title: Low-Frequency Interlayer Raman Modes to Probe Interface of Twisted Bilayer MoS2.
abstract: van der Waals homo- and heterostructures assembled by stamping monolayers together present optoelectronic properties suitable for diverse applications. Understanding the details of the interlayer stacking and resulting coupling is crucial for tuning these properties. We investigated the low-frequency interlayer shear and breathing Raman modes (<50 cm(-1)) in twisted bilayer MoS2 by Raman spectroscopy and first-principles modeling. Twisting significantly alters the interlayer stacking and coupling, leading to notable frequency and intensity changes of low-frequency modes. The frequency variation can be up to 8 cm(-1) and the intensity can vary by a factor of ∼5 for twisting angles near 0° and 60°, where the stacking is a mixture of high-symmetry stacking patterns and is thus sensitive to twisting. For twisting angles between 20° and 40°, the interlayer coupling is nearly constant because the stacking results in mismatched lattices over the entire sample. It follows that the Raman signature is relatively uniform. Note that for some samples, multiple breathing mode peaks appear, indicating nonuniform coupling across the interface. In contrast to the low-frequency interlayer modes, high-frequency intralayer Raman modes are much less sensitive to interlayer stacking and coupling. This research demonstrates the effectiveness of low-frequency Raman modes for probing the interfacial coupling and environment of twisted bilayer MoS2 and potentially other two-dimensional materials and heterostructures.
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Shengxi
authors.initials: S
authors.lastname: Huang
authors.affiliation: Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute , Troy, New York 12180, United States.
authors.forename: Liangbo
authors.initials: L
authors.lastname: Liang
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Xi
authors.initials: X
authors.lastname: Ling
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Puretzky
authors.forename: David B
authors.initials: DB
authors.lastname: Geohegan
authors.forename: Bobby G
authors.initials: BG
authors.lastname: Sumpter
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Kong
authors.affiliation: Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute , Troy, New York 12180, United States.
authors.forename: Vincent
authors.initials: V
authors.lastname: Meunier
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.|Department of Physics, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Mildred S
authors.initials: MS
authors.lastname: Dresselhaus
keywords: Molybdenum disulfide; density functional theory; interlayer coupling; interlayer phonon modes; low-frequency Raman spectroscopy; twisted bilayer