"Our lab has been working on two-dimensional semiconductors with layered structures similar to graphene," said associate professor Xuan Gao at Case Western.
Inspired by the favourable thermoelectric properties of nanomaterials with its nanometer-scale dimensions, the researchers grew nanometer-thick nanoflakes and thin films of SnSe to further study its thermoelectric properties.
The work centres on the thermoelectric effect, looking at how the the temperature difference in a material can cause charge carriers to redistribute and generate a voltage across the material, converting thermal energy into electricity.
"Applying a voltage on a thermoelectric material can also lead to a temperature gradient, which means you can use thermoelectric materials for cooling," said assistant prof Gao. "Generally, materials with a high figure of merit have high electrical conductivity, a high Seebeck coefficient - generated voltage per Kelvin of temperature difference within a material - and low thermal conductivity.”
According to the team, a thermoelectric figure of merit, ZT, indicated how efficiently a material converts thermal energy to electrical energy. The team focused on the power factor, which is proportional to ZT and apparently indicated a material's ability to convert energy. As a result, the researchers measured the power factor of the materials they made.
To grow SnSe nanostructures, the team used a chemical vapor deposition process. A tin selenide powder source was thermally evaporated inside an evacuated quartz tube. The tin and selenium atoms reacted on a silicon or mica growth wafer placed at the low-temperature zone of the quartz tube, which caused the SnSe nanoflakes to form on the surface of the wafer.
Adding a dopant element, like silver, to SnSe thin films during material synthesis further optimised its thermoelectric properties, the researchers added.
To begin with the nanostructure SnSe thin films the researchers fabricated had a power factor of only 5% of that of single crystal SnSe at room temperature. However, after experimenting with a variety of dopants to improve the material's power factor, the team said they found silver was most effective, resulting in a 300% power factor improvement compared to undoped samples.
The belief is that the silver-doped SnSe nanostructured thin film holds promise for a high figure of merit.
The researchers hope that in the future, SnSe nanostructures and thin films may be useful for miniaturised, environmentally friendly, low-cost thermoelectric, and cooling devices.
