The Raman spectra were recorded using LabRam confocal Raman spectroscopy instrument (Horiba Jobin Yvon, France). An Ar ion laser with the emission wavelength of 514.532 nm (ca. 514.5 nm), a 600 grooves/mm grating and an Olympus 10x objective (numerical aperture 0.25) were used in conjunction with a charge-coupled device (CCD) detector. Each of the spectra was collected for 5 seconds of integration time averaged over 100 accumulations. The beam power at the position of the sample was around 15 mW for all the measurements. The spectra were corrected for the wavenumber shift by calibrating the position of laser line to zero by fitting the laser line spectrum to a Gaussian function.