Revealing the Interlayer Interaction of 2D-TMDs with Ultra-Low Frequency Raman Spectroscopy

Hui-Yu Cheng^1,2, Li-Syuan Lu², Wen-Hao Chang², Yu-Ming Chang^1*

¹Center for Condensed Matter Sciences, National Taiwan University, Taipei City, Taiwan
²Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan

* Presenter:Yu-Ming Chang, email:ymchang@ntu.edu.tw

Two-dimensional transition metal dichalcogenides (2D-TMDs) and their heterostructures have emerged as new building blocks for developing novel electronic and optoelectronic devices recently. For example, the heterobilayer formed by vertical stacking of different monolayer TMDs, its electronic and optical properties strongly depend on the stacking configuration which causes the symmetry-dependent interlayer coupling. It has been established that the stacking configuration of TMD homobilayer can be characterized by low-frequency (< 100 cm^-1) shear and breathing Raman modes, which correspond to in-plane and out-of-plane layer vibrations respectively. Layered PdSe₂ crystal is theoretically predicted to exhibit a widely tunable bandgap varying from zero (bulk) to ∼1.3 eV (monolayer). The layer-number-dependent electronic and optical properties of PdSe₂ have attracted much attention because of its potential application in optoelectronics. In this work, the technology of ultra-low frequency Raman spectroscopy is developed to investigate the interlayer interaction in few-layered PdSe₂ crystals. The observed interlayer Raman modes and their peak frequencies can be well predicted and described by the conventional linear chain model corrected by a minor interlayer restoring force. Furthermore, by performing the polarized-dependent Raman spectroscopy, one can determine the Raman selection rule of these interlayer vibration modes. In brief summary, we have demonstrated that ultra-low frequency Raman spectroscopy can be a powerful and noninvasive method to identify the stacking configuration and the interlayer interaction of 2D-TMDs and their heterostructures.

Keywords: Ultra-low frequency Raman spectroscopy, Interlayer interaction, 2D-TMD, heterobilayer, homobilayer