The Raman spectra are measured in a home-built micro-Raman setup using a 100X Olympus objective for focusing the excitation laser at 532 nm on the sample and collecting the Raman signal. The setup includes a Jobin-Yvon spectrometer (HR460) equipped with a CCD camera and 600 grooves/mm grating, resulting in ~ 5.5 cm -1 resolution with a 50 m slit setting. The laser power is kept below 1 mW to avoid overheating of the sample at the laser spot.