Cao Mao-Feng, Peng Xiao-Hui, Zhao Xiao-Jiao, Bao Yi-Fan, Xiao Yuan-Hui, Wu Si-Si, Wang Jun, Lu Yao, Wang Miao, Wang Xiang, Lin Kai-Qiang, Ren Bin
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China.
Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province, Xiamen, 361102, China.
Adv Mater. 2024 Aug;36(35):e2405433. doi: 10.1002/adma.202405433. Epub 2024 Jul 15.
Collective excitations including plasmons, magnons, and layer-breathing vibration modes emerge at an ultralow frequency (<1 THz) and are crucial for understanding van der Waals materials. Strain at the nanoscale can drastically change the property of van der Waals materials and create localized states like quantum emitters. However, it remains unclear how nanoscale strain changes collective excitations. Herein, ultralow-frequency tip-enhanced Raman spectroscopy (TERS) with sub-10 nm resolution under ambient conditions is developed to explore the localized collective excitation on monolayer semiconductors with nanoscale strains. A new vibrational mode is discovered at around 12 cm (0.36 THz) on monolayer MoSe nanobubbles and it is identified as the radial breathing mode (RBM) of the curved monolayer. The correlation is determined between the RBM frequency and the strain by simultaneously performing deterministic nanoindentation and TERS measurement on monolayer MoSe. The generality of the RBM in nanoscale curved monolayer WSe and bilayer MoSe is demonstrated. Using the RBM frequency, the strain of the monolayer MoSe on the nanoscale can be mapped. Such an ultralow-frequency vibration from curved van der Waals materials provides a new approach to study nanoscale strains and points to more localized collective excitations to be discovered at the nanoscale.
包括等离激元、磁振子和层呼吸振动模式在内的集体激发在超低频（<1太赫兹）下出现，对于理解范德华材料至关重要。纳米尺度的应变会极大地改变范德华材料的性质，并产生诸如量子发射器之类的局域态。然而，目前尚不清楚纳米尺度的应变如何改变集体激发。在此，我们开发了在环境条件下具有亚10纳米分辨率的超低频针尖增强拉曼光谱（TERS），以探索具有纳米尺度应变的单层半导体上的局域集体激发。在单层MoSe纳米气泡上，在约12厘米⁻¹（0.36太赫兹）处发现了一种新的振动模式，并将其确定为弯曲单层的径向呼吸模式（RBM）。通过在单层MoSe上同时进行确定性纳米压痕和TERS测量，确定了RBM频率与应变之间的相关性。证明了RBM在纳米尺度弯曲单层WSe和双层MoSe中的普遍性。利用RBM频率，可以绘制出纳米尺度上单层MoSe的应变图。这种来自弯曲范德华材料的超低频振动为研究纳米尺度应变提供了一种新方法，并指出在纳米尺度上还有更多局域集体激发有待发现。
