Correlated electron transport across atomic and molecular tunnel junctions

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McDermott, Shane
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University College Cork
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As transistors continue to miniaturise the importance of describing electronics on an atomic scale increases. A molecular junction consists of a molecule connected to to metal electrodes via linker molecules and may be thought of as the prototype system for electronics on a few nanometre length scale. For charge transport calculations such systems are usually treated with a single particle approximation such as NEGF+DFT non-equilibrium Green’s function plus density functional theory. Typical single particle treatments are incomplete due to approximations made in the treatment of the electronic structure leading to discrepancies between theory and experiment by orders of magnitude, believed to be due to electron correlation. A solution to this is an accurate many body treatment of charge transport explicitly accounting for electron correlation. In this thesis the comparison of many body method MECS (Many Electron Correlated Scattering) to experiment and single particle methods, in particular the (NEGF+DFT) is performed. Comparison with single particle methods is established for alkane-based and silane-based molecular junctions utilising both thiol and amine linker molecules. In addition, components of the method such as electrostatic behaviour and screening, electronegativity, sensitivity to boundary conditions, and the level of treatment of electron correlation are tested. Comparisons with single particle methods yield agreement for systems with a lower degree of electron correlation such as alkane-based molecular junctions, with a larger disagreement between single particle and MECS methods for the moderately correlated silane-based junctions. A complex band structure analysis was performed on silane and alkane junctions with an emphasis on the dependence with respect to the linker molecules was undertaken to further investigate energy level alignment and demonstrate how alignment is affected by end groups. Electrostatic calculations have been used to investigate and quantify the effects of the screening effect on point contact and molecular junction voltages focussing on the screening length into the metal contacts was performed. This allows for more accurate estimates of the applied voltage across the junctions. The application of single particle open system boundary conditions through the use of the Wigner function is shown to be robust with respect to electrode dimensions and geometry, and is demonstrated to have little impact on the current for molecular junctions. Electronegativity calculations consist of a hexatriene-di-thiol model system with variable treatment of the electron correlation in comparison with conventional electronic structure treatments and demonstrate that correcting ionisation potentials and electron affinities with electron correlation leads to increasing the overlap to the exact one-electron reduced density matrix thereby improving theoretical predictions of electron currents on the nanoscale.
Electron transport , Molecular junctions
McDermott, S. 2018. Correlated electron transport across atomic and molecular tunnel junctions. PhD Thesis, University College Cork.