Study of Structure, Morphology and Optical Properties of Cobalt-Doped and Co/Al-co-Doped ZnO Thin Films Deposited by Electrospray Method

Round 1

Reviewer 1 Report

Manuscript ID: ISSN 2076-3417

Paper review for the study of the structure, morphology and optical properties of cobalt doped and Co/Al co-doped ZnO thin films deposited by electrospray method. 

Dear Editor,

Marinov et al.'s manuscript, titled "Study of the structure, morphology and optical properties of cobalt-doped and Co/Al co-doped ZnO thin films deposited by electrospray method," provides a comprehensive exploration of doped ZnO thin films produced through variations in cobalt (Co) and aluminum (Al) additives via the electrospray technique. This study thoroughly analyzed the resulting films' morphology, structure, phase composition, surface roughness, and optical properties. The process involved the deposition of ZnO films with Co (5%) and Co (2.5%) as well as Al (2.5%) additives by an electrospray technique. This method encompassed aerosolizing solutions containing zinc acetate, cobalt acetate, and aluminum nitrate through exposure to high voltage (18 kV), followed by their direction onto a grounded collector maintained at 300°C. The fabricated films were subjected to meticulous examination of their morphology, structure, and phase composition using techniques such as SEM, TEM, XRD, and SAED. Additionally, 3D profile measurements were employed to determine surface roughness, while optical properties were investigated through ellipsometry, optical band gap analysis, and Urbach energy calculations. Notably, the study also encompassed an evaluation of the impact of additives on the final film properties. Notably, while the cobalt doping led to an increase in grain size from 22 nm to 29 nm, intriguingly, no additional peaks attributed to dopant impurities or oxides were observed at a doping concentration of 5%.

The overall problems in the manuscript are listed below:

1. Even though there might be a limited number of articles in the literature related explicitly to ZnO and doped ZnO thin films by the electrospray method, I recommend including them in the introduction section to compare the results of similar structures.

2. As you have mentioned in detail in your article titled "Aluminum-doped zinc oxide thin films deposited by electrospray method," there have been many reports on Al-doped ZnO thin films. Still, I recommend more information about growing Co-doped ZnO thin films by electrospray method.

3. Although the optical properties section has explained the substrate, due to the off-center property of AZO, it would be advisable to specify in the “Materials and Methods” section for the substrate used, along with its dimensions.

4. In your article, it is recommended to give more information about transparent conductive thin films such as AZO in the introduction and to support thin films with reference. To further increase the support of your work, please consider citing the following article titles in the references section. Including these articles can add to the overall quality of your work.

 Ø  Ekmekcioglu, M., Erdogan, N., Astarlioglu, A. T., Yigen, S., Aygun, G., Ozyuzer, L., & Ozdemir, M. (2021). High transparent, low surface resistance ZTO/Ag/ZTO multilayer thin film electrodes on glass and polymer substrates. Vacuum, 187, 110100.

Ø  Turkoglu F., Koseoglu H., Zeybek, S., Ozdemir, M., G. Aygun, G., Ozyuzer, L. ''Effect of substrate rotation speed and off-center deposition on the structural, optical, and electrical properties of AZO thin films fabricated by DC magnetron sputtering'', Journal of Applied Physics, 123, 165104, (2018).

Ø  Ozbay, S., Erdogan, N., Erden, F., Ekmekcioglu, M., Rakop, B., Ozdemir, M., Ozyuzer, L. ''Surface free energy and wettability properties of transparent conducting oxide-based films with Ag interlayer'', Applied Surface Science, 567, 15090, (2021)

5. It is recommended to perform the energy dispersive X-ray (EDX) analysis to determine atomic concentrations of ZnO, AZO, CZO, and CAZO thin films. In addition, it is necessary to strengthen it with references.

6. In the conclusion section, it would be appropriate to emphasize one of the main findings of this paper: This research has assessed three distinct scenarios in which ZnO and ZnO thin films were doped with Al (aluminum) and Co (cobalt) through comprehensive analysis. Particularly regarding characterization results or specific values, the superiority of a particular material should be highlighted.

7. AFM measurements are an essential characterization technique to investigate thin films' morphological, mechanical, and surface properties, assess film quality, and confirm their suitability for specific applications. If your laboratory has an XPS instrument, I recommend specifying the produced films' AFM analyses.

8. XPS measurements play a crucial role in determining the structural characteristics of films and optimizing production processes. If your laboratory is equipped with an XPS instrument, I recommend obtaining XPS measurements for all thin films, as this would enhance the scientific aspect.

9. Although the article contains innovation and originality, you can emphasize that it is more of an original scientific article by expressing its originality with a few small touches.

10. All references should be rechecked in this paper.

Comments for author File: Comments.docx

The english of the manuscript seems OKEY. However, there are minor effects can be checked carefully. 

Author Response

Dear reviewer,

thank you very much for your positive assessment and fruitful comments. Please find as follows our point-by-point responses.

1. Even though there might be a limited number of articles in the literature related explicitly to ZnO and doped ZnO thin films by the electrospray method, I recommend including them in the introduction section to compare the results of similar structures.

Answer: We have already included in the introduction our papers for properties optimization of Al-doped films deposited by the method of electrospray (ES). According to our knowledge ES method has not been applied yet for deposition of Co-doped ZnO films. If the reviewer could give us papers about CZO thin films deposited by ES we will appreciate it very much. We would like to mention here that our main purpose is to demonstrate the successful implementation of ES as a simple, versatile and low-cost method for deposition of pure and doped ZnO films in ambient environment and moderate temperatures. Moreover we have demonstrated in the manuscript that high quality thin films of pure and doped ZnO films with comparable properties to these produced by other means can be produced by ES without postdeposition annealing.

2. As you have mentioned in detail in your article titled "Aluminum-doped zinc oxide thin films deposited by electrospray method," there have been many reports on Al-doped ZnO thin films. Still, I recommend more information about growing Co-doped ZnO thin films by electrospray method.

Answer: According to our knowledge ES method has not been applied yet for deposition of Co-doped ZnO film. If the reviewer could give us papers about CZO thin films deposited by ES we will appreciate it very much. However, as we have mentioned in the answer to question 1 our main purpose is to obtain high quality thin films of pure and doped ZnO films with comparable properties to these produced by other means using simple, versatile and low-cost method such as electrospray that we have already used for AZO films.

3. Although the optical properties section has explained the substrate, due to the off-center property of AZO, it would be advisable to specify in the “Materials and Methods” section for the substrate used, along with its dimensions.

Answer: We have used two types of substrates – silicon wafers (2 inches) and rectangle optical glass substrate with dimensions of 24mm x 36 mm. The last is needed for transmission measurements. Additional information has been added in the revised manuscript.

4. In your article, it is recommended to give more information about transparent conductive thin films such as AZO in the introduction and to support thin films with reference. To further increase the support of your work, please consider citing the following article titles in the references section. Including these articles can add to the overall quality of your work.

 Ø  Ekmekcioglu, M., Erdogan, N., Astarlioglu, A. T., Yigen, S., Aygun, G., Ozyuzer, L., & Ozdemir, M. (2021). High transparent, low surface resistance ZTO/Ag/ZTO multilayer thin film electrodes on glass and polymer substrates. Vacuum, 187, 110100.

Ø  Turkoglu F., Koseoglu H., Zeybek, S., Ozdemir, M., G. Aygun, G., Ozyuzer, L. ''Effect of substrate rotation speed and off-center deposition on the structural, optical, and electrical properties of AZO thin films fabricated by DC magnetron sputtering'', Journal of Applied Physics, 123, 165104, (2018).

Ø  Ozbay, S., Erdogan, N., Erden, F., Ekmekcioglu, M., Rakop, B., Ozdemir, M., Ozyuzer, L. ''Surface free energy and wettability properties of transparent conducting oxide-based films with Ag interlayer'', Applied Surface Science, 567, 15090, (2021)

Answer: We would like to thank to the reviewer for the recommendation. AZO films were subject of our previous papers. In this manuscript we focused our attention on Cobalt and Co/Al co-doped films. Thus we have presented results of AZO only for comparative purposes because we study Co/Al co-doping and we think that it will be more convenient for readers to compare the obtained results for co-doping with these for Al-doping. To summarize, AZO films are not a main subject of the manuscript and we think that including more details about them will distract readers attention from the main topic without improving the manuscript.

5. It is recommended to perform the energy dispersive X-ray (EDX) analysis to determine atomic concentrations of ZnO, AZO, CZO, and CAZO thin films. In addition, it is necessary to strengthen it with references.

Answer: We have used EDS in STEM (Scanning Transmission Electron Microscopy). The mapping of elements confirms the homogenous distribution of elements on entire surface of the films. There are no clusters or areas rich or poor or some elements. The chemical composition is close to the assumed one.

6. In the conclusion section, it would be appropriate to emphasize one of the main findings of this paper: This research has assessed three distinct scenarios in which ZnO and ZnO thin films were doped with Al (aluminum) and Co (cobalt) through comprehensive analysis. Particularly regarding characterization results or specific values, the superiority of a particular material should be highlighted.

Answer: We would like to mention here that it is hard to highlight the superiority of a particular material because the focus of manuscript is not on the particular application. Our main purpose is to demonstrate the successful implementation of electrospray (ES) as a simple, versatile and low-cost method for deposition of pure and doped ZnO films in ambient environment and moderate temperatures. Moreover we have demonstrated in the manuscript that high quality thin films with comparable properties to these produced by other means can be obtained through ES without postdeposition annealing.

7. AFM measurements are an essential characterization technique to investigate thin films' morphological, mechanical, and surface properties, assess film quality, and confirm their suitability for specific applications. If your laboratory has an XPS instrument, I recommend specifying the produced films' AFM analyses.

Answer: We absolutely agree with the reviewer about the power of AFM for characterization of films. In our Institute we have AFM instrument and have measured all samples. The results about the surface morphology from AFM and SEM are similar but we finally decided to present the SEM studies. The main message for both analyses is that only Co influenced the granular morphology and we think that it is up to the authors how to illustrate it. According to the surface roughness, we think that in this particular case 3-dimensional optical profiler is most suitable than AFM because the measurements are performed over larger surface and the local spot differences are overcome to the great extent.

8. XPS measurements play a crucial role in determining the structural characteristics of films and optimizing production processes. If your laboratory is equipped with an XPS instrument, I recommend obtaining XPS measurements for all thin films, as this would enhance the scientific aspect.

Answer: We absolutely agree with the reviewer about the power of XPS for optimizing the production processes. In our previous paper (ref.31) we have used XPS for optimizing the electrospray deposition of ZnO films using different substrate temperatures. However, considering that XPS is expensive, time-consuming and complicated method and the instruments are not commonly available we did not consider XPS measurements in the recent study.

9. Although the article contains innovation and originality, you can emphasize that it is more of an original scientific article by expressing its originality with a few small touches.

Answer: In the revised manuscript we have made some changes considering all reviewers recommendations and hope that the originality is better expressed.

10. All references should be rechecked in this paper.

Answer: All references have been carefully rechecked

Reviewer 2 Report

Authors reported some experimental results about metal-doped ZnO thin films for potential optical application. That is interesting although little lack of novelty. More data and discussion should be provided before it could be accepted. 

1. Line 85, 5ml shoud be 5 mL. The same mistake was seen in line 87, 103, and so on.

2. Line 174-175, "peaks shift with 0.08  and 0.06 degree". That is unreliable. More parameters of instrument (like X-Ray Diffractometer,) and test condictions must be provided detailedly.

3. Only 5% doping concentration was investigated in this paper. why not much more options of doping ratio for comparison?

The English language is acceptable, but the grammar needs to be revisedcarefully.

Author Response

Dear reviewer,

thank you very much for your positive assesment and fruitful comments. Please find as follows our point-by-point responses.

“Authors reported some experimental results about metal-doped ZnO thin films for potential optical application. That is interesting although little lack of novelty. More data and discussion should be provided before it could be accepted. “

Answer: We would like to thank to the reviewer for the assessment that our results are interesting. We would like to mention that the successful implementation of electrospray (ES) as a simple, versatile and low-cost method for deposition of pure and doped ZnO films in ambient environment and moderate temperatures could be regarded as a novelty. Moreover we have demonstrated in the manuscript that high quality thin films with comparable properties to these produced by other means can be produced by ES without postdeposition annealing.

“1. Line 85, 5ml shoud be 5 mL. The same mistake was seen in line 87, 103, and so on.”

Answer: In the revised version of manuscript “ml” was replaced by “mL” in all instances.

“2. Line 174-175, "peaks shift with 0.08  and 0.06 degree". That is unreliable. More parameters of instrument (like X-Ray Diffractometer,) and test conditions must be provided detailed.”

Answer: The measurement step was 0.025 deg because the systematic error of the instrument was determined to be less than 0.015 degree. Therefore the shift of 0.06 degree is 4 time higher than the error. In the case of 0.08 degree shift the difference is more than 5 times. We apologies, but we could not agree with the reviewer that peaks shift of 0.08 and 0.06 degree “is unreliable” because 400%  and 500%  difference could be considered as significant.

“3. Only 5% doping concentration was investigated in this paper. why not much more options of doping ratio for comparison?”

Answer: Actually we have prepared and characterized 10% doped films but we have decided not to show the results for two reasons. Firstly, in our previous paper (ref. 30 in the submitted manuscript) we have investigated 10% Al-doped ZnO films and have observed deterioration of films’ properties with increasing the doping level. For example, the transmittance and crystallinities size decrease, while extinction coefficient, surface roughness and structural disorder increase as compared to 5% doped films. The same trend was observed with Co and Co/Al co-doping (results are not shown in the manuscript). Secondly, as we have already explained in the manuscript, with increasing the dopant concentration to 10% local areas can be found where lattice spacing increases as compared to pure ZnO films. This means that at higher concentration dopant ions have occupied interstitial sites between zinc and oxygen atoms. The last is not beneficial for possible applications as transparent conductive oxide and dilute magnetic semiconductor because may lead to deterioration of properties.

“The English language is acceptable, but the grammar needs to be revised carefully.”

Answer: The language was revised in the entire manuscript.

Reviewer 3 Report

Manuscript ID: applsci-2559160

Title: Study of structure, morphology and optical properties of cobalt doped and Co/Al co-doped ZnO thin films deposited by electrospray method

This manuscript presents the synthesis and characterization of thin ZnO films doped with cobalt (Co) and/or aluminium (Al). The primary focus is on elucidating the impact of the doping and co-doping processes on the film properties. While the paper is well-written and the experimental results appear to be meticulously obtained, certain aspects require attention and improvement by the authors during the paper's revision:

1. The abstract section necessitates rewriting. It should concisely convey the key findings and highlights of the study. Detailed experimental procedures should be excluded from this section.

2. Discuss both the advantages and drawbacks of employing the electrospray method in the production of ZnO films compared to other techniques.

3. Cobalt doping was observed to induce a substantial reduction in particle size, whereas surface morphology changes were marginal for aluminium doping. The authors should provide scientific rationale for these divergent outcomes. Similarly, explanations for the discrepancies observed in the XRD analysis should be elaborated upon.

4. The authors calculated crystallite size using the Debye–Scherrer's formula based on peak positions and full width at half maximum. Did they also utilize the Williamson-Hall Plot method for this calculation?

5. Address the uncertainties associated with the calculated crystallite sizes presented in Table 1. A discussion on the potential sources of measurement error and their impact on the results would be valuable.

6. The authors employed the Tauc plot method to determine the band gap. To further strengthen this approach, I recommend referencing the following articles which provide solid support for the utilization of this method: "Optical Materials 58 (2016), 51-60” and "Applied Physics B 119 (2015), 273-279”. Additionally, including a reference for Equation 3 would enhance the clarity and credibility of the methodology.

7. Provide information on the thickness of the film samples in your study.

8. The observation that pure ZnO exhibits a lower band gap than ZnO doped with Co and Al warrants further explanation. Discuss the main reasons behind this outcome and explore whether different film thicknesses yield similar findings.

9. The authors should elaborate on the potential applications of Co and Al-doped ZnO films. This discussion could serve as a platform for stimulating new avenues of scientific inquiry.

Best regards.

Minor editing of English language required.

Author Response

Dear reviewer,
thank you very much for your positive assesment and fruitful comments. Please find as follows our point-by-point responses.

“1. The abstract section necessitates rewriting. It should concisely convey the key findings and highlights of the study. Detailed experimental procedures should be excluded from this section.”

Answer: We appreciate very much the recommendation of the reviewer. We have thoroughly revised the abstract according to the reviewer’s recommendations: the details about the deposition and characterization of films are removed from the abstract and the main results are highlighted.

“2. Discuss both the advantages and drawbacks of employing the electrospray method in the production of ZnO films compared to other techniques.”

Answer: The advantages of employing electrospray (ES) as deposition methods are many and they are discussed in details in Introduction part of the manuscript. Briefly, ES is simple, versatile, scalable and low-cost deposition techniques that can be performed in ambient environment. There is no need of complicated equipment because ES is being performed in air and uses non-toxic and inexpensive precursors and solvents. Unlike sol-gel method (that is also simple and low-cost method), in ES post deposition annealing is not required because both the moderate substrate temperatures (200-300 ^oC) and small size of the droplets ensure the precursor decomposition during film deposition. In our previous paper (Ref. 30) and in the present manuscript we have demonstrated that ES can produce high quality thin films with comparable properties to these produced by other means.

According to the disadvantages, the low deposition rate (about 3 nm/min), the high voltage (18 kV) used and higher surface roughness of the films compared to ALD films for example could be considered as main disadvantages. However, if the voltage is temporally stable the low deposition rate cannot be regarded as a drawback. If some precautions such as save grounding are taken than the high voltage was not a problem. The roughness of the films is higher but the films are still transparent and their transmittance is more than 80% in average.

3. Cobalt doping was observed to induce a substantial reduction in particle size, whereas surface morphology changes were marginal for aluminium doping. The authors should provide scientific rationale for these divergent outcomes. Similarly, explanations for the discrepancies observed in the XRD analysis should be elaborated upon.

Answer: It is seen from SEM pictures (Figure 2) that for all studied films the grain sizes are bigger than the crystallites size calculated from the XRD patterns (Figure 3). This means that the grains on the surface were built from few crystallites, so an agglomeration of crystallites took place. The decrease of grain size in the case of cobalt doping (figure 2c) could be associated with suppression of crystallites’ agglomeration and the possible reason could be the bigger solubility of Co in ZnO matrix as compared to Al. Similar suppression of aggregation is clearly evident in [10], but unfortunately there is now a possible reason presented.

On the other hand the decrease of grain size leads to grow of the attraction forces between crystallites causing them slightly to aggregate in the case of cobalt-doping. Another possible explanation was given by Abdelkrim et al. [5]. The authors attributed the increase in crystallite size to the distortion in the ZnO host lattice by Co impurities which stimulate the coalescence of the ZnO crystallites [5]. Mir et. al [32] explained the preferential growth in particular plane in terms of surface energy minimization: the preferred orientation is a result of self-ordering caused by the minimization of the crystal surface force energy. Obviously, more studies are needed for unveiling the possible reasons behind the observed preferential growth in (101) plane for CZO films. These are out of the scope of the present manuscript.

The manuscript is revised accordingly and the explanations above are included.

 “4. The authors calculated crystallite size using the Debye–Scherrer's formula based on peak positions and full width at half maximum. Did they also utilize the Williamson-Hall Plot method for this calculation?”

Answer: No, we have not used the Williamson-Hall plot for calculating the crystallites size and the stress in the films. Thank you for the idea, we will consider it in our future studies.

5. Address the uncertainties associated with the calculated crystallite sizes presented in Table 1. A discussion on the potential sources of measurement error and their impact on the results would be valuable.

Answer: The results of the size of crystallites presented in Table 1 are obtained using the position and width of the half maximum of the XRD peaks. The idea behind this study was to estimate the impact of doping on crystallite size. We did not claim that the calculated values are the absolute size values because we are aware that some error sources exist. However, considering that the error sources influence all samples in similar manner (because we used one and the same instrument and the same measurement parameters) we can be sure that the observed trend of the size with doping is correct. Moreover, TEM measurements show similar size. We agree with the reviewer that a discussion of the impact of the potential error sources on the calculated crystallites size is valuable but it is not in the scope of the present study.

6. The authors employed the Tauc plot method to determine the band gap. To further strengthen this approach, I recommend referencing the following articles which provide solid support for the utilization of this method: "Optical Materials 58 (2016), 51-60” and "Applied Physics B 119 (2015), 273-279”. Additionally, including a reference for Equation 3 would enhance the clarity and credibility of the methodology.

Answer: The Tauc plot method is famous and widely used method for determination of optical bandgap. This was the reason not to include a reference in the manuscript. We appreciate the reviewer recommendation and in the revised manuscript we have added a new reference (ref. 39) that is Tauc’s paper. We are confident that the reviewer will agree that citing Tauc’s paper is better than citing "Optical Materials 58 (2016), 51-60” and "Applied Physics B 119 (2015), 273-279”. Moreover the first one considers organic semiconductors and the second one is about modified fitting of absorption coefficient and both are not very relevant to our case.

“7. Provide information on the thickness of the film samples in your study.”

Answer: The thickness of the films is about 200 nm. This information is included in section 3.3.1. In the revised manuscript we included it also in the abstract and in the “Materials and method” section.

8. The observation that pure ZnO exhibits a lower band gap than ZnO doped with Co and Al warrants further explanation. Discuss the main reasons behind this outcome and explore whether different film thicknesses yield similar findings.

Answer: The increase of optical band gap of ZnO after doping was explained in the manuscript by the phenomena called Burstein–Moss effect where the increase of carrier concentration generated from substitution of Zn ions with cobalt and aluminum ions leads to a shift of Fermi level toward the conduction band. This has been already observed for doped ZnO by many authors [10,17,37,40]. It is also observed for many doped semiconductors.

According to thickness dependence, we would like to mention here that for calculation of optical band gap a commonly used approach (that is not quite right) is to use approximate equation for absorption coefficient (alfa) including transmittance and thickness of the films that are measured separately. In our study we used alfa that is calculated from extinction coefficient (k). The last is measured ellipsometrically along with refractive index (n) and thickness of the film. It is well known that n and k are material constants and they do not depend on thickness of the material if the material’s structure and chemical composition are the same for all thicknesses. In our case ZnO films with thickness in the range 80-300 nm exhibit similar properties and have similar optical constants n and k. This means that films with thickness in the range 80-300 nm will have similar optical band gaps.

9. The authors should elaborate on the potential applications of Co and Al-doped ZnO films. This discussion could serve as a platform for stimulating new avenues of scientific inquiry.

Answer: As we have already stated in the introduction, ZnO is regarded as a suitable platform for doping with different dopants thus achieving the appropriate properties for diverse applications such as transparent conductive electrodes [3], thin film photocatalysts [4-6], active media for sensing [7-9], antibacterial agent [4,10] and photoanodes for efficient perovskite solar cells [11].

According to Cobalt doping, one of the sources that fueled the scientific interest is the theoretical prediction of the existence of room temperature ferromagnetism in transition metal doped ZnO [12] and its experimental confirmation in Co-doped ZnO [13-15] as the most striking example. In the introduction we have pointed out some examples of Co-doped films: dilute magnetic semiconductor (DMS) through doping with transition metals (Co, Ni, Mn, etc.) [16-18], optospintronic applications [19] and photocatalysis [20].

We would like to mention here that the manuscript is not so application oriented. Our main purpose is to demonstrate the successful implementation of electrospray (ES) as a simple, versatile and low-cost method for deposition of pure and doped ZnO films in ambient environment and moderate temperatures. Moreover we have demonstrated in the manuscript that high quality thin films with comparable properties to these produced by other means can be produced by ES without postdeposition annealing.

 

Reviewer 4 Report

1. The advantages brought by the research method are not reflected in the abstract.

2. The introduction can supplement the previous research progress in this area, so as to introduce the characteristics of my own research method.

3. EDS can be used to characterize the material distribution on the surface of the film to better explain the relationship between particle size and doping materials under SEM.

4. Explain why Co enhances particle nucleation and crystal growth in ZnO matrix.

 

No specific comments.

Author Response

Dear reviewer,
thank you very much for your positive assesment and fruitful comments. Please find as follows our point-by-point responses.

“1. The advantages brought by the research method are not reflected in the abstract.”

Answer: We have thoroughly revised the abstract according to the recommendations of reviewer 1: the details about the deposition and characterization of films are removed from the abstract and the main results are highlighted.

“2. The introduction can supplement the previous research progress in this area, so as to introduce the characteristics of my own research method.”

Answer: As we have already stated in the introduction, ZnO is regarded as a suitable platform for doping with different dopants thus achieving the appropriate properties for diverse applications such as transparent conductive electrodes [3], thin film photocatalysts [4-6], active media for sensing [7-9], antibacterial agent [4,10] and photoanodes for efficient perovskite solar cells [11]. According to Cobalt doping, one of the sources that fueled the scientific interest is the theoretical prediction of the existence of room temperature ferromagnetism in transition metal doped ZnO [12] and its experimental confirmation in Co-doped ZnO [13-15] as the most striking example.

In the introduction we have pointed out some examples of Co-doped films: dilute magnetic semiconductor (DMS) through doping with transition metals (Co, Ni, Mn, etc.) [16-18], optospintronic applications [19] and photocatalysis [20]. Moreover we have reviewed the deposition techniques used for CZO thin film: atomic layer deposition [18], pulsed laser deposition [21,22], magnetron sputtering [16,23], chemical bath deposition [24,25], sol-gel combined with spin coating or dip-coating [4,26-28] and spray pyrolysis [5, 29].

According to our knowledge, Electrospray (ES) method has not still applied for deposition of CZO films although it has a lot of advantages clearly stated in the manuscript. This is why we have selected it and have demonstrated its successful implementation as a simple, versatile and low-cost method for deposition of pure and doped ZnO films in ambient environment and moderate temperatures. Moreover we have demonstrated that high quality thin films with comparable properties to these produced by other means can be produced by ES without postdeposition annealing.

“3. EDS can be used to characterize the material distribution on the surface of the film to better explain the relationship between particle size and doping materials under SEM.”

Answer: Instead of EDS in SEM we have used EDS in STEM (Scanning Transmission Electron Microscopy). The mapping of elements confirms the homogenous distribution of elements on entire surface of the films. There are no clusters or areas rich or poor or some elements.

“4. Explain why Co enhances particle nucleation and crystal growth in ZnO matrix.”

Answer: In the revised manuscript we have added more details about this topic. Briefly, it has been assumed that the decrease of grain size in the case of CZO films leads to grow of the attraction forces between crystallites causing them slightly to aggregate. Another possible explanation was given by Abdelkrim et al. [5]. The authors attributed the increase in crystallite size to the distortion in the ZnO host lattice by Co impurities which stimulate the coalescence of the ZnO crystallites [5].

We would like to mention that the reasons behind the decrease of grans size in the case of CZO films and preferential growth are also discussed in the revised manuscript.

Reviewer 5 Report

This manuscript described an electrospray synthesis method for doped ZnO films, coming with the characterization and analysis. Mulitple characterizations were made. It is to some extent useful to applied industry. I would suggest to publish after these comments addressed. 

1. In abstract, the authors need to mention the innovation/importance of this work, not only preparation methods, characterizations.

2. Personally I feel AZO,CZO,CAZO is not very good abbreviation, may cause some confusion, but it is good for now.

3. I would suggest in optical properties and conclusion section, authors could add more reasons for doing the tests and why this doped ZnO films is useful, why is variable with other research work.

Thanks and I would suggest publish after revision.

Author Response

Dear reviewer,

thank you very much for your positive assessment and fruitful comments. Please find as follows our point-by-point responses.

1. In abstract, the authors need to mention the innovation/importance of this work, not only preparation methods, characterizations.

Answer: We have thoroughly revised the abstract according to the recommendations: the details about the deposition and characterization of films are removed from the abstract and the main results are highlighted.

2. Personally I feel AZO,CZO,CAZO is not very good abbreviation, may cause some confusion, but it is good for now.

Answer: The abbreviations AZO, CZO, CAZO have been already used in the literature. In order to avoid confusion we defined them at first instance in the manuscript.

3. I would suggest in optical properties and conclusion section, authors could add more reasons for doing the tests and why this doped ZnO films is useful, why is variable with other research work.

Answer: As we have already stated in the introduction, ZnO is regarded as a suitable platform for doping with different dopants thus achieving the appropriate properties for diverse applications such as transparent conductive electrodes [3], thin film photocatalysts [4-6], active media for sensing [7-9], antibacterial agent [4,10] and photoanodes for efficient perovskite solar cells [11].

According to Cobalt doping, one of the sources that fueled the scientific interest is the theoretical prediction of the existence of room temperature ferromagnetism in transition metal doped ZnO [12] and its experimental confirmation in Co-doped ZnO [13-15] as the most striking example. In the introduction we have pointed out some examples of Co-doped films: dilute magnetic semiconductor (DMS) through doping with transition metals (Co, Ni, Mn, etc.) [16-18], optospintronic applications [19] and photocatalysis [20].

Determination of optical properties like refractive index and absorption coefficient and their use for further determination of optical band gap and Urbach energy is essential part of the comprehensive characterization of material and can be very useful from the viewpoint of possible applications. According to the comparison with other work, we tried to compare all results obtained with data already published. Moreover, we tried to unveil and discuss the reasons behind all phenomena observed. Additional discussion and comparison is presented in the revised manuscript according to the recommendations of referees. 

Reviewer 6 Report

In general, the article is written at a fairly high level. But there are some improvements to be made.

1. In the methodology section, you should indicate how many samples were made for different types of studies.

2. Structuring the Methodology section. Make subsections: Coating, Structure study, ...

3. In figure 2, there are no dimensions above the micrometer rulers.

4. It would be nice if you could explain or put forward hypotheses what is the reason for the presence of the concentration threshold of the influence of cobalt (lines 155-165)

5. When you describe Figure 2 and talk about grain sizes, give numerical values. Expand the description, perhaps add a table with grain size values.

6. In the conclusions, you should also give the numerical values of the grain sizes. Also give percentage changes in properties.

7. Write in the conclusions or at the end of the research part about the possibilities of applying the results of your work.

Author Response

Dear reviewer,
thank you very much for your positive assessment and fruitful comments. Please find as follows our point-by-point responses.

1. In the methodology section, you should indicate how many samples were made for different types of studies.

Answer: For deposition of thin films two types of substrates are used: silicon wafers 2 inches in diameter and double-polished optical glass with rectangle shape and dimensions of 24 mm x 36 mm. The text in “Materials and methods” section was revised accordingly.

2. Structuring the Methodology section. Make subsections: Coating, Structure study, ...

Answer: According to the referee’s recommendation the “Materials and methods” section was divided to three subsections: 2.1 Thin films deposition; 2.2 Characterization of morphology and structure; 2.2. Characterization of optical properties

3. In figure 2, there are no dimensions above the micrometer rulers.

Answer: The dimension of scale bar was denoted in the caption to the figure 2

4. It would be nice if you could explain or put forward hypotheses what is the reason for the presence of the concentration threshold of the influence of cobalt (lines 155-165)

Answer: We use the hypotheses of concentration threshold in order to explain why we observed crystallites growth for CZO films but did not observed it for CAZO films. In Ref. 17 Chanda et al. have found that crystallites size increases from 27 nm for ZnO films to 30 nm and 33 nm for 5% and 10% cobalt doped films, respectively. They explained it with “increased cobalt concentration in ZnO matrix which has increased the nucleation of particles as well as has enhanced the growth of crystallites”. On this basis we assumed that 2.5% concentration in CAZO films is not enough to “increase the nucleation of particles” and to “enhance the growth of crystallites”. We would like to point the referee attention to the expanded discussion in section 3.2 in the revised manuscript that relates the grain size with crystallites size.

5. When you describe Figure 2 and talk about grain sizes, give numerical values. Expand the description, perhaps add a table with grain size values.

Answer: We have measured the grain size of all samples and presented the values in Table 1. We expanded the discussion of morphology changes including the numerical values of the grain size.

6. In the conclusions, you should also give the numerical values of the grain sizes. Also give percentage changes in properties.

Answer: We have revised the conclusion, added the numerical values of the grain size and calculated the percentage of the decrease in the case of cobalt doping.

7. Write in the conclusions or at the end of the research part about the possibilities of applying the results of your work.

Answer: In the revised conclusion we have mentioned the possible applications of the results.

 

Round 2

Reviewer 3 Report

The authors have made improvements to their manuscript. The paper is now suitable for acceptance in its current form.

 

 

Minor editing of English language required.

Reviewer 4 Report

Accept as it is.

Reviewer 6 Report

Thanks to the authors for their work. The authors corrected my comments. The article may be published.