Crystallization and Investigation of the Structural and Optical Properties of Ce³⁺-Doped Y_3−xCa_xAl_5−ySi_yO₁₂ Single Crystalline Film Phosphors

Round 1

Reviewer 1 Report

This article describes the growth and properties of Ce-doped garnet single-crystal films. The standard YAG structure is modified by alloying with Si and Ca, which effectively promotes the formation of Ce4+. This greatly affects the properties of the Ce3+ luminescent center, e.g. by reducing the luminescence decay time and scintillator properties. The paper is well written and the experiments are well performed. Two minor revisions are requested; i.e. explaining the meaning of segregation coefficient and elaborating on IVCT transitions that will occur naturally if Ce3+ and Ce4+ coexists. More detailed comments can be found below.

• Please explain the meaning of segregation coefficient and how it is obtained.
• A potentially important effect that accelerates the Ce3+ decay when Ce4+ is present are direct intervalence charge transfer (IVCT) transitions that induce fast non-radiative decay channels. This effect has been described for Ce3+/Ce4+ in elpasolites and Eu2+/Eu3+ in fluorides by the group of Luis Seijo. It is desirable that the authors discuss whether this is a possibility in their garnet samples as well.
• The English language is overall good, however very often articles are lacking. The authors are hence recommended to revise the English, especially by adding articles. Some additional typo’s were found:
  • Line 82 row -> raw
  • Line 232 bans -> bands

Author Response

Answers  to the Reviver’s Comments and Suggestions

Reviewer 1

This article describes the growth and properties of Ce-doped garnet single-crystal films. The standard YAG structure is modified by alloying with Si and Ca, which effectively promotes the formation of Ce4+. This greatly affects the properties of the Ce3+ luminescent center, e.g. by reducing the luminescence decay time and scintillator properties. The paper is well written and the experiments are well performed. Two minor revisions are requested; i.e. explaining the meaning of segregation coefficient and elaborating on IVCT transitions that will occur naturally if Ce3+ and Ce4+ coexists. More detailed comments can be found below.

• Please explain the meaning of segregation coefficient and how it is obtained.

The segregation coefficient is the ratio between the concentrations of the garnet cations in the melt-solution and as-grown film. Namely, for the film sample, grown from the melt with a nominal charge content of Y_2.5Ca_0.5Al_4.5Si_0.5O₁₂:Ce, the real content is Y_2.825Ca_0.13Ce_0.065Al_4.935Si_0.065O₁₂ (see Table 1). Therefore, the segregation coefficients of Ca and Si cations will be 0,13/0.5=0.28 and 0,065/0,5=0.13, respectively.

• A potentially important effect that accelerates the Ce³⁺ decay when Ce⁴⁺ is present are direct intervalence charge transfer (IVCT) transitions that induce fast non-radiative decay channels. This effect has been described for and Eu2+/Eu3+ in fluorides by the group of Luis Seijo. It is desirable that the authors discuss whether this is a possibility in their garnet samples as well.

Thank you for your comment. This is an interesting point of view. We have added the possible relationship on the presence of Ce³⁺/Ce⁴⁺ intervalence charge transfer transitions in garnet compounds. The respective publications on observation of such transitions in the other materials were added to the revised version of the manuscript as well.

• The English language is overall good, however very often articles are lacking. The authors are hence recommended to revise the English, especially by adding articles.

Thank you for your suggestion. We have checked again the English in the revised version of the manuscript.

Some additional typo’s were found:

• Line 82 row -> raw
• Line 232 bans -> bands

Thank you for your comments. We have corrected these mistakes in the revised version of the manuscript.

Reviewer 2 Report

It is an interesting and well-written manuscript in my opinion.

I suggest the publication of this manuscript as it is.

Author Response

No comments

Reviewer 3 Report

The authors are reporting investigation of the optical properties of single crystalline films of Ce3+ doped Y3-xCaxAl5-ySiyO12 garnet with different x and y composition.

This study is a full investigation and worthwhile for publication, clarification or more information are suggested to make strengthen this article.

The following is my comments.

1) Describe how to obtain real SCF contents in Table 1. What is the accuracy of composition measurement?

2) Grown crystal quality need to be quantified. Show photography of the grown crystals, Rocking curve as well as transmittance plot.

3) Authors report luminescence property related with Pb2+. How much impurity of Pb2 in the grown SCF?

4) Authors used alpha source for the light yield comparison, are they all have the same alpha/beta ratio?

5) To have better understanding of light yield and quantum efficiency, it will be helpful if authors can add internal quantum efficiency in Table 3 (Ex :Using Quantalus for QE measurement). Table 3 show that light yield decreases when decay time decreases, it could have a relation with internal quantum efficiency.

Author Response

Answers  to the Reviver’s Comments and Suggestions

Reviewer 3

The authors are reporting investigation of the optical properties of single crystalline films of Ce3+ doped Y3-xCaxAl5-ySiyO12 garnet with different x and y composition.

This study is a full investigation and worthwhile for publication, clarification or more information are suggested to make strengthen this article.

The following is my comments.

• Describe how to obtain real SCF contents in Table 1. What is the accuracy of composition measurement?

Typical one-point accuracy of the composition measurements using EDX detectors at scanning electronic microscope lies in  the 50-100 ppm (0.005-0.01 at. %) range. Meanwhile, due to the measurements of the content in several places of the film samples (typically 5-10 points) and following averaging of the results, the accuracy level can be improved even up to 0.001-0.003 at. %, depending on the cation type.

• Grown crystal quality need to be quantified. Show photography of the grown crystals, Rocking curve as well as transmittance plot.

Thank you for your comments. The photo of as grown  Y_2.42Ca_0.52Ce_0.05Al_4.5Si_0.5O₁₂ SCF in comparison with reference YAG:Ce SCF sample is presented in Fig.1.

• Authors report luminescence property related with Pb2+. How much impurity of Pb2 in the grown SCF?

Typically the content of the Pb flux related impurity is in the 30-50 ppm rangre depending on the SCF growth temperature and overcooling growth rate of the melt solution; e. g. typically below the detectable level  of the apparatus for content measurements. But Pb²⁺ or Pb⁴⁺ ions can be simply detected in the absorption and/or luminescence spectra measurements, which are much more sensitive for determination of the presence of such types  of dopants in the SCF samples.

• Authors used alpha source for the light yield comparison, are they all have the same alpha/beta ratio?

No. The alpha/beta ratio of the scintillation light yield is typically equal to 0.2 for the garnet compounds.

• To have better understanding of light yield and quantum efficiency, it will be helpful if authors can add internal quantum efficiency in Table 3 (Ex :Using Quantalus for QE measurement). Table 3 show that light yield decreases when decay time decreases, it could have a relation with internal quantum efficiency.

Thank you for your comment. But please take into account that it is impossible to perform correct QE measurement of the SCFs under study due to the fact that such samples are the LPE grown epitaxial structures which include YAG substrate with much larger (~0.5 mm ) thickness than that of SCF with a thickness of several microns on the both sides of substrate. For this reason, the LY measurements under α-particles excitation of the film sample in comparison with standard YAG:Ce SCF with the known LY in Photons per KeV are  much more correct and informative for application of SCFs of different oxides as scintillating materials (see also [15], [16], [28], [29], [32] and reference therein).

Reviewer 4 Report

This article demonstrates Ce3+ doped YAG phosphors. The manuscript was well written and scientific evidences were clearly presented. Thus, this manuscript is accepted for publication.

Author Response

No comments

Round 2

Reviewer 1 Report

The authors changed the manuscript according to the questions of the referees, it can now be accepted for publication.