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Alkanethiol-oxidized copper interface: The critical influence of concentration
Journal of Colloid and Interface Science 326 (2008) 333–338
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Alkanethiol-oxidized copper interface: The critical influence of concentration G. Fonder, F. Laffineur, J. Delhalle, Z. Mekhalif ∗ Laboratory of Chemistry and Electrochemistry of Surfaces (CES), University of Namur, FUNDP, Rue de Bruxelles 61, B-5000 Namur, Belgium
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 15 April 2008 Accepted 18 June 2008 Available online 8 August 2008
In this contribution, self-assembled monolayers of n-dodecanethiol (C12 H25 SH) at different concentrations on polycrystalline copper have been elaborated. Using XPS, PM-IRRAS, and electrochemical methods (cyclic voltammetry curves and cathodic desorption), the effect of the C12 H25 SH concentration on the reduction of the oxide layer has been studied. In all cases, a monolayer of good quality has been obtained. Results provide proof that while the concentration is increased, the thickness of the oxide layer is decreased, to a point that leads to metallic copper for the higher concentration. The results presented in this publication indicate the importance of controlling the interface when forming SAMs of organothiols on oxidizable metals. © 2008 Elsevier Inc. All rights reserved.
Keywords: Copper SAM Oxide reduction Thiol
1. Introduction Copper has a wide range of industrial applications, primarily related to its excellent thermal and electrical conductivity. It also recently became a metal of choice as interconnects in semiconductor integrated circuits. For most of those applications, copper needs to be protected from the environment in which it is required to operate. One approach that can be taken to address this problem is surface modification using a self-assembled monolayer (SAM) that has the potential to inhibit surface oxidation [1–38]. When SAMs are dense and highly organized, they form a real protective film on the surface, making it resistant to ambient oxidation. Laibinis et al. have extensively studied the self-assembly of a range of alkanethiols on evaporated copper [1–4]. As a direct result of this extensive study, they have shown that the SAM protects the underlying copper from oxidation and that a relationship exists between the hydrocarbon chain length and the effectiveness of the SAM at achieving this objective. Rubinstein and co-workers have studied the influence of the solvent and the presence of a copper oxide layer on the surface prior to SAM deposition [15], while Feng et al. specifically studied the behavior of SAM on copper in chloride ion environments [13]. It has been shown that SAMs exhibit superior corrosion protection to benzotriazole [13]. These studies also indicate that the use of thiols as a means of forming a protective layer on a substrate proved to be effective at achieving this objective. However, the long-term stability of the SAM layers was found to be limited. Aramaki and co-workers developed the approach of bilayer structure
*
Corresponding author. E-mail address: [email protected] (Z. Mekhalif).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.06.038
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2008 Elsevier Inc. All rights reserved.
formation using alcohol-terminated alkanethiols and modifying the monolayer with an alkyltrichlorosilane to form a 2-dimensional polymer monolayer [5,39–41]. Similarly, we performed in previous work a molecular bilayer as well as mixed monolayers on copper with great protection efficiency for the bilayer system [27,42–45]. We have shown in more recent work that electroplated copper has a typical behavior [46,47]. Significant reduction of its oxide layer by alkanethiol or alkaneselenol has been evidenced. This efficiency has been shown to be dependent on the nature of the anchoring group. Selenol- and thiol-based molecules were compared with their homologues disulfide and diselenide. In the present work we focus on bulk copper, which seems to exhibit different behavior than electroplated copper under conditions similar to those in the previous work. The objective is to better understand the oxide reduction process of the oxide layer on bulk copper compared to electroplated copper. We also emphasize the role of the concentration of the organothiol and establish its impact on the efficiency of the oxide reduction. 2. Experimental 2.1. Materials n-Dodecanethiol (Aldrich, 98% D22,140-6), NaCl (Acros, 99.5%, 207790010), LiClO4 (Acros, 99+%, 194715000), acetonitrile (Lab-scan, HPLC analysis, C27C11X), absolute ethanol (Merck, 1.00983.2500), and ultrapure water (18.2 M cm) were used without further purification. The 1-, 3-, and 9-μm diamond paste and colloidal silica used in the polishing process were supplied by Beuhler. The copper sheets were obtained from Goodfellow, 99.99% (CU000748).
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2.2. Sample preparation The polycrystalline copper substrate was supplied as a sheet approximately 1 mm thick. These sheets were cut onto 10 × 20 mm coupons, polished (Phoenix 4000 machine) with 1200 grit silicon carbide paper, and subjected to further polishing with 9-, 3-, and 1-μm diamond paste suspensions; the final polishing stage was carried out using a 0.5-μm colloidal silica suspension. The samples were washed with Milli-Q water (18.2 M cm) after a polishing step. The resulting copper substrates were rinsed in absolute ethanol. Before modification, copper substrates were subjected to 15 min of UV/O3 activation and sonicated 20 min in absolute ethanol. For the modification, the SAMs were formed by immersing the copper in n-dodecanethiol solutions of ethanol for 2 h at different concentrations from millimolar to solvent-free. Immediately after modification, the copper samples were removed from the solution, rinsed with absolute ethanol, and dried under a stream of argon. The copper pieces were then subjected to a further 10 min sonication in absolute ethanol to remove excess reactants, dried, and stored under argon. 2.3. Analysis techniques 2.3.1. XPS measurements In this study XPS is used to reveal the surface chemical interactions that occur between organic thiols and the copper surface. Spectra were collected on a Surface Science SSX-100 spectrometer. The photoelectrons were excited using AlK α radiation as the excitation source (1486.6 eV). The photoelectrons were collected at 35◦ from the surface normal and detected with a hemispherical analyzer. The spot size of the XPS source on the sample was 600 μm, and the analyzer was operated with a pass energy of 20 eV. During data acquisition the pressure was kept below (1 × 10−9 Torr), and the binding energies of the peaks obtained were made with reference to the binding energy of the C1s line, set at 285.0 eV. The spectrum was fitted using an 80%/20% linear combination of Gaussian and Lorentzian profiles. To obtain information about the thickness of the oxide layer, progressive etching was carried out by bombarding the surface with 2-keV argon ions. 2.3.2. Electrochemical analysis All cyclic voltammetry measurements were carried out in 0.1 M NaOH aqueous solutions (EG&G Princeton Applied Research, Potentiostat/Galvanostat Model 273A). The electrode was scanned over a −1.0 to 0.4 V potential range, at a scan rate of 20 mV s−1 . All electrochemical measurements were made with reference to a SCE (saturated calomel electrode) and a platinum counter electrode. All polarization measurements were performed in deaerated 0.5 M NaCl aqueous solution and recorded from −1.0 to 1.0 V, at a sweep rate of 5 mV s−1 . Cathodic desorption was performed in a deareated acetonitrile solution with LiClO4 (0.1 M) using cyclic voltammetry (scan rate 50 mV s−1 ). Potential was measured versus the Ag/AgCl reference electrode. 2.3.3. PM-IRRAS Polarization modulation infrared reflection absorption spectroscopy data were collected from a Bruker Equinox55 PMA37 equipped with a liquid-nitrogen-cooled mercury–cadmium-telluride (MCT) detector and a zinc-selenide photoelastic modulator. The infrared light, reaching the sample surface at an angle of 85◦ , was modulated between s- and p-polarization at a frequency of 50 kHz. Signals generated from each polarization (R s and R p ) were detected simultaneously by a lock-in amplifier and used to calculate the differential surface reflectivity R / R = ( R p − R s )/
Fig. 1. Cyclic voltammetry of bare copper and copper modified with C12 SH elaborated at different concentrations.
( R p + R s ). The spectra were taken by collecting 640 scans at a spectral resolution of 2 cm−1 . 3. Results and discussion Cyclic voltammetry curves of naturally oxidized copper modified with n-dodecanethiol (C12 SH) ethanolic solution at different concentrations are shown in Fig. 1. The comparison of the CV curves of bare copper and the modified substrates highlights the formation of particularly insulating SAMs, indicating effective blocking of the surface. This outcome could be due to a synergetic effect of the copper oxide layer and the adsorbed n-dodecanethiol molecules. The analysis of the oxidation blocking effect versus the concentration reveals several evident distinctions. In all cases, a slight oxidation current starts at a potential close to +140 mV. This current is higher as the C12 SH concentration decreases. The relative ratio of the oxidation charges of copper with and without modification indicates a surface blockage of about 95% for copper treated with C12 SH solvent-free and a 10−1 M solution. This value is 10% lower (85%) when the monolayer is elaborated from more diluted solutions (10−2 and 10−3 M). The concentration has been shown to be of less importance when noble metals such as gold are used [48]. It has indeed a significant effect when oxidizable metals constitute the platform for organothiol adsorption [49,50]. Modification from lower concentrated solutions seems to provide modified surfaces less stable and more easily prone to oxidative desorption. This could be a consequence of weaker interface bonds between the organothiol and the substrate, most likely because of still remaining oxides on the surface. Another parameter that supports the instability and differences in the interface nature and quality as a function of the C12 SH concentration is the reverse cathodic peak arising at about −540 and −880 mV. The current density of this peak, as well as the cathodic charge, increases as the concentration decreases. This is an addi-
G. Fonder et al. / Journal of Colloid and Interface Science 326 (2008) 333–338
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Table 1 Ratio C/S, S/C, and O/Cu calculated from the XPS core levels for monolayers of C12 SH at different concentrations on oxide copper Concentration (M)
C/S
S/Cu
O/Cu
Solvent-free 10−1 10−2 10−3
20 20 18 17
0.2 0.2 0.4 0.3
0.5 0.4 0.8 1.0
Fig. 2. PM-IRRAS spectra for C12 SH monolayer elaborated at different concentrations.
tional proof that the monolayers from diluted C12 SH solutions are less stable and therefore the copper is more oxidized at high potential. This could be also due to the reduction of the oxide still present on copper after modification with SAM. This will be discussed later using XPS investigations. To evidence the difference of behavior between bulk copper and electroplated copper, it is necessary to compare the present results to previous results on C12 SH SAM on electroplated copper, where it is found that for a similar solution (C12 SH 10−2 M) the electrochemical stability of the SAMs is better than that noted here on the bulk polished copper [46,47]. Good and very stable SAMs have been obtained on electroplated copper even after 5 min immersion time. At the reverse cycle, the current density in that case was almost zero, a strong indication of the stability of the monolayer versus oxidation. We relate this to the absence of surface oxide, completely reduced by the thiol during the adsorption process. To understand this difference, we discuss in the sequel the spectroscopic analysis of C12 SH SAMs differing by the concentration of the solutions where they have been elaborated. PM-IRRAS and XPS are very complementary; the first technique emphasizes the organization of the monolayer, while the second gives information on the composition and interface nature. The results should be helpful in emphasizing differences between electroplated and polished copper as well as the concentration effect on the selfassembly process. Fig. 2 displays the PM-IRRAS spectra. In all cases, four peaks are observed at similar wavelengths ±2 cm−1 , corresponding to the resolution of the spectrometer. These peaks are related to asymmetric CH3 vibrations (νa (CH3 )) at 2960 cm−1 , asymmetric CH2 (νa (CH2 )) at 2921 cm−1 , symmetric CH3 (νs (CH3 )) at 2873 cm−1 , and symmetric CH2 (νs (CH2 )) at 2851 cm−1 . The values of νa (CH2 ) and νs (CH2 ) are close to 2918 and 2850 cm−1 , respectively, values that are indicative of organized monolayers [51,52]. The SAMs spectra exhibit a small shift of 3 cm−1 toward higher frequencies for νa (CH2 ). The concentration effect does not seem to have an effect on the organization of the monolayer. This small difference can be a consequence of the presence of the oxide or an influence of the roughness, which is larger for the polished than electroplated copper. Table 1 displays the data extracted from XPS measurements of copper modified with C12 SH with different concentrations from
Fig. 3. Oxygen core-level for C12 SH formed at different concentrations.
10−3 M to pure C12 SH (solvent-free). One has to emphasize that the carbon core C1s level for all these monolayers shows a unique component at 285.0 eV, typical of the alkyl chain of the molecule. The sulfur core-level S2p is also present as a doublet centered at 162.4 eV. The SAMs are free from contaminants and from oxidized or free sulfur. The C/S ratio varies very slightly; a small decrease is noticed for the diluted solution (10−3 M), but not in a significant way. The S/Cu ratio does not show great differences except for the 10−2 M solution, for which the ratio is slightly higher. However, more differences are seen for the O/Cu ratio, which decreases from a value close to 1 for SAMs elaborated from 10−3 M solution of C12 SH down to 0.4 for the more concentrated solutions (10−1 M). It is interesting to note that pure C12 SH, does not allow full reduction of all the oxide. The oxide layer of polished bulk copper seems to be more resistant to reduction than the electroplated copper. In particular, with a selenol anchoring group, copper is oxide-free after modification. With C12 SH, the oxide is present in trace amounts already after 5 min immersion time [46]. Fig. 3 displays the oxygen core level O1s and Fig. 4 shows the LMM Auger lines for oxidized copper modified with C12 SH at different concentrations. The O1s peak, as well as the LMM Auger lines, varies significantly with the concentration of C12 SH.
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The second step is the reaction of the thiol with the Cu2 O. Two types of reactions are proposed in the literature as depicted in the following equations: 2RSH + Cu2 O → 2RSCu + H2 O,
(1)
or 2RSH + Cu2 O → 2Cu + RS-SR + H2 O.
Fig. 4. CuLMM spectra for C12 SH formed at different concentrations.
The different peaks appear at the following binding energies: 529.5 eV (CuO) and 530.2 eV (Cu2 O). Peaks at 531.7 and 532.8 eV may correspond to (Cu(OH)2 ) and adsorbed water, respectively. At low concentrations (10−3 M), the oxygen peak at 529.5 eV indicates the presence of the cupric oxide, with an atomic composition of about 19%. Three other components are detected: 58% for the oxygen related to Cu2 O (530.2 eV), 18% for that coming from Cu(OH)2 (531.7 eV), and the rest (5%) due to the adsorbed water (532.8 eV). The CuO disappears with the increase of the concentration. Significant modifications are obtained with C12 SH solvent-free and a solution at 10−1 M, a small quantity of adsorbed water still remains (532.8 eV), and Cu(OH)2 is of lower intensity. This is confirmed with the changes in the LMM Auger line intensities. For presentation clarity, three lines in Fig. 4 indicate three types of copper: Cu(OH)2 and Cu2 O at 570.6 eV, CuO at 569.8 eV, and metallic copper at 567.8 eV. A net transition in the modification of the LMM peaks’ shape is observed when the concentration varies from 10−2 to 10−1 M, with a large decrease of the peak at 570.6 eV (Cu(OH)2 and Cu2 O), together with an increase of the metallic copper at 567.8 eV. This result emphasizes the close relation between the organothiol concentration and the efficiency of the oxide reduction. From the above results, an adsorption mechanism can be proposed to explain the formation of organothiol SAMs and to establish a possible structure of the system “SAMs-oxidized copper.” The first step corresponds to the physisorption of the molecule on the surface. The thiol is oxidized to disulfide and the CuO is reduced to Cu2 O.
(2)
It is evident that the concentration in the organothiol is a very important parameter. The formed thiolates in contact with the oxide − layer can be oxidized to sulfinate (RSO− 2 ) or sulfonate (RSO3 ) and desorbed afterward due to the exchange reaction with free thiol molecules. The global reaction after adsorption and desorption can be summarized according to Eq. (1). In this case, the concentration effect is expressed by the large quantity of thiols able to promote the desorption of oxidized species such as disulfides, sulfinates, and sulfonates. This mechanism for copper reduction is also valued for nickel and zinc [49,50,53–59]; however, the stability of the oxide, as well as the higher activity of these metals toward oxidation, makes the reduction by the thiols limited to a few monolayers of oxides, therefore the need to assist the process by electrochemical reduction before SAMs elaboration. To understand the lower efficiency for the thiol in completely reducing the surface oxide layer of bulk copper compared to electroplated copper, we have performed ionic etching of two surfaces. Bulk polished copper and the electroplated copper substrates have been aged for 1 wk after polishing or after electrodeposition. Figs. 5a and 5b show the LMM Auger lines at different etching times. After 90 s, the LMM line of electroplated copper is characteristic of metallic copper, which signals the complete removal of the oxide layer, while for polished copper, metallic copper is reached after 120 s, which indicates that the oxide layer is thicker in the case of bulk copper. Furthermore, the stability of the oxide also seems to be different, since immersion times longer than 2 h do not remove it completely. Fig. 6 and Table 2 emphasize the effect of the concentration during the elaboration process on the final properties of the molecular films of C12 SH in term of corrosion resistance in NaCl aqueous medium. The corrosion current density i cor decreases from 43 μA/cm2 for bare copper down to 1.9 μA/cm2 when copper is modified with C12 SH (10−1 M). Lower concentrations of C12 SH and pure C12 SH lead to poorer results. This result supports the idea that a well-controlled interface and a well-organized organic monolayer are both needed for the blocking of the surface and the stability of the organic film. This concentration effect is also evidenced by cathodic desorption of the monolayers (Fig. 7 and Table 3). For all cases the desorption occurs in one cycle at a potential of −1.2 V/Ag/AgCl except for the monolayer elaborated from solvent-free thiol, which desorbs at a lower cathodic potential. Differences are, however, evident from the cathodic desorption charge, which increases for with the C12 SH concentrated solutions. These results indicate that more molecules are adsorbed onto the surface as the C12 SH concentration increases and correspondingly the oxide quantity decreases, thus confirming the blocking effect discussed above. This also supports the idea that the insulating effect is a result of the monolayer and not the oxide. 4. Conclusions The effect of n-dodecanethiol concentration on the quality of the resulting molecular film is revealed to be very important. This has been shown to be a consequence of better efficiency in reducing the oxide when the concentration increases.
G. Fonder et al. / Journal of Colloid and Interface Science 326 (2008) 333–338
(a)
337
(b)
Fig. 5. CuLMM spectra for (a) electroplated copper and (b) bare copper before and after etching of 30, 60, 90, 120, and 150 s. Table 2 Corrosion potential and corrosion current density for bare copper and copper modified with C12 SH at different concentrations Concentration (M) Bare copper Solvent-free 10−1 10−2 10−3
Fig. 6. Polarization curves of bare copper and copper modified with C12 SH elaborated at different concentrations.
E cor (mV)
−389 −358 −272 −302 −298
i cor (μA/cm2 ) 43 4.5 1.9 2.1 3.5
The copper oxide CuO, initially present, is completely removed during the chemisorption process. The metallic copper is detected for copper modified with SAMs elaborated at concentration 10−1 M and higher. The PM-IRRAS gives similar information about all SAMs, indicating a similar organization. Electrochemical assessment with cyclic voltammetry and linear polarization settles on the difference of the impact of the elaboration condition, for instance the concentration, on the copper coverage and protection in different media. All these results indicate the importance of the interface of organothiols when the oxidizable metals are used as a platform. The oxide reduction with the thiol is less efficient for polished sheet copper compared to electroplated copper. The slight difference of the oxide may participate; however, we believe that the structure of the oxide of the copper sheet makes the reduction more difficult. The perspective of this work is to use physical methods or electrochemical ones to remove the oxide before modification. This should allow a copper-oxide-free surface even after the oxidation during the transfer to the solution. Thiol has been shown to reduce the oxide; therefore the few monolayers created should be removed easily.
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Fig. 7. Cathodic desorption curves of copper modified with C12 SH elaborated at different concentrations. Table 3 Potential and charge desorption for bare copper and copper modified with C12 SH elaborated at different concentrations Concentration (M)
Charge (mC/cm2 )
Desorption potential (V)
Solvent-free 10−1 10−2 10−3
0.323 0.286 0.245 0.210
−1.09 −1.14 −1.14 −1.18
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Alkanethiol-oxidized copper interface: The critical influence of concentration G. Fonder, F. Laffineur, J. Delhalle, Z. Mekhalif ∗ Laboratory of Chemistry and Electrochemistry of Surfaces (CES), University of Namur, FUNDP, Rue de Bruxelles 61, B-5000 Namur, Belgium
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 15 April 2008 Accepted 18 June 2008 Available online 8 August 2008
In this contribution, self-assembled monolayers of n-dodecanethiol (C12 H25 SH) at different concentrations on polycrystalline copper have been elaborated. Using XPS, PM-IRRAS, and electrochemical methods (cyclic voltammetry curves and cathodic desorption), the effect of the C12 H25 SH concentration on the reduction of the oxide layer has been studied. In all cases, a monolayer of good quality has been obtained. Results provide proof that while the concentration is increased, the thickness of the oxide layer is decreased, to a point that leads to metallic copper for the higher concentration. The results presented in this publication indicate the importance of controlling the interface when forming SAMs of organothiols on oxidizable metals. © 2008 Elsevier Inc. All rights reserved.
Keywords: Copper SAM Oxide reduction Thiol
1. Introduction Copper has a wide range of industrial applications, primarily related to its excellent thermal and electrical conductivity. It also recently became a metal of choice as interconnects in semiconductor integrated circuits. For most of those applications, copper needs to be protected from the environment in which it is required to operate. One approach that can be taken to address this problem is surface modification using a self-assembled monolayer (SAM) that has the potential to inhibit surface oxidation [1–38]. When SAMs are dense and highly organized, they form a real protective film on the surface, making it resistant to ambient oxidation. Laibinis et al. have extensively studied the self-assembly of a range of alkanethiols on evaporated copper [1–4]. As a direct result of this extensive study, they have shown that the SAM protects the underlying copper from oxidation and that a relationship exists between the hydrocarbon chain length and the effectiveness of the SAM at achieving this objective. Rubinstein and co-workers have studied the influence of the solvent and the presence of a copper oxide layer on the surface prior to SAM deposition [15], while Feng et al. specifically studied the behavior of SAM on copper in chloride ion environments [13]. It has been shown that SAMs exhibit superior corrosion protection to benzotriazole [13]. These studies also indicate that the use of thiols as a means of forming a protective layer on a substrate proved to be effective at achieving this objective. However, the long-term stability of the SAM layers was found to be limited. Aramaki and co-workers developed the approach of bilayer structure
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Corresponding author. E-mail address: [email protected] (Z. Mekhalif).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.06.038
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2008 Elsevier Inc. All rights reserved.
formation using alcohol-terminated alkanethiols and modifying the monolayer with an alkyltrichlorosilane to form a 2-dimensional polymer monolayer [5,39–41]. Similarly, we performed in previous work a molecular bilayer as well as mixed monolayers on copper with great protection efficiency for the bilayer system [27,42–45]. We have shown in more recent work that electroplated copper has a typical behavior [46,47]. Significant reduction of its oxide layer by alkanethiol or alkaneselenol has been evidenced. This efficiency has been shown to be dependent on the nature of the anchoring group. Selenol- and thiol-based molecules were compared with their homologues disulfide and diselenide. In the present work we focus on bulk copper, which seems to exhibit different behavior than electroplated copper under conditions similar to those in the previous work. The objective is to better understand the oxide reduction process of the oxide layer on bulk copper compared to electroplated copper. We also emphasize the role of the concentration of the organothiol and establish its impact on the efficiency of the oxide reduction. 2. Experimental 2.1. Materials n-Dodecanethiol (Aldrich, 98% D22,140-6), NaCl (Acros, 99.5%, 207790010), LiClO4 (Acros, 99+%, 194715000), acetonitrile (Lab-scan, HPLC analysis, C27C11X), absolute ethanol (Merck, 1.00983.2500), and ultrapure water (18.2 M cm) were used without further purification. The 1-, 3-, and 9-μm diamond paste and colloidal silica used in the polishing process were supplied by Beuhler. The copper sheets were obtained from Goodfellow, 99.99% (CU000748).
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2.2. Sample preparation The polycrystalline copper substrate was supplied as a sheet approximately 1 mm thick. These sheets were cut onto 10 × 20 mm coupons, polished (Phoenix 4000 machine) with 1200 grit silicon carbide paper, and subjected to further polishing with 9-, 3-, and 1-μm diamond paste suspensions; the final polishing stage was carried out using a 0.5-μm colloidal silica suspension. The samples were washed with Milli-Q water (18.2 M cm) after a polishing step. The resulting copper substrates were rinsed in absolute ethanol. Before modification, copper substrates were subjected to 15 min of UV/O3 activation and sonicated 20 min in absolute ethanol. For the modification, the SAMs were formed by immersing the copper in n-dodecanethiol solutions of ethanol for 2 h at different concentrations from millimolar to solvent-free. Immediately after modification, the copper samples were removed from the solution, rinsed with absolute ethanol, and dried under a stream of argon. The copper pieces were then subjected to a further 10 min sonication in absolute ethanol to remove excess reactants, dried, and stored under argon. 2.3. Analysis techniques 2.3.1. XPS measurements In this study XPS is used to reveal the surface chemical interactions that occur between organic thiols and the copper surface. Spectra were collected on a Surface Science SSX-100 spectrometer. The photoelectrons were excited using AlK α radiation as the excitation source (1486.6 eV). The photoelectrons were collected at 35◦ from the surface normal and detected with a hemispherical analyzer. The spot size of the XPS source on the sample was 600 μm, and the analyzer was operated with a pass energy of 20 eV. During data acquisition the pressure was kept below (1 × 10−9 Torr), and the binding energies of the peaks obtained were made with reference to the binding energy of the C1s line, set at 285.0 eV. The spectrum was fitted using an 80%/20% linear combination of Gaussian and Lorentzian profiles. To obtain information about the thickness of the oxide layer, progressive etching was carried out by bombarding the surface with 2-keV argon ions. 2.3.2. Electrochemical analysis All cyclic voltammetry measurements were carried out in 0.1 M NaOH aqueous solutions (EG&G Princeton Applied Research, Potentiostat/Galvanostat Model 273A). The electrode was scanned over a −1.0 to 0.4 V potential range, at a scan rate of 20 mV s−1 . All electrochemical measurements were made with reference to a SCE (saturated calomel electrode) and a platinum counter electrode. All polarization measurements were performed in deaerated 0.5 M NaCl aqueous solution and recorded from −1.0 to 1.0 V, at a sweep rate of 5 mV s−1 . Cathodic desorption was performed in a deareated acetonitrile solution with LiClO4 (0.1 M) using cyclic voltammetry (scan rate 50 mV s−1 ). Potential was measured versus the Ag/AgCl reference electrode. 2.3.3. PM-IRRAS Polarization modulation infrared reflection absorption spectroscopy data were collected from a Bruker Equinox55 PMA37 equipped with a liquid-nitrogen-cooled mercury–cadmium-telluride (MCT) detector and a zinc-selenide photoelastic modulator. The infrared light, reaching the sample surface at an angle of 85◦ , was modulated between s- and p-polarization at a frequency of 50 kHz. Signals generated from each polarization (R s and R p ) were detected simultaneously by a lock-in amplifier and used to calculate the differential surface reflectivity R / R = ( R p − R s )/
Fig. 1. Cyclic voltammetry of bare copper and copper modified with C12 SH elaborated at different concentrations.
( R p + R s ). The spectra were taken by collecting 640 scans at a spectral resolution of 2 cm−1 . 3. Results and discussion Cyclic voltammetry curves of naturally oxidized copper modified with n-dodecanethiol (C12 SH) ethanolic solution at different concentrations are shown in Fig. 1. The comparison of the CV curves of bare copper and the modified substrates highlights the formation of particularly insulating SAMs, indicating effective blocking of the surface. This outcome could be due to a synergetic effect of the copper oxide layer and the adsorbed n-dodecanethiol molecules. The analysis of the oxidation blocking effect versus the concentration reveals several evident distinctions. In all cases, a slight oxidation current starts at a potential close to +140 mV. This current is higher as the C12 SH concentration decreases. The relative ratio of the oxidation charges of copper with and without modification indicates a surface blockage of about 95% for copper treated with C12 SH solvent-free and a 10−1 M solution. This value is 10% lower (85%) when the monolayer is elaborated from more diluted solutions (10−2 and 10−3 M). The concentration has been shown to be of less importance when noble metals such as gold are used [48]. It has indeed a significant effect when oxidizable metals constitute the platform for organothiol adsorption [49,50]. Modification from lower concentrated solutions seems to provide modified surfaces less stable and more easily prone to oxidative desorption. This could be a consequence of weaker interface bonds between the organothiol and the substrate, most likely because of still remaining oxides on the surface. Another parameter that supports the instability and differences in the interface nature and quality as a function of the C12 SH concentration is the reverse cathodic peak arising at about −540 and −880 mV. The current density of this peak, as well as the cathodic charge, increases as the concentration decreases. This is an addi-
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Table 1 Ratio C/S, S/C, and O/Cu calculated from the XPS core levels for monolayers of C12 SH at different concentrations on oxide copper Concentration (M)
C/S
S/Cu
O/Cu
Solvent-free 10−1 10−2 10−3
20 20 18 17
0.2 0.2 0.4 0.3
0.5 0.4 0.8 1.0
Fig. 2. PM-IRRAS spectra for C12 SH monolayer elaborated at different concentrations.
tional proof that the monolayers from diluted C12 SH solutions are less stable and therefore the copper is more oxidized at high potential. This could be also due to the reduction of the oxide still present on copper after modification with SAM. This will be discussed later using XPS investigations. To evidence the difference of behavior between bulk copper and electroplated copper, it is necessary to compare the present results to previous results on C12 SH SAM on electroplated copper, where it is found that for a similar solution (C12 SH 10−2 M) the electrochemical stability of the SAMs is better than that noted here on the bulk polished copper [46,47]. Good and very stable SAMs have been obtained on electroplated copper even after 5 min immersion time. At the reverse cycle, the current density in that case was almost zero, a strong indication of the stability of the monolayer versus oxidation. We relate this to the absence of surface oxide, completely reduced by the thiol during the adsorption process. To understand this difference, we discuss in the sequel the spectroscopic analysis of C12 SH SAMs differing by the concentration of the solutions where they have been elaborated. PM-IRRAS and XPS are very complementary; the first technique emphasizes the organization of the monolayer, while the second gives information on the composition and interface nature. The results should be helpful in emphasizing differences between electroplated and polished copper as well as the concentration effect on the selfassembly process. Fig. 2 displays the PM-IRRAS spectra. In all cases, four peaks are observed at similar wavelengths ±2 cm−1 , corresponding to the resolution of the spectrometer. These peaks are related to asymmetric CH3 vibrations (νa (CH3 )) at 2960 cm−1 , asymmetric CH2 (νa (CH2 )) at 2921 cm−1 , symmetric CH3 (νs (CH3 )) at 2873 cm−1 , and symmetric CH2 (νs (CH2 )) at 2851 cm−1 . The values of νa (CH2 ) and νs (CH2 ) are close to 2918 and 2850 cm−1 , respectively, values that are indicative of organized monolayers [51,52]. The SAMs spectra exhibit a small shift of 3 cm−1 toward higher frequencies for νa (CH2 ). The concentration effect does not seem to have an effect on the organization of the monolayer. This small difference can be a consequence of the presence of the oxide or an influence of the roughness, which is larger for the polished than electroplated copper. Table 1 displays the data extracted from XPS measurements of copper modified with C12 SH with different concentrations from
Fig. 3. Oxygen core-level for C12 SH formed at different concentrations.
10−3 M to pure C12 SH (solvent-free). One has to emphasize that the carbon core C1s level for all these monolayers shows a unique component at 285.0 eV, typical of the alkyl chain of the molecule. The sulfur core-level S2p is also present as a doublet centered at 162.4 eV. The SAMs are free from contaminants and from oxidized or free sulfur. The C/S ratio varies very slightly; a small decrease is noticed for the diluted solution (10−3 M), but not in a significant way. The S/Cu ratio does not show great differences except for the 10−2 M solution, for which the ratio is slightly higher. However, more differences are seen for the O/Cu ratio, which decreases from a value close to 1 for SAMs elaborated from 10−3 M solution of C12 SH down to 0.4 for the more concentrated solutions (10−1 M). It is interesting to note that pure C12 SH, does not allow full reduction of all the oxide. The oxide layer of polished bulk copper seems to be more resistant to reduction than the electroplated copper. In particular, with a selenol anchoring group, copper is oxide-free after modification. With C12 SH, the oxide is present in trace amounts already after 5 min immersion time [46]. Fig. 3 displays the oxygen core level O1s and Fig. 4 shows the LMM Auger lines for oxidized copper modified with C12 SH at different concentrations. The O1s peak, as well as the LMM Auger lines, varies significantly with the concentration of C12 SH.
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The second step is the reaction of the thiol with the Cu2 O. Two types of reactions are proposed in the literature as depicted in the following equations: 2RSH + Cu2 O → 2RSCu + H2 O,
(1)
or 2RSH + Cu2 O → 2Cu + RS-SR + H2 O.
Fig. 4. CuLMM spectra for C12 SH formed at different concentrations.
The different peaks appear at the following binding energies: 529.5 eV (CuO) and 530.2 eV (Cu2 O). Peaks at 531.7 and 532.8 eV may correspond to (Cu(OH)2 ) and adsorbed water, respectively. At low concentrations (10−3 M), the oxygen peak at 529.5 eV indicates the presence of the cupric oxide, with an atomic composition of about 19%. Three other components are detected: 58% for the oxygen related to Cu2 O (530.2 eV), 18% for that coming from Cu(OH)2 (531.7 eV), and the rest (5%) due to the adsorbed water (532.8 eV). The CuO disappears with the increase of the concentration. Significant modifications are obtained with C12 SH solvent-free and a solution at 10−1 M, a small quantity of adsorbed water still remains (532.8 eV), and Cu(OH)2 is of lower intensity. This is confirmed with the changes in the LMM Auger line intensities. For presentation clarity, three lines in Fig. 4 indicate three types of copper: Cu(OH)2 and Cu2 O at 570.6 eV, CuO at 569.8 eV, and metallic copper at 567.8 eV. A net transition in the modification of the LMM peaks’ shape is observed when the concentration varies from 10−2 to 10−1 M, with a large decrease of the peak at 570.6 eV (Cu(OH)2 and Cu2 O), together with an increase of the metallic copper at 567.8 eV. This result emphasizes the close relation between the organothiol concentration and the efficiency of the oxide reduction. From the above results, an adsorption mechanism can be proposed to explain the formation of organothiol SAMs and to establish a possible structure of the system “SAMs-oxidized copper.” The first step corresponds to the physisorption of the molecule on the surface. The thiol is oxidized to disulfide and the CuO is reduced to Cu2 O.
(2)
It is evident that the concentration in the organothiol is a very important parameter. The formed thiolates in contact with the oxide − layer can be oxidized to sulfinate (RSO− 2 ) or sulfonate (RSO3 ) and desorbed afterward due to the exchange reaction with free thiol molecules. The global reaction after adsorption and desorption can be summarized according to Eq. (1). In this case, the concentration effect is expressed by the large quantity of thiols able to promote the desorption of oxidized species such as disulfides, sulfinates, and sulfonates. This mechanism for copper reduction is also valued for nickel and zinc [49,50,53–59]; however, the stability of the oxide, as well as the higher activity of these metals toward oxidation, makes the reduction by the thiols limited to a few monolayers of oxides, therefore the need to assist the process by electrochemical reduction before SAMs elaboration. To understand the lower efficiency for the thiol in completely reducing the surface oxide layer of bulk copper compared to electroplated copper, we have performed ionic etching of two surfaces. Bulk polished copper and the electroplated copper substrates have been aged for 1 wk after polishing or after electrodeposition. Figs. 5a and 5b show the LMM Auger lines at different etching times. After 90 s, the LMM line of electroplated copper is characteristic of metallic copper, which signals the complete removal of the oxide layer, while for polished copper, metallic copper is reached after 120 s, which indicates that the oxide layer is thicker in the case of bulk copper. Furthermore, the stability of the oxide also seems to be different, since immersion times longer than 2 h do not remove it completely. Fig. 6 and Table 2 emphasize the effect of the concentration during the elaboration process on the final properties of the molecular films of C12 SH in term of corrosion resistance in NaCl aqueous medium. The corrosion current density i cor decreases from 43 μA/cm2 for bare copper down to 1.9 μA/cm2 when copper is modified with C12 SH (10−1 M). Lower concentrations of C12 SH and pure C12 SH lead to poorer results. This result supports the idea that a well-controlled interface and a well-organized organic monolayer are both needed for the blocking of the surface and the stability of the organic film. This concentration effect is also evidenced by cathodic desorption of the monolayers (Fig. 7 and Table 3). For all cases the desorption occurs in one cycle at a potential of −1.2 V/Ag/AgCl except for the monolayer elaborated from solvent-free thiol, which desorbs at a lower cathodic potential. Differences are, however, evident from the cathodic desorption charge, which increases for with the C12 SH concentrated solutions. These results indicate that more molecules are adsorbed onto the surface as the C12 SH concentration increases and correspondingly the oxide quantity decreases, thus confirming the blocking effect discussed above. This also supports the idea that the insulating effect is a result of the monolayer and not the oxide. 4. Conclusions The effect of n-dodecanethiol concentration on the quality of the resulting molecular film is revealed to be very important. This has been shown to be a consequence of better efficiency in reducing the oxide when the concentration increases.
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(a)
337
(b)
Fig. 5. CuLMM spectra for (a) electroplated copper and (b) bare copper before and after etching of 30, 60, 90, 120, and 150 s. Table 2 Corrosion potential and corrosion current density for bare copper and copper modified with C12 SH at different concentrations Concentration (M) Bare copper Solvent-free 10−1 10−2 10−3
Fig. 6. Polarization curves of bare copper and copper modified with C12 SH elaborated at different concentrations.
E cor (mV)
−389 −358 −272 −302 −298
i cor (μA/cm2 ) 43 4.5 1.9 2.1 3.5
The copper oxide CuO, initially present, is completely removed during the chemisorption process. The metallic copper is detected for copper modified with SAMs elaborated at concentration 10−1 M and higher. The PM-IRRAS gives similar information about all SAMs, indicating a similar organization. Electrochemical assessment with cyclic voltammetry and linear polarization settles on the difference of the impact of the elaboration condition, for instance the concentration, on the copper coverage and protection in different media. All these results indicate the importance of the interface of organothiols when the oxidizable metals are used as a platform. The oxide reduction with the thiol is less efficient for polished sheet copper compared to electroplated copper. The slight difference of the oxide may participate; however, we believe that the structure of the oxide of the copper sheet makes the reduction more difficult. The perspective of this work is to use physical methods or electrochemical ones to remove the oxide before modification. This should allow a copper-oxide-free surface even after the oxidation during the transfer to the solution. Thiol has been shown to reduce the oxide; therefore the few monolayers created should be removed easily.
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Fig. 7. Cathodic desorption curves of copper modified with C12 SH elaborated at different concentrations. Table 3 Potential and charge desorption for bare copper and copper modified with C12 SH elaborated at different concentrations Concentration (M)
Charge (mC/cm2 )
Desorption potential (V)
Solvent-free 10−1 10−2 10−3
0.323 0.286 0.245 0.210
−1.09 −1.14 −1.14 −1.18
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