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  • 发布时间:2018-06-08 10:25:53
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  • 报告题目:Raman Spectroscopy of Interlayer Phonons and ChargeDensityWaves in Transition Metal Dichalcogenide AtomicLayers

    报告嘉宾:Rui He, Associate Professor, Texas Tech University




    Atomically thin ReS2 and NbSe2 crystals are new types of 2D materials that have differentcrystal structures and electronic properties from common transition metal dichalcogenides(TMDs), e.g. MoS2, MoSe2, WS2, and WSe2. We measured ultralow frequency Raman responseof ReS2 and NbSe2 atomic layers. ReS2 has unique distorted 1T structure. We found that the twoshear phonon modes in bilayer ReS2 are nondegenerate and clearly resolved in the Ramanspectrum, in contrast to the doubly degenerate shear modes in other TMD materials. By carryingout comprehensive first-principles calculations, we can account for the frequency and Ramanintensity of interlayer modes and determine the stacking order of bilayer ReS2. Few-layer ReS2exhibits rich Raman peaks at frequencies below 50 cm-1, where apanoply of interlayer shear andbreathing modes are observed. Atomically thin NbSe2 is a metallic layered TMD with novelcharge-density-wave (CDW) and superconductive phases. We observed both the interlayerbreathing modes and shear modes at frequencies below 40 cm-1 for samples of 2 to 15 layers.Their frequencies, Raman activities, and environmental instability depend systematically on thelayer number. We find that, although NbSe2 has different stacking order from MoS2, they sharethe same crystal symmetry groups and exhibit similar Raman selection rules for interlayerphonons. In addition, the interlayer phonon modes evolve smoothly from T = 300 K to 8 K, withno observable response to the CDW formation in NbSe2. Our results reveal that the interlayerphonons can serve as an effective probe of the interface properties and interlayer interactions inthese 2D atomic layers. In the third part of my talk, I will present our recent studies of nearlycommensurate to commensurate CDW transitions in ultrathin 1T-TaS2. We identified additionalCDW modes from surface layers of 1T-TaS2. Phase nucleation energetics are proposed tounderstand the bulk and surface CDW transitions.


    Dr. HeRui obtained her B.S. degree from Fudan University in China in 1999. Shereceived her Ph.D. degree in Applied Physics from Columbia University in the City of NewYork in 2006. After her graduation from Columbia, she joined the Hong Kong University ofScience and Technology as a postdoc in the physics department and as a research assistant in themathematics department. In 2009 she returned to Columbia University where she worked as apostdoctoral research scientist. She joined the Physics Department at the University of NorthernIowa as an assistant professor in August 2011 and became an associate professor in 2016. Shejoined Electrical and Computer Engineering Department at Texas Tech University as anassociate professor in July 2017. Her research interests include the general area of optical studiesof nanostructures, especially 2D materials and their heterostructures.