Ce^3+/Eu²⁺ Doped Al₂O₃ Coatings Formed by Plasma Electrolytic Oxidation of Aluminum: Photoluminescence Enhancement by Ce³⁺→Eu²⁺ Energy Transfer

Round 1

Reviewer 1 Report

The overall quality of the manuscript is very high. The findings are very interesting and presented in a clear and logical manner. I recommend this paper for publishing after some minor revision. I would like the following question to be answered in the revised manuscript:

How many crystallographical sites there are for Ce³⁺ and Eu²⁺ in gamma-Al₂O₃?

What is the expected charge compensation mechanism for Eu²⁺ at Al³⁺ site?

Line 137 “Although XPS indicates that Eu incorporated into Al₂O3 is also in the 3+ oxidation state, typical f-f transitions of Eu³⁺ ions have not been identified in PL excitation and emission spectra” The emission of Eu³⁺ does not have to be visible under 240 nm excitation. Have you tried exciting at 395 nm?

Line 143 Why there are two maxima for Eu2+? There’s only one transition. Moreover, Fig. 5 b shows, that the Eu²⁺ emission spectrum has a different shape under 260 and 285 nm excitation. In the latter, the band with a maximum at 500 nm is more intense. This suggests the location of Eu2+ at two distinct sites. Maybe the non-equivalent sites result from an uncompensated charge.

Figure 5. Both Eu²⁺ and Ce³⁺ are being excited. The decreasing intensity of Ce³⁺ does not have to result from the ET, but can result from the increase of Eu²⁺ concentration and more competition for the exciting photons. To confirm the energy transfer beyond doubt, one should excite the Ce³⁺ ions selectively, which is in this host very difficult. The sufficient demonstration of the energy transfer is in Figure 7., which should be mentioned before the calculation of energy transfer efficiency. Another way to demonstrate the occurrence of the energy transfer is the investigation of the decay curves: if the decay curves of Ce³⁺ are single exponential in absence of Eu²⁺ and derive from the single-exponential shape in presence of Eu²⁺, this would serve as a good demonstration of the energy transfer. But it’s not necessary in this case.

Figure 10 – too much data, would you consider skipping the 285 nm excitation for the clarity of an influence of Eu and Ce content on the color coordinates.

Author Response

The overall quality of the manuscript is very high. The findings are very interesting and presented in a clear and logical manner. I recommend this paper for publishing after some minor revision.

Thank you for your affirmative response.

I would like the following question to be answered in the revised manuscript:

1. How many crystallographical sites there are for Ce³⁺ and Eu²⁺ in gamma-Al2O3?
Line 143 Why there are two maxima for Eu²⁺? There’s only one transition. Moreover, Figure 5 b shows, that the Eu²⁺ emission spectrum has a different shape under 260 and 285 nm excitation. In the latter, the band with a maximum at 500 nm is more intense. This suggests the location of Eu2+ at two distinct sites. Maybe the non-equivalent sites result from an uncompensated charge.

Thank you for the suggestions. We have added to the text:

“This transition is structurally sensitive to the local environment around the Eu²⁺ in Al₂O₃ [21]. The gamma Al₂O₃ has two different sites for Al³⁺ ions. The appearance of two emission bands for a single transition of Eu²⁺ in Al₂O₃:Eu²⁺ is thus attributed to the Eu²⁺ substituting the Al³⁺ ions at both crystallographic sites.”

2. What is the expected charge compensation mechanism for Eu²⁺ at Al³⁺ site?

Thank you for observing that we missed the description of the mechanism behind the charge compensation. We have corrected this:

“Recent investigations have shown that PEO process of aluminum is capable for preparation of Ce³⁺ and Eu²⁺ doped Al₂O₃ coatings in electrolyte enriched with CeO₂ and Eu₂O₃ particles, respectively [8,9]. As ionic radii of Al³⁺ (0.53 Å), and Eu³⁺ (0.95 Å) and Ce⁴⁺ (0.97 Å) significantly differ, the substitution of Al³⁺ by either Eu³⁺ or Ce⁴⁺ produces oxygen vacancies that in turn distort the crystal lattice. The excess O^2- ions give their electrons to the lattice (2O^2- - 4e^- ® O₂), which are then captured by Ce⁴⁺ and Eu³⁺, leading to their reduction to Ce³⁺ and Eu²⁺, respectively.”

3. Line 137 “Although XPS indicates that Eu incorporated into Al₂O₃ is also in the 3+ oxidation state, typical f-f transitions of Eu3+ ions have not been identified in PL excitation and emission spectra” The emission of Eu3+ does not have to be visible under 240 nm excitation. Have you tried exciting at 395 nm?

Thank you for the observation. We have changed the sentence to:

“Although XPS indicates that Eu incorporated into Al₂O₃ is also in the 3+ oxidation state, typical f-f transitions of Eu³⁺ ions have not been identified in PL excitation and emission spectra, not even under the 395 nm excitation corresponding to the ⁷F₀→⁵L₆ transition of Eu³⁺.”

4. Figure 5. Both Eu²⁺ and Ce³⁺ are being excited. The decreasing intensity of Ce³⁺ does not have to result from the ET, but can result from the increase of Eu²⁺ concentration and more competition for the exciting photons. To confirm the energy transfer beyond doubt, one should excite the Ce³⁺ ions selectively, which is in this host very difficult. The sufficient demonstration of the energy transfer is in Figure 7., which should be mentioned before the calculation of energy transfer efficiency. Another way to demonstrate the occurrence of the energy transfer is the investigation of the decay curves: if the decay curves of Ce³⁺ are single exponential in absence of Eu²⁺ and derive from the single-exponential shape in presence of Eu²⁺, this would serve as a good demonstration of the energy transfer. But it’s not necessary in this case.

Thank you for your suggestion. Unfortunately, we are not able to record the emission decays. As you suggested, we have rearranged the Fig. 7 to be mentioned before the calculation of energy transfer efficiency.

5. Figure 10 – too much data, would you consider skipping the 285 nm excitation for the clarity of an influence of Eu and Ce content on the color coordinates.

Thank you for your suggestion. We have removed the 285 nm from the CIE diagram, Table 2, and from the discussion.

Reviewer 2 Report

The authors analyze the PL and Energy transfer enhancement in Al₂O₃ matrix doped with Ce³⁺ and Eu²⁺ ions. The study clearly characterizes the phenomenon of energy transfer as a function of the concentration of Eu₂O₃. The results are sound and the conclusions the authors draw on the reason why the resonant energy contributes significantly on the enhancement of the photoluminescence are convincing. 

As a couriosity more than a concern, if the authors already have these experiments, I would ask them to add Time-Correlated Single Photon Counting measures showing the decay lifetime of the optical transitions for all the concentrations of Eu₂O₃ (figure 5a,b) and for all the concentrations of CeO₂ (Figure 7a,b). If they cannot perform the experiments, they can always try to discuss the dynamics of the process as well. 

Moreover, a plethora of ways to enhance the PL and structure the energy transfer between fluorescent interacting materials have been shown so that the authors may want to include these references in the bibliography: 

1. D. Martín-Cano, L. Martín-Moreno, F. J. García-Vidal, E. Moreno, Nano Lett. 2010, 10, 8, 3129-3134

2. V. Caligiuri, M. Palei, M. Imran, L. Manna, R. Krahne, ACS Photonics 2018, 5, 6, 2287-2294

3. Rashmi & Dwivedi, Y. Appl. Phys. A (2019) 125: 542. https://doi.org/10.1007/s00339-019-2830-3

Author Response

The authors analyze the PL and Energy transfer enhancement in Al₂O₃ matrix doped with Ce³⁺ and Eu²⁺ ions. The study clearly characterizes the phenomenon of energy transfer as a function of the concentration of Eu₂O₃. The results are sound and the conclusions the authors draw on the reason why the resonant energy contributes significantly on the enhancement of the photoluminescence are convincing.

Answer: Thank you for the affirmative response.

1. As a couriosity more than a concern, if the authors already have these experiments, I would ask them to add Time-Correlated Single Photon Counting measures showing the decay lifetime of the optical transitions for all the concentrations of Eu₂O₃ (figure 5a,b) and for all the concentrations of CeO₂ (Figure 7a,b). If they cannot perform the experiments, they can always try to discuss the dynamics of the process as well. 

Thank you for the suggestion. Unfortunately, we lack instrumentation for time resolved measurements of photoluminescence. We have added to the text:

The energy transfer Ce³⁺→Eu²⁺ can be also observed via the shortening of the Ce³⁺ emission decay times with the increasing Eu²⁺ concentration, as demonstrated in Ref. [7].

2. Moreover, a plethora of ways to enhance the PL and structure the energy transfer between fluorescent interacting materials have been shown so that the authors may want to include these references in the bibliography: 

D. Martín-Cano, L. Martín-Moreno, F. J. García-Vidal, E. Moreno, Nano Lett. 2010, 10, 8, 3129-3134 V. Caligiuri, M. Palei, M. Imran, L. Manna, R. Krahne, ACS Photonics 2018, 5, 6, 2287-2294 Rashmi & Dwivedi, Y. Appl. Phys. A (2019) 125: 542. https://doi.org/10.1007/s00339-019-2830-3

Thank you for your suggestion. We have added the suggested references where appropriate.