Self-Assembled Monolayers: Chemical and Physical Modification Under Vacuum Conditions
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Self-assembled monolayers (SAMs) are highly organized nanoscale films formed by the chemisorption of organic molecules (e.g., alkanethiols and alkanesiloxanes) onto solid substrates (e.g., Au and Si). The organized structure, chemical design flexibility, and facile preparation of SAMs have made them popular models for the study of interfacial phenomena and important systems in the development of new technologies. For example, there is current interest in using SAMs as the template for molecular resists, nanopatterned surfaces, and highly selective sensors. However, some of these applications require specific properties that demand the selective chemical and physical modification of existing SAMs. In addition to their potential technological applications, a study of the chemical transformations that accompany the modification of SAMs can also provide a detailed molecular-level understanding of the interfacial processes that accompany reactions at polymeric surfaces.
Modification of SAMs can be accomplished using either wet chemical or vacuum-based treatments. A number of advantages are inherent in the use of vacuum conditions during the modification of SAMs, including better control over the extent of reaction and the greater ease of in situ reaction monitoring. In this chapter, we focus on the chemical processes that accompany the vacuum-based modification of several chemically distinct types of SAMs adsorbed on Au substrates, specifically alkanethiolate SAMs, semifluorinated SAMs, and alkanethiolate SAMs functionalized at the vacuum / film interface. The chemical modification of these SAMs has been investigated during exposure to three distinctly different types of reactive species, specifically ionizing radiation, atomic radicals, and vapor-deposited metal atoms.
One of the frequent motivations for the chemical and physical modification of SAMs is the desire to selectively modify or control the interfacial properties of the organic interface. Results from our investigations indicate that the surface selectivity of a given modification treatment is strongly dependent on both the nature of the reactive species and the chemical composition of the SAM. For example, because of the greater penetration depth of electrons and X-rays relative to the thickness of typical SAMs, the modification of SAMs by ionizing radiation results in a nonsurface-selective process initiated by electron-stimulated bond cleavage events throughout the film. These events produce an initial period of modification characterized by desorption and structural disordering, leading to the formation of a cross-linked carbonaceous overlayer.