Self-Assembled Monolayers and Multilayers for Molecular Scale Device Applications

Soto-Villatoro, Ernesto R

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Self-Assembled Monolayers and Multilayers for Molecular Scale Device Applications

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Self-assembled monolayers (SAMs) are organized molecular assemblies that are formed by spontaneous adsorption of a compound in solution to a surface (e.g. alkanethiols on gold). The design, preparation, and characterization of several self-assembled monolayers and multilayers on surfaces (gold, indium tin oxide and quartz) are described. The systems were chosen based on their ability to form ordered films and to perform a given device function. SAMs were fabricated with selected functional groups at the air-monolayer interface, capable of complexing metal ions (e.g. dicarboxypyridine, dicarboxybenzene, imidazole, 4-hydroxypyridine) with the purpose of using these SAMs to construct multilayered films. Deposition of a second layer consisting of metal ions (e.g. Cu(II), Co(II) and Fe(III)), occurs by non-covalent metal ligand binding interactions between the metal ion layer and the different organic ligands on the surface. Deposition of subsequent layers was achieved by the incorporation of the appropriate organic ligands and metal ions. These monolayers and multilayered films were characterized by contact angle measurements, ellipsometry, grazing angle FT-IR, cyclic voltammetry and impedance spectroscopy following deposition of each layer on the film. Electrochemical analysis of the multilayered films shows alternating insulating/conducting behavior (cyclic voltammetry) and alternating changes in films capacitance (impedance spectroscopy) depending on the outermost layer of the film. Films capped with an organic layer show low conductivity, while films capped with a metal layer show conducting behavior. The electrochemical behavior of the films is related to the degree of â€œleakinessâ€ or electrolyte solution permeation through the film, which is high for films with metal layers as the top layer and decreases once the film is capped with an organic layer. The alternating conducting/insulating behavior of the films allows for fabrication of multilayered thin films of variable thickness and tunable conducting properties. Ordered films were fabricated with up to seven layers of dicarboxypyridine and Cu(II), and 4-hydroxypyridine and Fe(III) metal-ligand units. The construction of these films provides an example of molecular films that could function as molecular wires or junctions due to their controllable electrochemical properties. Photocurrent generating films were fabricated by incorporation of chromophore groups (e.g. pyrene, porphyrins) into the multilayered structures. These films generate cathodic or anodic current upon photoexcitation of the chromophores. The monolayers functionalized with different organic ligands were also used to study lanthanide complexation on the surfaces. Successful deposition of different lanthanide ions was achieved from DMSO solutions. Monolayers of a bicyclic structure, 4, 7, 13, 16-tetraoxa-1,10,21-triaza-bicycle[8.8.5] tricosane-19,23-dione, attached to a hexadecanethiol molecule were used to study the ability of metal ion detection on the surface using electrochemical (cyclic voltammetry and impedance spectroscopy) techniques. The SAMs show higher complexation affinity for Li+ than for Na+ or K+. Preliminary studies were also carried out to investigate the ability of different SAMs to cell adhesion interactions. Future experiments will help elucidate a systematic relation of cell adherence and the bulk and molecular-level properties of the functionalized surfaces. The different multilayered films described in this dissertation served as preliminary models for different molecular scale device applications. Current work is focused in the design and preparation of more efficient photocurrent generating films, highly selective sensors for different types of ions, surfaces for cell adhesion and microbial interactions, and the study of other potential applications such as the design of micro and nanofluidic devices.

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