Quantum theory of electronic and thermal transport through nano-scale and single-molecule devices

Abstract

This thesis presents a series of studies into the electronic, thermal and thermoelectric properties of molecular junctions containing single organic molecules. The exploration and understanding the electronic and phononic characteristics of molecules connected to metallic leads is a vital part of nanoscience if molecular electronics is to have a future. This thesis documents a study for various families of organic and organometallic molecules, studied using a combination of density functional theory (DFT), which is implemented in the SIESTA code, and the Green’s function formalism of transport theory. The main results of this thesis are as follows: To elucidate the nature of the high and low conductance groups observed in break-junction measurements of single 4,4-bipyridine molecules, I present a combined experimental and theoretical study of the electrical conductance of a family of 4,4-bipyridine molecules, with a series of sterically-induced twist angles α between the two pyridyl rings. I show that their conductances are proportional to cos2(α), confirming that pi-pi overlap between adjacent rings plays a dominant role. Since both peaks exhibit a cos2 (α) dependence of conductance on torsion angle, this is evidence that the high and low conductances correspond to molecular orientations within the junctions, in which the electrical current passes through the C-C bond linking the pi systems of the two rings. Furthermore, this result demonstrates that the Fermi energy is located within the HOMO-LUMO gap and not close to a transmission resonance. A theoretical investigation into the Seebeck coefficient in pi-stacked molecular junctions is performed using a first principles quantum transport method. Using oligo (phenyleneethynylene) (OPE)-type molecules as a model system, I show that quantum interference produces anti-resonances in the gap between the HOMO and LUMO resonances and the stacking geometry can control the position of this quantum interference feature. The shifting of this resonance enhances the thermopower S is expected when the junction is tuned through a node in the transmission function. We found supramolecular π-π interactions between two molecules changed the sign of thermopower. I have investigated a family molecules with various side branched atoms to study the electron and phonon transport through nanoscale molecular junctions, with a view to understanding the performance thermoelectric materials. My calculations focus on the effect of heteroatoms formed from C, Si, Ge, and Sn on the thermal phonon conductance, electrical conductance, and Seebeck Coefficient. I also examine how the thermoelectric figure of merit is affected by side branched atoms, as the bond length and mass play an important role in determining the thermal phonon conductance of molecular wires. Due to the rigid nature of C-side branching, the thermal phonon conductance decreases monotonically with the bond length and mass, whereas thermal phonon conductance with Si-side branches increases with the length of the bond and mass. The low thermal conductance kel with S-bridging, combined with their higher thermopower and higher electrical conductance leads to a maximum thermoelectric figure of merit of ZT = 1.76, which is several orders of magnitude higher than that of bridges.