Ordered Organic Monolayers

We shall review the surface structures of monolayers of various homologues of organic compounds, the paraffins, the phthalocyanines, a few aromatic systems and amino acids that have been determined during recent investigations. 

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Glosli, J. N. and McClelland, G., "Molecular Dynamics Study of Sliding Friction of Ordered Organic Monolayers, Phys. Rev. Lett., Vol. 70,1993, pp. 1960-1963. 

Hovis, J. S. and Hamers, R. J. Structure and bonding of ordered organic monolayers of 1,3,5,7-cyclooctatetraene on the Si(001) surface Surface cycloaddition chemistry of an antiaromatic molecule. Journal of Physical Chemistry 102, 687 (1998). 

Examples of ordered organic monolayers are normal paraffins on platinum and silver (111) surfaces. If straight-chain saturated hydrocarbon molecules from propane (CsHg) to octane (CgHig) are deposited from the vapor phase onto Pt or Ag (111) between 100 and 200 K ordered monolayers are produced. As the temperature is decreased a thick crystalline film can condense. The paraffins adsorb with their chain axis parallel to the platinum substrate, and their surface unit cell increases smoothly with increasing chain length as shown in Fig. 15. 

Eor heterogeneous electron transfer, the use of ordered organic monolayers (self-assembled monolayers or SAMs) at electrode surfaces as blocking films either with electroactive species in the electrolyte [26] or with electroactive groups tethered at the opposite end of the blocking molecule from the covalent attachment end [27] has provided a method to study the effect of electrode-redox center distance and the effect of the electrode potential on the electron-transfer rate. 

Formation of monolayers by self-assembly at electrode surfaces has been widely used in recent years. How to bind molecules on electrode surfaces has been studied extensively among electrochemists since mid-1970s, and is called chemical modification of electrode surfaces [283-285]. More recently, chemisorbed ordered organic monolayers on gold, silver, and oxides have been studied extensively and are called sdfassemhled monolayers (SAMs) [286-289]. SAMs are... 

The expectation of the structural dependence of lubrication motivated great numbers of investigations that intended to prepare and use highly ordered organic films, such as the Langmuir-Blodgett (L-B) hlms and Self-Assembled Monolayers (SAMs), as solid lubricant, which will be discussed more specihcally in Section 4. 

These monolayers form themselves spontaneously by adsorption of suitable components from a diluted solution directly onto a surface. The formation of ordered and orientated monomolecular layers by spontaneous adsorption from a diluted solution is called self-assembling, the respective layers are called self-assembled monolayers (SAM) or self-organized monolayers. In the first part of this review, the coating of surfaces with functionalized SAMs will be examined. After that, some aspects of the attachment of the biomolecules onto these SAM-modified surfaces will be discussed. 

Organized multilayers of ferrocene alkyl thiol have been self-assembled on Au(lll) under conditions of controlled thiol concentration. Several methods, such as CV, ellipsometry, STM, AFM, and in situ FTIR spectroscopy have been applied in these studies in order to find out the differences between mono- and multilayers of the same compound [152]. Similar compounds, namely, short-chain alkyl thiols (n = 3-10) with ferrocene terminal group were allowed to form organized monolayers at Au(lll) surfaces [153]. 

Quayum, M. E., Kondo, T., Nihonyanagi, S., Miyamoto, D. and Uosaki, K. Formation of organic monolayer on a hydrogen terminated Si(l 11) surface via silicon-carbon bond monitored by ATR FT-IR and SFG spectroscopy Effect of orientational order on the reaction rate. Chemistry Letters, 208 (2002). 

In the 1940s, it was demonstrated in the pioneering work of Zisman and coworkers [8] that the LB technique is not the only way to create an organized organic monolayer on a solid substrate. It was demonstrated that when a compatible substrate is exposed to a solution of an amphiphilic compound, the dissolved molecules form a self-assembled monolayer on the substrate surface. Such films maintain their structural integrity after they are removed from solution. The most common examples of such films are organosulfur films on gold substrates [9] and alkyltrichlorosilane films on silicon dioxide substrates [10]. Compared with the LB films, the self-assembled films are somewhat less ordered. On the other hand, these films are easier to prepare, since they do not require special instrumentation and can easily be deposited on both planar and non-planar substrates. Also, in many cases the amphiphilic molecules which make the self-assembled film are chemisorbed on the substrate. Such films are more stable when heated or exposed to solvents than are typical LB films, which are held to the substrate by non-covalent interactions. 

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