2D materials such as graphene have been a focus for thermoelectric designs, and SnSe shows similar properties. "Our lab has been working on two-dimensional semiconductors with layered structures similar to graphene," said Xuan Gao, an associate professor at Case Western.
he group is looking at how the temperature difference across a material can cause charge carriers -- electrons or holes -- to redistribute and generate a voltage across the material, converting thermal energy into power.
"Applying a voltage on a thermoelectric material can also lead to a temperature gradient, which means you can use thermoelectric materials for cooling," said 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," he said.
A thermoelectric figure of merit, ZT, indicates how efficiently a material converts thermal energy to electrical energy. The group's work focuses on the power factor, which is proportional to ZT and indicates a material's ability to convert energy.
The SnSe nanostructures are grown with chemical vapor deposition (CVD), evaporating a tin selenide powder source inside an evacuated quartz tube. Tin and selenium atoms react on a silicon or mica growth wafer placed at the low-temperature zone of the quartz tube, creating SnSe nanoflakes on the surface of the wafer. Adding a dopant element like silver to SnSe thin films during material synthesis can further optimize its thermoelectric properties.
At the start, "the nanostructure SnSe thin films we fabricated had a power factor of only around 5 percent of that of single crystal SnSe at room temperature," said researcher Shuhao Liu. Silver was the most effective dopant, resulting in a 300 percent power factor improvement compared to undoped samples, he said. "The silver-doped SnSe nanostructured thin film holds promise for a high figure of merit."