First-principles calculation on the electronic structure and optical properties of Eu2+ doped γ-AlON phosphor

Author’s Accepted Manuscript First-principles calculation on the electronic structure and optical properties of Eu2+ doped γAlON phosphor Xian Zhang, Zhao Li, Qingfeng Zeng www.elsevier.com/locate/ceri

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S0272-8842(17)32236-8 https://doi.org/10.1016/j.ceramint.2017.10.044 CERI16463

To appear in: Ceramics International Received date: 4 August 2017 Revised date: 5 October 2017 Accepted date: 9 October 2017 Cite this article as: Xian Zhang, Zhao Li and Qingfeng Zeng, First-principles calculation on the electronic structure and optical properties of Eu2+ doped γAlON phosphor, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.10.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

First-principles calculation on the electronic structure and optical properties of Eu2+ doped γ-AlON phosphor

Xian Zhanga,*, Zhao Lia, Qingfeng Zengb a

School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071,

China b

International Center for Materials Discovery, School of Materials Science and

Engineering, Northwestern Polytechnical University, Xi’an 710072, China *Corresponding author. Tel./fax: +86 182 2050 7206. E-mail address: [email protected] (X. Zhang).

Abstract The crystal structure, electronic structure, and optical properties of Eu-doped γ-AlON at various Eu concentrations were obtained from density functional theory. Based on the calculated results, the luminescence properties and mechanism of Eu-doped γ-AlON are discussed. The calculated results demonstrate that AlON:Eu2+ phosphor exhibits a direct band gap, which is advantageous for luminescence. The absorption spectrum of AlON:Eu2+ phosphor has a single intense broad absorption band from 275 to 425 nm with a peak at 355 nm, which is consistent with corresponding experimental excitation spectra. The existence of Eu−N bonds enhanced the local covalence of Eu2+, hence the optical stability of AlON:Eu2+ phosphor.

Keywords: γ-AlON:Eu2+, first principles, electronic structure, optical property

1

1. Introduction InGaN-based white light-emitting diodes (LEDs) have driven demand for phosphor materials of high luminous efficiency [1,2]. The single-host tricolor phosphor and near ultraviolet (NUV) LED combination holds promise for obtaining white LEDs. Particularly, rare-earth-doped oxynitride phosphors have attracted extensive investigations due to their high quantum efficiency, adjustable composition, and antioxidation properties [3–5]. Recently, considerable attention has been paid to γ-AlON-based phosphors, Mg2+–Mn2+ co-doped and rare-earth Eu2+, Er3+, Ce3+, Tm3+, and Yb3+ doped AlON phosphors have been synthesized and characterized [6–17]. Among the latter, AlON:Eu2+ was found to emit intense blue light under 350–410 nm UV LED chip excitation [9,15], as a result of Eu2+ in the host absorbing the UV light emitted by the InGaN chip. γ-AlON is the solid-solution system of AlN-Al2O3, and the crystal has a cubic spinel structure with the Fd3m space group [18–20]. γ-AlON exhibits interesting mechanical, optical, and photoluminescent properties [21], which make it a promising host material for the development of advanced phosphors. A high internal quantum efficiency and tunable chromaticity coordinates make AlON:Eu2+ blue phosphor a suitable candidate for white LED applications [11]. Kikkawa et al. reported the preparation and luminescent properties of Eu2+ doped γ-AlON powders [15], and commented that the blue emission might be generated from interlayered Eu2+ ions in different coordinations with O2–/N3– in the crystal structure. Recently, Zhang et al. synthesized Eu2+-doped γ-AlON powders [11], and suggested that Eu2+ ions might 2

occupy some of the octahedral vacancies coordinated by six oxygen ions (VAlO6) in γ-AlON. The blue emission of AlON:Eu2+ was attributed to changes in the location, symmetry, crystal-field strength, and coordination of Eu2+ as well as to the bond length of Eu–N pairs in the AlON powders. Computational simulation is a commonly used research tool in materials science [22], which can satisfactorily provide detailed information on the electronic properties and luminescence properties of rare-earth-doped phosphors [23–30]. Wang et al. reported a first-principle calculation for the electronic, elastic, and thermodynamic properties and the structure disorder of γ-AlON [31]. Tu et al. studied the site preference of Al vacancies and N atoms in γ-AlON [32]. Their calculation results demonstrated that the N atoms prefer to distribute far away from each other, while the Al vacancies prefer coordinating with oxygen atoms in octahedral sites in γ-AlON. However, their calculation did not consider Eu2+ doping, and the effect of Eu2+ doping on the electronic structure and optical properties of Al23O27N5 was not researched. In this work, the crystal structural of γ-AlON host was firstly calculated to understand the crystal structure from theory aspect. Then the electronic structure and optical properties of Eu2+ doped γ-AlON were investigated. The luminescence properties and mechanism of Eu2+ dopedγ-AlON were discussed based on the calculated results. 2. Calculation details All calculations were performed based on density functional theory using the Cambridge Serial Total Energy Package (CASTEP) [33]. The interaction between the valence electrons and the ionic core was described using ultrasoft pseudopotentials 3

[34] with the following valence electron configurations: Al 3s23p1, O 2s22p4, N 2s22p3, and Eu 4s2p6d10f7. The exchange correlation energy was evaluated using the local-density approximation (LDA) [35]. Brillouin zone integrations were done using the Monkhorst–Pack scheme [36]. A plane wave cut-off energy of 500 eV and a 3 × 3 × 3 k-point mesh were sufficient to ensure convergence for the total energy. All the calculations were carried out for a maximum force on the atoms below 0.03 eV Å−1, a maximum stress of below 0.05 GPa, and a maximum displacement between cycles of below 0.001 Å.

Fig. 1. Structures of (a) the Al23O27N5 host, (b) VAlO4, (c) VAlO3N, (d) VAlO6, and (e) VAlO5N. The constant anion model of the γ-AlON structure was firstly considered, with an Al vacancy located at the octahedral center of (5/8, 5/8, 5/8) [37]. γ-AlON has a cubic structure with the F3dm space group, lattice constants a = b = c = 7.946 Å, and crystal plane angles α = β = γ = 90° [38]. As shown in Fig. 1(a), the anion sites are randomly occupied by 27 O atoms and 5 N atoms, while the cation sites are octahedrally and tetrahedrally coordinated by 16 and 8 Al atoms, respectively. 4

The crystal structure of γ-AlON has been investigated in detail by Tu et al. [32], and the Eu2+ ions were doped as the emission centers. Figures 1(b)–1(e) show the four sites generally considered to be occupied by Eu2+ ions. The VAlO4 site has tetrahedral vacancies coordinated by four oxygen ions, the VAlO3N site has tetrahedral vacancies coordinated by three oxygen ions and one nitrogen ion, the VAlO6 site has octahedral vacancies coordinated by six oxygen ions, and the VAlO5N site has octahedral vacancies coordinated by five oxygen ions and one nitrogen ion. The crystal structure, electronic structure, and optical properties of AlON-based powders, with the formula EuxAl23−xO27N5, were computed with various Eu2+ doping concentration (x = 0, 0.33, 0.50, and 1). The simulation results were compared with the experimental optical properties. To describe the influence of the Eu doping concentration on EuxAl23−xO27N5, 1 × 1 × 3, 1 × 1 × 2, and 1 × 1 × 1 supercell structures were adopted, with one Eu atom in the crystal for various Eu concentrations in EuxAl23−xO27N5 (when x = 0.33, 0.5 and 1, the crystal contains 165, 110, and 55 atoms, respectively). After full geometric optimization of the crystal, the influence of Eu doping on the crystal structure, electronic structure, and optical properties of EuxAl23−xO27N5 was investigated. 3. Results and discussion 3.1. Crystal structural of the γ-AlON host The absorption and emission spectra of Eu2+ usually comprise a broad band due to transitions between the 4f7 ground state and the 4f65d1 excited state configurations of Eu2+. Since the 5d orbital involved is external, the emission of Eu2+ is very strongly 5

dependent on the host lattice [39]. Thus, the host is very important for the optical properties of Eu2+ ions. The host lattice material AlON has a stoichiometry of Al23O27N5 [7]. The crystal structure of γ-AlON has been determined to be face-centered cubic with the Fd3m space group, by X-ray and neutron diffraction Rietveld refinement [40,41]. Prior to calculating the crystal structure of EuxAl23−xO27N5, the Al23O27N5 unit cell was optimized. To calculate the ground state structure of Al23O27N5, several different lattice parameters a were used to calculate the total energy E. For each a, the corresponding primitive cell volume V was calculated, then the energy–volume (E–V) curve of Al23O27N5 was obtained (Fig. 2). The calculated E–V data were fitted to Murnaghan’s equation of state (EOS) [42] to determine the ground state properties such as the equilibrium lattice constant a0, the bulk modulus B0, and its pressure derivative B0 : BV E V   E0  0 0 B0

 1  V0  B0 1 V0 B0        V B0  1  B0  1  V 

where E0 is the equilibrium energy, B0 is the bulk modulus, and B0 is the first derivative of B0 with pressure. The calculated equilibrium structure parameters, the bulk modulus B0, and its pressure derivative B0 for Al23O27N5 using various approximations are listed in Table 1, together with other theoretical and available experimental data for comparison. As is shown in Table 1, the lattice constant was underestimated by the LDA and overestimated by the GGA, which are consistent with the general trends of the LDA and GGA [43]. The bulk modulus B0 as well as its first-order derivatives B0′ show good agreement with the available literature values [32,37,44,45]. The calculated 6

crystal parameters of Al23O27N5 were a = b = c = 7.921 Å, corresponding to V0 = 497.070 Å3, which are consistent with the experimental values of a = b = c = 7.946 Å [38] and V0 = 503.670 Å3 [46]. This indicates that the calculation method in our work is reasonable, and that the calculated results are creditable. -403.5 -404.0

GGA

-404.5

BM EOS Fit

-405.0 -405.5

Energy (Hartree)

-406.0 -406.5 -407.0 -448.5

LDA

-449.0

BM EOS Fit

-449.5 -450.0 -450.5 -451.0 -451.5 470

480

490

500

510

520

530

540

550

3

Volum (bohr )

Fig. 2. Energy versus volume curve of γ-AlON using various approximations. Table 1. The calculated lattice constant (a0), volume (V0), bulk modulus (B0), and the pressure derivative of bulk modulus ( B0 ) for γ-AlON using various approximations.

This work Theoretical

Experiment

Method

a0 (Å)

V0 (Å)

B0 (GPa)

B0

GGA-PBE

8.0795

524.507

194.03

4.21

LDA

7.9210

497.070

192.89

3.85

GGA

514.220a

205.82a

3.86a

LDA

487.310a

227.92a

3.83a

LDA

492.800b

209.00b

LDA

7.91736c

GGA

8.01120d

514.153d

202.41d

3.92d

LDA

7.81760d

477.771d

225.78d

3.88d

7.94600e

503.670f

215.81f

7

References: a[32], b[37], c[44], d[45], e [38], f [46].

3.2. Crystal structural of Eu2+-doped γ-AlON

Fig. 3. (a) Eu2+ in the VAlO4 site, (b) Eu2+ in the VAlO3N site, (c) Eu2+ in the VAlO6 site, and (d) Eu2+ in the VAlO5N site.

There are four sites that Eu2+ might occupy in the Al23O27N5 host: the VAlO4, VAlO3N, VAlO6, and VAlO5N sites (Fig. 3). The potential energy of each site was determined by optimizing the structures with Eu2+ in these sites respectively (Table 2). As seen in Table 2, the potential energy of the VAlO5N site is lower than the other sites, indicating that Eu2+ prefers to occupy the VAlO5N site. Therefore, the property calculations were based on the structure with Eu2+ in the VAlO5N site. Table 2. The potential energy of Eu2+ in different sites. Eu2+ coordination

Eu2+ site

Energy (eV)

ΔE (meV)

Octahedral

VAlO5N

–17081.6593

0

VAlO6

–17080.6445

1015

VAlO3N

–17079.3300

2329

VAlO4

–17079.5560

2103

Tetrahedral

8

Structural optimization of doped Eu2+ in the VAlO5N site for various Eu concentrations was carried out. The optimized lattice parameters of EuxAl23−xO27N5 with different Eu2+ doping concentrations are summarized in Table 3. As is shown in Table 3, the lattice parameters and the primitive cell volume of EuxAl23−xO27N5 expand with increasing Eu2+ concentration. Accordingly ， a small shift of the diffraction peaks of AlON:Eu0.82+ towards lower 2θ values was found with respect to the undoped AlON, which indicated the incorporation of Eu2+ into AlON structure [11]. Table 3. Optimized lattice parameters of EuxAl23−xO27N5. x in EuxAl23−xO27N5

Lattice parameters of the primitive cell

(super cell)

Structure

a (Å)

b (Å)

c (Å)

V (Å3)

0 (1 × 1 × 1)

Al23O27N5

7.9210

7.9210

7.9210

497.07

0.33 (1 × 1 × 3)

Eu0.33Al22.66O27

7.9296

7.9265

7.9216

497.90

0.50 (1 × 1 × 2)

Eu0.50Al N522.5O27

7.9351

7.9374

7.9372

499.91

1 (1 × 1 × 1)

Eu1AlN 225O27N5

7.9576

7.9532

7.9557

503.50

3.3. Electronic structure To study the influence of Eu doping on the electronic structure of Al23O27N5, the band structure and density of states of EuxAl23−xO27N5 at different Eu concentrations were calculated. Fig. 4 presents the calculated band structure of EuxAl23−xO27N5 at various Eu concentrations. As seen in Fig. 4(a), both the top of the valence band and the bottom of the conduction band of Al23O27N5 are located at G with a band gap of 4.03 eV, which is less than the experimental value obtained from the absorption 9

spectrum (6.2 [47] and 6.5 eV [48]), as density functional theory always underestimates the size of the optical band gap [49]. Thus, it can be concluded that Al23O27N5 belongs to the category of materials with a large band gap, which are usually good hosts for many luminescent ions. The large band gap facilitates accommodation of both the ground and excited states of luminescent ions within the band gap [50]. 5

5

5

5

4

4

4

4

3

3

3

3

2

2

2

2

Energy (eV)

4.0eV 1

1

0

0

-1

-1

1

1.13eV

1

1.05eV

0

0

-1

-1

2.77eV

2.72eV

2.70eV

-2

-2

-2

-2

-3

-3

-3

-3

-4

-4

-4

-4

-5

Q

-5

Z

G

(a)

F

Q

Q

-5

Z

G

F

0.84eV

Q

(b)

Q

Z

G

F

-5

Q

Q

Z

(c)

G

F

Q

(d)

Fig. 4. The calculated band structure of EuxAl23−xO27N5 at various Eu concentrations: (a) x = 0, (b) x = 0.33, (c) x = 0.50, and (d) x = 1.

Compared with the band structure of undoped Al23O27N5 shown in Fig. 4(a), an additional energy level between the bottom of the conduction band and the top of the valence band around the Fermi level is observed in Figs. 4(b)–4(d). With the introduction of Eu2+, the bottom of the conduction band and the top of the valence band shift toward lower energy. AlON:Eu2+ phosphor shows a direct band gap, which 10

promotes luminescence since the transition probability of the direct band gap is higher than that of the indirect band gap due to no phonons being involved in the transition process [51]. As is shown in Figs. 4(b)–4(d), the Fermi energy level (EF) crosses this impurity energy band, indicating that it is a half-occupied energy band. The distance between the bottom of the conduction band and the top of the impurity energy band in these figures is 1.13, 1.05, and 0.84 eV, respectively, while the distance between the top of the valence band and the bottom of the impurity energy band is 2.77, 2.72, and 2.70 eV, respectively. The calculated band structure indicates that incorporation of Eu in the AlON host induces impurity energy states, which retain the main features of the electronic structure of AlON. Furthermore, both the valence and conduction bands are of low band energy dispersion in E(k), indicating a large joint density of states, hence the optical absorption and luminescence [52]. Al

30

s p

20 10 0

O

30

Density of States (electrons/ev)

s p

20 10 0 30

N

s p

20 10 0 30

Eu

20

d f

10 0 30

Total

20 10 0

-5

0

Energy (eV)

Fig. 5. Total and partial densities of states of EuxAl23−xO27N5 (x = 1). 11

5

Figure 5 presents the total and partial densities of states of EuxAl23−xO27N5 (x = 1), in which only the top of the valence band and the bottom of the conduction band are shown since the optical absorption is mainly determined by the states close to the band gap. As seen in Figure 5, the Fermi level is set to zero. The bottom of the conduction band mainly derives from Eu 5d, Al 2s, and Al 2p states, while the top of the valence band contribute mainly to the N 2p and O 2p states, partly derived from the Al 2p state. 3.4. Absorption spectra The function of phosphor in LEDs for solid-state lighting is to absorb NUV or blue LED chip and convert it into visible light, which requires high absorption in the NUV to blue spectral region [53]. Therefore, the intensity and shape of absorption spectra at different Eu2+ concentrations for EuxAl23−xO27N5 was investigated in this paper. Figure 6 shows the calculated absorption spectra of EuxAl23−xO27N5 at various Eu concentrations. The Al23O27N5 host shows a weak absorption band in the wavelength range 200–300 nm, corresponding to its wide band gap. In contrast, the Eu2+-doped Al23O27N5 exhibits a broad strong absorption in the range 275–425 nm with a maximum at 335 nm, which indicates that the energy is efficiently absorbed by Eu2+ [11]. It is clear that the introduction of Eu2+ into the Al23O27N5 lattice induces the enhanced a high absorption in the NUV to blue spectral region.

12

50000

x=0 x=0.33 40000

x=0.50

-1

)

x=1

Absorption(cm

30000

20000

10000

0 200

300

400

500

600

700

800

900

Wavelength (nm) Eu Al x

O N

23-x

27

5

Fig. 6. Calculated absorption spectra of EuxAl23−xO27N5 at various Eu concentrations: (a) x = 0, (b) x = 0.33, (c) x = 0.50, and (d) x = 1.

Based on the calculated absorption spectra of EuxAl23−xO27N5, it can be demonstrated that Eu2+ is an effective activator in the AlON host, and its absorption intensity increases with increasing Eu2+ concentration. The absorption spectrum of Al23O27N5:Eu2+ shows a single intense broad absorption band from 275 to 425 nm with a peak at 355 nm. According to the previous report, there is an obvious absorption band in the range of 330~500nm for Eu2+ doped AlON [11], which is attributed to 4f→5d transition of Eu2+ ions (Fig. 7). The absorption spectrum of AlON:Eu2+ in our work matches well with the emission wavelength of InGaN UV-LED chips (350-410nm), which is consistent with corresponding experimental excitation spectra[10]. Accordingly, it was found that AlON:Eu2+ was efficiently excited by the UV(350-410nm) of InGaN-based LEDs[11]. The intense absorption 13

peak indicates that Eu2+ concentration plays an important role in the luminescence efficiency of AlON:Eu2+ phosphor. 5 4

Al3s+O2s+N2s

Eu5d

Al3p+O2p+N2p

Eu4f

3

Energy (eV)

2 1 0

-1 -2 -3 -4 -5

Q

Z

EuAl

F

G

22

O

27

Q0

N

5

10

20

0

10

20

Density of States(electrons/ev)

Fig. 7. The calculated band structure and density of states of EuxAl23−xO27N5 (x = 1).

3.5. Mulliken population analysis The luminescence properties of Eu2+-doped phosphors can be characterized using the 5d–4f transition. In general, the electronic structure of the excited 5d level depends strongly on the crystal field and the covalency of the local structure of Eu2+ ion [54]. The radius of Eu2+ ion is larger than that of Al3+ ion, which results in that the Eu– O and Eu–N bonds in EuAl22O27N5 is longer than the Al−O and Al–N bonds in Al23O27N5. As shown in Table 4, the average length of Eu–O bonds in EuAl22O27N5 is 2.302 nm, while the average length of Al–O bonds in Al23O27N5 is 1.961 nm, and the average length of the Eu−N bonds (2.025 nm) in EuAl22O27N5 is longer than that of 14

the Al−N bonds (1.785 nm) in Al23O27N5.

Table 4. Band lengths and populations in Al23O27N5 and EuAl22O27N5. Al23O27N5

EuAl22O27N5

Bond

Length

type

(nm)

N1–Al

1.785

0.67

N1–Eu

2.025

0.31

O26–Al

1.836

0.38

O26–Eu

2.148

0.13

O20–Al

1.912

0.28

O20–Eu

2.206

0.08

O6–Al

2.008

0.24

O6–Eu

2.373

0.08

O17–Al

2.019

0.24

O17–Eu

2.381

0.14

O23–Al

2.030

0.27

O23–Eu

2.404

0.13

Population

Bond

Length

type

(nm)

Population

The Mulliken populations of the Al−O and Al−N bonds are 0.28 and 0.67, respectively, while the Mulliken populations of the Eu−O and Eu−N are 0.11 and 0.31, respectively. The Mulliken populations of the Al−O, Al−N, Eu−O, and Eu−N bonds are positive, which indicates that these bonds are covalent. The Mulliken population of the Eu−N bonds is 0.31, higher than the Eu−O bonds (0.11), which indicates that the Eu−N bonds has a higher covalency than the Eu−O bonds. In addition, the average length of the Eu−N bonds (2.025) is shorter than that of the Eu−O bonds (2.302). Since Eu2+ ions experience a strong nephelauxetic effect and crystal field due to the coordination of N atoms [55], the presence of Eu−N bonds enhances the stability of the valence state of Eu2+—resulting in the optical stability of γ-AlON:Eu2+ phosphor. 4. Conclusions A first-principle calculation for Eu-doped γ-AlON (EuxAl23−xO27N5) was undertaken to elucidate the effect of Eu2+ concentrations on its crystal structure, the 15

electronic structure, and photoluminescence properties. The calculated results showed that the Al23O27N5 host had a large band gap (4.03 eV), which is a favorable property for luminescent ions. AlON:Eu2+ phosphor exhibits a direct band gap, which is advantageous for luminescence. The absorption spectrum of Eu2+-doped Al23O27N5 has an intense absorption in the range 275–425 nm with a maximum at 335 nm, which was assigned to electronic transitions from the 4f7 levels to the 4f65d1 levels of Eu2+. Investigation of the Eu2+ sites showed that doped Eu2+ prefers to occupy the VAlO5N site, which is an octahedral vacancy coordinated by five oxygen ions and one nitrogen ion. The lattice parameters and primitive cell volume of EuxAl23−xO27N5 increase with increasing Eu2+ concentrations. This study demonstrated that the presence of Eu−N bonds enhanced the local covalence of Eu2+—responsible for the optical stability of γ-AlON:Eu2+. With improved quantum efficiency and thermal stability, γ-AlON:Eu2+ phosphors hold promise for application in white LEDs. Acknowledgments Financial support from the National Natural Science Foundation of China (Grants Nos. 51372203, 51332004, and 51571166) and computing support from the High Performance Computing Center in Xidian University are gratefully acknowledged. References [1] Mckittrick J, Shea‐Rohwer L E, Review: Down Conversion Materials for Solid‐State Lighting, J. Am. Ceram. Soc. 97 (2014) 1327-1352. [2] Ye S, Xiao F, Pan Y X, Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties, Materials 16

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