ELECTRICAL AND STRUCTURAL PROPERTIES OF GRAPHENE-BASED HETEROSTRUCTURES WITH SELF-ASSEMBLED ORGANIC TUNNELLING BARRIER

Abstract

In this thesis, a microfabrication process of synthesizing organic selfassembled Van der Waal heterostructures on rigid substrates is presented. The devices presented here consist of graphene monolayers functionalized with N,N´-bis[2-(4-pyridyl)ethyl]-naphthalenediimide molecules (NDI-Py) and clipped together to form organic tunnel junction. We will show that the NDI cores lie flat on the graphene surface bound by π-π interactions and the attached functional groups (pyridylethyl groups) are clipped by forming iondipole bonds with silver atoms. The molecular self-assembly takes place as molecules self-organise into quasi-crystalline monolayer due to the weak hydrogen bonding between molecular cores which yield a pinhole-free monolayer. Pyridinic endings have two rotational degrees of freedom which allow them to stand above the plane and adopt a conformation suitable for reaching out to another molecule. The molecular bridges serve as an insulating layer that does not require a control over its thickness and position like hexagonal Boron Nitride hBN. The components of the self-assembled devices are fabricated on rigid substrates (SiO2/Si) yet are designed to be transferred onto conformable substrates. The molecular self-assembly demonstrated in the thesis thus offers a route towards organic Van der Waals devices that benefit from the versatility of chemical synthesis and meet the requirement of flexible electronics. The structural properties of the organic self-assembled heterostructures are investigated at intermediate stages of the fabrication. Optical microscopy images show the advantages of the fabrication protocol presented in this thesis that include utilizing a thin insulating layer (SiO2) to locate insulated graphene electrodes on transparent substrates and the technique in using a two-polymer stamp to produce wrinkle-free graphene electrodes. The adsorption of NDI-Py molecules before and after clipping the graphene electrodes is verified by the blue-shift of the Raman peaks of graphene. This blue shift arises from the p-doping of the NDI-Py and the emergence of the NDI-Py peaks in the Raman spectrum. The low temperature scanning tunnelling microscope (STM) images demonstrate that the molecules are selforganised into quasi-crystalline monolayer due to the weak hydrogencarbonyl bonds between the cores of NDI-Py. This yields a monolayer of NDI-Py molecules free of pinholes. In addition, the STM images of graphene functionalized with NDI molecules show that the pyridylethyl groups are standing up right which promotes the ion-dipole bonds (Ag-N-Ag) to construct the molecular bridges. The X-ray photoelectron spectroscopy (XPS) measurements reveal that the filling fraction of the pyridinic-N sites with Ag is 65% which is necessary to form the molecular bridges in the heterostructures. The current-voltage (I-V) characteristics of the tunnel junction demonstrate coherent tunnelling consistent with microstructural properties. The I-V curves of the heterostructures at 77 oC yield consistent tunnel barrier width (1.62 nm). Also, the thermo-activated current observed is quenched at low temperature leaving a finite tunnelling component in the current which demonstrates the high-quality tunnel junctions. Tunnelling across the junctions depends on the chemical potential of the bottom graphene electrodes when tuned by a back-gate voltage. This action gives a zeroresistance peak at -8V when the Fermi levels of the electrodes crossed the Dirac points of graphene. This confirms the existence of energy conserving (coherent) tunnelling processes through the structure