A micro-Raman study of exfoliated few-layered n-type Bi2 Te2.7Se0.3

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.

Temperature Driven Unusual Reversible p‐ to n‐Type Conduction Switching in Bi2Te2.7Se0.3

“…Figure (a) shows the temperature dependent Raman spectra of Bi 2 Te 2.7 Se 0.3 . Usually bulk Bi 2 Te 2.7 Se 0.3 exhibit two signature Raman‐active optical phonons namely E g 2 and A 1g 2 , which are, respectively, centered around 100 cm −1 (E g 2 ), and 136 cm −1 (A 1g 2 ), and infect these modes are also seen in our samples . The other Raman mode seen at 119 cm −1 is an IR active A 1u 2 vibrational mode that that has been previously reported for nanoscale Bi 2 Te 3 compound at 116 cm −1 .…”

Multiple Magnetic Phases and Anomalous Hall Effect in Sb1.9Fe0.1Te2.85S0.15 Topological Insulators

“…The odd-parity IR-active phonon modes are Raman-forbidden and do not appear in the Raman spectrum for bulk crystals with crystal symmetry. Those IR-active modes also appear as Raman modes in pristine Bi 2 Te 3 and its derivatives − as a result of crystal symmetry breaking. Similar IR-active vibrational modes appear as Raman modes by the In-diffused Bi 2 Se 3 and in pristine Bi 2 Se 3 . , This particular mode appears possibly because of spin fluctuation occurring at 120 K. It disappears below this particular temperature, and we can observe only those Raman modes as expected in rhombohedral R 3̅ m structure.…”

Study of vibrational properties of Bi2 − xMnxTe3 nanocrystals in host glass: Effect of xMn‐concentration

“…Figure 3b shows the possible scenarios of the normal vibrational modes. In the chemical bonds of monoatomic plane Te (1)–Bi–Te (2)–Bi–Te (1) of quintuple layer (QL) of the Bi 2 Te 3 semiconductor, [ 37,39,40,50,51 ] Bi vacancy and Mn atoms are observed in the QL and vdW gap as a function of the xMn‐concentration from 0.00 to 0.20. Figure 3b also shows the vibrational mode characteristic of the Bi 2 OTe 2 unit cell.…”