Exploring the Impact of the HOMO-LUMO Gap on Molecular Thermoelectric Properties: A Comparative Study of Conjugated Aromatic, Quinoidal, and Donor-Acceptor Core Systems.

Thermoelectric materials have garnered significant interest for their potential to efficiently convert waste heat into electrical energy at room temperature without moving parts or harmful emissions. This study investigated the impact of the HOMO-LUMO (H-L) gap on the thermoelectric properties of three distinct classes of organic compounds: conjugated aromatics (isoindigos (IIGs)), quinoidal molecules (benzodipyrrolidones (BDPs)), and donor-acceptor systems (bis(pyrrol-2-yl)squaraines (BPSs)). These compounds were chosen for their structural simplicity and linear π-conjugated conductance paths, which promote high electrical conductance and minimize complications from quantum interference. Single-molecule thermoelectric measurements revealed that despite their low H-L gaps, the Seebeck coefficients of these compounds remain low. The alignment of the frontier orbitals relative to the Fermi energy was found to play a crucial role in determining the Seebeck coefficients, as exemplified by the BDP compounds. Theoretical calculations support these findings and suggest that anchor group selection could further enhance the thermoelectric behavior of these types of molecules.

Thermopower in Underpotential Deposition-Based Molecular Junctions.

Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule-electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance.

An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions.

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches.

Large Mechanosensitive Thermoelectric Enhancement in Metallo-Organic Magnetic Molecules.

Organic materials are promising candidates for thermoelectric cooling and energy harvesting at room temperature. However, their electrical conductance (G) and Seebeck coefficient (S) need to be improved to make them technologically competitive. Therefore, radically new strategies need to be developed to tune their thermoelectric properties. Here, we demonstrate that G and S can be tuned mechanically in paramagnetic metallocenes, and their thermoelectric properties can be significantly enhanced by the application of mechanical forces. With a 2% junction compression, the full thermoelectric figure of merit is enhanced by more than 200 times. We demonstrate that this is because spin transport resonances in paramagnetic metallocenes are strongly sensitive to the interaction between organic ligands and the metal center, which is not the case in their diamagnetic analogue. These results open a new avenue for the development of organic thermoelectric materials for cooling future quantum computers and generating electricity from low-grade energy sources.

Contacting individual graphene nanoribbons using carbon nanotube electrodes.

Graphene nanoribbons synthesized using bottom-up approaches can be structured with atomic precision, allowing their physical properties to be precisely controlled. For applications in quantum technology, the manipulation of single charges, spins or photons is required. However, achieving this at the level of single graphene nanoribbons is experimentally challenging due to the difficulty of contacting individual nanoribbons, particularly on-surface synthesized ones. Here we report the contacting and electrical characterization of on-surface synthesized graphene nanoribbons in a multigate device architecture using single-walled carbon nanotubes as the electrodes. The approach relies on the self-aligned nature of both nanotubes, which have diameters as small as 1 nm, and the nanoribbon growth on their respective growth substrates. The resulting nanoribbon-nanotube devices exhibit quantum transport phenomena-including Coulomb blockade, excited states of vibrational origin and Franck-Condon blockade-that indicate the contacting of individual graphene nanoribbons.

Engineering Heat Transport Across Epitaxial Lattice-Mismatched van der Waals Heterointerfaces.

Artificially engineered 2D materials offer unique physical properties for thermal management, surpassing naturally occurring materials. Here, using van der Waals epitaxy, we demonstrate the ability to engineer extremely insulating thermal metamaterials based on atomically thin lattice-mismatched Bi2Se3/MoSe2 superlattices and graphene/PdSe2 heterostructures with exceptional thermal resistances (70-202 m2 K/GW) and ultralow cross-plane thermal conductivities (0.012-0.07 W/mK) at room temperature, comparable to those of amorphous materials. Experimental data obtained using frequency-domain thermoreflectance and low-frequency Raman spectroscopy, supported by tight-binding phonon calculations, reveal the impact of lattice mismatch, phonon-interface scattering, size effects, temperature, and interface thermal resistance on cross-plane heat dissipation, uncovering different thermal transport regimes and the dominant role of long-wavelength phonons. Our findings provide essential insights into emerging synthesis and thermal characterization methods and valuable guidance for the development of large-area heteroepitaxial van der Waals films of dissimilar materials with tailored thermal transport characteristics.

Controlling Spin Interference in Single Radical Molecules.

Quantum interference (QI) dominates the electronic properties of single molecules even at room temperature and can lead to a large change in their electrical conductance. To take advantage of this for nanoelectronic applications, a mechanism to electronically control QI in single molecules needs to be developed. In this paper, we demonstrate that controlling the quantum interference of each spin in a stable open-shell organic radical with a large π-system is possible by changing the spin state of the radical. We show that the counterintuitive constructive spin interference in a meta-connected radical changes to destructive interference by changing the spin state of the radical from a doublet to a singlet. This results in a significant change in the room temperature electrical conductance by several orders of magnitude, opening up new possibilities for spin interference based molecular switches for energy storage and conversion applications.