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An STM Study on Nonionic Fluorosurfactant Zonyl FSN Self-Assembly on Au(111): Large Domains, Few Defects, and Good Stability Yongan Tang, Jiawei Yan,* Xiaoshun Zhou, Yongchun Fu, and Bingwei Mao State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen, Fujian 361005, P.R. China ReceiVed August 16, 2008. ReVised Manuscript ReceiVed October 8, 2008 Nonionic Fluorosurfactant Zonyl FSN self-assembly on Au(111) is investigated with scanning tunneling microscopy under ambient conditions. STM reveals that the FSN forms SAMs on Au(111) with very large domain size and almost no defects. A (3 × 3)R30° arrangement of the FSN SAM on Au(111) is observed. The SAMs show excellent chemical stability and last for at least a month in atmospheric conditions. The structure and stability of the FSN SAMs are compared with those of alkanethiols SAMs. It is expected that FSN may serve as a new kind of molecule to form SAMs for surface modification, which would benefit wider applications for various purposes.
Introduction Surface self-assembly of organic molecules is an attractive way to functionalize solid surfaces.1 By designing functionalized tail groups of the self-assembled molecules, surfaces with different properties such as wettability, adhesion, and lubricity can be obtained. Preparing SAMs with larger domain size and few defects is highly desirable, which would benefit wider applications of the SAMs. For example, high-quality SAMs are required in the fields such as electronics and spintronics, where the presence of defects in a SAM would cause a short circuit between the deposited metal layer and the substrate during the process of metallization.2 Either chemically or physically natured molecule-surface interactions can promote the self-assembly process.3,4 Systems that bear strong chemical interactions include organosilicon derivatives on silicon, silicon dioxide, or glass substrates, and organosulfur on Au substrate. Among these systems, the SAMs of alkanethiolate on Au substrate are most extensively studied. Here, Au is frequently employed because of its ease of preparation and cleaning and resistance to oxidation and contamination in atmospheric condition. However, the SAMs of organosulfur on Au usually contain several kinds of defects such as vacancies of Au islands with monatomic depth, missing rows, and vacancies of molecules, as well as a considerable amount of domain boundaries where the molecules are disordered. Even by optimizing experimental parameters, a “perfect” self-assembled monolayer is still far from reality.2 For example, potential control5 and thermal annealing6 are applied in preparing SAMs, but some defects still remain such as vacancy Au islands. Cavalleri et al. treated SAMs by raising the temperature to 360 K to reduce the density of the defects. The quality of their SAMs is improved with domain sizes increasing to as large as 50-60 nm, which is, however, still *
[email protected]. (1) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (2) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (3) Ulman, A. Chem. ReV. 1996, 96, 1533. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (5) Brett, C. M. A.; Kresak, S.; Hianik, T.; Oliveira Brett, A. M. Electroanalysis 2003, 15, 557. (6) Yang, G. H.; Liu, G. Y. J. Phys. Chem. B 2003, 107, 8746.
Figure 1. Schematic illustrations of the molecular structures of FSN (A) and Brij 56 (B).
not large enough.7 The discontinuity of the SAMs degrades their properties such as electron-transfer blocking ability.8 Though factors such as assembling time and temperature, chain length and tail groups of the molecules, and substrate quality may influence the defect density, it is the strong chemical interaction between the thiol and Au surface that accounts mainly for the assembly quality. The structural changes of the Au surface, such as lifting of the surface reconstruction and restructuring of the surface, can take place during the assembly processes because of the mobile Au atoms and adsorbed thiol molecules. In recent years, SAMs based on physically natured surfacemolecule interactions have attracted increasing attention, motivated by their potential applications in lubrication, thin filmbased electronic devices, and novel molecular devices. These SAMs provide more choices for investigating interactions between molecules and substrates.9-11 Nonionic Fluorosurfactant Zonyl FSN (F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH) is a kind of nonionic surfactants. A schematic illustration for the structure of FSN molecule is shown in Figure 1. Its hydrophilic part consists (7) Cavalleri, O.; Hirstein, A.; Bucher, J.; Kern, K. Thin Solid Films 1996, 28, 4–285, 392. (8) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (9) De Feyter, S.; De Schryver, F. C. J. of Phys. Chem. B 2005, 109, 4290. (10) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (11) Wan, L. J. Acc. Chem. Res. 2006, 39, 334.
10.1021/la802682n CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
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Figure 2. (A) Large-scale (500 × 500 nm2) and (B) high-resolution (6 × 6 nm2) STM images of FSN SAM on Au(111).
of a polyoxyethylene chain, while its hydrophobic part consists of a fluorocarbon chain. Being a fluorosurfactant, FSN has high surface activity as well as good chemical and thermal stability for use in acid or alkaline solutions. In addition, FSN has been used for electrogenerated chemiluminescence and HPLC assay.12-14 It is inferred that FSN can form SAMs on Au surface through physical interaction between its hydroxyl group at the end of the hydrophilic part and Au substrate. By employing cyclic voltammetric and differential capacitance measurements, Cachet et al. investigated adsorptive behavior of a perfluorinated Forafac F1110 on Au electrode, whose molecular structure is similar to that of FSN.15 It was suggested that, at low concentrations, F1110 adsorbed in a flat orientation, and at higher concentrations and in the positive charge region, the electrode surface is progressively filled by the molecules from partly upright to upright orientation. However, no direct observations of the structure of the adlayer have been reported. Scanning tunneling microscopy (STM) with high spatial resolution down to molecules or even atoms can be used in different conditions including air atmosphere, UHV, and electrolyte environments. Hence, it has become a powerful tool for investigating structures of self-assembled monolayers and providing valuable information relating to molecule-surface interactions. In this paper, we report an STM study of FSN selfassembly on Au(111) surface. Our STM results show that the FSN SAM forms a very large domain size with almost no defects. Moreover, the assembled sample of FSN is highly stable in atmospheric conditions for at least a month. To our knowledge, SAMs of fluorocarbon molecules with highly uniform structure and good stability have not been reported.
Experimental Section STM and AFM measurements were performed on a Nanoscope IIIa multimode SPM (Digital Instrument, USA). All STM images were obtained in the constant current mode in ambient environment at room temperature. Tungsten tips were etched by electrochemical method in 0.8 mol L-1 KOH solutions. Contact mode and commercial cantilevers with spring constants 0.58 N/m were used for AFM experiments. (12) Li, F.; Zu, Y. B. Anal. Chem. 2004, 76, 1768. (13) Zu, Y. B.; Li, F. Anal. Chim. Acta 2005, 550, 47. (14) Lu, C.; Zu, Y. B.; Yam, V. W. W. Anal. Chem. 2007, 79, 666. (15) Cachet, C.; Keddam, M.; Mariotte, V.; Wiart, R. Electrochim. Acta 1992, 37, 2377.
X-ray photoelectron spectrometer (XPS, QUANTUM 2000) was employed to further verify the formation of FSN SAMs. Contact angle measurements were performed on an SL200B contact angle meter (Solon, China). Au(111) substrate was prepared following the Clavilier method.16 Briefly, one end of a Au wire with a diameter of 0.5 mm was melted in a hydrogen-oxygen flame to form a single crystal bead with a diameter of about 3 mm. The bead was then fixed on a Au foil with one of the (111) facets facing upward and served as the substrate for STM. Prior to each experiment, the Au surface was subjected to electrochemical polishing followed by a flame annealing to obtain a clean surface with high quality. Zonyl FSN-100 (F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH) and Brij 56 were purchased from Aldrich and used as received. All solutions were prepared with Milli-Q water (18.2 MΩ cm, Millipore). FSN aqueous solutions were freshly prepared prior to self-assembly experiments. Three kinds of concentrations of FSN, 1%, 0.1%, and 0.05%, were used. The FSN SAMs were prepared by immersing an electrochemically polished and flame-annealed Au(111) substrate into an FSN-containing aqueous solution for 3 h, which was then thoroughly rinsed with Milli-Q water and dried in air.
Results and Discussion The STM images of FSN SAMs on Au(111) are shown in Figure 2A,B. The Au(111) surface has a very large terrace size with only one step on the right side in an area of 500 nm × 500 nm (Figure 2A). More surprisingly, the FSN SAM presents a uniform structure without any domain boundaries. The molecular resolution image of FSN SAM is given in Figure 2B, which appears in a hexagonal arrangement of bright spots, each spot corresponding to an FSN molecule. The nearest distance of two bright dots is 0.50 ( 0.03 nm, and a unit cell is highlighted on the image. It should be pointed out that the STM images obtained from all samples or different places of one sample show the same characteristic as in Figure 2A,B. Imaging areas were selected by either adjusting the X offset and the Y offset of the piezo scanner, or moving the sample stage manually. Within the largest scan area reachable by the piezo (800 nm × 800 nm), we never observed domain boundaries that arose from the FSN SAMs in the STM images regardless the terrace steps from the Au(111) surface itself. This conclusion is valid on the basis of measurements from about 100 samples of (16) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205.
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Figure 3. High-resolution STM image of FSN SAM on Au(111). The upper part shows the atomically resolved Au(111) surface acquired at constant tunneling current of 5 nA and bias voltage of 5 mV; the lower part shows the ordered structure of FSN SAM acquired at constant tunneling current of 1 nA and bias voltage of 50 mV. Scan size: 10 × 10 nm2.
the FSN SAMs. The Au(111) substrates we used were facets of Au single crystal beads, the width of the terraces ranges from several hundred nanometers to about 1 µm. The FSN domains size is thus limited only by the size of the Au(111) terraces. None of the samples or selected areas has been found with missing rows, pinholes, or vacancy Au islands. In addition, FSN SAMs obtained from different concentrations of FSN, 1%, 0.1%, and 0.05%, give the same structure. These morphological features of FSN SAMs are different from those of alkanethiolate SAMs. In order to reveal the structural registration of FSN SAMs with the Au(111) substrate, we acquired atomic resolution images of Au(111) substrate in the presence of FSN by adjusting tunneling conditions. Under the condition of a bias voltage of 5 mV and tunneling current of 5 nA, the atomic structure of Au(111) substrate was imaged as shown in the upper part of Figure 3. Due to the existence of FSN SAMs, the image quality of the Au(111) substrate is not very good, but the [11j0] direction of the Au(111) surface can be clearly identified as is indicated by the short arrow in the upper part of the image. The nearest distance of two bright lines is 0.29 ( 0.02 nm, which is consistent with the atomic distance of the Au(111) substrate. By raising the bias to 50 mV and decreasing tunneling current to 1 nA, an image of different structure is obtained with the nearest distance of two bright dots 0.50 ( 0.03 nm, which should originate from the adsorbed molecules; see the lower part of Figure 3. There is a rotation of 30 ( 2° between the close-packed direction of the adsorbed molecules (indicated by the long arrow) and the [11j0] direction of the Au(111) substrate. For obtaining STM images with high quality, the tunneling current and bias voltage need to be adjusted to appropriate values. Similar high-quality STM images of the FSN SAMs could be acquired under the tunneling current of 0.4-1 nA and bias voltage of 50-400 mV. Lower current and higher bias, i.e., higher tunneling gap impedance, would degrade significantly the quality of the STM images. Since bright spots in the STM images appear to be similar in contrast, it is likely that the STM tip penetrated into the SAM and scanned inside the SAM in order to maintain
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the preset feedback current. In this case, the variable chain length of the FSN will not produce different contrast in the STM images. Due to the complexity of STM imaging mechanism, even for alkanethiolate SAMs, it is not yet completely clear whether the bright spots in the STM images correspond to the S head or to the alkyl chains.2 Widrig et al proposed that the tunneling current is mainly through sulfur atoms, i.e., sulfur atoms are imaged by STM.17 However, others suggested that the terminal alkyl group is imaged by STM with high tunneling gap impedance.18,19 If the FSN molecule is bound to the substrate via its oxygen atom at one end of FSN chain, we prefer to believe that it is the oxygen atom imaged by STM, which would give relatively high tunneling current. We conducted XPS measurements on bare Au(111) and FSN SAM modified Au(111). The peak from F1s is clearly identified in Figure S1a of the Supporting Information from the modified surface, which is absent for the bare Au(111) in Figure S1b (Supporting Information). The peaks from C1s are observed on both the modified and bare Au(111), which may originate from the FSN SAMs as well as carbon contamination. Note, however, that the peak from the FSN modified Au(111) appears broadened in comparison with that of bare Au(111), and the peak widths at half-height are 3.3 and 2.2 eV for the modified and bare Au(111), respectively (Supporting Information Figure S2a,b). No other obvious difference exists between XPS spectra of FSN SAMs modified Au(111) and bare Au(111). These results confirm that FSN SAMs do exist on Au(111) surface. Since the hydrophilic part of the FSN molecule consists of a hydroxyl group, the structure of FSN may be similar to that of alkanethiolate to a certain extent. For alkanethiolate SAMs on Au(111), Porter et al. observed a primitive (3 × 3)R30° structure in ambient condition and a c(4 × 2) superlattice of a basic (3 × 3)R30° in UHV condition.17,19,20 In Figure 2B of the present work, molecular resolution STM image of FSN SAMs on Au(111) shows the same characteristics as those of alkanethiolate SAMs in ambient conditions. On the other hand, Yeo et al. characterized self-assembled n-decanol monolayers at the liquid/Au(111) interface by STM, they believe that n-decanol molecules stand on end with the OH polar groups facing the gold substrate.21 Therefore, it is inferred that FSN molecules adsorb on Au(111) surface with a structure similar to that of alkanethiolate molecules, but with the hydroxyl group anchored to the surface. Contact angle measurements were performed to investigate the hydrophobicity of the FSN SAM, which would help estimate the possible orientation of the FSN molecules. In order to make the measurements reliable, we first measured the water contact angle of octanethiol SAMs on Au(111) (99 ( 3°), which is consistent with that of previous studies.22 Then, we measured the water contact angle of FSN SAMs on Au(111), which is 95 ( 3°. This value suggests a hydrophobic nature of the FSN SAMs. If it were the hydroxyl groups of the FSN molecules that were exposed on the outer surface, the contact angle of FSN SAMs would be about 20°, given that the contact angle of a hydrophilic HO(CH2)11S-Au surface is 22°.22 Therefore, our contact angle experiments suggest that fluorocarbon chains are exposed on the outer surface of the FSN SAMs. (17) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (18) Delamarche, E.; Michel, B.; Gerber, C. Langmuir 1994, 10, 2869. (19) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (20) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (21) Yeo, Y. H.; McConigal, G. C.; Yackoboski, K.; Guo, C. X.; Thomson, D. J. J. Phys. Chem. 1992, 96, 6110. (22) Banks, J. T.; Yu, T. T.; Yu, H. Z. J. Phys. Chem. B 2002, 106, 3538.
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Figure 4. Schematic illustration for (3 × 3)R30° adlayer structure on Au(111) surface.
Figure 4 gives a proposed structural model explaining the (3 × 3)R30° registration of the FSN SAM with the Au(111) surface. In this model, the nearest distance of two FSN molecules is 3, and the close-packed direction of FSN molecules rotates by 30° relative to the [11j0] direction of the Au(111) substrate. In the STM images of alkanethiolate SAMs, the bright spots are attributed to the standing-up alkanethiolate molecules. However, even for the (3 × 3)R30° structure, there was controversy about the adsorption site of alkanethiolate SAMs on Au(111).2,23,24 The formation of alkanethiolate SAMs involves several processes including physical adsorption, chemical adsorption, and the structural changes of the Au surface such as the appearance of vacancy Au islands. A two-site model is proposed corresponding to the formation of incoherent domains of alkanethiolate molecules at top and fcc hollow sites. 25 In the two-site model, during the initial stage of physisorption, alkanethiolate molecules adsorb at the less favorable top sites for steric reasons. After the molecules are chemisorbed, the presence of energy barriers for alkanethiolate surface diffusion “freezes” some of them at the top sites, hindering their movement toward fcc hollow sites. These processes result in the existence of different adsorption sites and thus the complexity of the SAM structures. Recently, Cossaro et. al studied hexanethiol and methylthiol on Au(111) by density functional theory based molecular dynamics simulations and grazing incidence X-ray diffraction. They found that the sulfur atoms of the molecules bind at two distinct surface sites and the first gold surface layer contains gold atom vacancies as well as gold adatoms that are laterally bound to two sulfur atoms.26 However, for FSN SAMs on Au(111) surfaces, no domain boundaries and vacancy Au islands are observed. Instead, a uniform structure with very large size is obtained. Therefore, we infer that FSN molecules take only one kind of site on the Au(111) surface. In Figure 4, we propose a model assuming that FSN molecules bind to threefold hollow sites of the Au(111) surface. (23) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokohama, T.; Ohta, T.; Shimomura, M.; Kono, S. Phys. ReV. Lett. 2003, 90, 66102. (24) Roper, M. G.; Skegg, M. P.; Fisher, C. J.; Lee, J. J.; Dhanak, V. R.; Woodruff, D. P.; Jones, R. G. Chem. Phys. Lett. 2004, 389, 87. (25) Torrelles, X.; Vericat, C.; Vela, M. E.; Fonticelli, M. H.; Millone, M. A. D.; Felici, R.; Lee, T. L.; Zegenhagen, J.; Munoz, G.; Martin-Gago, J. A.; Salvarezza, R. C. J. Phys. Chem. B 2006, 110, 5586. (26) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; Scoles, G. Science 2008, 321, 943.
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Various mechanisms about the presence of the vacancy Au islands have been proposed for alkanethiolate SAMs on Au(111). Poirier et al. suggested a model in which excess Au atoms are forced out of the surface layer by relaxation of the compressed herringbone reconstruction.27 This creates adatoms on the surface layer and vacancies in the surface layer. The complex interactions between the assembling thiols and the herringbone reconstruction account for the final monolayer surface. Yang et al. refined Poirier’s model;6 they proposed that chemisorbed thiol molecules may still be mobile on the surface, and the bare surface reorganizes into a new (3 × 23) structure with domains that are punctuated by gold vacancies. The vacancy defects on gold are mobile, creating the vacancy islands observed in SAMs. In the present work, neither the reconstruction structure nor the islands are observed after FSN SAM is formed (Figures 2 and 3). With regard to that, a flame-annealed Au(111) surface is reconstructed. It is likely that during the FSN self-assembly the reconstruction is lifted, but the Au islands thus generated are mobile enough to incorporate into the step edges of the Au surface. In addition, the uniform structure of the FSN SAMs also implies that FSN molecules are mobile in the domain boundaries, which possibly existed before the completion of the self-assembly, and rearrangement of molecules takes place, which is facile enough to reach a final uniform structure of the SAM. The above-described characteristics of the FSN SAMs are the result of the subtle balance of molecule-surface and intermolecular interactions. The weak interactions between the FSN and substrate account for the high mobility of Au atoms, which also reveal the physisorbed nature of the system. Nevertheless, the interaction between the FSN molecules and the Au substrate is still strong enough to lift the reconstruction of the Au substrate. We mention that, unlike the FSN on Au(111), the physisorbed systems, such as metal-organic coordination monolayers28 and terephthalic acid29 on Au(111), usually consist of many domains with small size and defect sites. In order to investigate the influence of the moleculemolecule interactions on the structure of the FSN monolayer, we tried to assemble Brij56 molecules on Au(111), whose molecular structure is similar to that of FSN. Brij56 is also a kind of nonionic surfactant; its hydrophilic end consists of a polyoxyethylene chain like FSN, but its hydrophobic end consists of a hydrocarbon chain instead of fluorocarbon chain. However, no ordered structures were observed by STM, which implies that the fluorocarbon chain in the FSN plays an important role in forming the ordered structure of monolayer. The main difference between the FSN and the Brij 56 molecules is that the FSN consists of a fluorocarbon chain as the hydrophobic end while the Brij 56 consists of a hydrocarbon chain. The fluorocarbon chain is more rigid than the hydrocarbon chain, which is expected to enhance its ability to form ordered self-assembled monolayers even though the fluorosurfactant structure is more complicated than that of alkylthiols. STM images taken on the FSN SAM samples that were kept under atmospheric conditions for one month show almost identical features to those obtained from the freshly prepared ones. In addition, FSN SAMs is stable in 0.1 M H2SO4, and the same STM images can be obtained. However, no ordered structure can be observed after the FSN SAMs were immersed in 0.1 M KOH for an hour, which indicates that FSN SAMs are not stable in basic conditions. (27) Poirier, G. E. Langmuir 1997, 13, 2019. (28) Zhang, H. M.; Zhao, W.; Xie, Z. X.; Long, L. S.; Mao, B. W.; Xu, X.; Zheng, L. S. J. Phys. Chem. C 2007, 111, 7570. (29) Clair, S.; Pons, S.; Seitsonen, A. P.; Brune, H.; Kern, K.; Barth, J. V. J. Phys. Chem. B 2004, 108, 14585.
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AFM was used to characterize the thickness of the FSN SAMs. First, STM was used to confirm that the ordered FSN SAM was formed on Au(111). Then, AFM tip scanned across an area of 100 nm by 100 nm to remove the FSN molecules by using larger contact force (about 435 nN). As a result, three square pits were generated as shown in Supporting Information Figure S3. Note that no pits can be formed on the bare Au(111) under the above experimental conditions. Finally, an area of 2 µm × 2 µm was imaged that enclosed the three pits. Sectional analysis gives the depth of the three pits of about 1.7 nm, i.e., the thickness of FSN SAMs. This value is comparable to the length of the FSN molecules, considering that they have a variable chain length.
Conclusions STM measurements have revealed that FSN molecules can form SAMs with large domains and few defects on Au(111). A (3 × 3)R30° registration of SAM with the Au(111) surface is discerned. Furthermore, the SAMs show excellent chemical stability, lasting for at least a month in atmospheric conditions. The physical interaction of FSN with the substrate and the high mobility of the molecules as well as the surface Au atoms during
self-assembly are responsible for the formation of such uniform SAMs; while the chemical stability of the FSN molecules accounts for the stability of SAMs. The continuity of the film and excellent chemical stability makes the FSN a potential candidate for applications in a variety of aspects such as sensors for biomolecular interaction analysis. It should be pointed out that with the presently available STM data the detailed mechanism of FSN self-assembly on Au(111) is still unclear. Further experiments employing approaches other than STM as well as theoretical calculations are highly desirable. Acknowledgment. This work was supported by the Natural Science Foundation of China (NSFC No. 20303013) and the Special Funds for Major State Basic Research Project of China (“973” Project Nos. 2007CB935600, 2009CB220102). We sincerely thank Dr. Yanbing Zu for his suggestion to perform STM studies on FSN self-assembly. Supporting Information Available: Additional spectra and AFM image. This material is available free of charge via the Internet at http://pubs.acs.org. LA802682N
