Peer review of: Fast growth of large-grain and continuous MoS2 films through a self-capping vapor-liquid-solid me (2020)


RR2         
 Reviewers' comments:
Reviewer #1 (Remarks to the Author):
  In this revised manuscript, several comments are still not clearly answered. There are so many
 hypotheses in the growth mechanism which make most of the discussion superficial. The overall
 image quality is still poor and below the standard. The illustrations need to be improved. The
 optical images should optimal brightness and contrast, otherwise, they are hard to view. I insisted
 the authors take all the comments seriously, improve the quality of figures, do additional
 experiments and provide solid data to support their claims. Therefore, I will not recommend
 publication in Nature Communications after all the comments are well addressed.
 1. The authors change the title to “Fast Growth ...” so how fast? Is it comparable to the previous
 record: 300 μm/min? See reference, Yuping Shi et al Na-assisted fast growth of large single-
 crystal MoS2 on sapphire, Nanotechnology 2019 30 034002. I would like to see a more clear
 discussion on "the fast growth".
 Abstract:
 2. “Conventional chemical vapor deposition methods for two-dimensional materials generally use
 an extremely small amount of precursor to render large single-crystal flakes, which inevitably
 causes low coverage of the materials on the substrate.” The judgment is only concluded from one
 reference, I didn’t think it can be generalized to “conventional chemical vapor deposition method”.
 3. “An intermediate liquid phase Na2Mo2O7 is formed through a eutectic reaction,”. Please revise
 this sentence to “An intermediate liquid phase Na2Mo2O7 is formed through a eutectic reaction of
 MoO3 and NaF,”.
 4. “The field-effect transistors fabricated from the full-coverage films show high mobility (33 and
 49 cm2V−1s−1 for the mono and bilayer regions)”. It should be the average mobility and the
 value 33 cm2/Vs is conflict to the data present in Page 11: “90% of devices have pure monolayer
 channel and show high mobilities (34 ± 7 cm2 V-1s-1) with a small variation of Vth (4.9±2.3 V)”.
 Introduction:
Results:
 5. Page 4: “However, because of the relatively low melting temperature of NaCl and rapid eutectic
 reaction, this method can only generate MoS2 nanoribbons, which considerably limits its
 application.” This comment totally misunderstands the paper. The reason for growing 1D MoS2
 nanoribbon only because of the low wettability between the growth substrate and the liquid
 droplets, the liquids prefer to keep spheres during the sulfurization process. In the reference 31,
 wafer-scale 2D monolayer MoS2 films can be grown with these liquid precursors, e.g., Na2MoO4.
  Material synthesis and growth mechanism.
 One obvious drawback in this part is that the authors use too many hypotheses and schematic
 illustrations instead of direct experimental data. That makes all the discussions weak and no roots.
 6. Page 4: “Figure 1a and S1 show that a smooth MoO3 layer was grown on c-plane sapphire
 through plasma-enhanced atomic layer deposition.” There is no data indicating a smooth MoO3
 layer, only a schematic illustration.
 7. Page: “Figure S4 shows a full-coverage monolayer MoS2 film grown on a sapphire of 3 × 3 cm2
 (size was only limited by the CVD tube size)”. It is not a fully-coverage monolayer MoS2 film. It

 also contains voids, thick layers. Please mention the ratio of monolayer, thick-layers, and voids.
 Controlling the coverage and thickness of MoS2.
 8. The understanding of the growth mechanism can be wrong. We need more direct evidence, just
 propose a schematic illustration is meaningless.
 Here are some core questions must be answered with direct evidence. The authors mentioned the
 later introduction of the sulfur, the Na2MoO7 will grow up to large droplet and lead to the growth
 of bilayer MoS2. So please do more controlled experiments to prove it.
 In control experiments without introducing sulfur, what are the area-density and size of the
 Na2MoO7 particles with different annealing times at the growth temperature?
 9. Page 7: “The average grain size of the full-coverage film was approximately 450 μm” The
 authors estimate the nucleation only happened in the first 1 minute, but the truth is the nucleation
 of MoS2 can happen at the whole growth process. And the grain size of approximately 450 μm is
 strongly against the data presents in Figure S4.
 10. Page 8: “Although there are some small islands on the film (Figure 3d)” I don’t think thick
 layers with a dimension of hundreds of micrometers is “small”. From Figure 3d, we can see the
 thick layers are very large with a high ratio in the as-grown film. This non-uniformity will cause
 problems when integrating it into electronic devices.
 11. Page 8: “the bottom continuous film can be peeled off and transferred to various substrates
 with the recently developed layer-resolved 2D material splitting technique34.” If the authors think
 the method reported in Ref. 34 is applicable to their samples, please demonstrated it with direct
 efforts. This kind of proposal is meaningless.
 Technical issue:
 12. The mention of supplementary figures is manuscript is in quite random order. No mention of
 Figure S12-18 in the manuscript.
 Supplementary Information
 13. Figure S1c: It should be “temperature ramping profile”.
 14. Figure S6: Please change “3-7 nm MoS2 precursor” to “3-7 nm MoO3”
 15. Figure S7: What are the thickness of SiO2 and MoO3 layers?
 16. Figure S8: What are the thickness of SiO2, MoO3 and NaF layers?
 17. Figure S9: What are the thickness of NaF and MoO3 layers?
 Rebuttal Letter
 18. Question 1: In Figure R1, the authors proposed the growth of tungsten-based TMDs and TMD
 alloys, but we DO want to see more experimental data on these aspects.
 19. Question 2: The AFM results (Figure S12) only show small MoS2 flakes, but cracks can be seen
 in large-grain MoS2 films as shown in Figure S4.
 20. Question 11 and 17 references
 1. Batch production of 6-inch uniform monolayer molybdenum disulfide catalyzed by sodium in
 glass, Nature Communications 9(2018) 919. This is very important progress in CVD growth of
 large-area (6-inch) MoS2 film with a grain size of 400 um in 8-min growth on a cheap glass
 substrate. This paper shows many advantages over the current paper. It must be included in Table
 1 and Figure 3g. Intentionally overlook this important literature is unacceptable.
 2. Kinetic modulation of graphene growth by fluorine through spatially confined decomposition of
 metal fluorides. Nature Chemistry 11(2019) 730-736. This is the first research paper that uses
 fluoride as growth promoters. It works well not only for the growth of TMDs but also for graphene
 and h-BNs. It is an important progress in the “salt-assisted growth of 2D materials”. It is also

 strongly related to this work, using NaF as a growth promoter.
 3. Nanoscale 11(2019) 16122-16129. The citation in Reference should change from “Li, S. et al.
 Wafer-Scale and Deterministic Patterned Growth of Monolayer MoS2 via Vapor-Liquid-Solid Method.
 arXiv Prepr. arXiv1906.05436 (2019).” to “Li, S. et al. Wafer-Scale and Deterministic Patterned
 Growth of Monolayer MoS2 via Vapor-Liquid-Solid Method. Nanoscale 11 16122-16129
 Reviewer #2 (Remarks to the Author):
 After reading the response letter thoroughly, as well as the revised manuscript, the reviewer feels
 this manuscript still falls short to the standard in nature communications for the following reasons.
 1, The VLS method has been used previously.
 2, The self-capping effect is not well controlled, as the multilayer region cannot be completely
 suppressed.
 3, The monolayer quality has only incremental improvement over previous results in terms of
 mobility or current.
 4, The wafer scale result is not convincing with very few surface and material characterization.
 Previous work (ACS Nano 2017) has demonstrated even better growth results.
 5, The overall figure quality and data presentation is of low-quality, far below the standard of
 nature communications. The reviewer has raised this issue in the first round process and the
 quality has not been improved.
 Overall, this work lacks the novelty, significance and quality to be accepted in Nature
 Communications.
 Reviewer #3 (Remarks to the Author):
 The questions have been addressed by the point to point responses of the author and I agree that
 this work is published in Nature Communications.