Self-assembled Monolayer - Types of SAMs

Types of SAMs

Selecting the type of head group depends on the application of the SAM. Typically, head groups are connected to a molecular chain in which the terminal end can be functionalized (i.e. adding –OH, –NH3, –COOH, or –SH groups) to vary the wetting and interfacial properties. An appropriate substrate is chosen to react with the head group. Substrates can be planar surfaces, such as silicon and metals, or curved surfaces, such as nanoparticles. Alkanethiols are the most commonly used molecules for SAMs. Alkanethiols are molecules with an alkyl chain, (C-C)ⁿ chain, as the back bone, a tail group, and a S-H head group. Other types of interesting molecules include aromatic thiols, of interest in molecular electronics, in which the alkane chain is (partly) replaced by aromatic rings. An example is the dithiol 1,4-Benzenedimethanethiol (SHCH₂C₆H₄CH₂SH)). Interest in such dithiols stems from the possibility of linking the two sulfur ends to metallic contacts and this was first used in molecular conduction measurements. Thiols are frequently used on noble metal substrates because of the strong affinity of sulfur for these metals. The sulfur gold interaction is semi-covalent and has a strength of approximately 45kcal/mol. In addition, gold is an inert and biocompatible material that is easy to acquire. It is also easy to pattern via lithography, a useful feature for applications in nanoelectromechanical systems (NEMS). Additionally, it can withstand harsh chemical cleaning treatments. Recently other chalcogenide SAMs: selenides and tellurides have attracted attention in a search for different bonding characteristics to substrates affecting the SAM characteristics and which could be of interest in some applications such as molecular electronics. Silanes are generally used on nonmetallic oxide surfaces; however monolayers formed from covalent bonds between silicon and carbon or oxygen cannot be considered self assembled because they do not form reversibly. Self-assembled monolayers of thiolates on noble metals are a special case because the metal-metal bonds become reversible after the formation of the thiolate-metal complex. This reversibility is what gives rise to vacancy islands and it is why SAMs of alkanethiolates can be thermally desorbed and undergo exchange with free thiols.