Since the "Raman spectrum" of MAPbI3 was reported in one paper (http://pubs.acs.org/doi/abs/10.1021/jz402589q  ), a couple of other papers have mentioned some raman spectroscopy study of MAPbI3 to help demonstrate the expected conclusions or assumptions.(http://pubs.acs.org/doi/abs/10.1021/jz501877h, http://pubs.acs.org/doi/abs/10.1021/jz502111u. etc.) More paper with some raman study of MAPbI3 may come out soon since I heard some sounds about the application of raman spectroscopy in different MAPbI3 studies in the recent MRS fall meeting 2014.

Since Raman spectroscopy is a very common and effective spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a perovskite system, these studies sound interesting and promosing.

However, before more extensive studies of the raman spectroscopy on MAPbI3 materials, it might be of substantial necessity to observe "real Raman spectrum repsonse" from the MAPbI3 at the first step. While the as-reported studies are encouraging and interesing, they seem not to have enough information to make the best reliablity of the observed signals .

The following suggestion are appreciated, and for a "model" publication about first report of raman spectra of new materials, we may refer to Journal of Raman Spectroscopy - the very specialized journal.(e.g. http://onlinelibrary.wiley.com/doi/10.1002/jrs.4180/full  )

1) Raman scattering is inelastic.The peaks should be observed at use of two different laser sources if the observed peaks at certain laser are raman signial.(See book J Ferraro,Introductory Raman Spectroscopy)  This is suggested to be done while actually not.

2) MAPbI3 is laser-sensitive materials, the damages of the sample can easily occur during strong laser radiation during the raman measurment. The possible products associated with such damages involves PbI2 http://onlinelibrary.wiley.com/doi/10.1002/pssb.2221760124/abstract  , organic component, PbO http://iopscience.iop.org/0953-8984/7/45/010 , etc, all of which are raman-active. It is likely that the resultant signals contains of the signals from the damaged samples.

3)It is ultra-important to have a smooth phase-pure sample to work on, for which the damaging can be visiable under microscopy before and after measurements. Some reports are based on sample with Al2O3. And even many reports have indicated the passivative PbI2 is intrinsicly accompanied with the MAPbI3, making the measurements are difficult.

4)Fluorescence peaks are usually asscoaited with the raman spectroscopy. And therefore a long wavelength (lower photon energy) laser (red laser or near infare laser) is highly suggested to suppress the fluorescence and reduce the possiblity of sample damages as well.

5)To prevent the effect from filter edges and allow the showing of all the excitation peaks, a low-wavelength number filter should be used, for example, a Near-Excitation Tuneable filter.

Based on the above concerns, we have not able to observe raman signals that we are sure since 2012. Nevertheless a basic and reliable exprimental report on raman spectrum of MAPbI3 is really appreciated for real advance of the understanding of vibrational and rotational mode in this hybrid material. More dicussion or argument are appreciated, and contact me at yuanyuan_zhou@brown.edu or yyzhou1987@gmail.com