title: Low-Frequency Raman Fingerprints of Two-Dimensional Metal Dichalcogenide Layer Stacking Configurations.
abstract: The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks; however, fast optical characterization techniques are essential for rapid development of the field. Here, using two- and three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low frequency (LF) Raman modes (<50 cm(-1)) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries.
authors.affiliation: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Puretzky
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: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Xufan
authors.initials: X
authors.lastname: Li
authors.affiliation: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Kai
authors.initials: K
authors.lastname: Xiao
authors.affiliation: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Kai
authors.initials: K
authors.lastname: Wang
authors.affiliation: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Masoud
authors.initials: M
authors.lastname: Mahjouri-Samani
authors.affiliation: ∥Departamento de Física, Escuela Politécnica Nacional, Quito 170525, Ecuador.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Basile
authors.affiliation: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Juan Carlos
authors.initials: JC
authors.lastname: Idrobo
authors.affiliation: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.|§Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: Bobby G
authors.initials: BG
authors.lastname: Sumpter
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: †Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors.forename: David B
authors.initials: DB
authors.lastname: Geohegan
keywords: first-principles calculations; low-frequency Raman spectroscopy; stacking configurations; transition metal dichalcogenides; two-dimensional materials