Session: 10-03: Alternative Energy Conversion Technology (including Wind, Geothermal, Hydro, and Ocean)

Paper Number: 142267

142267 - 2d Material Thermal Conductivity From Optothermal Raman and Stokes/anti-Stokes Thermometry 

Abstract: 

2D materials have begun to see an increase in interest due to their potential applications in energy conversion applications such as thermoelectrics, protective coatings in clean energy technologies, as well as microelectronics. Tin diselenide (SnSe₂) has been of particular interest due to its viability as a thermoelectric material and its ability to achieve an ultralow thermal conductivity [~0.67 W/(m-K)] perpendicular to the plane in which its covalently bonded atoms lie, which makes the study of the thermal conductivity of SnSe₂ of particular interest.

The optothermal Raman technique is useful in determining the thermal conductivity of 2D materials both suspended over a hole or supported on a substrate, and is a non-contact probe of local heating due to stimulated heating by photons. Not studied as frequently in the application of the optothermal Raman thermometry technique is that of anti-Stokes scattering, which can play a role in both calibration of Raman peaks and absolute phonon temperature calculations. This work demonstrates that the anti-Stokes scattering plays an important role in determining the thermal conductivity of 2D materials, with the use of SnSe₂ on a Cu substrate as the sample material. Theoretical calculations reveal the Eg mode frequency is at 120.90 (1/cm) and A1g mode is 182.69 (1/cm). In the Eg mode, the Se atoms vibrate in-plane in opposite directions while for the A1g mode, Se atoms vibrate out-of-plane in opposite directions. The calculation was performed using plane-wave density functional theory in Quantum ESPRESSO, version 7.3,  using local density approximation functional. The experiment was repeated five times and focused on the higher intensity A1g mode, resulting in a 1.3% decrease in average thermal conductivity when the phonon temperature is back calculated raising the Raman frequency to the third power against the traditional optothermal Raman method and a 6.6% increase in thermal conductivity when the phonon temperature is back calculated raising the Raman frequency to the fourth power compared to the traditional optothermal Raman method that uses temperature dependent peak position shifts. The Stokes/Anti-Stokes method of calculating the thermal conductivity results in a higher uncertainty, although when both methods are combined the thermal properties of the 2D material are quantified within a well-defined uncertainty band. Additionally, an analysis of graphene thermal conductivity using the traditional optothermal Raman method is presented here using the same methods to calculate and analyze the thermal conductivity and quantify the uncertainty in both techniques.

Through these studies, we show that anti-Stokes Raman scattering, while not often studied, is an important process in determining the thermal conductivity and establishing power and temperature dependence trends. The governing equation found to be most appropriate to calculate the thermal conductivity is that which considers convection, radiation, and substrate resistance effects. Overall, the anti-Stokes Raman scattering remains an important factor in calculating the thermal conductivity of a 2D material such as SnSe₂ or graphene.

Presenting Author: Micah Vallin Los Alamos National Laboratory

Presenting Author Biography: Micah Vallin is a graduate research assistant at Los Alamos National Laboratory. He is obtaining his Ph.D. from the University of North Texas.

Authors:

Micah Vallin Los Alamos National Laboratory
Michael Pettes Los Alamos National Laboratory
Richard Zhang University of North Texas
Rijan Karkee Los Alamos National Laboratory