The Raman spectra were recorded using a micro-Raman setup containing a single monochromator (Jobin Yvon HR460) equipped with a CCD camera and a 600 grooves/mm grating resulting in a resolution of ~ 5cm -1 . The 532-nm laser of ~ 15 mW power is focused on the sample by an Olympus objective (100X magnification) which also collects the signal.