Charge Transport and Thermoelectric Properties of Carbon Sulfide Nanobelts in Single-Molecule Sensors

The design, synthesis, and operation of single-molecule sensor devices are outstanding challenges in the field of molecular electronics. These devices are of significant interest as they could report the presence of harmful substances in different environments. Motivated by this, by means of non-equilibrium Green's function density functional theory, we investigate the properties of pristine single-molecule junctions comprised of single-walled heterocyclic nanobelts terminated with sulfur in contact with gold electrodes. These nanobelts are single slices of nanotubes, resulting in a molecular belt of fused aromatic rings, which have the shortest possible length of a nanotube; thus, fundamental studies of transport properties can be performed because of their finite size. We probe the charge transport and thermoelectric properties as a function of voltage bias and temperature. We find the radius of the nanobelt, and thus the number of Au-S contacts, has a strong impact on their electronic properties. With density functional theory methods and Green's functions, we compute the current-voltage (J-V) curves and observe typical characteristics consistent with semimetallic nanotubes around the Ohmic region, with semiconducting behavior at a higher voltage bias. We also observe bias-independent upshifting of the transmission spectra and identify one possible source as a coherent-tunneling analogue of the quadratic Stark effect, although the exact origin is unclear. Their projected density of states shows strong metallic behavior with transport primarily through the partially filled conduction bands. From the thermopower (Seebeck) function, we observe these nanobelts to be intrinsically n-type. In addition, an applied voltage bias does not change their conduction type, inferring these nanobelts are bias-independent n-type organic transistors across a 4 V window. The applicability of these molecular junctions as single-molecule gas sensors is also studied. We focus on the diatomics CO, HF, N₂, NO, F₂, and O₂ and observe typical electron donor-acceptor responses. The carbon and nitrogen monoxides give the strongest responses via inhibition of conduction. The fluorine and oxygen covalently functionalize across one double bond, each on separate nanobelts, resulting in a strong modulation of the transmission coefficient. The conduction (G_Ef) values of the functionalized @O₂ and @F₂ species at the Fermi energy and zero voltage bias are 78.2 and 59.9 μS, respectively, both of which are significantly enhanced versus that of the "bare" nanobelt (37.4 μS) and correlate with conduction values of molecular wires such as OBV. These results, in conjunction with the transmission spectra reported herein, are examples of the distinguishable response of a junction to the adsorption of a single molecule, which could motivate future work in nanobelt molecular bridges.