Controlling molecular assemblies

by Dameron, Arrelaine A.

iii Using molecules designed to have only specific differences in their functionality, we have explored the influence of molecular conformation on the structural, electronic, and physical properties of self-assembled monolayers using both scanning probe and ensemble techniques. In the former case, we used two structurally similar molecules that differ in the degrees of freedom afforded to each. We found that this influenced the degree of order and conductance of self-assembled monolayers of each molecule, but had little influence of conductance switching of individual molecules inserted in alkanethiolate self-assembled monolayers. We further demonstrated how molecular structure influences phase separation, displaceability, and molecular mobility of self-assembled monolayers by assembling 1-adamantanethiol on Au{111}. Molecular-resolution imaging of the self-assembled monolayers with the scanning tunneling microscopy confirmed a highly ordered hexagonally close-packed molecular lattice. We found that the 1-adamantanethiolate self-assembled monolayers were susceptible to replacement by the presence of another thiolated species, both from solution and vapor phases. Additionally, we determined that the displacement process is a nucleation and growth mechanism and the structure of the resulting self-assembled monolayers is dependent on the strength of the intermolecular interactions of the displacing molecules It was hypothesized that 1-adamantanethiolate displacement was driven by a combination of energies gained from the exchange of one self-assembled monolayer for a denser self-assembled monolayer and from the increased stability due to intermolecular interaction forces. Exploiting the susceptibility of the 1-adamantanethiolate self-assembled monolayers to displacement, we have designed a novel patterning strategy, termed ‘microdisplacement printing’, by combining these sacrificial self-assembled monolayers with microcontact printing. During microdisplacement printing, a molecular ink is patterned by contact with a patterned stamp directly atop an existing adamantanethiolate self-assembled monolayer, displacing the self-assembled monolayer everywhere the self-assembled monolayer is contacted. In this way, artificial diffusion barriers are created that block lateral mobility of the stamped molecules during patterning, allowing the patterning of molecules that are otherwise not patternable by conventional techniques. iv We also studied the influence of deposition time and the substrate species presence on molecular transport during dip-pen nanolithography. The transport rate was dependent on the size of the patterned features, and slowed down as a function of total deposition time. Additionally, the transported molecules interacted with preexisiting substrate molecules, and the transport rate was influenced by the functionality of substrate species. These studies demonstrate how complex assembles can be controlled by manipulating the properties of the individual components of the molecular device.