Molecular Electronics - Modeling, Devices and Architecture

Doctoral thesis, 2010

Molecular electronics is an area of micro/nanoelectronics with a number of farreaching goals and challenges: denser implementation of electronic memory and logic, less expensive manufacturing, smaller or more sensitive measurement devices, selfassembly of devices and systems and, last but not least, exploring new science and applications. In recent years, molecular electronics has expanded into a broader field of molecular-scale electronics, partly because molecular components have to be defined in solid-state contexts via lithography and self-assembly, and effectively become hybrid devices with new and interesting properties. This thesis therefore provides an overview and critical assessment of recent experimental and theoretical development in the field of molecular-scale electronics, with focus on molecular-scale components for circuits and systems capable of performing information processing. The central part of this thesis is an in-depth investigation of one such device, the Nanocell, a self-assembled multi-terminal nanoelectronic switching network built from molecular (scale) linker elements with strongly non-linear and hysteretic current-voltage characteristics (IVC). The non-linearity involves negative differential resistance (NDR). The ground breaking generic new result is a demonstration, by programming from the edges without direct external access to individual links, how to configure the untrained Nanocell after fabrication and also how to reconfigure an already configured Nanocell to becoming a specific type of logic gate. The thesis also makes credible that the reconfiguration scheme is robust to most variations in the initial network topology. The thesis also contributes two theoretical investigations of normal and superconducting electron transport through molecular scale objects. One study demonstrates the importance of the molecular adsorption site (metal-molecule contact) for electron transport through a gold-sulfur-benzene-sulfur-gold (Au-DTB-Au) single-molecule junction. In particular, DTB and similar molecules with the terminal S-atoms buried in Au-vacancies shows prominent narrow transmission peaks close to the Fermi level, suggesting opportunities for devicing molecular switches and rectifiers. In the other transport investigation, it is shown that a quantum dot coupled to a phonon and positioned between two superconductors, can be used for Andreev Level spectroscopy.