The recent focus in thermal management in nanostructures and energy harvesting using thermolectric devices has motivated the interest towards understanding the role of phononic thermal transport in nanostructured materials. Engineered nanostructured semiconductors have been shown to improve the efficiency of thermoelectric systems, by reducing the thermal conductivity of the crystalline materials while preserving their electronic properties. Recent experiments have reported a reduction in the group velocities of phonons, thereby leading to a strong reduction in the thermal conductivity in sub-10 nm free-standing Si membranes [1]. However, a microscopic understanding of thermal transport in ultra-thin membranes especially the correlation between phonon surface scattering and thermal transport, is still lacking. The theoretical studies reported, mostly rely on bulk phonon properties and use of specularity parameters to model surface scattering of phonons[2]. A detailed understanding of the role and behavior of phonons in confined structures is necessary in the design of nanostructured materials with tailored thermal transport properties.

We use lattice dynamics (LD) and classical molecular dynamics (MD) to investigate the nature of phononic thermal transport in nanostructured silicon membranes with thicknesses of the order of 20 nm and below. We find that dimensionality reduction has a significant effect on the phonon dispersion and has the direct consequence of suppression of group velocities of phonons in the silicon membranes. The dimensional reduction leads to a 3-fold reduction in the thermal conductivity of the membranes with respect to bulk silicon. The presence of surface nanostructures, by means of pattern formation and surface oxidation, has even a stronger influence on the phonon dispersion, leading to a 25-fold reduction in the in-plane thermal conductivity of the rough oxidized membranes, implying a 25-fold enhancement of the thermoelectric figure of merit at room temperature. Such figures make nanostructured silicon membranes viable materials for thermoelectric units.

References:

[1] J. Cuffe et al, “Phonons in slow motion: dispersion relations in ultra-thin Si membranes.,” Nano Lett.,12, 3569-3573, (2012).

[2] J. E. Turney et al, “In-plane phonon transport in thin films,” J. Appl. Phys.,107, 024317 (2010).

Acknowledgment: This project is funded by the program FP7-ENERGY-2012-1-2STAGE under contract number 309150.

Sanghamitra Neogi

Max-Planck Institute for Polymer Research

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