Tuning the Optical Properties of WO₃ Films Exhibiting a Zigzag Columnar Microstructure

Round 1

Reviewer 1 Report

The authors have described the deposition method to create WO3 zigzag structure via GLAD technique by varying titled angle during deposition. This manuscript is well written, however there is some minor comments that needs to be addressed:

• What is supplier of substrates in this work? Which type of Glass is used? Why was different substrates introduced in this work?
• How was N Zigzag calculated? I don’t understand on basis of which calculation was it made.
• Figure 2: SEM images are nice but it is pity that scale bars are not shown. On which substrate it was deposited? Maybe it would be nice if the authors can draw zigzag structure schematically on cross-sectional SEM images? For example, zigzag structure is bit difficulty to see on Figure 2f due to high N number.
• The section 3.3 (Microstructure vs RI) is well written but it would be nice to see the table overview of all obtained results to have good overview of this work.

Author Response

The authors have described the deposition method to create WO3 zigzag structure via GLAD technique by varying titled angle during deposition. This manuscript is well written, however there is some minor comments that needs to be addressed:

1. What is supplier of substrates in this work? Which type of Glass is used? Why was different substrates introduced in this work?

Answer

(100) silicon wafer and standard microscope glass have been used for depositing zigzag WO₃ films. We used these two types of substrates as function of the characterization, i.e. glass for optical properties and silicon for cross-section views by SEM. An explanation has been added in the manuscript, accordingly.

1. How was N Zigzag calculated? I don’t understand on basis of which calculation was it made.

Answer

The number of zigzags N has been determined from experimental conditions, i.e. the number of 180° rotations gives the number of zigzags. A sentence has been added in the manuscript.

1. Figure 2: SEM images are nice but it is pity that scale bars are not shown. On which substrate it was deposited? Maybe it would be nice if the authors can draw zigzag structure schematically on cross-sectional SEM images? For example, zigzag structure is bit difficulty to see on Figure 2f due to high N number.

Answer

The scale is given on the top right and for all pictures. Films have been deposited on (100) Si for these SEM observations. A sentence has been added in the figure caption to specify it. As suggested by the reviewer, a scheme has been added representing the zigzag architecture.

1. The section 3.3 (Microstructure vs RI) is well written but it would be nice to see the table overview of all obtained results to have good overview of this work.

Answer

We think that a table would be redundant with results presented in figure 7. So, we prefer keeping only figure 7.

Reviewer 2 Report

Tha manuscript shows some experimental results on tungsten oxide thin films deposited on glass by magnetron sputtering together with
GLancing Angle Deposition to produce zig-zag columnar structures. In particular the authors show the trasmittance of samples with different 
number of zig-zag in the visible range. By means of the Swanepoel's method they were able to extract from them the refractive index, the 
extinction coefficient and the absorption coefficient. The paper is well written and very well organized. The introduction focuses on the main
purpose, the results are well represented in the figures and their discussion is convincing. My only concern is about the absence of a figure 
showing the optical transmittance also at tilt angle of 75°, since the the authors measured it at 75°, 80° and 85°, as it is also reported in Figure 7.
My last comment is then on line 115. Here the authors describe the mask used for measuring the optical properties, by mentioning possible 
probing of a thickness gradient, but they do not quantify how much it can be. Beyond those comments which represent mostly curiosities, I consider
the manuscript worthy to be published on Coatings.

Author Response

1. The manuscript shows some experimental results on tungsten oxide thin films deposited on glass by magnetron sputtering together with GLancing Angle Deposition to produce zig-zag columnar structures. In particular the authors show the transmittance of samples with different number of zig-zag in the visible range. By means of the Swanepoel's method they were able to extract from them the refractive index, the extinction coefficient and the absorption coefficient. The paper is well written and very well organized. The introduction focuses on the main purpose, the results are well represented in the figures and their discussion is convincing. My only concern is about the absence of a figure showing the optical transmittance also at tilt angle of 75°, since the the authors measured it at 75°, 80° and 85°, as it is also reported in Figure 7.

Answer

It is right that optical transmittance has been measure for tilt angles of 75, 80 and 85°. If we only plot optical transmittance vs. wavelength for 80°. If we also plot the other angles, it will be tricky to distinguish all spectra. In addition, the other angles (i.e. 75 and 85°) do not bring supplementary information, since interference fringes with different amplitudes are recorded.

2. My last comment is then on line 115. Here the authors describe the mask used for measuring the optical properties, by mentioning possible probing of a thickness gradient, but they do not quantify how much it can be.

Answer

That’s a good remark suggested by the reviewer. A mask with a hole of 1 mm diameter has been placed for recording optical transmittance spectra due to thickness gradient. With a hole of 1 mm diameter, the gradient has been estimated to be less than 1% of the thickness. A sentence has been added in the text to specify it.

Beyond those comments which represent mostly curiosities, I consider the manuscript worthy to be published on Coatings.

Reviewer 3 Report

The manuscript "Tuning the optical properties of WO₃ films exhibiting a zigzag columnar microstructure" by Charalampos Sakkas et al. deals with the fabrication and optimization of optical properties of WO₃ microscale structures based on zigzag columns on non-flexible substrates. The topic seems interesting and actual, but I do have some suggestions as to the content of the manuscript before finalizing the publication of this manuscript in Coatings.

To begin with, a few mistakes of typos and missing texts are observed in their write-up, for example, In figure caption (1) and (2) they used different symbols for the angle representation. Please use a uniform format throughout the manuscript. Please check line # 170.

• Q # 1: It is suggested to include a few other reports of oxygen-to-metal concentrations ratio to make the introduction part more comprehensive, you may include the following articles which describe a comprehensive review about this kind of details;

1. Ajmal, H.M.S.; Khan, F.; Nam, K.; Kim, H.Y.; Kim, S.D. Ultraviolet photodetection based on high-performance co-plus-ni doped ZnO nanorods grown by hydrothermal method on transparent plastic substrate. Nanomaterials 2020, 10, 1–20.
2. Ajmal, H.M.S.; Khan, F.; Ul Huda, N.; Lee, S.; Nam, K.; Kim, H.Y.; Eom, T.H.; Kim, S.D. High-performance flexible ultraviolet photodetectors with Ni/Cu-codoped ZnO nanorods grown on PET substrates. Nanomaterials 2019, 9, 1–18

• Q #2: I would recommend the authors include the recent publications for the references instead of old reports.
• Q #3: I would recommend the authors to mention the potential use of the Glancing Angle Deposition method (GLAD), why they prefer this method. Furthermore, there are a lot of other transition metal-based compounds for the sensor’s application as an active layer, therefore, authors should explain carefully why they picked WO₃ over the other available binary compounds.
• Q #4: Can the authors include the XPS analysis (if they have the data) to understand the elemental composition in WO How about the oxygen to metal ratio or vice versa? Please include such data or some analysis from the EDX scan.
• Q #5: is there any test of the absorbance vs. wavelengths with varying numbers of zigzag architectures or varying thickness of coated nanostructures.
• Q #6: Authors never mentioned about durability and robustness tests of the zigzag architectures. Can you provide some clue or any explanation about these kinds of characterizations?

Author Response

The manuscript "Tuning the optical properties of WO₃ films exhibiting a zigzag columnar microstructure" by Charalampos Sakkas et al. deals with the fabrication and optimization of optical properties of WO₃ microscale structures based on zigzag columns on non-flexible substrates. The topic seems interesting and actual, but I do have some suggestions as to the content of the manuscript before finalizing the publication of this manuscript in Coatings.

1. To begin with, a few mistakes of typos and missing texts are observed in their write-up, for example, In figure caption (1) and (2) they used different symbols for the angle representation. Please use a uniform format throughout the manuscript. Please check line # 170.

Answer

Typos and missing texts have been corrected, accordingly. Symbols for α and ϕ angles are the same in the manuscript and figures. Line #170 has been checked.

2. Q # 1: It is suggested to include a few other reports of oxygen-to-metal concentrations ratio to make the introduction part more comprehensive, you may include the following articles which describe a comprehensive review about this kind of details;

• Ajmal, H.M.S.; Khan, F.; Nam, K.; Kim, H.Y.; Kim, S.D. Ultraviolet photodetection based on high-performance co-plus-ni doped ZnO nanorods grown by hydrothermal method on transparent plastic substrate. Nanomaterials 2020, 10, 1–20.
• Ajmal, H.M.S.; Khan, F.; Ul Huda, N.; Lee, S.; Nam, K.; Kim, H.Y.; Eom, T.H.; Kim, S.D. High-performance flexible ultraviolet photodetectors with Ni/Cu-codoped ZnO nanorods grown on PET substrates. Nanomaterials 2019, 9, 1–18

Answer

We downloaded the suggested references and read it. Unfortunately, we don’t think they can bring some significant information about the oxygen-to-metal concentrations ratio since both articles concern ZnO films and not WO₃ films prepared by GLAD.

3. Q #2: I would recommend the authors include the recent publications for the references instead of old reports.

Answer

As suggested by the reviewer, some more recent publications have been included, instead of old reports, especially in the introduction part.

4. Q #3: I would recommend the authors to mention the potential use of the Glancing Angle Deposition method (GLAD), why they prefer this method. Furthermore, there are a lot of other transition metal-based compounds for the sensor’s application as an active layer, therefore, authors should explain carefully why they picked WO₃ over the other available binary compounds.

Answer

As suggested by the reviewer, we added some comments in the introduction part in order to mention the potential use of the GLAD method. In addition, a comment has been added about the choice of WO₃ compound.

5. Q #4: Can the authors include the XPS analysis (if they have the data) to understand the elemental composition in WO. How about the oxygen to metal ratio or vice versa? Please include such data or some analysis from the EDX scan.

Answer

As suggested by the reviewer, a reference about the chemical composition, especially concerning the oxygen-to-tungsten atomic concentration ratio, has been added in order to justify why we selected the given duty cycle (i.e. dc = 80% of the pulsing period P). These operating conditions correspond to the deposition of stoichiometric WO₃ thin films.

6. Q #5: is there any test of the absorbance vs. wavelengths with varying numbers of zigzag architectures or varying thickness of coated nanostructures.

Answer

Absorbance vs. wavelength is another representation of optical transmittance vs. wavelength. For optical thin films, optical transmittance is commonly used rather than absorbance.

7. Q #6: Authors never mentioned about durability and robustness tests of the zigzag architectures. Can you provide some clue or any explanation about these kinds of characterizations?

Answer

That’s a good question. The question related to aging effect. To the best of our knowledge, we have never seen studied reporting durability of zigzag thin films. Till now, our samples are stable after a few months (even a few years for some of them) of deposition.