Understanding Electron Transport Properties of Molecular Electronic Devices

University dissertation from Stockholm : KTH

Abstract: his thesis has been devoted to the study of underlying mechanisms for electron transport in molecular electronic devices. Not only has focus been on describing the elastic and inelastic electron transport processes with a Green's function based scattering theory approach, but also on how to construct computational models that are relevant to experimental systems. The thesis is essentially divided into two parts. While the rst part covers basic assumptions and the elastic transport properties, the second part covers the inelastic transport properties and its applications.It is discussed how di erent experimental approaches may give rise to di erent junction widths and thereby di erences in coupling strength between the bridging molecules and the contacts. This di erence in coupling strength is then directly related to the magnitude of the current that passes through the molecule and may thus explain observed di erences between di erent experiments. Another focus is the role of intermolecular interactions on the current-voltage (I-V) characteristics, where water molecules interacting with functional groups in a set of conjugated molecules are considered. This is interesting from several aspects; many experiments are performed under ambient conditions, which means that water molecules will be present and may interfere with the experiment. Another point is that many measurement are done on self-assembled monolayers, which raises the question of how such a measurement relates to that of a single molecule. By looking at the perturbations caused by the water molecules, one may get an understanding of what impact a neighboring molecule may have. The theoretical predictions show that intermolecular e ects may play a crucial role and is related to the functional groups, which has to be taken into consideration when looking at experimental data.In the second part, the inelastic contribution to the total current is shown to be quite small and its real importance lies in probing the device geometry. Several molecules are studied for which experimental data is available for comparison. It is demonstrated that the IETS is very sensitive to the molecular conformation, contact geometry and junction width. It is also found that some of the spectral features that appear in experiment cannot be attributed to the molecular device, but to the background contributions, which shows how theory may be used to complement experiment. This part concludes with a study of the temperature dependence of the inelastic transport. This is very important not only from a theoretical point of view, but also for the experiments since it gives experimentalists a sense of which temperature ranges they can operate for measuring IETS.