Single crystal tin selenide (SnSe) is a perfect thermoelectric material and a semiconductor; it can instantly change waste heat to electrical energy or be used for cooling. When a team of researchers from Case Western Reserve University in Cleveland, Ohio, saw the graphene-like layered crystal structure of SnSe, they had one of those mystic "aha!" flashes.
Electric charges in a nanostructured tin selenide (SnSe) thin film flow from the hot end to the cold end of the material and generate a voltage. (Image credit: Xuan Gao)
The team stated in the Journal of Applied Physics, from
AIP Publishing, that they instantly knew this material had the potential to be fabricated in nanostructure forms. " Our lab has been working on two-dimensional semiconductors with layered structures similar to graphene," said Xuan Gao, an associate professor at Case Western.
Nanomaterials with nanometer-scale dimensions -- such as grain size and thickness -- have promising thermoelectric properties. This motivated the team to grow nanometer-thick thin films and nanoflakes of SnSe to further explore its thermoelectric properties.
The team’s work concentrates on the thermoelectric effect. They examine how the temperature variance in a material can make charge carriers -- electrons or holes -- to redistribute and produce a voltage across the material, changing 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. 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.
A thermoelectric figure of merit, ZT, specifies how effectively a material changes thermal energy to electrical energy. The team's work concentrates on the power factor, which is proportional to ZT and specifies a material's ability to change energy, so they measured the power factor of the materials they created.
To grow SnSe nanostructures, they made use of a chemical vapor deposition (CVD) process. They thermally evaporated a tin selenide powder source within an evacuated quartz tube. Tin and selenium atoms react on a mica or silicon growth wafer positioned in the low-temperature zone of the quartz tube. This makes SnSe nanoflakes to develop on the wafer’s surface. Incorporating a dopant element like silver to SnSe thin films during material synthesis can additionally enhance its thermoelectric properties.
In the beginning, "
the nanostructure SnSe thin films we fabricated had a power factor of only ~5 percent of that of single crystal SnSe at room temperature," said Shuhao Liu, an author on the paper. But, after trying a range of dopants to improve the material's power factor, they established that "silver was the most effective - resulting in a 300 percent power factor improvement compared to undoped samples,"
Liu said. "
The silver-doped SnSe nanostructured thin film holds promise for a high figure of merit."
Going forward, the researcher hopes that SnSe nanostructures and thin films may be beneficial for miniaturized, low-cost, and environmentally friendly thermoelectric and cooling instruments.