Field Of Single-molecule Electronics Research Articles

High temporal resolution electrical characterization technology of single-molecule devices

<p indent=0mm>Single-molecule electronics is a cross-discipline which aims at employing the individual molecule as the functional element for further miniaturization of the silicon-based microelectronics, and also focuses on the physical and chemical phenomena at single-molecule level. With the development of the single-molecule electronics, numerous novel physical and chemical effects such as Kondo physics, superconductivity and chemical reactions catalyzed by local strong electric fields, and single-molecule electronic devices including filed-effect transistors, diodes, and photo-switches have been widely explored. However, most of the abovementioned investigations are implemented at the millisecond level owing to the low temporal resolution of the measurement instrument, which makes it difficult to monitor the reaction events at faster time scales as well as meet the requirement of the high operating speed of the single-molecule devices. As a result, more attentions have been paid to develop the high temporal resolution electrical characterization techniques. In conventional single-molecule electrical measurement techniques, there is a big challenge that the extremely weak current signal through the single-molecule junction puts higher demands on the current amplification system of the instrument. However, the response speed of such amplifying circuits is relatively low, which limits the temporal resolution for single-molecule electrical measurements. In order to break through the limitation of temporal resolution, much efforts have been made in recent years. On the one hand, the low response speed caused by DC circuits can be improved through increasing the response speed of the current amplifying circuits. On the other hand, AC signal provides a promising approach to improve the temporal resolution, considering that the AC signal has already proven its application potential in the study of other micro-scale high-speed processes. This method requires the addition of AC signal measurement equipment in the circuit, including AC-DC separation circuits, RF amplifiers and lock-in amplifiers, etc., in addition, how to conduct effective data analysis is also a challenge worth considering. In this article, we will give a comprehensive review on the high temporal resolution electrical characterization techniques in the field of single-molecule electronics. This review will describe the challenges, developments, and applications of high temporal resolution electrical characterization techniques. We will briefly explain the significance of the research on the high temporal resolution electrical characterization techniques in the first part, the second part will describe the technical bottleneck of conventional single-molecule electrical measurement techniques, the third part will focus on the applications of the high temporal resolution electrical characterization techniques, and we will draw some expectations and conclusions in the fourth part. Although the high temporal resolution electrical characterization technology of single-molecule devices is still staying at the primary stage for its high requirements for circuit technology and measurement equipment, but this characterization technology can provide us with effective means to study higher-speed processes and more essential properties at the single-molecule scale, which includes the rearrangement of atoms, the structural transformation of molecules, and even the charge or energy transfer at the molecule-electrode interface. Those properties are also the guarantee for the manufacture of high-performance single-molecule electronic devices in the future. Therefore, we believe that the high temporal resolution electrical characterization techniques of single-molecule devices is full of development prospects and will be employed widely in the future.

Single molecule electronic devices with carbon-based materials: status and opportunity.

The field of single molecule electronics has progressed remarkably in the past decades by allowing for more versatile molecular functions and improving device fabrication techniques. In particular, electrodes made from carbon-based materials such as graphene and carbon nanotubes (CNTs) may enable parallel fabrication of multiple single molecule devices. In this perspective, we review the recent progress in the field of single molecule electronics, with a focus on devices that utilizes carbon-based electrodes. The paper is structured in three main sections: (i) controlling the molecule/graphene electrode interface using covalent and non-covalent approaches, (ii) using CNTs as electrodes for fabricating single molecule devices, and (iii) a discussion of possible future directions employing new or emerging 2D materials.

Fabrication and functions of graphene-molecule-graphene single-molecule junctions.

The past two decades have witnessed increasingly rapid advances in the field of single-molecule electronics, which are expected to overcome the limitation of the miniaturization of silicon-based microdevices, thus promoting the development of device manufacturing technologies and characterization means. In addition to this, they can enable us to investigate the intrinsic properties of materials at the atomic- or molecular-length scale and probe new phenomena that are inaccessible in ensemble experiments. In this perspective, we start from a brief introduction on the manufacturing method of graphene-molecule-graphene single-molecule junctions (GMG-SMJs). Then, we make a description on the remarkable functions of GMG-SMJs, especially on the investigation of single-molecule charge transport and dynamics. Finally, we conclude by discussing the main challenges and future research directions of molecular electronics.

Charge Transport and Quantum Interference Effects in Oxazole-Terminated Conjugated Oligomers.

Charge transport in single molecule junctions critically depends on the chemical identity of anchor groups used to connect molecular wires to electrodes. In this work, we report the charge transport properties of conjugated oligomers with oxazole anchors, focusing on the role of the heteroatom substitution position in terminal oxazole groups. Our results show that oxazole serves as an efficient anchor group to form stable gold-molecule-gold junctions. We further observe quantum interference (QI) effects in oxazole-terminated phenylene molecular junctions, including destructive QI in meta-substituted phenyl rings and constructive QI in para-substituted phenyl rings containing terminal oxazole groups with the same chemical constitution on both termini (i.e., O5O5 (5-oxazolyl) or O4O4 (4-oxazolyl) linkages on both termini). Surprisingly, meta-substituted phenyl rings with nonequivalent constitutions (i.e., O4O5 oxazole terminal linkages) show unexpectedly higher conductance as compared to para-substituted analogues. These results suggest that charge transport in oxazole-terminated molecules is determined by the heteroatom substitution position of the oxazole anchor in addition to the aryl substitution pattern of the π-conjugated core. Our results further show that conjugated molecules with homogeneous oxazole linkages obey a quantum circuit rule such that GO4-p-O4/GO4-m-O4 = GO5-p-O5/GO5-m-O5, where G is molecular conductance. Overall, our work provides key insight into the development of new chemistries for molecular circuitry in the rapidly advancing field of single molecule electronics.

Captodative Substitution Enhances the Diradical Character of Compounds, Reduces Aromaticity, and Controls Single-Molecule Conductivity Patterns: A Valence Bond Study.

The present contribution uses a valence bond (VB) perspective to consider the captodative substitution strategy, a method to enhance the diradical character of (potentially aromatic) compounds. We confirm the qualitative reasoning that has generally been used to rationalize the diradical-character-enhancing effect of captodative substitution: this type of substitution scheme disproportionally stabilizes specific Dewar/diradical(oid) VB structures, thus increasing their weight in the full ground-state wave function. Furthermore, we assess the effect of captodative substitution on the aromaticity of the considered compound. We observe a clear trade-off between diradical character and aromaticity for our model systems: as one of these properties increases, the other decreases. This finding is especially significant within the field of single-molecule electronics because it enables unification of the previously observed inverse proportionality between the aromaticity of a compound and the magnitude of conductance through that molecule, with the observed proportionality between diradical character and the magnitude of conductance associated with a compound. To some extent, both properties, i.e., aromaticity and diradical character, appear to be the flip-sides of the same coin.

Concepts in the design and engineering of single-molecule electronic devices

Over the past two decades, various techniques for fabricating nano-gapped electrodes have emerged, promoting rapid development in the field of single-molecule electronics, on both the experimental and theoretical sides. To investigate intrinsic quantum phenomena and achieve desired functionalities, it is important to fully understand the charge transport characteristics of single-molecule devices. In this Review, we present the principles that have been developed for fabricating reliable molecular junctions and tuning their intrinsic properties from an engineering perspective. Through holistic consideration of the device structure, we divide single-molecule junctions into three intercorrelated components: the electrode, the contact (spacer–linker) interface and the molecular backbone or functional centre. We systematically discuss the selection of the electrode material and the design of the molecular components from the point of view of the materials, the interface and molecular engineering. The influence of the properties of these elements on the molecule–electrode interface coupling and on the relative energy gap between the Fermi level of the electrode and the orbital energy levels of the molecule, which directly influence the charge transport behaviour of single-molecule devices, is also a focus of our analysis. On the basis of these considerations, we examine various functionalities demonstrated in molecular junctions through molecular design and engineering. In this Review, the principles developed for fabricating reliable molecular junctions and tuning their intrinsic properties are examined from the point of view of the electrode, the interface and molecular engineering. The various functionalities demonstrated in molecular junctions through molecular design are discussed, along with the open challenges in the field.

Synthetic Methodologies for Structurally Defined Linked-[n]Rotaxanes with Permethylated Cyclodextrins: Platform for Functionalized Molecular Electronics

Abstract Insulated molecular wires, π-conjugated molecules insulated by nonconductive macrocycles, have been regarded as key components in the field of single-molecule electronics. Their various functionalities were attributed to their π-conjugated systems and enhanced by their macrocyclic insulation. We have recently proposed that the linked-structures resulting from π-conjugation with cyclic molecules, especially with permethylated cyclodextrins, were effective for preparing various types of functionalized insulated molecular wires for use in molecular electronics. The unique self-threading supramolecules called linked-[n]rotaxanes, are reported to have the enhanced functional properties of molecular wires. The development of such insulated molecular wires has been strongly supported by precise engineering protocols with fine synthetic techniques for making self-threading supramolecular structures. In this account, we systematically describe and categorize synthetic procedures for making structurally defined linked-[n]rotaxanes via various reaction systems with focus on the threading and fixation systems peculiar to linked-[n]rotaxane structures.

Statistical I–V measurements of single-molecule junctions with an asymmetric anchoring group 1,4-aminobenzenethiol**Invited talk at 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2016), 8–12 November 2016, Ha Long City, Vietnam.

Molecular diodes are an interesting topic in the field of single-molecule electronics. Rectification of molecules such as 1,4-aminobenzenethiol (ABT) having different contact areas was reported. However, a more statistical approach is necessary to clarify the rectification of the ABT single-molecule junctions. In this research, we statistically measured the single molecular conductance and I–V characteristics of ABT single-molecule junctions using the scanning tunneling microscope break junction (STM-BJ) method. Two single molecular conductances caused to difference of bridging geometries were observed in the conductance measurements. Statistically significant rectification was not observed for ABT junctions. We concluded rectification does not appear only due to the difference of two anchoring groups in case of a small conjugate molecule such as ABT.

Reversible Tuning of Interfacial and Intramolecular Charge Transfer in Individual MnPc Molecules.

The reversible selective hydrogenation and dehydrogenation of individual manganese phthalocyanine (MnPc) molecules has been investigated using photoelectron spectroscopy (PES), low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based near edge X-ray absorption fine structure (NEXAFS) measurements, and supported by density functional theory (DFT) calculations. It is shown conclusively that interfacial and intramolecular charge transfer arises during the hydrogenation process. The electronic energetics upon hydrogenation is identified, enabling a greater understanding of interfacial and intramolecular charge transportation in the field of single-molecule electronics.

Conductance through Carbosilane Cage Compounds: A Computational Investigation

Silicon is still the dominating material in microelectronics, yet primarily π-conjugated hydrocarbons are investigated in the field of single-molecule electronics even though linear oligosilanes are σ-conjugated. A drawback with the latter is their high conformational flexibility which strongly affects conductance. Here we report on a first principles density functional theory investigation of a series of rigid [2.2.2]bicyclic carbosilanes with 3, 2, 1, or 0 disilanylene bridges, providing all-silicon paths for charge transport. It is explored if these paths can be seen as independent and equivalent current paths acting as parallel resistors. For high conductance through the carbosilanes they need to be anchored to the gold electrodes via groups that are matched with the σ-conjugated paths of the oligosilane cage segment, and we find that silyl (SiH3) groups are better matched than thiophenol groups. Even for the carbosilane with three disilanylene bridges we find that the most transmitting conductance channel is not equally distributed on the three parallel bridges. In addition, there is significant communication between the various pathways, which results in destructive interference lowering the conductance. Taken together, the different disilanylene bridges in the cage compounds do not act as parallel resistors.

Molecular Electronics: Effect of External Electric Field

The effect of electric field, applied on systems in the nanoscale regime has attracted a lot of research in recent times. We highlight some of the recent results in the field of single molecule electronics and then move on to focus on some of our own results in this area. We have first shown how important it is to obtain the spatial profile of the external bias potential across the system, and how this would change in the presence of electron−electron interactions. We have also studied different kinds of insulators in the presence of the spatially varying external bias and have explicitly shown that a two sublattice structure, caused either by a lattice distortion, or by the presence of substituents with strong dipolar nature, can result in negative differential resistance (NDR) in the transport characteristics. We also find this to be true in case of correlated insulators. Additionally, we have shown clear NDR behavior in a correlated double quantum dot by tuning the electron−electron interaction strength in the system.