Thermoelectric Studies of Nanoporous Thin Films and 2D Materials

Abstract

Thermoelectric (TE) materials can be used for direct conversion between the thermal and electrical energy, which is promising for power generating and refrigeration. TE performance can be evaluated by the TE figure of merit ZT=S^2 σT/k , where S, σ, k, T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature. One challenge of TE research is to achieve a high power factor S^2 σ but a low k in the same material. This requirement can be satisfied in various nanostructured materials such as thin films or 2D materials with periodic nanopores. In these materials, a significant reduction in the lattice part of the thermal conductivity (kL) can be observed. The power factor S^2 σ can be further improved in periodic nanoporous graphene called a graphene antidot lattice (GAL). When the neck width between the nanopores is around 10 nm or less, an electronic band gap can be opened in a semi-metal graphene to dramatically increase S^2 σ. Further S^2 σ enhancement can be achieved by tuning the Fermi level with a gate voltage. In this dissertation, the in-plane heat conduction has been studied using periodic nanoporous Si films as the test bed. Different from previous studies focusing on the thermal conductivity measurements only, the specific heat has been simultaneously measured to further justify when wave effects can be critical to the thermal transport. Along another line, the cross-plane thermal studies have been carried out on nanoporous In0.1Ga0.9N alloy films directly grown on a substrate, as the first attempt for thermal studies of such nanoporous films. Pore-edge defects introduced by typically drilling techniques is minimized in this case to simplify the thermal analysis. For electrical studies, representative GALs have been measured for their enhanced power factors due to the opened electronic band gap and applied gate voltage. By analyzing the maximum gate-tuned Seebeck coefficient, the dominant scattering mechanism of charge carriers can be identified as the scattering by pore-edge-trapped charges.