Elsevier

Applied Surface Science

Volume 379, 30 August 2016, Pages 467-473
Applied Surface Science

Fine tuning of emission property of white light-emitting diodes by quantum-dot-coating on YAG:Ce nanophosphors

https://doi.org/10.1016/j.apsusc.2016.04.116Get rights and content

Highlights

  • Surface-functionalization makes uniform coating of QDs on YAG nanoparticles.

  • Luminescence spectra of QD@YAG:Ce are finely tuned by QD contents.

  • Warm white LED with a high CRI of 92.82, and low CCT of 2617 K is obtained.

Abstract

We report fine tuning of emission color of Ce-doped yttrium aluminum garnet (Y3Al5O12:Ce3+, YAG:Ce) nanophosphor-based white light-emitting diodes (WLED), by coating CdSe/CdS/ZnS quantum dots (QDs) onto the surface of the YAG:Ce nanoparticles via surface functionalization of both the QDs and the YAG:Ce. Mixture of bromo-functionalized QDs and amino-functionalized YAG:Ce nanoparticles results in conformal coating of the QDs onto the YAG:Ce nanoparticles (QD@YAG:Ce). By varying the QD to YAG:Ce weight ratios, the luminescence spectra of the QD@YAG:Ce are tuned. A high-quality warm-white-light emission is achieved by appropriate combination of the yellow and red emissions from the QD@YAG:Ce, and the blue emission from InGaN LED chip. However, without surface functionalization, irregular mixtures of YAG:Ce and QDs are formed, which consequently make it hard to control the emission spectra. This study demonstrates a promising way to prepare uniformly QD-coated nanophosphors and an approach to control the emission spectra the nanophosphors.

Introduction

Compared to incandescent and fluorescent lighting, white light-emitting diodes (WLEDs) are currently receiving much attention as an environmentally friendly and high-efficiency solid-state lighting alternative because of their low power consumption and fast response time, as well as the low amount of toxic materials used in the fabrication process [1], [2]. In addition, these advantages mean they have great potential for a wide range of applications, e.g., display backlights, general lighting, automobile headlights, and therapeutic illumination [3], [4]. Although several methods have been used to fabricate WLEDs, combining a blue-light-emitting InGaN LED chip with the yellow-light-emitting inorganic phosphor of Ce-doped yttrium aluminum garnet (Y3Al5O12:Ce3+, YAG:Ce) has been widely adopted because of the easy fabrication process and low cost [5]. However, YAG:Ce-based WLEDs have a drawback which is a red spectral deficiency resulting in a bluer and colder white-light emission, with a high correlated color temperature (CCT) of 4500–6500 K, than that of traditional incandescent and fluorescent devices [6]. In general, the bluish and cold white-light emission of YAG:Ce-based WLEDs impedes the fabrication of WLEDs with a low CCT and high color-rendering index (CRI). Therefore, many studies have been conducted to produce WLEDs with a low CCT (2500–4500 K) and high CRI (>85) [7], [8], [9], [10]. Of the various methods developed, the incorporation of orange/red-light-emitting sulfide or nitride phosphors into the YAG:Ce phosphor has been widely adopted to compensate for the red spectral deficiency and produce warm WLEDs [11]. However, the high reaction temperature and pressure required for the fabrication process of the sulfide and nitride phosphors are critical issues [12].

In an attempt to find an alternative to the microscale red/orange phosphors, colloidal semiconductor nanocrystals, i.e., quantum dots (QDs), have received rapidly growing attention because of their attractive properties, such as low-temperature process, size-tunable optical properties, a narrow spectral bandwidth, a high photoluminescence (PL) quantum efficiency, and non-scattering properties [13], [14], [15], [16]. Recently, several studies have reported the fabrication of warm WLEDs with high CRI values that were achieved by combining a blue LED chip with a mixture of YAG:Ce and red-light-emitting QDs [17], [18]. By combining a blue LED chip with a mixture of the YAG:Ce phosphor and orange-light-emitting CdS:Mn/ZnS core/shell QDs (weight ratio = 1:1), the resulting product exhibited a CRI value of 85. In addition, combining CdS:Cu/ZnS QDs with a blue LED chip and YAG:Ce phosphor resulted in a CRI value of 86 [17], [18]. Despite these efforts to obtain the desired CRI, the improvement of the CRI value is limited because finely controlling the white-light emission by varying the concentration of QDs is difficult in WLEDs that are based on blue LED chips with a mixture of YAG:Ce phosphor and red-light-emitting QDs.

In this study, we attached CdSe/CdS/ZnS QDs to the surface of YAG:Ce nanoparticles (QD@YAG:Ce), achieved by performing ligand exchange reactions on the red-light-emitting CdSe/CdS/ZnS QDs, and by modifying the surface of the yellow-light-emitting YAG:Ce nanoparticles. The QD@YAG:Ce nanophosphor was synthesized with different QD to YAG:Ce weight ratios and the resultant optical properties were investigated. Furthermore, WLEDs were fabricated by combining the QD@YAG:Ce nanophosphors with blue LED chips. We found that by controlling the QD to YAG:Ce weight ratio, the white-light emission spectrum of the WLEDs can be manipulated, and high-quality warm WLEDs with high and low CRI and CCT values, respectively, were achieved. However, without surface functionalization, a mere mixture of YAG:Ce nanoparticles and QDs is formed, which consequently make it hard to control the emission spectra of WLEDs.

Section snippets

Synthesis of aluminum hydroxide particles and YAG:Ce

The YAG:Ce nanoparticles were prepared according to a previously published method [19], [20]. First, aluminum hydroxide (Al(OH)3) particles were synthesized with a urea-based method by dissolving aluminum nitrate nonahydrate (AlN3O9·9H2O, Sigma-Aldrich), aluminum sulfate octadecahydrate (Al2S3O12·18H2O, Sigma-Aldrich), and 0.1 M of urea (CH4N2O, Sigma-Aldrich) in deionized water. After heating this mixture at 98 °C for 4 h, spherical Al(OH)3 particles were precipitated. By adjusting the [SO43−]/[NO

YAG:Ce nanoparticles and QDs

The XRD pattern (Fig. S2, in Supplementary material) and the SEM image (Fig. S3, in Supplementary material) of the as-prepared Al(OH)3 particles confirms the formation of uniform, spherical Al(OH)3 nanoparticles. The particle-size is controlled by adjusting the [SO43−]/[NO3] molar ratio (hereinafter referred to as K) [19]. When K is 0.15, the average size of the Al(OH)3 particles is 164 ± 18 nm. As K increases, the average size of the Al(OH)3 particles also increases (Fig. S3, in Supplementary

Conclusions

In summary, we synthesized YAG:Ce nanoparticles, conformally coated by CdSe/CdS/ZnS quantum dots (QD@YAG:Ce) via covalently bonding bromo-functionalized QDs to the surface of amino-functionalized YAG:Ce nanoparticles. The surface modification of QDs and YAG:Ce nanoparticles were verified by FT-IR studies, and the conformal coating of QDs were confirmed by HRTEM, EDS, and PL analysis. The YAG:Ce emission peak at 534 nm gradually decreases, while the intensity of the QD emission peak at 624 nm

Acknowledgements

This work was supported by the Global Frontier R&D Program at the Center for Multiscale Energy System (2012M3A6A7054855). This work was also supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2014R1A2A2A01007722 & 2014R1A2A2A04005614).

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