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

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 


In this communication, Chang et al. reported the fast growth of large-area MoS2 single crystal 
continuous film via a novel self-capping vapor-liquid-solid (SCVLS) method. The as-grown 
millimeter-scale MoS2 single crystal has high electron mobility up to 33 cm2/Vs on-off ration of 
5×108 and current density up to 230 uA/um. However, this paper needs more solid data to prove 
the advances of the SCVLS method. I would like the see all the following comments are well 
addressed before recommending publication in Nature Communications. 

The following are detailed comments: 
1. The SCVLS method is quite simple and universal with great potential in growing many other 2D 
TMDs and doped-TMDs. Here I suggest the authors demonstrated more data on these aspects. 
Only report the growth of millimeter MoS2 single crystals is kind of weak for publication in Nature 
Communications. 
2. Since the morphology of SiO2 film changes a lot after growth, I suggest the authors 
demonstrate more data on the flatness of as-grown and transferred large MoS2 flakes. Are there 
any cracks in the crystals which caused by the incompatible thermal expansion between MoS2 
flakes and growth substrates? 
3. Uniformity of the large MoS2 single crystals. 
A. Optical uniformity: Please provide fluorescence images of the large MoS2 flakes. 
B. Electrical uniformity: It is not reasonable to choose the three far-apart regions (Edge of MoS2 
flakes). The variations of crystallinity or electrical property are always seen in the edge and center 
of MoS2 flakes. More devices should be tested to clarify the electrical uniformity across the large 
MoS2 flakes, from the center to edge. Here, only a few large MoS2 flakes were studied. How about 
the electrical uniformity of MoS2 flakes and films over the large, centimeter-scale substrates? More 
FET devices should be tested and demonstrated. In addition, when test the FETs, please apply the 
back-gate bias both forward and backward. I strongly suggest the authors demonstrate all the 
transport curves which will give the readers more information about the hysteresis and the 
crystallinity of MoS2 flakes. 
4. Layer-controlled growth is a very important aspect in CVD growth of 2D TMDs. Please 
demonstrate more data about the electrical quality of as-grown bilayer and trilayer MoS2 flakes. 
This information can be a guideline for the community. 
5. Page 6, after the reaction between NaF and MoO3, it is not proper to say the residual is NaF 
matrix, most of the NaF may be consumed. It is better to have cross-section TEM image to 
evaluate the structure after MoS2 growth. Please change the “NaF matrix” with a more proper 
phrase in the manuscript. 
6. How to evaluate the crystallinity of MoS2 monolayer by Raman spectroscopy, any reference? 
The peak separation of 19.5 cm-1 can only give us the information of monolayer, not crystallinity 
information. It is not accurate to simply conclude high-crystallinity from Raman spectra. 
7. Page 7 Line 78, Figure 3e, NO direct evidence of large-area continuous film. NO information of 
average grain size of 450 μm. 
8. How can you control the sulfurization timing before and during the liquid’s rise to the surface? 
9. Page 11 Line 263. NO evidence of 100% coverage of MoS2 monolayer. Thick layers can be seen 
with a high coverage on the monolayer MoS2 film (Figure 3d). 
10. Page 19 Figure 2. The insets in Figure 2b and 2c are too small to view. 
11. Page 20 Figure 3g. The comparison should be more comprehensive. I would like to see more 
comprehensive reference data are plotted in Figure 3g, e.g. 1. Nature Communications 9(2018) 
919; 2. Nanoscale 11(2019) 16122-16129, etc. 
12. Page 22 Figure 5d and 5f. The insets can be presented in SI with a broad view. 
13. Figure S3. It is better to have atomic-resolution STEM images for the as-grown MoS2 crystals. 
14. Figure S4. It is kind of weak to say full overage of MoS2 film with only one photo. More solid 
data should be presented. 
15. Figure S6b. There is no height bar for the AFM images. It is quite random format when the 
scale bars were added in the figures. I suggest the authors pay more attention to these details. 
16. Figure S8. What is the meaning of “MoS2 vapor-pressure”? 
17. There are several related papers were published recently, please clearly address the 
advancements of this work over the following works: 
1. Nature Communications 9(2018) 919; 
2. Nature Chemistry 11(2019) 730-736; 
3. Nanoscale 11(2019) 16122-16129. 



Reviewer #2 (Remarks to the Author): 

The authors report a self-capping method to fabricate large grain continuous MoS2 mono- and bi-
layers. The fast growth rate and coverage of the resulting film are quite impressive. This work is 
interesting in general to the field and the approach adopted quite novel. However, there are still 
some issues to be addressed. 
1, Insufficient relevant references including large grain bilayer and multilayer growth technique by 
using different substrate and gas ambient. 
2, Details of the MoO3 layer and NaF layer growth are lacking. Also the MoS2 growth dependence 
on the NaF thickness around 15 nm needs to be provided. Why 15 nm is the optimal thickness? 
Will 10 nm or 20 nm be better? A step from 5 nm to 15nm to 30 nm is too large. 
3, The excessive seeds and extra smaller multi-layer regions in the monolayer grains can cause 
performance degradation and uniformitu issues in devices. How to elimiminate these using the 
current growth technique? 
4, In Fig.5, the device electric characteristics are not solid. Detailed IdVd needs to be provided and 
the detailed mobility calculation is also questionable. The IdVg curve shows a superlinear trend at 
higher Vbg which could cause overestimation of mobility. 
5, The claims and analysis on MIT are not convincing, since this cross-over can be caused by the 
ohmic contact and carrier injection at various temperatures as previous reports pointed out. 
6, Vds=8V is considered unusually large even for a 1.48 um gate device, and the resulting on off 
ratio is questionable in Fig. 5f, without multiple devices and gate leakage current. 
7, The overall quality of the figures are bad, also the formality is quite careless. For example, in 
Fig.5f, Vd=8 is even without unit. 



Reviewer #3 (Remarks to the Author): 

In this work, the authors reported the controlled growth 2D MoS2 by SCVLS method using MoO3, 
NaF and S as the reaction sources, resulting in large-area single-crystal MoS2 flakes. And the 
excellent electrical properties of MoS2 FETs indicated as-synthesized MoS2 flakes with low defects 
have been obtained. Meanwhile, through SCVLS method, monolayer MoS2 can full-coverage a 3 × 
3 cm2 c-plane sapphire substrate with a relatively large average grain size. Therefore, I'd like to 
recommend its publication in Nature Communications. However, the following issues need to be 
clarified/addressed before acceptance. 

(1) There are some white dots in the most of MoS2 flakes, as showed in optical microscopy images 
(Figure 1g, 1h), suggesting that as-obtained MoS2 nanosheet with triangular shape and 1mm in 
length is not single-crystal sample. And I think the white dots play an important role in growth 
mechanism and the quality of MoS2 flakes, so it is necessary to characterize the white dots. In 
addition, the author just judges the layer number of MoS2 flakes by optical microscopy images, 
which is not precise enough. I think the precise method of thickness, e.g., AFM, etc, should be 
shown. 

(2) The authors claimed that MoO3 vaporized and penetrated through the SiO2 diffusion 
membrane at growth temperature, so the author should explain the mechanism of SiO2 
penetrated by MoO3, which is important for readers to understand the mechanism of the SCVLS 
method proposed by the authors. In addition, the authors claimed that liquid precursor rose to the 
NaF matrix surface through reactive digging and capillary effect, yet, generally speaking, MoO3 
vapor will break SiO2 layer and remain holes and trenches on its surface because there is not 
reaction between MoO3 and SiO2, so after the growth finishes, the SiO2 layer should be carefully 
characterized to check the surface of SiO2 layer can deeply explaining the growth mechanism. In 
addition, the reaction of MoO3 and NaF will expand NaF and provide room for liquid precursor, so it 
should be explored further to make clear whether the holes and trenches appear on the NaF, which 
is important for how the liquid precursor rise to the NaF matrix surface. 

(3) For FET, the authors fabricated and measured 18 FETs, which show 33 cm2V-1s-1 of average 
mobility and 5*108 of on/off ratio, but some things as followed are indistinct: 1) the mobility, 
on/off ratio, image, thickness of each device were not found in main text and SI, pls give AFM 
tests and some channel materials may be not monolayer because some white dots are found in 1-
mm MoS2 crystal ; 2) the author should add one column in the Table 1, which indicates the 
thicknesses of the measured samples because the comparison of FET properties of different 
thicknesses MoS2 is meaningless.