Single-Layer MoS2: A Two-Dimensional Material with Negative Poisson’s Ratio
Round 1
Reviewer 1 Report
The manuscript is devoted to the study of the reasons for the origin of the negative Poisson's ratio (NPR). The authors focused onto atomically-thin molybdenum disulfide MoS2. Here my first question concerning citation practice - why the authors did not mention the original article about 2D MoS2 (https://doi.org/10.1103/PhysRevLett.105.136805)?
Next, in the abstract the authors state that ten different configurations of MoS2 was explored. But where the information/data concerning ten different configuration of MoS2 in the manuscript/supplementary? Such data are absent... There are only 3-4 of MoS2 structures in the text, while it is not clear where the authors took the structural data from, proper citation is absent.
The novelty of manuscript is low - the authors used well-known materials, apply well-known methods and conclude that the origin of NPR is linked to unique structure (it is obvious) and some interaction between M-S orbitals, but it is not well argued.
Bad english translation, overall text need polishing.
Author Response
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Author Response.docx
Reviewer 2 Report
This manuscript describes periodic DFT calculations on 2D layered MoS2 structures with the aim to identify new materials with negative Poisson’s ratio. As outlined in the introduction the topic is of great relevance in materials science. The most interesting part of the work is the correlation between electronic structure and material properties. Here the concept of an overlap area is introduced. However, it is unclear how such an overlap area is defined. Figure 3 gives a kind of qualitative definition, but I find this not satisfying for the introduction of this concept. A more quantitative analysis is needed. In this respect, I want also to point out that orbital overlap is not indicative for chemical bonding. Take as example Löwdin orthogonalized orbitals that by construction have no overlap. Nevertheless, they can be used to describe the same chemical bonding as non-orthogonal orbitals. Therefore, it is mandatory that the authors explain in a more quantitative way their concept of overlap area.
In general, the manuscript is well written and straightforward to read. However, there are some details I do not understand or they need further specification:
1.) The CASTEP SCF tolerance is explicitly given in the Computational Methods. Was this SCF convergence reached with an energy smearing?
2.) I do not understand the logic of “…in the generation of NPR for MoS2. In contrast, …” Why is this a contrast?
3.) The sentence “… used to calculate, among ten different …” in the introduction reads strange to me.
4.) The authors perform BOMD simulations of their optimized structures and conclude from them thermal stability. However, these simulations seemed to be performed within a given space group. Therefore, the authors claim is not true (see for example Theor. Chem. Acc. 140, 44, 2021). Although the temperature in Fig. 6 shows an unusual behaviour. Is this the instantaneous or average temperature? Which thermostat is used for the NVT simulation?
Minor points:
· Please check reference 9.
In conclusion, the manuscript is of scientific interest and deserves publication if the above outlined points are elaborated and rectified.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
The overall paper is well written, all calculations seems to be reasonable and well described.
Minor typos and marks detected:
a. The references should appear in the same order as the authors.
b. English must be checked. E.g. in abstract: “Here, first-principles are used to calculate, among ten different configurations….”
c. The non-breaking spaces should be added between Figure and number of it as well as numbers and units. Besides, the caption for pictures and figures have to be on one page.
d. Figures and text have to be aligned.
Besides, there are no MD calculation details given in the Method section.
Author Response
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Reviewer 4 Report
In their paper, the Authors study the origin of the negative Poisson ratio observed in some configurations of MoS2. Performing numerical simulations and comparing the behavior of the considered structure with that of other 2D materials, they are able to show how this effect is related to the geometric (zigzag) structure and to the electronic structure (antibonding state, strong interaction between p and d orbitals).
The paper is particularly interesting and I strongly suggest its publication, with some minor revisions that I list in the following:
1) page 1, the second sentence of the abstract (Here...directions) has to be corrected (the part "are used to calculate" has to be completed);
2) page 1, line 6 of the introduction: "this phenomenon" -> "the phenomenon"
3) page 1, line 15 of the introduction: change "fetchable" with something more proper
4) page 1, last line: add some other references on 2D materials, such as:
Miró, P.; Audiffred, M.; Heine, T. An atlas of two-dimensional materials. Chem. Soc. Rev. 2014, 43, 6537–6554. https://doi.org/10.1039/C4CS00102H
Jiang, L.; Marconcini, P.; Hossian, M. S.; Qiu, W.; Evans, R.; Macucci, M.; Skafidas E. A tight binding and k.p study of monolayer stanene. Sci. Rep. 2017, 7, 12069. https://doi.org/10.1038/s41598-017-12281-y
5) page 2, line 13: "is there is" -> "is that there is"
6) page 2, line 27: add a reference for the software "CASTEP"
7) page 2, lines 28-30: write better this sentence (in particular, modify "In course of the")
8) page 2, line 33: is a 5x5x5 mesh sufficient? have the Authors tried to see if the results significantly change using a denser mesh?
9) page 3, line 9: "shows" -> "is shown"
10) page 5, last line of the caption of the caption of Fig. 2: "layers and do" -> "layers; this does"
11) page 5, line 7: "PDOS of four" -> "PDOS of the four"
12) page 5, line 9: "orbit" -> "orbital" (twice)
13) page 5, last sentence before Fig. 3: this sentence (The material...this material) has to be written better
14) page 6, sentence before Fig. 4: please clarify "caused by the interlayer electronic structure" ("inter-" or "intra-"?)
15) page 8, line 5: write the meaning of the acronyms "MD" (molecular dynamics) and "NVT" (constant number, volume and temperature)
16) page 8, last line of the caption of Fig. 6: write the meaning of the acronym "AIMD" (ab initio molecular dynamics)
Author Response
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Author Response File: Author Response.docx
Reviewer 5 Report
The work , in the statement at least, examines some 2D materials with a negative Poisson's ratio. Unfortunately, the work contains fundamental methodological flaws, namely:
- Authors mistake nomenclature for 2D and 3D materials, see https://en.wikipedia.org/wiki/Space_group
- Page 1, “Young’s modulus and Poisson’s ratio (PR) are two vital parameters…”: true only for isotropic materials i.e. for 2D such that have hexagonal symmetry
- Page 2, “ … 1T-phase MoS2 … ”: There are 3 different phases of monolayer MoS2 and the authors don't even mention it, see https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.165412
- Page 2, “…. density functional theory (DFT)…“ : Please add citations of fundamental papers on DFT
- Page 2, “The k-point meshes are set as 5 × 5 × 5 for the bulk.”: 2D or 3D
- Page 2, “ … so 2D crystals have only four standalone elastic constants …”: not true, they have 6, see https://iopscience.iop.org/article/10.1088/2053-1583/ab2ef3/pdf
- Page 2, Table S1.: for hexagonal symmetry must C11=C22 and here it is not, see https://books.google.pl/books/about/Physical_Properties_of_Crystals.html?id=ugwql-uVB44C&redir_esc=y
- Page 3, “Young’s modulus attains a maximum value … ”: Numerical artifact, since the material is isotropic then Young’s modulus and Poisson’s ratio do not depend on direction
- Page 3, “And the band structure …”: what is it for?
- Page 4, “…. Figure S2.”: If these are 2D materials then there is no c-axis and there is a vacuum in the z-direction
- Page 6, “…. (c) The relationship between PR and layer spacing for two different arrangements of structures.”: If these are 2D materials then the layers are far enough apart that they do not interact with each other
- Page 8, “… the steady 2D structure follows the Born−Huang criteria in which ….”: this condition is wrong, see https://iopscience.iop.org/article/10.1088/2053-1583/ab2ef3/pdf
Author Response
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Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
English is very difficult to understand and sometimes I can't even realise if it's a grammatical error or a incorrect research. For example, page 3 "As Figure 1a is showns, Young’s modulus of MoS2 shows unobvious anisotropy isotropy". I do not see any anisotropy at the figure 1a.
The organisation of the manuscript is also very unclear and complicated. For example, page 3 "selected materials are MoS2 (mp-
558544), MoS2 (mp-2815), MoS2 (mp-1405065), WS2 (mp-9813), and MoTe2 (mp-602)." and then at the page 4 "three additional materials were selected for comparison: the MoS2 with zigzag structure (space
group P63/mmc), the MoS2 without the special undulations (space group F4Ì… 3m)..." What is the correspondence between structures?
Next, I believe that conclusions are not supported adequately by the presented data. The text concerning interlayer/intralayer electronic interactions is obscure. For example I do not see the difference between Fig 4a and 4b, the explanation in the text is also unclear. Fig 4c does not support conclusion that "the PR first increase and then decrease as the layer spacing increases" - we see the chainsaw-like dependence for the red plot. And such small variations can be caused by computational reasons, not physics.
Author Response
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Author Response.pdf
Reviewer 2 Report
The answer to my BOMD question is not satisfying. Are the periodic boundary conditions enforced in the BOMD? If yes, how can you conclude structural stability?
Without answering these questions, the manuscript is not fit for publication!
Author Response
Response to Reviewer 2 Comments
Point 1: The answer to my BOMD question is not satisfying. Are the periodic boundary conditions enforced in the BOMD? If yes, how can you conclude structural stability?
Response 1: Thank you for pointing this out. In order to ensure the accuracy of the simulation and verify the stability of the structure, a vacuum layer of 15 Å was added to the four models to eliminate the influence of periodic boundary conditions, and the bottom layer of MoS2 was fixed in the calculation process. These problems are caused by our negligence in not describing the molecular dynamics simulations in detail in the text. Therefore, we have refined the description of molecular dynamics in the second part of the calculation method, see line 39 on page 2.
Reviewer 5 Report
I don't see any novelty in this paper.
Author Response
Response to Reviewer 5 Comments
Point 1: I don't see any novelty in this paper.
Response 1: The novelty of this paper lies in the study of the mechanism of the emergence of NPR in materials. At the atomic scale, DFT is used to reveal the structural characteristics and electron interactions of NPR materials. The work has deepened our understanding of, as well as paved the way for future discovery of the NRP materials. In addition, this paper also explores the effect of the number of 2D layers on the Poisson's ratio.
Round 3
Reviewer 1 Report
The manuscript can be accepted in present form.
Author Response
Thank you for all your suggestions on the manuscript, which have been an essential guide to our paper writing and scientific work.
Reviewer 2 Report
Unfortunately, a vacuum layer, independent of its size, does _not_ eliminate periodic boundary conditions. It removes to a certain degree periodic interactions but not the constraints to the degrees of freedom. Therefore, the authors answer is not satisfying. To bring this to an end, I suggest the authors write explicitly in the text that their MD is performed with periodic boundary conditions. Also, please remove the statement that this ensures stability of the structure. It only ensures stability of the structures under the periodic boundary conditions. Furthermore, please define the meaning of “Universal” and “smart optimization”.
Author Response
Response to Reviewer 2 Comments
Point 1: Unfortunately, a vacuum layer, independent of its size, does _not_ eliminate periodic boundary conditions. It removes to a certain degree periodic interactions but not the constraints to the degrees of freedom. Therefore, the authors answer is not satisfying. To bring this to an end, I suggest the authors write explicitly in the text that their MD is performed with periodic boundary conditions. Also, please remove the statement that this ensures stability of the structure. It only ensures stability of the structures under the periodic boundary conditions. Furthermore, please define the meaning of “Universal” and “smart optimization”.
Response 1: Thank you for pointing this out. On page 2, line 40, we have revised the manuscript in accordance with your suggestions. The meaning of“Universal” in the manuscript refers to the fact that the force field used in the MD simulation is a universal force field, which allows for fast energy calculations, geometry optimization, and dynamics simulation studies of material systems. It allows for analysis of various structural parameters, thermodynamic properties, mechanical properties, kinetic properties, and statistical properties of material systems. The meaning of “smart optimization” refers to the designation of the algorithm used in the geometry optimization calculations as a smart algorithm. The Smart algorithm is a cascade of the steepest descent, ABNR, and quasi-Newton methods. On page 2, line 45, and on page 3, line 1, we have added their meanings.
Round 4
Reviewer 2 Report
Okay, I think the manuscript is almost ready for submission. However, please modify the "Computational Methods" such that is clear that the MD calculations are performed with the universal force field (UFF). Don't use the term "Universal" for referring to the UFF. Furthermore, instead of "smart" optimization please write that you used a quasi-Newton method for the structure optimization. With this changes the manuscript is ready for acceptance.
Author Response
Response to Reviewer 2 Comments
Point 1: Okay, I think the manuscript is almost ready for submission. However, please modify the "Computational Methods" such that is clear that the MD calculations are performed with the universal force field (UFF). Don't use the term "Universal" for referring to the UFF. Furthermore, instead of "smart" optimization please write that you used a quasi-Newton method for the structure optimization. With this changes the manuscript is ready for acceptance.
Response 1: Thank you for pointing this out. On page 2, line 42 and page 3, line 1, we have made the relevant changes to the manuscript.
