title: Molecular electronics: From Single Molecule to Large Area Junctions
Creators: name: Qiao, Xiaohang
Publisher: name: University of Liverpool Repository
Year: 2023

description: Molecular electronics is a field of research and technology that focuses on electronic devices which use individual molecules, nanoscale assemblies of molecules or ultrathin molecular films as the building blocks. It is a bottom-up approach, which involves studying the electrical and optical properties of molecules and aims to design and construct functional electronic components at the molecular scale. Initially introduced and postulated as an alternative to silicon-based materials in semiconductors, this field has evolved into a comprehensive interdisciplinary subject as researchers have progressively delved into the nanoscopic realm. Scanning Probe Microscopy (SPM) and Mechanically Controlled Break Junction (MCBJ) are the most common methods for single molecular measurements, while EGaIn contacting can examine large-area systems. Together these research tools extend measurements from single molecules to molecular thin films. These instruments and their enhanced versions can be used to study a range of properties of importance to molecular electronics, such as electrical conductivity, optics, photonics, spintronics, etc., by forming junctions of this general type: electrodes//single-molecule or molecular layers//electrodes. The analysis environment has a significant impact on molecular electronics measurements. Changing the test environment may enable evaluation of more molecular structures, while by modifying the target molecules, the conductance and related electrochemical properties of molecular junctions can also be fine-tuned. For the study of molecular thin films, it is necessary to arrange one-dimensional molecules into two-dimensional planes through self-assembly methods. In this thesis, a new testing environment, deep eutectic solvents (DESs), an analogue of ionic liquids (ILs), has been put forward and evaluated by scanning tunnelling microscopy (STM) techniques. Various molecular targets which form 2-terminal junctions, have been analysed in this type of solvent and studied using a scanning tunnelling microscope break junction technique (STM-BJ). The electrochemical behaviour of molecular junctions can be monitored using a four-electrode bipotentiostatic configuration with an in situ electrochemical scanning tunnelling microscope (EC-STM) and it is demonstrated how this can be applied in DESs. The thesis describes the attributes and advantages of using DESs for single molecule electronics measurements of 2-terminal junctions and electrochemically controlled junctions. Modification by altering the molecular structure or substituents can change the compatibility of different solvents and the electrochemical behaviour of molecular junctions. In the thesis, a typical phenothiazine molecule was designed for single molecular junctions and successful electrochemistry redox gating was achieved in a propylene carbonate (PC) environment with a supporting electrolyte. For EC-STM measurements, various mechanisms were considered to explain conductance variations, and the "soft gating" model best matched the experimental data. Alongside the electrolyte solvent, the concentration of the supporting electrolyte and the different reference electrode materials can be factors which influence the behaviour or stability of the electrochemical gating of molecular junctions. This thesis also describes scaling up of single-molecule devices to large-area ones in which electron poor chemical “dopants” (TCNE, TCNQ and Chloranil) are inserted into SAMs containing terthiophene moieties. During SAM assembly the molecular arrangement achieves a dynamic equilibrium through a self-assembly approach and exhibits a boost in conductivity with the addition of dopant. Changes in molecular structure also affect the corresponding changes in the conductivity of the self-assembled monolayers (SAMs), but these changes do not necessarily mirror the single-molecule behaviour, since the SAM structuring has a significant influence.