Stacking-dependent interlayer phonons in 3R and 2H MoS ₂

Abstract

We have investigated the interlayer shear and breathing phonon modes in MoS2 with pure 3R and 2H stacking order by using polarization-dependent ultralow-frequency Raman spectroscopy. We observe up to three shear branches and four breathing branches in MoS2 with thickness from 2 to 13 layers. The breathing modes show the same Raman activity behavior for both polytypes, but the 2H breathing frequencies are consistently several wavenumbers higher than the 3R breathing frequencies, signifying that 2H MoS2 has slightly stronger interlayer lattice coupling than 3R MoS2. In contrast, the shear-mode Raman spectra are strikingly different for 2H and 3R MoS2. While the strongest shear mode corresponds to the highest-frequency branch in the 2H structure, it corresponds to the lowest-frequency branch in the 3R structure. Such distinct and complementary Raman spectra of the 3R and 2H polytypes allow us to survey a broad range of shear modes in MoS2, from the highest to lowest branch. By combining the linear chain model, group theory, effective bond polarizability model and first-principles calculations, we can account for all the major observations in our experiment.KEY WORDS: 3R MoS2, 2H MoS2, stacking order, shear mode, breathing mode, ultralowfrequency Raman.

Probing stacking configurations in a few layered MoS2 by low frequency Raman spectroscopy

“…In the 2H stacked region, the peak around 23 cm −1 is identified as the in-plane shear mode, and the peak at 40 cm −1 corresponds to the out of plane breathing mode. In the 3R stacked region, while the shear mode stays at 23 cm −1 , the breathing mode is found to shift to 37.4 cm −1 , which is in good agreement with theoretical calculations 52 . The peak position of the breathing mode is red-shifted in 3R stacking, while the shear mode shows almost no change.…”

“…Further, in the two-layered region, the intensity of the shear mode is higher in the 2H stacked region as compared to the 3R stacked region, as seen in Fig. 3(c), which is also in good agreement with theoretical calculations 52 . The effect of stacking configuration on the shear mode intensity can be explained by the interlayer bond-polarizability model 62 .…”

“…In case of three-and more layered samples, one can estimate a volume average of the fraction of stacking sequence within the diffraction-limited measurement area. Similar to the twolayered regions, the I s /I b value is expected to be around 4 for a perfect 2H-2H configuration and 1 for a perfect 3R-3R configuration 52 . The intensity of the shear mode decreases as the stacking order changes from 2H-2H to 2H-3R and then to 3R-3R.…”

“…However, no extensive studies of the low-frequency modes to probe the stacking configurations of MoS 2 are reported so far. Previous studies that probed stacking configuration in MoS 2 had relied only on single point spectroscopic measurements of Raman modes 5,44,[52][53][54] . While single point Raman measurements are sufficient to measure the properties of a homogenous sample, they do not provide an accurate picture about the distribution of any physical properties, such as the stacking configurations, within the MoS 2 sample.…”

“…2(d). The most widely found stacking configuration in a four-layered www.nature.com/scientificreports/ MoS 2 is 2H-2H-2H 52,63 . The shear mode for this configuration appears at 29.7 cm −1 , and the breathing mode is found at 21 cm −1 .…”