Nanotechnology

Investigating interactions at molecular junctions for new electronic devices

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July 13, 2023

(Nanowerk News) π-π interactions are a type of noncovalent interactions that occur when the electron clouds in the π orbitals of aromatic rings or π-conjugated molecular systems overlap. These interactions allow efficient movement of electrons between molecules, offering the potential to design materials with unique electronic properties. The junction structure formed by these molecules plays a decisive role in electron transport. However, insufficient structural information on these junctions has made it difficult to establish a clear relationship between the structure and electron transport properties.

To address this knowledge gap, a group of researchers from Japan, led by Assistant Professor Satoshi Kaneko and Associate Professor Tomoaki Nishino of the Tokyo Institute of Technology (Tokyo Tech), recently single-junctioned dimers and monomers from naphthalenethiol (NT) molecules and performed detailed examination of their structure and electron transport properties using combined optical and electrical measurements.

Their study was published recently in Journal of the American Chemical Society (“Intermolecular and Electrode-molecular Bonds in Naphthalenethiol Single Dimer Junctions as Revealed by Surface Enhanced Raman Scattering Combined with Transport Measurements”).

The researchers made the connection by first depositing a gold electrode on a phosphor bronze plate coated with a layer of polyimide. Next, they selectively removed the polyimide material beneath the central region of the gold electrode, forming a freestanding structure. Finally, they added an ethanol solution containing NT dropwise to the substrate, resulting in the formation of a single layer of NT molecules connecting the gold electrodes.

After making a junction, researchers then do it simultaneously in place surface-enhanced Raman scattering (SERS) and current-voltage (IV) measurement using a mechanically controllable disconnect-connection technique. “This was followed by a correlation analysis of measured vibration energy and electrical conductance values, which made it possible to identify intermolecular and molecule-electrode interactions and transport properties at the NT junction,” explained Dr. Kaneko.

Current-voltage measurements reveal distinct high conductivity and low conductivity states. While the high conductance state originates from the NT-monomer junction, where the molecule interacts directly with the gold electrode via direct π bonds, the low conductance state arises due to the NT dimer formed by intermolecular π-π interactions.

However, considering the vibrational energy along the conductance confirms three distinct structures at the junction, each corresponding to a high conductance state and two low conductance states. When the naphthalene ring — in dimer and monomer configurations — interacts directly with the gold electrode via π coupling, a highly conductive junction is formed. Conversely, the weak interaction between the naphthalene ring and the gold electrode via chamber coupling results in a weak conductive junction.

“The simultaneous application of the SERS and IV techniques can distinguish various noncovalent interactions at the junctions of NT molecules, highlighting their electron transport properties. In addition, the noncovalent character is also expressed by the power density spectrum,” said Dr. Nishino.

These findings thus provide important insights into π-π interactions that may pave the way for utilizing aromatic molecules in the design of future electronic devices and technologies.



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