Atomic force microscopy based nanofabrication of organic thin films

A nanofabrication method, nanografting, has been developed to fabricate nanometer scale patterns on surfaces with specified size and geometry. The nanografting process combines the displacement of adsorbates within a self-assembled monolayer (SAM) by an atomic force microscopy tip and self-assembly reactions. The present work discusses the procedure for nanografting, and relevant technical issues regarding the requisite displacement forces, the level of spatial selectivity, and the stability of the resulting nanopatterns. Compared with other methods for microfabrication, nanografting allows a more precise control over the size and geometry of patterned features and their locations on surfaces. Fabrication precision better than 1 nm is routinely obtained. Nanopatterns comprising various thiol-based components can be fabricated. As examples, we have demonstrated fabrication of nanopatterns with either the same or different chain lengths and terminal groups from the matrix SAM. Furthermore, nanografting allows fabricated patterns to be altered in situ without the need to change masks or repeat entire fabrication processes.; The reaction mechanisms and kinetics under natural self-assembly conditions and in a spatially confined environment are studied and compared. In an unconstrained assembly environment, thiol molecules initially physisorb on gold with the molecular axis of their hydrocarbon chains oriented parallel to the surface. As the surface coverage increases to near saturation, a two-dimensional phase transition occurs and produces islands composed of molecules with their hydrocarbon axis oriented ∼30° from the surface normal. These islands continue to growth until a mature SAM is formed. The self-assembly occurs more than ten times faster in a spatially confined environment than on unconfined bare substrates. During nanografting, the thiol molecules present in the solution above the SAM rapidly assemble onto the exposed nanometer-size gold area that is confined by the scanning tip and surrounding SAM. The accelerated rate is attributed to a change in the pathway for the self-assembly process as the spatial confinement makes it geometrically and energetically more favorable for the initially adsorbed thiols to adopt a standing-up configuration directly in this microenvironment.