UC Riverside

Nanostructured materials have seen rapid development in the past few decades because of their promising technological applications. Compared with individual nanoparticles or random aggregates, ordered arrangements of nanoparticles often lead to extraordinary collective properties that can delicately exploit the synergy between adjacent building blocks, inducing pronounced enhancement in optical, plasmonic, catalytic, electronic, magnetic, and mechanical properties. Currently, there are two different strategies to fabricate ordered superstructures in the nanoscale: top-down and bottom-up methods. The top-down method, which utilizes techniques such as thin-film deposition, lithography and etching, is able to fabricate ordered arrays of nanostructures with arbitrary lattices in a precisely controllable manner. However, the high cost of this method significantly impedes its application in producing large-scale nanoparticle superlattices. Besides, it remains a challenge for the current top-down methods to fabricate perfect periodic nanoparticle arrays in all three dimensions.In contrast, bottom-up assembly represents a promising method to address the above issues since colloidal synthesis is a cost-effective way to produce nanoscale building blocks on a large scale, and their assembly into 3D arrays can be carried out conveniently. In addition, building blocks produced through colloidal synthesis sometimes possess better structural, physical, and chemical properties resulting from the high synthesis temperature, surface protection and passivation, compared with nanostructures produced by the top-down method. Despite the advantages, obtaining ordered nanoparticle arrays with desired structures by self-assembly of colloidal nanoparticles is a nontrivial task, which requires delicate control of the positions and orientations of nanoparticles by manipulating the interactions acting on nanoparticles. This dissertation studies colloidal interactions commonly employed as the driving forces in the assembly process of colloidal nanoparticles. The first part of the dissertation reports a quantitative study on the self-assembly behavior of magnetic nanospheres under external magnetic fields using a computational model. Various interactions, including magnetic dipole-dipole interaction, electrostatic interaction, van der Waals interaction, and steric effect, were considered to explain the field strength-dependent color change of the 1D photonic chains. A model based on Langevin dynamics was then adopted to visualize the dynamic assembly process of the formation of 1D chains. In addition, potential and force analyses were also performed to figure out the key factors contributing to the phase transition from 1D chains to 2D sheets. It provides insights into the assembly process of magnetic nanospheres. The second part of the dissertation introduces the fabrication of photonic crystals capable of responding to electrostatic surface charges. First, 1D photonic chains were fabricated using Fe3O4 nanospheres through the magnetic field-mediated assembly. To reduce the screening of the electric field which occurs in polar solvents, the photonic chains were made hydrophobic through a surface modification and then dispersed in a nonpolar solvent. Such a system showed a distinct color change upon approaching an object carrying electrostatic surface charges. The relationship between the performance of the solution and the dielectric constant of the solvents was also investigated. The third part reports the fabrication of colloidal photonic crystals using SiO2 nanospheres of exceptionally low concentrations. It has remained a great challenge to assemble normal SiO2 nanospheres into ordered arrays due to the difficulty of maintaining a strong interparticle electrostatic repulsion in such a large particle separation. However, a simple proton exchange by acid treatment has been shown to reduce ionic strength and enable long-range electrostatic repulsion. Previous research focused only on the structures and optical properties of colloidal photonic crystals, while their assembly dynamics have been neglected for a long time. It was found that there is a concentration-dependent assembly rate where an optimal concentration exists for a fast assembly. Further theoretical investigations revealed that this optimal concentration resulted from the balance between thermodynamics and kinetics.