While identical characterization conditions have been used for the MAPbI
3, PbI
2 and MAI samples (short integration time: 0.2 s, 10 accumulations; 532 nm laser; with low wavelength holographic notch filter) in Figs. 4 and 5, well-resolved Raman scattering spectra have been observed only in the case of PbI
2 and MAI. This implies that significant Raman scattering under laser excitation does not occur in the MAPbI
3 perovskite case under ambient conditions. Note that we also conducted Raman spectroscopy on phase-pure MAPbI
3 perovskites (single crystals or thin films) that were deposited using other typical methods (“one-step,” “two-step,” solvent engineering, etc
.) [
10–
13], with consistent results. Thus, the characterization of the relatively weak Raman scattering in MAPbI
3 perovskite has been highly challenging using typically available laser excitation sources. While the tradeoff between sample damage (chemical stability in the ambient under laser irradiation) and signal intensity by adjusting the integration time is well known, this tradeoff is particularly severe in the MAPbI
3 perovskite case — low integration time yield little to no Raman signal, while longer integration time needed for obtaining Raman signal damages the sample, and the spectra are essentially from the degradation products rather than the MAPbI
3 perovskite itself. This appears to be responsible for the striking variability in the Raman spectra of MAPbI
3 perovskite reported in the literature. To circumvent this issue, Ledinský et al. [
25] used a 785 nm laser, but still found damage to MAPbI
3 perovskite at room temperature. Brivio et al. [
23] also used a 785 nm laser, but they were able to perform their Raman experiments on MAPbI
3 perovskite only at 100 K (orthorhombic phase), where they found no damage. Ledinský et al. [
25] have suggested that the optimal laser wavelength is 830 nm, which is above the absorption edge and peak photoluminescence (PL) in MAPbI
3 perovskite. Unfortunately, such long-wavelength lasers are not readily available commercially. While one can experiment with power and integration time parameters using conventional excitation lasers used in commercial Raman spectrometers, questions will remain regarding the validity of those measurements. Thus, to remove any ambiguity regarding the Raman results for MAPbI
3 perovskites under ambient conditions, it is imperative that infrared (>830 nm wavelength) excitation lasers must be used in the future.