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1.3 Self-Assembled Monolayers

Self-assembled monolayers have been useful in dramatically changing the interfacial properities of a wide variety of materials.  Coating of metals, semiconductors, and insulating crystals with thin layers of organic compounds could not be deposited in a well-ordered systematic manner until recently¹⁶ when researchers began applying self-assembled monolayers (SAM).  Here, organic reagents such as alkylthiols can be deposited onto substrates such as Au, where the thiol head group coordinates with the substrate surface.  Amazingly, these chains can pack onto the surface in a close-packed arrangement in a crystalline fashion.  The application of these monolayers can dramatically change the properties of the surface.  By modifying the tail of the monolayer, you can change the surface free energy from being either hydrophobic or hydrophilic.  Wetting or contact angle measurements¹⁷ clearly demonstrate the modification of the surface properties of the material.  By changing the wetting characteristics of a surface, improved adhesion to the substrate can be realized.  It has been noted¹⁸ that most of the studies in which a SAM is applied have been successful due to the use of relatively inert substrate materials such as Au.  This implies that the successful application of a SAM requires a pristine surface on which to assemble.  The large change in surface properties after the deposition of only a monolayer dramatically emphasizes the unique character of self-assembled monolayers.  If these monolayers can be applied to the surface of high-temperature superconductors, they will be found useful in modifing the interfacial character of superconductors when combined with other materials such as conducting polymers.