Theory and modelling of electronic properties of molecular-scale systems

Aljasim, Abdulrahman and Lambert, Colin and Hou, Songjun (2025) Theory and modelling of electronic properties of molecular-scale systems. PhD thesis, Lancaster University.

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Abstract

Single-molecule junctions (SMJs) provide an excellent platform for understanding charge transfer (CT) in molecular-scale structures and are a crucial building block for investigating electronic devices. Over the past two decades, numerous efforts have been focused on studying electron transport in molecular systems. The aim is vital for synthesising stable molecules and designing highly efficient energy devices. Furthermore, explorations of electronic and thermoelectric properties of single-molecule junctions have established the validity of applying quantum tools to molecular systems. This thesis addresses theoretical simulations for electric and thermoelectric transport properties such as transmission coefficient T(E), Seebeck coefficient S, and electrical conductance G. The charge transfer (CT) in single-molecule junctions can tune their thermoelectric properties. Particularly, CT complexes such as chlorine perylene bisimide (Cl-PBIs) combined with tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF) can exhibit distinct electron transport behaviours due to donor-acceptor interactions and also molecular-orbital hybridisation. Furthermore, porphyrin-based wires incorporating hexafluorophosphate [PF6] - counterions can influence redox activity through electrostatic gating. Thus, theoretical tools are employed to explain many phenomena, including quantum interference effects, rectification, switching, and transistor effects. In Chapter 2, density functional theory (DFT) is implemented within the SIESTA code. In Chapter 3, Green’s function methods and thermoelectric coefficients are addressed. In Chapter 4, I investigated charge transfer complexes of chlorine perylene bisimide (Cl-PBI) with TCNE as a pendant molecule in SMJs. In Chapter 5, thermoelectric properties are explored in the chlorine perylene bisimide (Cl-PBI) with a TTF as a pendant molecule, which affects an electron transport behaviour close to the Fermi energy EF at room temperature (∽300K). In Chapter 6, I focused on tuning the thermoelectric properties of single-porphyrin wires by studying various scenarios.