题 目： Raman Spectroscopy of Interlayer Phonons and Charge Density Waves in Transition Metal Dichalcogenide Atomic Layers
报告人： Prof. Rui He （Department of Electrical and Computer Engineering, Texas Tech University）
时 间： 06月04日（周一），上午 10:00 - 11:00
地 点： 南校区第一实验楼物理馆502会议室
Atomically thin ReS2 and NbSe2 crystals are new types of 2D materials that have different crystal structures and electronic properties from common transition metal dichalcogenides (TMDs), e.g. MoS2, MoSe2, WS2, and WSe2. We measured ultralow frequency Raman response of ReS2 and NbSe2 atomic layers. ReS2 has unique distorted 1T structure. We found that the two shear phonon modes in bilayer ReS2 are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other TMD materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of interlayer modes and determine the stacking order of bilayer ReS2. Few-layer ReS2 exhibits rich Raman peaks at frequencies below 50 cm-1, where a panoply of interlayer shear and breathing modes are observed. Atomically thin NbSe2 is a metallic layered TMD with novel charge-density-wave (CDW) and superconductive phases. We observed both the interlayer breathing 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 the layer number. We find that, although NbSe2 has different stacking order from MoS2, they share the same crystal symmetry groups and exhibit similar Raman selection rules for interlayer phonons. In addition, the interlayer phonon modes evolve smoothly from T = 300 K to 8 K, with no observable response to the CDW formation in NbSe2. Our results reveal that the interlayer phonons can serve as an effective probe of the interface properties and interlayer interactions in these 2D atomic layers. In the third part of my talk, I will present our recent studies of nearly commensurate to commensurate CDW transitions in ultrathin 1T-TaS2. We identified additional CDW modes from surface layers of 1T-TaS2. Phase nucleation energetics are proposed to understand the bulk and surface CDW transitions.
Dr. He, Rui obtained her B.S. degree from Fudan University in China in 1999. She received her Ph.D. degree in Applied Physics from Columbia University in the City of New York in 2006. After her graduation from Columbia, she joined the Hong Kong University of Science and Technology as a postdoc in the physics department and as a research assistant in the mathematics department. In 2009 she returned to Columbia University where she worked as a postdoctoral research scientist. She joined the Physics Department at the University of Northern Iowa as an assistant professor in August 2011 and became an associate professor in 2016. She joined Electrical and Computer Engineering Department at Texas Tech University as an associate professor in July 2017. Her research interests include the general area of optical studies of nanostructures, especially 2D materials and their heterostructures.