A novel single-composition tunable color emission KSrY(PO4)2:Ce3+, Tb3+, Mn2+ phosphor based on energy transfer

A novel single-composition tunable color emission KSrY(PO4)2:Ce3+, Tb3+, Mn2+ phosphor based on energy transfer

Materials Research Bulletin 57 (2014) 231–237 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.c...

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Materials Research Bulletin 57 (2014) 231–237

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

A novel single-composition tunable color emission KSrY(PO4)2:Ce3+, Tb3+, Mn2+ phosphor based on energy transfer Mengfei Zhang a,b , Yujun Liang a,b, *, Wenzhu Huang b , Zhanggen Xia b , Dongyan Yu b , Yazhu Lan b , Guogang Li a,b , Wei Zhou a,b a b

Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 21 November 2013 Received in revised form 1 May 2014 Accepted 10 June 2014 Available online 11 June 2014

A novel single-composition white-emitting phosphor KSrY(PO4)2:Ce3+, Tb3+, Mn2+ has been synthesized by a high-temperature solid-state reaction. The spectral overlap between the emission band of Ce3+ and the excitation band of Mn2+ supported the occurrence of the energy transfer from Ce3+ to Mn2+, and demonstrated to be the dipole–dipole interaction. The energy transfer efficiency (Ce3+ ! Mn2+) obtained by decay curves was consistent with the result calculated by the emission intensity, which gradually increased from 14.5% to 65.8% by increasing the Mn2+ doping content from 0.02 to 0.36. The mechanism of transferring energy from Ce3+ to Tb3+ and Ce3+ to Mn2+ was also discussed in this study. A single composition white-light emitting phosphor KSr0.94Y0.98(PO4)2:0.01Ce3+, 0.01Tb3+, 0.06Mn2+, which shows correlated color temperature of 5110 K, color rendering index of 46, and color coordinates of (0.341, 0.337), was obtained. The results indicate that the KSrY(PO4)2:Ce3+, Tb3+, Mn2+ phosphors have potential applications as an ultraviolet-convertible phosphor. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: Optical materials Luminescence Optical properties

1. Introduction The increasing demand for fossil fuels and the environmental impact of their use are continuing to exert pressure on an alreadystretched world energy infrastructure [1]. Conventional incandescent and fluorescent lamps rely on either heat or discharge of gases. Until now, the conventional white light sources have almost reached their physical limit of efficiency, while whitelight-emitting diodes (W-LEDs) have not [2]. Phosphor-converted W-LEDs have gained enormous commercial interest because of their high luminous efficiency, long lifetimes, environmentally friendly features, etc. [3–5]. The most dominant way to create a W-LED is to combine blue InGaN chip with Y3Al5O12:Ce3+ (YAG: Ce)-based yellow phosphors [6,7]. However, the device based on this phosphor exhibits a poor color-rendering index (CRI) and a high correlated color temperature (CCT) because of the lack of red light at long wavelength and limits expansion of the LED application [8–10]. Current lighting technology employs ultraviolet (UV) LED chips with red, green, and blue phosphors to

* Corresponding author at: Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China. Tel.: +86 27 67884814; fax: +86 27 67883733. E-mail address: [email protected] (Y. Liang). http://dx.doi.org/10.1016/j.materresbull.2014.06.009 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.

improve this problem. This approach provides W-LEDs with excellent CRI and can generate warm white light, but the manufacture cost is high and the blue emission efficiency is poor because of the strong reabsorption of blue light by the red, green-emitting phosphors. To circumvent these disadvantages, researchers are turning their attention to the investigation of efficient, durable, and single phase white-light-emitting phosphors with the red, green, and blue (RGB) components through energy transfer between activators [11,12]. It is well-known that energy transfer plays an important role in the optical properties of luminescent materials both from theoretical and practical points of view. Rare earth ions have been playing an irreplaceable role in modern lighting and display fields due to the abundant emission colors based on their 4f ! 4f or 5d ! 4f transitions [13,14]. The Ce3+ with the 4f1 configuration in solids shows efficient broad band luminescence due to the 4f  5d parity allowed electric dipole transition. In addition, the Ce3+ ion can also act as an efficient sensitizer to enhance the emission of coactivators by transferring part of its excitation energy to them, such as Tb3+ and Mn2+ [15,16]. So far, Ce3+ ! Tb3+/Mn2+ energy transfer has been generally investigated, Ce3+ ions serving as effective sensitizer ions not only help Tb3+ and Mn2+ ions emit efficiently but also tune their emission colors from blue to green and from blue to orange/red, respectively [17]. Furthermore, a single-phased white-light-emitting phosphor utilizing energy

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transfer can avoid the reabsorption for blue or UV light by the orange/green emitting phosphors and the mixing of RGB phosphors. Consequently, it can enhance the luminescence efficiency and color reproducibility. The structure of double phosphate KSrY(PO4)2 (KSYP) has been reported previously by Zhang et al. [18] KSYP has the similar crystal structure with hexagonal LaPO4, except that the large 8coordinated K+ occupies only half of the tunnel sites (positions 3b in P6222), and La3+ position is statistically occupied by Y3+ or Sr2 + ions [19]. It contains chains of – (Sr2+ or Y3+) – PO4 – (Sr2+ or Y3+) – along the c axis, linked to similar chains by corner shared with four PO4 tetrahedra in the (0 0 1) plane [18]. To the best of our knowledge, there are no reports about the detailed photoluminescence properties and energy transfer phenomena of Ce3 + /Tb3+/Mn2+ in the KSYP host lattice. Accordingly, in this paper, we reported the synthesis of KSYP:Ce3+, Tb3+, Mn2+ phosphors via the solid state reaction process, and also investigated energy transfer of Ce3+ ! Tb3+/Mn2+ in these phosphors. It can be found that they have potential application as an ultraviolet-convertible phosphor. 2. Experimental 2.1. Sample preparation A series of polycrystalline KSr1zY1xy(PO4)2: x mol% Ce3+, y mol% Tb3+, z mol% Mn2+ powder samples were prepared by conventional high temperature solid state reaction process. On the basis of the similar effective ionic radius and valence of the cations, we suggested that Ce3+/Tb3+ ions prefer to occupy Y3+ sites, while Mn2+ ions replace Sr2+ ions more easily. The expression of KSr1zY1xy(PO4)2: x mol% Ce3+, y mol% Tb3+, z mol% Mn2+, in the following sections are abbreviated as KSYP:xCe3+, yTb3+, zMn2+, for example, the KSr0.99Y0.95(PO4)2:1 mol% Ce3+, 4 mol% Tb3+, 1 mol % Mn2+ are denoted as KSYP:1Ce3+, 4Tb3+, 1Mn2+. The doping concentrations of Ce3+, Tb3+ and Mn2+ were chosen as 1 mol% of Y3 + , 0–2 mol% of Y3+, and 0–36 mol% of Sr2+ in KSrY(PO4)2, respectively. Typically, stoichiometric amounts of K2CO3 (A.R.), SrCO3 (A.R.), Y2O3 (A.R.), (NH4)2HPO4 (A.R.), CeO2 (99.99%), MnCO3 (A.R.), Tb4O7 (99.99%) and an excess of 5 mol% of H3BO3 as a flux were thoroughly mixed in an agate mortar, then the homogeneous mixture was transferred to an alumina crucible and calcined in a furnace at 1273 K for 8 h under a reducing atmosphere of 15% H2/ 85% N2, and slowly cooled to room temperature.

3. Results and discussion 3.1. XRD All XRD patterns of the KCaY(PO4)2 JCPDS 51-1632, KSYP, KSYP:8Mn2+, KSYP:1Ce3+, zMn2+ (z = 0, 2, 12, 24 and 36) and KSYP:1Ce3+, yTb3+, zMn2+ (y = 1–2, z = 4–6) phosphors are showed in Fig. 1. The XRD patterns of doped and undoped KSrY(PO4)2 are in good agreement with those reported of KCaY(PO4)2 JCPDS 51-1632. For KCaY(PO4)2 and KSrY(PO4)2, no similar structural data of these compounds are available for comparison. However, due to the similar ionic size of Ca2+ (0.99 Å) and Sr2+ (1.18 Å), it is reasonable to assume that a complete solid solution is very likely to form in the series of KMY(PO4)2 (M2+ = Ca, Sr) compounds, which is indeed experimentally observed [18]. As can be observed in Fig. 1, when the Ca2+ ions are substituted completely by Sr2+ ions, the XRD patterns are almost the same except that there is a discernible shift in the position of the XRD peaks, which can be explained by the difference between the ionic radii of these metals. Furthermore, the similar phenomena have been observed by Xia et al. and the results are in accordance with our experiment conclusions [20]. Fig. 1 clearly presents the two strongest diffraction peaks ((2 0 0) and (1 0 2)) and it is observed that the two peaks vary to the low degree. Furthermore, the XRD patterns in Fig. 1 indicate that the obtained phosphors are single phase and the co-doped Ce3+ and Mn2+ ions do not cause any significant change. 3.2. Luminescence properties and energy transfer in KSYP:Ce3+, Mn2+ Significant spectral overlap between the broad Ce3+ emission band (Fig. 2(a), blue solid line) ascribed to the 5d1 ! 4f1 transition and several excitation bands of Mn2+ centered at 321, 355, 402, 418, and 467 nm (Fig. 2a, black dash line), which corresponded to the transitions from the 6A1 (6S) ground state to the excited states 4E (4D), 4T2 (4D), [4E (4G), 4A1 (4G)], 4T2 (4G), and 4T1 (4G) levels [15,21,22], as shown in Fig. 2(a), is observed. These observations have also been observed and reported by Huang et al. [22]. It has been reported that the PLE spectrum of Mn2+ 6A1 (6S) ! [4E (4G), 4 A1 (4G)] centered at 402 nm exhibits substantial overlap with the emission band of Ce3+ 5d1 ! 4f1 centered at 401 nm, indicating a favorable condition for possible energy transfer between the sensitizer Ce3+ and the activator Mn2+. Because the d–d transitions of the Mn2+ ion are spin- and parity-forbidden by the selection rules, the intensity of this absorption is low in the UV region.

2.2. Sample characterization The crystal structure of the phosphors were characterized by Xray powder diffractometer (XRD) (Bruker D8 Focus, Bruker, Kalsruhe, Germany) with Ni-filtered Cu-Ka (l = 1.540598 Å) radiation at 40 KV tube voltage and 40 mA tube current. The XRD data were collected in a 2u range from 10 to 60 , with the continuous scan mode at the speed of 0.05 s per step with step size of 0.01. Photoluminescence excitation (PLE) and emission (PL) spectra were measured by fluorescence spectrometer (Fluoromax-4P, Horiba Jobin Yvon, New Jersey, U.S.A.) equipped with a 450 W xenon lamp as the excitation source and both excitation and emission spectra was set up to be 1.0 nm with the width of the monochromator slits adjusted as 0.50 nm. Photochromic properties of the resulting phosphors were tested by UV–vis diffuse reflectance spectroscopy (Shimadzu UV 2550) with BaSO4 as the baseline correction. The photoluminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO). The other measurement conditions (PMT detector sensitivity, scan peed) were kept consistent from sample to sample in measurements. All the measurements were carried out at room temperature.

Fig. 1. The XRD patterns of the KCaY(PO4)2 JCPDS 51-1632 standard pattern, KSYP, KSYP:8Mn2+, KSYP:1Ce3+, zMn2+ (y = 0, 2, 12, 24, and 36) and KSYP:1Ce3+, yTb3+, zMn2+ (y = 1–2, z = 4–6).

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Fig. 2. (a) PLE/PL spectra of KSYP:Ce3+ (green solid and blue solid line) and PLE spectrum of KSYP:Mn2+ (black dash line). (b) PLE/PL spectra of KSYP:Ce3+, Mn2+ (green/red solid and orange dash line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2(b) illustrates the PLE and PL spectra of KSYP:1Ce3+, 8Mn2+. The PLE spectrum shows a similar band with KSYP:1Ce3+ (Fig. 2(a)) monitored at 401 nm. In addition, it is observed that the PLE spectrum monitoring at 570 nm due to the emission of Mn2+ is also similar to that responding to the blue emission of Ce3+, demonstrating the existence of energy transfer from Ce3+ to Mn2+ in the host of KSYP. Under the excitation of 328 nm, the PL spectrum of KSYP:1Ce3+, 8Mn2+ shows not only a broad blue band peaked at 401 nm due to the transition of 5d ! 2FJ (J = 5/2, 7/2) of Ce3+ but also an orange band of the Mn2+ ions. Compared with the PL spectra of the single doped samples, the co-doped powder shows the multi-color emission. A good evidence of absorption in the near-UV region from activator ions was obtained through comparison of the reflectance spectra of host, KSYP:1Ce3+ and KSYP:1Ce3+, 8Mn2+ phosphors (Fig. 3). The KSYP host shows a decrease in reflectance from 300 to 400 nm. However, as Ce3+ ions are introduced into the KSYP host, two obvious broad absorption bands with maximum at 275 and 320 nm appear in the 250–350 nm region, which are assigned to the f–d absorption of the Ce3+ ions. This phenomenon is also consistent with the PLE spectrum of KSYP:1Ce3+ in Fig. 2(a). Moreover, as also shown in Fig. 3, the absorption band of KSYP:1Ce3+, 8Mn2+ is similar to that of KSYP:1Ce3+, which can also be found in the corresponding PLE spectra in the Fig. 2(b).

Fig. 4. PL spectra of KSYP:1Ce3+, zMn2+ phosphors (z = 0, 2, 4, 6, 8, 12, 16, 20, 24, 28 and 36). In the inset energy transfer efficiency from Ce3+ to Mn2+ is provided.

Fig. 4 illustrates the dependence of PL spectra on the Mn2+ dopant content for KSYP:1Ce3+, zMn2+ phosphors (z = 0, 2, 4, 6, 8, 12, 16, 20, 24, 28 and 36). The PL intensity of Ce3+ at 401 nm is found to decrease with the increasing Mn2+ content (z), whereas the PL intensity of Mn2+ at 589 nm is observed to increase with the increasing Mn2+ content (z) until the emission intensity of Mn2+ is saturated when z is above 28, and then the concentration quenching appears when the Mn2+ dopant content (z) is greater than 28, which is related to the energy transfer probability occurring from Ce3+ to Mn2+. Therefore, the emitting colors of KSYP:1Ce3+, zMn2+ samples are tuned from blue to orange by changing the Mn2+ content (z), as suggested by the Commission Internationale de L'Eclairage (CIE) coordinates in the solid line of Fig. 9. Moreover, similar phenomenon was observed and discussed by Liu et al. [23] Generally, the energy transfer efficiency from a sensitizer (Ce3+) to activator (Mn2+) can be expressed as the following equation [24]:

hT ¼ 1 

Is Iso

(1)

where hT is the energy transfer efficiency and Iso and Is are the luminescence intensity of a sensitizer in the absence and presence of an activator, respectively. The inset of Fig. 4 shows the results of energy transfer efficiency from Ce3+ to Mn2+ calculated by Eq. (1) under the excitation of 328 nm. When the Mn2+ doping content is increased from 2 to 36, the energy transfer efficiency gradually increases from 14.5% to 65.8%. To further elucidate the energy-transfer process, we measured the Ce3+ decay curves and then calculated the energy-transfer efficiencies by the lifetimes. The decay curves of KSYP:1Ce3+, zMn2+ phosphors (z = 0, 2, 6, 12, 20, 24, 28 and 36) excited at 355 nm and monitored at 401 nm are shown in Fig. 5(a). The decay curves are well fitted with a typical single-order decay mode and the average decay times (t ) are calculated to be 38.03, 34.36, 32.04, 31.63, 30.11, 24.53, 20.05, and 16.87 ns for KSYP:1Ce3+, zMn2+ with z = 0, 2, 6, 12, 20, 24, 28 and 36, respectively. The energy transfer efficiency ðht Þ can be calculated using the following equation by Paulose et al. [25]:

ht ¼ 1 

Fig. 3. Reflectance spectra of KSYP host (a), KSYP:1Ce3+ (b), and KSYP:1Ce3+, 8Mn2+ (c) phosphors.

233

ts t so

(2)

where t so is the lifetime of the sensitizer Ce3+ of the sample in the absence of Mn2+, and t s is the lifetime of Ce3+ in the presence of Mn2+, the energy-transfer efficiency from Ce3+ to Mn2+ in KSYP:1Ce3+, zMn2+, calculated by using Eq. (2) as a function of z

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Fig. 5. (a) Decay curves of Ce3+ emission monitored at 401 nm for KSYP:1Ce3+, zMn2+ (z = 0, 2, 6, 12, 20, 24, 28 and 36) under excitation at 355 nm. Round circles: experimental data, red solid line: fitting results. (b) The dependence of energy transfer efficiency ðhT Þ on the Mn2+ content. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

under 355 nm UV excitation, is shown in Fig. 5(b). The energytransfer efficiency is found to increase gradually from z = 0 to 36 with an increase of the Mn2+ concentration. The hT obtained by decay curves agrees well with the results calculated by the emission intensity. All the obtained results elucidate that the energy-transfer process from Ce3+ to Mn2+ in the KSYP host is efficient. According to Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained [26,27]:   hSO (3) /C ln

hS

hSO / C n=3 hS

(4)

where C is the sum of the content of Mn2+; hSO and hS are the luminescence quantum efficiency of Ce3+ in the absence and presence of Mn2+. ln ðhSO =hS Þ / C corresponds to the exchange interaction and hSO =hS / C n=3 with n = 6, 8, and 10 corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole– quadrupole (q–q) interactions [28–30], respectively. The value hSO=hS is approximately calculated by the ratio of related luminescence intensities as [17,31]:   ISO (5) /C ln IS ISO / C n=3 IS

(6)

where ISO is the intrinsic luminescence intensity of Ce3+, and IS is the luminescence intensity of Ce3+ in the presence of the Mn2+. The relationships between ln ðISO =IS Þ and C as well as ISO =IS and C n=3 are illustrated in Fig. 6(a–d). The linear relationship reaches the optimal one for ln ðISO =IS Þ / C by comparing the fitting factors of R values in Fig. 6(b), implying that energy transfer from Ce3+ to Mn2+ occurs via the dipole–dipole interactions which is similar to those previously observed by Sun et al. [32] Therefore, the electric dipole–dipole interactions predominates in the energy-transfer mechanism from the Ce3+ and Mn2+ ions in KSYP host. 3.3. Luminescence and chromaticity of KSYP:Ce3+, Tb3+, Mn2+ As we know, energy transfer from sensitizer to activator is a feasible route to realize color-tunable emission, and a white light emission can be obtained through mixing the RGB light sources at a

suitable ratio [11]. In our case, the KSYP:Ce3+ emits bright blue light due to the 5d–4f transition of Ce3+, and the efficient energy transfer of Ce3+ ! Mn2+ is also validated in Ce3+/Mn2+-coactivated KSYP phosphors. Thus, it is reasonable to predict that the color-tunable emissions from blue light to orange light in KSYP:Ce3+, Mn2+ systems can be obtained. In addition, a color-tunable emission from blue to green can also be realized in KSYP:Ce3+, Tb3+ samples through the Ce3+ ! Tb3+ energy transfer. Thus, it is possible to produce white emission due to the simultaneous appearance of RGB light by appropriately adjusting of the doping concentration of Mn2+ and Tb3+ at a fixing the Ce3+ concentration in the present KSYP host. Our experiments have confirmed the above situation. A series of KSYP:1Ce3+, yTb3+, zMn2+ samples with different Tb3+ and Mn2+ concentrations (y = 1  2, z = 4  5) have been synthesized. Fig. 7 shows the typical PL spectra of KSYP:1Ce3+, yTb3+, zMn2+ phosphor (y = 1  2, z = 4  5) under 328 nm excitation. The PL spectra are found to consist of six emission bands in the visiblewavelength region, one at 401 nm (Ce3+5d1 ! 4f1 transition), four green-emitting Tb3+ peaks located at about 481, 543, 581 and 622 nm (due to 5D4 to 7F6, 7F5, 7F4, 7F3 transitions), and a broad orange-emitting band at 570 nm (due to a Mn2+ d-level spinforbidden transition) [33]. With the increasing of Mn2+ dopant concentration, the Ce3+ blue emission gradually decreases and Mn2 + orange emission gradually increases. Moreover, when the Tb3+ dopant concentration increases, the PL intensity of Tb3+ emission is found to increase, and that of Ce3+ emission is observed to decrease, yet the PL intensity of Mn2+ does not increase nor decrease with the increasing Tb3+ dopant concentration. Thus, there could not be any energy transfer between Mn2+ and Tb3+ as expected. In view of the Ce3+ ! Tb3+ transition in the KSYP host, Ce3+ ions can strongly absorb UV light from the ground state (2F5/2) to the excited state and then efficiently transfer the energy to the 5D3 level of Tb3+ ions; subsequently, the 5D3 level gives its characteristic transitions or continues to transfer the energy to the 5D4 level via cross relaxation, as shown in Fig. 8. Mn2+ ions doped in host matrices generally show a broad band emission because of the 4 T1 ! 6A1 transition within the 3d shell in which the electrons are strongly coupled to lattice vibrations and are affected by crystal field strength and site symmetry. If Mn2+ ions lie in the weak crystal-field condition, the splitting of the excited-state d energy levels will be small, resulting in Mn2+ emission with higher energy as a green emission; if it is in strong crystal-field condition, a yellow/red emission is usually obtained [13,34]. According to the Sugano–Tanabe diagram [35], the d–d transitions of Mn2+ are forbidden so their excitation transitions are difficult to pump and

M. Zhang et al. / Materials Research Bulletin 57 (2014) 231–237

235

Fig. 6. Dependence of ln ðISO =IS Þ of Ce3+ on (a) C and ISO =IS of Ce3+, (b) C 6=3 , (c) C 8=3 and (d) C 10=3 .

the emission intensity is very weak. It is noteworthy that Ce3+ and Tb3+ ions can absorb near-UV light and have many excited energy levels at 20–30  103 cm1 (blue light), which largely overlap with the major excitation bands of Mn2+ ions. So in other words, the energy levels of the excited states from Mn2+ match with some energy levels of Ce3+ and Tb3+ ions [6,34,36]; thus, Ce3+ and Tb3+ can be introduced as sensitizers to enhance the emission of Mn2+ ions. Ce3+ ions can strongly absorb UV light from the ground state (2F5/2) to the excited state and then efficiently transfer the energy to the 4T1 level of Mn2+ ions; subsequently, the 4T1 level gives its characteristic transitions [37]. The X and Y values of CIE chromaticity coordinates for KSYP:1Ce3+, yTb3+, zMn2+ phosphors with different dopant contents were measured and presented in Fig. 9 and Table 1, respectively. The tristimulus values (R, G, B) for a test light source,

Fig. 7. PL spectra of KSYP:1Ce3+, yTb3+, zMn2+ phosphor (y = 1–2, z = 4 5) under 328 nm excitation.

which has a spectral energy distribution P(l), are calculated with the following formulate: Z 780 PðlÞxðlÞdl (7) R¼ 380

Z G¼

380

Z B¼

780

780

380

PðlÞyðlÞdl

(8)

PðlÞzðlÞdl

(9)

where xðlÞ, yðlÞ , and xðlÞ are the spectral stimulus values for 50. The chromaticity coordinates of color of the light sources X, Y and Z are given by:

Fig. 8. Illustration of the energy-transfer mechanism for Ce3+/Tb3+ and Ce3+/Mn2+ pairs in the KSYP host.

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from point 1 (0.304, 0.282) to point 3 (0.321, 0.312), 5 (0.341, 0.337) with the increase of z values from 4 to 5, 6, namely, the increase of orange component. The same situation is held for KSYP:1Ce3+, 2Tb3 + , zMn2+ (z = 4, 5, 6) samples, for example, the point 2 (0.302, 0.313), 4(0.327, 0.344), 6 (0.347, 0.3707) in Fig. 9. When fixing the z at 5, the CIE coordinate positions of KSYP:1Ce3+, yTb3+, 5Mn2+ samples move from point 3 (0.321, 0.312) to point 4 (0.327, 0.344) with the increase of y values from 1 to 2, namely, the increase of green component. The similar results are suitable to point 1 (0.304, 0.282) ! 2 (0.302, 0.313) and point 5 (0.341, 0.337) ! 6 (0.347, 0.371). The correlated color temperature and color rendering index (Ra) can be expressed by: n¼

X  0:3320 Y  0:1858

CCT ¼ 437  n3 þ 3601  n2  6861  n þ 5514:31 Ra ¼

Fig. 9. CIE chromaticity diagram for KSYP:1Ce3+, yTb3+, zMn2+ samples under 328 nm UV excitation: (1) y = 1, z = 4; (2) y = 2, z = 4; (3) y = 1, z = 5; (4) y = 2, z = 5; (5) y = 1, z = 6; and (6) y = 2, z = 6.



R RþGþB

(10)



G RþGþB

(11)



B RþGþB

(12)

8 1X R 8 i¼1 i

(13) (14)

(15)

where X, Y are the chromaticity coordinates and Ri is obtained for each of eight selected object colors. In addition, by properly tuning the relative composition of Tb3+/Mn2+, a single composition whitelight emitting phosphor KSYP:1Ce3+, 1Tb3+, 6Mn2+ shows chromaticity coordinates of (0.341, 0.337), Ra = 46, and CCT = 5110 K. The above results indeed indicate that a tunable white emission can be obtained by precisely controlling the Ce3+, Tb3+, Mn2+ concentrations in the KSYP host. 4. Conclusions

From Fig. 9, it is clearly seen that the CIE coordinates of KSYP:1Ce3+, zMn2+ (z = 0–36) samples under 328 nm UV excitation move from blue region (0.169, 0.019) to white region (0.342, 0.304) and eventually to orange region (0.453, 0.441) with the increase of z values. More importantly, a wide-range-tunable white emission can be obtained by precisely controlling the contents of Mn2+ and Tb3+ ions, as shown in points 1–6. When fixing the y at 1, the CIE coordinate positions of KSYP:1Ce3+, 1Tb3+, zMn2+ samples move

Table 1 CIE chromaticity coordinates (X, Y) and CCT of KSYP:1Ce3+, yTb3+, zMn2+ phosphors excited at 328 nm. Point

Composition

CIE (X, Y)

CCT (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

KSYP:1Ce3+, 1Tb3+, 4Mn2+ KSYP:1Ce3+, 2Tb3+, 4Mn2+ KSYP:1Ce3+, 1Tb3+, 5Mn2+ KSYP:1Ce3+, 2Tb3+, 5Mn2+ KSYP:1Ce3+, 1Tb3+, 6Mn2+ KSYP:1Ce3+, 2Tb3+, 6Mn2+ KSYP:1Ce3+ KSYP:1Ce3+, 2Mn2+ KSYP:1Ce3+, 4Mn2+ KSYP:1Ce3+, 6Mn2+ KSYP:1Ce3+, 8Mn2+ KSYP:1Ce3+, 12Mn2+ KSYP:1Ce3+, 16Mn2+ KSYP:1Ce3+, 20Mn2+ KSYP:1Ce3+, 24Mn2+ KSYP:1Ce3+, 28Mn2+ KSYP:1Ce3+, 36Mn2+

0.304, 0.282 0.302, 0.313 0.321, 0.312 0.327, 0.344 0.341, 0.337 0.347, 0.371 0.169, 0.019 0.256, 0.164 0.315, 0.267 0.342, 0.304 0.366, 0.340 0.390, 0.374 0.406, 0.392 0.418, 0.406 0.431, 0.419 0.448, 0.436 0.453, 0.441

7858 7336 6159 5719 5110 4996

In summary, a series of single-phased novel color-tunable KSYP: Ce3+, Tb3+, Mn2+ phosphors have been synthesized and the luminescence properties as well as energy transfer from Ce3+ to Tb3+ and Mn2+ are investigated in detail for the first time. The obtained phosphors exhibit a broad excitation band ranging from 250 to 400 nm, which can perfectly match UV excitation light. The KSYP:Ce3+, Tb3+, Mn2+ phosphors show three emission colors: blue band of 401 nm, green band of 543 nm and orange band of 570 nm. The energy transfers from Ce3+ to Tb3+ and Ce3+ to Mn2+ in KSYP host matrix are demonstrated by luminescence spectra, energytransfer efficiency, and lifetimes of phosphors. The excitation spectrum of Mn2+ 6A1 (6S) ! [4E (4G), 4A1 (4G)] centered at 402 nm exhibits substantial overlap with the emission band of Ce3+ 5d1 ! 4f1 centered at 401 nm, indicating a favorable condition for possible energy transfer between the sensitizer Ce3+ and the activator Mn2+. The energy transfer phenomena have been investigated and the high efficient energy transfer from Ce3+ to Mn2+ with an efficiency of over 65% by the luminescence spectra. We also have demonstrated that the energy transfer from the Ce3+ to Mn2+ ions is dominated by the electric dipole–dipole interaction. By controlling the doping content of the Mn2+ ions with a fixed Ce3+ content, the emission color of the phosphor varied from blue region (0.169, 0.019) to white region (0.342, 0.304) and eventually to orange region (0.453, 0.441). More importantly, a single composition white-light emitting phosphor KSYP:1Ce3+, 1Tb3+, 6Mn2+ shows chromaticity coordinates of (0.341, 0.337), color rendering index Ra = 46, and CCT = 5110 K. In conclusion, the results indicate that the color-tunable phosphor KSYP:Ce3+, Tb3+, Mn2+ can be a promising candidate to serve as the potential phosphor. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21171152), and by the Guangdong

M. Zhang et al. / Materials Research Bulletin 57 (2014) 231–237

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