Structural and optical properties of Dy3+:YAlO3 phosphors for yellow light-emitting diode applications
Graphical abstract
The spectroscopic properties of a series of Dy3+ single-doped and Dy3+/Nd3+, Dy3+/Tb3+ and Dy3+/Tm3+ co-doped YAlO3 phosphors were investigated and compared through the spectral measurements of optical absorption, emission and fluorescence decay curves, etc. Under 447 nm excitation, the strong yellow waveband emission peak at 572 nm was obtained in all the samples, and 2 at% Dy3+:YAP phosphor exhibits the highest emission intensity.
Introduction
In recent years, the demand for advanced, energy-saving, and environmentally-friendly high-resolution displays and lighting has grown rapidly, which has led to the search for high-quality phosphors.1, 2, 3, 4 Rare-earth (RE)-doped phosphors have promising applications in plasma display panels (PDPS), light-emitting diodes (LEDs), solid-state lighting, and luminescent devices due to their excellent optical properties.5, 6, 7, 8, 9, 10 LED is widely used in displays as a solid-state light source owing to its high efficiency, long lifetime, and energy-saving advantages.11,12 The yellow emission laser is close to the most sensitive wavelength of the human eye (555 nm),13 so the use of yellow LED light has a protective effect on the human eye and can prevent the disruption of natural circadian rhythm effectively.14 The yellow laser within 550–600 nm wavelength range also has important applications in LIDAR (light detection and ranging), Bose-Einstein condensates, biomedical instruments, and atomic cooling and trapping.15,16 The yellow LED can be realized by coating the ultraviolet LED chip with a yellow fluorescent powder.14,17 The multiple physical properties of the phosphors enhance their application, especially the unique optical properties of Dy3+ doped phosphors, which make them the components of most yellow light LEDs.2,18 The trivalent dysprosium ion (Dy3+) has abundant energy levels, the yellow (570–660 nm) emission corresponding to its 4F9/2 → 6H13/2 transition makes it a useful element for yellow light emitters.19 The Dy3+ ion also shows other typical emissions within the visible light wavelength region, such as 4F9/2 → 6H15/2 (blue, 470–500 nm),20,21 which is required for a full primary color display.22 Thus, Dy3+ doped phosphors are effective functional materials,23,24 and it is meaningful to study Dy3+ doped phosphors for yellow light applications.
In this work, the host material was selected from the Y2O3-Al2O3 binary system. It is well known that, in this system, Y performs highly reactive chemical properties, large magnetic moment of electrons, and strong spin–orbit coupling characteristics, thus it can connect with other elements such as Al and O to form diverse crystal structures with a coordination number between 3 and 12.25, 26, 27 Therefore, Y2O3-Al2O3 binary system contains abundant compounds, which normally possess the following advantages: high melting point, good physical and chemical properties, high mechanical strength, high hardness, average phonon energy, high dielectric constant and wide bandgap, where some of them melt congruently.28, 29, 30 Among them, Y3Al5O12 (YAG) and YAlO3 (yttrium aluminum perovskite, YAP) are two kinds of popular optical functional materials, which exhibit similar physical and chemical characteristics, and the main difference is in the crystal structure: YAG belongs to the cubic system with space group Ia3d-O10h, while YAP is orthorhombic with space group Pnma-D162h. Up to now, YAP phosphors have been widely investigated, such as YAlO3:Mn3+ for the energy response of the thermal luminescence (TL) detector,31 and YAlO3:Ce3+ for granular phosphor screens.32 The above reports indicate that the compounds in the Y2O3–Al2O3 system are useful as matrix materials, so YAlO3 is chosen as the host material from this binary system. The Dy3+ is configured with the rich energy levels of YAlO3, so that Dy3+ ion can show unique optical properties, and the intermediate energy is discharged in various ways. Also, Dy3+:YAP phosphors show excellent light stability and sharp emissions. Though many reports on the properties of rare earth ions in YAlO3 crystals have been published,33,34 according to the best of our knowledge, there are still no reports on Dy3+-doped YAlO3 phosphors for yellow LEDs yet.
In this work, a series of Dy3+-activated YAlO3 phosphors were prepared by solid-state synthesis, and their absorption spectra, emission spectra as well as the fluorescence decay curves were measured and compared to optimize the concentration of Dy ion in achieving the strongest yellow emission. The CIE chromaticity coordinates were calculated and presented. The spectroscopic parameters including J-O intensity parameters of Dy3+ in YAP phosphors were calculated. The structure of YAlO3 was also analyzed.
Section snippets
Sample synthesis
A series of Dy3+ singly-doped and co-doped YAlO3 phosphors were prepared by the high-temperature solid-state reaction method. For Dy3+ singly-doped phosphors, the doping concentrations of Dy3+ were set to be 0.5 at%, 1 at%, 2 at%, 3 at% and 4 at%, respectively. As for Dy3+ co-doped samples, the concentration of Dy3+ ion was 2 at% and the concentrations of Nd3+, Tb3+, and Tm3+ were all set as 0.5 at%. The chemicals used were Al2O3, Y2O3 (99.999% purity), Dy2O3, Nd2O3, Tb4O7 (4N purity), and Tm2O3
X-ray diffraction patterns and crystal structure
Fig. 1 shows the X-ray diffraction (XRD) patterns of Dy3+ singly-doped and co-doped YAP phosphors including the JCPDF card of YAP [No. 33-0041] as a reference. The diffraction peaks match well with the standard card data except for two weak peaks located at 29.7° and 30.7°, which belongs to Y2O3 [JCPDF#43-0661]. During the sample synthesis, for the sintering process, as the number of cycles increases, the relative intensity of the impurity peaks gradually reduces. From Fig. 1, we also found
Conclusions
In summary, a series of Dy3+-doped and co-doped YAP phosphors were synthesized and characterized by XRD, EDS, optical absorption, luminescence, and decay curves measurements. Under the excitation of 351 nm, the strongest emissions are obtained when Dy3+ concentration is 3 at%, while under the excitation of 447 nm, the emission bands are enhanced in intensity up to 2 at% Dy3+ doping. The introduction of Nd ion does not promote the yellow emission in YAP phosphors. The emission intensity of Dy/Tb
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Foundation item: Projects supported by the National Natural Science Foundation of China (51872286, 51832007, 51472240, 61675204), Science and Technology Plan Leading Project of Fujian Province (2018H0046) and State Key Laboratory of Rare Earth Resource Utilization (RERU2018004, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences), the National Key Research and Development Program of China (2016YFB0701002), the fund of the State Key Laboratory of Solidification Processing (SKLSP201908, Northwestern Polytechnical University) and Natural Science Foundation of Jiangxi Province (20181BAB211009).