Fig. 2: Spectra acquisition, statistics, and comparison for bulk and membrane material.

a Confocal map of a 25 × 25 μm² region in membrane D1 with circles marking the manually selected positions of recorded photoluminescence spectra. The bold white circle indicates where the background correction spectrum was recorded, which we subtracted from all subsequent measurements. The brown, red, and yellow circles correspond to three representative spectra, which are shown below: brown/top: multiple peaks within one spectrum; red/middle: a single V2-peak; yellow/bottom: no peak. b Stacked photoluminescence spectra from different spots with peaks on a bulk sample that was only polished, on the bulk part of the etched membrane sample, on the membrane C1, and on the membrane D1. The upper (lower) panels show data before (after) the post-fabrication processing. Each line corresponds to a background corrected spectrum taken at one spot in the respective region. The insets on the right zoom into the wavelength interval 914–919 nm where dashed red lines indicate the region 915.7–917.1 nm in which peaks are ascribed to emission from a V2 centre for c. The respective membrane thickness is given in the title. Peaks in the range of 861.0–862.4 nm are considered to be V1 centres, the reasoning for these intervals is given in the main text. c Percentages of V1-peaks, V2-peaks, both V1- and V2-peaks (V1& V2), and unidentified peaks (O) in the spectra, obtained as in a, before and after the post-fabrication processing.