1. Introduction
Rare-earth-based luminescent materials have been widely studied for their excellent spectral properties, which include a high and adjustable luminescence, long fluorescence lifetime, and large Stokes shift [
1,
2]. Consequently, they have extensive uses in light-emitting diodes (LEDs), lasers, optical temperature sensors, optical communications, display panels, luminescence dosimeters, and biomedical diagnostics [
3,
4,
5,
6]. Nevertheless, considerable efforts are still being employed to improve the luminescence properties of such rare-earth-doped materials [
7,
8,
9], and the color, intensity, and luminescence efficiency have been shown to strongly depend on the structure and composition of the luminescent center [
8,
10]. Furthermore, the crystal structure, ionic radius, luminescence efficiency, refractive index, and phonon energy are key factors in determining the suitability of hosts and dopants, and the usefulness of certain fluorescent materials is dependent on their unique compositions [
11].
Oxides with a garnet structure are commonly used as hosts for rare-earth-doped luminescent materials, and commercial w-LED lamps are currently manufactured using a combination of YAG: (Ce
3+, Sm
3+) yellow and red phosphors [
12,
13]. However, there are manufacturing problems associated with the use of these materials, including an uneven dispersion of the phosphor particles, short lifetimes, and low thermal conductivity [
13,
14], and the search for new white LED fluorescent materials to replace traditional phosphors is an ongoing research topic [
15,
16]. Furthermore, although rare-earth-doped materials with a garnet structure have desirable properties, including high excitation and emission efficiency, uniform distributions of rare-earth ions, a physical and chemical stability, long life, and high thermal conductivity, which are suitable for use in high-quality w-LEDs [
17], there is still a need to improve the color temperature and color purity of white light by obtaining more efficient red light emission [
18,
19]. Thus, the fact that Sm
3+ can achieve efficient orange-red light emission as a result of high absorptions near 404 nm and 468 nm, and thus can be efficiently excited by InGaN-based blue or UV LEDs, indicates that Sm
3+-doped materials with a garnet structure could be appropriate for use in w-LED devices [
18,
20].
Recently, it has been reported that Eu
3+ transitions in Eu
3+-doped YAG are enhanced by replacing the Al
3+ with Ga
3+ [
21] and that the luminescence intensity in YGG crystals is stronger than in YAG [
22]. Furthermore, do** of YGG with other rare-earth ions, such as Er
3+ and Tm
3+, has also been described [
23], but we are unaware of any reports on the fluorescence properties of Sm
3+-doped YGG crystals.
Compared with ceramics and polycrystalline materials, single crystals have a greater atomic uniformity, better mechanical properties, and a higher electrochemical and thermal stability [
24,
25]. Furthermore, garnet single crystals are transparent materials, have a low defect density, no grain boundaries that affect the properties of ceramics, and negligible surface effects that influence the properties of powders. As a consequence, they have high photoluminescence quantum yields [
26,
27], and their use can improve the power and lifetime of w-LEDs [
18,
28]. Additionally, although traditional high-temperature methods for obtaining single crystals often suffer from contamination from crucibles, this problem is overcome with the optical floating zone method [
29,
30], which does not require a crucible and has a rapid crystal production cycle [
31], which is advantageous for investigating the properties of new crystal materials [
32,
33].
Sm3+-doped yttrium gallium garnet single crystals were prepared by the optical floating zone method, and their physical and optical properties were compared with those of YAG: Sm3+ crystals. These materials were then characterized by XRD, photoluminescence (PL) spectroscopy, and fluorescence lifetime measurements.
2. Materials and Methods
2.1. Crystal Preparation
Nanometer-size powders of Ga
2O
3 (99.99%, Maklin), Sm
2O
3 (99.99%, Aladdin), Y
2O
3 (99.99%, Maklin) and Al
2O
3 (99.99%, Maklin) were purchased in China and used to produce crystals with the empirical composition Y
2.
96Sm
0.
04Ga
5O
12 and Y
2.
96Sm
0.
04Al
5O
12 by the optical floating zone (OFZ) method. A more detailed description of the preparative procedures has been presented in previous work [
34,
35,
36].
As shown in
Figure 1, the prepared samples were light yellow in color and had no cracks or inclusions. Slices were cut and polished on both sides to produce 1.0 mm thick discs for spectroscopic measurements. In addition, crystal fragments were ground in an agate mortar to produce fine powders for X-ray diffraction (XRD) measurements (Dandong Hao Yuan Company, Dandong City, Liaoning Province, China).
2.2. Physical Measurements
Powder samples ground from crystal were measured by XRD (DX-2700, Dandong Hao Yuan Company, Dandong City, Liaoning Province, China) at room temperature using Cu-Kα radiation (λ = 1.54060 nm). Measurements were performed over the range 10–90° 2θ in steps of 0.02° with sampling times of 3 s, and the resulting XRD patterns were analyzed using Jade software (MDI Jade 6.0).
Crystal densities were measured by a high-precision density tester (DE-120M, Daho Meter Company, Dongguan, Guangdong Province, China). By measuring the weight of YGG: Sm
3+ and YAG: Sm
3+ single crystal rods in air and pure water, respectively, the volume of the crystal
V can be obtained based on the Archimedes principle:
where
M is the weight of the crystal in air (in grams),
F is the weight of the crystal in pure water (in grams), and
w is the density of pure water at room temperature, which is 1.0 g/cm
3. The density of the crystal
can be calculated by the following formula:
Absorption spectra were obtained in the 250–550 nm range with a UV-Vis Spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan).
A photoluminescence spectrometer (ZLF-325, Zolix Instruments Co., Ltd., Bei**g, China) was used to measure the photoluminescence emission (PL) and excitation spectra (PLE), with a 150 W xenon lamp as the excitation light source.
Fluorescence lifetimes were obtained with an Edinburgh steady/transient fluorescence spectrometer (FLS1000, Edinburgh, UK) under excitation with 407 nm light, and then the average lifetime of the samples was determined by tri-exponential fitting.
4. Conclusions
High-quality Y2.96Sm0.04Ga5O12 (YGG: Sm3+) single crystals were successfully prepared for the first time by the optical floating zone method and were compared with Y2.96Sm0.04Al5O12 (YAG: Sm3+) single crystals. The density of Y2.96Sm0.04Ga5O12 single crystal (5.756 g/cm3) is larger than that of Y2.96Sm0.04Al5O12 single crystal (4.481 g/cm3), because of the larger atomic mass of Ga compared with Al. XRD analysis showed that Sm3+ successfully entered into the cubic-phase structure of the garnet crystals. Ten absorption peaks were observed at 345, 362, 377, 390, 407, 419, 439, 468, 482 and 495 nm, corresponding to 6H5/2 → 4D7/2, 4D3/2, 6P7/2, 4L15/2, 4F7/2, 6P5/2, 4G9/2, 4I13/2, 4I9/2 and 4G7/2 transitions of Sm3+, respectively. Excitation peaks at similar wavelengths were observed in the PLE spectra detected at 613 nm, including strong peaks at 407 nm and 468 nm. Both YGG: Sm3+ and YAG: Sm3+ crystals emit orange-red light with a wavelength of about 611 nm under excitation at 407 and 468 nm, respectively, and the luminescence intensity is much stronger with 407 nm excitation. Furthermore, with both PL spectra, the emission peaks from YGG: Sm3+ crystal are both significantly more intense than those from YAG: Sm3+, and both experience a blue shift. The YGG: Sm3+ crystal has a highly efficient orange-red emission. In addition, under the excitation of 407 nm, the color purity of the orange-red light emitted by the YGG: Sm3+ crystal (85%) is higher than that emitted by the YAG: Sm3+ crystal (83%). Additionally, the fluorescence lifetime at the 4G5/2 → 6H7/2 transition of the YGG: Sm3+ crystal (0.705 ms) is longer than that of the YAG: Sm3+ crystal (0.466 ms). This shows that the optical properties of YGG: Sm3+ crystal are better than those of YAG: Sm3+ crystal and that they have a potential use in w-LEDs and as orange-red solid-state lasers. In other words, YGG: Sm3+ crystals are promising new materials for use in w-LEDs and orange-red solid-state lasers.