Surface modification of copper with 2-dodecylpropane-1,3-dithiol: The key effect of the solvent
Introduction
Self-assembled monolayers (SAMs) have been extensively studied over the past two decades [1], [2], [3]. These organic thin films are potentially useful in many areas ranging from corrosion resistance [4], [5], [6], [7] and thin-film lubrification [8] to lithographic patterning [9], [10] and microelectronic device fabrication [11], [12]. Their popularity arises from their ease of preparation and high reproducibility to form densely packed and highly ordered SAMs to impart and control surface chemical and physical properties. The most widely studied SAMs are those generated by the adsorption of alkanethiols on gold [1], [13].
In the literature [14], [15], [16], [17], [18], [19], [20], [21], [22], the most common solvent for gold modification by alkanethiols is ethanol. Its popularity arises from its relatively low price, availability in high purity, low toxicity and low tendency to be incorporated into SAMs [23].
Bain et al. [23] compare the effect of various solvents (THF, DMF, hexadecane, CCl4, toluene, ethanol, acetonitrile and cyclooctane) on the self-assembly of hexadecanethiol onto gold. It comes out that the solvents used have little effect on the thickness and wettability of those SAMs. The only exception arises from the adsorption of hexadecanethiol in hexadecane, where the incorporation of hexadecane into the film is noticed. Some studies [24], [25], [26] suggest that nonpolar solvents (heptane, hexane) improve the kinetics of the grafting by comparing with ethanol. However, the organization of the SAMs is lower than SAMs formed in ethanol due to the strong solvent–adsorbate interaction.
In the case of self-assembly on copper, more pronounced differences are observed according to the solvents used. It was found that the properties of the monolayer differ with the solvent used. Laibinis et al. [21] have shown that isooctane leads to better reproducibility than ethanol thanks to the higher solubility of the organothiols in isooctane. Ron et al. [27] also proved than ethanol is not the most appropriate solvent to modify copper surface, better properties being obtained in toluene than in ethanol.
The modification of metals (Au, Cu, etc) by alkanethiols largely depend on the interaction between the thiol anchoring group and the metal surface. But in the case of copper, the interaction between ethanol and copper has to be taken into account. Copper catalyses the conversion of ethanol to aldehydes which is associated to the formation of ethoxy intermediates [28], [29], [30], [31]. These intermediates are bound chemically to copper, this chemisorption occuring through interaction with chemisorbed oxygen [28], [29], [30], [31]. Ron et al. [27] have studied the immersion of C18OH monolayer in C18SH solution and they observed improvement in the monolayer properties. Part of the alcohol molecules adsorbed on the copper are exchanged with the alkanethiol molecules. They suggest that the same phenomenon occurs with the ethoxy species chemically adsorbed onto copper. Those species could be exchanged with thiol molecules.
Less interference is expected with isooctane and toluene in the adsorption of alkanethiols on copper because those solvents are only physisorbed to the copper surface. In the case of ethanol, the alkanethiol monolayer formation is hindered by the presence of chemisorbed ethoxy moieties. The different interactions between copper and the solvent used lead to variation of monolayers properties.
Recently, we have investigated the relative stability of tetradecanethiol (RSH) and 2-dodecyl-1,3-propanedithiol (R(SH)2) adsorbed on copper from ethanolic solutions [32]. It appears that R(SH)2 monolayers exhibit a higher coverage and better electrochemical stability when subjected to both anodic and cathodic desorption tests, than RSH. However, the position of the Auger line obtained after the modification by R(SH)2 points to the presence of copper oxide, which is not the case of RSH. The copper oxide remaining after the modification by R(SH)2 could be attributed to ethoxy intermediate moieties still present on the surface. The oxide layer could also arise from other parameters. Shon et al. [33] have shown that the formation of the chelating dithiol monolayer on gold is slower than alkanethiols. This low kinetics of SAM formation could favor more oxygen diffusion during the SAMs formation and lead to a partial re-oxidation of the substrate.
In this work, SAMs of 2-dodecylpropane-1,3-dithiol (R(SH)2) have been elaborated in various solvents (toluene, dichloromethane, tetrahydrofuran, dimethylformamide, isooctane, acetonitrile, isopropanol and ethanol). The aim of this work is to define the origin of the copper oxide layer. The SAM formation in non-alcoholic solvent will define the effect of ethanol on the copper oxide layer. Furthermore, one of those solvents could lead to a monolayer with the same quality than in ethanol but with less or even without copper oxide left.
The quality of the SAMs is checked by polarization modulation infrared reflection-absorption (PM-IRRAS). The binding properties of the interface and the Auger line CuL3M45M45 are investigated by X-ray photoelectron spectroscopy (XPS). The stability of those self-assembly is assessed by cyclic voltammetry.
Section snippets
Materials
The substrates were prepared from pure polycrystalline copper (Goodfellow, 99.99+%, CU000749). NaOH, HClO4 were purchased from Acros Organic and were used without further purification. Absolute ethanol (Norma pur, Analytical reagent), acetonitrile (lab-scan, HPLC 99.9%), DMF (lab-scan, HPLC 99.8%), dichloromethane (lab-scan, HPLC 99.8%), isopropanol (Aldrich, HPLC 99.5%), toluene (lab-scan, HPLC 99.8%) and isooctane (Aldrich, HPLC 99%) were also used as received. THF (Acros 99+%) was distilled
XPS measurements
In our previous study [32], significant divergences were observed on the Auger line of copper modified by RSH and R(SH)2 in ethanolic solution. Essentially metallic copper was observed after the modification by RSH, while Cu was present in both metallic and oxidized forms for the R(SH)2. Shon et al. [33] have shown that the formation of the chelating dithiol monolayer on gold is slower than for normal alkanethiols. Diffusion of oxygen can occur during the SAMs formation and lead to
Conclusion and perspectives
Dichloromethane, toluene, acetonitrile, isooctane, THF afford a partial or complete removal of the copper oxide during R(SH)2 SAM formation. However, the monolayer formed contains lots of defects and contaminations incorporated into the film. Furthermore, the anodic stability of the monolayer is similar or lower than with ethanol. None of those solvents seem to be a good alternative to ethanol. Nevertheless, additionnal work should be made to define the effect of the concentration. The
Acknowledgement
J. Denayer is grateful to FNRS-FRIA for fellowship.
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