Measurement of Thermal Conductivities of Bi2Te3 and Sb2Te3 Nano-Powders at Extreme Temperature and Strain

In recent years, thermophotovoltaic cells such as Bi₂Te₃ and Sb₂Te₃ have become attractive due to their capability to convert heat into electricity directly. Different from solar cells which absorb visible and near infrared photons, the thermophotovoltaic cells can absorb long-wavelength infrared photons because they are made of narrow bandgap semiconductors. Considering the extensive application of the cells, many efforts have been paid and good progresses have been made.So far, all of the researches in this field focus on the thermophotovoltaic cells which work well under the radiation from the heat sources with high-temperature. It has not been reported about the thermophotovoltaic cells’ thermal transport properties under extreme conditions – high temperatures and large mechanical strains. Due to their potential applications, it is of significance to discover the thermal transport properties of thermophotovoltaic cells at high temperatures and large mechanical strains, and develop the thermophotovoltaic cells which can sustain the extreme environments. In this work, we demonstrate using optothermal Raman technique to measure the thermal transport properties of the thin film Bi₂Te₃ and Sb₂Te₃. We first utilize more direct measurements of the optical absorption, and then by comparing the response of the samples using different laser spot sizes, we are able to measure the thermal conductivities of the Bi₂Te₃ and Sb₂Te₃. The thermal conductivities at extreme temperatures and mechanical strains are obtained providing more scientific merits.

The optothermal Raman technique has been the most successful method for measurement of thermal conductivity of two-dimensional materials, and was used to measure the thin films for the first time. In this technique, a laser is focused at the center of a thin film and used to measure the peak position of a Raman-active mode. As the laser power is increased, the sample is heated which enables red-shift Raman mode due to thermal softening. Another comparison experiment is conducted by placing the samples on a heating platform and monitor the change of Raman-active mode peak position shift. Combining these two sections of experiments provide us the Thermal modeling can then be used to extract the thermal conductivity from the measured shift rate. In conclusion, we have used a refined version of the optothermal Raman technique to study thermal transport of Bi₂Te₃ and Sb₂Te₃, at extreme temperatures and mechanical strains. It is the first measurement on these two materials by optothermal Raman technique. This work also addresses several important issues in the measurement of thermal conductivity of thin films using Raman spectroscopy.