Figure 1: The ordered dense character of SAMs on gold defines layers in the interface that influence the chemistry, wetting, and transport properties of the surface. Chemistry occurs typically in the region comprising the endgroups. Wetting is influenced by characteristics of the film associated with a slightly thicker layer. Access to the underlying gold reflects the barrier properties fo the SAM that are largely the result of the length and organization of the alkyl spacers. Patterning of SAMs combines with the control of their properties to localize the function of the interface at mesoscopic scales.
The discovery in 1983 of the self-assembly of disulfides on gold and, soon thereafter, of alkanethiols coincided with the maturation of STM following its invention ine early 80's. SAMs on gold have turned out to be a valuable type of sample for investigation by STM: STM reveals the molecular structure of SAMs and SAMs in turn reveal information about the imaging mechanism of organic molecules on conductors. STM at picoampere tunneling currents can image the air-monolayer interface without noticable perturbation of the interfacial order of SAMs on gold(111). SAMs show collective organization of their molecules in at least four phases, constrained by disordered boundaries and substrate defects. The monolayers can also exhibit a type of collective order dominated by end-group packing that overwhelms the substrate topography and generates interfaces with a higher degree of perfection. Identification of endgroups in SAMs with mixtures of molecules promises a new level of rigor in the understanding of the structure-property relationships of SAMs at the microscopic level. Our data illustrate why SAMs have become excellent model systems for many phenomena in interfacial science. SAMs are not simply models, however. Increasing evdidence demonstrates that these organic monolayers provide the necessary resists for methods of fabrication like microcontact printing, that exploits wholly new concepts to achieve mesoscopic structures. This type of manipulation provides a basis for solving problems in the manufacturing of nanoscale devices and their use in a phlethora of derivative technologies with the attendant advantages of characteristically smaller size. Working out the detailed consequences of the organization of SAMs helps to assess their possible role in technology and defines paths towards ultimate control over organic interfaces.
Figure 2: STM image of dodecanethiol on gold(111) prepared by adsorption from a millimolar solution of dodecanethiol in ethanol for 2h at room temperature an 48 h at 50 degrees Celsius in the same solution. Monoatomic gold steps, depressions and a network of lines linking depressions delimit ordered domains in the SAM (left). Domains differ by their phase, their chain tilt or the origin of sulfur adsoption. Molecular arrangements are hexagonal with an addtional rectangular superlattice (right). The phases originate from the particular distribution in the unit cell of molecules with different twist angles that move end groups at different height levels in and out of the average plane of the monolayer by 0.03 nm.
