Raman spectroscopy. Raman investigation was performed using a confocal Raman microscope (Renishaw InVia) equipped with a suitable edge-filter for the elastically scattered intensities and a 50x LD objective lens using 785 nm laser excitation to reduce fluorescence. Prior to sample characterisation the grating and detector were calibrated against a single crystal silicon reference sample. Spectra were collected in the range 100-1250 cm -1 with ∼1.2 cm -1 resolution. Each measurement consisted of up to 60 individual accumulations at ∼1 s accumulation time to maximise the signal to noise ratio without oversaturation of the detector.