Upconversion Luminescence from Sol-Gel-Derived Erbium- and Ytterbium-Doped BaTiO₃ Film Structures and the Target Form

Round 1

Reviewer 1 Report

The authors show enhancement of upconversion luminescence in microcavity with BaTiO3:(Er,Yb) active layer, refractive index ? and absorption index ? for these two BaTiO3:(Er,Yb) films annealed at 450 °C and 800 °C are calculated from the reflection and transmission spectrum. The paper is overall well organized; however, some figures are not plotted properly. Especially figure 4a,4b and figure 5. Here are my comments:

1. In figure 4, the reflection spectrum of the cooling process (dashed lanes) are unclear to the readers, please correct it.

2. I suggest the authors to re-plot figure 5. It is difficult to identify the transmission and reflection spectrum of the 2 samples in one figure. In the main text you didn’t mention the difference in spectrum between films annealed at 450 °C and 800 °C,I suggest that separate them into two figures is better.

3. Why the upconversion luminescence shows a great difference between the BaTiO3:(Er,Yb)/SiO2 microcavity annealed at 450 °C and 800 °C? Is the film undergoing phase transition above 450 °C? Is the enhancement in PL in relation with the increase of absorption in the high region? (As shown in figure 6b)

Author Response

Reviewer 1:

The authors show enhancement of upconversion luminescence in microcavity with BaTiO3:(Er,Yb) active layer, refractive index ? and absorption index ? for these two BaTiO3:(Er,Yb) films annealed at 450 °C and 800 °C are calculated from the reflection and transmission spectrum. The paper is overall well organized; however, some figures are not plotted properly. Especially figure 4a,4b and figure 5. Here are my comments:

1. In figure 4, the reflection spectrum of the cooling process (dashed lanes) are unclear to the readers, please correct it.

Our response:

1. We are grateful to the Reviewer for useful comments, which were fully considered in the revised version of the manuscript. According to the Reviewer’s comment, this figure and its caption have been improved to demonstrate the differences more clearly between the heating and cooling data (new Figure 5 and page 7 in the revised version of the manuscript).
2. 2. I suggest the authors to re-plot figure 5. It is difficult to identify the transmission and reflection spectrum of the 2 samples in one figure. In the main text you didn’t mention the difference in spectrum between films annealed at 450 °C and 800 °C,I suggest that separate them into two figures is better.

Our response:

According to the Reviewer’s recommendations, Figure 5 has been improved and replotted as Figures 6a and 6b. The difference in the spectra of films annealed at 450 °C and 800 °C was emphasized and additionally discussed in the revised manuscript ( pages 8, 9). 

The higher reflectivity in the low-absorption spectral region from 400 to 800 nm (Fig.5a) results in smaller transmittance (Fig.5b) for the film annealed at 800 °C in comparison with that for 450°C.

3. Why the upconversion luminescence shows a great difference between the BaTiO3:(Er,Yb)/SiO2 microcavity annealed at 450 °C and 800 °C? Is the film undergoing phase transition above 450 °C? Is the enhancement in PL in relation with the increase of absorption in the high region? (As shown in figure 6b)

Our response:

Unlike BaTiO₃ target, the polycrystalline phase and phase transitions in the prepared sol-gel films were hardly detectable with our technique. Formation and detection of phases for sol-gel-derived films depends on film content, film thickness, and type of substrate. For the sol-gel derived films of a thickness of 50–100 nm, it is difficult to detect the phase either with XRD or Raman spectroscopy, we may see for example  [ Ref add1], [Ref add 2]. Some sol-gel derived films, like yttrium alumina on fused silica, are amorphous even after annealing at 1000 °C according to the XRD analysis. [Ref add 2].

 

[ Ref add1], E. I. Lashkovskaya, N. V. Gaponenko, M.V. Stepikhova, A. N. Yablonskiy, B. A. Andreev, V.D. Zhivulko, A.V. Mudryi, I.L. Martynov, A.A. Chistyakov, N. I. Kargin, V.A. Labunov, T. F. Raichenok, S. A. Tikhomirov and V.Yu. Timoshenko / Optical Properties and Upconversion Luminescence of BaTiO₃ Xerogel Structures Doped with Erbium and Ytterbium // Gels. – 2022. – Vol. 8. – P. 347. DOI: 10.3390/gels8060347

 

 

[Ref add 2]. J. C. Villegas Brito, N. V. Gaponenko, K. S. Sukalin, T. F. Raichenok, S. A. Tikhomirov, V. A. Yankovskaya, N. I. Kargin / Luminescence of Eu³⁺ ions from yttrium alumina films on fused silica substrates // Journal of Applied Spectroscopy. – 2017. – V.84 N 1. – P. 674-678. https://doi.org/10.1007/s10812-017-0528-x

 

The observed enhancement of the upconversion luminescence in the film annealed at 800 °C can be explained by thermal annealing of nonradiative centers, which control the population of the excited states of trivalent erbium. The most probable candidate are intrinsic defect states in the band gap of barium titanate matrix, which are detectable as the Urbach tail states in the absorption spectra (Fig.7), and hydroxyl groups, which are known to be responsible for the quenching of the erbium luminescence in oxide matrices [new Ref. 38]. The higher annealing temperature resulted in lower efficiency of both quenching processes in the investigated samples. The corresponding discussion has been added to the revised manuscript (page 10).

 

[new Ref.38] Yingchao Yan, Anne Jans Faber, Henk de Waal. Luminescence quenching by OH groups in highly Er-doped phosphate glasses. Journal of Non-Crystalline Solids 181 (1995) 283-290. https://doi.org/10.1016/S0022-3093(94)00528-1

 

 

Reviewer 2 Report

My comments are in attachment.

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

 

photonics-2271165

 

Upconversion luminescence from sol-gel-derived erbium and ytterbium-doped BaTiO3 film structures and the target

 

Nikolai V. Gaponenko, Nikolai I. Staskov, Larisa V. Sudnik, Petr A. Vityaz, Alexei R. Luchаnok, Yuliana D. Karnilava, Ekaterina I. Lashkovskaya, Margarita V. Stepikhova, Artem N. Yablonskiy, Vadim D. Zhivulko, Alexander V. Mudryi, Igor L. Martynov, Alexander A. Chistyakov, Nikolai I. Kargin, Vladimir A. Labunov, Yuriy V. Radyush, Eugene B. Chubenko, Victor Yu. Timoshenko

 

This manuscript contains a short description of the conditions used for the preparation of BaTiO3: Er3+,Yb3+-based phosphors by the sol-gel technique. To increase crystallinity, the samples were annealed at high temperature. The structural properties of the samples were obtained by the XRD analysis. To enhance the upconversion efficiency, the multilayer microcavity structure was fabricated onto the Si substrate and their photoluminescence characteristics were determined by conventional experimental methods. The experimental conditions used for the sample preparation and property measurements are given in detail that opens a possibility for future testing of the results by other researchers. In my opinion, this study is valuable because the upconversion properties were studied in specific multilayer microcavity structure that is promising for device integration. The general level of this study is high and manuscript could be considered for publication after minor revision reasonable to increase the paper quality. My several local corrections proposed for the text are listed below for author consideration.

 

Our response:

We are grateful to the Reviewer for the useful comments and recommendations, which were fully considered in the revised version of our manuscript.

 

Page 1

 

Optical properties of sol-gel derived structures depend on sol content, deposition and heat treatment conditions, film thickness and other factors.

 

Optical properties of sol-gel derived structures are dependent on sol composition, deposition and heat treatment conditions, film thickness and other factors.

 

Our response:

This comment was taken into account in the revised version of the manuscript, page 1 (Abstract).

 

 

Page 1

The BaTiO3:(Er,Yb)/SiO2 microcavity exhibits an enhancement of the upconversion luminescence when compared to the BaTiO3:(Er,Yb) double layer fabricated directly on a crystalline silicon substrate.

What is a "double layer"?

 

Our response:

Double layer is a structure, which is fabricated by using the layer-by-layer deposition. It consists of the first layer deposited by spinning the sol followed with drying in air and thermal annealing at 450 °C, and the second layer deposited onto the first one in the same manner. This double layer corresponds to a half-wave microcavity, unlike the single layer of Bragg reflector, also referred to as a quarter-wave layer.

In the abstract (page 1), we have omitted the term "double," while it was additionally described in the experiential section in the revised manuscript.

 

 

Page 1

The optical properties, upconversion luminescence and potential applications of BaTiO3:(Er,Yb) sol-gel derived specimens are discussed.

 

The optical properties, upconversion luminescence and potential applications of the BaTiO3:(Er,Yb) sol-gel derived specimens are discussed.

 

Our response:

This comment has been considered in the revised version of the manuscript, page 1 (Abstract).

 

 

Page 2

 

Also, strong luminescence of lanthanides from BaTiO3 host materials is currently widely investigated, particularly, Stokes luminescence under visible and UV light excitation [2,11] and upconversion photoluminescence (PL) excited under IR range illumination [12–14].

 

Also, strong luminescence of lanthanides in BaTiO3 host material is currently widely investigated, particularly, Stokes luminescence under visible and UV light excitation [2,11] and upconversion photoluminescence (PL) excited under IR range illumination [12–14].

 

Our response:

The corresponding correction has been done in the revised manuscript, page 2 (Introduction).

 

 

Page 2

 

In this work we present data on upconversion luminescence from a sol-gel derived BaTiO3 target doped with Er and Yb, i.e. rare earth doped material BaTiO3:(Er,Yb), and continue our investigation of the optical properties of BaTiO3/SiO2 microcavity annealed from 450 to 800 °C.

 

In this work, we present data on upconversion luminescence from a sol-gel derived BaTiO3 target doped with Er and Yb, i.e. rare earth doped material BaTiO3:(Er,Yb), and continue our investigation of the optical properties of BaTiO3/SiO2 microcavity annealed at the temperatures from 450 to 800 °C.

 

Our response:

The corresponding correction has been done in the revised manuscript, page 2 (Introduction).

 

 

Page 2

 

From the transmission and reflection spectra of a BaTiO3:(Er,Yb) layer we determined the refractive index and absorption index, optical band gap and Urbach energy for BaTiO3:(Er,Yb) films annealed at 450 and 800 °C.

 

From the transmission and reflection spectra of a BaTiO3:(Er,Yb) layer, we determined the refractive index and absorption index, optical band gap and Urbach energy for BaTiO3:(Er,Yb) films annealed at 450 and 800 °C.

 

Our response:

The corresponding correction has been done in the revised manuscript, page 2 (Introduction).

 

 

Page 2

For the synthesis of barium titanate sol (sol I), titanium isopropoxide (Ti(OC3H7)4), barium acetate (Ba(CH3COO)2), acetylacetone (CH3COCH2COCH3) and acetic acid (CH3COOH) were used as starting components.

Purity and supplier should be reported for each starting reagent.

 

Our response:

It has been considered in the revised manuscript, page 2 (Experimental).

The purity and supplier data were added in the revised version of the manuscript.

 

 

Page 2

 

The amounts of titanium isopropoxide and barium acetate were chosen so that the Ti/Ba ratio corresponded to the stoichiometric composition of the barium titanate in the films (i.e., Ti:Ba = 1:1).

 

The amounts of titanium isopropoxide and barium acetate were chosen so that the Ti/Ba ratio was correspondent to the stoichiometric composition of the barium titanate in the films (i.e., Ti:Ba = 1:1).

 

Our response:

The corresponding correction has been done in the revised manuscript, page 2 (Experimental).

 

 

Page 2

 

The solution of titanium isopropoxide (Ti(OC3H7)4) in acetylacetone (CH3COCH2COCH3) was stirred until it cooled. Separately, barium acetate (Ba(CH3COO)2) was dissolved in distilled water and stirred until completely dissolved. Erbium acetate hydrate (Er(CH3COO)3·xH2O) was added to the barium acetate solution and stirred until complete dissolution. Then, ytterbium acetate hydrate (Yb(CH3COO)3·xH2O) was added to the solution of barium and erbium acetates and stirred until completely dissolved.

To obtain silica xerogel (SiO2), silica sol (sol III) was prepared. Concentrated nitric acid (HNO3) was added to an alcohol-water mixture (volume ratio of distilled water and ethanol (C2H5OH) approximately 1:6) to pH = 1. Tetraethyl orthosilicate (Si(OC2H5)4) was added to this mixture, stirred and the pH was adjusted again to 1 by adding concentrated nitric acid.

 

Purity and supplier should be reported for each starting reagent.

 

Our response:

It’s corrected in the revised manuscript, page 2 (Experimental). The purity and supplier data were reported.

 

 

Page 2

 

The sol should age for at least 24 h in airtight conditions before the deposition.

 

The sol should be aged for at least 24 hours in airtight conditions before the deposition.

 

Our response:

The corresponding correction has been done in the revised manuscript, page 2 (Experimental).

 

 

Page 2

 

After the deposition of sol I it was dried at 200 °C for 10 min and annealed at 450 °C for 30 min.

 

After the deposition of sol I, it was dried at 200 °C for 10 minutes and annealed at 450 °C for 30 minutes.

 

Our response:

The corresponding correction has been done in the revised manuscript, page 3 (Experimental).

 

 

Page 2

 

Then the sol III was deposited on the BaTiO3 xerogel layer and subjected to the same heat treatment: drying at 200 °C for 10 min and annealing at 450 °C for 30 min to form SiO2 xerogel film. Then a double xerogel layer, BaTiO3:(Er, Yb), referred to as the "active cavity layer," was formed from the sol II by sequentially deposition and annealing at 450 °C for 30 min.

 

Then, the sol III was deposited on the BaTiO3 xerogel layer and subjected to the same heat treatment: drying at 200 °C for 10 min and annealing at 450 °C for 30 min to form SiO2 xerogel film. Then, a double xerogel layer, BaTiO3:(Er, Yb), referred to as the "active cavity layer," was formed from the sol II by sequential deposition and annealing at 450 °C for 30 min.

 

Over the text, please use h and min or hours and minutes, but not a mixture.

 

Our response:

The corresponding corrections have been performed in the revised manuscript (starting from page 3).

 

 

Page 3

 

For the reason of saving the expensive components containing Er and Yb we used two types of powder: BaTiO3 xerogel powder undoped with lanthanides prepared from the sol I and BaTiO3:(Er,Yb) xerogel powder prepared from the sol II.

 

For the reason of saving the expensive components containing Er and Yb, we used two types of powder: BaTiO3 xerogel powder undoped with lanthanides prepared from the sol I and BaTiO3:(Er,Yb) xerogel powder prepared from the sol II.

 

Our response:

It’s corrected in the revised version of the manuscript (page 3 Section 2.2).

 

 

Page 3

 

A target of diameter 48 mm and thickness 4 mm was formed from the BaTiO3 xerogel powders.

 

A target of diameter 48 mm and thickness 4 mm was formed from the BaTiO3 xerogel powder.

 

Our response:

We used two types of powders in the synthesis of the target, i.e. undoped BaTiO3 powder and BaTiO3 lanthanide-doped one.

Therefore, following the Reviewer’s remark we have written this sentence in the following way:

A target of diameter 48 mm and thickness 4 mm was formed from a mixture of the undoped BaTiO3 xerogel powder and lanthanides-doped one.

 

 

Page 3

 

Thus, the top part of target (front side) was enriched with Er and Yb whereas the bottom side was depleted with lanthanides.

 

What is the reason for this component redistribution?

 

Our response:

Since the lanthanides are rather expensive, it is no need to dope the target with the lanthanides along its entire thickness. From our viewpoint, the target described in the manuscript can be used for IR visualization. When we use two powders, one undoped and one doped with the lanthanides, we can reduce the cost of the target, which can be also used as an inexpensive IR visualizer.

 

 

Page 4

 

Reflection spectra studied in the temperature range 26–120 °C were measured using an Ocean Optics USB2000+ spectrometer equipped with a fiber reflection probe and a halogen lamp.

 

Reflection spectra in the temperature range of 26–120 °C were measured using an Ocean Optics USB2000+ spectrometer equipped with a fiber reflection probe and a halogen lamp.

 

Our response:

It’s corrected in the revised version of the manuscript, (page 4 Section 2.3).

 

 

Page 4

 

Figure 1. (a) scheme the microcavity BaTiO3:(Er,Yb)/SiO2 structure and (b) its SEM-image after the final heat treatment at 450 °C.

 

Figure 1. (a) scheme of the microcavity BaTiO3:(Er,Yb)/SiO2 structure and (b) its SEM-image after the final heat treatment at 450 °C.

 

Our response:

It’s corrected in the revised version of the manuscript (page 4, Figure 1).

 

 

Page 4

 

Table 1 displays the EDX data of the BaTiO3:(Er,Yb) target, prepared from the same sol (sol II) as the cavity layer.

 

Table 1 displays the EDX data of the BaTiO3:(Er,Yb) target prepared from the same sol (sol II) as the cavity layer.

 

Our response:

It’s corrected in the revised version of the manuscript (page 4, lines before Table 1).

 

 

Page 5

 

Raman spectra recorded from either of the sides of the target reveal several characteristic lines at 260, 293, 521 and 725 cm-1 corresponding to the three major vibration modes of crystalline BaTiO3: A(TO), E(TO-LO), (A+E)(LO) (Figure 2b) [15].

 

Raman spectra recorded from either of the sides of the target reveal the characteristic lines at 260, 293, 521 and 725 cm-1 corresponding to the three major vibration modes of crystalline BaTiO3: A(TO), E(TO-LO), (A+E)(LO) (Figure 2b) [15].

 

Our response:

It’s corrected in the revised version of the manuscript (page 5).

 

 

Page 5

 

It can be seen that barium titanate has a cubic perovskite structure with a lattice parameter of about 4.009 Å.

 

It can be seen that barium titanate has a cubic perovskite structure with a cell parameter of about 4.009 Å.

 

Our response:

It’s corrected in the revised version of the manuscript (page 5).

 

 

Page 5

 

The phase marked A on the X-ray diffraction pattern can be attributed to Er with the cubic structure of the Fm 3m (225) space group, similar to that recorded on ICSD Database card No. 53386, with the lattice parameter somewhat smaller than on the card.

 

The phase marked A on the X-ray diffraction pattern can be attributed to Er with the cubic structure of the Fm 3m (225) space group, similar to that recorded on ICSD Database card No. 53386, with the cell parameter somewhat smaller than on the card.

 

Is it true that the presence of metal Er is detected after the high-temperature annealing in the air?

 

Besides card number, the related original paper should be cited.

 

Our response:

We are especially grateful for this Reviewer’s comment.  Indeed, it is difficult to identify the phase marked with the symbol “A”. Similar spectra were assigned to metallic erbium  [ref. add1] or Er₂O₃ [ref. add 2]. In both cases, the spectra were described by the cubic structure of the Fm͞3m (225) space group, with somewhat different cell parameters.

Taking into account the high-temperature annealing of the target in air we agree with the Reviewer that the detected phase belongs more probably to Er2O3.

 

[Ref. add 1]. The observation of face centered cubic Gd, Tb, Dy, Ho, Er and Tm in the form of thin films and their oxidation / A.E. Curzon, H.G. Chlebek // Journal  of  Physics F: Metal Physics. –1973. –Vol.3(1). –P. 1-5.

[Ref. add 2]. Electron-diffraction and X-ray diffraction study of rare earth metal oxides in thin films / A.A. Kashaev, L.V. Ushchapovskii, A.G.Il'in // Soviet Physics, Crystallography –1975. –Vol.20. –P.114-115. (in rus.=Kristallografiya. –1975. –Vol. 20. –P.192-193.)

 

The corresponding corrections have been done in the revised version of the manuscript (page 5 and reference [15] on page 12).

 

 

Page 6

 

The target from the front side demonstrates bright orange room-temperature upconversion PL under excitation at 980 nm with the bands at 410, 523, 546, and 658 nm, which correspond to the 2H9/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 , and 4F9/2 → 4I15/2 transitions in Er3+ ions (Figure 2c,d).

 

The target from the front side demonstrates bright orange room-temperature upconversion PL under the excitation at 980 nm with the bands at 410, 523, 546, and 658 nm, which are correspondent to the 2H9/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 , and 4F9/2 → 4I15/2 transitions in Er3+ ions (Figure 2c,d).

 

For comparison, several representative papers concerning the upconversion effect in different Er3+-doped oxide phosphors could be cited herein:

1. J. Solid State Che 228 (2015) 160-166 J. Lum. 215 (2019) 116703

Crystals 13 (2023) 362

 

Our response:

This comment has been taken into account in the revised manuscript (page 6 and references [17,18,19]) in the following way:

These upconversion luminescence bands were also observed in diverse matrices doped with erbium and ytterbium, e.g. CaIn₂O₄, CaGd₂(WO₄)₄,  Li_xNa_1-xCaLa_0.5(MoO₄)₃ [17, 18, 19]:

17. Lim, C.S.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A.; Atuchin, V. Microwave sol–gel synthesis and upconversion photoluminescence properties of CaGd₂(WO₄)₄:Er³⁺/Yb³⁺ phosphors with incommensurately modulated structure. J. Solid State Chem. 2015, Volume 228, pp. 160–166. https://doi.org/10.1016/j.jssc.2015.04.032
18. Liu, M.H..; Li, T.T.; Wang, X.; Liu, D.Y.; Yuan N.; Zhang D.L.; Tian Y. Synthesis and Er³⁺ spectroscopic property of Er³⁺/Yb³⁺-codoped CaIn₂O₄ nano-fibers for thermometry. Lumin. 2019, Volume 215, pp. 116703 (8 pages). https://doi.org/10.1016/j.jlumin.2019.116703
19. Lim, C.S.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A.; Atuchin, V. Structural and spectroscopic effects of Li⁺ substitution for Na⁺ in Li_xNa_1−xCaLa_0.5Er_0.05Yb_0.45(MoO₄)₃ upconversion scheelite-type phosphors. Crystals 2023, Volume 13, pp. 362 (15 pages). https://doi.org/10.3390/cryst13020362

 

 

Page 6

 

The PL intensity was approximately 6 times higher as compared with the previously fabricated BaTiO3 target doped with ~3.8 at.% of Er without sensitized Yb ions as described in Ref.[16]. Figure 2d illustrates PL excitation (PLE) spectrum of the BaTiO3:(Er,Yb) target.

 

The PL intensity was approximately 6 times higher as compared with the previously fabricated BaTiO3 target doped with ~3.8 at.% of Er without sensitized Yb ions, as described in Ref.[16]. Figure 2d illustrates the PL excitation (PLE) spectrum of the BaTiO3:(Er,Yb) target.

 

Our response:

It’s corrected in the revised manuscript (page 6).

 

 

Page 6

 

Figure 4 shows reflection spectra of microcavities recorded at different temperature.

 

Figure 4 shows the reflection spectra of microcavities recorded at different temperatures.

 

Our response:

The corresponding correction has been done in the revised version of the manuscript:

Figure 5a,b shows the reflection spectra of microcavities recorded at different temperatures ( page 7).

 

 

Page 7

 

The following physical values for these two single layer BaTiO3:(Er,Yb) films annealed at 450 or 800 °C were determined from the reflection and transmission spectra: n, k, E_g, ?_u, where n is

 

the real part of the refractive index of the film,  k is imaginary part of the refractive index or absorption index, ?_g is optical band gap, ?_u is Urbach energy.

 

The following physical values for these two single layer BaTiO3:(Er,Yb) films annealed at 450 or 800 °C were determined from the reflection and transmission spectra: n, k, E_g, ?_u, where  is the real part of the refractive index of the film,  is the imaginary part of the refractive index or absorption index, ?_g is the optical band gap and ?_u is the Urbach energy.

 

Our response:

It’s corrected in the revised version of the manuscript (page 8):

 

 

Page 7

 

The obtained solution of the inverse spectrophotometry problem gives the following results presented in Figures 5–7.

 

The obtained solution of the inverse spectrophotometry problem gives the results presented in Figures 5–7.

 

Our response:

It’s corrected in the revised version of the manuscript (page 8).

 

 

Page 7

 

Figure 5 depicts reflection and transmission spectra of single-layer BaTiO3:(Er,Yb) films deposited on fused silica substrates and annealed at 450 or 800 °C. The triangles and circles represent the experimental point, and the solid lines are the calculated spectra.

 

Figure 5 depicts the reflection and transmission spectra of single-layer BaTiO3:(Er,Yb) films deposited on fused silica substrates and annealed at 450 or 800 °C. The triangles and circles represent the experimental points, and the solid lines are the calculated spectra.

 

Our response:

It’s corrected in the revised version of the manuscript (page 8).

 

 

Page 7

 

An increase in the annealing temperature leads to a decrease in the thickness of BaTiO3:(Er,Yb) film, that is typical for sol-gel derived films and particularly for BaTiO3:(Er,Yb) films and for undoped BaTiO3 films this was confirmed with the use of SEM [9,10]. Decrease in the film thickness means increase in the film density ρ  and increase in the refractive index in accordance with the Lorentz-Lorenz formula (n2 − 1)/(n2 + 1)~ρ [23].

 

An increase in the annealing temperature leads to a decrease in the BaTiO3:(Er,Yb) film thickness that is typical for sol-gel derived films and particularly for BaTiO3:(Er,Yb) films, and for undoped BaTiO3 films this was confirmed with the use of SEM [9,10]. Decrease in the film thickness means an increase in the film density ρ and increase in the refractive index in accordance with the Lorentz-Lorenz formula (n2 − 1)/(n2 + 1)~ρ [23].

 

Our response:

It’s corrected in the revised version of the manuscript (page 8)…

 

 

Page 8

 

Figure 6 shows spectra of the refractive index n and absorption index k for these two BaTiO3:(Er,Yb) films annealed at 450 and 800 °C.

 

Figure 6 shows spectra of the refractive index n and absorption index k for the BaTiO3:(Er,Yb) films annealed at 450 and 800 °C.

Our response:

It’s corrected in the revised version of the manuscript (pages 8, 9)...:

 

 

Page 9

 

In the formula (2) B is a constant and  E= 1240 λ−1 is the photon energy in electron volts, and the wavelength λ is taken in nm.

 

In (2), B is a constant,  e= 1240 λ−1 is the photon energy in electron volts, and the wavelength λ is taken in nm.

 

Our response:

It’s corrected in the revised version of the manuscript (page 9).

 

 

Page 9

 

The intersection of the linear range of the dependence [(λ)E]m with the E axis yields an estimate of E_g.

 

The intersection of the linear range of the dependence [(λ)E]m with the E axis yields the estimated value of E_g.

 

Our response:

It’s corrected in the revised version of the manuscript  (page 9).

 

 

Page 9

 

In this case the following function:

 

In this case, the following function:

 

Our response:

It’s corrected in the revised version of the manuscript (page 9, before eq. 3).

 

 

Page 9

 

The lower Urbach energy for the higher annealing temperature can be explained by a decrease in the crystalline lattice disorder in nanocrystalline BaTiO3 film.

 

The lower Urbach energy for the higher annealing temperature can be explained by a decrease in the crystal lattice disorder in nanocrystalline BaTiO3 film.

 

Our response:

It’s corrected in the revised version of the manuscript (page 10).

 

 

Page 9

 

The optical band gap values reported in this work are comparable to those for undoped

300-nm-thick BaTiO3 films with a grain size of approximately 95 nm that were produced using pulsed laser deposition and had an optical band gap of 3.77 eV [27].

 

The optical band gap values determined in this work are comparable to those of undoped

300-nm-thick BaTiO3 films with a grain size of approximately 95 nm that were produced using pulsed laser deposition and had an optical band gap of 3.77 eV [27].

 

Our response:

This comment has been considered and the corresponding corrections were done in the revised manuscript (page 10, lines 33-332).

 

 

Page 10

 

From the analysis of the transmission and reflection spectra, we determined that for the single layer of BaTiO3:(Er,Yb) spin-on film annealed at 450 °C, the optical band gap for indirect electron transitions ?_g= 3.82 eV, while for the film annealed at 800 °C, ?_g =3.87 eV.

 

From the analysis of the transmission and reflection spectra, we determined that, for the single layer of BaTiO3:(Er,Yb) spin-on film annealed at 450 °C, the optical band gap for indirect electron transitions is ?_g= 3.82 eV, while for the film annealed at 800 °C, it is equal to ?_g =3.87 eV.

 

Our response:

 

It’s corrected in the revised version of the manuscript (page 10).

 

 

Page 10

 

The produced target has potential applications as an IR laser radiation visualizer and for the sputtered production of BaTiO3 film structures in vacuum.

 

The produced target has potential applications as an IR laser radiation visualizer and for the production of BaTiO3 film structures by sputtering in vacuum.

 

Our response:

It’s corrected and the corresponding sentence was moved below Figure 5 in the revised version of the manuscript.

 

 

Page 10

 

The effect of sample temperature on the amplitude of the reflection spectrum, as well as an intense upconversion luminescence for structures treated at 800 °C, and the previously noted shift of the cavity mode for less dense xerogel structures treated at 450 °C [10], encourages further research into the characteristics of barium titanate structures doped with lanthanides for optical adsorption and temperature sensors and remote monitoring systems [31–33].

 

The effect of sample temperature on the amplitude of the reflection spectrum, as well as an intense upconversion luminescence for structures treated at 800 °C, and the previously noted shift of the cavity mode for less dense xerogel structures treated at 450 °C [10], encourage further research into the characteristics of barium titanate structures doped with lanthanides for optical adsorption and temperature sensors and remote monitoring systems [31–33].

 

Conventionally, in Conclusions section, citations are not appropriate.

 

Our response:

We have made the corresponding corrections and moved these citations to the Discussion section (below figure 5) in the revised version of the manuscript.

 

 

Page 10

 

Funding: This research was funded by the grant F22KITG-008 and grant F22MLDG-002 of State Committee on Science and Technology of the Republic of Belarus;

 

Funding: This research was funded by the grant F22KITG-008 and grant F22MLDG-002 of the State Committee on Science and Technology of the Republic of Belarus;

 

Our response:

It’s corrected in the revised version of the manuscript (page 11).

 

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript reports a detailed study of the synthesis and characterization of three different Er and Y-doped BaTiO3 films with different upconversion luminescence properties. The film structure and elements were identified clearly using SEM and EDX. The PL properties difference were also explained in detail. I would recommend accepting this manuscript after minor revisions and I only have two minor suggestions.

1.      There are multiple grammar errors presented in the manuscript. Please proofread it repeatedly and try to fix them.

 

2.      There are too many sub-figures in figure 2 and each of them is important to illustrate the film property. I would recommend dividing Figure 2 into multiple figures.

Author Response

Response to the Reviewer 3

 

Reviewer 3:

This manuscript reports a detailed study of the synthesis and characterization of three different Er and Y-doped BaTiO3 films with different upconversion luminescence properties. The film structure and elements were identified clearly using SEM and EDX. The PL properties difference were also explained in detail. I would recommend accepting this manuscript after minor revisions and I only have two minor suggestions.

1. There are multiple grammar errors presented in the manuscript. Please proofread it repeatedly and try to fix them.
2. There are too many sub-figures in figure 2 and each of them is important to illustrate the film property. I would recommend dividing Figure 2 into multiple figures.

 

Our responses:

1. We are grateful to the Reviewer for the careful reading of our manuscript and helpful suggestions for its improvement. We have corrected grammar errors and improved readability of the manuscript.

 

2. According to the Reviewer’ recommendations, this figure was divided in two new ones, i.e. Figures 2 and 3 as follows:

Figure 2. XRD and Raman spectroscopic characterization of BaTiO3(Er:Yb)/BaTiO3 target: (a) XRD angular spectrum; (b) Raman spectra from the front (red line) and bottom (black line) sides of the target.

Figure 3. Upconversion PL spectra of BaTiO₃(Er:Yb)/BaTiO₃ target: (a) room-temperature upconversion PL spectrum under the excitation wavelength of 980 nm; (b) Energy level diagram of the upconversion PL excitation, where GSA is the ground state absorption, ET is the energy transfer; (c) Figure 3. Upconversion PL spectra of BaTiO3(Er:Yb)/BaTiO3 target: (a) room-temperature upconversion PL spectrum under the excitation wavelength of 980 nm; (b) Energy level diagram of the upconversion PL excitation, where GSA is the ground state absorption, ET is the energy transfer; (c) PLE spectra for the PL wavelength of 550 nm for the BaTiO3(Er:Yb)/BaTiO3 target (red line with circles) and for a sample of BaTiO3 with similar Er content (black line with circles), respectively.

Author Response File: Author Response.docx