(5cb) Alkanethiol Self-Assembled Monolayers as Capping Agents, Molecular Resists and Lubricants

Self-assembled monolayers of alkanethiols have been exploited in a wide range of fields such as bioengineering (to render surfaces biocompatible), biology (to dictate the spatial location of growing cells and to pattern surfaces with areas for specific binding), surface science (to tune the surface energy of a material), materials science (to protect surfaces against corrosion), colloidal science (to stabilize particles against agglomeration and as capping agents) and in molecular electronics. Yet the mechanisms used by SAMs in these applications and their behavior under different conditions are not well understood.

In prior work, I have studied SAMs of alkanethiol in three specific applications; stabilizers in the synthesis of nanoparticles, molecular resists in the electrodeposition of metallic structures, and as lubricants using model systems.

(i) SAMs of octanethiol are shown to be effective capping agents to quench the growth of ZnO nanocrystals. XPS is used to determine the binding strength of octanethiol to different facets of bulk ZnO single crystals. UV spectroscopy is used to monitor the growth and quenching of growth of the nanoparticles in situ. Although the binding of the SAM to the surface of ZnO is weak, it is sufficiently strong to act as an effective capping agent.

(ii) SAMs of alkanethiol are used as molecular resists for the site-selective patterning of modified electrodes. AFM and SEM are used to characterize the electrodeposited structures. A wide range of unexpected behavior is found and explored as a potential means to create 3 dimensional, multicomponent, complex patterned structures ranging from the microscale to nanoscale over large areas. A gold electrode is patterned by microcontact printing to create regions of bare electrode and SAM-covered regions. At low overpotentials, the SAM acts as a positive resist and deposition occurs on the bare electrode regions. At intermediate overpotentials, the SAM in ineffective as a positive resist with deposition occurring on the SAM covered regions but still preferentially on the bare electrode regions. At high overpotentials, the SAM acts as a catalyst and the deposition of Ag occurs predominantly over the SAM-covered regions. The tunable resist behavior of the SAM to silver electrodeposition is explored as a function of alkanethiol chainlength.

(iii) SAMs of alkanethiol are used to modify surfaces resulting in a change in their frictional behavior. A modified SFA is used to measure the tribology of the SAM. The effect of potential on the stability of the SAM is studied and the use of SAMs under the influence of a potential to reversibly tune friction is explored.

The unresolved questions are: (i) why do you surfactants preferentially bind to certain facets and is there a way to a priori exploit the anisotropic capping behavior of a particular surfactant to fabricate nanoparticles of different shapes? (ii) why do alkanethiol surfactants on an electrode become leaky at a critical potential and is the tunable resist behavior of these surfactants unique to the Ag deposition system? (iii) How does an electric field affect the properties (i.e. phase, packing) of a SAM?

In future work, I plan to study these issues thereby providing greater insight into the abilities and limitations of SAMs in given applications.