Self-Assembly of Nanoparticles at Liquid-Liquid Interfaces

• Main Author: Du, Kan
• Format: Others
• Published: ScholarWorks@UMass Amherst 2010
• Subjects: adsorption energy; Coulomb blockade; liquid interfaces; nanoparticles; self-assembly; Physics
• Online Access: https://scholarworks.umass.edu/open_access_dissertations/274; https://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1274&context=open_access_dissertations

In this thesis, we studied the self-assembly of nanoparticles at liquid metal-water interfaces and oil-water interfaces. We demonstrated a simple approach to form nanostructured electronic devices by self-assembly of nanoparticles at liquid metal surfaces. In this approach, two liquid-metal droplets, which were coated with a monolayer of ligand-stabilized nanoparticles, were brought into contact. They did not coalesce but instead remained separated by the nanoparticles assembled at the interface. Devices formed by this method showed electron transport between droplets that was characteristic of the Coulomb blockade, where current was suppressed below a tunable threshold voltage because of the energy of charging individual nanoparticles. Further studies of this approach demonstrated the potential of interfacial assembly in fabricating microscopic electronic devices over a large area in a cost-effective and time-efficient fashion. Micrometer-scale Ga droplets coated with nanoparticles were fabricated using ultrasonication and then deposited on patterned substrates to form microscopic devices. I-V measurements showed Coulomb blockade effect in the devices containing more than one nanoparticle junction. The measured threshold voltages increased with number of junctions as expected for devices arranged in series. We also studied experimentally the energy of adsorption of nanoparticles and microparticles at the oil-water and Ga-water interfaces by monitoring the decrease of interfacial tension as the particles bind. For citrate-stabilized gold nanoparticles assembling on a droplet of octafluoropentyl acrylate, we found adsorption energy =-5.1 kBT for particle radius R = 2.5 nm, and adsorption energy scales R^2 for larger sizes. Gold nanoparticles with (1-mercaptoundec-11-yl) tetra(ethylene glycol) ligand had a much larger binding energy (-60.4 kBT) and an energy barrier against adsorption. For polystyrene spheres with R = 1.05 micrometer, we found adsorption energy =-0.9*10^6 kBT. We also found that the binding energy depended on the composition of the oil phase and could be tuned by the salt concentration of the nanoparticle suspension. At Ga-water interfaces, we found that adsorption energy of Au-cit and Au-TEG nanoparticles were much larger. We have also studied desorption of polystyrene microparticles from oil-water interfaces by changing experimental conditions, including addition of nanoparticles, cross-linking ligand molecules or in response to chemical interactions between the particles and the oil. We found that microparticles can desorb even though the adsorption energy is large. We also found that the desorbed particle formed a surprising `tail'-like structure.