title: Raman Signatures of Polytypism in Molybdenum Disulfide.
abstract: Since the stacking order sensitively affects various physical properties of layered materials, accurate determination of the stacking order is important for studying the basic properties of these materials as well as for device applications. Because 2H-molybdenum disulfide (MoS2) is most common in nature, most studies so far have focused on 2H-MoS2. However, we found that the 2H, 3R, and mixed stacking sequences exist in few-layer MoS2 exfoliated from natural molybdenite crystals. The crystal structures are confirmed by HR-TEM measurements. The Raman signatures of different polytypes are investigated by using three different excitation energies that are nonresonant and resonant with A and C excitons, respectively. The low-frequency breathing and shear modes show distinct differences for each polytype, whereas the high-frequency intralayer modes show little difference. For resonant excitations at 1.96 and 2.81 eV, distinct features are observed that enable determination of the stacking order.
authors.affiliation: Department of Physics, Sogang University , Seoul 04107, Korea.
authors.forename: Jae-Ung
authors.initials: JU
authors.lastname: Lee
authors.affiliation: Department of Physics, Sogang University , Seoul 04107, Korea.
authors.forename: Kangwon
authors.initials: K
authors.lastname: Kim
authors.affiliation: Department of Physics, Sogang University , Seoul 04107, Korea.
authors.forename: Songhee
authors.initials: S
authors.lastname: Han
authors.affiliation: School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST) , Ulsan 44919, Korea.
authors.forename: Gyeong Hee
authors.initials: GH
authors.lastname: Ryu
authors.affiliation: School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST) , Ulsan 44919, Korea.
authors.forename: Zonghoon
authors.initials: Z
authors.lastname: Lee
authors.affiliation: Department of Physics, Sogang University , Seoul 04107, Korea.
authors.forename: Hyeonsik
authors.initials: H
authors.lastname: Cheong
keywords: 3R stacking; HR-TEM; MoS2; Raman spectroscopy; molybdenum disulfide