Nanoelectricity is achieved with a new organometallic compound

The Seebeck effect is a thermoelectric phenomenon that creates a voltage or current when there is a temperature difference across a conductor. This effect is the basis of both established and new thermoelectric applications, such as heat-to-electricity harvesters, sensing devices, and temperature control.

In line with the relentless demand for ever-smaller devices, scientists are looking for new ways to exploit the Seebeck effect at the nanoscale. One way to achieve this is by using molecular junctions, which are miniature devices consisting of two electrodes bridged by one or several individual molecules. Depending on how sensitive these molecules are to temperature, it is possible to fine-tune the thermoelectric properties of the molecular compounds to suit their purpose.

Until now, most studies of molecular thermoelectricity have been limited to fairly simple organic molecules. This led to molecular junctions with a low Seebeck coefficient, which means poor temperature-to-voltage conversion and performance. Therefore, there is a constant challenge to design molecular compounds with better characteristics and, most importantly, a higher Seebeck coefficient.

Fortunately, a recent study by a research team including Assistant Professor Yuya Tanaka of the Tokyo Institute of Technology (Tokyo Tech), Japan, and Professor Hyo Jae Yoon of Korea University, Korea, may lead to significant progress in this field. As noted in their paper published in the journal Nano Letters, the researchers set their sights on a particular type of organometallic compound that may hold the key to this puzzle: ruthenium alkynyl complexes. But unlike previous studies, the team was curious whether multinuclear ruthenium alkynyl complexes based on multiple Ru(dppe)2 [where Ru is ruthenium and dppe is 1,2-bis(diphenylphosphino)ethane] fragments can lead to more powerful molecular compounds, thanks to their unique electronic structure.

To test their theory, the scientists prepared different self-assembled monolayers (SAMs) consisting of two opposite planar electrodes connected by organometallic compounds with different numbers of ruthenium alkynyl complexes. The hot electrode was made of ultra-smooth gold to provide a good substrate for anchoring organometallic molecular compounds, while the cold electrode was made of liquid metal, eutectic gallium-indium, covered with a layer of gallium oxide (Figure 1).

Through various experiments and theoretical methods, the team studied how the Seebeck coefficient of these SAMs changed depending on the number of ruthenium atoms in the molecular compound, as well as the state of oxidation and the detailed chemical composition of its organic backbone. Significantly, they found that the prepared molecular compounds achieved unprecedented thermoelectric performance, as Assistant Professor Tanaka notes: “Our organometallic compounds exhibited much higher Seebeck coefficient values ​​than their purely organic counterparts. Moreover, to the best of our knowledge, the Seebeck coefficient of 73 μV/K obtained for the trinuclear ruthenium complex is remarkably excellent compared to conventional molecules reported in the literature.” In addition, the prepared molecular compounds had exceptional thermal stability, which expands their potential fields of application.

These results are very encouraging for those working in the field of thermoelectronics, as they could point to new strategies for finally achieving breakthroughs in nanoscale semiconductor manufacturing. “This work offers important insights into the development of devices at the molecular level for efficient thermoregulation and the conversion of heat into electricity,” Assistant Professor Tanaka points out.

Be sure to keep an eye out for new developments in thermoelectric molecular junctions in the future; they could be the key to sustainable power generation from heat and thermal control in next-generation electronic devices.

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