Bringing the power of a single atomic layer into a bulk material

A research team led by Professor Takayoshi Katase at Institute of Science Tokyo therefore focused on a material called TlFe₁.₆Se₂. Inside its crystal structure, many FeSe layers less than 1 nanometer thick are confined and stacked. The researchers wondered whether these layers could retain their special properties inside a bulk material, making it possible to combine the performance of atomically thin layers with the easier handling of a larger solid.

The atomically thin FeSe layers inside TlFe₁.₆Se₂ contain places where iron atoms are missing. Such atomic vacancies are often considered defects that reduce a material’s performance. However, the research team found that the arrangement of these missing iron atoms plays an important role in controlling the flow of electricity and heat.

First, when the iron vacancies in TlFe₁.₆Se₂ were arranged in an orderly pattern, the material became better at converting a temperature difference into electricity. The measure of this ability was about 30 times higher than that of bulk FeSe.

To generate electricity efficiently from a temperature difference, however, a material must not only convert that difference into electricity effectively, but also prevent heat from passing through too easily. If heat quickly reaches the colder side, the temperature difference needed for power generation becomes smaller.

In TlFe₁.₆Se₂, the heavy thallium atoms between the thin FeSe layers are only weakly bonded to the surrounding atoms. This makes it difficult for the atomic vibrations that carry heat to travel smoothly through the crystal. In addition, the missing iron atoms create variations in the strength of the bonds between neighboring atoms. This uneven structure scatters the atomic vibrations and further reduces the flow of heat.

As a result, TlFe₁.₆Se₂ not only became better at converting a temperature difference into electricity, but also retained that temperature difference more effectively. Together, these two effects raised its overall thermoelectric performance to about 100 times that of bulk FeSe.

The material’s performance will need to be improved further before it can be used in power-generation devices. The researchers now aim to achieve higher performance by adjusting the proportion of missing iron atoms and the number of electrons that carry electricity.

The idea of stacking atomic layers inside a bulk material and even using the arrangement of missing atoms within those layers as part of the design may also be applicable to other materials. This approach could provide a new way to apply properties available only in thin films to practical bulk materials.

Defects in a material are not necessarily a bad thing. If their arrangement is used effectively, they can provide a way to control the flow of electricity and heat. We are greatly encouraged that this study demonstrates the possibility of harnessing the special properties of ultrathin FeSe in a bulk material by incorporating single-atomic-layer structures within a crystal.

(Takayoshi Katase, Professor, Materials and Structures Laboratory, Institute of Science Tokyo)

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