Xu Chaochen, Yao Quanzhou, Du Hongzhi, Hong Changyi, Xue Tao, Kang Yilan, Li Qunyang
Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P. R. China.
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P. R. China.
ACS Appl Mater Interfaces. 2021 May 12;13(18):22040-22046. doi: 10.1021/acsami.1c03941. Epub 2021 Apr 27.
The Raman peak position shift rate per strain (RSS) coefficient of graphene is crucial for quantitative strain measurement by Raman spectroscopy. Despite its essential role, the experimentally measured RSS values are found to be highly scattered and many times significantly lower than the theoretical prediction. Here, using in situ Raman spectroscopy with a tensile test system, we resolve this controversy by examining the Raman characteristics of graphene derived from chemical vapor deposition (CVD) transferred on polymer substrates. Our experiments show that the Raman 2D-peak position of CVD graphene can shift nonlinearly with applied strain, in contrast to its intrinsically linear trait. More importantly, the resultant RSS coefficient at the steady state is much lower than the theoretical prediction. By analyzing atomic force microscopy (AFM) phase images and full width at half-maximum (FWHM) of Raman spectra, we attribute the abnormal behavior to nanometer-scale inhomogeneity of the graphene/substrate contact interface. Assisted by a simplified discrete interface slip model, we correlate the evolution of nanometer-scale inhomogeneity with that of the apparent Raman response. The theoretical model provides a useful tool for understanding and optimizing the contact interface behavior of various two-dimensional materials on substrates; the revealed mechanism is critical for correct interpretation of data obtained by Raman or any other spectroscopies based on homogenized laser signals.
石墨烯的每应变拉曼峰位置偏移率（RSS）系数对于通过拉曼光谱进行定量应变测量至关重要。尽管其具有重要作用，但实验测量的RSS值却高度分散，且多次显著低于理论预测值。在此，我们使用带有拉伸测试系统的原位拉曼光谱，通过研究化学气相沉积（CVD）转移到聚合物基底上的石墨烯的拉曼特性，解决了这一争议。我们的实验表明，与固有线性特征相反，CVD石墨烯的拉曼2D峰位置会随施加的应变发生非线性偏移。更重要的是，稳态下所得的RSS系数远低于理论预测值。通过分析原子力显微镜（AFM）相图像和拉曼光谱的半高宽（FWHM），我们将这种异常行为归因于石墨烯/基底接触界面的纳米级不均匀性。借助简化的离散界面滑移模型，我们将纳米级不均匀性的演变与表观拉曼响应的演变关联起来。该理论模型为理解和优化各种二维材料在基底上的接触界面行为提供了有用工具；所揭示的机制对于正确解释通过拉曼或任何其他基于均匀激光信号的光谱学获得的数据至关重要。
