научная статья по теме LUMINESCENCE PROPERTIES OF NOVEL SINGLE-HOST WHITE-LIGHT-EMITTING PHOSPHOR KBABP2O8:DY3+ Физика

Текст научной статьи на тему «LUMINESCENCE PROPERTIES OF NOVEL SINGLE-HOST WHITE-LIGHT-EMITTING PHOSPHOR KBABP2O8:DY3+»

ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 1, с. 139-145

СПЕКТРОСКОПИЯ ^^^^^^^^

КОНДЕНСИРОВАННОГО СОСТОЯНИЯ

УДК 535.37

LUMINESCENCE PROPERTIES OF NOVEL SINGLE-HOST WHITE-LIGHT-EMITTING PHOSPHOR KBaBP2O8:Dy3+

© 2015 г. Bing Han, Jie Zhang, Pengju Li, and Hengzhen Shi

School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002,

People's Republic of China E-mail: hanbing@zzuli.edu.cn, shihz@zzuli.edu.cn Received March 4, 2014

A series of white-light-emitting KBaBP2O8:Dy3+ phosphors were synthesized by using a solid state reaction technique at high temperature. X-ray diffraction and fluorescence spectroscopy measurements were utilized to characterize the structure and luminescence characteristics including excitation and emission spectra, decay curves, chromaticity coordinates of the as-prepared phosphors. The influence of the doping concentration of Dy3+ on the relative emission intensity of Dy3+ was investigated. The critical distance as well as concentration quenching mechanism was calculated and confirmed. The as-prepared phosphors can be effectively excited with near ultraviolet, and exhibit white light emission with short decay time of milliseconds. The above work indicates these phosphors could be potential candidates as single-host white-light-emitting phosphors for application in white light-emitting-diodes.

DOI: 10.7868/S0030403415010067

INTRODUCTION

Compared with conventional incandescent and fluorescent lamps, white light-emitting-diodes (w-LEDs) have attracted considerable attention due to their advantages of high luminescence efficiency, long lifetime, saving energy, and environmental-friendly characteristics, which have been widely considered as the next generation of solid-state lighting in the past years [1—4]. The current prevalent method to generate white light in w-LEDs is to combine blue LED chips with yellow-light-emitting phosphor Y3Al5O12:Ce3+. However, this type ofwhite light often shows low color rendering index (CRI) due to lack of a red light component and chromatic aberration after a long working period [5]. Other approach to produce white light is to combine near ultraviolet (NUV)-LED chips with red/green/blue tricolor phosphors, but there are still lack of sufficient references on utilizing tricolor phosphors to convert NUV-LEDs as illumination sources [6]. A novel method has been suggested in which single-host white-light-emitting phosphors are pumped by NUV—LED chips to produce white light, which can be expected to obtain high luminous efficiency because it can avoid the re-absorption of emission colors in multi-phosphors [7]. Therefore, it is highly required to pay attention to the research on the single-host white-light-emitting phosphor under NUV excitation for NUV LED chips.

Dy3+ ion is an important rare-earth ion, and often emits white light originating from the combination of

its 4F9

9/2 '

H15/2 (~480 nm, blue light) and F

9/2 '

'H14/2 (575 nm, yellow light) transitions in many

host lattices. YVO4:Dy3+ is a well-known commercial white-light-emitting phosphor, which has been applied widely in high-pressure mercury lamps [8]. In the past years, many researchers have showed keen and persistent interest in the research field on the luminescence properties of Dy3+ in inorganic phosphors and developed many novel promising white-light-emitting phosphors, which have been proved to be with great value in many industrial regions [9—12]. For example, NaGd(PO3)4:Dy3+ phosphors are potential white-light-emitting phosphors for mercury-free lamps application because their emission intensities are comparable with that of commercial phosphors (Y, Gd)BO3:Eu3+ and Zn2SiO4:Mn2+ under vacuum ultraviolet (VUV) excitation; CaIn2O4:Dy3+ has potential applications in field-emission display (FED) devices due to its efficient white-light emission under low-voltage electron beam excitation; Sr3Bi(PO4)3:Dy3+ might be a potential phosphor for NUV-based w-LEDs in term of its multi-wavelength excitation bands in NUV region; SrSiO3:Dy3+ could be a potential white-light-emitting long-lasting phosphor. Moreover, as the hypersensitive 4F9/2 —*- 6H13/2 transition of Dy3+ is influenced strongly by the crystal-field environment of Dy3+ ion, the yellow-to-blue (Y/B) emission intensity ratio also reflects the coordination surroundings of Dy3+ to some extent, so Dy3+ ions can be useful to probe the local structure of luminescent centers in a definite host lattice [13]. In a word, the research on Dy3+-doped materials becomes more and

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Fig. 1. XRD pattern of K + xBa! _ 2xDyxBP2O8 (x = 0.04) sample and the calculated XRD pattern of KBaBP2O8 [18].

more significant not only for basic research but also for potential industrial applications.

As an important family of luminescent materials, borophosphate phosphors have attracted wide attention because of their excellent chemical and thermal stability, and low synthesis temperature. Over the past years, many researchers have carried out a lot of works on the optical properties and potential applications of rare-earth ions in borophosphate phosphors, and obtained some significant conclusions [14—17]. Thus, in order to develop some novel phosphors with favorable performances and potential application prospect, it is necessary to investigate the luminescence properties of borophosphate-based phosphor activated with rare-earth ions.

Among borophosphates, KBaBP2O8 is a new kind of borophosphate compound with three-dimensional diamond-like framework reported firstly by Zhao et al.

in 2009 [18]. It crystallizes in space group 14 2d with the following lattice parameters: a = 7.202(2), c = = 14.300(6) A. Though some researchers launched some investigations on the optical properties of rare earth ions (e.g. Eu2+, Eu3+) doped KBaBP2O8 and discussed their potential application in w-LEDs [19—22], there is no study on the luminescent properties of Dy3+ in KBaBP2O8 host. In this paper, we synthesized singlehost white-light-emitting phosphors KBaBP2O8:Dy3+ for the first time, and mainly investigated the spectroscopic properties of Dy3+ in KBaBP2O8 substrate in details.

EXPERIMENTAL

A series of polycrystalline powder samples with nominal chemical formula K + xBax _ 2xDyxBP2O8 (x = = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) were prepared by

a traditional high-temperature solid-state reaction. Since Dy3+ ions are expected to incorporate into the lattice sites of Ba2+ ions, K+ ions with excess of x in formula K1+ xBa1 _ 2xDyxBP2O8 were added as charge compensators. The reactants were K2CO3 [analytical reagent (A.R.)], BaCO3 (A.R.), H3BO3 (A.R.), NH4H2PO4 (A.R.), and Dy2O3 (99.9%), and used without further purification. The raw materials were carefully weighed stoichiometrically and ground in an agate mortar until the mixture appeared homogeneous. Then the mixture was preheated at 673 K for 2 h, reground, and finally fired at 1173 K for 8 h in a muffle furnace. The final products were cooled spontaneously to room temperature (RT) and crushed to fine particles.

The phase purity as well as the structure of the final products was characterized by a powder X-ray diffraction (XRD) analysis with Cu Ka (X = 1.5405 À) radiation on a Bruker D8 Advance X-Ray Diffractometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra as well as decay curves were measured on a fluorescence spectrometer (HITACHI F-7000) equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at RT.

RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of the as-prepared phosphor Kt+ xBat _2xDyxBP2O8 (x = 0.04) and the calculated data from the single-crystal structure of KBaBP2O8 [18]. The phase constitution of KBaBP2O8 is almost not changed when the doped Dy3+ and K+ ions enter into the host lattice. As expected, these ions occupy normal Ba2+ sites due to the similar ionic radii. Furthermore, this synthetic temperature is much mild

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250 300 350 400 450 500 550 600 650 700

Wavelength, nm

Fig. 2. Excitation spectrum (1) under emission at 574 nm and emission spectrum (2) (excitation at 349 nm) of Ki + xBa! _ 2xDyxBP2Os (x = 0.04).

compared with commercial phosphors synthesized at above 1273 K by the conventional solid state reaction technique [6].

The UV excitation spectrum by monitoring the 4F9/2 —»■ 6H13/2 transition at 574 nm of Dy3+ for the phosphor K1 + xBa1 _ 2xDyxBP2O8 (x = 0.04) is shown in Fig. 2a. A series of line-shaped excitation peaks can be observed in the curve (a), which correspond with the intra-configurational 4f 9—4f9 transitions of Dy3+ in

KBaBP2O8 host. As the ground state of Dy3+ is 6H

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Fig. 3. Energy levels and related transitions of Dy3+ ion.

the transitions from this state to different excitation levels [23, 24] can be read to be 298 (4K13/2 + 4H13/2), 324 (4Ki5/2), 349 (4Mi5/2 + ^7/2), 363 (4/n/2), 385 (4I13/2 + 4F7/2) nm, respectively, in Fig. 2a. Among all the excitation peaks, the peak at 349 nm possesses the maximum intensity. The host-related absorption band, O2— —»- Dy3+ charge-transfer band and 4f 9—4f 85d excitation band of Dy3+ are not observed in the UV wavelength range owing to their higher energy below 200 nm [9, 13]. These excitation bands locating in the wavelength 350—400 nm matches partly with the emitting of NUV chips, implying the potential application for NUV w-LEDs [24, 25]. The emission spectrum upon 349 nm excitation is exhibited in Fig. 2b. Two emission bands are observed in the curve (b), in w

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