Confocal Raman microscopy is performed with a WITec Alpha 500 Raman microscope. From each sample surface, a Raman map with 50 × 50 spectra with a resolution of 2 μm is recorded. A frequencydoubled Ne:YAG-Laser (532 nm) with a power of 10 mW and a 50 x lens with N.A. = 0.55 is used. The integration time is 0.5 s per spectrum. The dataset is then decomposed by non-negative matrix factorization (NMF) as described below to obtain the components and their spatial distribution (maps).