Photoluminescent properties of AlN: Mn2+ phosphors

Photoluminescent properties of AlN: Mn2+ phosphors

Accepted Manuscript Photoluminescent propertied of AlN: Mn phosphors Feifei Lei, Xiang Lei, Zhantong Ye, Nan Zhao, Xuwei Yang, Zhan Shi, Hua Yang PII:...

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Accepted Manuscript Photoluminescent propertied of AlN: Mn phosphors Feifei Lei, Xiang Lei, Zhantong Ye, Nan Zhao, Xuwei Yang, Zhan Shi, Hua Yang PII:

S0925-8388(18)32024-3

DOI:

10.1016/j.jallcom.2018.05.291

Reference:

JALCOM 46267

To appear in:

Journal of Alloys and Compounds

Received Date: 18 March 2018 Revised Date:

23 May 2018

Accepted Date: 24 May 2018

Please cite this article as: F. Lei, X. Lei, Z. Ye, N. Zhao, X. Yang, Z. Shi, H. Yang, Photoluminescent propertied of AlN: Mn phosphors, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.05.291. 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 proof before it is published in its final 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.

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Photoluminescent propertied of AlN: Mn phosphors

Feifei Leia, Xiang Leia, Zhantong Yea, Nan Zhaoa, Xuwei Yanga, Zhan Shib and Hua Yang*a a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

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b

College of Chemistry, Jilin University, Changchun, 130012, China

Chemistry, Jilin University, Changchun, 130012, PR China

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*Corresponding author

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Abstract

White light-emitting diodes (WLEDs), which has high luminous brightness, longevity, low energy consumption and friendliness of environment, could be employed in diverse fields. Nevertheless, commercial phosphors are short of red light component. New phosphors which can emit red light are required. Mn2+ doped aluminum nitride (marked as AlN) red phosphors were

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prepared by a simple solid-state reaction. X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HTEM), and X-ray photoelectron spectroscopy (XPS), as well as photoluminescence (PL) spectra were utilized to characterize the prepared samples. The preparing process of AlN phosphors, phase formation and crystal structure,

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morphology, and photoluminescence were detailedly investigated. For Mn2+ doped AlN phosphor(marked as AlN:Mn2+), it exhibits an intense red emission caused by the 4T1(4G)-6A1(6S)

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transition of Mn2+. The unusual red emission of Mn2+ is ascribed to the strong nephelauxetic and crystal field between Mn2+ and the tetrahedrally coordinated N3-. The oxygen-related defects in AlN have a great influence on the photoluminescence properties of the Mn2+ doped AlN. The AlN:Mn2+ phosphor exhibits a high brightness, high color purity, and lower saturation, which makes it a great candidate of red phosphors for white light-emitting diodes (WLEDs). Keywords: Mn-doped; AlN phosphor; photoluminescence properties

Introduction With the growing scarcity of fossil fuels accompanying global warming, which makes some 1

ACCEPTED MANUSCRIPT giant challenges such as energy shortage, atmospheric pollution, and drastic change of climate, it is required to make a revolution in the way we produce and consume energy.1 Although general lighting has a great proportion of consuming energy, it is particularly energetically inefficient so far. White light-emitting diodes(WLEDs), also so called solid-state lighting, may provide a

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resolution to the threats of global warming and the gradual depleting fossil fuels.2-5 WLEDs have advantages of low cost, easy fabrication, a tunable correlated color temperature, and a high color rendering index value, which are an important low-energy alternative to traditional incandescent and fluorescent light sources.4, 6

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With the spring of the WLEDs, researches on phosphors which can be applied to white LEDs have been found. Especially, rare-earth doped (silicon-) nitride based materials have turned out to

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be the best candidate phosphors.7-9 While III–V compound semiconductors, like AlN, GaN, and InN are kinds of them,10 which have drawn a wide attention due to their superior chemical ,physical properties and general applications in optoelectronic devices.11Aluminum nitride (AlN) is of particular interest owing to its distinctive features, for instance, wide band gap (6.2 eV), considerable thermal conductivity, high covalence, good thermal stability and small or

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even negative electron affinity. AlN has two polymorphs wurtzite and zinc-blende cubic similar to other III-N nitrides such as GaN and InN.

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Aluminum nitride has a promising future for

application in ultraviolet (UV) optical devices because of its wide band gap. It has been reported

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that an aluminum nitride LED was prepared with 210 nm emission.13 What is more, growing attention has been poured into the doping of AlN nanostructures. Nevertheless most of them are different kinds of rare-earth(RE) elements dopant, and there are few non-rare-earth doped nitride

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luminescent materials.1

In order to improve the color rendering property of such white LEDs for the usage as

backlights or general illumination lamps, efficient red phosphors are urgently required.14 In addition, as an activator, the luminescence behavior of Mn2+ ion has been extensively explored elsewhere.15-18 The Mn2+ ions are used to showing a broad band emission because of the 4

T1(4G)-6A1(6S) transition. The d electrons of Mn2+ are within 3d shell, which are strongly coupled

to the lattice vibration and affected by the crystal field strength. The different crystal field strength on Mn2+ can shift the emission color from green to red. In oxide, tetrahedrally coordinated (weak 2

ACCEPTED MANUSCRIPT crystal field) Mn2+ usually gives a green emission while octahedrally coordinated (strong crystal field) Mn2+ gives an orange to red emission. For instance, Mn2+ in Li2ZnGeO4,19 NaCaPO4,20 and CaLaGa3S6O2721 hosts give green, yellow, and red color emission, respectively. The luminescence properties of Mn2+ have been discussed in the nitride hosts. It is indicated that tetrahedrally

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coordinated Mn2+ in a nitride host also can emit red color, which is quite different from which in the oxide host. Generally, for the red emission, there are two main reasons, one of which is the nephelauxetic effect that shifts the Mn2+ d centre of gravity to a lower energy in the nitride host; the other is the high formal charge of N3- increasing the crystal field strength around Mn2+.22 In

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particular, the crystal field strength is the major reason for the changed emission color in the oxide hosts while it is the nephelauxetic effect in the nitride hosts. It is important that Mn2+ is

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inexpensive compared to rare earth ions but also doping with AlN have some especial applications such as excellent survivability in the mercury lamp conditions.23 Besides, Wang et al. reported that AlN:Mn has a great potential for full-color field emission displays (FEDs) due to its high color purity, low current saturation and stable cathodoluminescence (CL) chromaticity.22 Nitrides have been widely used as red phosphors in light-emitting diodes (LEDs), but traditional synthesis

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methods to nitride phosphors always experience high temperature and long-time reaction processes.24 Moreover, synthesizing the nitride-based materials is more complexed than oxides because of their lower stability .Therefore a rigorous control of oxygen concentration and moisture

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is needed during the synthesis. It is known that the most widely used method for synthesizing (oxy) nitridosilicates is the thermal treatment at high temperature under a high-pressure N2, or a N2/H2

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mixture if reducing conditions are required.1 It is no doubt that the rigorous synthesis conditions of nitride compounds give rise to a high cost as well as limit their further application. Hence, it is necessary to explore a more convenient, moderate, and less expensive method of preparing nitride luminescent materials. In this paper, we propose an unique and convenient route based on a simple solid-state reaction assisted with N2-flowfor preparing AlN and AlN:Mn2+. The corresponding formation process, crystal structure, component, morphology and photoluminescence properties were investigated in detail.

Experiment section 3

ACCEPTED MANUSCRIPT Preparation of AlN-based phosphors A series of Mn2+doped AlN phosphors with varying doping concentration were prepared by one step in a simple solid-state reaction method. Briefly, AlCl3·6H2O and CO(NH2)2 worked as the sources of Al and N, respectively. Mn2+ was introduced as activator by doping MnCl2·4H2O which

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was one of the raw materials. It was significant to add a certain amount of NH4Cl working as a fluxing agent but also restraining the hydrolyses of the AlCl3·6H2O, which is very different from our previous works. All of the raw materials were used as purchase without purification. Next, AlCl3·6H2O, NH4Cl and CO(NH2)2 with the mole ratio of 1:5:30, along with appropriate

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MnCl2·4H2O were thoroughly mixed and grounded in an agate mortar. And then the powder mixtures were transferred to porcelain boat located in the center of the tube furnace. The powder

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mixtures were heated to 880 ℃ for 2 h, followed by cooling down to room temperature with continuous N2 gas flow.

It is noteworthy that NH4Cl plays an important role during the process of obtaining AlN. At the very first, we find no phenomenon when heat a fixed ratio of AlCl3·6H2O and CO(NH2)2 slowly, after adding some NH4Cl, we find that the mixture melting and becoming clear liquid at

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90 ℃。It is obviously that NH4Cl works as a fluxing agent just like salt sprinkling in ice and snow to help the mixture melt. As a consequence, AlCl3·6H2O and CO(NH2)2 could be mixed uniformly. Secondly, NH4Cl can play a key role to avoid AlCl3·6H2O hydrolyzing into Al(OH)3. It is pointed

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that the adding of NH4Cl does not draw incidental elements into the system. In summary, NH4Cl

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plays two kinds of roles, one of which is fluxing agent and other is inhibitor.

Characterizations

The phase purity and composition of the prepared samples were studied by powder X-ray

diffraction(XRD) experiments that were implemented by a Shimadzu 6100 diffractometer with Cu Kα radiation of 1.54059 Å at room temperature. The morphology and composition of the samples were inspected using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi). Investigations on the microstructure of the samples were performed using transmission electron microscopy (TEM; Tecnai G2 F20), and the element composition was analyzed by X-ray photoelectron spectroscopy (XPS). Properties of photoluminescence were investigated at room 4

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Results and discussion

Fig. 1 XRD patterns of AlN samples doped at various molar percentages of Mn2+. The crystal structures were characterized by X-ray diffraction(XRD) patterns. Fig. 1 depicts the typical XRD patterns of AlN doped with various concentrations of Mn2+ ion. As it is shown in

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the patterns, the peaks at 2θ=33.22 °, 36.04 °, 37.92 °, 49.82 °, 59.35 °,66.05 °and 71.44 °can be properly assigned to the (100), (002), (101), (102), (110), (103) and (112) planes of wurtzite AlN (JCPDS file no.25-1133). All of the diffraction peaks are in accordance with the crystalline phase of wurtzite AlN. No impurity peaks have been observed, indicating that the powders prepared by

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the solid-state reaction route are pure in both chemistry and crystalline phase.25What is more,

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doping with Mn2+ did not induce a second phase.

Fig. 2 SEM images of AlN:Mn2+ 0.8% NPs. The microstructure and component of the as-prepared samples were analyzed by SEM images, 5

ACCEPTED MANUSCRIPT TEM images and XPS spectrum. Fig. 2 presents the SEM images of the as-synthesized AlN:Mn2+ product. Consistent with our previous work, AlN consists of nanosphere with a smooth surface and a perfect sphere structure. It could be seen that near uniformly dispersed particles were obtained in Fig. 2a. Moreover, these conglobated nanoparticles accumulate up to bulk whose size

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was in range of 100-1000nm. When the bulk was amplified further, it is clearly found that the bulk is composed of some spherical particles with a size in the range of 30-100nm in the Fig. 2b. These

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particles exhibit a regular array that are aggregated by small crystallites.

Fig. 3 TEM images of the as-prepared AlN:Mn0.8% phosphors. Fig. 3 depicts the TEM images of AlN:Mn2+ phosphors prepared at 880 ℃.On the basis of Fig. 3a and 3b, it can be seen that the sample phosphors are irregular particles with a broad grain

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size distribution in accordance with Fig.2 above. However, a closer examination reveals that irregular aggregates consist of several tiny AlN nanocrystals.26 It is no doubt that calcination

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process exalts energy of the particles, while nanoparticles agglomerate together to minimze their own energy . Besides, the synthetic strategy of solid state reaction commonly results in aggregates of the products.11 Lattice fringes can be clearly seen in the HRTEM image. From Fig.3d, the sample has large uniform crystal domains with well-defined grain boundaries. The interplanar distance between adjacent lattice fringes is determined to be 0.249 nm, corresponding the (002) plane, which shows well-defined wurtzite AlN. It is indicated that the trace of Mn2+ dopants has almost no influence on the lattice space of AlN.

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Fig. 4. XPS spectra for the as-prepared AlN:Mn0.8% phosphors.

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The X-ray photoelectron spectroscopy (XPS) was employed to confirm the chemistry composition of the surface of the AlN:Mn0.8%. Fig. 4a shows the XPS survey scans of the sample

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in the binding energy range of 0-1200 eV. Elements of Al, C, and N can be detected, as remarked in Fig. 4a. And the corresponding binding energy data for Mn2+ are displayed in Fig. 4b. The peaks located at 641.3 and 652.8 eV could be assigned to the Mn 2p3/2 peaks and Mn 2p1/2, respectively. A satellite peak at 647.0 eV may be ascribed to the presence of Mn2+.27 On the basis

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into AlN lattice.

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of above analysis, the peaks corresponding to Mn2+ confirm the successful incorporation Mn2+

Fig. 5 Photoluminescence excitation and emission spectra of AlN samples doped with 0.3, 0.6, 0.8, 1.0 and 2.0 mol% Mn from curves a to e. All the AlN samples exhibit similar locations of excitation and emission peaks. We wanted to single out the difference of excitation spectra. From the Fig.5a, we can find there are two main bands centered at 250 nm and 350 nm detected by 600 nm light. The broad band at 350 nm is 7

ACCEPTED MANUSCRIPT attributed to the defects from the AlN host while the 250 nm is ascribed to the 6A1(6S) – 4T1(4G). With the concentration of Mn2+ increasing, the later begins to appear and reach a maximum at 0.8%. As demonstrated in Fig. 5b, with the doping concentration increased from 0.3% to 0.8%, the emission intensity increases with doping concentration, and reaches a maximum at 0.8%. In

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relatively low doping concentration range, the increase of Mn2+ content is equivalent to the increase of the number of luminescence centers in AlN lattice. As a consequence, the more Mn2+ in ground state can be excited making the stronger luminescence emission band centered at 600 nm under the excited wavelength of 250 nm. This red emission is ascribed the transmission of T1(4G)-6A1(6S) from Mn2+. Above this content, concentration quenching occurs, which is caused

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by nonradiative energy transfer between neighboring doped ions. In our work, the AlN:Mn2+

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phosphor was prepared under a protective atmosphere, indicating that Mn2+ was hardly oxidized to Mn4+. Meanwhile, the Mn4+ usually gives a line emission rather than a band one.28 Therefore, we believe that the Mn2+ in the AlN host are divalent. As mentioned above, the emission light of Mn2+ strongly depends on the strength of the crystal field. In the crystal structure of AlN, Al atoms are coordinated with four N atoms. According to previous reports, Mn2+ ions are expected to occupy

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the Al3+ lattice sites.20 As a consequence, Mn2+ will be located in the tetrahedral nitrogen coordinated site (weak crystal field) in AlN and is expected to give a green emission. However, the nephelauxetic effect and increased crystal field strength caused by the high formal charge of

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phosphors.29, 30

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N3-, Mn2+ in the AlN host lattice gives a red emission, as with other Mn2+-doped nitride

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Fig. 6. CIE chromaticity diagram of AlN samples doped with 0.3, 0.6, 0.8, 1.0 and 2.0 mol%

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Mn2+.

The CIE chromaticity diagram (Fig. 6.) AlN:Mn phosphors show the emission colors for AlN:Mn phosphors in the red-orange light region. The emission from Mn2+doped AlN lead to the red-organe light. Furthermore, the emission color of AlN:Mn2+ phosphors can be tuned by the

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LEDs.

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concentrations of Mn2+. Accordingly, AlN:Mn phosphors is a promising material utilized in white

Formation Mechanism for AlN From previous research, it can be discovered that some of the following chemical reactions might occur synchronously or asynchronously during the synthesis process of AlN.31 AlCl3·6H2O NH4Cl

Al(OH) 3 +3HCl+3H 2 O

NH 3 +HCl

Al(OH)3+3HCl

(1) (2)

lCl 3 +3H 2 O

(3)

AlCl3+6CO(NH2)2

Al(CON 2 H 4 ) 6 Cl 3

(4)

Al(CON 2 H 4 ) 6 Cl 3

AlCl 3 +6CO(NH 2 ) 2

(5) 9

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HCNO+NH 3

(6)

CO(NH 2 ) 2

H 2 CN 2 +H 2 O

(7)

AlCl3+ NH3

AlCl 3 · NH 3

(8)

1/2(Cl 2 AlNH 2 ) 2 +HCl

1/2(Cl2AlNH2)2

1/n(ClAlNH) n +HCl

1/4(ClAlNH)4

c-AlN+HCl

1/6(ClAlNH)6

h-A lN+HCl

c-AlN

h-AlN

(9) (10)

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AlCl3·NH3

(11)

(12) (13)

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The nucleation of AlN could be proposed as follows: firstly, the AlCl3·6H2O hydrolyzed into Al(OH)3 which would react to HCl becoming AlCl3. AlCl3 and CO(NH2)2 became a metastable

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Al(CON2H4)6Cl3 which would decompose into AlCl3 and CO(NH2)2 making them uniform mixture. Then CO(NH2)2 generated NH3 that would be adsorbed by AlCl3 to form AlCl3·NH3. With the temperature heating up, AlCl3·NH3 decomposed to obtain intermediate compounds (ClAlNH)4 or (ClAlNH)6 which play an important role to generate AlN.32 It is noted that NH4Cl can avoid Al(OH)3 hydrolyzing further Al2O3. Residual NH3 and HCl generate NH4Cl that

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deposited on the shell of pipe.

Conclusions

In this study, AlN semiconductors doped by an array of concentration Mn2+ were prepared by

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a solid-state route. The optimal doping concentration of Mn2+ in AlN is 0.8% mole percentage. It

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is a very important point that adding NH4Cl in the system can not only restrain AlCl3·6H2O generating Al2O3 by hydrolyzing but also work as fluxing agent making the system form a uniform eutectic mixture. It is a particular prospect for preparing rest of nitride because it restrains oxide generating effectively. AlN:Mn2+ displays a strong red emission band of 4T1(4G)-6A1(6S) centered at 600 nm. The AlN:Mn2+ phosphor exhibits a high brightness, high color purity, and lower saturation, which makes it a great candidate of red phosphors for white light-emitting diodes (WLEDs).

Acknowledgments 10

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ACCEPTED MANUSCRIPT 1. Mn doped aluminum nitride (AlN) red phosphors were prepared by a simple solid-state reaction. 2. Their structure, morphology and photoluminescence were detailedly investigated. 3. The photoluminescence properties of the materials are affected by the Mn doped

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4. The formation mechanism for AlN was briefly discussed.

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concentration.