Date of Award
Program or Major
Doctor of Philosophy
Howard R Mayne
Sterling A Tomellini
In this work we considered the adlayer self-assembly of three model molecules di-, tri- and tetratopic with different sizes and potential energy parameters on square and hexagonal (triangular) lattices. For each case, we carry out minimization using an analytical gradient to find the most stable minima. In all cases we use “coarse-grained” site-to-site pairwise additive potential. We have explored how the change in the size of the molecule affects the pattern formation in the molecular adlayer. A primary focus of this work restricts the exploration of the landscape to a “unit cell” of 2x2 angles, labeled [1, 2, 3, 4] and extrapolate this to an infinite lattice by the application of tessellation. The model we study represents a 2-dimensional surface with fully occupied lattice sites and with boundary conditions to resemble the infinitely occupied surface.
To investigate the patterns we have used several order parameters that can distinguish between the adlayers. We have found several adlayers varied as the shape and the size of the molecules’ change. We also have reported the chirality of the adlayer by using the order parameters. We note that homochiral patterns can be formed by using achiral molecules, and comment on the areas of parameter space where this occurs.
The molecular pattern hierarchy of the ditopic molecule on a square lattice distributed from highly ordered motifs such as a linear sheet “short stripe” geometry to fourfold achiral windmill structure and chiral windmill pattern. On the other hand we have reported a pinwheel chiral structure of ditopic molecule on a triangular lattice. On both square and triangular lattices we also found several herringbone structures.
Depending on the shape of the molecule and the surface lattice, the porous shape and size of the adlayer change wildly. We note several porous shapes such as square, rectangle, hexagon and octagon with their sizes depend on the molecular distance parameters. For instance we note a honeycomb structure of tritopic molecule on a triangular lattice distorts to a semi-hexagonal pattern as the size of the molecule increases.
We have also conducted Monte Carlo simulation for a range of molecular sizes of ditopic molecule on both square and triangular lattices. We note that the adlayer patterns of the simple minimization method and the Monte Carlo simulation are quite consistent.
Alomari, Moh'd Ibrahim, "A MODEL STUDY OF ADLAYER PATTERN FORMATION OF RIGID DI- TRI- AND TETRATOPIC MOLECULES ON SQUARE AND TRIANGULAR LATTICES" (2015). Doctoral Dissertations. 2230.