DQI in electron transport through graphene nanoribbons

Graphene has been extensively investigated as a promising material for use in high performance nano-electronic devices due to their unique physical properties such as their excellent thermal stability, high charge carrier mobility or their ability to form atomically precise nano-junctions. Due to the very large changes in conductivity that are possible as a result of charge doping of graphene sheets with a number of adsorbates together with its uniquely high surface-to-volume ratio the material has also been proposed for gas sensing, as a bio sensor and even DNA base sequence analysis. There are two major challenges for their further development: i) The mechanism on which the gas sensing is based is not very well understood and sometimes several competing theoretical explanations exist; ii) While there is little doubt that thin graphene films show great sensitivity, unfortunately they are sensitive to many different types of adsorbates and thus not chemically selective. Both problems are linked to the same source, namely the dependence of the sensing mechanism on the detailed device structure caused by any particular fabrication process, which are all beyond control in devices made of larger graphene sheets. These problems can therefore only be overcome by replacing such extended sheets by atomically well defined graphene nanoribbons (GNRs), for which then new paradigms for the detection mechanism need to be proposed. Tremendous progress has been made recently in the chemical synthesis of structurally well-defined and liquid-phase- processable, while destructive quantum interference (DQI) effects have been suggested as a new paradigm for logic devices based on GNRs with extremely low power consumption as well as a tool for increasing the selectivity of graphene-based gas sensors. This project aims to systematically investigate the relationship between the structure (or topology) of GNRs connected to a source and drain electrode on multiple contact sites and the resulting conductance, which crucially depends on the occurrence or absence of DQI effects. This is of fundamental interest in basic research on graphene, where the basic capability to synthesize GNRs of arbitary shapes and sizes now inspires the question which particular structures would be of interest scientifically and for potential technological applications. Since there are different types of QI encountered at different length scales, it is scientifically of interest if there might be any connection of these different types of QI that can be drawn from scaling up topologies from the molecular scale to a mesoscopic length scale. In terms of applications chemical sensors are amongst the most interesting candidates, where Density Functional Theory based calculations will be employed in this project for an atomistic description of the source and drain electrodes as well as for the characterization of the charge transfer between particular types of adsorbate molecules and GNRs.

Gas sensing technologies are of paramount importance for environmental safety and medical applications. Already a decade ago the detection of a single NO2 molecule with a graphene based sensor was reported and the sensitivity of graphene-based sensors has been increased in recent experiments, in particular for H2O and NO2. The reviews on gas sensors based on graphene identify the same two major challenges for their further development: (i) the mechanism on which the gas sensing is based is not very well understood and sometimes several competing theoretical explanations exist; (ii) while there is little doubt that thin graphene films show great sensitivity, unfortunately they are sensitive to many different types of adsorbates and this cross-sensitivity naturally diminishes another important property of any chemical sensor, namely chemical selectivity. These problems can potentially be overcome by replacing extended graphene sheets with graphene nanostructures, such as atomically well-defined graphene nanoribbons (GNRs), for which new paradigms for the detection mechanism need to be proposed. Tremendous progress has been made recently in the chemical synthesis of atomically precise and liquid-phase-processable GNRs, which can be functionalized for sensing applications and easily integrated in junctions or electric circuits. At the same time, quantum interference (QI) effects, which are well established in -conjugated single-molecule junctions, have also been theoretically predicted and experimentally observed in nanostructured graphene. In terms of applications, QI has also been suggested as a new paradigm for logic devices based on GNRs with extremely low power consumption as well as a tool for increasing the selectivity of GNR-based gas sensors. In this project we investigated the stability of destructive quantum interference (DQI) in electron transport through graphene nanostructures connected to source and drain electrodes in dependence on the system size, where we considered the influence of disorder, electron-electron and electron-phonon interactions. A thorough density functional theory (DFT) analysis allowed to disentangle the transmission features arising from the molecule and the electrodes. We demonstrate that by exploiting destructive QI arising in the meta-substituted pyrene, it is possible to calibrate a graphene-like sensor to enhance both its sensitivity and chemical selectivity by almost two orders of magnitude so that individual NO2, H2O, and NH3 molecules can be detected and distinguished. From DFT calculations we also derived a local orbital (LO) basis set, which allows one to perform post-processing of DFT calculations, ranging from the interpretation of electron transport to extracting effective tight-binding Hamiltonians, very efficiently and without sacrificing the accuracy of the results. From this LO basis we derived a tight-binding method which is suitable to perform simulations of chemical sensing in realistically complex systems, that can be compared directly to experiments.