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

Abstract

In this thesis, a microfabrication process of synthesizing organic self-assembled 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 ion-dipole 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 self-organised into quasi-crystalline monolayer due to the weak hydrogen-carbonyl 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 zero-resistance 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.

Date of Award	18 Nov 2020
Original language	English
Awarding Institution	University of Bath
Sponsors	Royal Embassy of Saudi Arabia & Qassim University
Supervisor	Alain Nogaret (Supervisor) & Sergey Gordeev (Supervisor)