Peer review of: Unzipping of black phosphorus to form zigzag-phosphorene nanobelts (2020)


RR2

 REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author):
  I would like to apologize to the authors for the time taken to return this review: I have had
 Covid19 and while now better, I was incapacitated for a number of weeks, which delayed my
 response and I apologise for the potential delay in publishing your work.
 I appreciate your efforts in answering my queries, which for the main part you have done
 satisfactorily. Even though the lab closures mean that not all data can be achieved to answer my
 questions, given the importance of the work, I recommend that the paper published on the
 condition that the following two points are fully addressed:
 1. The discussion is much improved, however, the following (rewritten) statement is still
 inaccurate: “The reported ionic scissoring method5 can generate high-quality, individual PNR;
 most (around 65%) are monolayer with POx content of 15 at.%, but the synthesized steps are
time-consuming and rigorous. By contrast, our electrochemical method has simple steps; and most (around 60%) of belts are less than trilayer and lower POx content (∼9.9 at.%).”
   Phosphorene is well known to highly air-sensitive and the XPS taken in the Watts paper was after
 air-exposure for 3-5 mins see their Methods: “The POx contribution for the ‘fresh’ PNRs (exposed
 to air for 2–5 min during transfer to the spectrometer) is 15% of the total area—the same as that
 reported for few-layer phosphorene sheets created through liquid exfoliation methods [Nat.
 Commun. 6, 8563 (2015)]. Here the surface sensitivity of XPS should be highlighted, as although
 the PNRs that are isolated or at the surface of collections of PNR oxidize, multilayer PNRs or those
 buried underneath are protected—evident from the Raman response even after six days.”
 Thus the POx content quoted in the Watts paper is following air-exposure and not of the “as made”
 ribbons and in fact the data shows the ribbons are intact below the surface (despite the POx signal
 fully dominating after 6 days). So please modify (or remove this aspect) in the above statement in
 your paper to reflect this. Also you should discuss the transfer method/time of air exposure for
 your own XPS results – considering that your own samples may well be much purer than you
 previously inferred if your sample are air-exposed upon transfer.
 2. I am still not convinced by the analysis of the heights. I agree that AFM of 2D materials often
 give an artificial height addition due to tip effects and the fact the 2D material will sit slightly proud
 of the surface. However, this ‘extra height’ will only add a single finite addition for each 2D
 material since subsequent layers will still have their crystallographically-defined layer spacing
 (unless restacked which is v unlikely for ribbons). i.e. if single layer will be 0.5 nm + X nm from
 measurement; 2 layer will be 1.0 nm + X nm; 3 layer will be 1.5 nm + X nm etc. Please modify
 your count and analysis accordingly.
 Reviewer #2 (Remarks to the Author):
The authors have appropriately addressed my previous comments. I recommend publishing.