Review Reports - Mechanical Analysis and Experimental Studies of the Transverse Strain in Wrinkled Metallic Thin Films

Round 1

Reviewer 1 Report

In the future, the authors should use nanoscale tomography to precisely analyze internal discontinuities in thin films. In the caption under Figure 4 (a), materials should be used instead of metals.

Author Response

Comment 1: In the future, the authors should use nanoscale tomography to precisely analyze internal discontinuities in thin films. In the caption under Figure 4 (a), materials should be used instead of metals.

Response: Thanks for your suggestions. Using nanoscale tomography to precisely analyze internal discontinuities can help us to have a deeper understanding of the formation of the microcracks in wrinkled metallic films, such as the influence of impurities in the film. We will actively consider using this method in the future. We have changed “metals” to “materials” in the caption of Figure 5a (the original Figure 4a).

Comment 2: Extensive editing of English language and style required

Response: We have proof-read all the manuscript to correct the grammatical mistakes overall.

Reviewer 2 Report

In this paper the authors attempt to explore the factors influencing the
formation of microcracks in wrinkled metallic films.The topic is actual.
The paper is of sufficient novelty.
However the authors employ a very simplified mechanical model to derive
formulas to be verified with experimental tests. In my opinion they should
first answer the following questions: is the wrinkled structure homogeneous?
may the microcracks due to the imperfect adhesion? Note that more accurate
models are present in literature.
Besides,experimental results (Figs.4 and 5)suggest that discussion concerning
the strains of continuous and strip films must be more accurate from a
theoretical point of view.

Author Response

Comment 1: In this paper the authors attempt to explore the factors influencing the formation of microcracks in wrinkled metallic films. The topic is actual. The paper is of sufficient novelty.

Response: Thanks for your positive comments on the novelty of our work.

Comment 2: However the authors employ a very simplified mechanical model to derive formulas to be verified with experimental tests. In my opinion they should first answer the following questions: is the wrinkled structure homogeneous? may the microcracks due to the imperfect adhesion? Note that more accurate models are present in literature.

Response: The morphology of wrinkled structure is uniform and controllable. The wavelength and amplitude of the wrinkled structure are well related to the thickness of the film and the prestrain. These have been reported in several previous works (including our previous work), such as Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 15607-15612, Appl. Phys. Lett. 2016, 108, 102409, ACS Nano 2016, 10, 4403-4409.

If the adhesion between film and substrate is imperfect, the substrate will not exert uniform force on the film after releasing the prestrain. Then the uniform wrinkled structure will not appear. In addition, the films are deposited by magnetron sputtering, which usually makes the thin film and the substrate have good adhesion. So we don’t think the microcracks are due to the imperfect adhesion between film and substrate.

Jiang et al. present a very good model to solve the problem of wavelength and amplitude of wrinkled structure (Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 15607-15612). In our model, we try to deal with microcracks caused by transverse strain in wrinkled structure.

Comment 3: Besides, experimental results (Figs.4 and 5) suggest that discussion concerning the strains of continuous and strip films must be more accurate from a theoretical point of view.

Response: For the strain of blank substrate, we conducted a more in-depth analysis, as shown in Table 1, error of the formula is about 3.7%, which is far less than the error of the strip film (~100 %). Considering the errors in the width measurement and prestrain measurement, the theoretical formula of blank substrate strain can predict the actual situation well. There is an error of the same order of magnitude as the film strain in the macroscopic measurement by optical microscope. So we can only make qualitative analysis of the results rather than quantitative analysis.

Reviewer 3 Report

Dear Authors,

I have read your paper titled "Mechanical Analysis and Experimental Studies of the Microcracks in Wrinkled Metallic Thin Films". In my opinion the topic is interesting and paper fulfill aims and scope of the journal.

General remarks:

I have some suggestions, which may be helpfull in improving your work.

- Line 29 - please extend this part of introduction to avoid citation of 9 work in one bracket [1-9].

- The description of figures 1 and 3 are illegible

- Conclusions are too general, no reference to the numerical results.

- I propose to write the most important conclusions in points. It will be more readable for potential readers.

Regards,

Author Response

Comment 1: I have read your paper titled "Mechanical Analysis and Experimental Studies of the Microcracks in Wrinkled Metallic Thin Films". In my opinion the topic is interesting and paper fulfill aims and scope of the journal. I have some suggestions, which may be helpfull in improving your work.

Response: Thanks for your positive comments and suggestions.

Comment 2: Line 29 - please extend this part of introduction to avoid citation of 9 work in one bracket [1-9].

Response: We have extended the Introduction and added more references to introduce the background.

“With the development of the application of wearable equipment, stretchable electronic devices have attracted wide attention [1-3]. They have a wide range of applications including disposable electronics, electronic skins and flexible displays, due to their reliable portability, excellent stretchability and bendability, fit to the human body, elastic deformation, etc. [4-10]”

“In recent years, wrinkling structures, which can greatly improve the stretchability of the thin films, have been widely used in the preparation of stretchable supercapacitors, field effect transistor, spin valve, diodes, integrated circuits, etc. [9-12, 17-25]”.

Comment 3: The description of figures 1 and 3 are illegible

Response: We have divided Figure 1 into two figures. The descriptions of Figures 1, 2 and 4 have been revised.

Comment 4: Conclusions are too general, no reference to the numerical results. I propose to write the most important conclusions in points. It will be more readable for potential readers.

Response: According to your suggestions, we have revised the Conclusions and added some numerical results.

“According to the experimental results, microcracks can be basically avoided when the 100 μm wide strip film is spaced 200 μm-300 μm, the specific parameters can be adjusted more conveniently according to the formula to meet the performance requirements.”

Reviewer 4 Report

REVIEW

on article

Mechanical Analysis and Experimental Studies of the Microcracks in Wrinkled Metallic Thin Films

Tongxin Nie, Baomin Wang, Bo Liu, Yali Xie, Huali Yang, Mingyuan Zhu and Run-Wei Li

SUMMARY.

The article discusses the stress-strain state and features of cracking thin wrinkled films. The authors applied thin metal films to the surface of the stretched substrate. After the pre-deformation is removed, the horizontal direction of the substrate will try to contract and in the vertical direction, the substrate will try to expand. If the preliminary deformation of the substrate is large, then after the removal of the preliminary deformation, many microcracks appear in the wrinkled film.

In this regard, the article devoted to the interesting problem of modeling stress-strain state and features of deformation of thin wrinkled films, which finds its application in microelectronics and other industries.

The reference list contains 18 items.

COMMENTS.

The Abstract must be extended and revised in accordance with the requirements of the journal. Editors strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: Describe briefly the main methods or treatments applied; (3) Results: Summarize the article's main findings; and (4) Conclusions: Indicate the main conclusions or interpretations. The abstract should be an objective representation of the article.
The Introduction is too small and observes only 18 items. The first 9 references are not reviewed, just mentioned.
The final part of the Introduction should contain the formulation of a scientific problem and the main aim of the article. Why is it so important? This will attract the readers' attention and strengthen the article.
The title does not match the content of the article. Cracking is not considered, only the deformed state of the films.
All formulations of deformations are valid only within the framework of the theory of small elastic deformations. Real materials have elastoplastic deformation, which is not taken into account in the article.
Line 70. The symbols of deformations and Poisson's ratio must be indicated at the first mention.
Line 108. Which integral? Please reformulate the phrase.
Figure 1a I recommend to give in a separate figure.
The article is missing a Discussion section. In this section, it is necessary to give a deep analysis of the results obtained and to compare them with the data obtained by other researchers. This will surely attract the readers' attention and strengthen the article.
There are many common phrases in the Conclusion. It is necessary to reflect the specific results obtained by the authors.

In general, the article solves an interesting problem. However, there are many ambiguities in the article, so I recommend the article for publication after major corrections.

Author Response

Comment 1: The Abstract must be extended and revised in accordance with the requirements of the journal. Editors strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: Describe briefly the main methods or treatments applied; (3) Results: Summarize the article's main findings; and (4) Conclusions: Indicate the main conclusions or interpretations. The abstract should be an objective representation of the article.

Response: Thanks for your suggestions. We have extended and revised the Abstract in accordance with the requirements of the journal.

Comment 2: The Introduction is too small and observes only 18 items. The first 9 references are not reviewed, just mentioned.

Response: We have extended the Introduction and added more references to introduce the relevant background. The newly added references are marked in red in the article.

Comment 3: The final part of the Introduction should contain the formulation of a scientific problem and the main aim of the article. Why is it so important? This will attract the readers' attention and strengthen the article.

Response: We further elaborated the existing problems and the significance of our research in the final part of the Introduction.

“However, the wrinkled metallic thin films are often accompanied by many microcracks parallel to the prestrain direction, which will affect the performance of the device. At present, there is no systematic study on the formation of microcracks, so it is difficult to determine how various factors affect the generation of microcracks, and what are the main and secondary factors, understanding these can better avoid the generation of microcracks. In this work, by preparing wrinkled structure with different prestrains and different strip interval of metallic thin films, we explored the factors influencing the formation of microcracks in wrinkled metallic thin films. The transverse strain caused by Poisson effect of film is the cause of microcracks, and the formula of transverse strain of the film is obtained through mechanical analysis, which is verified by experimental results in different wrinkled metallic thin films. The analysis and conclusions are very important for the preparation of stretchable wrinkled metallic thin films with good properties.”

Comment 4: The title does not match the content of the article. Cracking is not considered, only the deformed state of the films.

Response: After considering your suggestion, we changed the title as “Mechanical Analysis and Experimental Studies of the Transverse Strain in Wrinkled Metallic Thin Films”

Comment 5: All formulations of deformations are valid only within the framework of the theory of small elastic deformations. Real materials have elastoplastic deformation, which is not taken into account in the article.

Response: During the experiments, the prestrain is less than 50%. The PDMS substrate is elastic deformation in this range, which can be proved by the linear relationship of stress-strain curve in Figure 4c. For the metallic film, with the increase of strain, it will change from elastic deformation to elastic-plastic deformation, so we calculate the average modulus of the film in elastic deformation and elastic-plastic deformation process in Figure 4d to reduce the error, but if we want to further optimize the model, elastoplastic deformation must be taken into account.

Comment 6: Line 70. The symbols of deformations and Poisson's ratio must be indicated at the first mention.

Response: We've indicated them at the first mention.

Page 2, “Khang et al. [23] used chemical bonds to bond monocrystal strips to the prestrain (ε_pre) polydimethylsiloxane (PDMS)”

Page 2, “due to the Poisson effect (Poisson's ratio ν)”

Comment 7: Line 108. Which integral? Please reformulate the phrase.

Response: This sentence has been revised as “The average strain of the substrate can be calculated through dividing the strain integral by the substrate thickness.”

Comment 8: Figure 1a I recommend to give in a separate figure.

Response: We have divided Figure 1 into two figures.

Comment 9: The article is missing a Discussion section. In this section, it is necessary to give a deep analysis of the results obtained and to compare them with the data obtained by other researchers. This will surely attract the readers' attention and strengthen the article.

Response: we have made further analysis of the results and added some sentences in the corresponding parts.

Page 10, “As shown in Table 1, among the 15 groups of blank substrate strain data, only 4 groups of theoretical values exceed the range of experimental values, which are marked in red. In addition, all errors between theoretical values and experimental values are less than 10 %, and the average error is about 3.7%, which is far less than the error of the strip film (~100 %). Considering the errors in the width measurement and prestrain measurement, the theoretical formula of blank substrate strain predicts the actual situation well.”

Page 11-12, “The parameters can be adjusted according to the formula when preparing stretchable wrinkled films without microcracks for wearable devices. The maximum stretching amount of human skin can reach 50 % [26], so the prestrain should be more than 50 %, then the thickness of the films becomes the main factor affecting the wavelength and amplitude of the wrinkled structure [23], which will affect properties of the devices [10]. Therefore, the thickness of the films has an optimal range to ensure the properties of the device, which may be 30 nm or 50 nm. The remaining parameters that are most easily changed are the thickness of the substrate and the width and interval of the strip films. They can be adjusted flexibly according to different experimental conditions. For example, for 30 nm thick and 100 μm wide wrinkled strip film with 50 % prestrain and 200 μm thick substrate, the strip interval of 200 μm -300 μm can prevent the generation of microcracks, which is consistent with our previous work [25]. However, the model we established is still relatively simple, and many factors have been simplified. For example, the strain of the substrate under the strip film is approximately linear (Figure 6a), which is more complicated in practice. The stress-strain curve of the striped film is not a simple linear relationship, because elastic-plastic deformation will occur when the strain of the film is large. Therefore, the average modulus of the film in strain process is experimentally calculated to reduce the error in Figure 4d. In addition, there will always be some impurities in the strip films, where microcracks are more likely to occur, which is where we simplify. Generally speaking, our model needs further research and optimization, which is also the direction of our future research”

Comment 10: There are many common phrases in the Conclusion. It is necessary to reflect the specific results obtained by the authors.

Response: According to your suggestions, we have revised the Conclusions and added some numerical results.

Round 2

Reviewer 2 Report

The paper is almost worth publishing because the authors proposed a new version based on some of the comments of the reviewers and thus improving the quality of the previous paper. In my opinion, the authors should have emphasized better that their theoretical model is very approximate as it did not take into account important phenomena such as buckling, non-homogeneity and dynamic behaviour of the stucture.

Reviewer 4 Report

All my comments have been taken into account.

The necessary corrections in the text of the article have been made. The graphical part looks better. The list of references has been expanded to include 25 sources.

I recommend the article for publication in present form