Abnormal Raman Characteristics of Graphene Originating from Contact Interface Inhomogeneity

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.