Fig. 4

Effect of focusing height. The scanning area of 30 μm × 30 μm is squared in Fig. 3q and Raman spectra are collected under an objective lens of 100 × , integration time of 1 s for each pixel of 1 μm × 1 μm (to create a matrix of 30 × 30). (a) is the colour scale bar. (b) maps the PA via peak of ~ 2910 cm⁻¹, at different focusing heights along z-axis. (c, d) present them together at the indicated physical heights, using 3D presentation and white background, from different angle of view. (e) merges them together in the scanning plane (x-/y-axis) using their contours. At the marked position (#1), the collected spectra are shown in (f). (g) compares the peak intensities at ~ 2910 cm⁻¹ for the marked positions #1–3, once the focusing heights are changed. (h) shows the terrain map for imaging. (i) is the mapped Raman image overlapped on the terrain map, using the rainbow colour. For comparison, (j) shows an image mapped without the terrain map