Molecular Filtering

Electronic diodes are fundamental components of everyday integrated circuits. Their design allows for the unidirectional transport of charges. They are broadly employed as conductive elements for signal processing and amplification, and in logic systems.

Extending the diode concept to the transport of matter and heat is essential for a new generation of mechanical and thermal diodes based on the topological properties of materials, towards development of thermal management or conversion circuits, faster telecommunication and possibly longer-lasting batteries [R. Süsstrunk and S. D. Huber, Proc. Natl. Acad. Sci. USA (2016); Liu et al. Phys. Rev. B (2017)].

For an analogy of how topological diodes function, consider a fish trap (Fig 1A). Such contraptions allow fish to swim in one direction only, partly due to the trap’s geometry and topology (the number of sides, edges and genera). In practice, the realization of topological diode-like filters for the transport of heat and matter remains a complex task, and an open question at technologically-relevant frequencies. One solution could be right in front of us. Nature ubiquitously harnesses complex molecular architectures into filters and diode elements: Cell membranes possess channels for unidirectional transport of specific molecules and ions to the cytoplasm [Preston et al. Science (1992); Doyle et al. Science (1998)]. Thus, complex molecular materials could hold the key to thermal technology.

Recently, we studied cyanotriangulene molecules [Gottardi et al. Adv. Mater. Interfaces (2014)], which self-assemble on Au(111) into a chiral lattice (Fig 1B) by classical atomistic simulations. Restricting the two-dimensional periodicity to form a supramolecular nanoribbon (Fig 1C) results in new heat-carrying phonon modes. Mapping the new states, confirms the presence of edge phonon modes (Fig 1D), whose phonon spectrum is thermally insulated from the bulk – two characteristics pertaining to mechanical topological materials. We described the functional properties of such edge modes by mechanically exciting a single supramolecular bond, which results in unidirectional energy transport along the edge without considerable bulk or back-propagation (Fig 1E).

Within the Molecular Filtering project, we are exploring new filtering notions, applications and mathematical descriptors from the organized assembly of molecules at interfaces. We believe that by following nature’s example aided by the vast universe of supramolecular materials, supramolecular thermal waveguides, diodes, and logics could one day complement current technologies.