Self-Assembly of Two-Dimensional Non-Spherical Particles: A Monte Carlo Simulation Study

Open Access
- Author:
- Triplett, Derek Adam
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 09, 2009
- Committee Members:
- Kristen Ann Fichthorn, Dissertation Advisor/Co-Advisor
Janna Kay Maranas, Committee Member
Evangelos Manias, Committee Member
Darrell Velegol, Committee Member
Kristen Ann Fichthorn, Committee Chair/Co-Chair - Keywords:
- liquid crystal
Monte Carlo
assembly
nematic
non-spherical particles
smectic - Abstract:
- For applications in device fabrication, such as sensors or nanoelectronics, it is often desirable to utilize the self-assembly behavior of particles in two dimensions to produce the structures. However, using particle self assembly in achieving long range or complex order can be difficult due to the inability to control or a lack in understanding of the various interparticle interactions. The work presented in this dissertation contributes to the understanding necessary to achieve self-assembled structures using non-spherical particles in two dimensions. Non-spherical particle geometries result in phases that symmetric particles, such as disks or spheres, do not exhibit. Rectangles, spherocylinders, ellipses, cubes, and squares have the ability to produce diverse structures depending on their aspect ratio, concentration, and subtle differences in their geometry. Using Monte Carlo (MC) simulations, this work studies the assembly of hard rectangles confined between two infinitely long hard walls and establishes the orientational and structural characteristics imparted by the confining walls as compared to bulk rectangles. In all cases, the rectangles align with their long axis parallel to the confining wall. Systems of rectangles that are in the nematic phase in the bulk exhibit layering next to the wall when confined. The dependence of the depletion interaction between two squares on depletant shape (either rectangles or disks), size relative to the squares, and concentration is established. At short separations between the squares, the effect of the depletant is found to induce attraction between the squares. Further, the rectangles and disks are found to align the squares so that they approach via a preferred pathway. The depletion interaction is extended to ensembles of rectangles and disks, where the influence of the disks on the structure of the rectangles is studied. At sufficiently high concentrations of rectangles and disks, the two components microphase separate into alternating rectangle-rich and disk-rich layers. Based on unique structural features of the rectangles and disks, a phase diagram delineating three different phases is constructed. MC simulations are also used to understand recent experimental results on the the self assembly of electrostatically stabilized gold nanowires on a surface. The simulations resolve the physical origin of the nanowires’ preference for the smectic phase and capture the experimentally observed trend of decreasing order as nanowire length increases.