New Semifluorinated Dithiols Self-Assembled Monolayers on a

Aug 23, 2008 - New Semifluorinated Dithiols Self-Assembled Monolayers on a Copper Platform. Claire Amato, Sébastien Devillers, Patrick Calas, Joseph ...
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Langmuir 2008, 24, 10879-10886

10879

New Semifluorinated Dithiols Self-Assembled Monolayers on a Copper Platform Claire Amato,†,§ Se´bastien Devillers,†,§ Patrick Calas,‡ Joseph Delhalle,† and Zineb Mekhalif*,† Laboratory of Chemistry and Electrochemistry of Surfaces (CES), UniVersity of Namur, (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium, and Institut Gerhardt, CMOS, UMR 5253, UniVersite´ Montpellier II, 34095 Montpellier Cedex 05, France ReceiVed February 15, 2008. ReVised Manuscript ReceiVed July 4, 2008 New R,ω-semifluorinated dithiols HS-(CH2)11-(CF2)n-(CH2)11-SH, called DTn, and corresponding dithioacetate molecules CH3COS-(CH2)11-(CF2)n-(CH2)11-SCOCH3, called DTAn (n ) 4, 6, 8), were synthesized and used to create self-assembled monolayers (SAMs) on both untreated copper surfaces and electrochemically reduced ones. The aim of this study is to assess the organization of the resulting SAMs, particularly the effect of the presence of two perhydrogenated segments surrounding the perfluorinated one, and the ability of these difunctional molecules to bind copper substrates by only one end per molecule. In each case, the organization of the SAM is rather poor and only DTA8 molecules seem to adopt an upright position on reduced copper. In addition, the layers have been investigated by cyclic voltammetry (CV) to assess their coverage. DT4 SAMs reveal a covering ratio higher than 99%.

Introduction Polyfluorinated SAMs with thiol on noble metals such as gold have been extensively studied. They can be considered as bilayers containing a perhydrogenated and a perfluorinated segment. The larger covalent radius of fluorine atoms relative to hydrogens and the helical conformation adopted by the perfluorinated chains lead to bulkier segments than the alkyl moieties, with Van der Waals diameters of 5.6 Å and 4.2 Å, respectively. As a consequence, composition, structure, and interfacial properties of the polyfluorinated SAMs are different than in perhydrogenated ones.1-15 The critical influence of the fluorinated chain length in the self-assembly of terminally perfluorinated alkanethiols CF3-(CF2)n-(CH2)m-SH monolayers on gold surfaces reveals that the monolayer’s organization largely depends on the n and m values. Molecules with longer fluorinated segments tend to self* Prof. Zineb Mekhalif, Tel.: +32-(0)81-72 52 30; fax: +32-(0)81-72 46 00, E-mail address: [email protected]. † University of Namur. § equal participation. ‡ CMOS, UMR 5253.

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organize more readily into dense monolayers and adopt upright positions in the films.16-19 To date, gold has been the most frequently studied substrate because of its good inertness to most potential contaminants as well as the high affinity of sulfur for Au and the relative ease of obtaining high-quality monolayers from a large variety of organothiol solutions. To extend the potential of the field, growing research interest also develops for the modification of active (oxidizable) metal substrates such as Ag,20-26 Fe,27-30 Cu,31-48 Ni,49-53 Zn,54-58 and a few alloys like CuNi,59 ZnCu,60 and (16) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 327-329, 150. (17) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417. (18) Naud, C.; Calas, P.; Commeyras, A. Langmuir 2001, 17, 4851. (19) Scho¨nherr, H.; Vancso, G. J. Langmuir 1997, 13, 3769. (20) Laffineur, F.; Auguste, D.; Plumier, F.; Pirlot, C.; Hevesi, L.; Delhalle, J.; Mekhalif, Z. Langmuir 2004, 20, 3240. (21) Angelova, P.; Hinrichs, K.; Esser, N.; Kostova, K.; Tsankov, D. Vibrational Spectrosc. 2007, 45(1), 55. (22) Yihong, W.; Song, W.; Jie, Z.; Ning, G.; Wesche, K. D. Appl. Surf. Sci. 2006, 252(23), 8264. (23) Schweizer, M.; Kolb, D. M. J. Electroanal. Chem. 2004, 564, 85. (24) Ohgi, T.; Fujita, D.; Deng, W.; Dong, Z.-C.; Nejoh, H. Surf. Sci. 2001, 493(1-3), 453. (25) Hutt, D. A.; Cooper, E.; Leggett, G. J. Surf. Sci. 1998, 397(1-3), 154. (26) Burleigh, T. D.; Shi, C.; Kilic, S.; Kovacik, S.; Thompson, T.; Enick, R. M. Corrosion 2002, 58(1), 49. (27) Volmer-Uebing, M.; Reynders, B.; Stratmann, M. Werkst. Korros. 1991, 42, 19. (28) Nozawa, K.; Nishihara, H.; Aramaki, K. Corros. Sci. 1997, 39, 1625. (29) Feng, Y.; Chen, S.; Zhang, H.; Li, P.; Wu, L.; Guo, W. Appl. Surf. Sci. 2006, 253(5), 2812. (30) Zhou, J.; Chen, S.; Zhang, L.; Feng, Y.; Zhai, H. J. Electroanal. Chem. 2008, 612(2), 257. (31) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, M. G. J. Am. Chem. Soc. 1991, 113, 7152. (32) Imanishi, A.; Isawa, K.; Matsui, F.; Tsuduki, T.; Yokoyama, T.; Kondoh, H.; Kitajima, Y.; Ohta, T. Surf. Sci. 1998, 407, 282. (33) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130. (34) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (35) Sung, M. M.; Sung, K.; Kim, C. G.; Lee, S. S.; Kim, Y. J. Phys. Chem. B 2000, 104, 2273. (36) Ron, H.; Cohen, H.; Maltis, S.; Rappaport, M.; Rubinstein, I. J. Phys. Chem. B 1998, 102, 9861.

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ZnNi.61 Copper is particularly interesting in the context of SAMs because it is largely used in modern microelectronics and is expected to become even more crucial in the developing nanoelectronics. It is also a challenging substrate to modify with organothiols31-33,39,62-64 because of its oxidative nature.65 In contrast to gold, Cu requires specific surface preparation to bring it into the metallic state before modification and keep it in that state during the modification. Increasing attention has been devoted to R,ω-difunctional thiols and their incorporation in SAMs on different metals. Such molecules bearing a reactive functional group at each end allow the preparation of chemically reactive surfaces and provide a way of bonding adlayers to target substrates. The self-assembly of R,ω-difunctional molecules is complicated by the fact that both terminal functional groups per molecule are reactive toward the target substrate.66 Although the role of the molecular structure of alkanedithiols on the ultimate quality of the SAMs is still not fully understood, several experimental results suggest that alkanedithiol molecules are arranged on the gold surface with their molecular axis parallel to the substrate due to the strong affinity between sulfur and gold atoms.67,68 On the basis of STM measurements in the liquid phase, Esplandiu´ et al. have suggested (37) Laffineur, F.; Delhalle, J.; Guittard, S.; Ge´ribaldi, S.; Mekhalif, Z. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 198-200, 817. (38) Whelan, C. M.; Kinsella, M.; Carbonell, L.; Ho, H. M.; Maex, K. Microelectron. Eng. 2003, 70, 551. (39) Mekhalif, Z.; Sinapi, F.; Laffineur, F.; Delhalle, J. J. Electron Spectrosc. Relat. Phenom. 2001, 121(1-3), 149. (40) Yamamoto, Y.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 436. (41) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (42) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 1839. (43) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 3696. (44) Haneda, R.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1997, 144, 1215. (45) Haneda, R.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1998, 145, 1856. (46) Haneda, R.; Aramaki, K. J. Electrochem. Soc. 1998, 145, 2786. (47) Ishibashi, M.; Itoh, M.; Nishihara, H.; Aramaki, K. Electrochim. Acta 1996, 41, 241. (48) Taneichi, D.; Haneda, R.; Aramaki, K. Corros. Sci. 2001, 43, 1589. (49) Mekhalif, Z.; Delhalle, J.; Pireaux, J.-J.; Noe¨l, S.; Houze´, F.; Boyer, L. Surf. Coat. Technol. 1998, 100-101, 463. (50) Tortech, L.; Mekhalif, Z.; Delhalle, J.; Guittard, F.; Ge´ribaldi, S. Thin Solid Films 2005, 491(1-2), 253. (51) Mekhalif, Z.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Langmuir 1997, 13, 2285. (52) Noe¨l, S.; Houze´, F.; Boyer, L.; Mekhalif, Z.; Caudano, R.; Delhalle, J. J. IEEE Trans. Comp. Pack. Technol. 1999, 22, 79. (53) Mekhalif, Z.; Laffineur, F.; Couturier, N.; Delhalle, J. Langmuir 2003, 19, 637. (54) Mekhalif, Z.; Massi, L.; Guittard, F.; Geribaldi, S.; Delhalle, J. Thin Solid Films 2002, 405, 186. (55) Sinapi, F.; Forget, L.; Delhalle, J.; Mekhalif, Z. Appl. Surf. Sci. 2003, 212-213, 464. (56) Noble-Luginbuhl, A. R.; Nuzzo, R. G. Langmuir 2001, 17, 3937. (57) Sinapi, F.; Issakova, T.; Delhalle, J.; Mekhalif, Z. Thin Solid Films 2007, 515, 6833. (58) Zhang, H.; Baldelli, S. J. Phys. Chem. B 2006, 110, 24062. (59) Laffineur, F.; Delhalle, J.; Mekhalif, Z. Mater. Sci. Eng. 2002, 22(2), 331. (60) Sinapi, F.; Deroubaix, S.; Pirlot, C.; Delhalle, J.; Mekhalif, Z. Electrochim. Acta 2004, 49, 2987. (61) Berger, F.; Delhalle, J.; Mekhalif, Z. Electrochim. Acta 2008, 53(6), 2852. (62) Ferral, A.; Paredes-Olivera, P.; Macagno, V. A.; Patrito, E. M. Surf. Sci. 2003, 525, 85. (63) Sinapi, F.; Lejeune, I.; Delhalle, J.; Mekhalif, Z. Electrochim. Acta 2007, 52(16), 5182. (64) Mekhalif, Z.; Fonder, G.; Laffineur, F.; Delhalle, J. J. Electroanal. Chem., in press. (65) Maurice, V.; Strehblow, H.-H.; Marcus, P. J. Electrochem. Soc. 1999, 146, 524. (66) Pranger, L.; Goldstein, A.; Tannenbaum, R. Langmuir 2005, 21, 5396. (67) Kobayashi, K.; Yamada, H.; Horiuchi, T.; Matsushige, K. Appl. Surf. Sci. 1999, 144-145, 435. (68) Leung, T. Y. B.; Gerstenberg, M. C.; Lavrich, D. J.; Scoles, G.; Schreiber, F.; Poirer, G. E. Langmuir 2000, 16, 549.

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that alkanedithiol SAMs consist of molecular rows with a parallel arrangement as well as disordered structures.69 Mixed SAMs on gold composed of 1-decanethiol and 1,10-decanedithiol have been investigated by noncontact AFM; in that case, phase separation has been identified.70 Nevertheless, it has also been shown that the ultimate molecular organization in the SAMs depends on the self-assembly conditions as well as on the molecular structure.4,16,17,31,36,66,69,72,73,75 In that respect, dithioacetate end functions tend to favor SAMs with molecules in the upright position. Hence, combining molecular structure together with preparation conditions remains a promising avenue for the preparation of SAMs based on R,ω-difunctional molecules. To the best of our knowledge, no polyfluorinated alkanedithiol molecules have been incorporated in SAMs on oxidizable metal substrates like Cu. The goal of this paper is the study of the self-assembly of polyfluorinated dithiols and corresponding dithioacetates on untreated copper substrates, on one hand, and electrochemically reduced substrates, on the other hand. For this purpose, we report the synthesis of highly fluorinated alkyldithiol HS-(CH2)11-(CF2)n-(CH2)11-SH with n ) 4, 6, and 8, called DT4, DT6, and DT8, respectively. These new compounds are involved in the formation of DTn SAMs on untreated copper, i.e., cleaned Cu surfaces used without any subsequent treatment and on electrochemically reduced copper substrates. For comparison, the corresponding polyfluorinated dithioacetates DTA4, DTA6, and DTA8 are also used to form DTAn SAMs. These new molecules are of particular relevance owing to the presence of two anchoring terminal groups and a perfluorinated segment surrounded by two perhydrogenated moieties: alkyl/perfluoroalky/alkyl. It is interesting to find out if the balance of the Van der Waals interactions due to these segments between neighboring molecules is propitious to SAMs with molecules in the upright position. An interest in such SAMs would be to form an insulating ultrathin yet thick enough layer between a copper substrate and another metallic adlayer (Cu, Au, etc.). Having the two functionalities in the desired configuration would have the advantage of avoiding post-treatments on the SAMs before attaching the adlayer. An assembled system of this kind corresponds to the so-called MIMs (metal/insulator/metal) needed in electronic devices. XPS analysis is used first to assess the chemical state of polyfluorinated molecules after self-assembly. The ability of dithiol and dithioacetate to bind copper substrate by only one terminal group per molecule is appraised as a function of perfluorinated chain length (n ) 4, 6, or 8), state of copper (untreated or electrochemically reduced), and type of terminal function (thiol or thioacetate). Second, it is used to investigate the oxidation state of copper substrate after SAMs formation and the formation mechanism of the alkylthiolate monolayers on the copper substrate. Monolayers organization is assessed on the basis of PM-IRRAS measurements. Finally, cyclic voltammetry experiments are performed to estimate their coverage. (69) Esplandiu´, M. J.; Hangenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828. (70) Ichii, T.; Fukuma, T.; Kobayashi, K.; Yamada, H.; Matsushige, K. Appl. Surf. Sci. 2003, 210, 99. (71) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563. (72) Weckenmann, U.; Mittler, S.; Naumann, K.; Fischer, R. A. Langmuir 2002, 18, 5479. (73) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (74) Tsao, M.-W.; Rabolt, J. F.; Scho¨nherr, H.; Castner, D. G. Langmuir 2000, 16, 1734. (75) Lau, K. H. A.; Huang, C.; Yakovlev, N.; Chen, Z. K.; O’Shea, S. J. Langmuir 2006, 22, 2968.

Semifluorinated Dithiols SAMs on Cu

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Scheme 1. Synthesis of r,ω-semifluorinated dithiols 5a, 5b, and 5c called DT4, DT6, and DT8, respectively, starting from r,ω-diiodoperfluoroalkane I(CF2)nI (n ) 4, 6, or 8)a

a

Conditions: (i) AIBN, 1,2-dichloroethane; (ii) Zn, EtOH/AcAc (25:1); (iii) PPh3, DIAD, THF; (iv) NH2NH2.H2O, EtOH.

Experimental Section Chemicals. Absolute ethanol (Merck), acetic acid (Prolabo), azobis isobutyronitrile AIBN (Acros Organics), 1,2-dichloroethane (Acros Organics), dichloromethane (Romil SpS), n-dodecanethiol (Acros Organics, 98%), 1,6-diiodoperfluorohexane and 1,8-diiodoperfluorooctane (FluoroChem), diisopropyle azodicarboxylate DIAD (Acros Organics), hydrazine hydrate (Avocado), perchloric acid HClO4 (Janssen Chimica), sodium chloride NaCl (Acros Organics), sodium fluoride NaF (Acros), sodium hydroxyde NaOH (Acros Organics), tetrahydrofurane (Carlo Erba), n-tributyl tin hydride (Aldrich), thioacetic acid (Acros Organics), triphenylphosphine (Merck), undecyl-10-ene-1-ol (Acros Organics), ultrapure water H2O (18 MΩ.cm), and zinc powder (Merck) were used without any further purification. Substrate Preparation. The foil copper substrates were supplied by Metalor. They are cut in 2 × 1 cm2 coupons. Their average roughness (Rrms) measured with a DEKTAK8 surface profilometer is 45 nm. These samples are modified through SAMs formation in two different conditions: without any pretreatment (except for a cleaning in ethanol for 15 min under sonication) and just after an electrochemical reduction. In the latter case, the samples were dipped for 10 min in a 0.5 M perchloric acid solution under argon at an applied potential of -800 mV vs the saturated calomel electrode (SCE). Monolayer Preparation and Characterizations. The reduced substrate is directly dipped in the modification bath for 15 h after being dried under an argon flow in order to remove droplets of electrolytic solution. Modification bath is a 10-3 M organothiol solution. Dichloromethane dried on magnesium sulfate is used as solvent. The organothiol solutions are bubbled under argon just before the modification to eliminate a maximum of oxygen. After modification, the samples are copiously rinsed with absolute ethanol, subjected to sonication for 15 min in the same solvent, and rinsed again with absolute ethanol. The samples were finally blown dry in an argon flow and used immediately for characterization. Each surface modification has been repeated at least three times for each characterization method used in this work in order to ascertain the reproducibility of the results presented here. The monolayers were characterized by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and cyclic voltammetry (CV). XPS is used to evaluate the elemental composition of the monolayers and the oxidation state of sulfur and to determine the chemical state of the copper. The photoelectron spectra of the monolayers have been obtained with a SSX-100 spectrometer using a monochromatized X-ray Al KR radiation (1486.6 eV), the photoemitted electrons being collected at 35° takeoff angle. Nominal resolution was measured as full width at half-maximum of 1.0-1.5 eV for core levels and survey

spectra, respectively. The binding energy of core levels was calibrated against the C1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian-Lorentzian curves (80% of Gaussian character). The S2p line, which is a doublet structure where the S2p3/2 and S2p1/2 components are spaced by 1.18 eV and have an intensity ratio S2p3/2/S2p1/2 of 2, was analyzed accordingly. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) data are collected to assess the monolayers organization. They were registered on a Brucker Equinox 55-PMA37 equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector and a zinc-selenide photoelastic modulator. The infrared light was modulated between s- and p-polarizaton at a frequency of 50 kHz and an incident angle upon the sample surface of around 85°. Signals generated from each polarization (Rs and Rp) were detected simultaneously by a lock-in amplifier and used to calculate the differential surface reflectivity (∆R/R) ) (Rp - Rs)/(Rp + Rs). All spectra are the average of 640 scans at a spectral resolution of 2 cm-1. Electrochemical techniques provide additional information on the monolayer quality through coverage and resistance to oxidation. Experiments were carried out with an EG&G Instruments potensiostat, model 263A, monitored by computer and M270 electrochemistry software. A three-electrode electrochemical cell was used with SCE as reference electrode and a platinum foil as counter electrode. The cell used enables analysis of a well-defined and reproducible spot on the sample. Cyclic voltammetry was carried out in a 0.1 M hydroxide sodium solution by sweeping a range of potential from -1.1 to 0.6 V at 20 mV/s. Covering has been calculated by measuring the area of the copper oxidation peaks for unmodified substrates (Au) and for modified ones (Am) and by applying to following formula C(%) ) 100 × (Au - Am)/Au.

Results and Discussion GeneralSynthesisRoutetowardPerfluorinatedAlkyldithiols. The synthesis route to target compounds 5a, 5b, and 5c called DT4, DT6, and DT8, respectively, is depicted in Scheme 1 and synthesis data are collected in the Supporting Information. Starting from R,ω-diiodoperfluoroalkane I-(CF2)n-I 1a (n ) 4), 1b (n ) 6), and 1c (n ) 8), DT4, DT6, and DT8 are obtained in four steps. The first step is the radical chain addition of 1a,b,c to undecyl-10-en-1-ol affording diadducts HO-(CH2)9-CHI-CH2(CF2)n-CH2-CHI-(CH2)9-OH 2a (n ) 4), 2b (n ) 6), and 2c (n ) 8) in 88%, 90%, and 95% yield, respectively. The second step is the reduction of the iodide functions in 2a,b,c to form the reduced diadduct compounds HO-(CH2)11-(CF2)n-(CH2)11-OH

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Figure 1. C1s core level XPS spectra of DT4 (a), DT6 (b), and DT8 (c) SAMs on untreated copper and DTA4 (d), DTA6 (e), and DTA8 (f) on electrochemically reduced copper.

3a (n ) 4), 3b (n ) 6), and 3c (n ) 8) in 73%, 70%, and 73% yield, respectively. The third step is the conversion of alcohols 3a,b,c into thiolacetates H3COCS-(CH2)11-(CF2)n-(CH2)11SCOCH3 4a (n ) 4), 4b (n ) 6), and 4c (n ) 8) using the Mitsunobu reaction yielding 62%, 60%, and 64%, respectively. Finally, the dithioacetate 4a,b,c are treated by hydrazine hydrate in order to deprotect the thiol functions. Perfluorinated dithiols HS-(CH2)11-(CF2)n-(CH2)11-SH 5a (n ) 4), 5b (n ) 6), and 5c (n ) 8) are obtained in 82%, 84%, and 80% yield, respectively. The details for each compound are provided as Supporting Information. XPS Investigations. XPS experiments were performed for DTn and DTAn modified copper substrates immersed in dichloromethane solution for 15 h. Figure 1 shows the C1s core level XPS spectra of DTn and DTAn SAMs on untreated and electrochemically reduced copper, respectively. On untreated copper substrates, the presence of fluorine atoms is clearly demonstrated in DTn SAMs by the two characteristic peaks of difluoromethylene groups at 290.8 and 291.6 eV (Figure 1a-c). When DTAn molecules are used, no characteristic peak of the fluorinated segment is observed in the region above 290 eV, revealing the inefficiency of thioacetate function to reduce copper oxide and thus the impossibility of forming DTAn SAMs on untreated copper substrates with the present self-assembly conditions. This is confirmed by the absence of a characteristic peak in the S2p core level spectra of DTAn SAMs (Figure 2). For DTn SAMs on untreated copper, the S2p XPS spectrum (Figure 3a-c) exhibits four doublets, S2p3/2 components centered at 162.6, 163.8, 165.5, and 168.0 eV, corresponding to thiolates, unbound thiols, sulfinates, and sulfonates, respectively. The presence of thiolate species proves the chemical binding of DTn to the untreated copper surface. Sulfinate and sulfonate species are formed by oxidation of chemisorbed sulfur atoms by the copper oxide layer present on the substrate. It has been shown that these species are less strongly bound and usually desorbed and replaced by other thiol molecules present in solution.34,36,39,63,64,71 In our case, sulfinate and sulfonate species are still present after 15 h of modification and no other thiol molecule has reached the surface to replace them. We could also note that sulfinate and sulfonate characteristic peaks increase with the length of DTn molecules. This phenomenon could be explained by the nature of the grafted molecules. DTn are large molecules (roughly 50 Å length); thus, they probably have limited mobility that disadvantages the exchange of molecules at the metal surface

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and therefore the complete replacement of oxidized sulfur atoms by chemisorbed thiolates. The signal at 163.8 eV, assigned to sulfur atoms in free thiol groups, indicates that a certain amount of thiol groups does not react with the copper surface to form S-Cu bonds. On electrochemically reduced copper substrate, both DTn and DTAn SAMs show characteristic peaks of fluorinated segment in the C1s core level spectra. An additional peak centered at 288.8 eV is obtained with DTAn SAMs, corresponding to the carbonyl group of the thioacetate function (Figure 1d-f). On the S2p core level spectra of DTn SAMs (Figure 3d-f), the sulfinate and sulfonate species are not detected. Thus, we conclude that dithiol molecules bind to a reduced copper surface without being oxidized to sulfonate or sulfinate species as when they bind to an untreated copper surface. In the case of DTAn SAMs, sulfonate and sulfinate species are also absent, and two S2p3/2 components centered at 162.6 and 163.8 eV corresponding to thiolate and thioacetate, respectively, are observed (Figure 4). It indicates that the thioacetate function could bind to reduced copper substrates. We now assess how difunctional molecules bind to copper substrates, via one end function or both. We report in Table 1 the “bound sulfur”/“unbound sulfur” ratio (Sb/Su) after 15 h of modification for DTn and DTAn SAMs. According to LambertBeer’s law,72 if dithiol or dithioacetate molecules bind to the substrate via only one terminal function per molecule, the Sb/Su ratio must be less than 1. Indeed, for difunctional molecules which adopt a standing-up orientation with their molecular backbone oriented perpendicular to the surface, electrons emerging from the sulfur atom bound to the copper substrate are attenuated by the overlying organic monolayer -(CH2)11-(CF2)n(CH2)11-SX (X ) H or COCH3). On the contrary, electrons emerging from the sulfur atoms in thiol or thioacetate groups exposed to the surface of the SAM do not experience such attenuation. The Sb/Su ratio for DT4, DT6, and DT8 SAMs on untreated copper substrate is estimated by considering sulfinate and sulfonate species as bound sulfur. This ratio is found to be greater than unity. Thus, we conclude that a significant quantity of dithiol molecules bind to the substrate by both terminal functions per dithiol. This proportion is lower with DT8 or DT6 than with DT4. When an electrochemically reduced copper substrate is employed, the Sb/Su ratio value decreases for DT4 and DT6 SAMs. It reveals that, when the substrate is reduced before modification, the tendency of DT4 and DT6 molecules to chemisorb via only one of their thiol functions increases. With DT8 SAMs, no difference is noticed for the Sb/Su ratio whether the substrate is previously reduced or not. With DTAn SAMs on electrochemically reduced copper substrate, the Sb/Su ratio is lower than with DTn SAMs (Table 1). We could emphasize that the tendency of difunctional molecules (DTn and DTAn) to bind the substrate via one terminal function per molecule improves with the increase of the perfluorinated chain length and with the use of a previously reduced copper substrate. Moreover, DTAn molecules favor the formation of self-assembly by only one terminal function, especially for n ) 8. This may be due to the lower reactivity of thioacetate groups for copper. XPS spectra of the Cu2p core levels and Cu LMM Auger line for DT4 and DTA4 modified copper substrates and for bare, untreated copper are reported in Figure 5. DT4 SAMs on untreated copper substrate show the loss of Cu2p peak at 933.5 eV and the extinction of the satellite peaks around 938-945 eV, which are characteristic of the presence of CuO (Figure 5b). It confirms that modification of the substrate induces the reduction of the CuO present in the oxide layer. As was already men-

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Figure 2. S2p (left) and C1s (right) core level XPS spectra of DTA4 (a,d), DTA6 (b,e), and DTA8 (c,f) SAMs on untreated copper substrate.

Figure 3. S2p core level XPS spectra of DT4 (a), DT6 (b), and DT8 (c) SAMs on untreated copper and DTA4 (d), DTA6 (e), and DTA8 (f) on electrochemically reduced copper.

tioned,35,36,39,63,64 the oxide layer CuO could be reduced by oxidation of the alkanethiols into disulfide, sulfinate, and sulfonate species. When DTA4 molecules are used, XPS analyses of S2p and C1s core levels has revealed that no SAM on untreated copper substrate is obtained with the present self-assembly conditions. This is confirmed in the Cu2p XPS core level spectrum of DTA4 SAMs (Figure 5c) by the CuO satellite peaks around 938-945 eV and characteristic peaks at 934.7 and 933.5 eV assigned to Cu(OH)2 and CuO, respectively. The absence of the Cu(0) peak at 568 eV in LMM Auger line of DT4 and DTA4 SAMs on untreated copper substrates (Figure 5h-i) clearly indicates that metallic copper is not formed during SAMs elaboration as is possible with smaller molecules such as n-dodecanethiol or n-dodecaneselenol.64 XPS analyses of DT4 SAMs on electrochemically reduced copper substrate reveal no cupric oxide CuO characteristic peak in the Cu2p core level spectrum (Figure 5d) and the presence of metallic copper, by the shoulder at 568.1 eV in the LMM

Figure 4. S2p core level XPS spectra of DTA4 (a), DTA6 (b), and DTA8 (c) SAMs on electrochemically reduced copper substrate. Table 1. Sb/Su Ratio (Bounded to Unbounded Sulfur; See Text) Calculated from Experimental XPS S2p Spectra of DTn and DTAn SAMs on Copper Substrates untreated copper reduced copper

reduced copper

DT4 SAMs

DT6 SAMs

DT8 SAMs

16.6 4.8

2.5 1.3

1.4 1.4

DTA4 SAMs 1.5

DTA6 SAMs 1.2

DTA8 SAMs 0.6

Auger X line (Figure 5j). In that case, the adsorption reaction is considered formally as an oxidative addition of the S-H bond to the metallic copper, followed by a reductive elimination of the hydrogen. Figure 5e shows Cu2p core level XPS spectrum of DTA4 SAMs on electrochemically reduced copper. The presence of Cu(+2) is indicated by the appearance of broad satellite peaks around 938-945 eV and also by the characteristic peak at 569.6 eV in the LMM Auger line (Figure 5k). Thus, metallic copper oxidizes to cupric oxide when DTA4 molecules are used. This suggests that DTA4 molecules bind to reduced

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Amato et al. Table 2. IR Bands Characteristic of Adsorbed DTn and DTAn on Copper DT4

DT6

DT8

DTA4 DTA6 DTA8

Copper untr red untr red untr red νCH3 2951 νaCH2 2925 2925 2924 2924 2924 2923 2928 νsCH2 2854 2854 2853 2853 2853 2852 2855 νCdO 1697

Figure 5. Cu2p core level XPS spectra and Cu LMM Auger line of bare, untreated copper substrate (a,g), DT4 (b,h), and DTA4 (c,i) SAMs on untreated copper substrate, DT4 (d,j) and DTA4 (e,k) SAMs on electrochemically reduced copper substrate.

Figure 6. PM-IRRAS spectra of DT4 (a,b) and DTA4 (c) SAMs on untreated copper substrate (a) and on electrochemically reduced one (b,c).

copper substrates to form a less protective layer than DT4 molecules. Similar observations could be obtained by analyzing the Cu2p core level XPS spectra and the Cu LMM Auger line of DT6, DT8, DTA6, and DTA8 SAMs on untreated or reduced copper substrates. PM-IRRAS Analyses. The organization state of hydrogenated and fluorinated segments in DTn and DTAn SAMs copper substrate has been evaluated on the basis of PM-IRRAS analyses. The two regions of the IR spectrum containing the characteristic bands of polyfluorinated difunctional molecules have been examined. The first one is the C-H stretching region ranging from 2800 to 3000 cm-1. Representative spectra are presented in Figure 6. The CH2 symmetrical and asymmetrical vibration frequency values are related to hydrogenated chain organization:4 when the conformational order of the hydrocarbon chain increases, the νa(CH2) and νs(CH2) vibration frequencies approach 2918 and 2850 cm-1, respectively, indicating predominantly transextended conformation for the hydrogenated segment. An increase of these values is typical of lower organization of the monolayer, indicating that methylene groups possess random conformations.

red 2951 2929 2859 1696

2951 2925 2853 1698

The CH2 symmetrical and asymmetrical vibration frequencies values of DTn SAMs on the untreated copper substrates (Table 2) point to a lack of organization of the chains in the films. The main factor responsible for this phenomenon is the adsorption of dithiol molecules by either one or two of the terminal groups. This induces heterogeneity in the monolayer as well as the presence of a bulky fluorinated segment in the middle of the bound molecules. This bulky segment causes an important steric hindrance in the fold inducing a relatively long distance between the two anchoring groups of the doubly bound molecules and thus between their two hydrocarbon moieties. PM-IRRAS analysis of DTn SAMs on electrochemically reduced copper substrates gives very similar results to those ones obtained with the untreated copper substrates. The wavenumber values, which are reported in Table 2, are indicative of a random conformation of the hydrogenated moieties. On the infrared spectra of DTAn SAMs, the C-H stretching bands appear more intense than in the case of DTn SAMs. A shoulder at 2951 cm-1 corresponds to the CH3 stretch of thioacetate function, which is confirmed by the characteristic peak of carbonyl group at 1696 cm-1. The CH2 symmetrical and asymmetrical vibration frequency values of DTAn SAMs are still indicative of poor organization (Table 2). In Figure 7 (right) are reported the CF2 stretching bands of DTn SAMs on untreated copper substrates and DTAn SAMs on the reduced ones. According to the literature,4,17,73,74 the bands around 1200 cm-1 are referred to perpendicular CF2 stretching bands (νpdCF2) and those between 1300 and 1400 cm-1 to axial CF2 stretching bands (νaxCF2). In a previous work on octadecanethiol on Ag exposed for two hours to ambient atmosphere, a band around 1200 cm-1 has been shown to develop which has been assigned to a sulfonate stretch.31 In our case, however, this band cannot be assigned to a sulfonate stretch according to the fact that monolayers on reduced copper substrate lead to the same absorption band although no sulfonate has been detected by XPS as seen in Figure 8 (no peak at 168 eV). Note that, for each freshly prepared sample, PM-IRRAS analysis has been carried out just before XPS. The comparison of the relative

Figure 7. PM-IRRAS spectra of DT4 (a), DT6 (b), and DT8 (c) SAMs on untreated copper substrate and DTA4 (e), DTA6 (f), and DTA8 (g) SAMs on electrochemically reduced one.

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Figure 8. S2p core level XPS spectra (a,b) and corresponding PMIRRAS spectra (c,d) of DT6 SAMs on untreated copper (a,c) and on electrochemically reduced copper (b,d).

intensity of the νpdCF2 and the νaxCF2 bands gives information on the orientation of the helical fluorocarbon moiety to the surface normal. In DTn SAMs on untreated copper (Figure 7, left), νpdCF2 is very intense compared to νaxCF2. Consequently, fluorocarbon moieties seem to be mainly parallel to the metal surface. This can be explained by a weak folding of the fluorinated moieties in line with the long distance between the hydrocarbon moieties of doubly bound molecules. In the case of single bound molecules, this tendency can be explained by the weakness of Van der Waals interactions between the chain moieties, which are located “outside” of the monolayer and separated by doubly bound molecules, resulting in the impossibility for these chain moieties to stand vertically. Similar observations were obtained when analyzing DTn SAMs on electrochemically reduced copper (results not shown): the intensity of νpdCF2 is still very large compared to νaxCF2, which implies that fluorinated segments are mainly parallel to the surface. When DTAn molecules are used, the intensity of the characteristic bands of axial CF2 stretch increases, especially with DTA8. This observation is in good agreement with the results obtained from XPS analyses of S2p core level, which indicate that DTA8 molecules show the best faculty to bind reduced copper substrate by one terminal function per molecule. Electrochemical Characterizations. The electrochemical study of the different modified copper substrates involved in this work was carried out by cyclic voltammetry (CV). By scanning the range of copper oxidation and reduction potentials, we could assess the coverage of DTn and DTAn SAMs on untreated and comparatively on electrochemically reduced copper. The coverage represents the quantity of surface metallic sites involved in binding with a grafted molecule and thus inaccessible to reactive species. In Figure 9 are shown the cyclovoltammograms (CV) of DT6 and DTA6 SAMs. Similar CVs (not shown) have been recorded for DT4, DTA4, DT8, and DTA8 SAMs on both types of copper substrates. Coverages of DTn and DTAn SAMs have been estimated; they are given in Table 3. DTn SAMs exhibit good coverage (minimum 94% obtained with DT8) with a slight improvement when the substrate is reduced before modification (minimum 97.2% obtained with DT8). The steric hindrance generated by the presence of large molecules with hydrophobic character disadvantages the diffusion of reactive species to the surface, and thus high coverage can be obtained even if a large spacing exists between the two anchoring groups of a doubly grafted molecule. However, the coverage decreases

Figure 9. Cyclic voltammograms of a bare untreated copper substrate (solid line) (a) DT6 SAMs on untreated copper substrate (dotted line) and (b) DTA6 SAMs on electrochemically reduced copper substrate (dashed line) obtained in a 0.1 M hydroxide sodium aqueous solution at 20 mV/s (all potentials expressed vs SCE). Table 3. Covering Ratios Calculated from Experimental CV Curves of DTn and DTAn SAMs on Untreated and Electrochemically Reduced Copper Substrates untreated copper reduced copper

reduced copper

DT4

DT6

DT8

98.6 99.8

95.3 99.4

94 97.2

DTA4

DTA6

DTA8

47.7

67.0

88.3

as the length of the fluorocarbon segment increases (from 98.6% obtained with DT4 to 94% obtained with DT8, both on untreated substrate), indicating that the space between two anchoring moieties is larger. When an electrochemically reduced copper substrates are used, improved coverage is observed, which is in agreement with the noted better bonding of DTn molecules to this kind of substrates. Indeed, as we already mentioned in the XPS study, sulfinate and sulfonate species as anchoring moieties are not observed in that case. Thus, thiolates are the only possible desorbed molecules, which are more strongly bound to the

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substrate than sulfinates or sulfonates. The presence of the reduction peaks on the cathodic sweep is due to an alteration of DTn SAMs at high potentials, thus rendering the oxide copper layer accessible for electrochemical reduction. DTn SAMs on electrochemically reduced copper substrates are more resistant to high potentials than when untreated copper substrates are used. As expected, when DTAn molecules are used no protection against oxidation is observed for untreated copper substrates. As noted previously, thioacetate cannot reduce the copper oxide layer, and thus no SAMs on untreated copper substrate can be formed with the thioacetate function. On electrochemically reduced copper substrate, the surface coverage is considerably lower when DTn are replaced by DTAn. A possible explanation for this result could be the formation of an acetyl-copper bond, a side reaction occurring during the Cu-S bond formation in which DTAn chemisorbed via the catalytic deacetylation of the thiol by Cu to form the Cu-(CH2)11-(CF2)n-(CH2)11-SCOCH3.75 Thus, the presence of a Cu-acetyl region in the layer induced an attenuation of the protection against oxidation. In contrast to DTn SAMs, the coverage improves as the length of the fluorocarbon segment increases. This difference could be linked to the different way of organizing DTn and DTAn SAMs. DTn molecules bind the substrate principally via two thiols per molecule, while DTAn molecules have a higher tendency to bind the substrate by only one function per molecule. In the case of DTn molecules, it is found that the best coverage is obtained for the shortest fluorocarbon segments. On the other hand, when difunctional molecules adopt a standing-up orientation as in DTAn SAMs, the protective property against copper oxidation improves as the length of the polyfluorinated moiety increases.

Conclusion We have synthesized new polyfluorinated dithiol molecules HS-(CH2)11-(CF2)n-(CH2)11-SH called DTn and corresponding

Amato et al.

dithioacetate CH3CdOsS-(CH2)11-(CF2)n-(CH2)11-SsCdOCH3 called DTAn, with n ) 4, 6, and 8. The study of their adsorption on electroplated copper substrates in a dichloromethane solution during 15 h of modification time was investigated. First, we have shown that, in contrast to DTn molecules that could bind to untreated copper substrates to form SAMs, DTAn molecules cannot reduce copper oxide and thus cannot self-assemble. Second, we have compared the ability of dithiol and dithioacetate to bind on reduced copper substrates by only one terminal group per molecule. When thioacetate terminal functions are used on electrochemically reduced copper substrates, the proportion of doubly grafted molecules decreases as the perfluorinated segment increases. Furthermore, we have evaluated the protecting properties of the SAMs toward oxidation of copper. About 99% coverage was calculated with DT4 SAMs on both kinds of copper substrates. This percentage decreases as the length of the fluorocarbon segment increases. On the contrary, coverages observed with thioacetate coatings are improved with a longer fluorinated segment. However, their protecting properties are very poor compared to those obtained with DTn SAMs, even if the disorder induced in the layer decreases the coverage. Acknowledgment. C.A. and S.D. thank the Belgian National Interuniversity Research Program on ”Quantum size effects in nanostructure materials” (IUAP P5/01) and the Fonds de la Recherche Scientifique (FNRS), respectively. The authors acknowledge Drs. Zohra Benfodda and Abdel Dahmani, from the Max Mousseron Institute of Montpellier II University, for their contribution to the chemical analysis of molecules involved in the syntheses of DT4. Supporting Information Available: The synthesis, general experimental procedures, as well as additional experimental spectra have been added in this section. This material is available free of charge via the Internet at http://pubs.acs.org. LA800496D