Modulating Nanoparticle Film Assembly Using Amphiphiles
Nanocomposite thin films comprised of nanoparticles have shown great promise for use in electronics, photonics, biomedical as well as energy storage and conversion devices. One versatile method for fabricating such thin films is layer-by-layer (LbL) assembly, a process that involves sequential deposition of oppositely charged species to create conformal thin films. The advantage of LbL assembly lies in the fact that the properties and structure of films can be tuned by varying assembly conditions such as pH and ionic strength. Furthermore, a variety of nanomaterials with useful properties can be incorporated within LbL assembled thin films. Despite these advantages, there are a few limitations to using LbL assembly to fabricate nanoparticle films: (1) Favorable film growth of all-nanoparticle LbL assembly in aqueous phase occurs within a narrow processing window thus limiting the versatility of LbL assembly. (2) nanoparticle LbL assembly has generally been limited to aqueous phase due to the ease of charging nanomaterials in water. (3) The fabrication of nanoparticle films via LbL assembly is slow and typically takes several hours to complete. In this thesis, amphiphiles will be used to address these three limitations of nanoparticle LbL assembly. The first limitation is addressed by using a small amphiphilic molecule, hexylamine to broaden the narrow nanoparticle LbL assembly window. In addition, an array of experimental techniques is used to reveal the mechanism leading to a broad processing window. It will be demonstrated that the second limitation of nanoparticle LbL assembly to aqueous phase can be overcome by using a surfactant Aerosol-OT (AOT) to charge stabilize particles in toluene for non-polar LbL assembly. Furthermore, the effect of the surface chemistry of particles and dispersion moisture content on the charge of particles in non-polar media is probed along with the role of relative humidity on the LbL assembly process in non-polar media. Lastly, electrophoretic deposition (EPD) of surfactant-charged particles in a non-polar solvent is used to rapidly assemble nanocomposite films, thus overcoming the third limitation of nanoparticle LbL assembly.