Self-Assembled Monolayers Derived from Bidentate Organosulfur Adsorbates
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ABSTRACT The adsorption of a series of 1,2-bis(mercaptomethyl)-4,5-dialkylbenzenes (1), spiroalkanedithiols (2), and aliphatic dithiocarboxylic acids (3) on gold yielded new types of self-assembled monolayers (SAMs). The new SAMs were characterized by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These results were compared to those obtained from SAMs derived from normal alkanethiols (4) of analogous chain length. Comparisons of ellipsometric thickness and contact angle wettability showed that the new SAMs were well packed and highly oriented. Comparisons of the PMIRRAS data showed that the SAMs generated from 1 and 2 exhibited slightly less crystallinity than their alkanethiolate-based analogs, while the SAMs generated from 3 exhibited comparable crystallinity to their alkanethiolate-based analogs. Moreover, the SAMs derived from 1 and 2 showed no "odd-even" wettability or PM-IRRAS effects, while those derived from 3 showed remarkably large "odd-even" effects compared to those derived from normal alkanethiols.
INTRODUCTION The spontaneous adsorption of organic compounds onto solid substrates can be used to generate chemically and structurally well-defined self-assembled monolayers (SAMs) that have found wide reaching applications in a variety of fields such as wetting [2], adhesion [3], corrosion resistance [4], and microelectronic materials [5]. Because of their ease of sample preparation, highly ordered structure, and rich end-group chemistry, SAMs derived from alkanethiols on gold are the most extensively studied self-assembled monolayer system [6,7]. Despite these attractive features, these SAMs exhibit at least two shortcomings.
First, the long-term stability of
alkanethiolate-based SAMs is less than satisfactory; they decompose in solution upon exposure to moderate heat (e.g., 70 'C in hexadecane) [7]. Moreover, displacement of the original molecular adsorbates of SAMs occurs when the films are immersed in a solution comprised of different thiols [7,8]. Second, it is difficult to generate well-defined multicomponent surfaces from a mixture of two or more alkanethiols [9,10]. Coadsorption of different alkanethiols often leads to inhomogeneous mixing and/or domain formation (known as "islanding") [11,12]. A major objective of our research is to synthesize and study new types of SAMs generated by the adsorption of bidentate compounds (e.g., 1, 2, and 3 in Figure 1) [13-15]. These bidentate adsorbates are specifically designed to provide enhanced stabilities via multiple interactions of sulfur with the surface of gold. Schlenoff and co-workers have suggested that multiple attachments of sulfur to the surface of gold might provide monolayers with enhanced stabilities [8]. In addition, we anticipated that studies of these monolayers should provide an opportunity to explore the structural features of SAMs on gold because it might be possible to influence the structure of the films by altering the nature of the binding between t
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