In recent years, heterojunctions and devices consisting of two-dimensional material stacks held together through van der Waals (vdW) forces have gained a lot of attention. In particular, graphene is a promising 2D material for use in single-molecule junctions, due to its high mechanical strength and robust chemical stability at room temperature. In the case of conventional metal electrode junctions, the molecules are attached to the metal electrodes through anchor groups at both ends. Through this configuration, electrons are transported along the molecular backbone, which means they travel from the left electrode to the right electrode via anchors attached to the molecule. Due to this, the size of the device corresponds to the length of the molecule. In contrast, in this thesis, a series of selected molecules was successfully sandwiched between two-dimensional graphene electrodes via vdW interactions to create single-molecule two-dimensional van der Waals heterojunctions (M-2D-vdWHs). In this case, electrons are transported between two graphene electrodes in a cross-plane manner, and the size of the device is determined by the molecule's thickness, rather than it’s length.
This thesis investigates cross-plane charge transport in graphene-based single-molecule van der Waals heterojunctions (M-2D-vdWHs). The results presented in this thesis are computed using SIESTA, which is a density functional theory (DFT) code that solves the Kohn-Sham self-consistent equations. This is then combined with the Gollum code to obtain electron transport properties. The resulting predictions are compared with the experimental results obtained using a newly developed cross-plane break junction (XPBJ) technique. In this thesis, two collaborative research projects have been undertaken, whose results are summarized below.
First, I have investigated charge transport through three well-defined molecular bilayer-graphenes (MBLGs), which consist of two vertically stacked graphene nanoflakes bound together via - stacking interactions, and molecular single-layer graphene (MSLG). DFT calculations indicate that the size of molecular graphene could be used to tune charge transport through vdW heterojunctions. Additionally, molecular junctions based on molecular single-layer graphene (MSLG) are more conductive than those based on molecular bilayer-layer graphenes (MBLGs). Moreover, the DFT calculations also indicate that the angles between the core of molecular graphene and peripheral mesityl groups significantly affect charge transport through MSLG junctions, where a decrease in the angle results in an increase in electrical conductance.
Secondly, I studied the influence of substituents and -conjugation on cross-plane charge transport in graphene-based molecular junctions. The theoretical results demonstrated that the electrical conductance of molecular graphene junctions based on pyrene increases after being substituted by both electron-withdrawing and electron-donating groups. This suggests that both types of substituents can be used to tune charge transport in graphene-based junctions, which differ from conventional metal electrode junctions. Furthermore, I investigated the electrical conductance of the hydrogenated derivatives of pyrene, which have different degrees of conjugation and consequently different degrees of planarity. I found that the conductance of the molecular junctions decreases gradually with a weakening of the molecular conjugation.