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

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.


Phase, Morphology, and Elemental Composition of SrAl 2 O 4 :Dy 3+ ,Mn 4+
The phase of SrAl 2 O 4 :Dy 3+ ,Mn 4+ phosphors was determined by their X-ray diffraction (XRD) profiles. The XRD profiles of SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ phosphors was characterized with a scanning electron microscope (SEM) (model S-4800, Hitachi, Tokyo, Japan). The elemental composition of SrAl 2 O 4 :Dy 3+ ,Mn 4+ phosphors was provided by the energy dispersive X-ray (EDX) spectrum of the synthesized phosphor. The nanostructures and crystal lattice of SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 Mn 4+ and Dy 3+ in SrAl 2 O 4 :Dy 3+ ,Mn 4+ , 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.

PL Spectra of SrAl 2 O 4 :Dy 3+ ,Mn 4+
The steady-state PL spectra of SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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.

PL Decay Curves of SrAl 2 O 4 :Dy 3+ ,Mn 4+
On a picosecond fluorescence lifetime spectrometer (LifeSpec II, Edinburgh Instruments, Edinburgh, UK), the PL decay curves of intrinsic defect-related emissions of SrAl 2 O 4 :Dy 3+ before Mn 4+ 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 SrAl 2 O 4 :Dy 3+ after Mn 4+ diffusion was measured at room temperature on a transient-state fluorescence spectrometer (FS5, Edinburgh Instruments, Edinburgh, UK). The PL decay curve of Mn 4+ emission was recorded at 651 nm for SrAl 2 O 4 :Dy 3+ after Mn 4+ diffusion at 1000 • C. A pulsed Xenon lamp was employed to provide the pulsed excitation.

Results and Discussions
3.1. Phase and Morphology of SrAl 2 O 4 :Dy 3+ ,Mn 4+ Figure 1 represents the X-ray diffraction (XRD) curves of SrAl 2 O 4 :Dy 3+ before Mn 4+ diffusion (a) and after Mn 4+ 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 SrAl 2 O 4 (JCPDS card no.  are shown at the bottom of Figure 1 for comparison. Evidently, pure phase SrAl 2 O 4 is formed when the diffusion temperatures are 900 and 1000 • C [2][3][4][5]. In contrast, a secondary phase is formed in SrAl 2 O 4 before Mn 4+ diffusion (curve a) and after Mn 4+ 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 SrAl 2 O 4 has a stuffed tridymite structure with the space group P2 1 and Z = 4. As described in our previous work, monoclinic SrAl 2 O 4 has a three-dimensional network of corner-sharing [AlO 4 ] tetrahedra with channels present in the aand c-directions. Two crystallographically different Sr 2+ sites are present in the unit cell of SrAl 2 O 4 , and these Sr 2+ ions are located along the channels [5]. Therefore, it is easy for Dy 3+ and Mn 4+ to dope SrAl 2 O 4 . 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 Sr 2+ , Dy 3+ , Al 3+ , and Mn 4+ are 0.145 (CN = 9), 0.122 (CN = 9), 0.053 (CN = 4), and 0.053 nm  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 Sr 2+ sites are present in the unit cell of SrAl2O4, and these Sr 2+ ions are located along the channels [5]. Therefore, it is easy for Dy 3+ and Mn 4+ 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 Sr 2+ , Dy 3+ , Al 3+ , and Mn 4+ 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 Dy 3+ and Mn 4+ tend to replace the Sr 2+ and Al 3+ 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:Dy 3+ after Mn 4+ 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:Dy 3+ ,Mn 4+ 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 ( 2 11) crystallographic planes of monoclinic SrAl2O4. Thus, the micrographs in Figure 2 demonstrate that the synthesized SrAl2O4:Dy 3+ , Mn 4+ exhibits good crystallinity.   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 SrAl 2 O 4 nanocrystals in a single aggregate [5]. The high-resolution TEM micrograph in Figure 2c illustrates the crystal lattices of one SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 (211) crystallographic planes of monoclinic SrAl 2 O 4 . Thus, the micrographs in Figure 2 demonstrate that the synthesized SrAl 2 O 4 :Dy 3+ , Mn 4+ exhibits good crystallinity.  Figure 3 illustrates the EDX spectrum of SrAl2O4:Dy 3+ after Mn 4+ diffusion at 800 °C for 4 h. As mentioned in the experimental section, the doping concentration of Dy 3+ is 1.6 mol%. As can be seen in Figure 3, the characteristic X-ray emissions of O(K), Al(K), and Sr(L1) are located at 0.525, 1.486, and 1.806 keV, respectively. In the meantime, the characteristic emissions of Dy(L1) and Dy(L1) can be identified at 6.495 and 7.248 keV, respectively. Additionally, the X-ray emissions of Au(Mα1) and Au(L1) 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:Dy 3+ ,Mn 4+ verifies the presence of elements Al, Sr, O, and Dy in the phosphor.  Figure 3 illustrates the EDX spectrum of SrAl 2 O 4 :Dy 3+ after Mn 4+ diffusion at 800 • C for 4 h. As mentioned in the experimental section, the doping concentration of Dy 3+ is 1.6 mol%. As can be seen in Figure 3, the characteristic X-ray emissions of O(Kα), Al(Kα), and Sr(Lα 1 ) are located at 0.525, 1.486, and 1.806 keV, respectively. In the meantime, the characteristic emissions of Dy(Lα 1 ) and Dy(Lβ 1 ) can be identified at 6.495 and 7.248 keV, respectively. Additionally, the X-ray emissions of Au(Mα 1 ) and Au(Lα 1 ) 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ verifies the presence of elements Al, Sr, O, and Dy in the phosphor.

XPS Spectrum of SrAl2O4:Dy 3+ ,Mn 4+
The oxidation states of Mn and Dy in SrAl2O4:Dy 3+ ,Mn 4+ are examined with the XPS analysis. Figure 4 represents the high-resolution XPS spectra of Mn 2p (a) and Dy 3d (b) in SrAl2O4:Dy 3+ ,Mn 4+ . Mn 4+ ions are diffused into SrAl2O4:Dy 3+ at 900 °C for 4 h. As can be seen in Figure 4a, the peaks of Mn 2p3/2 and Mn 2p1/2 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 (3d5/2 and 3d3/2) of Dy 3+ are located at approximately 1297.6 and 1335.1 eV, respectively. The high-resolution XPS spectrum of O 1s in SrAl2O4:Dy 3+ ,Mn 4+ nanocrystals is shown in Figure S1. Peaking at 531.78 eV, the XPS spectral profile of O1s can be deconvoluted into several components.

XPS Spectrum of SrAl 2 O 4 :Dy 3+ ,Mn 4+
The oxidation states of Mn and Dy in SrAl 2 O 4 :Dy 3+ ,Mn 4+ are examined with the XPS analysis. Figure 4 represents the high-resolution XPS spectra of Mn 2p (a) and Dy 3d (b) in SrAl 2 O 4 :Dy 3+ ,Mn 4+ . Mn 4+ ions are diffused into SrAl 2 O 4 :Dy 3+ 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 Dy 3+ are located at approximately 1297.6 and 1335.1 eV, respectively. The high-resolution XPS spectrum of O 1s in SrAl 2 O 4 :Dy 3+ ,Mn 4+ nanocrystals is shown in Figure S1. Peaking at 531.78 eV, the XPS spectral profile of O1s can be deconvoluted into several components.

XPS Spectrum of SrAl2O4:Dy 3+ ,Mn 4+
The oxidation states of Mn and Dy in SrAl2O4:Dy 3+ ,Mn 4+ are examined with the XPS analysis. Figure 4 represents the high-resolution XPS spectra of Mn 2p (a) and Dy 3d (b) in SrAl2O4:Dy 3+ ,Mn 4+ . Mn 4+ ions are diffused into SrAl2O4:Dy 3+ at 900 °C for 4 h. As can be seen in Figure 4a, the peaks of Mn 2p3/2 and Mn 2p1/2 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 (3d5/2 and 3d3/2) of Dy 3+ are located at approximately 1297.6 and 1335.1 eV, respectively. The high-resolution XPS spectrum of O 1s in SrAl2O4:Dy 3+ ,Mn 4+ nanocrystals is shown in Figure S1. Peaking at 531.78 eV, the XPS spectral profile of O1s can be deconvoluted into several components.   Figure 5 represents the PL spectra of SrAl 2 O 4 :Dy 3+ before Mn 4+ diffusion (a) and after Mn 4+ 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 SrAl 2 O 4 :Dy 3+ before Mn 4+ diffusion consists of a broad PL band peaking at about 400 nm, and two characteristic emission bands of Dy 3+ ions peaking at 482 and 572 nm, respectively. The broad emission band of SrAl 2 O 4 :Dy 3+ comes from intrinsic defects (namely oxygen and/or strontium vacancies) in the lattice of SrAl 2 O 4 [5]. The narrow emission bands peaking at 482 and 572 nm are due to the 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 transitions of Dy 3+ [2][3][4]6]. Before Mn 4+ diffusion, the broad emission band of SrAl 2 O 4 :Dy 3+ 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 SrAl 2 O 4 :Dy 3+ before Mn 4+ diffusion. Thus, the perception color of the emissions of SrAl 2 O 4 :Dy 3+ before Mn 4+ diffusion is blue ( Figure S2). Actually, quite similar intrinsic defect-related emissions have been recorded for a large number of aluminates, such as undoped CaAl 2 O 4 [20,23], Dy 3+ -doped BaAl 2 O 4 [21], Tb 3+ -doped CaAl 2 O 4 [24], and Tb 3+ -doped SrAl 2 O 4 [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 SrAl 2 O 4 :Dy 3+ . 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 SrAl 2 O 4 :Dy 3+ disappears when the diffusion temperature is elevated to 600 • C and higher, as shown in Figure 5b-f.  Figure 5 represents the PL spectra of SrAl2O4:Dy 3+ before Mn 4+ diffusion (a) and after Mn 4+ 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:Dy 3+ before Mn 4+ diffusion consists of a broad PL band peaking at about 400 nm, and two characteristic emission bands of Dy 3+ ions peaking at 482 and 572 nm, respectively. The broad emission band of SrAl2O4:Dy 3+ 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 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy 3+ [2][3][4]6]. Before Mn 4+ diffusion, the broad emission band of SrAl2O4:Dy 3+ 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:Dy 3+ before Mn 4+ diffusion. Thus, the perception color of the emissions of SrAl2O4:Dy 3+ before Mn 4+ 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], Dy 3+ -doped BaAl2O4 [21], Tb 3+ -doped CaAl2O4 [24], and Tb 3+ -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:Dy 3+ . 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:Dy 3+ 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:Dy 3+ after Mn 4+ 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:Dy 3+ ,Mn 4+ 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 Dy 3+ , peaking at 482 nm when SrAl2O4:Dy 3+ is subject to Mn 4+ diffusion at 600, 700, and 800 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 SrAl 2 O 4 :Dy 3+ after Mn 4+ 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 Dy 3+ , peaking at 482 nm when SrAl 2 O 4 :Dy 3+ is subject to Mn 4+ diffusion at 600, 700, and 800 • C, respectively. Surprisingly, the red PL band of SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 Mn 4+ -doped SrAl 2 O 4 [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 Dy 3+ peaking at 482 nm. Thus, the PL spectra e and f verify that strong red emission can be achieved via Mn 4+ diffusion into SrAl 2 O 4 :Dy 3+ 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 Y 3 Al 5 O 12 :Ce 3+ is known to be around 97%, and the green silicate Ba 2 SiO 4 :Eu 2+ 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ are quite low.

PL Spectra of SrAl 2 O 4 :Dy 3+ after Mn 4+ Diffusion at Different Temperatures
Once Mn 4+ is diffused into SrAl 2 O 4 :Dy 3+ , a set of energy levels are introduced into the bandgap of SrAl 2 O 4 . As documented in the literature, the ground state and the excited state of Mn 4+ can be denoted as 2 E g and 4 A 2g , respectively [15], and the red emission peaking at 651 nm is associated with the 2 E g → 4 A 2g transition of Mn 4+ . One might wonder how it would be possible to make the red emission much stronger than the characteristic emissions of Dy 3+ in SrAl 2 O 4 :Dy 3+ ,Mn 4+ . The answer lies in the much higher dose of Mn 4+ in SrAl 2 O 4 :Dy 3+ ,Mn 4 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: where E A is the activation energy for diffusion (in eV), D 0 is the diffusion coefficient at infinite temperature (in cm 2 /s), D is the diffusion coefficient (in cm 2 /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: where Q is the dose in the host (atoms/cm), N 0 is the surface concentration (atoms/cm 2 ), D is the diffusion coefficient (cm 2 /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.

Emission Colors of SrAl2O4:Dy 3+ ,Mn 4+
Along with the characteristic emissions of Dy 3+ peaking at 482 nm (blue) and 572 nm (yellow), the emergence of the red emission of Mn 4+ makes SrAl2O4:Dy 3+ ,Mn 4+ 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:Dy 3+ before Mn 4+ diffusion (a) and after Mn 4+ 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 Mn 4+ diffusion, the PL color is blue with the chromaticity coordinates of (0.174, 0.157) for SrAl2O4:Dy 3+ , as shown in Figure 6a. When the diffusion temperature arises from 600 to 800 °C, the emission color of SrAl2O4:Dy 3+ ,Mn 4+ 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:Dy 3+ is subject to Mn 4+ diffusion at a temperature higher than 800 °C. As illustrated in Figure 6e,f, the emission colors of SrAl2O4:Dy 3+ ,Mn 4+ 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 Mn 4+ . The CIE chromaticity diagram of the emissions of SrAl2O4:Dy 3+ before Mn 4+ diffusion (a) and after Mn 4+ 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:Dy 3+ ,Mn 4+ by tuning the diffusion temperature of Mn 4+ .   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 2 E g → 4 A 2g transition of tetrahedrally-coordinated Mn 4+ [16]. The quenching concentration of Mn 4+ in SrAl 2 O 4 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 Mn 4+ in SrAl 2 O 4 :Dy 3+ ,Mn 4+ can be well controlled before reaching the quenching concentration. According to Equation (2)

PL Spectra of SrAl2O4:Dy 3+ after Mn 4+ 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 Mn 4+ in SrAl2O4:Dy 3+ ,Mn 4+ can be tuned by the diffusion time. Figure 7 depicts the PL spectra of SrAl2O4:Dy 3+ after Mn 4+ 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 2 Eg→ 4 A2g transition of tetrahedrally-coordinated Mn 4+ [16]. The quenching concentration of Mn 4+ 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 Mn 4+ in SrAl2O4:Dy 3+ ,Mn 4+ can be well controlled before reaching the quenching concentration. According to Equation (2) Figure 8a represents the PL decay curve of SrAl 2 O 4 :Dy 3+ before Mn 4+ 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 SrAl 2 O 4 . Figure 8a represents the PL decay curve of SrAl2O4:Dy 3+ before Mn 4+ 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:Dy 3+ after Mn 4+ 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 Figure 8b represents the PL decay of SrAl 2 O 4 :Dy 3+ after Mn 4+ 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  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 PbMoO 4 and ZnWO 4 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 PbMoO 4 and ZnWO 4 single crystals, the sol-gel-derived SrAl 2 O 4 :Dy 3+ ,Mn 4+ 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ nanocrystals is responsible for the short PL decay time. Such an interpretation is supported by the averaged PL decay times of several nanoseconds for ZnWO 4 nanocrystals [38,39], ZnMoO 4 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 ZnWO 4 nanocrystals can be extended from 2.25 to 1770 ns when the population density of oxygen vacancies in ZnWO 4 nanocrystals is decreased via thermal annealing in air [43].

Conclusions
Employing a constant-source diffusion technique, SrAl 2 O 4 :Dy 3+ ,Mn 4+ phosphors were obtained by diffusing Mn 4+ ions into SrAl 2 O 4 :Dy 3+ . 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 SrAl 2 O 4 :Dy 3+ ,Mn 4+ became dominant over the characteristic emissions of Dy 3+ when the diffusion temperature was higher than 800 • C; (ii) the red emission intensity of SrAl 2 O 4 :Dy 3+ ,Mn 4+ was far more sensitive to diffusion temperature than to diffusion time; and (iii) full-color tunable emissions were realized for SrAl 2 O 4 :Dy 3+ ,Mn 4+ by constant source diffusion. Owing to the high degree of control over the dose of Mn 4+ in SrAl 2 O 4 :Dy 3+ , diffusing Mn 4+ into SrAl 2 O 4 :Dy 3+ provided a unique opportunity to prepare full-color tunable phosphors. This approach could be the basis for convenient color control of Dy 3+ -doped materials by controlling the red emission intensity of Mn 4+ in the host.