Quantum and Classical Dynamics of Molecular Scale Structures

TL;DR

This thesis explores the electronic properties and conductance of molecular junctions using density functional theory, green function formalism, and molecular dynamics, revealing differences in electrode performance and molecular assembly behaviors.

Contribution

It applies advanced computational methods to compare molecular junctions with different electrodes and studies molecular assembly, providing new insights into molecular electronics.

Findings

Graphene electrodes show higher conductance than gold in certain junctions.

Oligoether chains have lower conductance than alkanes of the same length.

Molecular dynamics reveal distinct assembly structures of pyrene-based molecules on graphene.

Abstract

In this thesis, I investigate the molecular electronic properties of molecular junctions formed from single molecules. I started my thesis by discusses the main theoretical methods using in this work density functional theory and the green function formalism of electron transport. These two techniques are used to study the charge transport through dicarboxylic acid terminated alkanes in chapter 4, with graphene gold nanogap electrodes. The results are then compared with those using symmetric gold electrodes and reveal that there is a difference between the two situations due to the difference in Fermi energies relative to the frontier orbitals of the molecules. Furthermore, the electrical conductance in the asymmetric junction is higher than that in the symmetric junction, which suggests that graphene offers superior electrode performance when utilizing carboxylic acid anchor groups. In chapter 5, I study affected of the saturated chain conductance by adding oxygen to the chain. Comparisons between my electronic structure calculations on oligoether chains and previous work on alkanes show that the conductance of oligoether is lower than that of alkane chains with the same length. I found that the beta tunneling factor per methyl unit of the alkanes is lower than the beta factor of oligoether. In the final chapter, molecular dynamic simulations are used to examine the molecular assembly of two candidate molecules for graphene-based molecular electronics, one with one pyrene anchor, Pyrene PEGn exTTF, PPT, and the other with three pyrene anchors, tri pyrene derivative, TPPT, on a disordered graphene surface. PPT is seen to form flat structures whilst TPPT is seen to form semicircular cone like micelle structures on the graphene surface. In the presence of water, the PPT tends to aggregate whereas the TPPT micelle expands.