Side-Group Effect on Electron Transport of Single Molecular Junctions

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.

Substituent-mediated quantum interference toward a giant single-molecule conductance variation

Quantum interference (QI) in single molecular junctions shows a promising perspective for realizing conceptual nanoelectronics. However, controlling and modulating the QI remains a big challenge. Herein, two-type substituents at different positions ofmeta-linked benzene, namely electron-donating methoxy (-OMe) and electron-withdrawing nitryl (-NO₂), are designed and synthesized to investigate the substituent effects on QI. The calculated transmission coefficientsT(E) indicates that -OMe and -NO₂could remove the antiresonance and destructive quantum interference (DQI)-induced transmission dips at position 2. -OMe could raise the antiresonance energy at position 4 while -NO₂groups removes the DQI features. For substituents at position 5, both of them are nonactive for tuning QI. The conductance measurements by scanning tunneling microscopy break junction show a good agreement with the theoretical prediction. More than two order of magnitude single-molecule conductance on/off ratio could be achieved at the different positions of -NO₂substituent groups at room temperature. The present work proves chemical substituents can be used for tuning QI features in single molecular junctions, which provides a feasible way toward realization of high-performance molecular devices.

Side-group-mediated thermoelectric properties of anthracene single-molecule junction with anchoring groups

Charge transfer characteristics of single-molecule junctions at the nanoscale, and consequently, their thermoelectric properties can be dramatically tuned by chemical or conformational modification of side groups or anchoring groups. In this study, we used density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) formalism in the linear response regime to examine the thermoelectric properties of a side-group-mediated anthracene molecule coupled to gold (Au) electrodes via anchoring groups. In order to provide a comparative inspection three different side groups, i.e. amine, nitro and methyl, in two different positions were considered for the functionalization of the molecule terminated with thiol or isocyanide anchoring groups. We showed that when the anchored molecule is perturbed with side group, the peaks of the transmission spectrum were shifted relative to the Fermi energy in comparison to the unperturbed molecule (i.e. without side group) leading to modified thermoelectric properties of the system. Particularly, in the thiol-terminated molecule the amine side group showed the greatest figure of merit in both positions which was suppressed by the change of side group position. However, in the isocyanide-terminated molecule the methyl side group attained the greatest thermoelectric efficiency where its magnitude was relatively robust to the change of side group position. In this way, different combinations of side groups and anchoring groups can improve or suppress thermopower and the figure of merit of the molecular junction depending on the interplay between charge donating/accepting nature of the functionals or their position.

An In-Depth Assessment of the Electronic and Magnetic Properties of a Highly Ordered Hybrid Interface: The Case of Nickel Tetra-Phenyl-Porphyrins on Fe(001)-p(1 × 1)O

In this paper we focus on the structural, electronic, and magnetic properties of Ni tetra-phenyl-porphyrins (NiTPP) grown on top of Fe(001)–p(1 × 1)O. Ordered thin films of metal TPP molecules are potentially interesting for organic electronic and spintronic applications, especially when they are coupled to a ferromagnetic substrate. Unfortunately, porphyrin layers deposited on top of ferromagnetic substrates do not generally show long-range order. In this work, we provide evidence of an ordered disposition of the organic film above the iron surface and we prove that the thin layer of iron oxide decouples the molecules from the substrate, thus preserving the molecular electronic features, especially the HOMO-LUMO gap, even when just a few organic layers are deposited. The effect of the exposure to molecular oxygen is also investigated and an increased robustness against oxidation with respect to the bare substrate is detected. Finally, we present our results for the magnetic analysis performed by spin resolved spectroscopy, finding a null magnetic coupling between the molecules and the substrate.

Single-Molecule Sensing of Interfacial Acid-Base Chemistry

Bronsted acid and base interactions are a cornerstone of chemistry describing a wide range of chemical phenomena. However, probing such interaction at the solid-liquid interface to extract the elementary and intrinsic information at a single-molecule level remains a big challenge. Herein, we employ an STM break junction (STM-BJ) technique to investigate the acid-base chemistry of carboxylic acid-based molecules at a Au (111) model surface and propose a prototype of a single-molecule pH sensor for the first time. The single-molecule measurements in different environmental conditions verify that the formation probability of molecular junctions is determined by the populations of deprotonated -COO^- form in a self-assembled monolayer. Furthermore, the variation of the intensity of the conductance peaks (i.e., junction-forming probability) with the pH of the bulk solution fits well to the Henderson-Hasselbalch type equation. From the equation, a good linear relation is found between the degree of dissociation of the immobilized -COOH group and the environmental pH, providing a feasible way to design chemicals and biosensors and a detector at the single-molecule scale.