Journal
NANO LETTERS
Volume 13, Issue 3, Pages 1007-1015Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nl304169w
Keywords
Transition metal dichalcogenides; shear modes; breathing modes; Raman spectroscopy; first principle calculations; linear chain model
Categories
Funding
- Singapore National Research Foundation [NRF-RF2009-06]
- Ministry of Education [MOE2011-T2-2-051, MOE2012-T2-2-086]
- Nanyang Technological University (NTU) [M58110100]
- Institute of High Performance Computing Independent Investigatorship
- Singapore National Research Foundation under a CREATE programme: Nanomaterials for Energy and Water Management
- NTU [M4080865.070.706022]
- [M58113004]
- [ONR-MURI-N00014-09-1-1063]
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Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently attracted tremendous interest as potential valleytronic and nanoelectronic materials, in addition to being well-known as excellent lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low-frequency phonon modes and how they evolve with the number of layers are important for both the mechanical and the electrical properties of 2D TMDs. Here we uncover the ultralow frequency interlayer breathing and shear modes in few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman spectroscopy and first principles calculations. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbor interactions. We show that the derived in-plane (shear) and out-of-plane (breathing) force constants from experiment remain the same from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale interlayer frictional characteristics of these excellent lubricants should be independent of the number of layers.
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