Electron transfer through a single barrier inside a molecule: from strong to weak coupling

In all theoretical treatments of electron transport through single molecules between two metal electrodes, a clear distinction has to be made between a coherent transport regime with a strong coupling throughout the junction and a Coulomb blockade regime in which the molecule is only weakly coupled to both leads. The former case where the tunnelling barrier is considered to be delocalized across the system can be well described with common mean-field techniques based on density functional theory (DFT), while the latter case with its two distinct barriers localized at the interfaces usually requires a multideterminant description. There is a third scenario with just one barrier localized inside the molecule which we investigate here using a variety of quantum-chemical methods by studying partial charge shifts in biphenyl radical ions induced by an electric field at different angles to modulate the coupling and thereby the barrier within the $\pi$-system. We find steps rounded off at the edges in the charge versus field curves for weak and intermediate coupling, whose accurate description requires a correct treatment of both exchange and dynamical correlation effects is essential. We establish that DFT standard functionals fail to reproduce this feature, while a long range corrected hybrid functional fares much better, which makes it a reasonable choice for a proper DFT-based transport description of such single barrier systemsComment: 8 pages, 4 figures; J. Chem. Phys., in print (2012

Controlling the transmission line shape of molecular t-stubs and potential thermoelectric applications

Asymmetric line shapes can occur in the transmission function describing electron transport in the vicinity of a minimum caused by quantum interference effects. Such asymmetry can be used to increase the thermoelectric efficiency of molecular junctions. So far, however, asymmetric line shapes have been only empirically found for just a few rather complex organic molecules where the origins of the line shapes relation to molecular structure were not resolved. In the present work we introduce a method to analyze the structure dependence of the asymmetry of interference dips from simple two site tight-binding models, where one site corresponds to a molecular $\pi$ orbital of the wire and the other to an atomic $p_z$ orbital of a side group, which allows us to analytically characterize the peak shape in terms of just two parameters. We assess our scheme with first-principles electron transport calculations for a variety of {\it t-stub} molecules and also address their suitability for thermoelectric applications.Comment: 11 pages, 5 figures; J. Chem. Phys., in print (2011

Noninvasive ultrasound techniques for assessment of early atherosclerosis

Thesis (Ph. D.)--Massachusetts Institute of Technology, Whitaker College of Health Sciences and Technology, 1996.Includes bibliographical references (p. 135-150).by Robert William Stadler.Ph.D