Non-universal scaling of thermoelectric efficiency in 3D and 2D thermoelectric semiconductors

• Published in: Advances in natural sciences. Nanoscience and nanotechnology Vol. 12; no. 1; p. 15017
• Main Authors: Octavian, Kevin, Hasdeo, Eddwi H
• Format: Journal Article
• Language: English
• Published: Hanoi IOP Publishing 01.03.2021
• Subjects: Band theory; Bismuth tellurides; Dimensional changes; Electrical conductivity; Electrical resistivity; Energy gap; Figure of merit; First principles; Intermetallic compounds; Miniaturization; Semiconductors; Silicon germanides; Thermal conductivity; Thermoelectricity; Two dimensional materials
• DOI: 10.1088/2043-6262/abe93c
• ISSN: 2043-6262; 2043-6254

We performed the first-principles calculation on common thermoelectric semiconductors Bi 2 Te 3 , Bi 2 Se 3 , SiGe, and PbTe in bulk three-dimension (3D) and two-dimension (2D). We found that miniaturisation of materials does not generally increase the thermoelectric figure of merit ( ZT ) according to the Hicks and Dresselhaus (HD) theory. For example, ZT values of 2D PbTe (0.32) and 2D SiGe (0.04) are smaller than their 3D counterparts (0.49 and 0.09, respectively). Meanwhile, the ZT values of 2D Bi 2 Te 3 (0.57) and 2D Bi 2 Se 3 (0.43) are larger than the bulks (0.54 and 0.18, respectively), which agrees with HD theory. The HD theory breakdown occurs because the band gap and band flatness of the materials change upon dimensional reduction. We found that flat bands give a larger electrical conductivity ( σ ) and electronic thermal conductivity ( κ el ) in 3D materials, and smaller values in 2D materials. In all cases, maximum ZT values increase proportionally with the band gap and saturate for the band gap above 10 k B T . The 2D Bi 2 Te 3 and Bi 2 Se 3 obtain a higher ZT due to the flat corrugated bands and narrow peaks in their DOS. Meanwhile, the 2D PbTe violates HD theory due to the flatter bands it exhibits, while 2D SiGe possesses a small gap Dirac-cone band.