Assembly of Nanowire Arrays: Exploring Interparticle Interactions, Particle Orientation, and Mixed Particle Arrays

Open Access
Author:
Kirby, David Joseph
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 19, 2015
Committee Members:
  • Christine Dolan Keating, Dissertation Advisor
  • Thomas E Mallouk, Committee Member
  • Raymond Edward Schaak, Committee Member
  • James Hansell Adair, Committee Member
Keywords:
  • van der Waals
  • electrostatics
  • sedimentation
  • gold
  • nanotube
  • silica
  • vertical array
  • horizontal array
  • microwells
Abstract:
Assembly represents an important step between nanowire synthesis and their use in next generation technologies. It is an interesting challenge because the particle shape necessitates control over its position and orientation with respect to the assembly substrate and neighboring particles. This dissertation examines the fundamental forces involved in the self-assembly of nanowire arrays. The effects of gravity, van der Waals (VDW) attractions, and electrostatic repulsions were explored by manipulating the geometry, material, and surface chemistry of the particles and substrates. Optical microscopy was used to examine arrays that were formed in an aqueous suspension. Single or multi-segment metal nanowires (2 12 µm long and 190-300 nm in diameter) were fabricated by the template electrodeposition process and small molecule, nucleic acid, polyelectrolyte, or silica coating was applied to provide electrostatic repulsions. The silica shell provided a rigid coating that allowed specific core segments to be chemically etched while maintaining the overall particle shape. These particles were referred to as partially etched nanowires (PENs) and assembled in vertical or horizontal arrays as the particle center of mass and aspect ratio were varied. In vertical arrays, nearly 70% of particles were perpendicular to the surface. In horizontal arrays, smectic rows formed and alignment of like segments was observed. Experiment and simulation together suggested that differences in VDW attractions between the particle segments were responsible for this alignment. The rich behavior of the different particle types was explored further by assembling binary mixtures of PENs. Here, the roles of gravity and VDW attractions were emphasized as they dictated the particle orientation in the final observed structure. Particle-substrate interactions were probed by creating patterned assembly surfaces with optical lithography. Microwell patterns were used to improve the percentage of vertically aligned PENs and allowed assemblies to be dried in place. These microwells also enabled the vertical assembly of single segment nanowires which typically oriented parallel to the underlying substrate. Glass surfaces patterned with gold provided high VDW regions that localized nanowire row formation. Finally, a reconfigurable system was explored that used light-induced temperature gradients and VDW/gravitational forces to switch between horizontal and vertical arrays.