Fig. 3

Photo images (a, b), typical Raman spectra (c) and images (d–q). The squared area in (a) of 60 μm × 60 μm was zoomed in as (b) and scanned. Raman spectra were collected under an objective lens of 40 × , integration time of 1 s for each pixel of 2 μm × 2 μm (to create a matrix of 30 × 30). (c) shows the Raman spectra of PA, to compare with 1 single spectrum (10 s integration) and 3 typical scanning spectra (1 s) collected from the marked positions in (b), and the average spectrum of 900 (30 × 30) spectra. The intensity images (d–l) are mapped at a blank wavenumber window (d), a characteristic peak ofTiO₂ (e) and seven characteristic peaks of PA (f–l), as marked under each image (and the peak width), after 10% colour off-setting. (m, n) and (o, p) merge images (f–k) using either logic-based algorithm (m, n) or algebra-based algorithm (o, p), as suggested under the images. (q) is another version of (k), using a 3D presentation and white a background. The squared area is further checked in Fig. 4