High Stability of Liquid-Typed White Light-Emitting Diode with Zn_0.8Cd_0.2S White Quantum Dots

Round 1

Reviewer 1 Report

The use of electric lighting involves costs that must be incurred in connection with the demand for electricity. The growing level of demand for it forces the look for solutions consisting in limiting the power installed in lighting devices. There are many ways to minimize the electricity consumption. One of them is the use of light sources with high luminous efficiency, which is a measure of energy efficiency. It is also worth emphasizing that one of the priority actions of the European Union authorities is to reduce the consumption of electricity, including these for lighting purposes. The effect of this is the process of phasing out the energy-consuming light sources and promoting the introduction of modern light sources to the lighting market, especially those built on semiconductor elements (LED light-emitting diodes). Taking into account the above-mentioned facts, the subject of the paper should be considered as important and topical.

Obviously, the implementation of energy-saving lighting should take place without deteriorating the quality of the emitted light. Such a parameter describing the quality of the emitted light is, among others, the Color Rendering Index (CRI), which was pointed out by the authors of the work when presenting the parameters of the LED lamp.

The paper is of experimental nature. It mainly presents the test results for the designed white LED diode. The content of the paper is presented in a coherent and logical manner.

In the introduction, the authors presented various techniques for producing white light using LED lamps, giving the values of two basic (but very important) parameters: luminous efficiency and Color Rendering Index. Then the experiment was described in a concise and matter-of-fact manner. Section 3 entitled “Measurement and Analysis” is at the heart of the work.

There are some remarks while readinbg the paper:

1. I find it a bit unfortunate to treat Table 1 as part of Figure 3.

A better solution in my opinion seems to include this table in the text of the work as a separate item. When including Table 1 in the text (apart from parameters such as: Efficiency and CRI), it is worth considering to specify other parameters (eg. R9, CCT and x, y chromatic coordinates).

Thank you for your helpful comment.We have corrected Figure 3 and Table 1.

2. To improve readability, consider increasing Figures 3, 4a, 4b and 7.

Thank you for your helpful comment.We have corrected Figure 3and 7.Because of the correction of the manuscript content, Figure 4 has been removed. The original Figure 7 has been adjusted to Figure 6.

3. The location of the chromatic coordinates in figures 4c, 4d and 8a, 8b is not readable. I have a doubt whether enlarging the charts alone will improve readability enough. I would consider limiting the range on the x and y axes.

The idea for a graphic illustration of changing the position of chromatic coordinates can be taken from the work:

Marc Dyble, Nadarajah Nerendran, Andrew Bierman, and Terence Klein. Impact of Dimming White LEDs: Chromaticity Schifts Due to Different Dimming Methods

https://www.lightingassociates.org/i/u/2127806/f/tech_sheets/Impact_of_Dimming_white_LEDs.pdf (look at Figure 4 and Figure 5 from this paper)

Thank you for your helpful comment.We have corrected Figure 8a and 8b.Because of the correction of the manuscript content, Figure 4 has been removed. The original Figure 8has been adjusted to Figure 7.

4. In equation 1, when specifying the unit, for the normalization factor it is worth considering inserting the subscript "op" next to the letter "W"

(see equation 11 in the paper: Talha Erdem and Hilmi Volkan Demir. Color science of nanocrystal quantum dots for lighting and displays. Nanophotonics 2013; 2(1): 57–81 DOI 10.1515/nanoph-2012-0031

Thank you very much for your suggestion. One of the reviewers suggested to remove the manuscript data presentation of lm/Wop. He thought it might confuse readers with lm/Wele. Therefore, equation 1 has been removed from the manuscript during the revision process.

5. Section “Conclusion” seems to be not enough extensive. Including only 5 sentences may be somewhat unsatisfied. Please rewrite Conclusions to underline your achievements.

Thank you very much for your suggestion.We have tried to make corrections to the manuscript.

Concluding, in terms of content, the work deserves a positive assessment. It should be noted that the work is a logical and internally coherent whole. The comments posted do not contribute to the high value of the work. The results obtained by the authors of the study are important from a scientific point of view.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript: coatings-1138831

The manuscript titled 'High Stability of Liquid-typed White Light-Emitting Diode with Zn0.8Cd0.2S White Quantum Dots' by authors Chin-Chuan Huang et al present results on the facile synthesis of colloidal ternary Zn0.8Cd0.2S white quantum dots (WQD-WLEDs) by chemical route and preparation of WLEDs there after. I have following observations:

1// Please discuss the motivation for using ternary ZnCdS. Sulfide materials are known for their poor chemical stability and reaction with air atmosphere. 

Thank you for your helpful comment.The main motivation for using ZnCdS in this study is the high luminescence efficiency of the material.The main focus of this manuscript is to achieve high stability by isolating the material from the atmospheric reaction in a liquid package.

2// Authors claim that the synthesis of WQDs is done by them and reported in Ref. 28 (line 99). However, the paper in Optics Letters does not have any author from the present study. I am confused with the statement.

Thank you for your helpful comment.In this study, the material was developed in collaboration with other research teams and not by us alone.We will correct the descriptions in the manuscript to avoid misinterpretation.WQD was prepared by chemical method. From Wang, K.W. research team²⁸.

Reference:28. Chen, H.S.; Wang, K.W.; Chen, S.S.; Chung, S.R. ZnxCd1−xS quantum dots–based white light-emitting diodes. Optics Lett. 2013, 38, 2080-2082. doi: 10.1364/OL.38.002080.

3// Please provide spectral response of PDMS dispense alone along with the current-EL intensity response for the same.

Thank you for your helpful comment.The actual amount of material used in this study is quite limited.Therefore, almost all of the material obtained was used in the packaging and pre-production operations to extract the parameters.Therefore, we have no choice but to keep the material, and the components have been degraded by aging.If you agree, we will keep your reminder in mind and improve the manuscript in subsequent continuity studies.

4// Thermal quenching is another important parameter that should be discussed in the manuscript. Authors have provided a graph for operating RWQD-WLED under various operation time (Figure 7). However, it does not give any information for thermal quenching of quantum dots. 

Thank you very much for your suggestion.Please refer to Figure 5 in the manuscript and the following data we observed with our thermal imager. (Not included in the manuscript due to space)

5// In Figure 7, what is the reason for differences in efficiency (Figure 7c) among two different samples. Please discuss.

Thank you very much for your suggestion.The main difference in efficiency comes from the difference in packaging form. RWQD-WLED refers to the mixing of white quantum dots in a polymer compound and then covering them around the wafer by dispense, which is the traditional LED packaging method.The LWQD-WLED is a liquid white light quantum dots, which are not easily permeable to oxygen and water, are covered with glass and placed on top of the LED.

6// Also, there is a sudden dip in the CRI of RWQD-WLEDs (Figure 7d). Please discuss the possible reasons.

Thank you very much for your suggestion.RWQD-WLEDs produce the self-aggregation effect of quantum dots.When the nanomaterial changes its particle size due to the self-agglomeration effect, the converted wavelength will change, and when the wavelength changes, it will affect the CRI value.This phenomenon, when the driving current is higher, the wavelength change will also be greater.

7// The images in Fig.8 is crowded. I recommend authors to include inset of inner part of CIE coordinate diagram to compare the influences and for better visualization.

Thank you very much for your suggestion.Figure 8 in the manuscript has been corrected.The original Figure 8has been adjusted to Figure 7.

8// The discussion requires detailed comparison with already reported literature, and how the numbers obtained by authors, are different (either good or bad). Right now it appears that authors have performed some measurements without comparison or information of the existing literature. A table and detailed comparison is required.

Thank you very much for your suggestion.This process is a relatively new approach and there seems to be little research literature available.If you agree, we will continue this research in the future and will compile the relevant research literature and conduct a table comparison

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

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This is a nice paper proposing the use of QD incorporated in a liquid media for converting UV Led radiation into white light. The concept is of interest and the paper can be seen as a proof-of-concept element. The subject fits rather well with the journal’s topics. The results and the associated discussion are sound. However, beyond these positive points the paper needs to be revised before being acceptable for publication. The main issues that should being fixed are listed in the attachment.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript is revised and can be accepted for publication.

Thank you for your appreciation for our work.

Reviewer 3 Report

In this new version of the paper authors respected partially the previous comments. The paper is now in acceptable form but some minor elements have to be addressed before final acceptance:

1. Following my previous recommendations the lm/Wop metrics is not acceptable. You switch for a major part of the paper into lm/Welec, this is fine. However, some references in lm/Wop are still present in the introduction part. All values in lm/Wop must be deleted there are meaningless for light sources and in addition this metrics is no more defined in the paper. This point is compulsory for accepting the paper in final form.

Thank you for your valuable comments, we have corrected the manuscript again.

1. Figure 2 is still unacceptable. What are the differences between blue and orange doted lines, both are marked as ‘excitation’? Why blue and orange dotted lines representing the excitation spectra are going up to very abruptly in 400nm and 500 nm respectively? If no explanation in this last point just stop the excitation spectra before 400nm and 500 nm respectively.

I am sorry for the misunderstanding caused by the unclear annotation on Figure 2. Figure 2 in the manuscript has been corrected again.The quantum dot material used in this study has some defects, i.e. the ball shell is incomplete.Therefore, the blue and orange dashed lines are used to show(1) The intact QD has a strong excitation intensity (orange dashed line), with a relative emission spectrum of 508 nm.(2) A defective QD has a poorer excitation intensity (blue dashed line), corresponding to an emission spectrum of 407 nm.

1. You explained in the ‘response to referee’ letter why you can’t give the value of volume density of QD in your device as requested in my recommendations. Your explanation is acceptable and the idea to link somehow the volume density to the CCT is a good proposal. However, you need explain that inside the paper not only to your response to the referee comments. Please add some lines in the paper at the appropriated place.

Thank you very much for the reminder again. We have corrected the manuscript again. In the text, line 129 to 134.

1. Add in your conclusions that more investigation is necessary concerning ageing of the device beyond the 1000 h.

Thank you very much for the reminder again. We have corrected the manuscript again. In the text, line 244 to 247.

1. I just seen that in line 105 of the manuscript you wrote ‘chip size is 45 mil . 45 mil’, I suppose that ‘mil’ is mm…. Please replace by ‘chip size is 45mm x 45mm’

Thank you very much for the reminder.The size of the LED chip used in this study is correct, it is indeed the common size of 45mil × 45mil.45 mil is about 1.143 millimeter. Most of the most common 1 watt (electrical power) high power LED chip are 45 mil or 40 mil.For example, Taiwan GPi's horizontal structured bluechipB4545ECI4, which measures 1143 um ×1143 um, i.e. 45 mil × 45 mil. The LED chipis shown in the following excerpt from the datasheet.

Author Response

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Author Response File: Author Response.pdf