Annealing Effect on Microstructure of Novel Ti Doped DLC Multilayer Films

Round 1

Reviewer 1 Report

The manuscript “Thermal Stability of novel Ti-doped DLC multilayer films using Raman Spectroscopy”, written by Shidong Zhang, Guang Jiang, Yang Yang, Hongtao Li, Fuyao Yan, Mufu Yan, and Yanxiang Zhang, offers a new perspective of thermal stability of the DLC compound doped with Ti, obtained in magnetron sputtering configurations. The manuscript respects the journal template, thus there are some questionable things. The manuscript is able to be published after major changes. Below you will find my comments/ suggestions/ questions.

1.      Line 64: The authors inserted “300 ´ 100 ´ 8 mm^3”. Please correct the unit dimensions. When are written cubic millimeters that signify that you want to express the volume, like in the above observation? For example, the authors said at line 82 the measured surface, and they typed correctly “3 x 3 µm^2”. If you want to write only the dimensions of the used target, then it can be emphasized like this: 300 mm x 100 mm x 8 mm, and not like 300 ´ 100 ´ 8 mm^3.

2.      Line 65: Based on your notations, “mA/cm^2”’, the authors expressed the “Ion current density”, and not the “plasma density”, as is written in the manuscript. Please make corrections. The plasma density is usually expressed in units like “cm^(-3)”.

3.      Line 69: Please verify again the inserted pressures during the manuscript. If the inserted pressure is 4 x 10 ^(-3) Pascal, that means around 4 x 10^(-5) mbar, which is hard to believe that the magnetrons were capable to work with. The magnetrons usually work at a minimum pressure of 10^(-3) mbar. If you still suggest that low pressure for your magnetrons, then please explain how you were able to accomplish it, and also, what vacuum system types have you used, because a turbo molecular pump cannot handle that low working pressure of the magnetrons.

4.      The authors described the annealing results, thus, how it influenced the surface morphology of the annealed samples? Have you seen cracks?

5.      What about the oxidation occurrence due to the annealing process? How did you manage to avoid the accumulation of oxygen?

Author Response

Dear Reviewer 1

Thanks a lot for your review work! We have carefully revised our manuscript base on your comments, suggestions and questions. Please see the details in the attached files. The main content of our new manuscript entitled "Thermal Stability of novel Ti doped DLC multilayer films" as follows.”

Diamond-like films (DLC) are an exceptional engineering material with excellent performance such as high hardness, low friction coefficient, superior wear resistance and chemical inertness. However, two major problems of high internal stress and poor thermal stability have seriously limited its industrial applications. In particular, the microstructures and properties of pure DLC films are highly sensitive to high temperature. Therefore, the purpose of this study is to investi-gate the effect of annealing temperature on the microstructures of the as-prepared films. Ti doped DLC multilayer films were synthesized by closed field unbalanced magnetron sputtering. The as-deposited films were annealed in the range of 200 to 800 °C. The surface morphology, phase structure and bonding structure of the films were characterized by SEM, AFM, GIXRD and Raman spectroscopy. The resulting films remained a smooth surface after annealing and maintained the nature of amorphous carbon up to 600 °C. The formed phases of graphite carbon and TiC nanocrystallines occur above 600 °C. Besides, the D- and G-bands showed a significant blue shift and the FWHMG shows a declining trend up to 600 °C. This result revealed that the films had high graphitization temperature and good thermal stability due to the formation of TiC nanocrystallines and its novel structure design containing elemental doping, multilayer structuring and functionally graded layering.. We ensure that the results obtained by our group have not been delivered to any other publications and manuscript is approved by all authors for publication.

Thank you again for your time and attention! We are looking forward to hearing from you soon!

Best wishes,

Sincerely yours

Prof. Mufu Yan (Corresponding Author)

E-Mail: [email protected]

Prof. Yanxiang Zhang (Corresponding Author)

E-Mail: [email protected] 

Harbin Institute of Technology

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript describes a thermal stability of Ti containing DLC. The manuscript contained a few interesting information for researchers on the DLC. Some technical problems are also contained in this manuscript, such as writing style as academic paper. It is hard to discuss the thermal stability of this study using Raman spectroscopy alone, as there is no specific evaluation of the fabricated film. Please consider following points.

1.       It should indicate the titanium concentration in these films.

2.       The bonding state between titanium and carbon should be indicated. Is there a metallic bond between titanium and titanium?

3.       If there are many Ti-C bonds among the bonding states of the fabricated film, the heat resistance of the film will increase. It is well known that titanium carbide has high heat resistance, so the novelty disappears.

4.       DLC films typically have a roughness of a few nm. Please indicate the reason for the large roughness of these films. If this large roughness is the effect of droplets, it is also necessary to describe how much sp2-bonded carbon is contained in this film.

5.       On page 4, line 116, it states "The D- and G-bands are distinctly separate at 500 °C and the separation increases with increasing temperature, indicating a distinct transformation of sp3 C to sp2 C [32]." However, visible Raman spectra can detect only sp2 bonds. This is because these peaks are due to resonant Raman scattering, and these peaks do not appear unless incident photons are absorbed. (sp3 is 5.4 eV and is UV region) Therefore, this statement is physically incorrect even with references.

6.       On page 4, line 132, it states "Figure 5 reveals annealing effect on the structure of D- and G-bands. The ID/IG ratio (ID/IG) is a measure of disorder of the amorphous carbon." 　This is also incorrect. This is order of sp2 not include sp3. Hence, these types of spectra are shown in Raman spectra in CNT or graphite.

7.       There is a problem with the fitting method in Figure 5. The starting point in base line is where the spectral is not flat and as a result the obtained ID/IG ratio is unreliable.

8.       The largest problem of this paper is that structural change by heating is only described by observed data by visible light Raman scattering spectroscopy. These results only indicated the fact that the sp2 carbon remained by 800 degree C heating. This data can not indicated that there is any DLC left or not. This phenomenon of sp2 remains is also observed in TiC films. This is that Ti exhibits a catalytic behavior and sp2 carbon is deposited. Therefore, these data alone cannot support the authors' claims. A lot of other data will need to be shown.

Some technical problems

9.       Why are there two spectra at 800°C in Figure 3?

10.   Use SI unit. “sccm ”is not Si unit.

11.   In the case of parallel notation, the unit only with the last value is enough.

12.   Line number 89, the dot in ”Figure. 1" should be deleted.

13.   Some variables are not written in italics, such as “I” in ID/IG or “R” in Ra . This is not only manuscript, but also figures and tables.

Author Response

Dear Reviewer 2

Thanks a lot for your review work! We have carefully revised our manuscript base on your comments, suggestions and questions. Please see the details in the attached files. The main content of our new manuscript entitled "Thermal Stability of novel Ti doped DLC multilayer films" as follows.”

Diamond-like films (DLC) are an exceptional engineering material with excellent performance such as high hardness, low friction coefficient, superior wear resistance and chemical inertness. However, two major problems of high internal stress and poor thermal stability have seriously limited its industrial applications. In particular, the microstructures and properties of pure DLC films are highly sensitive to high temperature. Therefore, the purpose of this study is to investi-gate the effect of annealing temperature on the microstructures of the as-prepared films. Ti doped DLC multilayer films were synthesized by closed field unbalanced magnetron sputtering. The as-deposited films were annealed in the range of 200 to 800 °C. The surface morphology, phase structure and bonding structure of the films were characterized by SEM, AFM, GIXRD and Raman spectroscopy. The resulting films remained a smooth surface after annealing and maintained the nature of amorphous carbon up to 600 °C. The formed phases of graphite carbon and TiC nanocrystallines occur above 600 °C. Besides, the D- and G-bands showed a significant blue shift and the FWHMG shows a declining trend up to 600 °C. This result revealed that the films had high graphitization temperature and good thermal stability due to the formation of TiC nanocrystallines and its novel structure design containing elemental doping, multilayer structuring and functionally graded layering.. We ensure that the results obtained by our group have not been delivered to any other publications and manuscript is approved by all authors for publication.

Thank you again for your time and attention! We are looking forward to hearing from you soon!

Best wishes,

Sincerely yours

Prof. Mufu Yan (Corresponding Author)

E-Mail: [email protected]

Prof. Yanxiang Zhang (Corresponding Author)

E-Mail: [email protected] 

Harbin Institute of Technology

Author Response File: Author Response.pdf

Reviewer 3 Report

The article needs improvement. The drawings were done carelessly. Here are some of my comments:

Line 19: in temperature range of 200⁰C-800⁰C

Line 34: Aisenberg and Chabot were the first who successfully deposited…..

Line 35: …at room-temperature……

lines 42-43 contradict lines 30-34

line 56: Raman Spectroscopy or „by analysing the raman spectra”

line 70:  The deposition temperature was lower than 200 °C

line 71: Table 1 describes……

line 114: Figure 3 illustrates Raman spectra of the films annealed at different temperatures

line 116: The D- and G-bands are distinctly separated……..

line 118: explain what you mean by “complete amorphous structure”. When the structure is completely amorphous and when not?

Line 124: ……annealed at different temperatures……….

Line137-139: In the first stage at  lower temperature  (200~400 C), the ID/IG slightly increases with the temperature increase. In the second stage at the middle temperature (400~600 °C), the ID/IG shows an almost linear declining trend as temperature increases.

Line 153-162: it was written very imprecisely, for example, about the role of the laser, which has no effect on changing the structure of the layer, and, for example, what is “graphite microcrystalline”, it should probably be microcrystalline graphite. There are also speculations about the transformation from sp3 to sp2. This can be examined, for example, using XPS spectroscopy

Fig.3 what does the spectrum shown in the highest part of the figure mean?

Fig.5a: does the ID/IG ratio mean the ratio of integral intensities? This needs to be explained.                                                                                                                                    

Tab2.: Were the values given in Table 2 really measured with such accuracy? What is the resolution of your Raman spectrometer?

Author Response

Dear Reviewer 3

Thanks a lot for your review work! We have carefully revised our manuscript base on your comments, suggestions and questions. Please see the details in the attached files. The main content of our new manuscript entitled "Thermal Stability of novel Ti doped DLC multilayer films" as follows.”

Diamond-like films (DLC) are an exceptional engineering material with excellent performance such as high hardness, low friction coefficient, superior wear resistance and chemical inertness. However, two major problems of high internal stress and poor thermal stability have seriously limited its industrial applications. In particular, the microstructures and properties of pure DLC films are highly sensitive to high temperature. Therefore, the purpose of this study is to investi-gate the effect of annealing temperature on the microstructures of the as-prepared films. Ti doped DLC multilayer films were synthesized by closed field unbalanced magnetron sputtering. The as-deposited films were annealed in the range of 200 to 800 °C. The surface morphology, phase structure and bonding structure of the films were characterized by SEM, AFM, GIXRD and Raman spectroscopy. The resulting films remained a smooth surface after annealing and maintained the nature of amorphous carbon up to 600 °C. The formed phases of graphite carbon and TiC nanocrystallines occur above 600 °C. Besides, the D- and G-bands showed a significant blue shift and the FWHMG shows a declining trend up to 600 °C. This result revealed that the films had high graphitization temperature and good thermal stability due to the formation of TiC nanocrystallines and its novel structure design containing elemental doping, multilayer structuring and functionally graded layering.. We ensure that the results obtained by our group have not been delivered to any other publications and manuscript is approved by all authors for publication.

Thank you again for your time and attention! We are looking forward to hearing from you soon!

Best wishes,

Sincerely yours

Prof. Mufu Yan (Corresponding Author)

E-Mail: [email protected]

Prof. Yanxiang Zhang (Corresponding Author)

E-Mail: [email protected] 

Harbin Institute of Technology

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have responded to all requests from the reviewer, improving the quality of the manuscript. The article can be published in its current form.

Author Response

Dear Reviewer 1

Thanks a lot for your review work! We are very grateful for your comments, suggestions and questions!

Best wishes,

Sincerely yours

Prof. Mufu Yan (Corresponding Author)

E-Mail: [email protected]

Prof. Yanxiang Zhang (Corresponding Author)

E-Mail: [email protected] 

Reviewer 2 Report

This manuscript describes a thermal stability of Ti containing DLC. The manuscript contained a few interesting information for researchers on the DLC. It is hard to discuss the thermal stability of this study using Raman spectroscopy alone, as there is no specific evaluation of the fabricated film. And Authors did not answer most of last comments.

1.       It should indicate the titanium concentration in these films.

*Authors indicated and determined from EDS spectra. However, it is difficult to determine the concentration of light elements by EDS. In particular, it is not possible to determine the concentration of mixtures of metals such as titanium and light elements such as carbon and oxygen. Use XPS or something.

2.       The bonding state between titanium and carbon should be indicated. Is there a metallic bond between titanium and titanium?

*Authors not answer this point.

3.       If there are many Ti-C bonds among the bonding states of the fabricated film, the heat resistance of the film will increase. It is well known that titanium carbide has high heat resistance, so the novelty disappears.

*Authors indicated XRD patterns. Based on this result, the improvement in heat resistance is attributed to carbide components such as TiC, and there is no novelty in this paper. This has already been discussed and put into practical use in TiC research.

4.       DLC films typically have a roughness of a few nm. Please indicate the reason for the large roughness of these films. If this large roughness is the effect of droplets, it is also necessary to describe how much sp2-bonded carbon is contained in this film.

*Authors not answer this point.

5.       On page 4, line 116, it states "The D- and G-bands are distinctly separate at 500 °C and the separation increases with increasing temperature, indicating a distinct transformation of sp3 C to sp2 C [32]." However, visible Raman spectra can detect only sp2 bonds. This is because these peaks are due to resonant Raman scattering, and these peaks do not appear unless incident photons are absorbed. (sp3 is 5.4 eV and is UV region) Therefore, this statement is physically incorrect even with references.

*Authors not answer this point.

6.       On page 4, line 132, it states "Figure 5 reveals annealing effect on the structure of D- and G-bands. The ID/IG ratio (ID/IG) is a measure of disorder of the amorphous carbon." 　This is also incorrect. This is order of sp2 not include sp3. Hence, these types of spectra are shown in Raman spectra in CNT or graphite.

*Authors not answer this point.

7.       There is a problem with the fitting method in Figure 5. The starting point in base line is where the spectral is not flat and as a result the obtained ID/IG ratio is unreliable.

*Authors not answer this point.

8.       The largest problem of this paper is that structural change by heating is only described by observed data by visible light Raman scattering spectroscopy. These results only indicated the fact that the sp2 carbon remained by 800 degree C heating. This data can not indicated that there is any DLC left or not. This phenomenon of sp2 remains is also observed in TiC films. This is that Ti exhibits a catalytic behavior and sp2 carbon is deposited. Therefore, these data alone cannot support the authors' claims. A lot of other data will need to be shown.

*Authors not answer this point.

Author Response

Dear Reviewer 2

Thanks a lot for your review work! We have carefully revised our manuscript base on your comments. In the first round, so sorry that we have only answered your questions in the attachment, whereas not in that box of MDPI system. Maybe you did not notice it. Besides, the title is changed to “Annealing effect on microstructure of novel Ti doped DLC multilayer films” in our new manuscript. The detailed answers for your comments including figures and references in the attached file.

Point 1: It should indicate the titanium concentration in these films.

*Authors indicated and determined from EDS spectra. However, it is difficult to determine the concentration of light elements by EDS. In particular, it is not possible to determine the concentration of mixtures of metals such as titanium and light elements such as carbon and oxygen. Use XPS or something.

Response 1: Thanks a lot for this kind suggestions! To be honest, This work was done a few years ago. Unfortunately, the original samples and raw data were lost in the Netherlands. We really hope to provide XPS data, but so pity that we can't provide it. But based on our previous work [1], XPS results shows that the carbon concentration of the films prepared by the same process is ~80 at.%.

Point 2: The bonding state between titanium and carbon should be indicated. Is there a metallic bond between titanium and titanium?

Response 2: As mentioned above, so sorry that the XPS data is not available. However, according to our previous XPS data [1] (using the films prepared by the same process) , the bonds of Ti-C and Ti-O were found and no Ti-Ti bond is observed, as shown in the below Figure 1.

Figure 1 (a) Ti 2p XPS spectra of the films deposited with different Ti target currents, (b) deconvoluted Ti 2p XPS spectrum of the films deposited with a Ti target current of 0.3A, (c) O1s XPS spectra of the films deposited with different Ti target currents, (d) deconvoluted O 1s XPS spectrum of the films deposited with a Ti target current of 0.3A.

Point 3: If there are many Ti-C bonds among the bonding states of the fabricated film, the heat resistance of the film will increase. It is well known that titanium carbide has high heat resistance, so the novelty disappears.

*Authors indicated XRD patterns. Based on this result, the improvement in heat resistance is attributed to carbide components such as TiC, and there is no novelty in this paper. This has already been discussed and put into practical use in TiC research.

Response 3: As mentioned above, there are some Ti-C bonds. Indeed, the formed TiC has good thermal stability. But our novel structures containing Ti doping, multilayer structuring and functionally graded layering thermal stability are also benificial to its thermal stability. As shown in our previous work[1], this novel DLC films obtained excellent mechanical, tribological and anti-corrosive performance. The new title of “Annealing effect on microstructure of novel Ti doped DLC multilayer films” may be more suitable.

Figure 2 Schematic diagram of the multilayer composite diamond-like films

Point 4: DLC films typically have a roughness of a few nm. Please indicate the reason for the large roughness of these films. If this large roughness is the effect of droplets, it is also necessary to describe how much sp2-bonded carbon is contained in this film.

Response 4:

• On the first question, ideally the as-deposited DLC films are very smooth with a few nm. But the film surface roughness depends on many factors such as roughness and cleanliness on substrate surface before dopisition, process parameters like Ti and C target curent, Ar gas flow rate, etc. The surface roughness of the as-deposited films (below 12 nm) remain relatively smooth. I think the main reasons are that the substrate surface isn't smooth enough, titanium doping and annealing effect. Sepcifically, the Ti target current and annealing temperature are beneficial to the growth of the TiC phase and amorphous carbon, thus it will increse its surface roughness.
• On the second question, Based on our newly added SEM results (Figure 3), no droplets were observed. The carbon concentration in these films is about 75 at.% by EDS. Besides, the D- and G-bands are attributed to sp² The G-band is ascribed to the bond stretching E_2g modes of all pairs of sp² hybridized carbon atoms in both rings and chains. The D-band arises from the breathing A_1g modes of sp² atoms in rings [2-7]. Moreover, As shown in our previous work[1], Figure 4 (a) shows that the I_D/I_G ratio is inversely proportional to the sp³ C content, which is consisted with the results of A.C. Ferrari and J. Robertson and [3,7]. So we can indirectly know the sp2-bonded carbon from the the I_D/I_G ratio except for other chemical bonds like the bonds of Ti-C and Ti-O or we can directly detect sp2-bonded carbon by XPS.

Figure 3 SEM surface morphologies of the as-deposited films annealed at different temperatures: (a)200 °C, (b) 400 °C, (c) 500 °C, (d) 600 °C, (e)800 °C.

Figure 4 (a) R_I vs. sp³ C content of the as-deposited films, (b) R_I vs. FWHM_G value of the as-deposited films.

Point 5: On page 4, line 116, it states "The D- and G-bands are distinctly separate at 500 °C and the separation increases with increasing temperature, indicating a distinct transformation of sp3 C to sp2 C [32]." However, visible Raman spectra can detect only sp2 bonds. This is because these peaks are due to resonant Raman scattering, and these peaks do not appear unless incident photons are absorbed. (sp3 is 5.4 eV and is UV region) Therefore, this statement is physically incorrect even with references.

Response 5: Thanks a lot for this comment! Indeed, Raman spectra cannot directly detect sp³ bonds and sp³ bonds can be directly detected by XPS. I have already deleted this incorrect statement in line 170 of our new manuscript. Besides, as we all known that the sp³ bonds of ta-C is usually over 80%. Figure 5 clearly shows that if the symmetry of D- and G-bands gradually decreases, Raman spectra of the amorphous carbon has a tendency to become one broad peak like ta-C, indirectly indicating more formation of sp³ C. Hence, the symmetry of D- and G-bands gradually increases, indirectly indicating less formation of sp³ C and more formation of sp² C.

Figure 5 Comparison of typical Raman spectra of carbons.[8]

Point 6: On page 4, line 132, it states "Figure 5 reveals annealing effect on the structure of D- and G-bands. The ID/IG ratio (ID/IG) is a measure of disorder of the amorphous carbon." This is also incorrect. This is order of sp2 not include sp3. Hence, these types of spectra are shown in Raman spectra in CNT or graphite.

Response 6: I have revised it in lines 186-187 of our new manuscript.

BTW: I have one question: Are there all the sp³ C which orderly existed in the amorphous carbon films? I am not sure about this. Thanks a lot in advance!

Point 7: There is a problem with the fitting method in Figure 5. The starting point in base line is where the spectral is not flat and as a result the obtained ID/IG ratio is unreliable.

Response 7: Thanks a lot for this good suggestion! As mentioned above, unfortunately, the original data was lost. So sorry that I cannot reanalyse this data. The base line is not flat because the base line is short. If the starting point start from 800 cm^-1, it will be much better, shown in the Firgure 6 [1]. We will definitely consider what you mentioned in my future work. Thanks again!

Firgure 6 Deconvoluted Raman spectra of the films deposited with different Ti target currents: (a) 0.3A, (b) 0.4A, (c) 0.5A, (d) 0.6A, (e) 0.8A, (f) 1.0A. [1]

Point 8: The largest problem of this paper is that structural change by heating is only described by observed data by visible light Raman scattering spectroscopy. These results only indicated the fact that the sp2 carbon remained by 800 degree C heating. This data can not indicated that there is any DLC left or not. This phenomenon of sp2 remains is also observed in TiC films. This is that Ti exhibits a catalytic behavior and sp2 carbon is deposited. Therefore, these data alone cannot support the authors' claims. A lot of other data will need to be shown.

Response 8: Thanks again for you all your comments and suggestions! Especically, you pointed out the biggest problem of this paper. Hence, we try our best to add some other data incluisng results of SEM, EDS and XRD in our new manuscript. As for XPS, so pity that we cannot provide this data. But we will definitely consider what you have mentioned in our future work.

Reference

1. Zhang, S.; Yan, M.; Yang, Y.; Zhang, Y.; Yan, F.; Li, H. Excellent mechanical, tribological and anti-corrosive performance of novel Ti-DLC nanocomposite thin films prepared via magnetron sputtering method. Carbon 2019, 151, 136-147. doi:https://doi.org/10.1016/j.carbon.2019.05.031
2. Tuinstra, F.; Koenig, J.L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 1126-1130. doi:https://doi.org/10.1063/1.1674108
3. Ferrari, A.C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B Condens. Matter. 2000, 61, 14095-14107. doi:https://doi.org/10.1103/PhysRevB.61.14095
4. Castiglioni, C.; Negri, F.; Rigolio, M.; Zerbi, G. Raman activation in disordered graphites of the A'₁ symmetry forbidden k≠0 phonon: The origin of the D line. J. Chem. Phys. 2001, 115, 3769-3778. doi:https://doi.org/10.1063/1.1381529
5. Castiglioni, C.; Di Donato, E.; Tommasini, M.; Negri, F.; Zerbi, G. Multi-wavelength Raman response of disordered graphitic materials: models and simulations. Synth. Met. 2003, 139, 885-888. doi:https://doi.org/10.1016/s0379-6779(03)00305-9
6. Castiglioni, C.; Tommasini, M.; Zerbi, G. Raman spectroscopy of polyconjugated molecules and materials: confinement effect in one and two dimensions. Philos. Trans. A Math. Phys. Eng. Sci. 2004, 362, 2425-2459. doi:https://doi.org/10.1098/rsta.2004.1448
7. Ferrari, A.C.; Robertson, J. Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond. Philos. Trans. A Math. Phys.Eng. Sci. 2004, 362, 2477-2512. doi:https://doi.org/10.1098/rsta.2004.1452
8. Robertson, J. Diamond-like amorphous carbon. Mater. Sci. Eng. R Rep. 2002, 37, 129-281. doi:https://doi.org/10.1016/S0927-796X(02)00005-0

Best wishes,

Sincerely yours

Prof. Mufu Yan (Corresponding Author)

E-Mail: [email protected]

Prof. Yanxiang Zhang (Corresponding Author)

E-Mail: [email protected] 

Author Response File: Author Response.pdf

Reviewer 3 Report

Thank you for considering my comments

Author Response

Dear Reviewer 3

Thanks a lot for your review work! We are very grateful for your comments, suggestions and questions!

Best wishes,

Sincerely yours

Prof. Mufu Yan (Corresponding Author)

E-Mail: [email protected]

Prof. Yanxiang Zhang (Corresponding Author)

E-Mail: [email protected]