Atomistic simulations of conducting and thermoelectric properties of single molecule junctions

Place: conference hall, IMDEA Nanociencia.

Abstract: 

In molecular electronics, where individual molecules are placed between two electrodes in a nanoscale gap, molecules act as the active component in the electronic circuit. Both the chemical structure of the molecule as well as the geometry of the metal-molecule contacts strongly influence the interface electronic properties, and atomistic simulations based on Density Functional Theory (DFT) are ideally suited to study these phenomena [1]

In my talk, I will describe our recent work in the simulation of conducting and thermoelectric properties of molecular junctions, carried jointly with experimental partners. I will first address the characterization of interface geometry through the combination of conductance measurements, surface-enhanced Raman spectroscopy, and atomistic simulations. For thiol-linked molecular junctions, it was possible to characterize the adsorption site at the junction [2].

I will also discuss junction thermoelectric properties. The thermopower or Seebeck coefficient, which governs how the temperature and voltage differences across the junction are related, depends on the value and slope of the electron transmission function at the Fermi energy. I will show how the thermoelectric properties of a series of molecules can be mechanically tuned by varying the size of the nanogap. Extensive DFT simulations supported by STM-BJ experiments show how molecular orbital energies shift as a function of electrode separation, which leads to the modulation of conducting and thermoelectric properties. By mechanically tuning the molecular resonance energy with respect to the Fermi level, and selecting HOMO- or LUMO-conducting molecules, it is possible to tailor the thermoelectric properties of single molecule junctions.

[1] J.C. Cuevas and E. Scheer, Molecular Electronics: An Introduction to Theory and Experiment, World Scientific (2017).

[2] S. Kaneko, E. Montes, S. Suzuki, S. Fujii, T. Nishino, K. Tsukagoshi, K. Ikeda, H. Kano, H. Nakamura, H. Vázquez and M. Kiguchi, Identifying the molecular adsorption site of a single molecule junction through combined Raman and conductance studies, Chem. Sci. 10 6261 (2019).

[3] S. Fujii, E. Montes, H. Cho, Y. Yue, M. Koike, T. Nishino, H. Vázquez and M. Kiguchi, Mechanically Tuned Thermopower of Single-Molecule Junctions, Adv. Electron.  Mater. 8 2200700 (2022).