Interference effects in molecular electronics

Molecular electronics is a promising field for nano-electronics. As found in the recent scientific work of the applicant, traditional schemes to build up computer architectures from logic gates and memories as used in semiconductor industry face fundamental difficulties on the scale of single molecules. This is partly due to quantum interference effects, which occur on the nano-scale. As a consequence new architectural concepts are needed, which make use of these inteference effects rather than try to bypass them. Objective I of the proposal addresses key issues of these interference effects. Most importantly, the work on a more realistic description of the electrode/molecule/electrode nano-junction as a whole (as compared to earlier simple models pursued by the applicant), has to be extended on the basis of density functional theory simulations. Here the geometrical and chemical details of the connection between nitro-benzene and the Au surface will be explored. In a next step, the applicant will move to a larger system and the substitution of anthracene with multiple nitro groups will be addressed. Objective II of the proposal is aimed at the question of the alignment of molecular orbitals with the metal electrodes Fermi level. The outcome of this alignment has a crucial effect on the zero bias conductance of the junction (varying it by at least an order of magnitude), where the major source determining it is equilibrium charge transfer between the molecule and the electrodes. The research proposed in the current application is concerned with the question how far it is possible to group different classes of molecules with regard to their charge transfer characteristics. In a first step this would mean the comparison of molecules with pyridil anchor groups focusing on benzene and anthracene. In a second step the question would be addressed whether the substitution of hydrogen atoms on the aromatic rings by nitro groups results in a changed energetic positions of their MOs. These electrostatic considerations of the level alignment in objective II are also complimentary to the interference scheme in objective I. In a simplified picture, the interference or hybridisation effects would determine the overall shape of the transmission function, whereas its energetic positioning is governed by charge transfer, where the interplay of both results in the final conductance of the junction. This project proposal is a resubmission of project NR. P19435-N16.

Single-molecule electronics has become a vibrant area of nano-electronics recently, because Moore`s law, which predicts a continuous rise in the performance of digital devices due to their continuous miniaturisation, cannot be upheld down to the atomic scale based on Silicon devices. For realising the potential of this field, it is necessary to design realistic devices by theoretical means, where the active part of the circuit would be performed by a single molecule junction, i.e. a single molecule sandwhiched between two metal electrodes. Ideally one would like to combine two things: i) A device scheme developed and justified by theoreticians should be so simple that it can be applied by experimentalists without significant theoretical knowledge; ii) the device scheme should be reliable enough, which means that its validity has to be derived from and assessed by first principles calculations. Both i) and ii) have been achieved in this project. A graphical scheme has been established, which can predict the occurrence or absence of quantum interference (QI) effects in relation to the molecular structure. The findings have been verified by density functional theory (DFT) calculations and provide an important tool for the design of data storage elements as well as logic gates in single molecule electronics. Another scheme has been also developed which relates the asymmetry of peak shapes of QI induced minima in electron transmission functions to just two parameters, which can be obtained from standard quantum chemical calculations and are intuitively meaningful in molecular orbital theory. This is significant for thermoelectric applications of single-molecule devices, where the thermopower and figure of merit are defined to a large extent by this asymmetry. A substantial amount of the research in this project was also invested in cross-checking important results for molecules weakly coupled to the electrodes, a so called Coulomb blockade scenario, with quantum chemical methods which exceed standard DFT in terms of accuracy. Finally, the role of chemical anchor groups for the energetic position of QI effects with respect to the Fermi energy has been investigated. The research conducted in this project is situated in the emerging field of nanoelectronics, which is one of the key areas supported by the European Community under the `Information Society Technologies` programme and there is substantial investment both industrial and by the government in the USA. Amongst the expected beneficiaries of these investments are the computer industry and also the medical sector, where nanotechnological links between both fields can be expected, because nanoelectronic devices would operate with organic molecules on an atomic scale.