In Situ Tuning of the Charge-Carrier Polarity in Imidazole-Linked Single-Molecule Junctions

Tuning the polarity of charge carriers in N-heterocyclic carbene-based single-molecule junctions via atomic manipulation

Tuning the polarity of charge carriers at a single-molecular level is essential for designing complementary logic circuits in the field of molecular electronics. Herein, the transport properties of N-heterocyclic carbene (NHC)-linked single-molecule junctions are investigated using the ab initio quantum transport approach. The results reveal that the hydrogen atoms in NHCs function as a switch for regulating the polarity of charge carriers. Dehydrogenation changes the chemical nature of NHC anchors, thereby rendering holes as the major charge carriers rather than electrons. Essentially, dehydrogenation changes the anchoring group from electron-rich to electron-deficient. The electrons transferred to molecules from the electrodes raise the molecular level closer to the Fermi level, thus resulting in charge carrier polarity conversion. This conversion is influenced by the position and number of hydrogen atoms in the NHC anchors. To efficiently and decisively alter charge carrier polarity via atomic manipulation, a methyl substitution approach is developed and verified. These results confirm that atomic manipulation is a significant method for modulating the polarity of charge carriers in NHC-based single-molecule devices.

Evolution of Single-Molecule Electronic Interfaces

Single-molecule electronics can fabricate single-molecule devices via the construction of molecule-electrode interfaces and also provide a unique tool to investigate single-molecule scale physicochemical processes at these interfaces. To investigate single-molecule electronic devices with desired functionalities, an understanding of the interface evolution processes in single-molecule devices is essential. In this review, we focus on the evolution of molecule-electrode interface properties, including the background of interface evolution in single-molecule electronics, the construction of different types of single-molecule interfaces, and the regulation methods. Finally, we discuss the perspective of future characterization techniques and applications for single-molecule electronic interfaces.

Silver electrodes provide higher conductance than gold for thiol-terminated oligosilane molecular junctions: the interfacial effect

The understanding of the interfacial effect on charge transport is essential in single-molecule electronics. In this study, we elucidated the transport properties of molecular junctions comprising thiol-terminated oligosilane with three to eight Si atoms and two types of Ag/Au electrode materials employing different interfacial configurations. First-principles quantum transport calculations demonstrated that the interfacial configuration determines the relative magnitude of the current between the Ag and Au electrodes, wherein the Ag monoatomic contact configuration presented a larger current than did the Au double-atom configuration. Further, the mechanism of electron tunneling from the interfacial states through the central σ channel was revealed. In contrast to Au double-atom electrodes, Ag monoatomic electrodes exhibit a higher current due to the presence of Ag-S interfacial states closer to the Fermi level. Our findings show that the interfacial configuration is a plausible way to generate the relative magnitude of current of thiol-terminated oligosilane molecular junctions with Au/Ag electrodes and provide further insight into the interfacial effect on the transport properties.

Polaron Induced Conductance Switching in Conjugated Oligophenylene: A First-Principles Analysis

For π-conjugated systems, polaron formation has a major impact on their optoelectronic properties. In fact, for such systems, an exquisite interplay between electron delocalization and the steric effect determines their ground state properties. However, an excess charge (positive or negative) injection causes structural reorientation because of extended conjugation. Herein, we investigate the effect of such an excess charge in an individual polyphenylene on its quantum conductance behavior. By combining the DFT and NEGF formalisms, we characterize both structural and electronic changes occurring upon electron and hole injection. We demonstrate that for both the cationic and anionic radicals, the excess charge is observed to be localized, inducing a partial planarization of the molecule and forming cationic and anionic polarons, respectively. The calculated low-bias conductance values determine the polaronic effect and could be implemented for easy determination and measurement of polaron formation. In fact, cationic and anionic polarons induce a large degree of conductance switching, involving a decrease and increase of conductance, respectively.