Red photoluminescent Eu3+-doped Y2O3 nanospheres as photon down-converters for LED-phosphor applications

Summary

This project is motivated by the potential applications of europium-doped yttrium oxide (Y2O3:Eu3+) nanospheres in the lighting industry. In recent years, solid-state lighting based on light-emitting diodes (LEDs) has become popular because of its energy efficiency and environmental friendliness. However, LED lighting is still far away from its theoretical efficiency limit.

The purpose of this project is to study the possibility of using Y2O3:Eu3+ nanospheres as a more efficient photon down-converter for white LEDs.

We have successfully synthesized Y2O3:Eu3+ nanospheres with a large size range (40-334 nm) using a cost-efficient co-precipitation method. X-ray measurements show an amorphous-to-crystalline phase transformation of the nanospheres at high annealing temperatures (> 500 °C). Optical measurements show strong red photoluminescence emission and ultraviolet (UV) excitation of the crystalline nanospheres with sizes over 100 nm, suggesting a potential application of implementing the nanospheres, as a photon down-conversion phosphor on GaAlN-based UV LEDs to generate white light. We also determined the optimal concentration of Eu3+ in Y2O3 for red emission (~8 mol%), which could be helpful for the future large-scale synthesis of Y2O3:Eu3+ nanospheres that are used as an LED-phosphor.

Our results contribute to the development of phosphor-converted white LEDs, which would advance solid-state lighting technology by increasing energy efficiency. Our work may lead to cheaper lighting and power costs, help reduce fossil fuel dependency, and help decrease carbon emissions.

This project is essentially fundamental research. The next step would be the implementation and optimization of Y2O3:Eu3+ nanospheres on fully functional LEDs for possible commercial use.

Question / Proposal

Development of photon down-conversion materials, which can be combined with blue/UV light-emitting diodes (LEDs) to generate high-quality white light, has recently attracted tremendous attention as it would advance solid-state lighting technology by increasing energy efficiency. Recently, europium-doped yttrium oxide (Y2O3:Eu3+) has been suggested as a photon down-converter. However, macroscopic Y2O3:Eu3+ has a low efficiency as a photon down-converter. On the other hand, development of nanostructured Y2O3:Eu3+ may provide a new direction.

The research question of this project is if Y2O3:Eu3+ nanospheres can be used as an efficient photon down-converter incorporated with UV LEDs to generate high-quality light. The hypothesis of this research is that Y2O3:Eu3+ nanospheres would generate intense red photoluminescence emission and strong UV excitation, which are necessary properties of an efficient photon down-converter for UV LEDs.

Expected outcomes would include the following:

(1) the successful synthesis of Y2O3:Eu3+ nanospheres with a wide range of sizes using cost-effective methods (a modified co-precipitation method followed by an annealing process is proposed);

(2) successful characterization/measurements of the structural and optical properties of the obtained nanospheres (with scanning electron microscopy, X-ray diffraction measurements, and photoluminescence spectra measurements), if Outcome (1) is achieved;

(3) understanding of the measured structural and optical properties based on fundamental knowledge of physics and chemistry, if Outcome (2) is achieved;

(4) identification of red photoluminescence emission and strong UV excitation, if any.

(5) determination of the optimal concentration of Eu3+ in Y2O3 for the purpose of red emission, if Outcome (4) is achieved.

Research

Europium-doped yttrium oxide (Y2O3:Eu3+) is a well-known red-emitting phosphor widely used in fluorescent lamps, plasma display panels, flat-panel and field emission displays, and cathode-ray tubes [1-9]. Y2O3:Eu3+ materials involve abundant 4f electronic configurations and rich energy levels [4,5,10]. Within bulk Y2O3, Eu3+ exhibits efficient luminescence emissions with excellent color quality. In addition, Y2O3:Eu3+ materials have high chemical durability and thermal stability, low toxicity, and excellent thermal conductivity [1-9]. Recently, Y2O3:Eu3+ materials have been suggested as a red-emitting photon down converter to generate white light emission with near ultraviolet (UV)- or UV-light-emitting diodes (LEDs) [11-14]. With group-III nitride-based LEDs (GaN, GaAlN, InGaN, and InGaAlN), which extend the emission spectrum to the green, blue, and ultraviolet regions, development of photon down-conversion materials (LED phosphors), which will combine with blue/UV LEDs to generate high-quality white light, would provide a solid-state light source for next-generation lighting industry and display systems, and advance solid-state lighting technology by increasing energy efficiency [15-17]. The synthesis and characterization of high-efficiency Y2O3:Eu3+ LED-phosphors are therefore significant.

While bulk and nano-sized Y2O3:Eu3+ materials with various morphologies have been synthesized [3,6,7,18-45], the most promising Y2O3:Eu3+ LED-phosphor could be Y2O3:Eu3+ nanospheres. With sizes of a few hundred nanometers, it is expected that the application of such nanospheres on the top of LEDs may lead to the enhancement of the light extraction efficiency. It is known that, for high-quality luminescence, the preferred morphology of phosphor particles is a perfectly spherical shape as spherical phosphors have high packing densities and low scattering of light [29,40,46-48]. Moreover, recent studies have shown that the deposition of TiO2 arrays with spheres of 500-900 nm in diameter on the top of GaN-based LEDs could enhance the light extraction efficiency by 4-5 times that of planar LEDs [49,50]. While similar studies with Y2O3:Eu3+ nanospheres and arrays with smaller sizes of nanospheres have not been reported, the mechanism for increasing the light extraction efficiency suggests that Y2O3:Eu3+ nanospheres with a few hundred nanometers in diameter may serve as an efficient LED-phosphor.

Y2O3:Eu3+ nanoparticles with spherical or sphere-like shapes have been synthesized with various approaches [3,6,11,20,22-26,29,32,35,36,40,43]. However, most of the previous syntheses focused on nanoparticles with sizes smaller than 100 nm [3,6,11,22-26,35,36,40,43]. Reports of syntheses of Y2O3:Eu3+ nanospheres with sizes of 100-500 nm are relatively rare [20,29,32]. Li et al. employed a homogeneous precipitation method to synthesize Y2O3:Eu3+ spheres with ~200 nm in diameter [32]. Using a similar approach, Yamagata et al. also obtained nanospheres with sizes of 200-300 nm for potential applications in orthodontic adhesive fluorescent [20]. On the other hand, Yan et al. used a low-temperature reflux method to synthesize Y2O3:Eu3+ nanospheres with sizes of 80-140 nm [29].

This project aims at the synthesis of Y2O3:Eu3+ nanospheres with a wide range of sizes using a single cost-effective method (a modified co-precipitation method), investigations of the structural and optical properties (with X-ray diffraction, SEM imaging, and photoluminescence measurements), and identification of strong red emission and UV excitation (which would suggest Y2O3:Eu3+ nanospheres as efficient photon down-conversion materials).

Method / Testing and Redesign

We employed a modified urea-assisted co-precipitation method similar to the one used by Fukushima et al. [51].  According to the desired concentration (5, 8, 10, and 15 mol% of Eu3+), a total of 0.0080 moles of Y2O3 and Eu2O3 were placed with 80 mL of nitric acid in a flask under a hot plate set to 300 °C while stirring magnetically. Once the solution had fully reacted into Y(NO3)3 and Eu(NO3)3, as indicated by a transparent solution, the hot plate was heated to 540 °C until the solution evaporated, forming a light yellow translucent solid. The flask was transferred to an unheated plate with a magnetic stirrer, and 25 g of urea was added. Deionized water (DIW) was added to make a solution of 180 mL. The solution was magnetically stirred until the solid on the bottom had dissolved and the solution was completely transparent. The flask was then placed in an 80 °C water bath for 1, 2, 3, 4, or 5 hours (reaction time) while magnetically stirring.  The solution turned translucent milky white as it was heated at 80 °C, eventually reaching a thick white color indicating that the reaction had completed. The resulting solution was centrifuged repeatedly at 15000 rpm for 15 minutes, each time being rinsed (once each with DIW and ethanol) and placed in an ultrasonic cleaner for 10 minutes to remove impurities.  This left a white solid that was either left overnight or heated in an oven for around an hour at 80 °C to dry. Finally, the dried powder was annealed at various temperatures ranging from 300 to 900 °C (annealing temperatures) for 1 hour.

Overall, this process is both simple and cost-effective, as all steps can be easily repeated in a factory setting and requires only relatively inexpensive equipment such as the centrifuging apparatus and the thermal annealing furnace.

Scanning electron microscopy (SEM) images of the obtained samples were taken on the Inspect S50 (FEI Company) SEM. X-ray diffraction (XRD) measurements were performed to study the structure of the samples, using a Rigaku SmartLab diffractometer with Cu Kα1 radiation, λ = 1.54 Å operating at 40 kV and 44 mA.  The XRD patterns were obtained at room temperature at an angular range (2θ) of 3°–80° with a step of 0.01°. Photoluminescence spectra were measured using a spectro-fluorophotometer (Shimadzu, RF-6000), with a xenon lamp as the excitation source.

All experimental work for this project was done at The University of Tulsa. The syntheses, XRD measurements, and photoluminescence spectra measurements were done in the laboratories of the Department of Physics and Engineering Physics, and the SEM images were taken in the Electron Microscope Laboratory of the Department of Biological Science.

 

Results

Figure 1 shows the SEM images of five samples with the Eu3+ concentration of 5 mol% synthesized at 80 °C for 4 hours and thermally annealed at various temperatures, if at all. All five samples are composed of nanospheres, with a size range of ~58-212 nm in diameter. Before annealing, the nanospheres have an average size of ~100 nm. After annealing at 300 °C, the nanoparticles are still spherical, but the size range becomes broader (58-141 nm) and the average size is slightly smaller. Annealing at 500 °C resulted in nanospheres with a size range of 87-212 nm and a greater average size of ~130 nm. The nanospheres obtained after annealing at 700 °C have a size range of 87-143 nm and the average size of ~120 nm is slightly reduced. Finally, the nanospheres obtained after annealing at 900 °C have an average size of ~145 nm in diameter, significantly greater than those of the nanospheres obtained with lower annealing temperatures.

Figure 2 shows the XRD patterns of the samples. The nanospheres obtained before annealing and after annealing at 300 and 500 °C do not exhibit any crystalline features. On the other hand, the nanospheres annealed at 700 °C and 900 °C have a cubic crystal structure. In addition, stronger XRD intensities of the Y2O3:Eu3+ nanospheres obtained after annealing at 900 °C suggest that higher annealing temperature may yield higher crystallinity.

Figure 3(a) shows the photoluminescence spectra of the nanospheres under the excitation of 250-nm UV light. Strong emission intensities are found in the two samples with a cubic structure (annealed at 700 and 900 °C). The strongest emission is located at 611.6 nm. In addition, the emission intensities of the 900 °C sample are more intense than those of the 700 °C sample. The three samples with an amorphous structure all show very weak emission intensities.

Figure 3(b) shows the excitation spectra of the nanospheres. The nanospheres with a crystalline structure have significantly stronger intensities than those with an amorphous structure.

Figure 4 shows the SEM images of the Y2O3:Eu3+ nanospheres with the Eu3+ concentration of 10 mol% synthesized at 80 °C for the synthesis times of 2, 3, 4, and 5 hours, respectively. We define the synthesis time to be the reaction time of the urea and the Y/Eu-compounds in the heated water bath. With synthesis times of 2 and 3 hours, the size ranges are ~40-70 nm and ~40-95 nm, respectively, as both are composed of nanospheres with sizes below 100 nm in diameter. With a synthesis time of 4 hours, the nanospheres obtained have a size range from ~100 nm to ~210 nm. The largest nanospheres, with a size range of ~164-334 nm, were obtained when the synthesis time was 5 hours. These results suggest that longer synthesis time generates large nanospheres.

Figure 5 shows the optimal concentration of Eu3+ for photoluminescence emission. The 8% sample has the strongest emission intensity, followed by the 10% and 5% samples, while the 15% sample shows the weakest intensity.

Conclusion

1. The results shown in Figures 1 and 4 demonstrate that we have successfully synthesized Y2O3:Eu3+ nanospheres with a wide range of sizes (40-334 nm) using a low-cost urea-assisted co-precipitation method [Outcome (1)]. The sizes of the nanospheres are controlled by the synthesis time and annealing temperature. The general trend is that longer reaction times and higher annealing temperatures result in the formation of larger nanospheres. This is an advance over previous studies, which involved Y2O3:Eu3+ nanospheres with small sizes (< 100 nm) or a narrow size range. This work demonstrates the synthesis of various sizes of nanospheres within a single frame of approach.

2. We have successfully measured the structural and optical properties of the obtained nanospheres using SEM, XRD, and photoluminescence spectra [Outcome (2)], as shown in Figures 1-5.

3. Analysis of the data allows us to understand the properties of the Y2O3:Eu3+ nanospheres [Outcome (3)]. (i) The size variation with the annealing temperature can be understood with information about the structure of the nanospheres (see the attached Google Docs); (ii) An important conclusion from the XRD measurements is that the nanospheres undergo a phase transformation from an amorphous structure to a cubic structure when annealing at 700 °C and 900 °C. A similar phase transformation of bulk Y2O3 at an annealing temperature of 600 °C was previously reported [44], but the present work provides direct evidence in nano-sized Y2O3:Eu3+; and (iii) Detailed analysis of the photoluminescence spectra is done using quantum transition theory, the atomic and electronic configurations, as well as the Judd-Ofelt theory. The attached Google Docs includes more details.

4. Equally important, we have identified intense red photoluminescence emission and strong UV excitation of the cubic crystalline Y2O3:Eu3+ nanospheres [Outcome (4)]. The three strongest emission peaks of the 900 °C sample are located at 611.6 nm (orange-red), 630 nm (red), and 709.8 nm (deep red), respectively. On the other hand, the nanospheres with an amorphous structure showed very weak emission and excitation. The nanospheres with a cubic crystal structure also exhibit large sizes (100-334 nm), which are suitable for enhancement of the light extraction efficiency when coated on LEDs. Our work, therefore, suggests a potential application of implementing the crystalline Y2O3:Eu3+ nanospheres with sizes over 100 nm on GaAlN-based blue/UV LEDs to generate white light.

5. Finally, our work shows that the optimal concentration of Eu3+ in Y2O3 for the purpose of red emission is ~8 mol% [Outcome (5)]. This result could be helpful for the future large-scale synthesis of Y2O3:Eu3+ nanospheres that are used as an LED-phosphor. Additional analysis is included in the attached Google Docs.

The experimental work, therefore, supports our hypothesis and expected outcomes, as stated in Section 3: Questions/Proposal. We believe that our experiment was successful.

The present work would impact solid-state lighting technology by offering more efficient photon down-conversion materials. Our work is expected to contribute to the efforts leading to cheap phosphor-converted white LEDs, which will indirectly help reduce fossil fuel dependency and decrease carbon emissions.

 

About me

My interest in science began when I visited the Adventure Science Center in Nashville at 16 months old. Since then, I’ve enjoyed exploring science and nature and I’ve participated in numerous STEM activities. My pre-school teachers even nicknamed me “naturalist”.

Since I was in elementary school, I’ve been concerned about global warming and its harmful effects on the environment and society. In 10th grade, I thought I was ready to do original scientific research. I contacted Dr. Peifen Zhu at the University of Tulsa to do volunteer research on efficient nanomaterials for optics/photonics applications, which could help reduce the cause of global warming by conserving energy. In May 2017, I joined Dr. Zhu’s lab. My first project was about the optical properties of nanosheets and nanotubes. The results have been published in IEEE Photonics Journal.

This school year, with the help of Dr. Zhu, I initiated and performed my Google Science Fair project.

The excitement of scientific discovery and the contributions to the knowledge of the scientific field makes research my favorite activity. I plan to obtain a Ph.D. degree in nanoscience/nanotechnology and to become a scientist and inventor.

I’ve been always inspired by Thomas Edison. His revolutionary invention of incandescent light bulbs using a carbon filament was the result of creativity, dedication, collaboration, and hard work.

Winning Google Science Fair would greatly encourage me to pursue my long-range goal of making new scientific discoveries and applying them to emerging technologies by inventing new nanotechnology with applications beneficial to society.

Health & Safety

This project involves the use of hazardous chemicals and devices such as nitric acid, high-temperature oven, and thermal annealing furnace. Before I started this project, I received training on the associated hazards and what to do in the event of exposure or a spill under the supervision of my research mentor, Dr. Peifen Zhu, assistant professor in physics and engineering physics, the University of Tulsa. Her contact information is as follows:

Professor Peifen Zhu, Ph.D., Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma 74104, USA. Phone: (918) 631-5125; Email: peifen-zhu@utulsa.edu

In addition, the University of Tulsa requires that every student doing experiments in any lab belonging to the university must receive safety training by watching the following YouTube videos (and students must follow the safety guidelines stated in the videos):

1. Proper Dress and PPE / Lab Safety Video Part 1

https://www.youtube.com/watch?v=GjAD83B4JaY&list=PL4qaj9envIYnBaQSPpcOMUqWiQUAgPoMq&index=4#t=4.045801

2. Safety Equipment / Lab Safety Video Part 2

https://www.youtube.com/watch?v=IiHEYtnKfF0&list=PL4qaj9envIYnBaQSPpcOMUqWiQUAgPoMq&index=2

3. Behavior / Lab Safety Video Part 3

https://www.youtube.com/watch?v=e7VkIuiT1kU&index=3&list=PL4qaj9envIYnBaQSPpcOMUqWiQUAgPoMq

4. Chemical Hazards / Lab Safety Video Part 4

https://www.youtube.com/watch?v=8queMM7VVfw&index=4&list=PL4qaj9envIYnBaQSPpcOMUqWiQUAgPoMq

5. Safe Chemical Handling Lab Safety Video Part 5

https://www.youtube.com/watch?v=uiB8qnlZTAM&index=5&list=PL4qaj9envIYnBaQSPpcOMUqWiQUAgPoMq

6. Other General HazardsLab Safety Video Part 6

https://www.youtube.com/watch?v=q8UiamEWz4Q&index=6&list=PL4qaj9envIYnBaQSPpcOMUqWiQUAgPoMq

During the process of this research project, I followed the safety guidelines, and no any safety issues were involved.

Bibliography, references, and acknowledgements

Reference for my entrant

The following paper includes some more details about the results and discussions for this research project:

William Wang and Peifen Zhu, “Red photoluminescent Eu3+-doped Y2O3 nanospheres for LED-phosphor applications: Synthesis and characterization,” Optics Express (in press).

This paper has recently been accepted for publication after peer-review and it is now in the stage of production. The paper will be available online any time soon. Optics Express, published by the Optical Society (OSA), provides “rapid publication for peer-reviewed articles that emphasize scientific and technology innovations in all aspects of optics and photonics”, according to the OSA website. Please visit the home page of Optics Express at https://www.osapublishing.org/oe/home.cfm for details.

Acknowledgment

I would like to thank my mentor, Professor Peifen Zhu, Department of Physics and Engineering physics at The University of Tulsa, for her guidance during the course of this research project. I would also like to thank Professor Alexei Grigoriev (The University of Tulsa) for help in the XRD measurement, Professor Hongyang Zhu (The University of Tulsa) for helpful discussions, and Mr. Richard Portman (The University of Tulsa) and Mr. Gopi Adhikari (The University of Tulsa) for help in the SEM measurements.  I appreciate the Department of Physics and Engineering Physics at The University of Tulsa for allowing me to use the laboratories for my nanostructure syntheses, XRD measurements, and photoluminescence spectra measurements, as well as the Department of Biological Science at The University of Tulsa for allowing me to use the electron microscopy laboratory for scanning electron microscopy measurements. In addition, I would like to thank my chemistry teacher, Dr. A. K. Fazlur Rahman at the Oklahoma School of Science and Mathematics, for his help to my presentations at the professional conferences.

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