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Ca Y Zr Al O :Ce : Solid Solution Design Toward the Green Emission Garnetstructure Phosphor for Near-UV LEDs and Their Luminescence Properties Yichao Wang, Jianyan Ding, and Yuhua Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09783 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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The Journal of Physical Chemistry

Ca2-xY1+xZr2-xAl3+xO12:Ce3+: Solid Solution Design toward the Green Emission Garnet Structure Phosphor for Near-UV LEDs and Their Luminescence Properties Yichao Wang, Jianyan Ding, Yuhua Wang* Key Laboratory for Special Function Materials and Structural Design of the Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou,730000, China. E-mail: [email protected]; Fax: +86-931-8913554; Tel: +86-931-8912772 Abstract A series of novel garnet structure phosphors Ca2-xY1+xZr2-xAl3+xO12: yCe3+(0≤x≤0.4) (0≤y≤0.07) were synthesised by the solid state reaction and developed by the Ca2YZr2Al3O12-YAG solid solution design. Structure information of the phosphors all were refined using the Rietveld method -

based on the XRD date, they all show the general cubic garnet structure with the space group Ia3d, the variation of the cell parameter and the average bond lengths is investigated, which is match well with the theoretical calculation. The band structure of the phosphors have been calculated using the density function theory method, the band gaps gradually become narrow with the increase of x, demonstrated by the diffuse reflectance spectra. The photoluminescence properties were investigated on aspects of the emission and excitation spectra, quantum efficiency, thermal stability and the decay curves. The possible mechanisms and reasons for the photoluminescence properties variations have been discussed in detail. All the phosphor can be efficiently excited by the near-ultraviolet chips, according to the double-substitution for Ca2+/Zr4+ by Y3+/Al3+, the emission light can be changed from cyan to green, with the peak shifting from 496nm to 514nm. The internal quantum efficiency all exceed 50%. An excellent WLED lamp was obtained by fabricating our Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ phosphor with BAM:Eu2+, (Sr, Ca)AlSiN3:Eu2+ and 395 nm GaN chip，its CIE coordinate(x, y), CCT and Ra are (0.3603, 0.348), 4404 K and 87. These results reveal the correlations between structure and properties for phosphors and provide a practical basis to engineer and develop novel phosphors for the n-UV LEDs.

1. Introduction In the field of displays and lighting technology, white light emitting diodes (w-LEDs) have received considerable attention due to their excellent properties compared with conventional

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lighting bulbs/lamps, including relatively stable physical and chemical properties, high energy efficiency, environmental friendliness and extensive adjustability for different requirements of the colour gamut.1-9 In order to producing white light with high color rendering index(CRI) and low correlated color temperature (CCT)，the use of a combination of near-ultraviolet (NUV) LED chips with three primary colors phosphors is proved to be an effective and preferable approach.10-12 The eventual performance of w-LED strongly depends on the luminescence properties of the phosphors; phosphors with high energy efficiency, good physical and chemical properties and tunable emission can not only improve the properties of w-LEDs but also exhibit adjustable advantages in the CRI, colour gamut and CCT.13-16 Focusing on the green emission phosphors, there are also some deficiency for the existing commercial phosphors, for examples, the (Ba,Sr)2SiO4:Eu2+ has a poor chemical stability and the β-sialon:Eu2+ has a harsh reaction condition. Thus, the design and development of new green phosphors used for nUV-LEDs with the excellent property are significant and urgent. In recent decades, garnet has been extensively investigated as a host material due to its -

outstanding physical and chemical stabilities. The garnet has a cubic structure with Ia3d symmetry, the chemical formula for a standard garnet is {A}3[B]2(C)3O12, the A, B and C are cations with different coordination environment, the atoms in A position occupy 8-fold dodecahedral coordination 24c sites, the atoms in B position are in 6-fold octahedral coordination 16a sites and the atoms in C position are in 4-fold tetrahedral coordination 24d sites.15-17 Each octahedron is connected to six tetrahedrons while each tetrahedron is connected to four octahedrons by sharing corners. It is the presence of the three different cation sites that give Ce3+-doped garnets their remarkable flexibility in tuning and optimization of luminescence properties for specific applications through cation substitution. Some garnet structure phosphors exhibit excellent luminescence properties, such as the commercial yellow phosphor YAG: Ce3 +, commercial green phosphors Ca3Sc2Si3O12:Ce3+ and Lu3Al5O12:Ce3+, etc.17-21 However, for the most of the reported garnet structure phosphors, their optimum excitation peaks are always located on the blue light range, which makes them more suitable for the use combined with the blue chips, they always show the low efficiency excited at the near ultraviolet chip, so it is significant and expectant to find or design the three primary colors phosphors with the garnet structure, which could be effectively excited at the near ultraviolet light.


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In 2015, Xicheng Wang et al. reported a novel garnet phosphor Ca2YZr2Al3O12 with the optimum excitation at the near ultraviolet region,22 the phosphor exhibit a blue-green emission(495nm) at the concentration quenching point with the high energy efficiency, it is expected that the emission spectrum has a red shift and shows a standard green light emission excited at n-UV chips. In general, solid solution design is an efficient route to develop new phosphors with diverse composition creations for changing the luminescence property and emission spectra, by the cation/anion substitution and the chemical unit substitution.23-25 Based on the known structure information, the solid solution Ca2YZr2Al3O12-YAG was designed and synthesized, this process could be regarded as chemical units of YO8/AlO6 replacing CaO8/ZrO6 polyhedra, respectively. Crystal structures of the series of phosphors were refined via the Rietveld method, their photoluminescence properties concerning excitation/emission spectra, decay lifetime, quantum efficiency and thermal quenching are studied in detail, the band structures were lucubrated to further elucidate the relationship between structure and luminescence properties. Via the structure design, a green emission garnet phosphor Ca1.6Y1.4Zr1.6Al3.4O12:Ce3+ with a large Stokes shift and high quantum efficiency is obtained, it has a shorter excitation wavelength compared with the commercial garnet structure green phosphors(LuAG:Ce3+, Ca3Sc2Si3O12:Ce3+), which make it could be effectively excited by the near ultraviolet light. Furthermore, the relationship between the luminescence properties and the crystal structure is discussed in detail.

2 Experimental A series of Ce3+ doped Ca2-xY1+xZr2-xAl3+xO12: yCe3+ phosphors were synthesized via the conventional solid state reaction. The initial materials are CaCO3(A.R.), Y2O3(A.R.), ZrO2(A.R.), Al2O3(A.R.) and CeO2(99.99%), the H3BO3(A.R.) is added as the flux. The ingredients were weighed on the basis of the stoichiometric amounts of the target product and mixed well with adding moderate alcohol. Then the powders were placed into boron nitride crucibles and sintered at 1525 ℃ for 8 h with the heating rate 5℃/min in a reducing atmosphere (5% H2–95% N2). Eventually, the samples were ground into the powder for various measurements after cooling to room temperature. The samples X-ray diffraction (XRD) data were collected on a Rigaku diffractometer with Ni-filtered Cu-Ka radiation (λ = 1.54056 Å), operating at 30 kV and 15 mA. The samples morphology was observed using scanning electron microscopy (SEM; Hitachi; S-4800). The



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elemental compositions of different samples were obtained using a scanning electron microscope equipped with the energy dispersive spectrometer (EDS) system. The sample high-resolution transmission electron microscopy (HRTEM) was recorded on a transmission electron microscope operating at 300kV (FEI Tecnai F30). The photoluminescence spectra at room temperature were measured with a fluorescence spectrophotometer (FLS-920T), equipped with the Xe 900 light source(450 W xenon arc lamp) and the double excitation monochromators. The high temperature luminescence property was measured using an aluminium plaque with cartridge heaters; the temperature was measured with thermocouples and controlled using a TAP-02 high temperature fluorescence controller. The PL decay curves were obtained by a FLS-920T fluorescence spectrophotometer equipped with an nF900 ns Flashlamp as the light source. The diffuse reflectance spectra of the samples were recorded on a PE lambda950 UV−vis spectrophotometer with the BaSiO4 as the reference.

3. Results and discussion 3.1 Phase identification and crystal structure Fig. 1 shows the XRD patterns of the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.6), for the samples of x≤0.4,

the diffraction data matches well with the calculated diffraction result and no

diffraction peaks of impurity can be observed. When the x exceeds 0.4, the YAG phase appears and the intensity gradually increases with the increase of x. In addition, with the increase of x, the diffraction peaks all shift to higher scattering(the magnified (420) crystallographic planes are shown as a sample), this variation implies the decrease of the lattice constants in the process of the double-substitution for Ca2+/Zr4+ by Y3+/Al3+.


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Fig. 1 The XRD patterns of the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.6)

For understanding the detailed structure variation after the double substitution, on the basis of the XRD data, the Rietveld analysis was implemented using the GSAS programme. Fig. 2(a-c) shows

the

Rietveld

refinements

results

of

Ca2YZr2Al3O12:0.03Ce3+,

Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ and Ca1.5Y1.5Zr1.5Al3.5O12:0.03Ce3+, respectively. The red lines and black crosses represent the calculated patterns and experimental patterns, the short vertical orange and red lines are Bragg reflection positions. The blue line shows the difference between the experimental results and calculated results. The structural parameters of Ca2YZr2Al3O12 and Y3Al5O12（Ref. 20 and ICSD #20090） are adopted as the initial parameters for the Rietveld analysis. According to the Rietveld refinements results, for the samples x≤0.4, they are all the single garnet structure, for the samples x=0.5 and x=0.6, the samples are composed of two garnet structure phase. The phase A (92.4% for x=0.5, 78.5% for x=0.6) has the similar Bragg reflection positions with the Ca2YZr2Al3O12, the phase B (7.6% for x=0.5, 21.5% for x=0.6) is the Y3Al5O12. The refined structure data of Ca2YZr2Al3O12:0.03Ce3+, Ca1.8Y1.2Zr1.8Al3.2O12:0.03Ce3+ and Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ are shown in Table 1. Fig. 2(d) shows the crystal structure of Ca2-xY1+xZr2-xAl3+xO12, the eight-coordinated sites(24c) are occupied by Ca2+ and Y3+ ions, the



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Fig.

2

(a)-(c)

The

Rietveld

refinements

results

of

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Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ and Ca1.5Y1.5Zr1.5Al3.5O12:0.03Ce3+. (d) The crystal structure of Ca2-xY1+xZr2-xAl3+xO12.

six-coordinated sites(16a) are occupied by Zr4+ and Al3+ ions and the four-coordinated sites(24d) all are occupied by Al3+ ions, the occupation proportion of the different ions in the eight and six-coordinated site would cause the changes of the lattice constants. Usually, the volume and the cell parameters of the solid solutions change linearly based on the variation of the chemical component.26-28 For the Ca2-xY1+xZr2-xAl3+xO12：0.03Ce3+, it can be seen as the solid solutions of Ca2YZr2Al3O12:Ce3+ and Y3Al5O12, the lattice constants a(=b=c) could be calculated according to the equation(1)： a(x)=b(x)=c(x)=Ax/2+B(1-x/2)(0≤x≤2)

(1)

where A is the lattice constant(a=b=c) of Ca2YZr2Al3O12:0.03Ce3+ (obtained from the calculated result), B is the lattice constant(a=b=c) of Y3Al5O12 (derived from the PDF#33-0040). The cell


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Table 1 Final refined structural parameters for 3+ 3+ Ca1.8Y1.2Zr1.8Al3.2O12:0.03Ce , Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce .


Atom Ca Y Ce Al(1) Al(2) Zr O

Position 24c 24c 24c 24d 16a 16a 96h

Occupancy 0.667 0.303 0.03 1 0 1 1

x 0.125 0.125 0.125 0.375 0 0 -0.0342

y 0 0 0 0 0 0 0.0513

z 0.25 0.25 0.25 0.25 0 0 0.1543

Ca1.8Y1.2Zr1.8Al3.2O12:0.03 Ce3+ crystal system:cubic Space group: Ia3d a = b = c = 12.372Å Rwp = 9.44% Rb = 7.61%



Occupancy 0.6 0.37 0.03 1 0.1 0.9 1

x 0.125 0.125 0.125 0.375 0 0 -0.0343

y 0 0 0 0 0 0 0.0510

z 0.25 0.25 0.25 0.25 0 0 0.1539

Ca1.6Y1.4Zr1.6Al3.4O12:0.03 Ce3+ crystal system:cubic Space group: Ia3d a = b = c = 12.341Å Rwp = 9.83% Rb = 7.91%



Occupancy 0.533 0.437 0.03 1 0.2 0.8 1

x 0.125 0.125 0.125 0.375 0 0 -0.0346

y 0 0 0 0 0 0 0.0497

z 0.25 0.25 0.25 0.25 0 0 0.1539

Ca2YZr2Al3O12:0.03Ce3+ crystal system:cubic Space group: Ia3d a = b = c = 12.418Å Rwp = 8.56% Rb = 7.23%

parameters as a function of x value obtained from Rietveld refinements of XRD data and calculated results of equation(1) are plotted in Fig. 3a, the average bond length of Ca/Y/Ce-O as a function of x value is shown in Fig. 3(b), it can be found that the cell parameters and the average bond length for the luminescence center gradually decrease with the increase of x, the variation could be reasonably attributed to the substitution of smaller Y3+ ions (1.159Å for CN=8) and smaller Al3+ ions (0.675 Å for CN=6) for larger Ca2+ ions (1.26 Å for CN=8) and Zr4+ ions (0.86 Å for CN=6). When x≤0.4, the Rietveld refinements results of XRD data match well with the calculated results of equation(1), which reflect that the Ca2YZr2Al3O12 and Y3Al5O12 have a good intermiscibility. For the samples of x≥0.5, the difference between the Rietveld refinements of XRD data and calculated results of equation(1) gradually increase, the appearing of second phase is the reason why the deviation becomes bigger with the increase of the YAG solid solubility, it can be found that the solid solution limit is around x=0.4-0.5.



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Fig. 3 (a) The cell parameters as a function of x value obtained from Rietveld refinements of XRD data and calculated results for Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.6). (b) The average bond length of Ca/Y/Ce-O as a function of x value for the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.6).

Fig.4a

and

4b

shows

the

SEM

images

of

Ca2YZr2Al3O12:0.03Ce3+

and

Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+, the two samples both exhibit the homogeneous irregular nubby morphology, and the sizes of the particles are mostly between 8µm and 12µm. The energy-dispersive X-ray (EDX) spectrum analyse of Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ are shown in Fig. 3c, it has the chemical composition of Ca, Y, Al, Zr, O, Cu and C((the C and Cu are ascribed to the instrument peaks, and the Ce3+ element could not be obviously observed due to the tiny concentration), thus, there are no impurity element in our samples. Fig. 3d shows the low-magnification TEM image and the inset exhibits the HRTEM image. The interplanar spacings


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are measured to be 4.33 Å and 3.25 Å, corresponding well to the (022) and (123) interplanar distances of Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+. These results show that the obtained sample has good crystallization.

Fig.

4

(a)-(b)

The

SEM

images

of

Ca2YZr2Al3O12:0.03Ce3+

and

Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+, (c) The EDS spectrum analyse of Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+. (d) The low-magnification TEM image and the inset exhibits the HRTEM image.

3.2 Photoluminescence properties 3.2.1 Photoluminescence properties of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.5) Fig .5a and 5b shows the normalised excitation and emission spectra of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.5). The inset in Fig. 5a shows the change of the two excitation peak positions, and the inset in Fig. 5b exhibits the emission peak position and the emission intensity variation. Monitored at the emission peaks for the different samples, the excitation spectra all contain three bands centering at around 275nm, 340nm, and 405nm. Among these bands, the 275 nm is ascribed



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Fig. 5 (a) The normalised excitation spectra of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.5). The inset shows the change of the two excitation peak positions. (b) The normalised emission spectra of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.5). The inset exhibits the emission peak position and the emission intensity variation.

to the Ca2-xY1+xZr2-xAl3+xO12 host absorption, the 340nm and 405nm bands are ascribed to the 4f →5d1 and 4f →5d2 transition of Ce3+ ions. It can be found that the atomic substitution leads to the


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small red shift of the excitation peak position for both the 4f→5d1 and 4f→5d2 transition. For the emission spectra, the emission spectra have the obvious red shift with the increase of x, the peak position change from 496 nm for x = 0 to 516 nm for x = 0.5. The emission spectra are attributed to the electric dipole-allowed transitions of Ce3+ ions from the lowest excited 5d to the ground 4f levels (2F5/2, 2F7/2). According to energetics,29-31 the 4f and 5d energy levels of Ce3+ ions in phosphors determine their excitation and emission properties. For the free Ce3+ ions, the energy different between the 4f ground state and the 5d excited state is about 6.2 eV (50000 cm-1). When the Ce3+ enters into the lattice, the energy levels of 5d orbitals would have obvious variation, which strongly depend on local structure around activator ions, crystal field strength, bond polarizability, structure symmetry and so on. Thus, the excitation and emission energies of the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.5) samples could be adjusted by changing the matrix structure and composition. In general, the crystal field spitting and Stokes shift are two important influence factors to understand the relationship between the luminescence property and the crystal structure. Crystal field splitting describes the breaking of degeneracies of electron orbital states, the strength of which can be reflected by the difference in value of the energy between the highest and lowest 5d energy levels. Generally, the crystal field splitting (Dq) can be determined by the following equation(2):32

=

(2)

where Dq is the magnitude of the 5d energy level separation; e represents the electron charge; r represents the radius of the d-wave function; Z is the charge or valence of the anion and R is the bond length. In our samples, z, e and r are equal, Dq is only inversely proportional to R5 and the bond length is gradually decrease with the decrease of x (Fig 3a and 3b), this variation would cause the increase of magnitude of the 5d energy level separation, leading to the decrease of energy between the lower 5d level and the 4f level of Ce3+ ions. For the Stokes shift, it is related to the ease of relaxation of the excited ions; when an electron is excited to the 5d level, the surrounding lattice relaxes to new equilibrium positions, which is resulted from coupling of the 5d electron with phonons. This relaxation to a new equilibrium position causes an effective decline of the energy difference between the 4f state and the lowest 5d states. Because the Stokes shift requires interactions with phonons, it is also affected by the host



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crystal.23,33 According to the excitation and emission spectra of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.5), the approximate results of Stokes shift are calculated to be 4409, 4570, 4746, 4902, 4935 and 4950 cm-1 with x = 0, 0.1, 0.2, 0.3 and 0.4. The Stokes shift becomes larger when the lattice sites become smaller. The similar phenomenon also could be found in some phosphors. 34, 35

It is beneficial for the samples to realize green emission under the near- ultraviolet excitation.

Fig. 6 The diffuse reflectance spectra of Ca2-xY1+xZr2-xAl3+xO12 (x=0, 0.2, 0.4), the inset shows the diffuse reflectance spectra treated by the Kubelka–Munk function

In addition, when a part of Ca2+/Zr4+ ions are substituted by Y3+/Al3+ ions, it can be seen from the inset in Fig. 5b that the intensity is subdued, the internal quantum efficiency are measured to be 60.2%, 56.7%, 53.4%, 52.1% and 50.6% for the samples from x=0 to x=0.4. It was common that the bigger bandgap of the host was supposed to be mainly reason for the higher luminescence efficiency in the Ce3+ doped garnet structure phosphors.19,36 Thus, the decrease of luminescence efficiency could probably suggest that double-substitution could modify the bandgap of the host. To proximately evaluate the variation of the bandgap, the diffuse reflectance spectra of Ca2-xY1+xZr2-xAl3+xO12 (x=0, 0.2, 0.4) are shown in Fig.6, the inset shows the diffuse reflectance


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Fig.7 Band structure of Ca2-xY1+xZr2-xAl3+xO12 (x=0, 0.2, 0.4) calculated using the density function theory (DFT) method on the basis of the refined results.

spectra

treated

by

the

Kubelka–Munk

function,

which

could

be

described

as

[F(R∞)hν]2=C2(hν-Eg),36,37 where F(R∞) represents the ratio of absorption and scattering coefficients, hν represents the photon energy, C2 is the constant, and Eg is the value of the



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bandgap. According to the equation, the bandgaps of Ca2-xY1+xZr2-xAl3+xO12 (x=0, 0.2, 0.4) are calculated to be 5.25eV, 5.13eV and 4.96eV, respectively. In order to further understanding the variation of the bandgap, the band structure of Ca2-xY1+xZr2-xAl3+xO12 (x=0, 0.2, 0.4) were also calculated using the density function theory (DFT) method on the basis of the refined results. The local-density approximation (LDA) was used as the theoretical foundation of density function.38,39 As shown in Fig. 7, the Ca2-xY1+xZr2-xAl3+xO12 (x=0, 0.2, 0.4) compounds have the band gap of 4.56 eV, 4.44 eV, 4.24 eV, their valence band tops and the conduction band bottoms all are located at the G point of the Brillouin zone, which illustrate that all the samples are the direct band gap. It is common that the results from DFT calculation underestimates the experimental results in some ways, thus, the calculated values of the bandgaps for the DFT calculation and reflectance spectra match well with each other, and they both demonstrate that the band gap is gradually reductive in the process of the double-substitution for Ca2+/Zr4+ by Y3+/Al3+ , then causing the decrease of the quantum efficiency and luminescense intensity. In addition, there are also two additional possible factors for the decrease of the intensity. First, the stronger crystal field splitting and the larger Stokes cause the decline of probability of the electron transition from excited level to the ground level. Second, the increase of structural defects induced by the increase of Y and Al content might lead to the reduction of the luminescence efficiency.


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Fig. 8 The schematic diagram for the photoluminescence property variations for the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(x=0, 0.4) .

As a consequence, the schematic diagram for the changes of the photoluminescence property is shown in Fig. 8 base on the experiment and measure data. The centroid shift originated from the nephelauxetic effect is related to the covalency between the activator ions and the surrounding anion ligands，therefore, the centroid shift should not be considered as the main cause for the photoluminescence properties variations of this series of garnet phosphors. The crystal field splitting and stokes shift play the major roles in the changes of the photoluminescence properties. With the increase of x, .the changes of the component cause the shrinkage of crystal structure, particularly highlighting the decrease of the average bond around the Ce3+ ions, then the crystal field splitting of Ce3+ and the Stokes shift gradually enhances, which mean that the 5d1 level of Ce3+ has a lower position and the electron has a bigger energy loss in the process of the jump from the 5d1 to 4f, then the changes lead to the red shift of excitation and emission spectrum. In addition, with the increase of x, the decreasing bandgap means that the energy difference between the bottom of the conduction and the 5d1 of Ce3+ (∆E) would be shrunken, which makes the photoionization of Ce3+ easy, therefore, the electron at the excited states would transition to the conduction band more easily, then it cause a small decline of the quantum efficiency and the PL intensity in the process of the double-substitution for Ca2+/Zr4+ by Y3+/Al3+. 3.2.2 Photoluminescence properties of Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+ According to the solid solution structure design, the Ce3+ doped Ca1.6Y1.4Zr1.6Al3.4O12 phosphor could be regarded as a potential green emitting phosphor for the near-UV LEDs. Fig. 9a shows the dependence of emission spectra on the Ce3+ concentration excited at 410nm for the Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+(0≤y≤0.07) samples and Fig. 9b exhibits the variations of their emission intensity and peak position. With increasing Ce3+ concentration, the concentration quenching phenomenon occurs at x > 0.03, the emission peak of the Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+ has a conspicuous red shift, changing from 504 nm for y = 0.01 to 530 nm for y = 0.07. This phenomenon could be caused by the energy transfer from the higher 5d energy states of Ce3+ to the lower energy states of Ce3+, resulting in the reduction of the radiation energy in the process of electron transition, which means that the emission spectrum would shifts toward the longer



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wavelength. As we known, the energy transition between the Ce3+ for the samples with different concentration could be reflected in the fluorescence decay curves, Fig.10 shows the fluorescence

Fig.9 (a) The dependence of emission spectra on the Ce3+ concentration excited at 410nm for the Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+(0≤y≤0.07). (b) Changes of the intensity and emission peak positions for the Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+(0≤y≤0.07).


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decay curves for the Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+(0≤y≤0.07). The average decay lifetime (τ) could be calculated using the equation (3):40, 41

τ=

(3)

where I(t) is the luminous intensity and t is the time. With the increase of Ce3+ concentration from 0.01mol% to 0.07mol%, the average distance between the Ce3+ ions would gradually decrease and the values of decay time change from 45.3ns to 41.3ns, the decrease of the decay time implies the enhancement of the energy transfer between the Ce3+, which is the reason of the occurrence for concentration quenching and the red shift.

Fig. 10 The fluorescence decay curves for the Ca1.6Y1.4Zr1.6Al3.4O12:yCe3+(0≤y≤0.07).

3.3. Temperature-dependent PL properties The thermal stability of the luminescence properties of phosphors is a vital factor for phosphor in near-UV LEDs. Generally, the thermal stability of phosphors is closely related to the rigidity of the crystal, the component elements, the Stokes shift, the energy band structure and so on. Fig. 11a and

11b

show

the

temperature

dependence

of

luminescence

property

for

Ca1.9Y1.1Zr1.9Al3.1O12:0.03Ce3+ and Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ with the range from 25–250℃ under the maximum excitation wavelength. It can be seen that the emission peak positions and



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Page 18 of 26

peak pattern have no obvious shift with the increase of temperature, the emission color or the chromaticity coordinate has a fine stability when increasing the operating temperature. Fig. 11c

Fig.

11

(a)

(b)

The

temperature

dependence

of

luminescence

property

for

Ca1.9Y1.1Zr1.9Al3.1O12:0.03Ce3+ and Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ with the range from 25–250℃ under the maximum excitation wavelength. (c) The emission intensity with the different temperature for the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) and the commercial green phosphor (Ba, Sr)2SiO4: Eu2+. (d) Configurational coordinate diagram of the ground state and split excited state of Ce3+, showing the change of the activation energy for Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4).

show the emission intensity with the different temperature for the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) and the commercial green phosphor (Ba, Sr)2SiO4: Eu2+. The emission peak intensity for Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) at 150℃ remains 62.1%, 59.2%, 57.1%, 53.7% and 50.3% of their initial intensity, and the intensity of the commercial (Ba, Sr)2SiO4: Eu2+ phosphors decreases to 48.2% at 150℃, thus, the Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) phosphors have a great thermal stability. In addition, the thermal quenching performance gradually


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decreases with the increase of x for Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4). The changes could be associated with the variations of activation energy, ∆E, as shown in Fig. 11(d), the activation energy is the distance between the crossing point of 4f and 5d energy level and the bottom of 5d energy level. According to the excitation and emission spectra, for the samples with the bigger x, the 5d energy levels of Ce3+ have a lower position and the bigger Stokes shift causes the right shift of the 5d energy level in the configurational coordinate diagram, thus, the activation energy becomes smaller and smaller with the increase of x. In general, the smaller activation energy would make it easier for the electrons to generate nonradiative transition, and the thermal stability of the luminescence properties declines. The activation energy can be estimate based on the Arrhenius equation(4)42 I =

(4)

∆! "#

I0 is the emission intensity at room temperature, IT is the intensity under different temperatures, c is a constant for a certain matrix, and k is the Boltzmann constant (8.629×10-5 eV). The activation energy ∆E of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) are calculated to be 2.948eV, 2.935 eV, 2.928 eV, 2.911 eV and 2.880 eV. The change of the data for activation energy is in accord with the theoretical analysis.

3.4 CIE coordinates of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) The

Commission

International

de

L’Eclairage

(CIE)

chromaticity

coordinates

of

Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) are shown in Fig. 12. Excited at their maximum excitation wavelengths, their CIE coordinates are calculated to be (0.2175, 0.3937), (0.2248, 0.406), (0.2481, 0.4413), (0.257, 0.4644) and (0.2659, 0.4738) for samples x = 0 to x = 5, respectively, it can be seen that with the double-substitution for Ca2+/Zr4+ by Y3+/Al3+, the emission color could be adjusted from the cyan to the relatively standard green, the excitation spectrum also matches well with the near-ultraviolet chip, the results show that the Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ could be used for providing the green composition for the three primary colors near UV-LEDs. In order to demonstrate its practical potential, the Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ sample is employed to fabricate the white LED lamps with the blue phosphor BAM:Eu2+, the red phosphor (Sr, Ca)AlSiN3:Eu2+.and 395 nm GaN chip. By adjusting the weight rate of three phosphors, a warm white light could be obtained and its patterns,



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emission spectrum are shown in the inset of Fig. 12.The CIE coordinate(x, y), CCT and Ra are (0.3603, 0.348), 4404 K and 87. These results demonstrate that the Ca1.6Y1.4Zr1.6Al3.4O12:0Ce3+ could be a promising candidate for a green-emitting phosphor for application of near-UV LEDs.

Fig. 12 The CIE coordinates of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) and white LEDs fabricated by BAM: Eu2+, Ca1.6Y1.4Zr1.6Al3.4O12:0.03Ce3+ (Sr, Ca)AlSiN3: Eu2+ and 395 nm GaN chip. (inset: The EL spectra of fabricated w-LEDs).

4. Conclusion We

have

successfully

developed

and

synthesized

a

series

of

novel

phosphors

Ca2-xY1+xZr2-xAl3+xO12: yCe3+(0≤x≤0.4) (0≤y≤0.07) by the solid solution design. They all show the


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-

cubic garnet structure with the space group Ia3d, the detailed crystal structure information are refined by the Rietveld method. The cell parameters and average bond lengths from the luminescence centres to the ligands gradually decreased with the increase of x, the variation matches well with the theoretical calculation result. All the phosphors can be efficiently excited by the near-ultraviolet chips, and the emission color changes from cyan to green by the double-substitution for Ca2+/Zr4+ by Y3+/Al3+. The stronger crystal field splitting and larger Stokes shift could comprehensively interpret the red shift of emission. The internal quantum efficiency of Ca2-xY1+xZr2-xAl3+xO12: 0.03Ce3+(0≤x≤0.4) all exceed 50% and the phosphors have a good thermal stability. The white LED device is also successfully fabricated, its CIE coordinate(x, y), CCT and Ra are (0.3603, 0.348), 4404 K and 87, which demonstrates the solid solution design of phosphors will be a promising way to develop novel potential phosphors for the n-UV LEDs.

Acknowledgements This work was supported by the Gansu Province Developmentand Reform Commission and the Fundamental Research Funds for the Central Universities (No. lzujbky-2016-234), the National Science Foundation under grant no. 51372105 and State Key Laboratory of Rare Earth Resource Utilization.(No.RERU2017007)

Notes and References

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TOC Graphic


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Zr - American Chemical Society

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