The concept of Nanoscale Thermoelectrics involves the use of nanoscale materials to convert heat into electrical energy and vice versa, which has potential applications in energy conversion and waste heat recovery.
Researchers from Tokyo Tech and Korea University have discovered that multinuclear organometallic junctions may hold the secret to creating high-performance thermoelectric devices at the nanoscale.
Researchers were able to obtain unheard-of heat-to-electricity conversion performance at molecular junctions thanks to the distinct electronic structure of organometallic ruthenium alkynyl complexes, opening the door to molecular-scale temperature sensors and thermal energy harvesters.
The Seebeck effect is a thermoelectric phenomenon that occurs when there is a temperature difference across a conductor and results in the generation of a voltage or current. This effect serves as the foundation for both established and new thermoelectric applications, including temperature management, sensor technology, and energy harvesters that convert heat to electricity.
In line with the unrelenting demand for ever-smaller devices, scientists are looking for new ways to leverage the Seebeck effect at the nanoscale. Utilizing molecular junctions, which are tiny devices made up of two electrodes connected by one or more individual molecules, is one approach to accomplish this. The thermoelectric properties of molecular junctions can be precisely tailored to fit their intended purpose depending on how temperature-sensitive these molecules are.
Most research on molecular thermoelectrics to date has been restricted to very basic organic compounds. This has led to molecular junctions with a low Seebeck coefficient, which translates to poor temperature-to-voltage conversion and performance. Designing molecular junctions with superior properties and, most crucially, a higher Seebeck coefficient is thus a constant challenge.
Fortunately, a recent study conducted by a research team including Assistant Professor Yuya Tanaka of Tokyo Institute of Technology (Tokyo Tech), Japan, and Professor Hyo Jae Yoon of Korea University, Korea, may lead to substantial progress in this field.
As stated in their paper published in Nano Letters, the researchers had set their eyes on a particular type of organometallic compound that could be the key to this conundrum: ruthenium alkynyl complexes.
But unlike previous studies, the team was curious as to whether multinuclear ruthenium alkynyl complexes based on multiple Ru(dppe)2 where Ru is ruthenium and dppe is 1,2-bis(diphenylphosphino)ethane fragments could lead to more powerful molecular junctions, thanks to their unique electronic structure.
To test their theory, the scientists prepared various self-assembled monolayers (SAMs) consisting of two opposing flat electrodes connected by organometallic compounds with different numbers of ruthenium alkynyl complexes.
The cold electrode was formed of a liquid metal, eutectic gallium-indium, coated in a gallium oxide layer, while the hot electrode was made of ultrasmooth gold to serve as an excellent anchoring substrate for the organometallic molecular junctions.
The scientists looked at how the Seebeck coefficient of these SAMs altered based on the number of ruthenium atoms in the molecular junction, as well as the oxidation state and precise chemical make-up of its organic backbone, using a variety of experimental and theoretical methodologies.
Notably, they found that the prepared molecular junctions achieved unprecedented thermoelectric performance, as Assistant Professor Tanaka remarks:
“Our organometallic compounds exhibited much higher Seebeck coefficient values than their purely organic counterparts. Moreover, to the best of our knowledge, a Seebeck coefficient of 73 μV/K, obtained for the tri-nuclear ruthenium complex, is remarkably superb compared to conventional molecules reported in literature.”
The constructed molecular junctions also exhibited extraordinary heat stability, which broadens the scope of their possible applications.
These findings are very intriguing for thermoelectronics researchers since they may suggest new avenues for making nanoscale semiconductors, which would be a major breakthrough.
“This work offers important insights into the development of molecular-scale devices for efficient thermoregulation and heat-to-electricity conversion,” highlights Assistant Professor Tanaka.
Future advancements in thermoelectric molecular junctions should be watched closely; they may hold the key to thermal management and sustained power generation from the heat in next-generation electronic devices.
