Zobeiri Hamidreza, Xu Shen, Yue Yanan, Zhang Qianying, Xie Yangsu, Wang Xinwei
Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, USA.
School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, 333 Longteng Road, Shanghai 201620, People's Republic of China.
Nanoscale. 2020 Mar 12;12(10):6064-6078. doi: 10.1039/c9nr10186a.
Temperature dependent Raman intensity of 2D materials features very rich information about the material's electronic structure, optical properties, and nm-level interface spacing. To date, there still lacks rigorous consideration of the combined effects. This renders the Raman intensity information less valuable in material studies. In this work, the Raman intensity of four supported multilayered WS2 samples are studied from 77 K to 757 K under 532 nm laser excitation. Resonance Raman scattering is observed, and we are able to evaluate the excitonic transition energy of B exciton and its broadening parameters. However, the resonance Raman effects cannot explain the Raman intensity variation in the high temperature range (room temperature to 757 K). The thermal expansion mismatch between WS2 and Si substrate at high temperatures (room temperature to 757 K) make the optical interference effects very strong and enhances the Raman intensity significantly. This interference effect is studied in detail by rigorously calculating and considering the thermal expansion of samples, the interface spacing change, and the optical indices change with temperature. Considering all of the above factors, it is concluded that the temperature dependent Raman intensity of the WS2 samples cannot be solely interpreted by its resonance behavior. The interface optical interference impacts the Raman intensity more significantly than the change of refractive indices with temperature.
二维材料的拉曼强度与温度相关，这一特性蕴含着有关材料电子结构、光学性质以及纳米级界面间距的丰富信息。迄今为止，对于这些综合效应仍缺乏严谨的考量。这使得拉曼强度信息在材料研究中的价值大打折扣。在这项工作中，我们研究了四个负载多层WS2样品在532 nm激光激发下，从77 K到757 K的拉曼强度。观察到了共振拉曼散射，并且我们能够评估B激子的激子跃迁能量及其展宽参数。然而，共振拉曼效应无法解释高温范围（室温至757 K）内的拉曼强度变化。高温下（室温至757 K）WS2与Si衬底之间的热膨胀失配使得光学干涉效应非常强烈，并显著增强了拉曼强度。通过严格计算并考虑样品的热膨胀、界面间距变化以及光学折射率随温度的变化，对这种干涉效应进行了详细研究。综合考虑上述所有因素，得出结论：WS2样品与温度相关的拉曼强度不能仅由其共振行为来解释。界面光学干涉对拉曼强度的影响比折射率随温度的变化更为显著。
