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

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this paper, the authors concoct a methodology to fabricate large-area monolayers of SnS---a 
prototypical group IV monochalcogenides. As motivated by the authors, several systems in this 
class of materials are piezoelectric and so is the monolayer made by the authors. The material is 
then characterized as is the usual practice. 

I have no qualms about this paper being published in some suitable materials journal. I can see 
that the work will add a useful set of data to the literature. However, there is nothing in this work 
that prompts me to believe it belongs to a journal like Nature Communications. I have learned 
nothing new. There is no new invention nor basic scientific discovery. No novel physical insights. 
Simply a nice use of existing methods to come up with yet another approach to create large scale 
monolayers. 

This paper should be perfectly fine in a normal materials journal which is where it belongs. 


Reviewer #2 (Remarks to the Author): 

In this communication, Prof. Kalantar-Zadeh and colleagues utilized a liquid metal-based synthesis 
method for growing monolayer SnS film and evaluated its performance as piezoelectric 
nanogenerator. This liquid metal-based synthesis method had been widely adopted by many 
research groups including Prof. Kalantar-Zade. In their last study, they synthesized SnO film (ACS 
Nano 2017, 11, 10974−10983). It is great to see this method is quite universal for growing low 
melting point metal-based compounds and potentially extends their applications in large-area 
functional devices. Here I have concerns about the size, uniformity, grain size, grain boundary, 
cracks, wrinkles, overlap layers and byproducts in the as-grown SnS film. However, these 
important data are missing in this work. Furthermore, any improvements in the abovementioned 
aspects of the as-grown SnS film when comparing with previous SnO film (ACS Nano 2017, 11, 
10974−10983)? In addition, some figures have poor quality, I suggest the authors to replace them 
with high quality figures. There are also many typos in the manuscript, reference and supporting 
information. I would like to see all the above and following comments are well addressed before 
recommending publication in Nature Communications. 

The following are some detailed comments: 
1. What is size of the typical SnS film grown by liquid-metal based synthesis method? Please 
demonstrate the detail data in the revised manuscript. 
2. What is the surface coverage of the transferred SnS film on target substrates? It can not be 
100% when we transfer a curved film from liquid droplet to a flat substrate. How do the 
discontinuities (e.g., cracks, wrinkles, overlap layers and byproducts) affect the electrical (hall 
mobility) and piezoelectricity? 
3. P6, line 109. “The near single crystallinity and low concentration of grain boundaries across the 
planes are also identified using dark field TEM”. The TEM gives very local information of a very 
small area. It cannot be used to claim that the SnS film is single crystalline. What is mean grain 
size of SnS film, any direct data? 
3. How to control the products to be SnS, instead of SnS2, what is the key factor (e.g., growth 
temperature, H2S)? If possible, I suggest the author to plot a growth map to link the products and 
growth conditions. 
4. What is the sample for Hall mobility measurements, monolayer of few-layer? What is the 
dimension of the devices? Please provide a SEM image in the SI for evaluation. 
5. What is the atomic ratio of Sn and S in the as-grown SnS film? Refer to the XPS data. 
6. P8, line156. “The homogeneity and reduced grain boundary…” No clear data can support this 
claim. 
7. Fig. S1. I suggest the author to prepare a schematic diagram and label all the main components 
of the setup. Otherwise, the readers may have difficulty to understand the experiment. 
8. Fig. S2. a. Nothing can be seen in Figure S2a. I suggest the authors carefully choose the silicon 
substrates with proper SiO2 thickness which can give the best contrast of monolayer and few-layer 
SnS film. Alternatively, I suggest the authors to use SEM image instead of optical image. b. Here, 
the optical images only give the information of a very small area, we cannot conclude the growth 
method can provide high uniformity wafer-scale SnS film without grain boundaries, cracks, 
wrinkles and overlap layers. 
10. Fig.3. a. What are the thick particles on SnS film. Is it possible to get rid of these byproducts? 
b. Please label the chemical formula of the thin film and the particles in Fig. S3a. 


Reviewer #3 (Remarks to the Author): 

The manuscript reports the fabrication of large-area high quality monolayer SnS and their 
subsequent deployment in a piezoelectric generator. The SnS monolayers are fabricated using a 
liquid-metal based synthesis method, that allows for the production of single crystal, grain-
boundary free 2D flakes of SnS, which can then be transferred to a different substrate to be made 
into a piezoelectric generator. 

I find the fabrication method quite compelling, particularly the fact the yield of this 2D material is 
greatly improved. The transport measurements seem robust and point to a narrow bandgap p type 
semiconductor with high mobility. However, I have concerns related to the piezoelectric properties 
being reported, particularly the device performance and subsequent analysis. There are also other 
issues that I will highlight below, which I hope the authors can address. 

1. In Figure 1a, the middle panel says "printed SnS" and "SnS prints" are also referred to on Page 
7. It is not clear what "print" here refers to, as no printing method has been discussed in the 
methods section, nor in the supporting info. 

2. The dimensions of the piezoelectric device are not given, and hence it is difficult to determine 
whether the whole device area is made up of monolayer SnS. AFM cannot possibly used to 
determine the thickness over millimeter-scale samples, for example, and so Fig 2b cannot be used 
as prove that the whole device is indeed monolayer SnS. 

3. Following the above point, there is no indication as to how good the coverage of the substrate is 
after the transfer process. Are the SnS monolayer connected, or are they not packed densely 
enough. What configuration do they take after transfer. There is no reason to believe that the 
atoms align as perfectly as shown in Fig 1 across the whole sample, as there does not seem to be 
anything in the transfer process that would guarantee the flakes remain aligned, or indeed in 
monolayer form. The authors should indicate how this is achieved in practice, and more 
importantly, how this is verified. 

4. Unfortunately I am not convinced by the arguments put forward regarding the piezoelectric 
output. For one, the very nature of the interdigitated electrodes would negate any piezoelectric 
voltage developed, unless the portions of the flakes between each pair of electrodes were 
physically aligned opposite to its neighbour such that the dipoles were oppositely aligned. In 
piezoelectrics that are ferroelectric, this is achieved through electrical poling, which is not the case 
in the present paper. The question is, how does the piezoelectric charge developed here add up 
across interdigitated electrodes, without any polar direction being explicitly defined between each 
pair of electrodes. 

5. The generated voltages due to impacting may be due to the triboelectric effect, which has not 
been explicitly ruled out, while that due to lateral stretching may be attributed to the flexoelectric 
effect. Both triboelectric and flexoelectric effects would be expected to manifest strongly in a 2D 
material. 

6. The authors have not actually shown any direct measurement of piezoelectric properties of their 
monolayers. Given that they have access to an AFM, the authors should provide piezo-response 
force microscopy (PFM) data to verify the piezoelectric properties, including showing how the 
dipoles are aligned in this material in the device configuration. This is crucial data to show the 
nanoscale piezoelectricity in this material. Without this, points 4 and 5 above cast doubt on the 
interpretation of the device data. 

7. In general, comparing piezoelectric "output" across different materials or devices in the 
literature is meaningless without proper context. For example, what was the mechanical excitation 
in the other studies? The output should at least be normalised to the input mechanical stimulus to 
provide a fair comparison. 

8. The authors have not commented at all on the frequency dependence of the electrical output 
that they observe in Fig 4b. More generally, the output profile shown in Fig4a(ii) would not be 
expected from a piezoelectric that is semiconducting, precisely due to the piezotronic effect that 
the authors demonstrate. One would expect carrier depletion due to the Schottky juntions to give 
rise to a steady-state piezoelectric response, rather than the transient response observed. This 
again points to the origin of the device output being something other than pure piezoelectric. 

In summary, while the fabrication method is quite interesting, I find that the paper does not 
provide compelling evidence of piezoelectricity in this material, particularly without PFM data (e.g. 
it would be interesting to report the measured d coefficient). Also the device geometry and output 
characteristics are not consistent with piezoelectric behaviour, particularly from a semiconductor. 
In light of the above, I am unable to recommend acceptance of this work.