been obtained at lower than molecular resolution.2 From eq 9, the ΔGz of heptadecane is estimated to be −18.7 kJ/mol (−7.6RT), which is similar in value to the threshold for the cohesive energy. The lack of molecular resolution of the STM image may be due to the higher surface mobility of heptadecane molecules on the graphite surface owing to their weak interaction with the graphite surface.

ΔGa ≈ −1.1 kJ/mol × [(number of methylene units) × 0.5 + (number of sulfur atoms)]

(5)

For dodecyl, hexadecyl, and octadecyl sulfides, the free energy changes are estimated to be ΔGa ≈ −14.3 kJ/mol = − 5.7RT (dodecyl sulfide)

(6)

ΔGa ≈ −18.7 kJ/mol = − 7.6RT (hexadecyl sulfide)

(7)

ΔGa ≈ −20.9 kJ/mol = − 8.5RT (octadecyl sulfide)

(8)

The free energy is related to the molecular adsorption process on HOPG. One of the indices for adsorption follows the empirical relationship known as Trouton’s rule, which states that a latent heat of vaporization, ΔH, is related to a normal boiling point of a liquid at 1 atm, Tb, by the expression ΔH /Tb ≈ 70 − 90 J/(K mol)

(9)

and this further corresponds to a cohesive energy of approximately 8RT−11RT.30 Equation 9 also indicates that the molecules will condense once their cohesive energies in the condensed phase exceed the threshold value of 8RT−11RT. The free energy change, ΔGa, of dodecyl sulfide at room temperature is much smaller than the threshold value, even if the effect of the entropy change is combined. Therefore, the dodecyl sulfide molecules cannot condense or form La lamellae structures on HOPG. On the other hand, the ΔGa of hexdecyl and octadecyl sulfides are similar in value to the threshold; thus, the molecules can form La domains on HOPG. In the parallel structure of the alkyl chain along the zigzag direction (Figure 6A), the alkyl chains lie parallel to the graphite surface, and the carbon atoms in the chains are equidistant from the graphite surface. Therefore, the interaction energy change between the methylene unit and graphite surface is assumed to be approximately 1.1 kJ/mol. The free energy change for the molecules along the zigzag direction of HOPG can be roughly estimated using the equation

4. CONCLUSIONS We have used scanning tunneling microscopy to demonstrate the coexistence of two types of molecular arrangements of alkyl sulfide self-assembled monolayers adsorbed on graphite at the liquid−solid interface. Although short dodecyl sulfide molecules form a monolayer consisting of Lz lamellae, which is already known to exist in the monolayers of n-alkane and their derivatives, long hexadecyl and octadecyl sulfide molecules were shown to exhibit an additional molecular arrangement along the armchair direction of the graphite lattice. The molecular arrangement along the zigzag direction was found to yield more favorable interactions with the graphite surface than along the armchair direction based on the lamellar surface coverage ratio (θz/θa). In both arrangements, the energies for alkyl sulfide adsorption needed to exceed a threshold value in order to form the monolayers on the graphite surface and also predicted the structural stability. Additionally, the existence of two different arrangements in the monolayers increased the interaction energy associated with the orientation entropy. In general, the dispersion interactions between alkyl chains cause the formation of ordered monolayers, which consist of all-trans extended molecules, and the interactions between the functional groups control the angles between the lamellae and molecular axes. Therefore, it is apt to consider that the relevant degrees of freedom are largely frozen out and the configurational entropy is reduced significantly during the formation of ordered monolayers of long alkanes and their derivatives. However, the unfavorable molecular arrangements, effects of entropy, and interaction energies must be considered when obtaining precise information on the formation of nanostruc-

ΔGz ≈ −1.1 kJ/mol × [(number of methylene units) + (number of sulfur atoms)]

(10)

For hexadecyl sulfide ΔGz ≈ −36.3 kJ/mol = − 14.7RT

(11)

Hence ΔGza = ΔG − ΔGz + ΔGa ≈ 6.1 kJ/mol

(12)

The free energy changes, ΔGz, and interaction energies, ΔGza, of alkyl sulfides are illustrated in Table 1. The ΔGza of octadecyl sulfide is larger than that of hexadecyl sulfide; this increase in interaction energy is attributed to an increase in entropy with greater molecular length. A molecular length greater than the threshold value leads to formation of La lamellae. The coexistence of Lz and La lamellae in the monolayers, i.e., the existence of two different molecular arrangements, leads to order/disorder in the system and thus increased entropy. Therefore, the following relationship has been established: (1) increasing molecular length leads to an increasing surface coverage of La lamellae, and (2) increasing surface coverage of La lamellae leads to a mixing of two different arrangements and increasing entropy. This indicates that the effect of entropy and interaction energy cannot be discounted when determining the 1490



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ture designs and molecular devices on the substrate. These results contribute to a better understanding of the mechanisms responsible for the self-assembly process and can potentially improve the design and manipulation of interfacial selfassemblies.

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AUTHOR INFORMATION

Corresponding Author

*Phone +81-143-46-5771; Fax +81-143-46-5771; e-mail [email protected] (M.H.). Present Address

H.T.: Display Device Development Division, Sharp Corporation, 2613-1 Ichinomoto-cho, Tenri 632-8567, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to Prof. I. Hatta for his encouragement and advisement throughout this work.

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REFERENCES

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Coexistence of Alkylated Sulfide Molecules along Two Orthogonal

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