From Chemistry to Functionality: Trends for the Length Dependence of the Thermopower in Molecular Junctions

We present a systematic ab-initio study of the length dependence of the thermopower in molecular junctions. The systems under consideration are small saturated and conjugated molecular chains of varying length attached to gold electrodes via a number of different binding groups. Different scenarios are observed: linearly increasing and decreasing thermopower as function of the chain length as well as positive and negative values for the contact thermopower. Also deviation from the linear behaviour is found. The trends can be explained by details of the transmission, in particular the presence, position and shape of resonances from gateway states. We find that these gateway states do not only determine the contact thermopower, but can also have a large influence on the length-dependence itself. This demonstrates that simple models for electron transport do not apply in general and that chemical trends are hard to predict. Furthermore, we discuss the limits of our approach based on Density Functional Theory and compare to more sophisticated methods like self-energy corrections and the GW theory

How dielectric screening in two-dimensional crystals affects the convergence of excited-state calculations: Monolayer MoS₂

We present first-principles many-body calculations of the dielectric constant, quasiparticle band structure, and optical absorption spectrum of monolayer MoS$_2$ using a supercell approach. As the separation between the periodically repeated layers is increased, the dielectric function of the layer develops a strong $q$ dependence around $q=0$. This implies that denser $k$-point grids are required to converge the band gap and exciton binding energies when large supercells are used. In the limit of infinite layer separation, here obtained using a truncated Coulomb interaction, a $45\times45$ $k$-point grid is needed to converge the G$_0$W$_0$ band gap and exciton energy to within 0.1 eV. We provide an extensive comparison with previous studies and explain agreement and variations in the results. It is demonstrated that too coarse $k$-point sampling and the interactions between the repeated layers have opposite effects on the band gap and exciton energy, leading to a fortuitous error cancellation in the previously published results.Comment: 10 pages, 11 figure

Quasiparticle GW calculations for solids, molecules, and two-dimensional materials

We present a plane wave implementation of the G0W0 approximation within the projector augmented wave method code GPAW. The computed band gaps of ten bulk semiconductors and insulators deviate on average by 0.2 eV (~ 5 %) from the experimental values - the only exception being ZnO where the calculated band gap is around 1 eV too low. Similar relative deviations are found for the ionization potentials of a test set of 32 small molecules. The importance of substrate screening for a correct description of quasiparticle energies and Fermi velocities in supported 2D materials is illustrated by the case of graphene/h-BN interfaces. Due to the long range Coulomb interaction between periodically repeated images, the use of a truncated interaction is found to be essential for obtaining converged results for 2D materials. For all systems studied, a plasmon pole approximation is found to reproduce the full frequency results to within 0.2 eV with a significant gain in computational speed. As alternative to G0W0, the efficient local GLLBSC potential yields significantly better results than the PBE0 hybrid. For completeness, we provide a mathematically rigorous and physically transparent introduction to the notion of quasiparticle states.Comment: 15 pages, 13 figure

Electron transport in molecular junctions with graphene as protecting layer

We present ab-initio transport calculations for molecular junctions that include graphene as a protecting layer between a single molecule and gold electrodes. This vertical setup has recently gained significant interest in experiment for the design of particularly stable and reproducible devices. We observe that the signals from the molecule in the electronic transmission are overlayed by the signatures of the graphene sheet, thus raising the need for a reinterpretation of the transmission. On the other hand, we see that our results are stable with respect to various defects in the graphene. For weakly physiosorbed molecules, no signs of interaction with the graphene are evident, so the transport properties are determined by offresonant tunnelling between the gold leads across an extended structure that includes the molecule itself and the additional graphene layer. Compared with pure gold electrodes, calculated conductances are about one order of magnitude lower due to the increased tunnelling distance. Relative differences upon changing the end group and the length of the molecule on the other hand, are similar

Bringing Research Data Management to Academic Libraries

Modern research institutions are faced with rapidly increasing challenges imposed by data-driven science and have to deal with new demands from funding agencies and publishers regarding open access to research data as well as from the Code of Conduct for Research Integrity on reproducibility of results. In reply to these requirements, the Technical University of Denmark (DTU) has adapted a two-fold strategy of bringing together decision-makers, relevant support units in the administration and researchers from all departments for the task of designing policies and services in alignment with common traditions in the different fields of research. This mixed “bottom-up” / “top-down” approach ensures that best practices  for research data management can be implemented in the daily routines of employees in a practical and supportive way while ensuring that the researchers can live up to all relevant administrative, legal and ethical standards.  In this presentation, we will show how DTU Library has taken over a central position in this process by investigating the needs and demands from researchers in the different fields, drafting procedures and guidelines for research data management and establishing support functions and training for all aspects  of the data life cycle. Within this new and complex area, the creation of new positions for research data management at the library in addition to an optimal utilization of existing resources and knowledge has been crucial for building up the necessary expertise and thus for the success of the strategy.  AuthorsFalco Jonas Hüser, PhDProject Officer – Research Data Managementemail: [email protected] 0000-0001-9645-6691Paula Maria Martinez Lavanchy, PhDProject Officer – Research Data Managementemail: [email protected] 0000-0003-1448-091