Magnetic Moment Assisted Layer-by-Layer Film Formation of a

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Magnetic Moment Assisted Layer-by-Layer Film Formation of a Prussian Blue Analog Abhijit Bera, Sukumar Dey, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India ABSTRACT: We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.

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In LbL films of PB analogues, magnetic moments of the compounds have so far remained unattended. It would be intriguing if the direction of the moment of the compounds in a monolayer could be aligned and then immobilized. The magnetic moment of the compound in a monolayer, if suitably oriented, may also take part in adsorption of subsequent monolayers. Electrical and dielectric properties of such magnetically aligned systems could be studied and compared with characteristics of unaligned LbL films. In this paper, we report such a comparison with LbL films of a PB analogue. We oriented the magnetic moment of a triply cationic PB analogue compound following our published recipe for simple LbLforming molecules.22 A similar method has been applied for ferrite nanoparticles by the present group of researchers23 and also in other magnetic nanoparticles24 including CoFe layered double hydroxide nanoplatelets by Shao et al.25 In such systems with several ionic functional groups that are active for LbL film formation process through the usual route of surface-charge reversal, e.g., anionic metal phthalocyanines (MePc), we could align magnetic moments of the compounds in a monolayer followed by immobilization of the compounds (and moments) with a monolayer of a polycation through electrostatic binding or electrostatic adsorption process.

INTRODUCTION Prussian Blue (PB) or ferric hexacyanoferrate and its analogues are a class of widely researched octahedrally coordinated compounds for various applications ranging from electrochemistry to electronics.1,2 They are now one of the most studied materials among organic magnetic materials for molecular magnetoresistance, magnetic memory devices, and spintronics applications.3,4 With their magnetic properties appearing due to FeIII ions,5 application possibilities of the compound have widened from optical to magnetic memories.2,6,7 For this purpose, thin films of the molecules or their composites with a range of materials have been formed to study optical, electrical, and more importantly magnetic properties.2,8,9 In forming thin films to study molecular magnetism, the layer-by-layer (LbL) electrostatic assembly process is a forerunner due to the uniqueness of the film formation method.10−15 The LbL film deposition procedure of PB analogues is all the more interesting due to the complex nature of the compounds. Instead of three cationic moieties that could have become handy in surface charge reversal for further electrostatic adsorption of a polyanionic layer, the compounds are triply cationic in nature. For such compounds, Bharathi et al. developed a general method of LbL formation that is based on self-assembly and “solution epitaxy” in sequence.16 Selfassembly of a polyanion, followed by adsorption of copper ions and hexacyanoferrate in sequence, was repeated in cycles in forming a LbL assembly of copper hexacyanoferrate. Multilayers of PB colloids within layers of polyelectrolytes have also been formed by a reiterative immersion−rinse−reimmersion approach.17 Such LbL films of PB analogues deposited along with a variety of materials, such as graphenes, carbon nanotubes, and other electron-accepting compounds, are being used for electrochemistry or sensing applications.18−21 © 2013 American Chemical Society

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MATERIALS AND METHODS

Materials. 3-Mercaptopropanoic acid (MPA) and potassium hexacyanoferrate(III) were purchased from Aldrich Chemical Co. Copper chloride was purchased from Loba Chemicals. Deionized water, as obtained from Millipore Academic System, had a resistivity of 18.2 MΩ·cm. Indium tin oxide (ITO) coated glass slides, obtained from Optical Filters, UK, had a surface resistance of 15 Ω/sq. Received: June 29, 2012 Revised: January 23, 2013 Published: January 24, 2013 2159

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LbL Film Formation. To form LbL films of the PB analogue,16 we first dipped precleaned quartz substrates in an aqueous solution of MPA (10 mM) for 20 min. The substrates were then washed in deionized water three times. They were then dipped in a CuCl2 solution (20 mM) for 15 min so that Cu2+ ions in the solution could react with the carboxylic acid groups of the absorbed MPA layer to form a monolayer of Cu2+−carboxylate complex on the substrate. To remove excess (physically absorbed) ions from the surface, the substrates were dipped in three separate deionized water baths for 3 min each. The substrates were then dipped in a potassium hexacyanoferrate(III) (an analogue of Prussian Blue) solution (10 mM) for 15 min followed by the same washing procedure. This led to the formation of a copper hexacyanoferrate (CuHCF) monolayer on the modified surface. In order to form multilayer films of CuHCF, the dipping procedure was repeated in sequence for a desired number of times. Magnetic Moment Assisted LbL Film Formation. We have aimed to form magnetic moment assisted LbL films of CuHCF. An external magnetic field was applied after formation of a monolayer of CuHCF. In practice, we placed the substrate containing a monolayer of CuHCF in a magnetic field of 320 mT with the direction of the field being perpendicular to the substrate. This oriented the magnetic moments of CuHCF along the field (perpendicular to the substrate and facing outward). After 2 min, the MPA solution was poured in the beaker so that the substrate became completely immersed in MPA solution. After 20 min, the magnetic field was switched off; the substrate containing the film was removed from the MPA solution and washed three times in deionized water. Here, the MPA monolayer formed in the last step immobilized the oriented CuHCF compounds through a sulfur bond. By repeating the three steps, namely, (i) formation of a monolayer of CuHCF, (ii) orienting the magnetic moment of the compounds perpendicular to the substrate and facing outward, and (iii) forming a monolayer of MPA to immobilize the oriented CuHCF compounds, we grew magnetic moment assisted assembly of LbL films of CuHCF. While adsorbing CuHCF in subsequent layers, magnetic moments of oriented CuHCF complexes on the substrate assisted the assembly process. In addition to LbL films formed (i) without any magnetic field and (ii) by orienting magnetic moments of CuHCF perpendicular to the substrate, we have also formed LbL films with moments of the compounds parallel to the substrate and facing a particular direction. That is, after forming a monolayer of CuHCF, the external magnetic field was applied parallel to the substrate. By repeating the dipping process, we formed films where moments of CuHCF compounds remained oriented parallel to the substrate and faced one particular direction. Film Characterization. To monitor growth of LbL films, optical absorption spectra of the films were recorded after adsorption of each layer. Atomic force microscopy (AFM) images of the films were recorded with a Nanosurf Easyscan 2 AFM under ambient conditions. To determine thickness of the films, we imaged an intentional scratch on the films and looked at the depth profile of the scratch. Magnetic force microscopy (MFM) images of multilayered films were recorded with a Veeco CP-II AFM system. Device Fabrication and Characterization. To fabricate sandwiched devices, films of CuHCF were deposited on indium tin oxide (ITO) coated glass substrates, where ITO electrodes were strips of 2 mm wide. As a top electrode, aluminum was thermally evaporated on CuHCF films as 2 mm strips orthogonal to ITO ones. Overlap of the electrodes defined the area of the devices (4 mm2). To characterize the devices, they were placed in a shielded vacuum chamber. Current− voltage (I−V) characteristics were recorded with a Keithley 6517 electrometer. Impedance spectroscopy of the devices was characterized in the 50 mHz to 10 MHz range with a Solartron 1260A impedance analyzer. The ac test voltage had an amplitude of 100 mV rms.

Article

RESULTS AND DISCUSSION

The LbL film formation of CuHCF is a multistep process. Here, the first monolayer or self-assembled monolayer (SAM) of MPA was formed on quartz substrates through thiol bonding. With scanning tunneling microscopy (STM) measurements, there are reports showing that surface Si atoms prefer the −SH of benzenethiol or 1,4-benzenedithiol over πconjugated aromatic groups. This is due to the fact that a sulfur atom prefers to bond with a charge-deficient adatom than to a charge-rich restatom.26 In other words, organothiol monolayers were formed on a bare silicon surface through a direct formation of a Si−S chemical bond.27 When the SAM of MPA was dipped in a CuCl2 solution, the Cu2+ ions reacted with the −COO− groups of the MPA layer to form a monolayer of a cationic copper carboxylate complex that subsequently adsorbed hexacyanoferrate ions through an electrostatic assembly process. This resulted in a monolayer of CuHCF. To form multilayer films of CuHCF, the dipping protocol was repeated (Scheme 1). Now, a SAM of MPA was formed on the CuHCF layer through the thiol end of MPA molecules due to the strong electronegativity of the −SH groups. Scheme 1. Schematic Representation of Formation Copper Hexacyanoferrate (CuHCF) Multilayers

To monitor the growth of LbL films of CuHCF, we have recorded optical absorption spectra after completion of each cycle of adsorption or dipping sequence that is defined as one bilayer. Figure 1a collects the spectra for 1−15 bilayers. All the spectra display a band at 468 nm, which represents the optical absorption of CuHCF. CuHCF formed separately in a solution returned a similar spectrum with a band at 477 nm. As the film grows layer after layer, the intensity of the absorption band increases, evidencing film formation in a regular or monotonic manner. The FTIR absorption spectrum of a CuHCF LbL film returned a band at 2090 cm−1 that represents the CN vibration consolidating the presence of CuHCF in the LbL films. The results hence demonstrate the presence of the PB analogue compound in LbL films. In Figure 1b, we present the optical absorption of 1−15 bilayered films when a suitable magnetic field was applied after adsorption of each CuHCF layer. The results show that absorbance grew at a faster rate than in Figure 1a, which represented films without any magnetic field. That is, the growth process of LbL films became accelerated through the magnetic moment assisted assembly process. The higher rate of growth of LbL films in Figure 1b occurs due to assistance from the oriented magnetic moments of the compounds present on the surface. When the moments of deposited CuHCF compounds were oriented perpendicular to the substrate facing outward or inward, the tiny magnets 2160

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Figure 1. Optical absorption of CuHCF films during LbL multilayer formation (a) without assistance from magnetic field, (b) when a magnetic field was applied perpendicular to the substrate after adsorption of each layer of CuHCF, and (c) when the applied magnetic field was parallel to the substrate. The traces with increasing absorbance in each of the figures show spectra of CuHCF films ranging from 1 to 15 bilayers. The chemical structure of the anionic part of potassium ferricyanide(III) is shown as an inset in panel a.

moment of deposited CuHCF compounds was oriented parallel to the substrate facing a particular direction and hence did not take part in the adsorption process of the next monolayers. A comparison of the growth of three types of LbL films can best be visualized if we plot the absorbance (of CuHCF) versus the number of bilayers. Such a plot, as presented in Figure 2,

attracted magnetic compounds during the adsorption process of the next layer. That is, oriented moments of the compounds on the substrate augmented or assisted adsorption of the anionic part of potassium hexacyanoferrate(III) compounds in the subsequent layer. A representation of the film deposition process is shown in part a of Scheme 2. Here the density of the Scheme 2. Schematic Representation of LbL Deposition Process with Magnetic Moments of CuHCF Oriented (A) Perpendicular and (B) Parallel to the Substratea

a

Red arrows denote the direction of the external magnetic field.

Figure 2. Absorbance at 468 nm of LbL films of CuHCF versus the number of deposited bilayers for the films deposited (i) without assistance from magnetic field, (ii) when a magnetic field was applied perpendicular to the substrate after adsorption of each layer of CuHCF, and (iii) when the applied magnetic field was parallel to the substrate.

compounds deposited in the subsequent layer became higher, leading to an increased absorbance. In other words, the magnetic moments supplemented the electrostatic attraction process of monolayer formation that is primarily responsible for the conventional LbL film formation. The magnetic force of attraction became operative in this magnetic moment assisted LbL adsorption process for the subsequent layers. The analysis can be further verified in another LbL deposition process where the magnetic field was applied parallel to the substrate along a particular direction. In Figure 1c, we present the optical absorption of 1−15 bilayered films when such a magnetic field was applied after adsorption of each CuHCF layer. The band corresponding to the compound grew in a usual manner; the adsorption process was not accelerated. In this case, since the magnetic moments of CuHCF compounds deposited on a layer were oriented along the plane of the substrate, the molecular magnets did not assist the formation of a monolayer of CuHCF compounds in the subsequent layer(s). A representation of the film deposition process is shown in part b of Scheme 2. Here, the magnetic

shows that the absorbance grew at a faster rate when an external magnetic field aligned the magnetic moment of the compound along the perpendicular to the substrate as compared to the other two cases, namely, when magnetic moments of the compounds were left as is or when they were oriented along the plane of the substrate. In fact, plots of absorbance versus layer number in the latter two cases were identical. For any number of bilayers, absorbance of the magnetic moment assisted LbL film was substantially higher (about 42%) than that of the other two films, showing the impact of the magnetic moment of CuHCF compound in forming a monolayer in the subsequent layer(s). It is important to know the magnitude of the magnetic field that was required to orient the CuHCF compound so that a LbL adsorption process could be accelerated or augmented. To 2161

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the results was much smaller than the effect of magnetic field in orienting the moments of the compounds of each monolayer. It is imperative to know if the orientation of the compound in LbL films does have an influence on the thickness of the films. We measured the thickness of the films for all the three types of orientations of the compounds, namely, when the CuHCF compounds were (a) unoriented, (b) oriented with their magnetic moments perpendicular to the substrate and facing the outward direction, and (c) oriented with their moments parallel to the substrate facing one particular direction (the direction of magnetic field). The thickness of the films was measured by recording the AFM topography of an intentional scratch on the film. A typical AFM topography and its corresponding depth profile for a 15-bilayered film (moments perpendicular to the substrate) are presented in part a and b of Figure 4, respectively. The AFM topography furthermore shows the profile of film surface deposited through this method. For each of the three types of films, we measured the thickness of different bilayered films. Plots of thickness versus the number of bilayers are shown in Figure 4c for the three types of LbL films. The figure shows that while the thickness increased uniformly with the progress of LbL film deposition process, the orientation of the compounds did not have an influence on the thickness. The thickness of 15bilayered films was 34 nm, implying that the CuHCF compound had a size of less than 2.2 nm since a bilayer consisted of a layer of MPA apart from the compound. The plots were indistinguishable for the three types of LbL films, presumably due to the symmetric nature of CuHCF’s chemical structure. Orientation of magnetic moments of the compounds in LbL films can be visualized by recording MFM images. For all the three types of films, we have recorded MFM images of different bilayered films. In Figure 5, we present such images for the three types of films. For each of the cases, 15-bilayered films have been imaged. The MFM image of unoriented film a, as presented in Figure 5a, shows randomness in magnetic moments, since the magnetic moment of the molecules remained oriented along all possible directions. Certain patterns however appeared in the image. Since adsorption of

do so, we carried out a magnetic moment assisted dipping procedure for different applied magnetic fields. For each case, we deposited 15-bilayered LbL films and measured the optical absorption of the films. A plot of CuHCF absorbance for the 15-bilayered films versus magnetic field that was applied after adsorption of each CuHCF layer is shown in Figure 3. The

Figure 3. Absorbance at 468 nm of 15-bilayered LbL films as a function of magnetic field that was applied perpendicular to the substrate after adsorption of each layer of CuHCF.

figure shows that the effect of orientation of magnetic moments of CuHCF compounds could be visible at around 100 mT. Absorbance of the LbL films increased with an increase in the magnitude of the magnetic field, implying that more CuHCF compounds in each monolayer could be oriented that in turn would assist or augment the LbL adsorption process. The absorbance reached saturation at around 200 mT; at such a field, orientation of all CuHCF compounds must have become complete. The moment of the compound could not be oriented further. We may state that the process of LbL film deposition with assistance from magnetic moments was reproducible and was always a success. The standard deviation of absorbance, presented as error bars in Figure 3, shows that the deviation of

Figure 4. (a) AFM topography and (b) depth profile of a scratch on a 15-bilayered LbL film of CuHCF deposited without assistance of any magnetic field. The area of the topography was 50 μm × 50 μm. (c) Plots of thickness of the three types of films as a function of number of deposited bilayers. The straight line is a best fit to the points. 2162

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Figure 5. MFM phase images of three types of 15-bilayered LbL films of CuHCF as described in Figure 1, that is, (a) without assistance from magnetic field, (b) when a magnetic field was applied perpendicular to the substrate after adsorption of each layer of CuHCF, and (c) when the applied magnetic field was parallel to the substrate. Each image represents 20 μm × 20 μm section of the film.

Figure 6. (a) Plots of real and imaginary components of complex impedance, Z′ and Z″, respectively, of devices based on three types of 15-bilayered LbL films of CuHCF films as described in Figure 1. (b) Dielectric constant versus frequency plots of devices based on three types of LbL films. The number of bilayers was 10, 15, and 20, as represented by different symbols for each type of film.

Table 1. Parameters of an Equivalent Circuit for Devices Based on Three Types of 15-Bilayered LbL Films of CuHCF As Described in Figure 1a

a Namely, (a) LbL films created without assistance from magnetic field, (b) when a magnetic field was applied perpendicular to the substrate after adsorption of each layer of CuHCF, and (c) when the applied magnetic field was parallel to the substrate.

could map the magnetic moment of the compound all over the surface of the film. In film c, that is the film where the tiny magnets remained parallel to the substrate and oriented along a particular direction, the magnetic tip of MFM, which was scanned from top of the monolayer, could not “see” or sense the magnetic moment of the compounds. Hence, no pattern was observed in the image, as has been presented in Figure 5c. As such, LbL films of a variety of active materials are being used in a range of electronic devices.28 It is hence important to know if the orientation of moments of the compounds has any influence on the dielectric and electrical properties of the thin films. To do so, we have formed sandwiched structures with the thin films as the active layer. We then characterized the devices

hexacyanoferrate(III) ions on a surface is a slow and isolated process, the magnetic compound, while becoming adsorbed on a charged substrate, is expected to align itself with the direction of moment of the neighboring compounds. Hence some clusters may form with the moments oriented along a particular direction. The clusters that have moments oriented perpendicular to the substrate (pointing inward or outward directions) therefore appeared in the MFM image of such films. The clusters with moments parallel to the plane of the substrate remained unnoticed by the MFM tip. The image of film b, where magnetic moments of the compound were oriented perpendicular to the substrate and faced the outward direction, is presented in Figure 5b. The image shows that the MFM tip 2163

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to study the dielectric and electrical properties of the active layers and to compare them. In Figure 6a, we present Nyquist plots of devices based on the three types of 15-bilayered LbL films of CuHCF. Plots of the real and imaginary components of complex impedance with the test frequency being an independent parameter could be fitted to semicircles. This implies that the devices could be modeled to an electrical analogue having a combination of a resistor and a capacitor connected in parallel (RP and CP, respectively). Here the diameter of each semicircle represents the bulk resistance of the device. The plot near the origin, when enlarged, can be found to meet the abscissa at a low positive value, indicating the presence of a resistance in series (RS). Values of RS, CP, and RP for the three devices, along with the proposed electrical analogue of the devices, are presented in Table 1. The figure and the table show that the bulk resistance was lower in a sandwiched device based on films where magnetic moments of the compounds were oriented along the direction of the applied electric field as compared to the other two cases, that is, where magnetic moments were perpendicular to the electric field or had no preferred orientation at all. Such a behavior was observed also for 10- and 20-bilayered films; the bulk resistance of thinner devices was proportionately smaller. Considering the fact that the thicknesses of the three types of films were the same, a lower bulk resistance may imply organized molecular films leading to an improved interlayer molecule-to-molecule conduction process. That is, the magnetic compounds, while becoming adsorbed on a substrate that had oriented moments on its surface, must have placed themselves on another compound of the preceding layer and maintained the orientation of the tiny magnets. Such an organized molecular film formed due to magnetic nature of the PB compound. Orientation of the moment of the compound in LbL films influenced the dielectric constant of the active layer. From the measurement of dielectric spectroscopy, we have calculated the dielectric constant of the three different types of active layers. Figure 6b shows the frequency response of the dielectric constant of the LbL films. For each type of film, we here have gathered results from films of three different thicknesses (10, 15, and 20 bilayers). Dielectric constant being an intrinsic property of the active material expectedly did not depend on the thickness of the thin films. The orientation of the magnetic moment of the compound, on the other hand, influenced the dielectric constant and hence the capacitance of the devices. As shown in Figure 6b and Table 1, the LbL films based on compounds having magnetic moments perpendicular to the substrate had a higher dielectric constant, and the devices based on such films had a higher capacitance as compared to those of the other two films or devices based on the two films, respectively. Oriented magnetic moments in the films must have resulted in an increased electrical polarizibility leading to a higher dielectric constant of the active material and hence an increased capacitance of the devices. We also recorded I−V characteristics of the devices under a dc voltage. Here also, we characterized the three types of films for three different thicknesses each. In order to normalize the characteristics with respect to the thickness, we have presented current versus electric field (I−F) plots for all the devices (Figure 7). For each type of film, I−F plots did not depend on the thickness. Current at any field was higher in films with compounds’ magnetic moments being oriented perpendicular to the substrate as compared to the other two films. Higher

Figure 7. Current versus electric field of devices based on three types of LbL films. The number of bilayers has been 10, 15, and 20, as represented by different symbols for each type of film. The three types of LbL films are (a) without assistance from magnetic field, (b) when a magnetic field was applied perpendicular to the substrate after adsorption of each layer of CuHCF, and (c) when the applied magnetic field was parallel to the substrate.

conductivity in such films could be due to the organized nature and higher density of the compounds in each monolayer of such films and hence an improved conduction process.

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CONCLUSIONS In conclusion, we have formed LbL films of a Prussian Blue analogue with their magnetic moments oriented perpendicular to the substrate in each monolayer. We also formed LbL films of the compound with the moments oriented parallel to the substrate facing the direction of applied external magnetic field. We showed that by orienting moments of the compounds in each monolayer along the perpendicular direction, the oriented moments assisted or augmented monolayer formation of the subsequent monolayer(s), as substantiated by a higher slope in the absorbance versus number of bilayers plot. In sandwiched structures based on multilayered LbL films having moments of the compounds (a) randomly oriented, (b) oriented perpendicular to the substrate, and (c) oriented parallel to the substrate and facing a particular direction, we measured and compared the electrical conductivity and dielectric constant of the active layers. The film b returned a higher conductivity and a higher dielectric constant as compared to the other two films. Higher conductivity in the LbL film b has been explained in terms of the organization of the compound and correspondingly an improved interlayer conduction process. We inferred that oriented magnetic moments in such films must have provided electric polarizibility leading to an increase in dielectric constant of the active material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +91-33-24734971. Fax: +91-33-24732805. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support through Nano Mission project SR/NM/NS-55/2008 and Ramanna Fellowship SR/S2/RFCMP-01/2009. A.B. and S.D. acknowledge CSIR Fellowship Nos. 09/080(0779)/2011-EMR-I (Roll No. 2164

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(20) Wang, L.; Guo, S. J.; Hu, X. O.; Dong, S. J. Layer-by-layer assembly of carbon nanotubes and Prussian Blue nanoparticles: A potential tool for biosensing devices. Colloid Surf. A 2008, 317, 394− 399. (21) Mao, Y.; Bao, Y.; Wang, W.; Li, Z.; Li, F.; Niu, L. Layer-by-layer assembled multilayer of graphene/Prussian Blue toward simultaneous electrochemical and SPR detection of H2O2. Talanta 2011, 85, 2106− 2112. (22) Dey, S.; Pal, A. J. Layer-by-layer electrostatic assembly with a control over orientation of molecules: Anisotropy of electrical conductivity and dielectric properties. Langmuir 2011, 27, 8687−8693. (23) Dey, S.; Mohanta, K.; Pal, A. J. Magnetic-field-assisted layer-bylayer electrostatic assembly of ferromagnetic nanoparticles. Langmuir 2010, 26, 9627−9631. (24) Bellido, E.; Domingo, N.; Ojea-Jimenez, I.; Ruiz-Molina, D. Structuration and integration of magnetic nanoparticles on surfaces and devices. Small 2012, 8, 1465−1491. (25) Shao, M. F.; Xu, X. G.; Han, J. B.; Zhao, J. W.; Shi, W. Y.; Kong, X. G.; Wei, M.; Evans, D. G.; Duan, X. Magnetic-field-assisted assembly of layered double hydroxide/metal porphyrin ultrathin films and their application for glucose sensors. Langmuir 2011, 27, 8233− 8240. (26) Baik, J.; Park, J.; Kim, M.; Ahn, J. R.; Park, C. Y.; An, K. S.; Hwang, C. C.; Hwang, H. N.; Kim, B. Adsorption of benzenethiol and 1,4-benzenedithiol on the Si(111)-7 × 7 surface. J. Korean Phys. Soc. 2007, 50, 690−694. (27) Lou, J. L.; Shiu, H. W.; Chang, L. Y.; Wu, C. P.; Soo, Y. L.; Chen, C. H. Preparation and characterization of an ordered 1dodecanethiol monolayer on bare Si(111) surface. Langmuir 2011, 27, 3436−3441. (28) Xiang, Y.; Lu, S. F.; Jiang, S. P. Layer-by-layer self-assembly in the development of electrochemical energy conversion and storage devices from fuel cells to supercapacitors. Chem. Soc. Rev. 2012, 41, 7291−7321.

510847) and 9/080(0647)/2009-EMR-I (Roll No. 507031), respectively.

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dx.doi.org/10.1021/la3036506 | Langmuir 2013, 29, 2159−2165