Abstract
Previously we showed that the thermoelectric (TE) performance of bulk n-type Bi2Te2.7Se0.3 can be enhanced by subjecting it to a combined process of chemical or mechanical exfoliation (C/ME) followed by a rapid densification and restacking of the exfoliated layers via the spark-plasma-sintering technique (SPS). Here, we present a systematic micro-Raman study of two-dimensional flakes of n-type Bi2Te2.7Se0.3 produced by the C/ME process, as a function of the flake thickness. We found Raman evidence for flakes with: (i) integer number of quintuples which exhibited a strong electron-phonon coupling, and (ii) non-integer number of quintuples, or sub-quintuples which exhibited the forbidden IR active mode due to symmetry lowering. Detailed atomic force microscopy was used to confirm the number of quintuples in all flakes examined in this study. The restacking and densification of these flakes by SPS promoted the formation of charged grain boundaries, which led to the enhanced TE properties via the energy filtering process.
Highlights
Bulk pristine and doped Bi2Te3 are some of the most efficient room temperature thermoelectric (TE) materials for sustainable power generation and refrigeration applications[1,2,3]
Chemical/mechanical exfoliation (C/ME) of layered materials has enabled the fabrication of two-dimensional (2D) nanosheets that exhibit superior TE properties compared to their bulk counterparts[10,11,12,13,14]
We employed the combined technique of chemical/mechanical exfoliation (C/ME) followed by spark-plasma-sintering (SPS), which led to preferential scattering of electrons at charged grain boundaries, and significantly improved the TE compatibility factor and stabilized the ZT peak at higher temperatures (350–500 K) in n-type Bi2Te2.7Se0.310
Summary
Bulk pristine (undoped) and doped Bi2Te3 are some of the most efficient room temperature thermoelectric (TE) materials for sustainable power generation and refrigeration applications[1,2,3]. In addition to improving electronic transport in low dimensional materials, nanostructuring via ball-milling and melt-spinning have been effective in reducing κL through increased phonon scattering effects, resulting in an enhanced ZT in TE nanomaterials[8,9]. We employed the combined technique of C/ME followed by spark-plasma-sintering (SPS), which led to preferential scattering of electrons at charged grain boundaries, and significantly improved the TE compatibility factor and stabilized the ZT peak at higher temperatures (350–500 K) in n-type Bi2Te2.7Se0.310. Stabilization of the ZT peak over a broad temperature range of ~150 K10 Both (i) and (ii) implied the creation of charged grain boundaries in Bi2Te2.7Se0.3 due to the C/ME-SPS process
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