New Semifluorinated Dithiols Self-Assembled Monolayers on a Copper Platform

New Semifluorinated Dithiols Self-Assembled Monolayers on a CopperPlatform

Claire Amato,†,§ Sebastien Devillers,†,§ Patrick Calas,‡ Joseph Delhalle,† andZineb 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 dithioacetatemolecules CH3COS-(CH2)11-(CF2)n-(CH2)11-SCOCH3, called DTAn (n ) 4, 6, 8), were synthesized and used to createself-assembled monolayers (SAMs) on both untreated copper surfaces and electrochemically reduced ones. The aimof this study is to assess the organization of the resulting SAMs, particularly the effect of the presence of twoperhydrogenated segments surrounding the perfluorinated one, and the ability of these difunctional molecules to bindcopper substrates by only one end per molecule. In each case, the organization of the SAM is rather poor and onlyDTA8 molecules seem to adopt an upright position on reduced copper. In addition, the layers have been investigatedby 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 goldhave been extensively studied. They can be considered as bilayerscontaining a perhydrogenated and a perfluorinated segment. Thelarger covalent radius of fluorine atoms relative to hydrogensand the helical conformation adopted by the perfluorinated chainslead to bulkier segments than the alkyl moieties, with Van derWaals diameters of 5.6 Å and 4.2 Å, respectively. As aconsequence, composition, structure, and interfacial propertiesof the polyfluorinated SAMs are different than in perhydrogenatedones.1-15 The critical influence of the fluorinated chain lengthin the self-assembly of terminally perfluorinated alkanethiolsCF3-(CF2)n-(CH2)m-SH monolayers on gold surfaces reveals thatthe monolayer’s organization largely depends on the n and mvalues. Molecules with longer fluorinated segments tend to self-

organize more readily into dense monolayers and adopt uprightpositions in the films.16-19

To date, gold has been the most frequently studied substratebecause of its good inertness to most potential contaminants aswell as the high affinity of sulfur for Au and the relative easeof obtaining high-quality monolayers from a large variety oforganothiol solutions. To extend the potential of the field, growingresearch 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

* Prof. Zineb Mekhalif, Tel.: +32-(0)81-72 52 30; fax: +32-(0)81-72 4600, E-mail address: [email protected].

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10.1021/la800496d CCC: $40.75 2008 American Chemical SocietyPublished on Web 08/23/2008

ZnNi.61 Copper is particularly interesting in the context of SAMsbecause it is largely used in modern microelectronics and isexpected to become even more crucial in the developingnanoelectronics. It is also a challenging substrate to modify withorganothiols31-33,39,62-64 because of its oxidative nature.65 Incontrast to gold, Cu requires specific surface preparation to bringit into the metallic state before modification and keep it in thatstate during the modification.

Increasing attention has been devoted toR,ω-difunctional thiolsand their incorporation in SAMs on different metals. Suchmolecules bearing a reactive functional group at each end allowthe preparation of chemically reactive surfaces and provide away of bonding adlayers to target substrates. The self-assemblyof R,ω-difunctional molecules is complicated by the fact thatboth terminal functional groups per molecule are reactive towardthe target substrate.66 Although the role of the molecular structureof alkanedithiols on the ultimate quality of the SAMs is still notfully understood, several experimental results suggest thatalkanedithiol molecules are arranged on the gold surface withtheir molecular axis parallel to the substrate due to the strongaffinity between sulfur and gold atoms.67,68 On the basis of STMmeasurements in the liquid phase, Esplandiu et al. have suggested

that alkanedithiol SAMs consist of molecular rows with a parallelarrangement as well as disordered structures.69 Mixed SAMs ongold composed of 1-decanethiol and 1,10-decanedithiol havebeen investigated by noncontact AFM; in that case, phaseseparation has been identified.70 Nevertheless, it has also beenshown that the ultimate molecular organization in the SAMsdepends on the self-assembly conditions as well as on themolecular structure.4,16,17,31,36,66,69,72,73,75 In that respect, dithio-acetate end functions tend to favor SAMs with molecules in theupright position. Hence, combining molecular structure togetherwith preparation conditions remains a promising avenue for thepreparation of SAMs based on R,ω-difunctional molecules.

To the best of our knowledge, no polyfluorinated alkanedithiolmolecules have been incorporated in SAMs on oxidizable metalsubstrates like Cu. The goal of this paper is the study of theself-assembly of polyfluorinated dithiols and correspondingdithioacetates on untreated copper substrates, on one hand, andelectrochemically reduced substrates, on the other hand. For thispurpose, we report the synthesis of highly fluorinated alkyldithiolHS-(CH2)11-(CF2)n-(CH2)11-SH with n) 4, 6, and 8, called DT4,DT6, and DT8, respectively. These new compounds are involvedin the formation of DTn SAMs on untreated copper, i.e., cleanedCu surfaces used without any subsequent treatment and onelectrochemically reduced copper substrates. For comparison,the corresponding polyfluorinated dithioacetates DTA4, DTA6,and DTA8 are also used to form DTAn SAMs. These newmolecules are of particular relevance owing to the presence oftwo anchoring terminal groups and a perfluorinated segmentsurrounded by two perhydrogenated moieties: alkyl/perfluoro-alky/alkyl. It is interesting to find out if the balance of the Vander Waals interactions due to these segments between neighboringmolecules is propitious to SAMs with molecules in the uprightposition. An interest in such SAMs would be to form an insulatingultrathin yet thick enough layer between a copper substrate andanother metallic adlayer (Cu, Au, etc.). Having the twofunctionalities in the desired configuration would have theadvantage of avoiding post-treatments on the SAMs beforeattaching the adlayer. An assembled system of this kindcorresponds to the so-called MIMs (metal/insulator/metal) neededin electronic devices.

XPS analysis is used first to assess the chemical state ofpolyfluorinated molecules after self-assembly. The ability ofdithiol and dithioacetate to bind copper substrate by only oneterminal group per molecule is appraised as a function ofperfluorinated chain length (n ) 4, 6, or 8), state of copper(untreated or electrochemically reduced), and type of terminalfunction (thiol or thioacetate). Second, it is used to investigatethe oxidation state of copper substrate after SAMs formation andthe formation mechanism of the alkylthiolate monolayers on thecopper substrate. Monolayers organization is assessed on thebasis of PM-IRRAS measurements. Finally, cyclic voltammetryexperiments are performed to estimate their coverage.

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10880 Langmuir, Vol. 24, No. 19, 2008 Amato et al.

Experimental SectionChemicals. Absolute ethanol (Merck), acetic acid (Prolabo), azobis

isobutyronitrile AIBN (Acros Organics), 1,2-dichloroethane (AcrosOrganics), dichloromethane (Romil SpS), n-dodecanethiol (AcrosOrganics, 98%), 1,6-diiodoperfluorohexane and 1,8-diiodoperfluo-rooctane (FluoroChem), diisopropyle azodicarboxylate DIAD (AcrosOrganics), hydrazine hydrate (Avocado), perchloric acid HClO4

(Janssen Chimica), sodium chloride NaCl (Acros Organics), sodiumfluoride 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 (18MΩ.cm), and zinc powder (Merck) were used without any furtherpurification.

Substrate Preparation. The foil copper substrates were suppliedby Metalor. They are cut in 2 × 1 cm2 coupons. Their averageroughness (Rrms) measured with a DEKTAK8 surface profilometeris 45 nm. These samples are modified through SAMs formation intwo different conditions: without any pretreatment (except for acleaning in ethanol for 15 min under sonication) and just after anelectrochemical reduction. In the latter case, the samples were dippedfor 10 min in a 0.5 M perchloric acid solution under argon at anapplied potential of -800 mV vs the saturated calomel electrode(SCE).

Monolayer Preparation and Characterizations. The reducedsubstrate is directly dipped in the modification bath for 15 h afterbeing dried under an argon flow in order to remove droplets ofelectrolytic solution. Modification bath is a 10-3 M organothiolsolution. Dichloromethane dried on magnesium sulfate is used assolvent. The organothiol solutions are bubbled under argon justbefore the modification to eliminate a maximum of oxygen. Aftermodification, the samples are copiously rinsed with absolute ethanol,subjected to sonication for 15 min in the same solvent, and rinsedagain with absolute ethanol. The samples were finally blown dry inan argon flow and used immediately for characterization.

Each surface modification has been repeated at least three timesfor each characterization method used in this work in order to ascertainthe reproducibility of the results presented here.

The monolayers were characterized by X-ray photoelectronspectroscopy (XPS), polarization modulation infrared reflectionabsorption spectroscopy (PM-IRRAS), and cyclic voltammetry (CV).XPS is used to evaluate the elemental composition of the monolayersand the oxidation state of sulfur and to determine the chemical stateof the copper. The photoelectron spectra of the monolayers havebeen obtained with a SSX-100 spectrometer using a monochromatizedX-ray Al KR radiation (1486.6 eV), the photoemitted electrons beingcollected at 35° takeoff angle. Nominal resolution was measured asfull width at half-maximum of 1.0-1.5 eV for core levels and survey

spectra, respectively. The binding energy of core levels was calibratedagainst the C1s binding energy set at 285.0 eV, an energycharacteristic of alkyl moieties. The peaks were analyzed using mixedGaussian-Lorentzian curves (80% of Gaussian character). The S2pline, which is a doublet structure where the S2p3/2 and S2p1/2

components are spaced by 1.18 eV and have an intensity ratioS2p3/2/S2p1/2 of 2, was analyzed accordingly.

Polarization modulation infrared reflection absorption spectro-scopy (PM-IRRAS) data are collected to assess the monolayersorganization. They were registered on a Brucker Equinox 55-PMA37equipped with a liquid nitrogen cooled mercury-cadmium-telluride(MCT) detector and a zinc-selenide photoelastic modulator. Theinfrared light was modulated between s- and p-polarizaton at afrequency of 50 kHz and an incident angle upon the sample surfaceof around 85°. Signals generated from each polarization (Rs and Rp)were detected simultaneously by a lock-in amplifier and used tocalculate the differential surface reflectivity (∆R/R) ) (Rp - Rs)/(Rp

+Rs). All spectra are the average of 640 scans at a spectral resolutionof 2 cm-1.

Electrochemical techniques provide additional information onthe monolayer quality through coverage and resistance to oxidation.Experiments were carried out with an EG&G Instruments poten-siostat, model 263A, monitored by computer and M270 electro-chemistry software. A three-electrode electrochemical cell was usedwith SCE as reference electrode and a platinum foil as counterelectrode. The cell used enables analysis of a well-defined andreproducible spot on the sample. Cyclic voltammetry was carriedout in a 0.1 M hydroxide sodium solution by sweeping a range ofpotential from-1.1 to 0.6 V at 20 mV/s. Covering has been calculatedby measuring the area of the copper oxidation peaks for unmodifiedsubstrates (Au) and for modified ones (Am) and by applying tofollowing formula C(%) ) 100 × (Au - Am)/Au.

Results and Discussion

GeneralSynthesisRoutetowardPerfluorinatedAlkyldithiols.The synthesis route to target compounds 5a, 5b, and 5c calledDT4, DT6, and DT8, respectively, is depicted in Scheme 1 andsynthesis data are collected in the Supporting Information. Startingfrom R,ω-diiodoperfluoroalkane I-(CF2)n-I 1a (n ) 4), 1b (n )6), and 1c (n ) 8), DT4, DT6, and DT8 are obtained in foursteps. The first step is the radical chain addition of 1a,b,c toundecyl-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 stepis the reduction of the iodide functions in 2a,b,c to form thereduced diadduct compounds HO-(CH2)11-(CF2)n-(CH2)11-OH

Scheme 1. Synthesis of r,ω-semifluorinated dithiols 5a, 5b, and 5c called DT4, DT6, and DT8, respectively, starting fromr,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.

Semifluorinated Dithiols SAMs on Cu Langmuir, Vol. 24, No. 19, 2008 10881

3a (n ) 4), 3b (n ) 6), and 3c (n ) 8) in 73%, 70%, and 73%yield, respectively. The third step is the conversion of alcohols3a,b,c into thiolacetates H3COCS-(CH2)11-(CF2)n-(CH2)11-SCOCH3 4a (n ) 4), 4b (n ) 6), and 4c (n ) 8) using theMitsunobu reaction yielding 62%, 60%, and 64%, respectively.Finally, the dithioacetate 4a,b,c are treated by hydrazine hydratein order to deprotect the thiol functions. Perfluorinated dithiolsHS-(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 SupportingInformation.

XPS Investigations. XPS experiments were performed forDTn and DTAn modified copper substrates immersed indichloromethane solution for 15 h. Figure 1 shows the C1s corelevel XPS spectra of DTn and DTAn SAMs on untreated andelectrochemically reduced copper, respectively. On untreatedcopper substrates, the presence of fluorine atoms is clearlydemonstrated in DTn SAMs by the two characteristic peaks ofdifluoromethylene groups at 290.8 and 291.6 eV (Figure 1a-c).When DTAn molecules are used, no characteristic peak of thefluorinated segment is observed in the region above 290 eV,revealing the inefficiency of thioacetate function to reduce copperoxide and thus the impossibility of forming DTAn SAMs onuntreated copper substrates with the present self-assemblyconditions. This is confirmed by the absence of a characteristicpeak 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 centeredat 162.6, 163.8, 165.5, and 168.0 eV, corresponding to thiolates,unbound thiols, sulfinates, and sulfonates, respectively. Thepresence of thiolate species proves the chemical binding of DTnto the untreated copper surface. Sulfinate and sulfonate speciesare formed by oxidation of chemisorbed sulfur atoms by thecopper oxide layer present on the substrate. It has been shownthat these species are less strongly bound and usually desorbedand replaced by other thiol molecules present in solu-tion.34,36,39,63,64,71 In our case, sulfinate and sulfonate species arestill present after 15 h of modification and no other thiol moleculehas reached the surface to replace them. We could also note thatsulfinate and sulfonate characteristic peaks increase with thelength of DTn molecules. This phenomenon could be explainedby the nature of the grafted molecules. DTn are large molecules(roughly 50 Å length); thus, they probably have limited mobilitythat disadvantages the exchange of molecules at the metal surface

and therefore the complete replacement of oxidized sulfur atomsby chemisorbed thiolates. The signal at 163.8 eV, assigned tosulfur atoms in free thiol groups, indicates that a certain amountof thiol groups does not react with the copper surface to formS-Cu bonds.

On electrochemically reduced copper substrate, both DTn andDTAn SAMs show characteristic peaks of fluorinated segmentin the C1s core level spectra. An additional peak centered at288.8 eV is obtained with DTAn SAMs, corresponding to thecarbonyl group of the thioacetate function (Figure 1d-f). On theS2p core level spectra of DTn SAMs (Figure 3d-f), the sulfinateand sulfonate species are not detected. Thus, we conclude thatdithiol molecules bind to a reduced copper surface without beingoxidized to sulfonate or sulfinate species as when they bind toan untreated copper surface. In the case of DTAn SAMs, sulfonateand sulfinate species are also absent, and two S2p3/2 componentscentered at 162.6 and 163.8 eV corresponding to thiolate andthioacetate, respectively, are observed (Figure 4). It indicatesthat the thioacetate function could bind to reduced coppersubstrates.

We now assess how difunctional molecules bind to coppersubstrates, via one end function or both. We report in Table 1the “bound sulfur”/“unbound sulfur” ratio (Sb/Su) after 15 h ofmodification for DTn and DTAn SAMs. According to Lambert-Beer’s law,72 if dithiol or dithioacetate molecules bind to thesubstrate via only one terminal function per molecule, the Sb/Su

ratio must be less than 1. Indeed, for difunctional moleculeswhich adopt a standing-up orientation with their molecularbackbone oriented perpendicular to the surface, electronsemerging from the sulfur atom bound to the copper substrate areattenuated by the overlying organic monolayer -(CH2)11-(CF2)n-(CH2)11-SX (X ) H or COCH3). On the contrary, electronsemerging from the sulfur atoms in thiol or thioacetate groupsexposed to the surface of the SAM do not experience suchattenuation. The Sb/Su ratio for DT4, DT6, and DT8 SAMs onuntreated copper substrate is estimated by considering sulfinateand sulfonate species as bound sulfur. This ratio is found to begreater than unity. Thus, we conclude that a significant quantityof dithiol molecules bind to the substrate by both terminalfunctions per dithiol. This proportion is lower with DT8 or DT6than with DT4. When an electrochemically reduced coppersubstrate is employed, the Sb/Su ratio value decreases for DT4and DT6 SAMs. It reveals that, when the substrate is reducedbefore modification, the tendency of DT4 and DT6 moleculesto chemisorb via only one of their thiol functions increases. WithDT8 SAMs, no difference is noticed for the Sb/Su ratio whetherthe substrate is previously reduced or not. With DTAn SAMson electrochemically reduced copper substrate, the Sb/Su ratio islower than with DTn SAMs (Table 1). We could emphasize thatthe tendency of difunctional molecules (DTn and DTAn) to bindthe substrate via one terminal function per molecule improveswith the increase of the perfluorinated chain length and with theuse of a previously reduced copper substrate. Moreover, DTAnmolecules favor the formation of self-assembly by only oneterminal function, especially for n ) 8. This may be due to thelower reactivity of thioacetate groups for copper.

XPS spectra of the Cu2p core levels and Cu LMM Auger linefor DT4 and DTA4 modified copper substrates and for bare,untreated copper are reported in Figure 5. DT4 SAMs on untreatedcopper substrate show the loss of Cu2p peak at 933.5 eV andthe extinction of the satellite peaks around 938-945 eV, whichare characteristic of the presence of CuO (Figure 5b). It confirmsthat modification of the substrate induces the reduction of theCuO present in the oxide layer. As was already men-

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.


tioned,35,36,39,63,64 the oxide layer CuO could be reduced byoxidation of the alkanethiols into disulfide, sulfinate, and sulfonatespecies.

When DTA4 molecules are used, XPS analyses of S2p andC1s core levels has revealed that no SAM on untreated coppersubstrate is obtained with the present self-assembly conditions.This is confirmed in the Cu2p XPS core level spectrum of DTA4SAMs (Figure 5c) by the CuO satellite peaks around 938-945eV and characteristic peaks at 934.7 and 933.5 eV assigned toCu(OH)2 and CuO, respectively. The absence of the Cu(0) peakat 568 eV in LMM Auger line of DT4 and DTA4 SAMs onuntreated copper substrates (Figure 5h-i) clearly indicates thatmetallic copper is not formed during SAMs elaboration as ispossible with smaller molecules such as n-dodecanethiol orn-dodecaneselenol.64

XPS analyses of DT4 SAMs on electrochemically reducedcopper substrate reveal no cupric oxide CuO characteristic peakin the Cu2p core level spectrum (Figure 5d) and the presenceof metallic copper, by the shoulder at 568.1 eV in the LMM

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

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.

Figure 4. S2p core level XPS spectra of DTA4 (a), DTA6 (b), andDTA8 (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

DT4 SAMs DT6 SAMs DT8 SAMs

untreated copper 16.6 2.5 1.4reduced copper 4.8 1.3 1.4

DTA4 SAMs DTA6 SAMs DTA8 SAMsreduced copper 1.5 1.2 0.6


copper substrates to form a less protective layer than DT4molecules.

Similar observations could be obtained by analyzing the Cu2pcore level XPS spectra and the Cu LMM Auger line of DT6,DT8, DTA6, and DTA8 SAMs on untreated or reduced coppersubstrates.

PM-IRRAS Analyses. The organization state of hydrogenatedand fluorinated segments in DTn and DTAn SAMs coppersubstrate has been evaluated on the basis of PM-IRRAS analyses.The two regions of the IR spectrum containing the characteristicbands of polyfluorinated difunctional molecules have beenexamined. The first one is the C-H stretching region rangingfrom 2800 to 3000 cm-1. Representative spectra are presentedin Figure 6. The CH2 symmetrical and asymmetrical vibrationfrequency 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 2918and 2850 cm-1, respectively, indicating predominantly trans-extended conformation for the hydrogenated segment. An increaseof these values is typical of lower organization of the monolayer,indicating that methylene groups possess random conformations.

The CH2 symmetrical and asymmetrical vibration frequenciesvalues of DTn SAMs on the untreated copper substrates (Table2) point to a lack of organization of the chains in the films. Themain factor responsible for this phenomenon is the adsorptionof dithiol molecules by either one or two of the terminal groups.This induces heterogeneity in the monolayer as well as thepresence of a bulky fluorinated segment in the middle of thebound molecules. This bulky segment causes an important sterichindrance in the fold inducing a relatively long distance betweenthe two anchoring groups of the doubly bound molecules andthus between their two hydrocarbon moieties. PM-IRRAS analysisof DTn SAMs on electrochemically reduced copper substratesgives very similar results to those ones obtained with the untreatedcopper substrates. The wavenumber values, which are reportedin Table 2, are indicative of a random conformation of thehydrogenated moieties. On the infrared spectra of DTAn SAMs,the C-H stretching bands appear more intense than in the caseof DTn SAMs. A shoulder at 2951 cm-1 corresponds to the CH3

stretch of thioacetate function, which is confirmed by thecharacteristic peak of carbonyl group at 1696 cm-1. The CH2

symmetrical and asymmetrical vibration frequency values ofDTAn SAMs are still indicative of poor organization (Table 2).

In Figure 7 (right) are reported the CF2 stretching bands ofDTn SAMs on untreated copper substrates and DTAn SAMs onthe reduced ones. According to the literature,4,17,73,74 the bandsaround 1200 cm-1 are referred to perpendicular CF2 stretchingbands (νpdCF2) and those between 1300 and 1400 cm-1 to axialCF2 stretching bands (νaxCF2). In a previous work on octade-canethiol on Ag exposed for two hours to ambient atmosphere,a band around 1200 cm-1 has been shown to develop which hasbeen assigned to a sulfonate stretch.31 In our case, however, thisband cannot be assigned to a sulfonate stretch according to thefact that monolayers on reduced copper substrate lead to thesame absorption band although no sulfonate has been detectedby XPS as seen in Figure 8 (no peak at 168 eV). Note that, foreach freshly prepared sample, PM-IRRAS analysis has beencarried out just before XPS. The comparison of the relative

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 onuntreated copper substrate, DT4 (d,j) and DTA4 (e,k) SAMs onelectrochemically reduced copper substrate.

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

Table 2. IR Bands Characteristic of Adsorbed DTn and DTAnon Copper

DT4 DT6 DT8 DTA4 DTA6 DTA8

Copper untr red untr red untr red redνCH3 2951 2951 2951νaCH2 2925 2925 2924 2924 2924 2923 2928 2929 2925νsCH2 2854 2854 2853 2853 2853 2852 2855 2859 2853νCdO 1697 1696 1698

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


intensity of the νpdCF2 and the νaxCF2 bands gives informationon the orientation of the helical fluorocarbon moiety to the surfacenormal. In DTn SAMs on untreated copper (Figure 7, left), νpdCF2

is very intense compared to νaxCF2. Consequently, fluorocarbonmoieties seem to be mainly parallel to the metal surface. Thiscan be explained by a weak folding of the fluorinated moietiesin line with the long distance between the hydrocarbon moietiesof doubly bound molecules. In the case of single bound molecules,this tendency can be explained by the weakness of Van derWaals interactions between the chain moieties, which are located“outside” of the monolayer and separated by doubly boundmolecules, resulting in the impossibility for these chain moietiesto stand vertically. Similar observations were obtained whenanalyzing DTn SAMs on electrochemically reduced copper(results not shown): the intensity of νpdCF2 is still very largecompared to νaxCF2, which implies that fluorinated segments aremainly parallel to the surface. When DTAn molecules are used,the intensity of the characteristic bands of axial CF2 stretchincreases, especially with DTA8. This observation is in goodagreement with the results obtained from XPS analyses of S2pcore level, which indicate that DTA8 molecules show the bestfaculty to bind reduced copper substrate by one terminal functionper molecule.

Electrochemical Characterizations. The electrochemicalstudy of the different modified copper substrates involved in thiswork was carried out by cyclic voltammetry (CV). By scanningthe range of copper oxidation and reduction potentials, we couldassess the coverage of DTn and DTAn SAMs on untreated andcomparatively on electrochemically reduced copper. The coveragerepresents the quantity of surface metallic sites involved in bindingwith a grafted molecule and thus inaccessible to reactive species.In Figure 9 are shown the cyclovoltammograms (CV) of DT6and DTA6 SAMs. Similar CVs (not shown) have been recordedfor DT4, DTA4, DT8, and DTA8 SAMs on both types of coppersubstrates. Coverages of DTn and DTAn SAMs have beenestimated; they are given in Table 3.

DTn SAMs exhibit good coverage (minimum 94% obtainedwith DT8) with a slight improvement when the substrate is reducedbefore modification (minimum 97.2% obtained with DT8). Thesteric hindrance generated by the presence of large moleculeswith hydrophobic character disadvantages the diffusion of reactivespecies to the surface, and thus high coverage can be obtainedeven if a large spacing exists between the two anchoring groupsof a doubly grafted molecule. However, the coverage decreases

as the length of the fluorocarbon segment increases (from 98.6%obtained with DT4 to 94% obtained with DT8, both on untreatedsubstrate), indicating that the space between two anchoringmoieties is larger. When an electrochemically reduced coppersubstrates are used, improved coverage is observed, which is inagreement with the noted better bonding of DTn molecules tothis kind of substrates. Indeed, as we already mentioned in theXPS study, sulfinate and sulfonate species as anchoring moietiesare not observed in that case. Thus, thiolates are the only possibledesorbed molecules, which are more strongly bound to the

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

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 solutionat 20 mV/s (all potentials expressed vs SCE).

Table 3. Covering Ratios Calculated from Experimental CVCurves of DTn and DTAn SAMs on Untreated and

Electrochemically Reduced Copper Substrates

DT4 DT6 DT8

untreated copper 98.6 95.3 94reduced copper 99.8 99.4 97.2

DTA4 DTA6 DTA8

reduced copper 47.7 67.0 88.3


substrate than sulfinates or sulfonates. The presence of thereduction peaks on the cathodic sweep is due to an alteration ofDTn SAMs at high potentials, thus rendering the oxide copperlayer accessible for electrochemical reduction. DTn SAMs onelectrochemically reduced copper substrates are more resistantto high potentials than when untreated copper substrates areused.

As expected, when DTAn molecules are used no protectionagainst oxidation is observed for untreated copper substrates. Asnoted previously, thioacetate cannot reduce the copper oxidelayer, and thus no SAMs on untreated copper substrate can beformed with the thioacetate function. On electrochemicallyreduced copper substrate, the surface coverage is considerablylower when DTn are replaced by DTAn. A possible explanationfor this result could be the formation of an acetyl-copper bond,a side reaction occurring during the Cu-S bond formation inwhich DTAn chemisorbed via the catalytic deacetylation of thethiol 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 anattenuation of the protection against oxidation. In contrast toDTn SAMs, the coverage improves as the length of thefluorocarbon segment increases. This difference could be linkedto the different way of organizing DTn and DTAn SAMs. DTnmolecules bind the substrate principally via two thiols permolecule, while DTAn molecules have a higher tendency tobind the substrate by only one function per molecule. In the caseof DTn molecules, it is found that the best coverage is obtainedfor the shortest fluorocarbon segments. On the other hand, whendifunctional molecules adopt a standing-up orientation as in DTAnSAMs, the protective property against copper oxidation improvesas the length of the polyfluorinated moiety increases.

Conclusion

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

dithioacetate CH3CdOsS-(CH2)11-(CF2)n-(CH2)11-SsCdOCH3

called DTAn, with n ) 4, 6, and 8. The study of their adsorptionon electroplated copper substrates in a dichloromethane solutionduring 15 h of modification time was investigated. First, we haveshown that, in contrast to DTn molecules that could bind tountreated copper substrates to form SAMs, DTAn moleculescannot reduce copper oxide and thus cannot self-assemble.Second, we have compared the ability of dithiol and dithioacetateto bind on reduced copper substrates by only one terminal groupper molecule. When thioacetate terminal functions are used onelectrochemically reduced copper substrates, the proportion ofdoubly grafted molecules decreases as the perfluorinated segmentincreases. Furthermore, we have evaluated the protectingproperties of the SAMs toward oxidation of copper. About 99%coverage was calculated with DT4 SAMs on both kinds of coppersubstrates. This percentage decreases as the length of thefluorocarbon segment increases. On the contrary, coveragesobserved with thioacetate coatings are improved with a longerfluorinated segment. However, their protecting properties arevery poor compared to those obtained with DTn SAMs, even ifthe disorder induced in the layer decreases the coverage.

Acknowledgment. C.A. and S.D. thank the Belgian NationalInteruniversity Research Program on ”Quantum size effects innanostructure materials” (IUAP P5/01) and the Fonds de laRecherche Scientifique (FNRS), respectively. The authorsacknowledge Drs. Zohra Benfodda and Abdel Dahmani, fromthe Max Mousseron Institute of Montpellier II University, fortheir contribution to the chemical analysis of molecules involvedin the syntheses of DT4.

Supporting Information Available: The synthesis, generalexperimental procedures, as well as additional experimental spectra havebeen added in this section. This material is available free of charge viathe Internet at http://pubs.acs.org.

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