Mechanism of rectification and negative differential resistance in single-molecule junctions with asymmetric anchoring groups: a DFT study.

CONTEXT: This study aims to investigate the electronic transport properties of tetracene molecule connected to gold (Au) electrodes with asymmetric anchoring groups. More specifically, we investigate the effect of asymmetric electrode coupling on the rectification ratio of tetracene-based molecular device. To introduce coupling asymmetry in these junctions, one end of the tetracene molecule is terminated with thiol (-SH) or isocyanide (-NC) while the other end with amine (-NH2) or nitro (-NO2) anchoring group. The results indicate that the electronic transport behavior is affected by the nature of molecule-electrode coupling, and the rectification ratio can be modulated by a proper choice of the anchoring groups. We reveal that the tetracene molecule when connected with isocyanide and amine combination exhibits remarkable rectifying performance (with a rectification ratio of 74) in contrast with other configurations. Furthermore, a prominent negative differential resistance (NDR) feature is observed when the molecule is connected with thiol as one of the anchors. Our present findings with excellent rectifying performance and negative differential resistance pave a new roadmap for designing multifunctional molecular devices. METHODS: By applying non-equilibrium Green's function (NEGF) formalism combined with density functional theory (DFT) Atomistic Tool Kit software package, the electronic transport properties of tetracene molecule connected to gold electrodes with asymmetric anchoring groups have been investigated. The calculations were performed using the Perdew-Burke-Ernzerhof (PBE) parameterization of DFT within generalized gradient approximation (GGA) exchange-correlation functional. To improve calculation precision and save computational efforts, the molecule and anchor groups were double-ζ (DZ) polarized, while single-ζ (SZ) polarized basis set was used for gold electrodes.

Tuning the transport properties of tetracene-based single-molecule junctions with chemical or structural variation of side and anchoring groups.

CONTEXT: This study aims to tune the transport properties of tetracene single-molecule junctions with the proper choice and placement of side and anchoring groups. For the operationalization of the molecule that was anchored with thiol or isocyanide groups, two different side groups, amine and nitro, in two different positions, were taken into consideration. For unperturbed tetracene molecule, a prominent negative differential resistance (NDR) feature at 1.8 V was observed with the isocyanide anchoring group while the thiol anchoring group exhibits a plateau region over a bias voltage of 2.2 to 3.2 V. At a bias voltage that is dependent on the chemical or structural change of side or anchoring groups, NDR feature of varying degree was seen in all configurations. Results show that the current flowing through the thiol-anchored molecule perturbed with the amine group at S' position is relatively larger than other configurations because of the smaller HOMO-LUMO gap and broader transmission peaks resulting in a peak to valley current ratio (PVCR) of 1.22. In addition, multiple NDR regions were realized in nitro-perturbed isocyanide-anchored molecule at S position. These results suggest their promising applications in switches, logic cells, and storage devices. METHODS: The modeling and simulation of side-group mediated anchored tetracene molecule through two electrodic systems were studied using density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) in Virtual NanoLab-AtomistixToolkit (ATK). The electron transport properties were calculated using Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation function. To optimize computing time, gold electrodes were single zeta polarized whereas the molecule, anchor groups, and side groups were double zeta polarized.

Investigating the influence of electrode Miller indices alteration on electronic transport in thiophene-based molecular junctions.

Electrical charge transport through thiophene-dithiol-based molecular wires attached to gold electrodes with three different types of crystallographic orientations (<1,1,1>, <1,1,0 > and <1,0,1 >) was investigated. Electron transport in the systems under consideration was evaluated systematically by analyzing current values, transmission spectrum, projected device density of states and zero bias orbital analysis utilizing density functional theory in conjunction with non-equilibrium Green's function. Investigations proved that tuning of conductance in nano-molecular junctions is possible through different electrode orientations. As the HOMO-LUMO gap in the <1,1,0 > oriented thiophene dithiol junction is drastically less than that of the other configurations under consideration, the <1,1,0 > configuration exhibited superior constructive conductance in comparison to other junction orientations. This provided us with ideas for designing pioneering hetero-cyclic nano-scale electronics devices. Also, <1,1,0 > has been found to show negative differential conductance behavior above +2.6 V and below -2.6 V, and hence has potential applications in oscillating and switching circuits.

The electronic transport properties of B40 fullerenes with chalcogens as anchor atoms.

Fullerenes are the most popular molecules to use in applications related to molecular electronics because of their superconductive nature. These molecules show a diverse range of properties, including optical, electronic, and structural characteristics. In this work, we focused on the electronic transport properties of molecular devices consisting of the fullerene B40 or B40 with different anchor atoms between two gold electrodes in a two-probe configuration. The elements used as anchor atoms in the B40 molecules were oxygen, selenium, and sulfur, i.e., chalcogens. The current characteristics of these fullerene-based molecular devices were calculated and analyzed. The analysis highlighted the superior electrical conductivity of the pure B40 device compared to the devices based on its chalcogen-anchored variants. The conductivities of the molecular devices were ranked as follows: pure B40 > selenium-anchored > sulfur-anchored > oxygen-anchored B40. It was also noted that the devices based on B40 and its chalcogen-anchored variants gave nonzero conductance values at zero bias. These results pave the way for the application of these molecules in future nanodevices utilizing extremely small bias voltages.

Reconnoitring the current characteristics of the double C20 fullerene molecular device in two probe configuration.

It is worth remarking that the C20 cage like isomer has been the topic of concentrated theoretical research. C20 single fullerene molecular devices gained a lot of popularity in the field of nano research due to their superlative doping dependent conductive properties. In this work, the double fullerene device has been considered. Here double fullerene molecular junction is created when two C20 fullerene molecules, one in pristine form and other in doped form, are positioned between gold electrodes. Doping was done firstly by second period elements, boron, nitrogen, oxygen, and fluorine and then by group 14 tetragens, silicon, germanium, tin, and lead. For both the cases current characteristics were investigated. Superior conductivity was observed in the boron doped double C20 molecular device while the fluorine doped device was the least conducting. Further for group 14 doping, the silicon doped double C20 device showed maximum current carrying feature, whereas, least value of current was noted in tin doped C20 device.