Trapping the Performance of a Single Atomic Layer Inside a Bulk Crystal — Thermoelectric Performance Increased About 100-Fold (Institute of Science Tokyo)

Explanation

What's new is that "performance previously achievable only in thin films has been achieved within a bulk material."

In the world of materials, there are substances whose performance jumps dramatically when reduced to a thickness of just a single atom. Iron selenide (FeSe) is one such material; it was known that when thinned to a single atomic layer, its ability to convert temperature differences into electricity (thermoelectric performance) increases greatly. However, single-atomic-layer thin films can only be made on a substrate, and as they are, they cannot be used in quantities sufficient for power-generation devices. The situation of "good performance but not in a usable form" had continued.

On August 21, 2026, a research group led by Professor Takayoshi Katase of the Institute of Science Tokyo's Laboratory for Materials and Structures announced a method to overcome this barrier. The group focused on the fact that a single-layer structure of iron selenide is originally embedded within the crystal of TlFe1.6Se2 (a compound composed of thallium, iron, and selenium), and extracted the properties of that single layer while keeping the material in bulk crystal form. The thickness of the embedded iron selenide layer is less than 1 nanometer (one-millionth of a millimeter).

The key was "defects." In this crystal, holes where iron atoms are missing (vacancies) are arranged regularly. Ordinarily, defects in materials are disliked as a cause of reduced performance, but the research group used this regular arrangement of vacancies to separately control electrical conductivity and thermal conductivity. In thermoelectric materials, the ideal combination is "electricity flows well but heat does not conduct easily," and these two properties normally cannot be achieved at the same time. The arrangement of vacancies helped make this compatibility possible.

As a result, the ability to convert a temperature difference into voltage became about 30 times that of bulk iron selenide, and overall thermoelectric performance improved about 100-fold.

Thermoelectric materials are being considered for applications that convert previously wasted heat into electricity, such as waste heat from factories or surplus heat from automobile engines. What this achievement demonstrated is not so much the performance value of a specific material, but rather a design concept: "properties obtainable only in thin films can be built into the internal structure of a bulk material." The research group states that this concept can be applied to a variety of other material systems as well.

The announcement includes the note that defects in materials are not necessarily villains. In semiconductors, it is common practice to intentionally introduce impurities and defects to change properties, and this research extends that idea to the way heat is conducted in thermoelectric materials.

When expressing thermoelectric performance, a value called the dimensionless figure of merit ZT is often used. This value is determined by three factors: the ability to convert a temperature difference into voltage (Seebeck coefficient), electrical conductivity, and thermal resistivity. The announcement's description that the Seebeck coefficient increased about 30-fold while overall performance increased about 100-fold indicates that factors other than the Seebeck coefficient were also improved simultaneously.

Source: Institute of Science Tokyo, "New technology extracts the power of a single atomic layer within a bulk material" (August 21, 2026)
https://www.isct.ac.jp/ja/news/npio5md0yosa