Newswise — A recent Tokyo Tech study uncovers the crucial role of molecular junction structure in electron transport. By employing concurrent measurements of surface-enhanced Raman scattering and current–voltage, researchers reveal that a lone dimer junction of naphthalenethiol exhibits three distinct bondings: π–π intermolecular, through-π, and through-space molecule–electrode interactions.
The π–π interaction, a noncovalent interaction, arises from the overlapping of electron clouds in the π orbitals of aromatic rings or π-conjugated molecular systems. This interaction facilitates the efficient transfer of electrons between molecules, presenting an opportunity to develop materials with distinct electronic characteristics. The significance of the molecular junction's structure in electron transport cannot be understated. Nonetheless, the limited structural data available for these junctions has hindered the establishment of a definitive correlation between structure and electron transport properties.
To bridge this informational void, a team of Japanese researchers, helmed by Assistant Professor Satoshi Kaneko and Associate Professor Tomoaki Nishino from Tokyo Institute of Technology (Tokyo Tech), have successfully produced a solitary dimer and monomer junction of the naphthalenethiol (NT) molecule. They performed an extensive analysis of these junctions' structure and electron transport characteristics through a comprehensive integration of optical and electrical measurements. Their findings were recently published in the Journal of the American Chemical Society.
To create the junction, the researchers followed a specific fabrication process. Initially, they placed a gold electrode onto a phosphor bronze plate that had been coated with a layer of polyimide. Then, they carefully eliminated the polyimide material from the central area of the gold electrode, generating a self-supporting structure. Lastly, they introduced an ethanol solution containing NT onto the substrate in a gradual manner, which led to the development of a sole layer of NT molecules that connected the gold electrodes.
After successfully fabricating the junction, the researchers proceeded to perform simultaneous in situ surface-enhanced Raman scattering (SERS) and current–voltage measurements (I–V) using the mechanically-controllable break-junction technique. By employing this approach, they were able to examine the vibrational energy and electrical conductance values. Dr. Kaneko clarifies that a correlation analysis of these measurements was subsequently conducted, allowing for the determination of the intermolecular and molecule-electrode interactions, as well as the transport properties within the NT junction.
The current-voltage measurements uncovered two distinct states: high conductivity and low conductivity. The high-conductance state was observed in the NT-monomer junction, where the molecule directly interacts with the gold electrodes through direct π-bonding. On the other hand, the low-conductance state arose from an NT dimer that formed as a result of intermolecular π–π interactions.
However, when taking into account the vibrational energy in conjunction with conductance, the researchers confirmed the presence of three distinct structures at the junction. These structures corresponded to a high-conductance state and two low-conductance states. In the high-conductance state, both the dimer and monomer configurations of the naphthalene ring interacted directly with the gold electrodes through π coupling, leading to the formation of highly conductive junctions. In contrast, the low-conductance states arose from weak interactions between the naphthalene ring and the gold electrode, specifically through space coupling, resulting in weakly conductive junctions.
Dr. Nishino highlights that the combined utilization of SERS and I-V techniques enabled the discrimination of different noncovalent interactions within the NT molecular junction, thus providing insights into its electron transport properties. Furthermore, the noncovalent nature of these interactions was also demonstrated through the analysis of power density spectra.
The current findings offer valuable insights into the role of π–π interactions, which could potentially open doors for leveraging aromatic molecules in the development of future electronic devices and technologies. This knowledge can aid in the design and optimization of novel materials that harness the unique properties of these interactions, leading to advancements in various areas of electronic engineering.
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