Micro-Raman scattering spectra were obtained using the confocal Raman spectrometer (WITec Inc., Germany). The spectra were excited by a laser that was operating with excitation wavelength of 488 𝑛𝑚. The laser was focused on the sample to obtain special resolution of 𝜙~610 𝑛𝑚 using a 50 × objective lens with numeric aperture 𝑁𝐴 = 0.75 (Leica Wetzlar, Germany) with the laser power at the sample's surface of 5 𝑚𝑊. The Raman Stokes signal was dispersed with a diffraction grating (2400 𝑔𝑟𝑜𝑜𝑣𝑒𝑠/𝑚𝑚) and data was recorded using a Peltier cooled charge-coupled device (CCD) detector (Renishaw, UK) (1024 × 256 pixels). The spectra of each specimen were collected over the wavenumber range of 10 ! -10 " 𝑐𝑚 51 , with scanning step size 1 𝑐𝑚 51 with an integration time constant of 1𝑠 where the signal was accumulated five times and then the average was reported. Silicon was used to calibrate the Raman setup for both Raman wavenumber and spectral intensity before the use of the equipment. The Positions of the Raman peaks were determined by fitting the data to the Lorentz line shape using a peak fit option in the OriginPro software (OriginLab Corp., USA). Silicon was used to calibrate the Raman setup for both the Raman wavenumber and spectral intensity.