Diffusing Mn⁴⁺ into Dy³⁺ Doped SrAl₂O₄ for Full-Color Tunable Emissions

Abstract

Dy³⁺ and Mn⁴⁺ codoped SrAl₂O₄ (SrAl₂O₄:Dy³⁺,Mn⁴⁺) phosphors were obtained by diffusing Mn⁴⁺ ions into Dy³⁺-doped SrAl₂O₄ via the constant-source diffusion technique. The influences of diffusion temperature and diffusion time on the emissions of SrAl₂O₄:Dy³⁺,Mn⁴⁺ were investigated. It was found that: (i) efficient red emission peaking at 651 nm can be readily achieved for SrAl₂O₄:Dy³⁺ by simply diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ at 800 °C and above; (ii) the red emission of Mn⁴⁺ becomes dominant over the characteristic emissions of Dy³⁺ when the diffusion temperature is 900 °C or higher; and (iii) the intensity of the red emission of SrAl₂O₄:Dy³⁺,Mn⁴⁺ is far more sensitive to diffusion temperature than to diffusion time. Our results have demonstrated that full-color tunable emissions can be realized for SrAl₂O₄:Dy³⁺, Mn⁴⁺ by tuning the parameters of diffusion temperature and diffusion time, which opens up a space for realizing easy color control of Dy³⁺-doped inorganic materials.

1. Introduction

Being a member of rare earth-activated luminescent materials, Dy³⁺-doped SrAl₂O₄ (SrAl₂O₄:Dy³⁺) has attracted much attention because of its interesting emission features [1,2,3,4]. Under ultraviolet excitation, the photoluminescence (PL) of SrAl₂O₄:Dy³⁺ is composed of two parts: (i) a broad emission band peaking at about 400 nm due to the intrinsic defects in SrAl₂O₄ [5]; and (ii) three narrow emission bands of Dy³⁺ activators centered at around 482, 574 nm and 663 nm, which are due to the optical transitions from ⁴F_9/2 to ⁶H_15/2, ⁶H_13/2, and ⁶H_11/2, respectively [6,7,8,9,10,11]. In principle, SrAl₂O₄:Dy³⁺ can be developed into a color-tunable light-emitting phosphor for applications in the solid state lighting industry because it contains blue, yellow, and red emissions. Unfortunately, the red emission band of Dy³⁺ is too weak to adjust the colorimetric performance of SrAl₂O₄:Dy³⁺. Consequently, the realization of full-color emissions for SrAl₂O₄:Dy³⁺ seems to be an intractable problem unless a facile technique is developed to endow efficient red emissions to SrAl₂O₄:Dy³⁺.

Red to deep red emission of Mn⁴⁺ (3d³ electron configuration) provides a solution to this problem. The ground state (⁴A_2g) and the lowest excited state (²E_g) of Mn⁴⁺ arise from the t³_2g configuration; the spin forbidden ²E_g→⁴A_2g emission generally consists of a sharp line and associated vibronic sidebands. Under ultraviolet or blue irradiation, deep red emissions are observed for a number of Mn⁴⁺-doped inorganic hosts [12,13]; examples include the red emissions for Mn⁴⁺-doped SrAl₂O₄ (653 nm) [14], Sr₄Al₁₄O₂₅ (662 nm) [15], Mg₂TiO₄ (665 nm) [16,17], CaAl₁₂O₁₉ (656 nm) [18], and CaAl₂O₄ (658 nm) [19]. Thus, codoping SrAl₂O₄:Dy³⁺ with Mn⁴⁺ seems to be a promising strategy to realize full-color tunable emissions for SrAl₂O₄:Dy³⁺. As documented in the literature, Chi et al. reported the red emission of Mn⁴⁺ in SrAl₂O₄ (around 660 nm) with a quenching concentration as low as 0.04 mol% [14]. Such a low quenching concentration poses stringent requirements on the doping technique, due to the fine control over the dose of Mn⁴⁺ in SrAl₂O₄:Dy³⁺,Mn⁴⁺. Making use of the advantages of the constant-source diffusion technique, our strategy in this work is to diffuse Mn⁴⁺ ions into SrAl₂O₄:Dy³⁺ at low doping levels by tuning the parameters of diffusion temperature and diffusion time. Our results demonstrate that intense red emission peaking at 651 nm can be achieved by diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ at 800ºC and higher, yielding full-color tunable emissions for Dy³⁺ and Mn⁴⁺ codoped SrAl₂O₄ (SrAl₂O₄:Dy³⁺,Mn⁴⁺).

2. Experimental Section

2.1. Preparation of SrAl₂O₄:Dy³⁺,Mn⁴⁺

The first step was to synthesize SrAl₂O₄:Dy³⁺. Using the sol-gel combustion technique, SrAl₂O₄:Dy³⁺ was synthesized by controlling the doping level of Dy³⁺ at 1.6 mol% [2,3,4]. Analytical grade reagents strontium nitrate (Sr(NO₃)₂), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), urea, and boric acid (H₃BO₃) were purchased from a local chemical supplier, i.e., Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). Dysprosium oxide (Dy₂O₃) provided the source of Dy dopant, and the purity of Dy₂O₃ was 4 N. Under stirring with a magnetic bar, Al(NO₃)₃·9H₂O (0.4 mol), Sr(NO₃)₂ (0.2 mol), H₃BO₃ (0.02 mol), urea (6.0 mol), and Dy₂O₃ (0.0016 mol) were dissolved in 600 mL of deionized water. A transparent solution was obtained after the mixture was vigorously stirred for 4 h in a glass beaker. After being aged at room temperature for two weeks, the solution could be used for the sol-gel combustion. Urea and boric acid in the solution functioned as the fuel and flux, respectively. Alumina crucibles were utilized as the containers for the sol-gel combustion; the volume capacity of each alumina crucible was 50 mL. After being filled with 25 mL of the aged solution, the alumina crucible was transferred into a box furnace where the sol-gel combustion reaction took place. The temperature in the box furnace was kept at 700 °C before the solution-containing crucible was added. During the combustion, the temperature in the furnace was elevated to about 820 °C, but the temperature in the flame could reach 1300 °C. After the sol-gel combustion, white powders were collected. According to the molar ratio of Dy³⁺ ions to Sr²⁺ ions in the solution, the nominal doping concentration of Dy³⁺ in SrAl₂O₄ was determined to be 1.6 mol%.

The second step was to yield SrAl₂O₄:Dy³⁺,Mn⁴⁺ by diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ via the constant-source diffusion technique. Cylindrical corundum crucibles (ϕ40 mm) with a height of 35 mm were employed as the vessel for diffusion. A thin layer of MnO₂ (about 1 μm in thickness) was deposited onto the bottom of the crucible for constant-source diffusion. SrAl₂O₄:Dy³⁺ powders were stored in the MnO₂-coated crucible, which was transferred into an air filled tubular furnace for Mn⁴⁺ diffusion. The diffusion was carried out under air atmosphere. The doses of Mn⁴⁺ in SrAl₂O₄:Dy³⁺,Mn⁴⁺ were controlled by tuning the diffusion temperature and diffusion time.

2.2. Phase, Morphology, and Elemental Composition of SrAl₂O₄:Dy³⁺,Mn⁴⁺

The phase of SrAl₂O₄:Dy³⁺,Mn⁴⁺ phosphors was determined by their X-ray diffraction (XRD) profiles. The XRD profiles of SrAl₂O₄:Dy³⁺,Mn⁴⁺ phosphors were recorded on an X-ray diffractometer (D/max 2500 PC, Rigaku Corporation, Akishima, Japan). The wavelength of the Cu Kα radiation was 0.15405 nm. The morphology of SrAl₂O₄:Dy³⁺,Mn⁴⁺ phosphors was characterized with a scanning electron microscope (SEM) (model S-4800, Hitachi, Tokyo, Japan). The elemental composition of SrAl₂O₄:Dy³⁺,Mn⁴⁺ phosphors was provided by the energy dispersive X-ray (EDX) spectrum of the synthesized phosphor. The nanostructures and crystal lattice of SrAl₂O₄:Dy³⁺,Mn⁴⁺ were investigated on a transmission electron microscope (TEM) (model JEOL JEM-2100, Japan Electronics Corp, Akishima, Japan). Samples for TEM analysis were prepared by dispersing the particles of the phosphor in ethanol. After being excited in an ultrasonic cleaner for 10 min, a drop of the suspension was dried on a carbon-coated copper grid.

To determine the valence states of dopants Mn⁴⁺ and Dy³⁺ in SrAl₂O₄:Dy³⁺,Mn⁴⁺, we performed X-ray photoelectron spectroscopy (XPS) analysis on an Escalab 250Xi spectrophotometer (Thermo Scientific, Waltham, MA, USA). Coming from Al Kα radiation, the energy of the incident X-ray was 1486.6 eV for the XPS characterization. Samples were not cleaned and not sputtered before XPS characterization. All the samples were analyzed by the charge correction. The binding energy of the standard C–C bond of the calibrated XPS instrument was 284.8 eV.

2.3. PL Spectra of SrAl₂O₄:Dy³⁺,Mn⁴⁺

The steady-state PL spectra of SrAl₂O₄:Dy³⁺,Mn⁴⁺ phosphors were acquired with a spectrophotometer (Tianjin Gangdong Ltd., Tianjin, China). A helium-cadmium laser (Kimmon Electric Co., Ltd., Tokyo, Japan) provided the excitation source for the PL measurement. The emission wavelength of the laser radiation was 325 nm; the output power of the laser radiation was 13 mW. Each PL spectrum was taken at room temperature.

2.4. PL Decay Curves of SrAl₂O₄:Dy³⁺,Mn⁴⁺

On a picosecond fluorescence lifetime spectrometer (LifeSpec II, Edinburgh Instruments, Edinburgh, UK), the PL decay curves of intrinsic defect-related emissions of SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion were measured at room temperature. The time correlated single photon counting technique was applied in the photon detection. A picosecond pulsed light-emitting diode provided the pulsed excitation. The excitation wavelength of the pulsed laser was 320 nm. The pulse width of the picosecond pulsed light-emitting diode was about 860 ps. For the PL decay curve, the detection wavelength was 400 nm, and the pulse repetition rate of the light source was fixed at 10 MHz, corresponding to a pulse every 100 ns. Details on the PL decay characterization could be found elsewhere [20,21]. In contrast, the PL decay curve of the SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion was measured at room temperature on a transient-state fluorescence spectrometer (FS5, Edinburgh Instruments, Edinburgh, UK). The PL decay curve of Mn⁴⁺ emission was recorded at 651 nm for SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion at 1000 °C. A pulsed Xenon lamp was employed to provide the pulsed excitation.

3. Results and Discussions

3.1. Phase and Morphology of SrAl₂O₄:Dy³⁺,Mn⁴⁺

Figure 1 represents the X-ray diffraction (XRD) curves of SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion (a) and after Mn⁴⁺ diffusion at temperatures of 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), and 1000 °C (f). The duration of each diffusion was 4 h. The diffraction data of standard monoclinic SrAl₂O₄ (JCPDS card no. 34-0379) are shown at the bottom of Figure 1 for comparison. Evidently, pure phase SrAl₂O₄ is formed when the diffusion temperatures are 900 and 1000 °C [2,3,4,5]. In contrast, a secondary phase is formed in SrAl₂O₄ before Mn⁴⁺ diffusion (curve a) and after Mn⁴⁺ diffusion at 600, 700, and 800 °C (curves b–d), due to the presence of a diffraction peak at 2θ = 25°. This phenomenon can be attributed to the formation of an aluminoborate complex in the combustion synthesis due to the addition of boric acid. Boric acid, which is used as a fluxing agent in promoting the formation of the required crystalline phase, can also behave as one of the precursor materials for the formation of aluminoborate complexes. The formed aluminoborate complexes can be decomposed and evaporated after annealing at high temperatures. This is the reason why the diffraction peak at 2θ = 25° gradually loses its intensity when the diffusion temperature is elevated from 600 to 800 °C.

Monoclinic SrAl₂O₄ has a stuffed tridymite structure with the space group P2₁ and Z = 4. As described in our previous work, monoclinic SrAl₂O₄ has a three-dimensional network of corner-sharing [AlO₄] tetrahedra with channels present in the a- and c-directions. Two crystallographically different Sr²⁺ sites are present in the unit cell of SrAl₂O₄, and these Sr²⁺ ions are located along the channels [5]. Therefore, it is easy for Dy³⁺ and Mn⁴⁺ to dope SrAl₂O₄. Depending on their coordination numbers (CN), the effective ionic radii of cations vary to some extent. In the case of site occupancy, the effective ionic radii of Sr²⁺, Dy³⁺, Al³⁺, and Mn⁴⁺ are 0.145 (CN = 9), 0.122 (CN = 9), 0.053 (CN = 4), and 0.053 nm (CN = 4), respectively [22]. Due to their comparable ionic sizes, dopants Dy³⁺ and Mn⁴⁺ tend to replace the Sr²⁺ and Al³⁺ sites in the lattice of SrAl₂O₄, respectively.

Figure 2 displays the SEM micrograph (a), low-magnification TEM micrograph (b), and high-resolution TEM micrograph (c) of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion at 800 °C for 4 h. As shown in Figure 2a, some of the particles are as small as 20 nm in diameter, but they are prone to forming large aggregates because of their high surface energy. As shown in Figure 2b, the aggregate consists of a large number of nanoparticles with their dimensions up to 80 nm. Our previous work has evidenced the formation of a number of SrAl₂O₄ nanocrystals in a single aggregate [5]. The high-resolution TEM micrograph in Figure 2c illustrates the crystal lattices of one SrAl₂O₄:Dy³⁺,Mn⁴⁺ nanoparticle. The spacing between two adjacent planes in Figure 2c is found to be 0.314 nm, which is in good agreement with the distance between two adjacent ($\overline{2}$11) crystallographic planes of monoclinic SrAl₂O₄. Thus, the micrographs in Figure 2 demonstrate that the synthesized SrAl₂O₄:Dy³⁺, Mn⁴⁺ exhibits good crystallinity.

3.2. EDX Spectrum of SrAl₂O₄:Dy³⁺,Mn⁴⁺

Figure 3 illustrates the EDX spectrum of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion at 800 °C for 4 h. As mentioned in the experimental section, the doping concentration of Dy³⁺ is 1.6 mol%. As can be seen in Figure 3, the characteristic X-ray emissions of O(Kα), Al(Kα), and Sr(Lα₁) are located at 0.525, 1.486, and 1.806 keV, respectively. In the meantime, the characteristic emissions of Dy(Lα₁) and Dy(Lβ₁) can be identified at 6.495 and 7.248 keV, respectively. Additionally, the X-ray emissions of Au(Mα₁) and Au(Lα₁) are located at 2.122 and 9.713 keV, respectively. However, element Mn is not detected with the EDX because the doping concentration is very low, which is outside the detection limit of the EDX facility. The detection limit of our EDX is about 1 wt. % for element Mn. As previously described, the Au element in the specimen is introduced in the process of Au sputtering for the convenience of SEM characterization. As expected, the EDX spectrum of SrAl₂O₄:Dy³⁺,Mn⁴⁺ verifies the presence of elements Al, Sr, O, and Dy in the phosphor.

3.3. XPS Spectrum of SrAl₂O₄:Dy³⁺,Mn⁴⁺

The oxidation states of Mn and Dy in SrAl₂O₄:Dy³⁺,Mn⁴⁺ are examined with the XPS analysis. Figure 4 represents the high-resolution XPS spectra of Mn 2p (a) and Dy 3d (b) in SrAl₂O₄:Dy³⁺,Mn⁴⁺. Mn⁴⁺ ions are diffused into SrAl₂O₄:Dy³⁺ at 900 °C for 4 h. As can be seen in Figure 4a, the peaks of Mn 2p_3/2 and Mn 2p_1/2 are located at about 642.2 and 653.8 eV, respectively. These binding energies indicate that the oxidation state of Mn ion is 4+. Figure 4b demonstrates that the peaks of the spin-orbit component (3d_5/2 and 3d_3/2) of Dy³⁺ are located at approximately 1297.6 and 1335.1 eV, respectively. The high-resolution XPS spectrum of O 1s in SrAl₂O₄:Dy³⁺,Mn⁴⁺ nanocrystals is shown in Figure S1. Peaking at 531.78 eV, the XPS spectral profile of O1s can be deconvoluted into several components.

3.4. PL Spectra of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ Diffusion at Different Temperatures

Figure 5 represents the PL spectra of SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion (a) and after Mn⁴⁺ diffusion at 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), and 1000 °C (f). The diffusion time is 4 h for each sample. As can be seen in Figure 5a, the PL spectrum of the SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion consists of a broad PL band peaking at about 400 nm, and two characteristic emission bands of Dy³⁺ ions peaking at 482 and 572 nm, respectively. The broad emission band of SrAl₂O₄:Dy³⁺ comes from intrinsic defects (namely oxygen and/or strontium vacancies) in the lattice of SrAl₂O₄ [5]. The narrow emission bands peaking at 482 and 572 nm are due to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ [2,3,4,6]. Before Mn⁴⁺ diffusion, the broad emission band of SrAl₂O₄:Dy³⁺ is very strong. Conversely, the two narrow emission bands centered at 482 and 572 nm are very weak. No red emission can be detected in the range of 600–700 nm for SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion. Thus, the perception color of the emissions of SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion is blue (Figure S2). Actually, quite similar intrinsic defect-related emissions have been recorded for a large number of aluminates, such as undoped CaAl₂O₄ [20,23], Dy³⁺-doped BaAl₂O₄ [21], Tb³⁺-doped CaAl₂O₄ [24], and Tb³⁺-doped SrAl₂O₄ [25]. At high annealing temperatures, oxygen atoms in air can migrate into the crystal lattice to repair the defects with the result of a decline in the population density of oxygen vacancies in SrAl₂O₄:Dy³⁺. Therefore, the intrinsic defect-related emissions should be weakened or even have disappeared after air annealing at high temperatures. Indeed, the broad emission band of SrAl₂O₄:Dy³⁺ disappears when the diffusion temperature is elevated to 600 °C and higher, as shown in Figure 5b–f.

The most striking feature in Figure 5 is that the red emission peaking at 651 nm gradually gains intensity as the diffusion temperature increases from 600 to 1000 °C. Figure S3 illustrates the PL spectra of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion at 800 °C (a), 900 °C (b), and 1000 °C (c) for 4 h. The inset in Figure S3 shows the zoomed section of the PL spectra of SrAl₂O₄:Dy³⁺,Mn⁴⁺ in the range of 620–700 nm. Quantitative analysis shows that the intensities of the red PL band are about 3%, 9%, and 30% that of the narrow blue PL band of Dy³⁺, peaking at 482 nm when SrAl₂O₄:Dy³⁺ is subject to Mn⁴⁺ diffusion at 600, 700, and 800 °C, respectively. Surprisingly, the red PL band of SrAl₂O₄:Dy³⁺,Mn⁴⁺ becomes dominant in intensity when the diffusion temperature is beyond 800 °C. As shown by the PL spectra e and f in Figure 5, the peak of the red PL band is located at 651 nm, which is in good agreement with the deep red emission of Mn⁴⁺-doped SrAl₂O₄ [14]. The integrated PL intensities of the red emission in PL spectra e and f are about 11.7 and 20.9 fold stronger than that of the narrow blue PL band of Dy³⁺ peaking at 482 nm. Thus, the PL spectra e and f verify that strong red emission can be achieved via Mn⁴⁺ diffusion into SrAl₂O₄:Dy³⁺ at diffusion temperatures higher than 800 °C. The PL quantum efficiency is a key parameter for the quantification of luminescent processes in phosphors. Characterized by Quantaurus-QY (Hamamatsu, Japan), the PL quantum yield values of these compounds are found to vary in the range of 27–60%, depending on the diffusion temperature and the diffusion time. The PL quantum yield values of commercial phosphors are higher than 75%. For example, the PL quantum yield of Y₃Al₅O₁₂:Ce³⁺ is known to be around 97%, and the green silicate Ba₂SiO₄:Eu²⁺ shows a PL quantum yield of about 79%. Compared to the high PL quantum yield values of these commercial phosphors, the PL quantum yield values of SrAl₂O₄:Dy³⁺,Mn⁴⁺ are quite low.

Once Mn⁴⁺ is diffused into SrAl₂O₄:Dy³⁺, a set of energy levels are introduced into the bandgap of SrAl₂O₄. As documented in the literature, the ground state and the excited state of Mn⁴⁺ can be denoted as ²E_g and ⁴A_2g, respectively [15], and the red emission peaking at 651 nm is associated with the ²E_g → ⁴A_2g transition of Mn⁴⁺. One might wonder how it would be possible to make the red emission much stronger than the characteristic emissions of Dy³⁺ in SrAl₂O₄:Dy³⁺,Mn⁴⁺. The answer lies in the much higher dose of Mn⁴⁺ in SrAl₂O₄:Dy³⁺,Mn⁴ at higher diffusion temperatures. The diffusion coefficient is known to depend on temperature. The diffusion coefficient in solids at different temperatures is found to be well predicted by the Arrhenius equation:

$$D=D_0\exp \left(-\frac{E_A}{kT}\right)$$

(1)

where E_A is the activation energy for diffusion (in eV), D₀ is the diffusion coefficient at infinite temperature (in cm²/s), D is the diffusion coefficient (in cm²/s), k is the Boltzmann constant, T is the absolute temperature (in K). The dose, which is the total amount of a dopant diffused into a solid, of a constant-source diffusion can be expressed as:

$$Q=\frac{2N_0}{\sqrt{\pi }}\sqrt{Dt}$$

(2)

where Q is the dose in the host (atoms/cm), N₀ is the surface concentration (atoms/cm²), D is the diffusion coefficient (cm²/s), and t is the diffusion time (s). In the constant source diffusion, the dose increases as a function of diffusion temperature and diffusion time. Equations (1) and (2) predict that the dose quickly increases with the diffusion temperature for given values of E_A and t. The activation energy for Mn⁴⁺ diffusion in SrA_l2O₄ has not yet been reported. An estimation of the activation energy for Mn⁴⁺ diffusion in SrAl₂O₄ becomes a choice. As documented in the literature, de Biasi and Grillo reported that the activation energies of 266 kJ/mol (2.76 eV) for the diffusion of Mn²⁺ in CaO and 203 kJ/mol (2.10 eV) for the diffusion of Mn²⁺ in MgO [26]. Additionally, Portavoce et al. reported that the activation energy for Mn in monocrystalline Ge is 2.37 eV [27]. After having considered the typical activation energies of As in polysilicon (3.9 eV), B in polysilicon (2.4–2.5 eV), B in SiO₂ (2.38–3.53 eV), Ga in SiO₂ (4.17 eV), and As in SiO₂ (3.7–4.7 eV), it seems reasonable to assume that the activation energy for Mn⁴⁺ diffusion in SrAl₂O₄ is in the range of 2–5 eV. Assuming that E_A takes the value of 3.0 eV, the doses of Mn⁴⁺ diffused into SrAl₂O₄ at 700, 800, 900, and 1000 °C are 7.8, 41, 163, and 523 times as large, respectively, as the dose of Mn⁴⁺ diffused into SrAl₂O₄ at 600 °C (Figure S4). The quick increase in the dose of Mn⁴⁺ at high diffusion temperature makes the red emission of Mn⁴⁺ dominant over the characteristic emissions of Dy³⁺ in SrAl₂O₄:Dy³⁺,Mn⁴. As a result, full-color tunable emissions are realized for SrAl₂O₄:Dy³⁺,Mn⁴ due to the complementary red emission of Mn⁴⁺. Rather than doping the host with red-emitting rare earth ions, such as Eu³⁺ (⁵D₀→⁷F₂ at 612 nm) [11,28], Sm³⁺ (⁴G_5/2→⁶H_7/2 at 605 nm) [29], or Pr³⁺ (³P₁→³F₃ at 642 nm) [30], diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ makes the red emission cost effective.

3.5. Emission Colors of SrAl₂O₄:Dy³⁺,Mn⁴⁺

Along with the characteristic emissions of Dy³⁺ peaking at 482 nm (blue) and 572 nm (yellow), the emergence of the red emission of Mn⁴⁺ makes SrAl₂O₄:Dy³⁺,Mn⁴⁺ suitable for multi-color emissions. Color coordinates, which are important parameters to quantitatively describe the emission color for luminescent materials, can be calculated from their PL spectral data [31,32]. Figure 6 shows photographs of the light-emitting SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion (a) and after Mn⁴⁺ diffusion at 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), and 1000 °C (f). The chromaticity coordinates are marked onto the corresponding photographs. Before Mn⁴⁺ diffusion, the PL color is blue with the chromaticity coordinates of (0.174, 0.157) for SrAl₂O₄:Dy³⁺, as shown in Figure 6a. When the diffusion temperature arises from 600 to 800 °C, the emission color of SrAl₂O₄:Dy³⁺,Mn⁴⁺ evolves from bluish green to white due to the weakened blue emission of the host at high diffusion temperature, as shown in Figure 6b–d. Interestingly, the emission color dramatically changes when SrAl₂O₄:Dy³⁺ is subject to Mn⁴⁺ diffusion at a temperature higher than 800 °C. As illustrated in Figure 6e,f, the emission colors of SrAl₂O₄:Dy³⁺,Mn⁴⁺ are pink and purplish red when the diffusion temperatures are 900 and 1000 °C, respectively. The dramatic changes in emission color are due to the enhanced red emission of Mn⁴⁺. The CIE chromaticity diagram of the emissions of SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion (a) and after Mn⁴⁺ diffusion at temperatures of 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), and 1000 °C (f) is shown in Figure S5. It is clear that full-color tunable emissions can be achieved for SrAl₂O₄:Dy³⁺,Mn⁴⁺ by tuning the diffusion temperature of Mn⁴⁺.

3.6. PL Spectra of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ Diffusion for Different Times

According to Equation (2), the dose of the constant-source diffusion increases with the diffusion time when the diffusion temperature is fixed. Therefore, the dose of Mn⁴⁺ in SrAl₂O₄:Dy³⁺,Mn⁴⁺ can be tuned by the diffusion time. Figure 7 depicts the PL spectra of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion at 800 °C for different times: 4 h, 8 h, 12 h, 16 h, 20 h, and 24 h. It can be seen in Figure 7 that each PL spectrum consists of one blue emission band centered at 482 nm, one yellow emission band centered at 572 nm, and one red band peaking at about 651 nm. Obviously, the sharp emission peak at 651 nm can be ascribed to the ²E_g→⁴A_2g transition of tetrahedrally-coordinated Mn⁴⁺ [16]. The quenching concentration of Mn⁴⁺ in SrAl₂O₄ was reported to be 0.04 mol% [14]. The red emission in Figure 7 increases slowly with the diffusion time, indicating that the concentration of Mn⁴⁺ in SrAl₂O₄:Dy³⁺,Mn⁴⁺ can be well controlled before reaching the quenching concentration. According to Equation (2), the doses of Mn⁴⁺ diffused into SrAl₂O₄:Dy³⁺ at 800 °C for 8, 12, 16, 20, and 24 h are 1.4, 1.7, 2.0, 2.2, and 2.5 times as large as the dose of Mn⁴⁺ diffused into SrAl₂O₄:Dy³⁺ at 800 °C for 4 h. Thus, it is understandable that the diffusion temperature, rather than the diffusion time, is more effective in enhancing the red emission of SrAl₂O₄:Dy³⁺,Mn⁴⁺. Obviously, diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ by the constant-source diffusion technique exhibits advantages because tuning the parameters of diffusion temperature and diffusion time can finely control the doping concentration at low doping levels.

Thermal diffusion of Mn⁴⁺ into SrAl₂O₄:Dy³⁺ has potential industrial sense because it modifies the PL properties of SrAl₂O₄:Dy³⁺ phosphor with the introduction of red emissions of Mn⁴⁺ ions. The diffusion time of 4 h seems quite long when compared to the diffusion times of B and P doping silicon in advanced semiconductor fabrication sectors. Optimizing the diffusion time and diffusion temperature is a key procedure for the application of SrAl₂O₄:Dy³⁺,Mn⁴⁺ as phosphors in the solid state lighting industry.

3.7. PL Decays of SrAl₂O₄:Dy³⁺,Mn⁴⁺

Figure 8a represents the PL decay curve of SrAl₂O₄:Dy³⁺ before Mn⁴⁺ diffusion. The excitation wavelength for this decay curve is 320; the detection wavelength is 400 nm. The instrument response function is shown in Figure 8a (in blue) for the decay curve. The exponential reconvolution of the raw data is represented by the green solid curve in Figure 8a. Our analysis reveals that this PL decay curve can be best described by a tri-exponential decay model with three largely different time constants. The fitting parameters of the PL decay are listed in Figure 8a. On one hand, the three time constants are largely different, suggesting the presence of three distinctly different channels in the phosphor for radiative recombination. On the other hand, the averaged lifetime of the PL decay is 4.78 ns, which provides complementary information about the intrinsic defect-related emissions of SrAl₂O₄.

Figure 8b represents the PL decay of SrAl₂O₄:Dy³⁺ after Mn⁴⁺ diffusion at 1000 °C for 4 h. The excitation wavelength for this decay curve is 320; the detection wavelength is 651 nm. The exponential reconvolution of the raw data in Figure 8b is represented by the red solid curve. Clearly, the PL decay curve in Figure 8b is well described by a mono-exponential decay model with a time constant of 0.7 ms. As documented in the literature, Cao et al. reported that the lifetime of Mn⁴⁺ emission in Mn⁴⁺-doped CaAl₂O₄ decreased from 1.35 to 0.94 ms as the concentration of Mn⁴⁺ increased from 0.2 to 1.6 mol% [19]. Sun et al. reported that the lifetime of Mn⁴⁺ emissions in Mn⁴⁺-doped Ba₂GdTaO₆ was around 0.3 ms [33]. Apparently, the PL lifetime of Mn⁴⁺ emissions in SrAl₂O₄:Dy³⁺,Mn⁴⁺ is in line with those of Mn⁴⁺ emissions in a large number of Mn⁴⁺-doped inorganic materials [19,33,34,35,36]. The lifetime of Mn⁴⁺ emission of SrAl₂O₄:Dy³⁺,Mn⁴⁺ is about five orders of magnitude higher than that of the intrinsic defect-related emissions of SrAl₂O₄:Dy³⁺, meaning that the red emission is caused by the parity-forbidden d-d transitions of the Mn⁴⁺ ions.

The PL decay time of the defect-related blue emission band is 4.78 ns, which is unusually fast when compared to the PL decay time of the defect-related emission in some complex oxides, such as tungstates and molybdates. For example, the PL decay times of PbMoO₄ and ZnWO₄ crystals are of the order of tens and hundreds of microseconds [37]. We tried to measure the decay of this broad blue band in microsecond or even millisecond scales, but no obvious changes in the PL decay time were found for our sol-gel-derived SrAl₂O₄:Dy³⁺,Mn⁴⁺ when the time scales of the PL decay measurements were extended to 200, 500, 1000, 2000, 5000, and 50,000 ns, respectively. It is known the PL decay time of a phosphor depends on the non-radiative recombination rates in the phosphor. The higher the non-radiative recombination rate, the shorter the PL decay time will be. When compared to PbMoO₄ and ZnWO₄ single crystals, the sol-gel-derived SrAl₂O₄:Dy³⁺,Mn⁴⁺ nanocrystals have a much higher density of oxygen vacancies, which in turn enhances the non-radiative recombination rate. Therefore, the high density of oxygen vacancies in the sol-gel-derived SrAl₂O₄:Dy³⁺,Mn⁴⁺ nanocrystals is responsible for the short PL decay time. Such an interpretation is supported by the averaged PL decay times of several nanoseconds for ZnWO₄ nanocrystals [38,39], ZnMoO₄ nanocrystals [40], zinc molybdenum oxide hydrate nanocrystals [41], and pentazinc dimolybdate pentahydrate [42]. In particular, our previous work revealed that the averaged PL decay time of ZnWO₄ nanocrystals can be extended from 2.25 to 1770 ns when the population density of oxygen vacancies in ZnWO₄ nanocrystals is decreased via thermal annealing in air [43].

4. Conclusions

Employing a constant-source diffusion technique, SrAl₂O₄:Dy³⁺,Mn⁴⁺ phosphors were obtained by diffusing Mn⁴⁺ ions into SrAl₂O₄:Dy³⁺. The influences of diffusion temperature and diffusion time on the red emission were investigated. It was found that: (i) peaking at 651 nm, the red emission of SrAl₂O₄:Dy³⁺,Mn⁴⁺ became dominant over the characteristic emissions of Dy³⁺ when the diffusion temperature was higher than 800 °C; (ii) the red emission intensity of SrAl₂O₄:Dy³⁺,Mn⁴⁺ was far more sensitive to diffusion temperature than to diffusion time; and (iii) full-color tunable emissions were realized for SrAl₂O₄:Dy³⁺,Mn⁴⁺ by constant source diffusion. Owing to the high degree of control over the dose of Mn⁴⁺ in SrAl₂O₄:Dy³⁺, diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ provided a unique opportunity to prepare full-color tunable phosphors. This approach could be the basis for convenient color control of Dy³⁺-doped materials by controlling the red emission intensity of Mn⁴⁺ in the host.