Crystal Growth and Spectroscopy of Yb²⁺-Doped CsI Single Crystal

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Submission ID: crystals-3027488

Journal: Crystals

Title:  Crystal growth........single crystal

Authors: D.Sofich et.al.

Comments: The authors discuss crystal growth of Yb2+ doped CsI. I have following observations:

1// The motivation of the present work is not clear. There should be inclusion of more recent articles in the introduction section.   

2// The English language should be modified with the help of a native speaker.

3// How are the results different than the other similar hosts. The clarity of the results should be improved. What is the basis for the choice of Yb2+ ions.

4//  What is the basis to fit thermally stimulated depolarisation current curve in Figure 7. Why fitting was done with only two components.

5// Please provide lifetime data of the sample. Does the sample shows persistent luminescence.

6// What is the basis of Yb2+ replacing Cs+ ions? What about the charge imbalance created during this process. 

Based on these, I recommend MAJOR revision for the present manuscript. 

Comments on the Quality of English Language

Should be improved

Author Response

Dear reviewer, thank you for your suggestions. We rewritten the manuscript in accordance with your comments.

1// The motivation of the present work is not clear. There should be inclusion of more recent articles in the introduction section.   

Thank you for the valuable comments. The introduction was extended. The motivation is rewritten.

2// The English language should be modified with the help of a native speaker.

We try to correct the English language.

3// How are the results different than the other similar hosts. The clarity of the results should be improved. What is the basis for the choice of Yb²⁺ ions.

The comparison with similar hosts was added to the manuscript. The motivation was added to the manuscript.

4//  What is the basis to fit thermally stimulated depolarisation current curve in Figure 7. Why fitting was done with only two components.

In the region 160-220 K two peaks is clearly seen. The fitting curve is well correlated with an experimental data. The physical meaning of two peaks is the presence of two different Yb²⁺ related dipole centers. We suppose that they are two Yb²⁺ and cation vacancy related dipoles when the cation vacancies are located in the nearest neighbor and next-nearest neighbor positions.

5// Please provide lifetime data of the sample. Does the sample shows persistent luminescence.

Thank you for the suggestion. We added luminescence decay measurements. No persistent luminescence and afterglow we registered.

6// What is the basis of Yb²⁺ replacing Cs⁺ ions? What about the charge imbalance created during this process. 

In the manuscript, we pointed that Yb²⁺ ion substitutes Cs⁺ ion. The charge compensation occurs with cation vacancies in nearest neighbor and next-nearest neighbor positions.

Reviewer 2 Report

Comments and Suggestions for Authors

In the article under review, a new scintillator single crystal CsI:Yb²⁺ was grown for the first time, and its luminescent properties were studied. The work was carried out on a current topic and can be supported after taking into account some points.
Questions and comments about the article:
-Is it possible to provide some quantitative estimates comparing the effectiveness of the new scintillator with the existing ones?
-in the “Funding” section, it is indicated that dielectric measurements were carried out, but this article presents data on absorption and luminescence;
-in a fairly large literature review, several related articles are not discussed

M. S. Alekhin, D. A. Biner, K. W. Krämer, P. Dorenbos. Optical and scintillation properties of SrI₂:Yb²⁺ // Optical Materials 37 382 (2014).
K. Mizoi, M. Arai, Y. Fujimoto, D. Nakauchi, M. Koshimizu, T. Yanagida, K. Asai. Evaluation of photoluminescence and scintillation properties of Yb²⁺-doped SrCl_{2− x}Br_x crystals // Journal of the Ceramic Society of Japan 129 406 (2021).
D. Sekine, Y. Fujimoto, M. Koshimizu, D. Nakauchi, T. Yanagida, K. Asai. Photoluminescence and scintillation properties of Yb²⁺-doped SrCl₂ crystals // Japanese Journal of Applied Physics 59 012005 (2020).

 

-What is the stoichiometry of the resulting crystal? What atmosphere was it grown in? Is the content of Yb²⁺ ions different in the melt and in the crystal?
- when discussing the method of optical measurements, the accuracy of the instruments used is not indicated;
-maybe it’s worth moving Figure 2 to the results section?
- when discussing the absorption and excitation spectra, the positions of the detected bands are given with different accuracy (one and two decimal places), and I would like to see the same number of decimal places;
-in line 166, it is necessary to correct the spelling of the point symmetry group;
-lines 171 and 173 must contain a capital letter "T";
-in reference 4, it is necessary to adjust the upper and lower indices;
-reference 6 to the translated version of the article?

Author Response

Dear Reviewer, thank you for your comments. We changed the manuscript following your suggestions.

-Is it possible to provide some quantitative estimates comparing the effectiveness of the new scintillator with the existing ones?

Yes, we provided the raw estimation comparing X-ray luminescence light outputs. In next articles, when the crystals with higher Yb²⁺ concentration will be grown we will provide pulse-height measurements.

“The X-ray luminescence output of grown CsI:Yb²⁺ is estimated by comparison with CaF₂:Eu²⁺ (21,000 photons/MeV). The X-ray luminescence is about 1.9 times higher than CaF₂:Eu and it is about 40,000 photons/MeV. The concentration of Yb²⁺ ions in the studied CsI is relatively low (10^-3 mol%). Increasing the Yb²⁺ concentration up to one order of magnitude should increase light output”

-in the “Funding” section, it is indicated that dielectric measurements were carried out, but this article presents data on absorption and luminescence;

We changed this to thermally stimulated depolarization

-in a fairly large literature review, several related articles are not discussed

We added the references and increase the Introduction.

-What is the stoichiometry of the resulting crystal? What atmosphere was it grown in? Is the content of Yb²⁺ ions different in the melt and in the crystal?

The crystal was grown under a pressure of 110 kPa in an argon atmosphere. We added a paragraph about the concentration of ytterbium in the crystal, and also described the process of single crystal growth in more detail.

"The concentration of Yb²⁺ in the raw material is much higher than in the bulk. The estimated concentration of Yb²⁺ in the studied single crystals is about 10^-3 mol. %. The difference in Yb²⁺ concentration between the melt and crystal could be due to a low Yb²⁺ impurity distribution coefficient. In some alkali halides, the divalent impurities could have low values of the impurity distribution coefficient. Another factor that influences the Yb²⁺ concentration in the bulk is the contamination by uncontrollable water molecules during the crystal growth, which leads to the formation of oxyiodides."

- when discussing the method of optical measurements, the accuracy of the instruments used is not indicated;

Thank you for the comment. We added information about accuracy.

“Optical absorption spectra of polished 10 mm CsI-Yb plates were measured using a Lambda 950 spectrophotometer (Perkin-Elmer, NY, USA) at room temperature. The spectral slit width was set to 2 nm. The limit of spectral resolution of the spectrophotometer was 0.2 nm. However, the absorption bands of CsI-Yb²⁺ are much broader than the limit of resolution.”

“The slits widths in the luminescence measurements were about 4 nm.”

“The spectral resolution of the excitation spectra measurements were about 3 meV”

-maybe it’s worth moving Figure 2 to the results section?

The figure was moved to the Results section

- when discussing the absorption and excitation spectra, the positions of the detected bands are given with different accuracy (one and two decimal places), and I would like to see the same number of decimal places;

Thank you for the comment. We fixed it.

-in line 166, it is necessary to correct the spelling of the point symmetry group;

Thank you, we fixed the spelling

-lines 171 and 173 must contain a capital letter "T";

The letter was changed to capital.

-in reference 4, it is necessary to adjust the upper and lower indices;

We fixed it

-reference 6 to the translated version of the article?

The reference is to Russian version of the article. We changed the references (added in Russian)

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript (crystals-3027488) by Dmitriy Sofich and his colleagues thoroughly examines the optical characteristics and possible uses of Yb²⁺ doped CsI single crystals. The research investigates several facets, such as crystal development methods, luminescence mechanism, excitation transmission kinetics, and theoretical calculations using density functional theory. The main discoveries are identifying the radiative transitions of Yb2+ ions, observing emission from self-trapped excitons, and clarifying how Yb²⁺ ions are compensated for in the CsI crystal lattice. Furthermore, the paper highlights the capability of Yb²⁺ ions as efficient additives for CsI scintillators and X-ray phosphors, mainly when used along with SiPM photodetectors. The results and discussion seem satisfactory overall, while some aspects still need additional improvement.

Comments

[1]   What are the radiative transitions of Yb²⁺ ions discovered?

[2]   How are cation vacancies related to Yb²⁺ ion transitions?

[3]   How do Yb^2+-cation vacancy combinations influence depolarization phenomena?

[4]   To improve the repeatability and rigour of the research, it is recommended that more detailed explanations of the individual experimental processes, especially those related to crystal formation using the Czochralski method and optical absorption measurements, be included.

[5]   Integrating the experimental findings with the theoretical insights derived from density functional theory could further elucidate the underlying mechanisms governing the optical properties and excitation dynamics of the CsI-Yb²⁺ crystal system, enhancing the coherence of the manuscript.

 

 

 

Comments on the Quality of English Language

Minor editing of the English language is required.

Author Response

Dear Reviewer, thank you for valuable question. We try to improve the manuscript.

What are the radiative transitions of Yb2+ ions discovered?

The radiative transitions in Yb²⁺ in CsI is attributed to 5d-4f transition. The observed spin-allowed and spin-forbidden transitions. We pointed this in the text

How are cation vacancies related to Yb²⁺ ion transitions?

Thank you for the question. We expanded the discussion: The ab initio calculation shows that the lowest energy bands in the absorption spectra correspond to transitions from 4f¹⁴ to triply degenerate 5d¹ (T2g) states, but higher energy transitions are attributed to transitions to doubly degenerate 5d¹ Eg states in the nnn configuration when the cation vacancy is located in the next-nearest neighbor position. The position of the 4f-5d absorption bands for the divalent Yb ion compensated by a cation vacancy in the nearest neighbor (nn) configuration have higher energy and larger intensity. The comparison of the observed and measured absorption spectra shows the contributions from both Yb²⁺ centers. All the calculated bands in the absorption spectra in the range 3-5 eV correspond to intracenter 4f-5d transitions. Transitions from the 4f level to the cation vacancy are not found in the calculation. Therefore, the cation vacancies do not participate in the observed electronic transitions. The differences between absorption spectra of Yb²⁺ compensated with the cation vacancy in nn and nnn positions are related to different point groups of Yb²⁺ ions.

How do Yb²⁺-cation vacancy combinations influence depolarization phenomena?

We try to elucidate this in the manuscript:

“The Yb²⁺-iodine vacancy and Yb²⁺-cation vacancy combinations form electrical dipoles. As a result, they can be polarized at low temperatures and subsequently depolarized during heating. This phenomenon gives rise to a thermally stimulated depolarization (TSD) curve….

The activation energies are 0.514 eV and 0.558 eV. The frequency factor, s = 5 THz, corresponds to the phonon modes present in CsI. The different types of dipoles have different activation energies. The observed peaks are attributed to Yb²⁺-cation vacancy dipoles, also known as I-V complexes. The lower energy peak is related to the reorientation of nearest-neighbor (nn) dipoles, while the peaks at 205 K correspond to the reorientation of next nearest-neighbor (nnn) dipoles.”

To improve the repeatability and rigour of the research, it is recommended that more detailed explanations of the individual experimental processes, especially those related to crystal formation using the Czochralski method and optical absorption measurements, be included.

The optical absorption measurement and crystal growth procedure description are extended.

Integrating the experimental findings with the theoretical insights derived from density functional theory could further elucidate the underlying mechanisms governing the optical properties and excitation dynamics of the CsI-Yb²⁺ crystal system, enhancing the coherence of the manuscript.

Thank you for the suggestion. We rewritten the part with integration of experimental results and theoretical insights for better understanding.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is modified and can be accepted for publication.

Comments on the Quality of English Language

Fine

Reviewer 2 Report

Comments and Suggestions for Authors

In the revised version of the article, the authors took into account the reviewer’s comments. I think that in its current form, the article can be accepted for publication.