Molecular Structure and Substrate Effects on Single-Molecule Thermoelectric Properties

The ability to tailor the electronic transmission function in molecular materials with the help of chemical design opens exciting prospects towards optimising their electronic and thermal transport properties as well as the thermopower. For example, it is now well-established that quantum interference effects, the targeted introduction of substituents or even the exchange of individual atoms in a molecular structure are factors that allow for finetuning of the energetic structure of the junction.[1,2] The notion of a "Molecular" Quantum Technology is emerging, here in the context of materials design with increasing evidence that favourable thermoelectric properties at the single-molecule level can be translated to "many molecule" devices.<br/>Employing Scanning Tunnelling Spectroscopy in combination with advanced data analysis tools,[3-5] we have recently started to explore different classes of molecules in this context. Based on a variation of distance-dependent I/V spectroscopy, we have measured the thermopower for simple organic molecules as well as new and more complex molecular designs. Specifically, our results agree well with previously reported values for 1,8-octanedithiol, 4,4'-bipyridine and OPE3,[6] while we also find interesting quantum interference and substrate effects for a series of anthracene-based molecules.[7] To this end, we observe a “pinning effect” for asymmetrically coupled molecules, where the stronger anchor dictates the overall charge transport and thermoelectric behaviour, and inversion of the thermopower for weakly bound molecules in Au/Au vs. Au/Pt junctions. In connection with recent thin-film studies, the latter lend themselves to a detailed comparison of thermoelectric performance at the single-molecule and thin-film levels.[8,9] <br/> <br/>[1] R.J. Nichols, S.J. Higgins, "Single-Molecule Electronics: Chemical and Analytical Perspectives", Ann. Rev. Anal. Chem. 2015, 8, 389-417.<br/>[2] C.J. Lambert, "Basic concepts of quantum interference and electron transport in single-molecule electronics ", Chem. Soc. Rev., 2015, 44, 875-888.<br/>[3] M.S. Inkpen et al., "New insights into single-molecule junctions using a robust, unsupervised approach to data collection and analysis", J. Amer. Chem. Soc. 2015, 137, 9971-9981.<br/>[4] M. Lemmer et al., "Unsupervised vector-based classification of single-molecule charge transport data", Nat. Commun. 2016, 7, art. no. 12922.<br/>[5] T. Albrecht, G. Slabaugh, E. Alonso, S.M. Masudur R Al-Arif, "Deep learning for single-molecule science", Nanotechnology 2017, 28, 423001.<br/>[6] J.M. Hamill, C. Weaver, T. Albrecht, “Multivariate Approach to Single-Molecule Thermopower and Electrical Conductance Measurements”, J. Phys. Chem. C 2021, 125, 47, 26256-26262.<br/>[7] J.M. Hamill, T. Albrecht et al. (manuscript in preparation)<br/>[8] X. Wang et al., "Scale-Up of Room-Temperature Constructive Quantum Interference from Single Molecules to Self-Assembled Molecular-Electronic Films", J. Amer. Chem. Soc. 2020, 142, 8555-8560.<br/>[9] A. Ismael et al., "Tuning the thermoelectrical properties of anthracene-based self-assembled monolayers", Chem. Sci. 2020, ,11, 6836-6841.