Electrochemical and structural characterization of self-assembled thiol monolayers on gold

Self-assembled alkanethiol monolayers on gold are used as model systems in a fundamental study of the potential-dependent wetting and of the galvanic metal deposition. For using such monolayers as model systems, well-defined and ordered monolayers are required. In order to control the quality of the monolayer, its structure was studied on a microscopic and a macroscopic scale. The experimental methods were scanning tunneling microscopy (STM), wetting and electrochemical measurements. The chain length and the type of terminal group of the monolayer molecules were varied systematically. The microscopic structure of monolayers of alkanethiol (HS(CH 2 ) n -1 CH 3 with n = 3, 8, 12, 18, and 22) on Au(111) is the subject of chapter 3. This structure is investigated with atomically resolved STM and wetting measurements. The characteristic depressions in these monolayers as observed with STM are proven to be holes in the underlying top gold surface layer. These depressions are filled with thiol. The holes originate from an etching process of the gold during the adsorption of the thiol. A distinct correlation is found between the number of holes and the amount of gold in the thiol solution after adsorption, as measured with atomic absorption spectroscopy. The etching which generates these holes is believed to be related to the mobility of the gold-thiolate molecules during the adsorption process, prior to self-assembly. In chapter 4, the potential-dependent wetting of thiol-modified gold electrodes is for the first time presented. A Wilhelmy plate technique is used to determine the potential- dependent wetting of the modified electrodes. These measurements are carried out simultaneously with differential capacitance measurements and cyclovoltammetry. For alkanethiols with n >10, the monolayer is very stable in the potential range where only double layer charging occurs. The extreme hydrophobicity, the low dielectric constant (≈2 for n >10), and the low double layer current (about a factor of 100 less than for clean gold) are all indicative of the dielectric character of these monolayers. Chapter 5 reports on the influence of the alkanethiol chain length on the electrowetting effect of the self-assembled monolayer. It is found that the shorter the chain the stronger the wettability changes as a function of the potential. A simple representation of the electrical double layer as a dielectric thiol layer in series with a diffuse double layer in the electrolyte accounts well for the observed chain length effect. The effect of the salt concentration can be qualitatively understood with the model. It is concluded that the potential-dependent wetting finds it origin in the formation of an electrical double layer and that potential-induced conformational changes within the thiol layer are insignificant. Functionalizing the alkanethiols with a terminal group (HS(CH 2 ) n -1 X , X = OH, CN, Cl and COOH) is found not to affect the stability of the monolayer, as follows from chapter 6. All thiols used are electroinactive except for the COOH group which can in part (5-10%) be reduced to the aldehyde compound. The difference in the capacitance of these thiol layers is determined by the different dielectric properties of the terminal group. The capacitance increases according to the sequence CH 3 lCllOHlCN. The potential in the electrocapillary maximum as determined from the electrowetting measurements also depends on the terminal group; this potential increases according to the sequence CH 3 lCllCNlOH. There are some indications that the orientation of the molecules changes with applied potential for thiols with CN, OH, and Cl as terminal group. Larger electrowetting effects are obtained by oxidation/reduction of a ferrocene- terminated alkanethiol monolayer, as described in chapter 7. Strong indications were found that the electrowettability is a result of specific anion binding upon oxidation of the ferrocene groups. This ion binding occurs to compensate the surface charge. The monolayers are not stable upon continuous oxidation/reduction of the ferrocene groups. The stability is strongly increased by mixing the ferrocenethiol with an alkanethiol of about the same chain length. The reversibility of the electrowetting is limited by contact angle hysteresis. The presence of a self-assembled thiol monolayer on a gold electrode strongly influences the morphology of galvanically deposited copper. This topic is discussed in chapter 8. On bare gold, copper is deposited as a rather homogeneous flat film, whereas on thiol-modified gold, independent of the type of terminal group, copper is deposited as hemispherical particles. The difference in morphology is ascribed to the difference between the surface energies of copper and the solid substrate. Generally, when a metal is deposited onto a solid material, flat films can only be obtained when the surface tension of the solid is high. Nucleation occurs on top of the thiol layer as long as the self-assembled monolayer is highly ordered. An overpotential is required to overcome the potential drop across the dielectric of the thiol layer. This potential drop increases with increasing thiol chain length and hence, the overpotential increases likewise. The influence of the terminal group of the thiol layer on galvanic copper deposition shows up most pronouncedly for the OH terminal group. About 100 times more particles are deposited on OH-terminated thiol compared to CH 3 -thiol. This is explained by a combination of a smaller potential drop across the thiol layer and a high chemical affinity of Cu atoms for the OH-group, thus decreasing surface diffusion. In the potential range between the equilibrium Nernst potential and the overpotential, no copper is deposited onto the monolayer. This makes the layers suitable as monolayer resists. In chapter 9 we focus on possible applications in this area. First, a thiol monolayer is treated by electron beam lithography to give very narrow patterns where the thiol has been removed. Subsequently, submicron metallized patterns can be produced by galvanic copper deposition in the openings. The smallest width of the copper patterns produced is about 75 run. The width is determined by the spot size of the electron beam. By optimizing the electron exposure, we expect that even finer patterns can be produced. Such fine metal structures may offer interesting applications in technologies such as ultra-high-density recording or disk mastering. This thesis concludes with an overview of this study and a comparison of our results with the current status of knowledge in the literature. In chapter 10, the information on the structure of the thiol monolayer as obtained with the various techniques is summarized. We conclude that methods like STM and metallization indicate the presence of small microscopic defects in the self-assembled monolayers. However, with electrochemical techniques like cyclovoltammetry and differential capacitance measurements it was shown that on a macroscopic scale the monolayers are ordered and densely packed. From the macroscopic point of view, the minor defects are shielded by the long chain alkane tails. Therefore, we have to conclude that the question whether a thiol monolayer can be considered as a model system depends on the particular type of goal one has in mind. In this thesis, we have demonstrated that thiol monolayers behaved as genuine model systems in the areas of electrowetting and monolayer lithography.