Peer review of: Liquid metal-based synthesis of high performance monolayer SnS piezoelectric nanogenerators (2020)


RR2
 Reviewers' comments:
Reviewer #2 (Remarks to the Author):
  The authors addressed most of the questions I asked.
 One more question is about the growth map. From the growth map, can I conclude that the as-
 grown SnS film change to SnS2 if the film exposes to H2S for a time longer than ~17 min? Do this
 mean the as-grown SnS film is chemically unstable? I suggest the authors prepare a more
 accurate growth map.
 The other question is still about the morphology of the SnS film. Optical images (the updated
 images) can only give poor contrast of the film. High-quality SEM images are required to prove the
 overall quality of the transferred SnS film. I suggest to authors present a group (array) of SEM
 images of the SnS film with a size of few tens micrometers by a few tens micrometers.
 Reviewer #3 (Remarks to the Author):
 The manuscript by Khan et al. reports on liquid metal-based synthesis of monolayer SnS, and their
 resulting piezoelectric properties. The authors have made changes based on reviewers' comments,
 however, there still remains significant issues. With regards the fabrication process itself, previous
 reviewers have raised concerns which the authors seem to have adequately addressed in their
 revised version. However, I am still not convinced about the validity of the measurements of
 piezoelectric properties, and here I feel there are major gaps in the understanding of the
 phenomenon. The fact that data related to the output of the interdigitated electrode geometry had
 to be removed without explanation raises severe concerns. It is highly unsatisfactory to simply
 remove data that was put forward as evidence of piezoelectricity, without explaining what the
 measured signals were in the first place. This, unfortunately raises doubts about subsequent
 measurements and interpretation. Also, the authors seem to have confused the piezoelectric and
 piezotronic effects, which are distinct effects in semiconductors, and there is insufficient discussion
 of the latter in this respect.
 I have concerns regarding the PFM measurements. In contact-mode operation, there is significant
 electrostatic contribution which can be considerable particularly for thin 2d materials. This effect
 has to be corrected for by performing KPFM measurements before the PFM measurements in order
 to offset the surface potential, and ideally afterwards as well to make sure the measurement is
 valid. Without this, the PFM measurements remain suspect (see Scientific Reports volume 7,
 Article number: 41657 (2017). It is unclear how many SnS flakes were measured by PFM - there
 does not seem to be any statistics. How reproducible are these results? Also, the allocation of polar
 directions seems a bit arbitrary - how are the 1 and 3 directions allocated? This has to with how
 the lattice is aligned, and PFM on random flakes cannot provide this information. Perhaps, PFM
 carried out by scanning along different directions might shed some light, but without this
 information, there is not much meaning to the calculated piezoelectric coefficients. I highly
 recommend the authors to refer to PFM studies on single-atomic layer MoS2 [Nature volume 514,
 pages470–474(2014)] as an excellente xample of how PFM should be conducted and interpreted
 for 2d materials.
 It is also not clear why SnS should have an "enhanced" d11 coefficient - I am not sure this is
 indeed the measured coefficient as the indices seem to have been designated arbitrarily (see
 previous point). Also, ideally, the PFM measurements should be carried out with the flakes in direct
 contact with a conducting substrate, and not SiO2. This introduces other complex interface effects
 that have not been accounted for.
 I am also concerned by the generator experiments themselves. The manuscript says that a

 "tapping force" was applied, but there is no information about the magnitude. This should be
 provided to set the results in context. Also, it is not clear as to why power per unit length is of any
 importance in the case of these types of generators.
 In conclusion, while the fabrication of these monolayers is interesting, I believe the main claims of
 the paper with respect to the piezoelectric properties and applications of SnS monolayers require
 further study and justification. Hence I am unable to recommend publication in Nature
 Communications.