Structural Color Control of CoFeB-Coated Nanoporous Thin Films

Round 1

Reviewer 1 Report

The paper by X. Zhu et al., entitled “Structural Color Control of CoFeB-coated Nanoporous Thin Films”
reports on the possibilities to tune the color of photonic crystals based on CoFeB@AAO.
The results are interesting, however many issues in the manuscript are present, that require a thorough
revision before publication.
1. A clear statement should be made in the abstract and introduction concerning the reason why the
authors chose the two materials CoFeB and AAO. Is there any intrinsic benefit in any of them? Are
these materials already studied in the literature? If yes, the necessary works should be cited.
2. In section 2.2: details of the instruments and experimental parameters should be given for each
technique listed in the section. E.g.: in ellipsometry, which instrument was used? Which was the
angle of incidence? Spectral resolution, spot size?
3. Ellipsometry results. The thickness of CoFeB is said to be obtained through ellipsometry, and the
values are reported with precision up to a tenth of nanometer (e.g. 16.3 nm). Such a high precision
should be backed by appropriate description of the model used to fit the data, and the relative
error estimation on the results. Was the CoFeB thickness measured on purposefully-grown films,
i.e. not on the photonic crystals themselves? If yes, this should be clearly reported in the
manuscript.
4. AAO templates. The authors report the interpore distance, hole depth, and thickness. How were
these measurements obtained? For example the interpore distance seems quite tricky to calculate,
it would be better to provide an average value and a dispersion range.
5. CoFeB deposition. The deposition angle should be specified. Is there any residual CoFeB that ends
up at the bottom, rather than on top of the AAO template? If yes, is it expected to play a role in the
overall optical properties of the samples?
6. Formula 2. The definition of a few symbols is missing.
7. Figure 2. What is the scale of the EDS/SEM images? What is the scale of the resulting colored
images? Most importantly: what is the lateral size of the samples? Since the authors mention
potential benefits to applications: is there any intrinsic limit to the lateral size of the samples?
Minor issues:
- Abstract, line 19: AAO should be defined
- Introduction, line 40: Low power consumption is cited as a benefit of structural color. Does a
structural color require power at all? If yes, an appropriate reference may help the reader to better
understand this point.
- Section 2.1, line 80-81. It is not clear what are the characteristics of sample series 2.

Author Response

1. A clear statement should be made in the abstract and introduction concerning the reason why the authors chose the two materials CoFeB and AAO. Is there any intrinsic benefit in any of them? Are these materials already studied in the literature? If yes, the necessary works should be cited.

CoFeB is a strong magnetic material, which has excellent performance in the construction of vertical magnetic anisotropic films, magnetic tunnel junctions and other structures. AAO template has been developed since 1953. It has become a mature, stable and efficient way to construct photonic crystals, and has good performance in the construction of a variety of photonic crystals. (Reference has been inserted into the article).

2. In section 2.2: details of the instruments and experimental parameters should be given for each technique listed in the section. E.g.: in ellipsometry, which instrument was used? Which was the angle of incidence? Spectral resolution, spot size?

Ellipsometry (J.A. Woollam RC2)

3. Ellipsometry results. The thickness of CoFeB is said to be obtained through ellipsometry, and the values are reported with precision up to a tenth of nanometer (e.g. 16.3 nm). Such a high precision should be backed by appropriate description of the model used to fit the data, and the relative error estimation on the results. Was the CoFeB thickness measured on purposefully-grown films, i.e. not on the photonic crystals themselves? If yes, this should be clearly reported in the manuscript.

When the sample thickness is measured, the sample thickness is not directly measured on the photonic crystal because it is difficult to find the boundary between CoFeB and AAO after the sample is completed. Therefore, we choose to measure the sputtering rate of CoFeB on the smooth film and control the sample thickness by controlling the time. Therefore, the actual thickness will have a deviation of about 0.5nm.

4. AAO templates. The authors report the interpore distance, hole depth, and thickness. How were these measurements obtained? For example the interpore distance seems quite tricky to calculate, it would be better to provide an average value and a dispersion range.

As the AAO templates are purchased by Shanghai Wood Technology, the above data are provided by the company.

5. CoFeB deposition. The deposition angle should be specified. Is there any residual CoFeB that ends up at the bottom, rather than on top of the AAO template? If yes, is it expected to play a role in the overall optical properties of the samples?

Sputtering Angle is fixed at 45°. There is residual CoFeB at the bottom, but so little that it is not expected to play a role in the overall optical properties.

6. Formula 2. The definition of a few symbols is missing.

Thank you for your valuable comments, which have been revised in the manuscript.

7. Figure 2. What is the scale of the EDS/SEM images? What is the scale of the resulting colored images? Most importantly: what is the lateral size of the samples? Since the authors mention potential benefits to applications: is there any intrinsic limit to the lateral size of the samples?

The attached artwork

3min: (Figure 2b)

6min: (Figure 2c)

9min: (Figure 2d)

The bottom was cut off before for the sake of the beautiful picture. The scale has been added in the manuscript. Thank you for your opinion.

Attached below are the original structure color images: (The colors shown in Figure 2 in article).

From left to right :9 min,  3 min,  6 min

As you can see from the original image, there is no restriction on the lateral size of the sample.

Reviewer 2 Report

The authors present a manuscript, titled "Structural Color Control of CoFeB-coated Nanoporous Thin Films". The manuscript is reasonably described. Overall the flow of the manuscript is good. With that I have a number of comments that require authors attention before considering publication.

1. What is the merit of CoFeB-based nanoporous compared with other material types? A brief statement on the comparison between them should be included in the introduction. It means that they should clearly show the advantage of CoFeB-based nanoporous thin film.

2. Why did the authors choose the AAO template with pore diameter of 90 nm instead of other ones (30 nm, 50 nm, and 70 nm)? In addition, the authors should clearly provide the pore diameter effect for this application.

3. What is the hole depth of AAO substrate the authors used? 300 nm (page 2, line 76) or 200 nm (page 5, line 135)? Make sure which one is correct.

4. I think that Figure 2(c) shows the planar image of the 49.0 nm film. Check it out one more time.

5. For the readers, put the scale bars in Figure 2(a)-(d).

6. The authors mention that when the thickness was increased, the amplitudes between the reflection and absorption peaks tended to decrease. Please add more detailed mechanism or explanation based on thickness effect. Also, it would be helpful to add some references as well.

Author Response

1. What is the merit of CoFeB-based nanoporous compared with other material types? A brief statement on the comparison between them should be included in the introduction. It means that they should clearly show the advantage of CoFeB-based nanoporous thin film.

CoFeB is a strong magnetic material, which has excellent performance in the construction of vertical magnetic anisotropic films, magnetic tunnel junctions and other structures. AAO template has been developed since 1953. It has become a mature, stable and efficient way to construct photonic crystals, and has good performance in the construction of a variety of photonic crystals. (Reference has been inserted into the article).

2. Why did the authors choose the AAO template with pore diameter of 90 nm instead of other ones (30 nm, 50 nm, and 70 nm)? In addition, the authors should clearly provide the pore diameter effect for this application.

Equation (1) 

Where, ,  and  represent the effective refractive index thickness and incidence Angle of  layer respectively as the resonance series, and The wavelength of the incident wave is The wavelength of the incident wave. Interference cancellation occurs when the wavelengths of the two beams interfering at the interface between the sample and the air differ by an odd factor of half a wavelength, and this is the main reason for the trough in reflectivity

For the peak position shift, a rough analysis is made here. For pure AAO/Al templates, the effective refractive index can be written in the form of (2).

n_{eff = (}π/2×3^1/2)•(D/S)² ∗ n_air + [1– π/2×3^1/2)•(D/S)²] ∗ n_alumina

From the formula, it is not difficult to find that when the aperture increases, the effective refractive index decreases. Under the condition that the right value of Equation (1) remains unchanged, the Angle on the left side must decrease.

3. What is the hole depth of AAO substrate the authors used? 300 nm (page 2, line 76) or 200 nm (page 5, line 135)? Make sure which one is correct.

300nm . Thank you for your valuable suggestions, which have been revised in the manuscript.

4. I think that Figure 2(c) shows the planar image of the 49.0 nm film. Check it out one more time.

Thank you for your valuable suggestions.

5. For the readers, put the scale bars in Figure 2(a)-(d).

Ok, it has been revised in the manuscript. Thank you for your comments

The attached artwork

3min: (Figure 2b)

6min: (Figure 2c)

9min: (Figure 2d)

The bottom was cut off before for the sake of the beautiful picture. The scale has been added in the manuscript. Thank you for your opinion.

Attached below are the original structure color images: (The colors shown in Figure 2 in article).

From left to right :9 min,  3 min,  6 min

6. The authors mention that when the thickness was increased, the amplitudes between the reflection and absorption peaks tended to decrease. Please add more detailed mechanism or explanation based on thickness effect. Also, it would be helpful to add some references as well.

As can be seen from Figure 1, when the thickness of CoFeB gradually increases, more and more parts of light are absorbed. When high reflection and low absorption in the visible area occur at the same time, a clear color will be produced. Therefore, when the thickness increases to a certain critical value, the color will gradually dim. (Relevant references have been inserted into the article).

Reviewer 3 Report

The author's, in the experiemental section, did not described in detail the sputtering target of FeCoB alloy, they only provided the stoichiometry of the target. In this section the author’s should provide morea data about the the target e.g., the manufacturer’s company, the size of the target. After the sputtering depositions, the stoichiometry of the coatings remain the same? Did the author’s stdudy the schoichiometry of the deposited thin films by means of XPS or another characterization technique?

In general, the results are well supported by the characterization techniques and the schematic illustration presented in Figure 1 helps to better understand the results. Nevertheless, the same scheme also shows the limitation of the sputtering technique in terms of coating porous nanostrucutres in a conformal manner.

Author Response

1. The author's, in the experiemental section, did not described in detail the sputtering target of FeCoB alloy, they only provided the stoichiometry of the target. In this section the author’s should provide morea data about the the target e.g., the manufacturer’s company, the size of the target. After the sputtering depositions, the stoichiometry of the coatings remain the same? Did the author’s stdudy the schoichiometry of the deposited thin films by means of XPS or another characterization technique?

We purchased CoFeB target material from Beijing Zhongnuo New Material Co., LTD. The target size is 50mm in diameter and 30 mm in thickness. After sputtering, we did maintain consistency and we measured it by EDS.

2. In general, the results are well supported by the characterization techniques and the schematic illustration presented in Figure 1 helps to better understand the results. Nevertheless, the same scheme also shows the limitation of the sputtering technique in terms of coating porous nanostrucutres in a conformal manner.

Thank you very much for your valuable advice. Relevant content has been revised in the manuscript.

Round 2

Reviewer 1 Report

The authors addressed most of the reviewer's comments and modified the manuscript accordingly. One relevant point, however, should be better addressed. The authors should describe the model that allowed them to obtain the various thicknesses, and should be aware that the precision they provide is in the range of 0.5 nm - that is, only a few atoms. The authors should not confuse the precision of the fitting procedure, which is given by an algorithm, with the physical meaning of the results. Providing a thickness precision of 0.5 nm may not be physically meaningful given that the height distribution of the AAO is probably much higher than that value. Indeed, the spot size (which the authors did not report) is at least several tens of microns in diameter, and across that size it is likely that the height distribution far exceeds 0.5 nm. In conclusion, this reviewer suggests that the authors include a critical description of the uncertainty they provide on the thickness calculation, otherwise their very precise results from ellipsometry analysis may lack a proper physical meaning. 

Author Response

1. The authors addressed most of the reviewer's comments and modified the manuscript accordingly. One relevant point, however, should be better addressed. The authors should describe the model that allowed them to obtain the various thicknesses, and should be aware that the precision they provide is in the range of 0.5 nm - that is, only a few atoms. The authors should not confuse the precision of the fitting procedure, which is given by an algorithm, with the physical meaning of the results. Providing a thickness precision of 0.5 nm may not be physically meaningful given that the height distribution of the AAO is probably much higher than that value. Indeed, the spot size (which the authors did not report) is at least several tens of microns in diameter, and across that size it is likely that the height distribution far exceeds 0.5 nm. In conclusion, this reviewer suggests that the authors include a critical description of the uncertainty they provide on the thickness calculation, otherwise their very precise results from ellipsometry analysis may lack a proper physical meaning. 

Dear reviewer：

Thank you very much for your advice. However, due to the old thickness data of ellipsometry, some specific details (such as the size of the spot) have been lost. In order to solve the problem, we re-calibrated the thickness of CoFeB.

As shown in the figure below:

1.T=3min40s     h=20.16nm

2.T=7min21s     h=40.03nm

3.T=11min01s     h=61.78nm

As shown in the figures, we chose the step profiler to calibrate the thickness, and the data was provided by the Scientific Compass test platform. In order to verify the accuracy of the data, we chose three points: 20nm,40nm and 60nm.

From the data, the margin of error for thickness is ±2nm

This part of the article has been corrected.

Thanks for your valuable comments.