Directed Assembly and Separation of Self-Assembled Monolayers via

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14410

J. Phys. Chem. B 2006, 110, 14410-14417

Directed Assembly and Separation of Self-Assembled Monolayers via Electrochemical Processing Thomas J. Mullen, Arrelaine A. Dameron, and Paul S. Weiss* Departments of Chemistry and Physics, The PennsylVania State UniVersity, 104 DaVey Lab, UniVersity Park, PennsylVania 16802-6300 ReceiVed: January 20, 2006; In Final Form: May 17, 2006

Separated domains of 1-dodecanethiolate were fabricated via solution displacement of preformed 1-adamantanethiolate self-assembled monolayers on Au{111}. Subsequently, the 1-adamantanethiolate domains were desorbed selectively, and the substrate was exposed to a 1-octanethiol solution, creating artificially separated self-assembled monolayers of 1-dodecanethiolate and 1-octanethiolate. The molecular order of each lattice type and the apparent height differences imaged with scanning tunneling microscopy and the two distinct cathodic peaks observed with cyclic voltammetry indicated distinct separated domains of each lattice type in the separated self-assembled monolayers. By manipulating the intermolecular interaction strengths of the patterned molecules, we are able to control the structure and properties of the separated self-assembled monolayers via the exploitation of competitive adsorption and the utilization of electrochemical processing, which can be extended to other self-assembly patterning techniques such as microdisplacement printing.

1. Introduction Chemically patterned surfaces at the supramolecular 1-100 nm scale have wide applications in areas ranging from microelectronics to biocompatible systems.1 As the dimensions of patterned surface structures have decreased, the difficulty and expense of fabricating and measuring these structures have increased.2,3 For the directed assembly of self-assembled monolayers (SAMs) to become a viable alternative to traditional lithography, the resolution and reproducibility of the chemical patterning techniques must be improved. Nanoscale separated SAMs offer an excellent opportunity to study how intermolecular interactions influence the assembly and separation of thin films, which is key to the further development of chemical patterning techniques. Few methods exist to create nanoscale-separated SAMs.4 Spontaneously separated SAMs have been fabricated by coadsorption of n-alkanethiols and ω-functionalized alkanethiols, where the driving force for nanoscale separation is the ω-functionalized group interaction.5,6 Another means of fabricating spontaneously separated SAMs employs varied backbones, such as using buried amide functional groups, where the intermolecular hydrogen bonding of the amide groups drives the separation when coadsorbed with n-alkanethiols.7 Both of these methods rely on the solution mole fraction of the two coadsorbed species to dictate the thin-film fractional coverage. This reliance on solution mole fraction results in poor control over the final makeup of the separated SAM. Additionally, the final fractional coverage of the thin film does not necessarily reflect the solution mole fraction.8 A more versatile method of fabricating nanoscale-separated SAMs employs n-alkanethiol solution displacement of preformed 1-adamantanethiolate SAMs.9 This method allows for the precise control over the fractional coverage by controlling the displacement time or solution concentration. The assembly of nanoscale-separated SAMs can be directed and characterized through electrochemical processing and * Corresponding Author. E-mail: [email protected].

measured via scanning tunneling microscopy (STM).1 For a gold electrode modified with a thiolated SAM, applying a sufficient cathodic potential will cause a one-electron reductive desorption of the thiol,

RS - Au + 1 e- f RS- + Au0

(1)

Studying this faradaic process with cyclic voltammetry, the intermolecular interaction strength, the molecular order, and the surface coverage of SAMs can be investigated.10-14 A complementary technique is STM, which investigates the electronic and topographic properties of a surface with molecular precision. From STM images, it is possible to visualize the lattice structure, the domain boundaries, and the molecular order of SAMs.15,16 Here, we use electrochemical desorption in concert with STM to investigate and to manipulate the properties of weakly interacting bicomponent separated SAMs of 1-dodecanethiolate and 1-adamantanethiolate. 2. Experimental Section 1-Dodecanethiol (C12) and 1-octanethiol (C8) were purchased from Lancaster (Pelham, NH) and used as received. 1-Adamantanethiol (AD) was synthesized by methods previously described.9 All SAMs were fabricated on commercially available Au{111} evaporated onto freshly cleaved mica substrates (Molecular Imaging, Tempe, AZ), which were annealed using a hydrogen flame just prior to deposition. All cyclic voltammograms were acquired employing a custom-built electrochemical cell and a three-electrode BAS Epsilon potentiostat (Bioanalytical Systems Inc., West Lafayette, IN). The working electrode was defined by a perfluoroelastomer O-ring (McMaster-Carr, Cleveland, OH) mounted on top of the Au{111} substrates, inside the electrochemical cell. The working electrode area was determined electrochemically to be ∼0.5 cm2 by using the Randle-Sevcik equation.17 The potential was referenced to a Ag/AgCl saturated KCl electrode (Bioanalytical Systems Inc., West Lafayette, IN) and a Pt wire counter

10.1021/jp0604342 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/29/2006

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Figure 1. Outline of the procedure to fabricate artificially separated C12 and C8 SAMs. Separated domains of C12 were created via solution displacement for 20 min of the preformed AD SAM. The gold substrate was rinsed in neat ethanol and blown dry with nitrogen twice. The gold substrate with the separated C12 and AD SAM was mounted in a custom-built electrochemical cell. Upon the application of a reductive potential of -900 mV for 120 s, the AD domains were desorbed selectively. Subsequently, the gold substrate was rinsed in neat ethanol and blown dry with nitrogen twice and immersed in a 1 mM ethanolic C8 solution for 24 h, thereby producing an artificially separated C12 and C8 SAM.

electrode (Bioanalytical Systems Inc., West Lafayette, IN). A supporting electrolyte, 0.5 M KOH (99.99%, semiconductor grade, Sigma-Aldrich, St. Louis, MO), was sparged using ultrahigh purity argon for 20 min prior to electrochemical measurements. Cyclic voltammograms were acquired at a sweep rate of 20 mV/s and were baseline corrected for solution resistance by using a straight line subtraction in the first 100 mV of the sweep where no faradaic processes occur. All STM measurements were performed under ambient conditions using a custom beetle-style STM.18,19 Images were recorded in constant-current mode and at high tunneling gap impedances (∼1012 GΩ) to ensure large tip-sample separation for minimal contact between the probe tip and the monolayer. Figure 1 outlines the experimental process for the fabrication of artificially separated C12 and C8 SAMs. Initially, a gold substrate was immersed into a 10 mM ethanolic AD solution for 24 h, creating a single-component AD SAM. Separated domains of C12 were created via solution displacement of the preformed AD SAM for 20 min. The gold substrate was rinsed in neat ethanol and blown dry with nitrogen twice. The gold substrate with the separated C12 and AD SAM was mounted into a custom-built electrochemical cell. Upon the application of a reductive potential of -900 mV for 120 s, the AD domains were desorbed selectively. Subsequently, the gold substrate was rinsed in neat ethanol and blown dry with nitrogen twice and immersed in a 1 mM ethanolic C8 solution for 24 h, thereby producing an artificially separated C12 and C8 SAM with domains of C12 and C8. Finally, the gold substrate was rinsed in neat ethanol and blown dry with nitrogen twice. Cyclic voltammograms and STM images were acquired for the

Figure 2. Voltammograms of single-component (A) C12, (B) C8, and (C) AD SAMs deposited on Au{111} substrates via exposure to solution for 24 h. Scan parameters: forward and backward from -200 to -1500 to -200 mV at 20 mV/s. Note: only the forward sweep is shown.

separated C12 and AD SAMs, the partially desorbed C12 SAMs, and the artificially separated C12 and C8 SAMs. 3. Results and Discussion 3.1. Single-Component SAMs. Figure 2 shows representative cyclic voltammograms of single-component C12, C8, and AD SAMs deposited on Au{111} substrates via exposure to solution for 24 h. Table 1 lists the average cathodic peak potential (Ep), the average cathodic peak full width at half-maximum (fwhm), the average cathodic peak current (Ip), and the average cathodic peak area for each film from several cyclic voltammograms. The average Ep for single-component C12 and C8 SAMs were measured to be -1113 ( 9 mV and -1060 ( 34 mV, respectively. These values are in agreement with Ep measured previously for well-ordered alkanethiolate SAMs.14 The average Ep for single-component AD SAMs is -996 ( 4 mV. This Ep for single-component AD SAMs is significantly more positive than those observed for single-component C12 and C8 SAMs, indicating that it takes less energy to remove the AD SAM from the surface (i.e., it is easier for the electrolyte to reach the electrode surface and to induce reductive desorption). It is hypothesized that this difference in energy is due to a combination of the increased lateral distance between the AD SAM molecules as observed in STM images, the decreased intermolecular interactions in AD SAMs, and the shorter physical size

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TABLE 1: Average Cathodic Peak Potential, Average Cathodic Peak Full Width at Half-Maximum, Average Cathodic Peak Current, and Average Cathodic Peak Area of Several Voltammograms Tabulated for Each Type of Single-Component SAM

single-component C12 SAMs single-component C8 SAMs single-component AD SAMs

average peak potential (mV)

average peak fwhm (mV)

average peak current (µA)

average peak area (µC)

-1113 ( 9 -1060 ( 34 -996 ( 4

45 ( 9 60 ( 7 57 ( 11

6.1 ( 1.0 3.9 ( 0.6 3.2 ( 0.5

14.2 ( 0.8 11.1 ( 1.0 8.5 ( 1.8

of the AD molecules in comparison to the C12 and C8 molecules, separating the electrolyte from the Au{111} electrode.9 Additionally, STM images (data not shown) and Fourier transform infrared spectroscopy have shown that singlecomponent C8 SAMs typically have more regions of disorder compared to those of single-component AD and C12 SAMs.20 This disorder is reflected in the larger standard deviation of the Ep for single-component C8 SAMs compared to that of the Ep of single-component AD and C12 SAMs. Another reflection of intermolecular interactions within the SAM is the fwhm.21,22 Kakicuchi et al. observed that a narrowing of the fwhm coincided with an increase in intermolecular interaction strengths.12 From Table 1, the fwhm values indicate that single-component C12 SAMs have greater intermolecular interactions when compared to single-component C8 SAMs or single-component AD SAMs, consistent with our knowledge of these films.9,23,24 Both the Ip and the cathodic peak area consist of a faradaic component, due to the reduction of the thiol molecules, and a nonfaradaic component, from double layer charging, due to the presence of a SAM on the electrode surface.25 Although the Ip and the cathodic peak area are convolutions of two components (faradaic and nonfaradaic), it is possible to compare the relative surface coverages of single-component C12, C8, and AD SAMs, assuming that the nonfaradic component is constant for all three SAMs. Single-component SAMs of C12 and C8 have an average Ip of 6.1 ( 1.0 µA and 3.9 ( 0.6 µA, respectively. These values are in agreement with Ip measured previously for well-ordered alkanethiolate SAMs.14 The average Ip for single-component AD SAMs is 3.2 ( 0.5 µA. The smaller average Ip and average cathodic peak area of AD SAMs compared to those generally seen for alkanethiolate SAMs, indicate a lower surface coverage than for the alkanethiolates. Using a (7 × 7) unit cell for the AD SAM and a (x3 × x3)R30° unit cell for the C12 SAM, 1.8 times more C12 molecules compared to AD molecules are present in the same area of Au{111}.9 For example, in a (7x3 × 7x3)R30° unit area of Au{111} (the unit area where the AD and C12 unit cells overlap), there are 49 C12 molecules (21.4 Å2/molecule) and 27 AD molecules (38.9 Å2/molecule). Because of the one electron loss per molecule during electrochemical desorption, this difference is reflected in the C12:AD ratios for both the Ip and the peak area. Molecular-resolution STM was employed to visualize the lattice of each single-component SAM to confirm that the surface coverage for single-component AD SAMs was lower compared to that of single-component C12 and C8 SAMs. Figure 3 shows representative molecularly resolved STM images of single-component C12, C8, and AD SAMs deposited on Au{111} substrates via exposure to solution for 24 h. The STM images of single-component SAMs show hexagonally closepacked lattices with nearest-neighbor spacings of 5.0 ( 0.2 Å for C12 and C8 SAMs and 6.9 ( 0.4 Å for AD SAMs.9,26 This difference in surface coverage observed both with cyclic voltammetry and STM is related directly to the bulky carbon cage of the AD molecules and to the relatively compact (predominantly all-trans) alkyl chains of the C12 and C8 molecules. Another feature of the STM images is the difference in the monolayer domain boundaries. Single-component AD

SAMs have “depressed” domain boundaries (in STM images) resulting from rotational domains of the AD molecules; n-alkanethiolate SAMs domain boundaries with a variety of appearances associated with regions of molecules with differing tilts, rotational boundaries, translational boundaries, and stacking faults.27-29 From this understanding in the differences between single-component C12, C8, and AD SAMs, the different lattice types in separated C12 and AD SAMs can be identified by both cyclic voltammetry and STM. 3.2. Separated C12 and AD SAMs. Figure 4A displays a representative voltammogram of a separated C12 and AD SAM and Table 2 lists the average Ep, average fwhm, average Ip, and average cathodic peak areas from several voltammograms. The two distinct cathodic peaks in the voltammogram indicate that there are separated domains of two lattice types.5,30,31 The morepositive cathodic peak, -903 ( 21 mV, is attributed to the AD lattice, and the less-positive cathodic peak, -1054 ( 18 mV, is attributed to the C12 lattice type. Both cathodic peaks are shifted to more-positive potentials compared to their singlecomponent counterparts (e.g., the C12 cathodic peak is shifted from -1113 ( 9 mV for a single-component SAM to

Figure 3. STM images of single-component (A) C12, (B) C8, and (C) AD SAMs formed on Au{111} substrates via exposure to solution for 24 h. Image parameters: STM image A, Vsample ) -1.0 V, Itunnel ) 1.0 pA, 300 Å × 300 Å; STM image B, Vsample ) -1.0 V, Itunnel ) 5.0 pA, 300 Å × 300 Å; STM image C, Vsample ) -1.0 V, Itunnel ) 2.0 pA, 300 Å × 300 Å.

Assembly and Separation of Self-Assembled Monolayers

J. Phys. Chem. B, Vol. 110, No. 29, 2006 14413 that is occupied by that molecule type because of the difference in lattice spacing between AD and C12 SAMs. Therefore, a conversion factor of 1.8 for the AD cathodic peak and 1.0 for the C12 cathodic peak was employed. The conversion factors were determined from molecular models, STM images, and electrochemical desorption, as discussed previously. The average cathodic peak area was also used to calculate the fractional C12 coverage in a similar fashion, giving the equations

χEchem/C12 )

(IpC12/1) (IpC12/1) + (IpAD/1.8)

(2)

and

χEchem/C12 )

Figure 4. (A) Voltammogram of a separated C12 (less-positive cathodic peak at ∼-1054 mV) and AD (more-positive cathodic peak at ∼-903 mV) SAM with two cathodic peaks representing the two lattice types present on the gold substrate. (B) STM image of a separated C12 (more-protruding molecular domains) and AD (less-protruding molecular domains) SAM. The most-depressed regions are substrate vacancy islands. Two substrate terraces are shown. (C) High-resolution STM image showing the molecular order of each lattice type. Scan parameters: forward and backward sweeps from -400 to -1300 to -400 mV at 20 mV/s. Note: only the forward sweep is shown. Image parameters: STM image B, Vsample ) -1.2 V, Itunnel ) 1.0 pA, 1000 Å × 1000 Å; STM image C, Vsample ) -1.0 V, Itunnel ) 1.0 pA, 200 Å × 200 Å.

-1054 ( 18 mV for a separated C12 and AD SAM). The positive shift in Ep for both lattice types indicates that ion flux through the different lattice types to the electrode surface is more facile in separated C12 and AD SAMs than in singlecomponent SAMs. This difference in ion flux implies that the displacement of AD molecules by C12 molecules disrupts both lattice types of the monolayer and suggests that only weak intermolecular interactions exist between the two lattice types.14 Additionally, the magnitude in the shift in Ep for both lattice types is related to the size of the molecular domains, which is determined by the displacement time. For example, a separated C12 and AD SAM that has been displaced for 40 min will have a more negative C12 Ep compared to that of a separated C12 and AD SAM that has been displaced for 20 min. In contrast, Imabayashi fabricated separated SAMs employing co-deposition of 3-mercaptopropionic acid and 1-hexadecanethiolate and showed no appreciable shift in the Ep of the two distinct cathodic peaks, indicating that there are significant intermolecular interactions between domains of the two molecules.5 The fractional C12 coverage (χC12) was calculated from electrochemical data by dividing the average Ip of the C12 lattice by the sum of the average Ip of the C12 lattice and the average Ip of the AD lattice. However, the fraction of molecules of one lattice type does not necessarily reflect the fraction of the surface

(AreaC12/1) (AreaC12/1) + (AreaAD/1.8)

(3)

By using the average Ip and the average cathodic peak area in Table 2, the fractional C12 coverage was calculated to be 0.38 ( 0.08 and 0.30 ( 0.06, respectively, as in Table 3. The fractional C12 coverages determined by the Ip and the average cathodic peak area were in general agreement. It is important to note that domains of less than 15 nm2, or approximately 50 thiolated molecules, cannot be resolved with cyclic voltammetry, limiting the fractional C12 coverage that can be studied.30 Parts B and C of Figure 4 show representative STM images of separated domains of two lattice types, consistent with the cyclic voltammograms. The apparent height, the lattice spacing, and the relative fractional C12 coverage were employed to differentiate between the two different lattice types. The mostprotruding lattice with smaller molecular spacings that originates at the substrate defects is attributed to C12, while the lessprotruding lattice with larger molecular spacings is attributed to AD.9 The most-depressed regions are substrate vacancy islands. The relative fractional coverage also was used to assign the domains of molecules. The fractional C12 coverage of the separated C12 and AD SAM was controlled by the C12 displacement time (i.e., longer C12 displacement times resulted in larger C12 domains). As the C12 displacement time was increased, the domains attributed to C12 increased in size, confirming the lattice assignments (data not shown). 3.3. Partially Desorbed C12 SAMs. Electrochemical processing was not only employed to characterize single-component SAMs and separated C12 and AD SAMs but also was used to desorb the AD lattice selectively from separated C12 and AD SAMs. Figure 5A displays a representative voltammogram of a partially desorbed C12 SAM. The single cathodic peak in the voltammogram is attributed to the C12 domains and indicates that there is only one lattice type remaining on the surface. Table 2 lists the average Ep, the average fwhm, the average Ip, and the average cathodic peak area of several voltammograms. These values are all in general agreement with values for the C12 lattice of a separated C12 and AD SAM, suggesting that the C12 lattice remains intact with similar domain size and order after electrochemical desorption of AD and that the AD is desorbed selectively from the surface. Figure 5B displays a representative STM image of the partially desorbed C12 SAM, also confirming that the C12 domains remain intact and retain their size and molecular order. However, it is difficult to distinguish between the exposed gold, resulting from the selective desorption of the AD molecules, and low density, mobile molecules on the surface.32 3.4. Artificially Separated C12 and C8 SAMs. Figure 6A displays a representative voltammogram of an artificially

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TABLE 2: Average Cathodic Peak Potential, Average Cathodic Peak Full Width at Half-Maximum, Average Cathodic Peak Current, and Average Cathodic Peak Area of Several Voltammograms Tabulated for Each Processing Step average peak potential (mV)

average peak fwhm (mV)

average peak current (µA)

average peak area (µC)

dodecanethiolate peak adamantanethiolate peak

-1054 ( 18 -903 ( 21

Separated C12 and AD SAMs 71 ( 9 107 ( 29

2.5 ( 0.5 2.4 ( 0.7

8.9 ( 1.3 11.8 ( 1.8

dodecanethiolate peak

-1059 ( 5

Partially Desorbed C12 SAMs 59 ( 8

3.1 ( 0.4

9.4 ( 0.4

dodecanethiolate peak octanethiolate peak

Artificially Separated C12 and C8 SAMs -1068 ( 6 56 ( 9 -965 ( 10 65 ( 13

0.6 ( 0.2 2.0 ( 0.2

1.5 ( 0.7 6.0 ( 0.8

TABLE 3: Fractional C12 Coverages of Separated C12 and AD SAMs and Artificially Separated C12 and C8 SAMs Calculated Using the Ip and the Cathodic Peak Area fractional C12 coverage (peak current)

fractional C12 coverage (peak area)

separated C12 and AD SAMs

0.38 ( 0.08

0.30 ( 0.06

artificially separated C12 and C8 SAMs

0.22 ( 0.09

0.20 ( 0.07

separated C12 and C8 SAM. The two distinct cathodic peaks in the voltammogram indicate that there are separated domains of two lattice types, and Table 2 tabulates the average Ep, the average fwhm, the average Ip, and the average cathodic peak area of several voltammograms.5,31,33 The more-positive cathodic peak, -965 ( 10 mV, is attributed to the C8 lattice, and the less-positive cathodic peak, -1068 ( 6 mV, is attributed to the C12 lattice. The average Ep and the average fwhm attributed to the C12 lattice agrees with those observed for the C12 lattice of separated C12 and AD SAMs and partially desorbed C12 SAMs. Similar to the positive shifts of the cathodic peaks observed in separated C12 and AD SAMs, the cathodic peak attributed to the C8 lattice type of artificially separated C12 and C8 SAMs was more positive than the cathodic peak of single-component C8 SAMs. It is hypothesized that this positive

Figure 5. (A) Voltammogram of a partially desorbed C12 SAM displaying only a single cathodic peak at ∼-1059 mV that is attributed to C12 domains. (B) STM image of a partially desorbed C12 SAM displaying the intact C12 domains on the gold substrate. Scan parameters: forward and backward from -400 to -1300 to -400 mV at 20 mV/s. Note: only the forward sweep is shown. Image parameters: STM image B, Vsample ) -1.0 V, Itunnel ) 1.0 pA, 1000 Å × 1000 Å.

shift in Ep was due to a less-ordered structure of the domains of C8 in addition to the size effects observed in separated C12 and AD SAMs. However, the fwhm of C8 lattice type of the artificially separated C12 and C8 SAM was comparable to a single-component C8 SAM, suggesting that significant intermolecular interactions still exist within the domains of the C8 molecules. In addition to the two distinct cathodic peaks, there was a small feature at Ep ) ∼-881 mV (data not shown). This peak was attributed to small, disordered domains of C8 molecules.34 The fractional C12 coverage of the artificially separated C12 and C8 SAMs was determined in a fashion similar to the

Figure 6. (A) Voltammogram of an artificially separated C12 (lesspositive cathodic peak at ∼-1068 mV) and C8 (more-positive cathodic peak at ∼-965 mV) SAM with two cathodic peaks representing the two lattice types present on the gold substrate. (B) STM image of an artificially separated C12 (most-protruding molecular terraces) and C8 (less-protruding molecular terraces) SAM displaying the presence of two lattice types on the surface. (C) High-resolution STM images showing the molecular order of each lattice type. Scan parameters: forward and backward from -400 to -1300 to -400 mV at 20 mV/s. Note: only the forward sweep is shown. Image parameters: STM image B, Vsample ) -1.0 V, Itunnel ) 1.0 pA, 1000 Å × 1000 Å; STM image C, Vsample ) -1.0 V, Itunnel ) 1.0 pA, 400 Å × 400 Å.

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calculation of the fractional C12 coverage of separated C12 and AD SAMs and is tabulated in Table 3. A conversation factor of 1 was used for both the C8 cathodic peak and C12 cathodic peak because the C12 and C8 lattice spacings are the same, giving the equations

χEchem/C12 )

(IpC12/1) (IpC12/1) + (IpC8/1)

(4)

and

χEchem/C12 )

(AreaC12/1) (AreaC12/1) + (AreaC8/1)

(5)

Using the average Ip and the average cathodic peak area from Table 2, the fractional C12 coverage for the artificially separated C12 and C8 SAM was determined to be 0.22 ( 0.09 and 0.20 ( 0.07, respectively. These fractional C12 coverage values are significantly smaller than the fractional C12 coverage values of separated C12 and AD SAMs, indicating that the C12 domains were reduced in size during the processing steps. The decrease in fractional C12 coverage was also reflected in STM images. Figure 6B displays a representative STM image of an artificially separated C12 and C8 SAM. The STM image shows separated domains of two lattices where the lessprotruding regions are attributed to the C8 domains, and the more-protruding regions are attributed to the C12 domains. The C12 domains are observed to remain intact, although smaller in size, after immersion of the substrate in octanethiol solution for 24 h. This decrease in fractional C12 coverage is due to a combination of two processes: the diffusion of the C12 molecules across the surface and the exchange of C12 molecules with C8 molecules at the periphery of the C12 domains. Figure 6C displays an STM image with molecular resolution of both lattices. From the molecularly resolved STM image, single molecules of C12 can be observed dispersed throughout the C8 domains. It is hypothesized that the C12 molecules diffuse across the surface to some extent after the selective desorption of the AD domains and before the adsorption of the C8 domains. At the C12 domain periphery, it is observed that C8 molecules are intermixed with C12 molecules. It is commonly understood that during the SAM formation process, solvated alkanethiols will exchange with alkanethiolates on the surface at domain boundaries, vacancy islands, and other defect sites.35-37 A co-deposited C12 and C8 SAM was fabricated to compare to an artificially separated C12 and C8 SAM. Figure 7A shows a representative STM image of a coadsorbed C12 and C8 SAM formed from an equimolar solution of C12 and C8 (2 mM in total thiol). It is apparent that a SAM is formed due to the presence of molecules on the terraces and the formation of substrate vacancy islands. Figure 7B displays a high-resolution STM image showing molecular order of the intermixed SAM in which the most-protruding features are attributed to C12 molecules, the less-protruding features are attributed to C8 molecules, and the most-depressed regions are attributed to substrate vacancy islands. There was no observable separation of the C12 and C8 molecules present on the surface. Instead, the molecules appeared to be intermixed throughout the SAM. This is in contrast to the artificially separated C12 and C8 SAM, Figure 6C, where separated domains of each lattice were observed. Although the nearest-neighbor distance between singlecomponent C8 SAMs and C8 domains of artificially separated C12 and C8 SAMs agree within experimental error, there are

Figure 7. (A) STM image of a coadsorbed C12 and C8 SAM formed from an equimolar solution of C12 and C8 (2 mM in total thiol) displaying the presence of an intermixed SAM on a Au{111} substrate. (B) High-resolution STM image showing the molecular order of the coadsorbed C12 and C8 SAM, where the most-protruding features are attributed to C12 molecules and the less-protruding features are attributed to C8 molecules. The most-depressed regions are attributed to substrate vacancy islands. Image parameters: STM image A, Vsample ) -1.0 V, Itunnel ) 3.0 pA, 1000 Å × 1000 Å; STM image C, Vsample ) -1.0 V, Itunnel ) 3.0 pA, 200 Å × 200 Å.

several differences between the two types of SAMs. First, the C8 domain sizes of artificially separated C12 and C8 SAMs (few nm) are significantly smaller compared with the domain sizes in typical single-component C8 SAMs (tens of nm). These smaller C8 domains of artificially separated C12 and C8 SAMs contribute to the appearance of a rougher surface with respect to a single-component C8 SAM. Another difference between the domains of single-component C8 SAMs and the C8 domains of artificially separated C12 and C8 SAMs is the number and size of the vacancy islands. The C8 domains of artificially separated C12 and C8 SAMs qualitatively have larger numbers of and physically smaller vacancy islands compared with singlecomponent C8 SAMs. It is hypothesized that these are a result of the combination of the vacancy islands remaining after electrochemical desorption of the AD domains and the vacancy islands resulting from the C8 SAM formation process. To ascertain the origins of the structural differences between domains of single-component C8 SAMs and C8 domains of artificially separated C12 and C8 SAMs, the effects of the supporting electrolyte and the electrochemical processing of the gold substrate were studied. Figure 8A displays a representative molecularly resolved STM image of a single-component C8 SAM formed on a gold substrate that was exposed to supporting electrolyte for 10 min, rinsed in neat ethanol, and blown dry with nitrogen twice, and immersed into a 1 mM ethanolic C8 solution for 24 h. Parts B and C of Figure 8 display a singlecomponent C8 SAM formed on an electrochemically processed (-900 mV for 120 s) gold substrate and the C8 domain of an artificially separated C12 and C8 SAM, respectively. The singlecomponent C8 SAM formed on a gold substrate after exposure to the supporting electrolyte for 10 min has comparable protruding domain boundaries, vacancy islands, and lattice structure to single-component C8 SAMs, Figure 3B. This structural similarity indicates that the supporting electrolyte does

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Mullen et al. erwise not possible by coadsorption methods. Cyclic voltammograms and STM images at each processing step have given insight into the molecular structure and order of the SAMs. Differences between electrochemically processed domains of C8 molecules and preexisting domains of C12 molecules were observed and determined to be caused by the electrochemical processing of the gold substrate. This exploitation of competitive adsorption and electrochemical processing to create nanoscale separated SAMs can be extended to other self-assembly patterning techniques such as those created by microdisplacment printing. Acknowledgment. We thank Profs. Jennifer Hampton, Mary Elizabeth Williams, and Andrew Ewing for their analytical support as well as for their helpful and insightful discussions. The Air Force Office of Scientific Research, Army Research Office, Defense Advanced Research Projects Agency, National Science Foundation, Office of Naval Research, and Semiconductor Research Corporation are gratefully acknowledged for their support. References and Notes (1) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1.

Figure 8. (A) High-resolution STM image of C8 SAM formed on a gold substrate that was immersed in 0.5 M KOH for 10 min and rinsed with ethanol prior to SAM formation. (B) High-resolution STM image of C8 domains formed on electrochemically processed gold. (C) Highresolution STM image of C8 domains of an artificially separated C12 and C8 SAM. Image parameters: STM image A, Vsample ) -1.0 V, Itunnel ) 5.0 pA, 300 Å × 300 Å; STM image B, Vsample ) -1.0 V, Itunnel ) 4.0 pA, 200 Å × 200 Å; STM image C, Vsample ) -1.0 V, Itunnel ) 3.0 pA, 150 Å × 150 Å.

not significantly influence the SAM formation process. However, the single-component C8 SAM formed on a gold substrate postelectrochemical processing possesses similar structure, including small domain sizes and similar molecular order of the C8 domains of an artificially separated C12 and C8 SAM. This similarity indicates that electrochemical processing alters the structure of the gold substrate, thus affecting formation of the C8 monolayer. Possible reasons for this structural change of the C8 SAM postelectrochemical processing include the supporting electrolyte changing the surface reconstruction of the gold surface and the supporting electrolyte adsorbing to the gold substrate. Hobara reported a structural change of Au{111} after the application of a reductive potential.38,39 However, it has also been reported that the supporting electrolyte adsorbs to a working electrode during electrochemical processing.40-43 Further studies are underway to determine the degree of contribution from each of these processes on the formation of SAMs on electrochemically processed gold substrates. 4. Conclusions and Prospects In summary, by using the aforementioned electrochemical desorption and labile SAM displacement techniques, a chemically patterned surface has been assembled and characterized with molecular precision. We fabricated separated SAMs of C12 and C8 employing electrochemical processing techniques oth-

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