The self-assembly of complex nanostructures guided by surface patterns has inspired an increased interest in novel schemes for localized chemical modifications. In the Fréchet group, we develop chemical methodologies that allow confining these processes to the nanoscale using scanning probe microscopes; these tools offer the unique ability to deliver a variety of stimuli onto a nanometer sized area to initiate the desired surface reactions. We are currently working on several chemical reactions that can be triggered by pressure, electric field, or electrochemical bias. We are also investigating photochemical patterning in the near field regime in collaboration with Prof. Zhang and Prof. Grigoropoulos (Mechanical Engineering, UCB).
In high field patterning, a moderate bias applied across the nanoscale gap between the sharp tip of an atomic force microscope and a sample induces a electric field in excess of 109 V m-1; such intense field causes organic molecules to fragment into highly reactive species. Using common organic solvents as substrates (figure 1) results in the deposition on the surface of glassy carbon nanostrips that offer excellent etch resistance for nanofabrication of silicon wafers. By careful screening of the reactants and the reaction conditions we were capable of patterning features as narrow as 25 nm at linear velocities as high as 1 cm/s; notably this is the fastest AFM lithography reported to date.
Another high field patterning approach is based on specially tailored molecules that always fragment at the same bond in virtue of a built in “weak link”. Upon exposure with the AFM probe, several surface functional groups can be activated; these afford the directed self assembly of nano-objects such as dendritic macromolecules, nanocrystals, and fullerenes (Figure 2).
An alternate approach involves specially designed self assembled monolayers based on a quinone functional group that upon tip induced reduction and oxidation produces distinct surface chemical functionalities (Figure3). This will enable the precise positioning of specific anchoring points for the directed assembly of complex nanostrucutres in a single patterning step.
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