The Seebeck effect is a thermoelectric phenomenon by which a voltage or current is generated when a temperature difference exists across a conductor. This effect underlies established and emerging thermoelectric applications, such as heat-to-electric energy harvesters, temperature sensing and temperature control devices.
In line with the relentless demand for ever-smaller devices, scientists are looking for new ways to harness the Seebeck effect at the nanoscale. One way to achieve this is to use molecular junctions, which are miniature devices consisting of two electrodes connected by one or a few individual molecules. Depending on the sensitivity of these molecules to temperature, the thermoelectric properties of the molecular junctions can be tuned to match the intended application.
So far, most studies of molecular thermoelectrics have been limited to fairly simple organic molecules. This has led to molecular junctions with a low Seebeck coefficient, which results in poor performance and temperature-to-voltage conversion. There is therefore an ongoing challenge to design molecular junctions with better characteristics and most importantly a higher Seebeck coefficient.
Fortunately, a recent study 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 could lead to substantial advances in this field. . As stated in their article published in Nano Letters, the researchers had their eye on a particular type of organometallic compound that may hold the key to this conundrum: alkynyl ruthenium 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 could lead to more powerful molecular junctions, thanks to their unique electronic structure.
To test their theory, the scientists prepared several self-assembled monolayers (SAMs) consisting of two opposing flat electrodes connected by organometallic compounds with different numbers of alkynyl ruthenium complexes. The hot electrode was made of ultra-smooth gold to provide a good anchoring substrate for the organometallic molecular junctions, while the cold electrode was made of a liquid metal, eutectic gallium-indium, coated with a layer of gallium oxide ( Figure 1).
The team investigated, through various experiments and theoretical methods, how the Seebeck coefficient of these SAMs changed depending on the number of ruthenium atoms in the molecular junction, as well as the oxidation state and detailed chemical composition of its organic structure. In particular, they found that the prepared molecular junctions achieved unprecedented thermoelectric performance, as Assistant Professor Tanaka notes: “Our organometallic compounds exhibited much higher Seebeck coefficient values than their purely organic counterparts. Furthermore, as far as we know, a Seebeck coefficient of 73 μV/K, obtained for the trinuclear ruthenium complex, is extraordinarily superb compared to conventional molecules reported in the literature. In addition to that, the prepared molecular junctions had remarkable thermal stability, which broadens their potential application fields.
These results are very encouraging for those working in thermoelectronics, as they could point to new strategies to finally achieve a breakthrough in nanoscale semiconductor manufacturing. “This work offers important insights into the development of molecular-scale devices for efficient thermoregulation and heat-to-electricity conversion,” points out Assistant Professor Tanaka.
Be sure to keep an eye out for new developments in thermoelectric molecular junctions in the future; they could be the key to producing sustainable energy from heat and thermal control in next-generation electronic devices.