Excellent Color Purity and Luminescence Thermometry Performance in Germanate Tellurite Glass Doped with Eu³⁺ and Tb³⁺

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors of this manuscript study the spectroscopic properties of 50GeO2-35 TeO2–15SrF2 doped with ions of Eu³⁺ and Tb³⁺. I think this paper can be published after proper responses to the questions below, also additional data analysis are needed.

1. In the introduction, the authors consider that the studied glasses can find applications in various fields, but there is a lack of information about the current materials used in a given field and why the studied glass compositions are a better alternative. It will be useful if authors rewrite the introduction to make it clearer.

2. It is well known that the advantages of the glass compositions are supported by differential thermal analysis data. The change in Tg (glass transition temperature) and Tx (glass crystallization temperature) gives information about the change in glass structure depending on the composition. Authors need to provide DTA for at least three representative samples and the advantages compared to simpler compositions should be considered.

3. In order to make a credible discussion excitation and emission spectra with different doping concentration must be shown, i.e. with Eu³⁺= 0.2, 1, 2, 4 % and with Tb³⁺ = 0.2, 1, 4.

4. The manuscript lacks structural data of the obtained glasses. It would be good to provide IR or Raman data.  

5. In the excitation spectra of Eu³⁺ doped glasses is observed broad absorbing band below 350 nm. As a result of what energy transitions is this band? Please assign it.

6. The concentration of europium and terbium ions may be expressed in ions/cm³

7. Please explain from structural point of view, why sample doped with 4% Tb₂O₃ exhibits the strongest luminescence?

8. What computer calculating software did you use to obtain CIE coordinates? Please compare the obtained values for Eu³⁺ with standard red light coordinates, for Tb³⁺ with standard green light respectively and also with the commercial red and green phosphor.

9. You are discussing only the possible energy transfer from Tb to Eu. What about host to active ions non- radiative energy transfer?

10. You are also discussing non-radiative relaxation mechanisms. Accordingly, the comments attributed to relaxation dynamic should be included, as well as energy level diagram of Eu³⁺ and Tb³⁺ with relative transitions should be included to make the discussion clearer.

Comments on the Quality of English Language

No comment.

Author Response

1. In the introduction, the authors consider that the studied glasses can find applications in various fields, but there is a lack of information about the current materials used in a given field and why the studied glass compositions are a better alternative. It will be useful if authors rewrite the introduction to make it clearer.

R: The examples of materials used in various mentioned fields were provided in the Introduction: “The current hosts for Eu³⁺ and Tb³⁺ used in the mentioned fields include polycrystalline oxides [16–18].”. We hope, that the Introduction now provides clearer view on why the glass hosts are better suited for listed applications.

2. It is well known that the advantages of the glass compositions are supported by differential thermal analysis data. The change in Tg (glass transition temperature) and Tx (glass crystallization temperature) gives information about the change in glass structure depending on the composition. Authors need to provide DTA for at least three representative samples and the advantages compared to simpler compositions should be considered.

R: We performed the DTA analysis, but the curves did not reveal any transitions up to 900 °C. This must be an equipment failure, since the similar glasses [24] exhibit T_g in the range 460 – 510 °C, but due to time and financial constrains we cannot repeat these measurements. Since we did not study crystallization of this glass system, we think the DTA measurements are not essential and we took the liberty to not include these results in the manuscript.

3. 3. In order to make a credible discussion excitation and emission spectra with different doping concentration must be shown, i.e. with Eu³⁺= 0.2, 1, 2, 4 % and with Tb³⁺= 0.2, 1, 4.

R: We are grateful for this valuable Reviewer’s suggestion. In relation, the Figures 2b and 3b were supplied with the emission spectra of samples with different dopants concentration. Since no change in the shape of the spectra can be observed for different concentrations, we believe it is reasonable to assume the excitation spectra also would not change, therefore the Figures 2a and 3a were left unchanged.

4. The manuscript lacks structural data of the obtained glasses. It would be good to provide IR or Raman data.  

R: The manuscript was supplied with IR and Raman data and relevant paragraph describing them.

5. In the excitation spectra of Eu³⁺doped glasses is observed broad absorbing band below 350 nm. As a result of what energy transitions is this band? Please assign it.

R: The band has been assigned to the host excitation and mentioned in the text of the manuscript.

6. 6. The concentration of europium and terbium ions may be expressed in ions/cm³

R: The europium and terbium concentration has been expressed in ions/cm3 as well in Figures 2b and 3b.

7. Please explain from structural point of view, why sample doped with 4% Tb₂O₃exhibits the strongest luminescence?

R: The reason behind sample doped with 4% Tb₄O₇ exhibiting the strongest luminescence is the fact, that this sample contain the high concentration of optically ions and accordingly, its absorption qualities are enhanced to reveal an effective luminescence.

8. What computer calculating software did you use to obtain CIE coordinates? Please compare the obtained values for Eu³⁺with standard red light coordinates, for Tb³⁺with standard green light respectively and also with the commercial red and green phosphor.

R: The CIE coordinates were calculated in Origin software by multiplying the spectra by the RGB color matching functions and integrating the resulting curves. The comparison of obtained values is the essence of the color purity calculation (7), where (xd,yd) are the standard red/green light coordinates. As for the commercial red and green phosphors, the data on the exact CIE coordinates are not available.

9. You are discussing only the possible energy transfer from Tb to Eu. What about host to active ions non- radiative energy transfer?

R: The energy transfer between the host and active ions does occur in this material, which can be observed in the excitation spectrum of Eu³⁺ ions – the wide band at 250-320 nm. However, this energy transfer is difficult to study and discuss, as the host does not exhibit intrinsic luminescence, that can be measured in function of the dopant’s concentration.

10. You are also discussing non-radiative relaxation mechanisms. Accordingly, the comments attributed to relaxation dynamic should be included, as well as energy level diagram of Eu³⁺and Tb³⁺ with relative transitions should be included to make the discussion clearer.

R: The energy level diagram of europium and terbium in GTS glass has been included as Figure 7 and discussion related to excited states relaxation mechanism has been extended.

Reviewer 2 Report

Comments and Suggestions for Authors

MANUSCRIPT REVIEW

 

The manuscript entitled: "Excellent color purity and luminescence thermometry performance in germanite tellurite glass doped with Eu³⁺ and Tb³⁺"

The melt-quenching method was used for synthesized germanite tellurite glasses doped with Eu³⁺ and Tb³⁺. The UV/VIS spectroscopy was used to study a unique GTS glass doped with Eu³⁺ and Tb³⁺. To assess their properties, the absorption, excitation, emission spectra, and luminescence decay periods are found. Eu³⁺ emission spectra were used to compute the Judd-Ofelt (J-O) parameters. The glass's heat treatment and its effect on its structural and spectroscopic properties evaluate the glass's potential to build a glass-ceramic based on the GTS glass under study.

Reviewer' comments:

1)      In the Introductory part, it is necessary to add a more extensive literature review. This is not a new topic, so I think it is necessary to review the publications more deeply.

2)      For conventional melt-quenching method it would be nice to add some short description or literature citation.

3)      The authors written: “Judd-Ofelt (J-O) parameters are calculated based on Eu³⁺ emission spectra” (line 54). In what way, nothing is described, and no reference. But authors showed the way, but only later (line 81). I believe that the reference should be given at the beginning, because you are mentioning the model for the first time.

4)      Figure 1b, XRD spectrum, please add standard for materials.

5)      Fig 2b, x-coordinate, the concentration of Eu is in %. wt.% or vol.%?

6)      The CIE coordinate (line 138) enter the full name first, then use the abbreviation.

7)      The conclusion needs to be rearranged. The main research contributions were not extracted.

8)      What is a specific application? You can tell us a little more about it. Material’s advantages, disadvantages, a little more engineering approach… Then what are the potentials of industrial application? Whether it can be used in practice or is it only within the scope of research?

9)      The graphics are very nicely done.

However, addressing the minor concerns outlined is crucial for improving the manuscript's quality and clarity. Thus, it is recommended Minor Revisions.

Author Response

1. In the Introductory part, it is necessary to add a more extensive literature review. This is not a new topic, so I think it is necessary to review the publications more deeply.

R: According to a meaningful Reviewer’s comment, the “Introduction” has been revised including more adequate references [5-8, 13].

2. For conventional melt-quenching method it would be nice to add some short description or literature citation.

R: The melt-quenching method was described in the Materials and Methods section: “The starting materials: (…) were weighed and mixed in a porcelain crucible. Then the mix was placed in a corundum crucible and put into a pre-heated furnace at the temperature of 1000 ⁰C for 45 minutes to melt the starting materials. Next, the melt was taken out of the furnace and poured onto a brass plate at room temperature. The resulting glass was annealed at 350 ⁰C for 12 h to remove residual stress.”.

3. The authors written: “Judd-Ofelt (J-O) parameters are calculated based on Eu³⁺emission spectra” (line 54). In what way, nothing is described, and no reference. But authors showed the way, but only later (line 81). I believe that the reference should be given at the beginning, because you are mentioning the model for the first time.

The citations for Judd-Ofelt model have been added.

4. Figure 1b, XRD spectrum, please add standard for materials.

R: We are not able to provide XRD standards for this type of amorphous material. The XRD results are meant to illustrate the lack of diffraction peaks, which would indicate crystallization or instability of the prepared glasses.

5. Fig 2b, x-coordinate, the concentration of Eu is in %. wt.% or vol.%?

R: For clarity, the Eu concentration have been written precisely as concentration of Eu₂O₃. It is a molar concentration, so it is in %.

6. The CIE coordinate (line 138) enter the full name first, then use the abbreviation.

R: The full name of Commission internationale de l'éclairage was added to the manuscript.

7. The conclusion needs to be rearranged. The main research contributions were not extracted.

R: The conclusions were rearranged for clarity.

8. What is a specific application? You can tell us a little more about it. Material’s advantages, disadvantages, a little more engineering approach… Then what are the potentials of industrial application? Whether it can be used in practice or is it only within the scope of research?

R: The Introduction was supplied with the examples of host materials for Eu³⁺ and Tb³⁺ with application in lighting, displays and luminescent thermometry, and how GTS glasses pose an advantage to those materials.

9. The graphics are very nicely done.

R: Thank you. We appreciate your favorable opinion on our work.

Reviewer 3 Report

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Perhaps you can run it for plagiarism - for me it is difficult to say, Or, add a button - "don't know"

Run through an AI  Language Tool will be a better measure for the English.

Author Response

1. If authors claim “excellent color purity” this should be carefully quantified and explained. The “purest color is a single wavelength in the visible range. In Eq. (7) “(xd, yd) is the coordinates of the dominant wavelength of the light source” - Is the light source the PL? If yes, what is the dominant wavelength of an emission  spectrum with two lines with approximately the same intensities? It seems that Eq. (7) is appropriate for a light source with continuous spectrum, with more or less bell-shaped curve.

R: The term “ligh source” was changed to “emission” for clarity. In our methodology, which is no different for other studies using color purity coefficient, we used the wavelength of the maximum of emission spectrum as a dominant wavelength, as the emission spectra of both Eu and Tb have clear maxima. In the hypothetical case of two lines with roughly the same intensity, the proper methodology would be to calculate the spectral centroid.

2. For temperature measurements - what is the advantage of measuring the color coordinates of the emission, rather than well, chosen FIR? If the importance of “color” purity is not well justified, it is a significant distraction from the main topic of the study.

R: The calculation of the chromaticity shift was done not instead of FIR, but as a separate issue. It was meant to illustrate how the CIE coordinates shift in function of temperature. We do not feel as if it is a distraction from the main topic of study, especially when the figure illustrating the shift was placed next to the spectra of the co-doped samples, analogically to previous results in Figure 4. It feels natural that the reader is able to transfer the information from the emission spectrum to the CIE coordinates and the impact of the temperature on the spectra is analogically displayed on the CIE coordinates as a chromaticity shift.

3. Fig. 1c – needs to be rescaled for better visibility of the absorption peaks. Indicate the electronic transitions.

R: An inset was added to show and name electronic transitions.

4. No legend in Fig 6 c. Is the red line the derivative Eq. (10) of the black line? What function is the black line? What are the points along the red line? More clarification is needed.

R: The black and red data in Fig. 6c were color coded to indicate they are FIR and Sr, respectively. The text clarifies how they are related to each other – equation (10). The figure caption was supplied with “The black/red line serves only to guide the eyes.”

5. “Where ΔFIR is the uncertainty of the determination of FIR with regard to the exponential function FIR = B + C exp(-ΔE’/kT) [37]” How exactly ΔFIR is assessed? What is ΔE’? How big is the preexponential coefficient C?

R: The equation was used as a fitting function. This equation (in this particular case) does not represent a real process and thus the parameters must be treated as phenomenological. Their values do not represent any physical values and therefore were not reported to avoid confusion. ΔFIR was calculated as a standard deviation of |FIR – fitted function|. The fitted function is the black line in Fig. 6c.

6. Is “thermometric” sensitivity the same as “thermal” or “temperature” sensitivity/ Using the latter is preferable.

R: The term “thermometric” was replaced with “temperature”.

7. Use Eq. (xxx) , instead of” formula (xxx)”, used 4 times. The equations need better formatting.

R: The references to equations were revised. The formatting of the equations was dictated by the journal’s requirements and we cannot change them.

8. Figs 5b; 7 c, d and Fig 8 c,d – list the coefficient of determination COD of the fits.

R: The requested COD of the fits were supplied as a figure caption for Figure 5b and in Table S1.

9. How “activation temperature” (line 182) is defined?

R: The term “activation temperatures” was corrected to say “activation energies”.

10. How the smooth lines in Fig 9c is calculated?

R: A paragraph about calculating δT^τ was added (please see page 13).

11. “The GTS glasses are an excellent host for Eu3+ and Tb3+ dopants” What do you

mean by “excellent”?

R: By “excellent host” we mean “a host that provides sufficiently advantageous environment for dopant ions for them to perform intended function”. In this case “intended function” mean “exhibiting luminescence with high color purity and high temperature sensitivity”.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript under review entitled “Excellent color purity and luminescence thermometry performance in germanite tellurite glass doped with Eu³⁺ and Tb³⁺” deals with optical properties of glass system. It is proposed to use intensities and life-times of Eu and Tb luminescence for temperature sensing. Authors performed extensive study, but some questions arose according to results and their discussion. So, I recommend Major revision. Some issues those should be solved before publication can be found below.

 1) What about stability of the glasses under study in temperature range up to 525 °C? It is known, that tellurite and germanite glasses can have quite low glass transition temperatures, e.g. below 400 C. It would be helpful to perform differential thermal analysis (or differential scanning calorimetry) and to include the results into the manuscript.

2) It is hard to agree with authors that refraction index for (50-x)GeO₂-35TeO₂–10SrF₂–5Nb₂O₅-xNd₂O₃ from ref [13] is suitable for calculation of Judd-Ofelt parameters for (50-x) GeO2 – 35 TeO2 – 15 SrF2 – xEu2O3 system. As samples in Fig. 1a looks like to have good quality, authors should try to measure/calculate refractive index for any of their sample.

3) It is not clear from introduction why the color characteristics are needed. Maybe the glass under study proposed not only for thermometry but for some lighting application?

4) Lines 55-57. It is said “The potential of forming a glass-ceramic based on the studied GTS glass is tested by the heat-treatment of the glass and its impact on the structural and spectroscopic properties” but no such discussion can be found in the manuscript. Please clarify.

5) Please add in supplementary some luminescence spectra for Eu-doped glasses in 410 – 720 nm. It is not clear from inset in Fig. 2a that transitions from 5D3, 5D2 and 5D1 levels has any significant intensity in comparison with transitions from 5D0 level.

6) Fig1b and Fig 2b. Please use “Eu2O3 conc” and “Tb4O7 conc.” instead of Eu and Tb conc. Alternatively, calculate Eu and Tb content in samples (e.g. ions/cm3).

7) Figure 4a. Spectrum for Eu+Tb sample has significant background. It is unlikely related with emission of glass host as no background can be found for other spectra. In my opinion, this background should be removed from spectra when chromaticity coordinates are calculated.

8)  Figure 5a. Why quantum yield of Tb-doped glass became negative at some excitation wavelength?

There are also some minor corrections:

9) lines 15-17. Check temperature range. Spectra and dependencies are shown for temperatures from -200 up to 400 C.

10) use degree sign instead of uppercase 0 or o.

11) add units of measure for absorbance in Fig. 1c.

12) lines 108-109. It is said “…scattered blue light introduced by the excitation source” but the UV light was used

13) line 129. It is said “as green emission consisting of four narrow lines” but bands at 480, 580 and 620 nm correspond to blue, yellow and red emission

14) line 132. It is said “The sample doped with 4% Tb2O3…” but it was Tb4O7

Author Response

 1) What about stability of the glasses under study in temperature range up to 525 °C? It is known, that tellurite and germanite glasses can have quite low glass transition temperatures, e.g. below 400 C. It would be helpful to perform differential thermal analysis (or differential scanning calorimetry) and to include the results into the manuscript.

R: We performed the DTA analysis, but the curves did not reveal any transitions up to 900 °C. This must be an equipment failure, since the similar glasses [24] exhibit T_g in the range 460 – 510 °C, but due to time and financial constrains we cannot repeat these measurements. Since we did not study crystallization of this glass system, we think the DTA measurements are not essential and we took the liberty to not include these results in the manuscript. The temperature dependent measurements were limited to 400 °C to ensure that the glasses remain stable.

2) It is hard to agree with authors that refraction index for (50-x)GeO₂-35TeO₂–10SrF₂–5Nb₂O₅-xNd₂O₃ from ref [13] is suitable for calculation of Judd-Ofelt parameters for (50-x) GeO2 – 35 TeO2 – 15 SrF2 – xEu2O3 system. As samples in Fig. 1a looks like to have good quality, authors should try to measure/calculate refractive index for any of their sample.

R: The refractive index was revised. Instead of the one reported for GTSN glass, we used one reported for GTS glass in ref [24]. The values have been revised but they did not vary significantly.

3) It is not clear from introduction why the color characteristics are needed. Maybe the glass under study proposed not only for thermometry but for some lighting application?

R: The applications were clarified in the Introduction: “These materials find applications in various fields, including lighting, displays, and sensors, especially in luminescence thermometry”.

4) Lines 55-57. It is said “The potential of forming a glass-ceramic based on the studied GTS glass is tested by the heat-treatment of the glass and its impact on the structural and spectroscopic properties” but no such discussion can be found in the manuscript. Please clarify.

R: The mentioned sentence has been removed from the manuscript.

5) Please add in supplementary some luminescence spectra for Eu-doped glasses in 410 – 720 nm. It is not clear from inset in Fig. 2a that transitions from 5D3, 5D2 and 5D1 levels has any significant intensity in comparison with transitions from 5D0 level.

R: The spectrum of Eu-doped sample in the 400-870 nm range was supplied as Figure S1.

6) Fig1b and Fig 2b. Please use “Eu2O3 conc” and “Tb4O7 conc.” instead of Eu and Tb conc. Alternatively, calculate Eu and Tb content in samples (e.g. ions/cm3).

R: The X axis labels were corrected to say “Eu2O3 conc” and “Tb4O7 conc” and concentration of Eu and Tb was expressed in [ion/cm3] as well in Figures 2b and 3b.

7) Figure 4a. Spectrum for Eu+Tb sample has significant background. It is unlikely related with emission of glass host as no background can be found for other spectra. In my opinion, this background should be removed from spectra when chromaticity coordinates are calculated.

R: Thank you for a valuable advice. The background of the spectrum has been removed and the CIE coordinates recalculated.

8) Figure 5a. Why quantum yield of Tb-doped glass became negative at some excitation wavelength?

R: The apparent negative values of the QY are due to uncertainties of the measurement method. When the values of absorbed light are very small, it may appear as if the scattered light of the reference measurement exhibit weaker intensity than the scattered light of the sample. This produces negative value, but should be interpreted as very low quantum yield.

There are also some minor corrections:

9) lines 15-17. Check temperature range. Spectra and dependencies are shown for temperatures from -200 up to 400 C.

R: Thank you for noticing. This mistake is a result of K->°C conversion. It has been corrected.

10) use degree sign instead of uppercase 0 or o.

R: The writing of the degree sign has been corrected throughout the manuscript.

11) add units of measure for absorbance in Fig. 1c.

R: The absorption spectra has been presented in unitless scale.

12) lines 108-109. It is said “…scattered blue light introduced by the excitation source” but the UV light was used

R: The excitation source is not monochromatic and although the maximum of the emission is in the UV, it does emit blue light.

13) line 129. It is said “as green emission consisting of four narrow lines” but bands at 480, 580 and 620 nm correspond to blue, yellow and red emission

R: The sentence has been rewritten to say: “as predominantly green emission being a result of four narrow emission lines” for clarity.

14) line 132. It is said “The sample doped with 4% Tb2O3…” but it was Tb4O7

R: The sentence has been corrected.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The article can be accepted.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript can be accepted for publication as is