Mn2+ phosphors for applications in white light-emitting diodes and optical thermometers

Journal Pre-proof Photoluminescence properties of Ca9La(PO4)5SiO4F2:Ce3+/Tb3+/Mn2+ phosphors for applications in white light-emitting diodes and optical thermometers

Jia Zhang, Yurong Shi, Songsong An PII:

S1386-1425(19)31276-4

DOI:

https://doi.org/10.1016/j.saa.2019.117886

Reference:

SAA 117886

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date:

21 September 2019

Revised date:

28 November 2019

Accepted date:

1 December 2019

Please cite this article as: J. Zhang, Y. Shi and S. An, Photoluminescence properties of Ca9La(PO4)5SiO4F2:Ce3+/Tb3+/Mn2+ phosphors for applications in white lightemitting diodes and optical thermometers, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2019), https://doi.org/10.1016/j.saa.2019.117886

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© 2019 Published by Elsevier.

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Photoluminescence properties of Ca9La(PO4)5SiO4F2:Ce3+/Tb3+/Mn2+ phosphors for applications in white light-emitting diodes and optical thermometers Jia Zhang1*, Yurong Shi2 and Songsong An3 1

Physics department and Jiangsu Key Laboratory of Modern Measurement

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Technology and Intelligence, Huaiyin Normal University, 111 West Chang Jiang Road, Huai'an 223300, China

The Key Laboratory of Rare Earth Functional Materials and Applications, Zhoukou

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2

Huaiyin Normal University, 111 West Chang Jiang Road, Huai'an 223300, China

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3

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Normal University, Zhoukou 466001, China

*Corresponding author. E-mail address: [email protected]

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Abstract A

series

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Ca9La(PO4)5SiO4F2:Ce3+,Tb3+,Mn2+

(CLPSF:Ce3+,Tb3+,Mn2+)

phosphors were obtained by a conventional solid-state reaction method, and the luminescence properties excited by ultraviolet light were investigated in detail. The Ce3+-doped CLPSF samples show near-ultraviolet luminescence with the dominant peaks around 361 nm. Different Ce3+ emission centers were identified from the

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emission spectra. When the Ce3+ and Mn2+ are codoped into the host, an energy

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transfer (ET) from Ce3+ to Mn2+ was found, owing to which the visible

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emitting-light-color has been tuned from blue to light brown. The corresponding ET

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mechanism was studied by employing Dexter’s theory. In the Ce3+-Tb3+ codoped

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CLPSF phosphors, the tunable emission was realized on the basis of the ET between

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Ce3+ and Tb3+. To further obtain the white emissions with tunable correlated color temperature, the Ce3+-Tb3+-Mn2+ tridoped CLPSF samples were designed, and the ET

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relationship in these phosphors were discussed. By studying the thermally luminescent properties, it was found that the Ce3+ and Mn2+ emission intensities in the CLPSF:Ce3+,Mn2+ samples showed different decrease rates with increasing temperature. The fluorescence intensity ratio (FIR) technique was used to investigate the temperature-sensing performance. On the other hand, the CLPSF:Ce3+,Tb3+ and CLPSF:Ce3+,Tb3+,Mn2+ phosphors exhibit relatively high thermally luminescent stability. The above discoveries indicate that the developed phosphors could have potential applications in LEDs and optical thermometer. Keywords: Phosphors; Optical thermometer; Energy transfer; Luminescence

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1. Introduction Rare earth (RE) ions doped luminescent materials have demonstrated a potential application prospect in fields such as the white light-emitting diodes (LEDs), solid state lasers, temperature sensors, optical communication, bio-labels and efficiency enhancement of solar cells [1-6]. Among these applications, the LEDs and optical

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thermometers have attracted much attention due to the advantages of high stability, good physical-chemical properties and environmental protection [7]. First, for the

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phosphor-converted LEDs, two main strategies were generally used to realize white

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emitting light. One is the combination of a blue InGaN chip and a yellow

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Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. Unfortunately, the while light produced in this

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way suffers a poor color rendering index (CRI, Ra) and high correlated color

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temperature (CCT). The other is the utilization of the tri-color phosphors based on a ultraviolet (UV) LED chip. This method can create tunable CCT and excellent CRI,

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which make it more and more popular. Second, for the optical thermometer, it is well known as a non-contact temperature measurement method that based on luminescent materials. The most used method is called fluorescence intensity ratio (FIR) technique, which employs two emissions from one or two activators. This technique can reduce the dependence on measurement conditions, which effectively improves the accuracy and resolution [8]. Lanthanide (Ln) ions doped inorganic materials could exhibit efficient luminescence in a suitable matrix. Due to the 4f-5d transition matching well with the emission of near-UV chips, the Ce3+ ion is considered as the optimal choice as a

Journal Pre-proof sensitizer to fabricate tunable phosphor for white-LEDs [9]. Besides, the Tb3+ and Mn2+ ions are also widely used green and orange/red emitting activators, respectively. However, the Tb3+ and Mn2+ ions exhibit weak emission intensity due to the spin-forbidden f-f transition of Tb3+ and d-d transition of Mn2+ [10,11]. Therefore, it is necessary to enhance the emissions of Tb3+ and Mn2+. An effective method is to introduce sensitizer such as Ce3+, where the excitation energy can be transferred from

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Ce3+ to Tb3+ and Mn2+ [12]. This energy transfer (ET) not only improves the green

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and red emissions but also generates an emission-tunable white light. As a result,

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co-doping Ce3+, Tb3+ and Mn2+ ions is the common strategy to obtain the single-phase

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white-emitting phosphors with the help of ET among kinds of white-LED-based

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phosphors [13-15]. For the temperature-sensing application, the Er3+ is the widely

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used RE ion, owing to its thermally-coupled levels 2H11/2 and 4S3/2 [16]. In order to increase the sensor sensitivity, other activators of single or couple ions have been

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widely developed, such as Ho3+, Ce3+-Mn2+ and Eu3+-Tb3+ [17-19]. So far, apatite-type compounds have been considered as excellent host materials for phosphors due to the efficient luminescence and good chemical durability [20,21]. Apatite compound is based on the formula of A10[TO4]6Z2, where A represents a mono-, di- or tri-valent cations such as Na+, Ba2+, La3+, TO4 is smaller anionic group (PO4, VO4, SO4) and Z is substituted by anion, such as F−, S2− or O2− [22,23]. In the previous

references,

the

Ca9La(PO4)5SiO4F2:Sm3+,Tb3+

and

Ca9La(PO4)5SiO4F2:Ce3+,Tb3+ phosphors have been respectively reported by Zhou and Mei et al. [24,25]. However, novel color-tunable phosphors by doping

Journal Pre-proof Ce3+-Tb3+-Mn2+ for LEDs and Ce3+-Mn2+ for optical thermometer in the Ca9La(PO4)5SiO4F2 (CLPSF) host have not been investigated in detail. Herein, we prepared Ce3+/Tb3+/Mn2+ doped CLPSF phosphors by a conventional solid-state reaction method. The influences of Ce3+/Tb3+/Mn2+ dopants on phase purity and luminescence properties were investigated by XRD, diffuse reflectance spectra

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(DRS) and photoluminescence (PL) spectra. 2. Experimental

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The Ca9(1-y)La(1-x-z)(PO4)5SiO4F2:xCe3+,yMn2+,zTb3+ (CLPSF:xCe3+,yMn2+,zTb3+,

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0 ≤ x ≤ 5%, 0 ≤ y ≤ 4%, 0 ≤ z ≤ 7%) samples were synthesized by high-temperature

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solid-state reaction method. The starting materials included CaCO3 (99%),

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(NH4)2HPO4 (99%), SiO2 (99%), NH4HF2 (99%), MnCO3 (99%), La2O3 (99.99%),

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CeO2 (99.99%) and Tb4O7 (99.99%). Stoichiometric amounts of the above materials were mixed and ground together in an agate mortar. The reactant mixture was then

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fired at 1300 °C for 5 h.

The phase purity was determined by using an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 35 mA. The morphology was examined by field emission scanning electron microscopy (FESEM, FEI, Quanta FEG, operated at 5.0 kV). DRS were measured by a UV/visible spectrophotometer (UV-3600, SHIMADZU) using BaSO4 as a reference in the wavelength region of 200-700 nm. The PL spectra were recorded on an EI-FS5 fluorescence spectrophotometer. The temperature dependent measurement was also carried out by the above spectrophotometer. The samples were mounted on a heating

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device whose temperature can vary from room temperature to 575 K. The sample was kept to stay for one minute before recording the emission spectra for every temperature. 3. Results and discussion 3.1 XRD, morphology and DRS analysis As shown in Fig. 1(a), the crystal phases of as-prepared CLPSF:Ce3+,

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CLPSF:Ce3+,Tb3+, CLPSF:Ce3+,Mn2+ and CLPSF:Ce3+,Tb3+,Mn2+ phosphors were

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investigated by XRD patterns. All of the diffraction peaks are agreement with the

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standard JCPDF card (no. 33-0287), and no other impurity was observed, indicating

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that the luminescent ions are doped into the CLPSF host lattices successfully. To

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check the morphology characteristics of the as-prepared samples, Fig. 1(b) shows the

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SEM image of the typical CLPSF:Ce3+ phosphor. It can be found that the particle sizes are non-uniform and some particles are agglomerated. The average particle scale

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was obtained to be about 4.3 μm from the particle size distribution that was taken from statistical analysis for approximately 150 particles. To study the optical behavior of the samples, the DRS of the typical CLPSF, CLPSF:Ce3+, CLPSF:Ce3+,Mn2+ and CLPSF:Ce3+,Tb3+ phosphors were measured, as shown in Fig. 2. For the CLPSF host, very high reflection can be seen in the visible region. In the ultraviolet (UV) range, strong absorption band appear, which can be attributed to the electron transition from valence band to conduction band. When the dopant ions were introduced, new absorption bands are observed. The absorption bands around 255 and 320 nm are mainly assigned to the 4f-5d transition of Ce3+ and

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the enhanced absorption band between 200 and 270 nm is mainly owing to the 4f-4f5d transition of Tb3+, which are in agreement with the discussion for the excitation spectra below. 3.2 PL properties of Ce3+/Tb3+Mn2+ doped CLPSF In order to understand the PL properties of different Ce3+ doping contents, the

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emission spectra of the CLPSF:xCe3+ (1% ≤ x ≤ 5%) phosphors under the excitation of 300 nm are shown in Fig. 3(a). All the spectra exhibit intense emission bands from

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300 to 550 nm, which can be ascribed to the 4f05d1-4f1 transition of Ce3+ ions. The

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emission shows an initial enhancement with increasing Ce3+ concentration till x = 2%.

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But further increasing the Ce3+ concentration will cause a decrease of emission

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intensity due to the concentration quenching effect. This is because that the distance

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between Ce3+ ions is shortened with the increasing of Ce3+ concentration, resulting in an increase in re-absorption or cross-relaxation probability, which occurs mainly in

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the form of non-radiative transitions. The inset of Figure 3(a) presents the normalized emission spectra of CLPSF:xCe3+ (1% ≤ x ≤ 5%) phosphors. It is obviously observed that the emission position exhibits a continuous red-shift with the increase of the Ce3+ concentration. The possible reason can be explained below. As reported in Ref. [26], there are two La3+ sites in the CLPSF compound, which are nine- and seven-fold coordinated by oxygen atoms (marked as La(1) and La(2), respectively). The crystal-field strengths around the two La3+ sites are different, which can be described by the following equation [27]

Dq 

1 2 r4 Ze 5 6 R

(1)

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where Dq corresponds to the energy level separation, Z is the valence of the anion ligand, e is the electron charge, r is the radius of the d wavefunction and R is the bond length. From Dq  R5 , it can be understood that even a small change in the bond length would largely affect the observed emission energy. Thus, the different R values for La(1) and La(2) cause the Ce3+ emission bands in different wavelengths. When the Ce3+ ions of small concentration are doped, they are supposed to dominantly

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substitute one kind of La2+ site in which the Ce3+ emits higher-energy photons. At a

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higher Ce3+ concentration, the other La2+ sites are occupied by Ce3+ gradually, which

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causes the continuous red-shift of the emission band. The emitting-light color of the

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Ce3+-activated CLPSF was evaluated by using the Commission International

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del’Eclairage (CIE) chromaticity coordinates. As indicated in Fig. 4 (see Point 1), the

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CLPSF:2%Ce3+ phosphor demonstrates a blue emission with the chromaticity coordinates of (0.138, 0.104). The excitation spectrum of the typical CLPSF:2%Ce3+

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by monitoring 361 nm is given in Fig. 3(b), which shows an broad excitation band with a maximum intensity at 300 nm. The strong excitation band can be assigned to the 4f1-4f05d1 transition of Ce3+ [28]. The Mn2+-activated CLPSF phosphor was prepared, and the luminescence spectra are shown in Fig. S1. By excited at 408 nm, a broad red emission band can be found, which is assigned to the 4T1-6A1 transition of Mn2+ [29]. The CIE chromaticity coordinates were calculated to be (0.542, 0.454), indicating a brown emission (see Point 2 in Fig. 4). When monitored at 595 nm, several excitation peaks appear. The dominant peak at 408 nm is attributed to the 6A1(6S)-[4A1(4G),4E(4G)] transition of

Journal Pre-proof Mn2+, other relatively weak excitation peaks around 313, 346 and 474 nm can be ascribed to the Mn2+ transitions from the ground state 6A1(6S) to 4T1(4P), 4E(4D) and 4

T1(4G), respectively [30]. To observe the energy transfer (ET) from Ce3+ to Mn2+, the

emission spectra of the CLPSF:2%Ce3+,yMn2+ (0 ≤ y ≤ 4%) phosphors excited at 300 nm were investigated, as shown in Fig. 5(a). Two strongest emission bands appear. The broad blue band is attributed to the 4f05d1-4f1 transition of Ce3+ and the orange

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emission band in the range of 525-700 nm corresponds to the 4T1-6A1 transition of

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Mn2+. It is noteworthy that the emission intensity of Ce3+ decreases remarkably with

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increasing Mn2+ concentration, while that of Mn2+ exhibits different change. This may

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be due to the efficient ET from Ce3+ to Mn2+. To further interpret this, a series of

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decay curves of CLPSF:2%Ce3+,yMn2+ (0 ≤ y ≤ 4%) phosphors with 300 nm

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excitation and 361 nm emission are shown in Figure 5(b). All the decay curves can be fitted by a double-exponential function based on the following equation [31]

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I  A1 exp(t / 1 )  A2 exp(t /  2 )

(2)

where τ1 and τ2 are the lifetimes for fast and slow exponential components, A1 and A2 are fitting constants, and I is the luminescence intensity. The corresponding τi and Ai values are given in Table 1. The average decay lifetime can be calculated by using the following formula [32]     ( A1 12  A2 22 ) / ( A11  A2 2 )

(3)

The calculated τ values for different Ce3+ concentrations are also shown in Table 1. We can see visually that the Ce3+ lifetime reduces gradually with increasing Ce3+ concentration, which could be due to the ET between Ce3+ and Mn2+. This ET can be

Journal Pre-proof also witnessed from the excitation spectra of the CLPSF:2%Ce3+,2%Mn2+ phosphor by monitoring the Ce3+ (361 nm) and Tb3+ (590 nm) emissions, as shown in Fig. 5(c). By monitoring both wavelengths, the broad excitation bands in the UV region are mainly attributed to the 4f-5d transition of Ce3+. This observation indicates the ET from Ce3+ to Mn2+. Besides, a very weak excitation peak at 407 nm is also found, which can be assigned to the 6A1(6S)-[4A1(4G),4E(4G)] transition of Mn2+. To study the

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ET mechanism from Ce3+ to Mn2+ in the CLPSF host, a modified equation was used,

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I S0  C / 3 IS

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based on Dexter’s ET expressions of multipolar interaction [33]:

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(4)

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where I0 and I are the emission intensity of Ce3+ in the absence and presence of Mn2+; C is the Mn2+ concentration. The values of n = 6, 8 and 10 correspond to dipole-dipole,

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dipole-quadrupole and quadrupole-quadrupole interactions, respectively. This

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equation has been widely used to study the multipolar interactions of luminescent ions in the nano- or micro-sized phosphors [34,35]. The relationships between I0/I and Cn/3 are illustrated in Fig. 5(d). It can be noticed that the linear relationship is obtained only when n = 6, indicating the ET between Ce3+ and Mn2+ occurs based on the dipole-dipole mechanism. Owing to this ET, the emission color of the phosphors can be tuned conveniently. From Fig. 4, it can be seen the emitting-light-color has been adjusted from blue to light brown with increasing Mn2+ concentration, and the warm white emission (0.425, 0.351) can be gained in the CLPSF:2%Ce3+,2%Mn2+ sample. In the CLPSF-based system, the luminescence characteristics of Tb3+ were also studied. Fig. S2 shows the spectra of the CLPSF:3%Tb3+ phosphor. Upon 377 nm

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excitation, several emission peaks can be found in the visible region from 400 to 650 nm, which are attributed to the 5D3,4-7FJ (J = 3-6) transitions [36]. By monitoring 541 nm, a very strong emission peak around 231 nm could be assigned to the 4f-4f5d transition of Tb3+. Moreover, the weak 4f-4f transitions of Tb3+ can be also observed from the enlarged spectrum. Fig. 6(a) depicts the emission spectra of the CLPSF:2%Ce3+,zTb3+ (0 ≤ z ≤ 4%) phosphors upon the excitation of 300 nm. It can

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be seen that the emission intensity of Ce3+ decreases monotonically with the increase

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of Tb3+ concentration, which supports the result of ET from Ce3+ to Tb3+. Meanwhile,

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the emission intensity of Tb3+ is strengthened first and reaches its maximum value for

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z = 3%, and then decreases due to the concentration quenching effect. Fig. 6(b)

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represents the excitation spectra of the typical CLPSF:2%Ce3+,3%Tb3+ sample by

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monitoring 361 and 541 nm. It is obvious that the two excitation spectra exhibit similar profiles from 250 to 350 nm, which is owing to the ET between Ce3+ and Tb3+.

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From the enlarged excitation spectrum monitored at 541 nm, weak excitation peaks belonging to 4f-4f transitions of Tb3+ appear. Fig. 6(c) presents the decay curves of the CLPSF:2%Ce3+,zTb3+ (0 ≤ z ≤ 7%) samples by exciting at 300 nm and monitoring 461 nm, which can be fitted by Eq. (2). The corresponding τi and Ai values are summarized in Table 2. According to Eq. (3), the average decay lifetimes were calculated, as shown in Table 2. Obviously, the decay lifetimes of Ce3+ decrease gradually with increasing Tb3+ concentration, which is an evidence for the ET occurrences between Ce3+ and Tb3+. Based on this ET, the adjustment of the emitting-light-color has been identified, that is, the chromaticity coordinates of

Journal Pre-proof CLPSF:2%Ce3+,zTb3+ can be adjustable from blue (0.183, 0.104) to green (0.315, 0.574) through controlling the Tb3+ doped content (see Fig. 4). To achieve tunable CCT for the white emission, the Ce3+-Tb3+-Mn2+ tridoped samples

were

designed.

Fig.

7(a)

shows

the

PL

spectra

of

the

CLPSF:2%Ce3+,1%Mn2+,zTb3+ (0 ≤ z ≤ 2%) phosphors under 300 nm excitation. The broad emission bands located at about 361 and 595 nm can be seen, which results

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from the transitions of 4f05d1-4f1 (Ce3+) and 4T1-6A1 (Mn2+), respectively. Besides,

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four obvious emission peaks also come out, which are derived from the 5D4-7FJ

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transitions (Tb3+). On the other hand, the emission intensity of Ce3+ remarkably

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decreases with increasing Tb3+ concentration, but the Tb3+ emission exhibits

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continuous enhancement. This result indicates the existence of ET from Ce3+ to Tb3+,

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which is in agreement with the above discussion. The CIE chromaticity coordinates of CLPSF:2%Ce3+,1%Mn2+,zTb3+ were calculated to be (0.361, 0.370), (0.371, 0.424)

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and (0.370, 0.475) for z = 0.5%, 1% and 2%, respectively. White emission can be gained for z = 0.5%, and the corresponding CCT was calculated to be 4519 K, which is very different from (2601 K) for the white emitting CLPSF:2%Ce3+,2%Mn2+. Therefore, this kind of phosphor may be used as a tunable luminescence material to promise application in white LEDs. Fig. 7(b) depicts the excitation spectrum of the CLPSF:2%Ce3+,1%,0.5%Tb3+ sample monitored at 540 nm. The broad excitation band from 250 to 350 nm is attributed to the 4f1-4f05d1 transition of Ce3+, which also reveals an ET from Ce3+ to Tb3+. The strong excitation peak below 250 nm is mainly ascribed to the 4f-4f5d transition of Tb3+, which is similar to the excitation spectrum

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in Fig. S2. From the enlarged excitation spectrum in Fig. 7(b), it can be found that both the Tb3+ and Mn2+ transitions also appear, which is in agreement with the above discussion. 3.3 Potential applications of Ce3+/Tb3+Mn2+ doped CLPSF For the optical thermometer, the FIR technique was used in the Ce3+-Mn2+ doped

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CLPSF system. Compared with the upconversion luminescent materials for which the excitation light sources are usually lasers, the downconversion phosphors in this work

ro

are excited by xenon lamp whose power density is much lower than the lasers. Thus,

Fig.

8(a)

and

(b)

show

re

phosphors.

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the heating effects of the excitation power densities were not considered in the present the

emission

spectra

of

the

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CLPSF:2%Ce3+,2%Mn2+ and CLPSF:2%Ce3+,4%Mn2+ phosphors excited at 300 nm

na

under different temperatures. When the temperature rises, the emission intensities of both Ce3+ and Mn2+ decrease gradually. This is because that the interaction of

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electron-phonon has been enhanced when the phosphor suffers from a high temperature, which can lead to the increasing non-radiative transition. However, the Mn2+ demonstrates a faster decrease rate in intensity compared with Ce3+, which can be found from the normalized (for 361 nm) emission spectra in Fig. S3(a) and (b). Based on this, the FIR for the Ce3+ and Mn2+ emissions will change with temperature. The dependences of ICe/IMn on absolute temperature for the CLPSF:2%Ce3+,2%Mn2+ and CLPSF:2%Ce3+,4%Mn2+ samples are depicted in Fig. 8(c) and (d), respectively. With rising temperature, the FIR increases continuously and the experimental data can be well fitted by a single-exponential function, as described in the figures. According

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to Refs. [37,38], the temperature sensor sensitivities, including absolute sensitivity (SA) and relative sensitivity (SR), are two important factors for the practical application. They can be written as follows:

SR 

d ( R) d (T )

(5)

1 d ( R) R d (T )

(6)

of

SA 

where R represents the FIR and T is absolute temperature. The absolute sensitivities as function

of

temperature

for

the

CLPSF:2%Ce3+,2%Mn2+

ro

a

and

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CLPSF:2%Ce3+,4%Mn2+ samples are depicted in Fig. 8(e). Both the phosphors show

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increasing SA values with rising temperature, which reach 8. 05  103 and

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6. 86  103 K-1 at 523 K for y = 2% and 4%, respectively. These SA values are much

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larger than those of the temperature-sensing luminescent materials doped with the most used Er3+, such as Ba5Gd8Zn4O21:Er3+-Yb3+ ( 3. 2  10- 3 K-1 at 490 K) and

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Ba9Y2Si6O24:Yb3+-Er3+ ( 5. 6  10- 3 K-1 at 543 K) [39,40]. Besides, it is noteworthy that the absolute sensitivity for y = 2% is remarkably larger than that for y = 4% at any temperature. Fig. 8(f) presents the relative sensitivities as a function of temperature for the above samples. Similarly, the SR values for y = 2% and 4% increase with rising temperature and reach 0.17% and 0.28% K-1, respectively. It also can be seen that the CLPSF:2%Ce3+,4%Mn2+ sample displays a much larger SR value than the CLPSF:2%Ce3+,2%Mn2+, which is opposite with the case of absolute sensitivities. Generally, the thermal stability of phosphors plays an important role for LED

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application. To evaluate the thermal quenching properties of the BYSO-based phosphors,

the

temperature-dependent

luminescence

of

the

typical

CLPSF:2%Ce3+,3%Tb3+ sample was recorded in Fig. 9. With the increase of temperature, the emission spectra profiles change little, but the integrated intensity of all the emission peaks declines gradually, which can be understood from the inset of

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Fig. 9. At 523 K, the brightness is decreased to about 80% of that for initial 303 K. The thermally luminescent stability of this phosphor is higher than many reported

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luminescent materials, such as the commercial YAG:Ce3+ [41]. For the white-emitting

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Ce3+-Tb3+-Mn2+ tri-doped sample, Fig. 10 exhibits the emission spectra of

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CLPSF:2%Ce3+,3%Tb3+,0.5%Mn2+ excited at 300 nm under various temperatures.

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Similar to that in Fig. 9, the emission intensities of all the emission peaks also exhibit

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continuous decrease with rising temperature. From the dependence of the integrated intensity on temperature in the inset, it can be seen that the brightness at 523 K is

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about 60% as strong as that for the initial value. Hence, this white-emitting phosphor shows a relatively good thermal stability. Moreover, the relative emission intensities of different luminescent ions exhibit slightly different decrease rates with increasing temperature. This point can be understood from the normalized emission spectra, as shown in Fig. S4(a). It reveals that the Tb3+ emission intensity displays a slowest decrease but the Mn2+ emission intensity shows the fastest. In this case, the emitting-light-color will change a little with temperature, which can be understood from the CIE chromaticity diagram in Fig. S4(b). 4. Conclusions

Journal Pre-proof In summary, a series of CLPSF:Ce3+/Tb3+/Mn2+ phosphors were successfully synthesized by a solid-state reaction method. The Ce3+-activated CLPSF shows broad emission band from 320 to 470 nm, and the optimal Ce3+ doping content was determined to be 2 mol%. When the Ce3+-Mn2+ ions are codoped into the CLPSF host, an ET was observed, due to which the emitting-light color can be tuned conveniently.

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The corresponding ET mechanism was also studied. By co-doping Ce3+ and Tb3+ ions, the emission color of the samples has been adjusted from blue to green with

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increasing Tb3+ concentration, owing to the ET role. The white-light-emission can be

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achieved by tri-doping Ce3+-Tb3+-Mn2+ ions. The investigations on thermally

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luminescent property of Ce3+-Mn2+ co-doped CLPSF reveal that the emission

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intensities of the two ions have different decrease rates with rising temperature. Good

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thermal stability of luminescence can be obtained in the CLPSF:2%Ce3+,3%Tb3+ and CLPSF:2%Ce3+,3%Tb3+,0.5%Mn2+ phosphors. On the basis of the above results, the

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CLPSF:Ce3+/Tb3+/Mn2+ phosphors could be considered for potential applications in LEDs and optical thermometer. Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51602117 and 51702378).

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Opt. Mater. 49 (2015) 266-270. [3] L.L. Feng, F. He, B. Liu, G.X. Yang, S.L. Gai, P.P. Yang, C.X. Li, Y.L. Dai, R.C.

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Lv, J. Lin, gC3N4 coated upconversion nanoparticles for 808 nm near-infrared light

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Table 1 τ2 (µs) 29.8 29.2 28.3 27.7 27.0

A1 10980.4 6895.3 5443.7 4309.9 4269.4

A2 13664.8 8903.0 6903.1 6448.9 5356.2

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τ1 (µs) 12.6 12.0 10.5 9.8 8.8

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y 0 1% 2% 3% 4%

τ (µs) 25.4 25.0 24.3 24.3 23.2

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Table 2 τ2 (µs) 29.8 29.2 28.7 27.2 26.9

A1 10980.4 11213.5 8674.2 8060.7 5939.0

A2 13664.8 11389.7 7562.7 5842.1 3939.6

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τ1 (µs) 12.6 12.4 12.2 10.8 10.6

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z 0 1% 3% 5% 7%

τ (µs) 25.4 24.2 23.3 21.4 20.8

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Table and figure captions Table 1 τi and Ai (i = 1, 2) values of CLPSF:2%Ce3+,yMn2+ (0 ≤ y ≤ 4%) with 300 nm excitation and 461 nm emission Table 2 τi and Ai (i = 1, 2) values of CLPSF:2%Ce3+,zTb3+ (0 ≤ z ≤ 7%) with 300 nm excitation and 461 nm emission Fig. 1 (a) XRD patterns of CLPSF:Ce3+, CLPSF:Ce3+,Tb3+, CLPSF:Ce3+,Mn2+ and

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CLPSF:Ce3+,Tb3+,Mn2+; (b) SEM image of CLPSF:Ce3+

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Fig. 2 DRS of CLPSF, CLPSF:Ce3+, CLPSF:Ce3+,Mn2+ and CLPSF:Ce3+,Tb3+

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Fig. 3 (a) Emission spectra of CLPSF:xCe3+ (1% ≤ x ≤ 5%), inset shows the

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normalized spectra; (b) excitation spectrum of CLPSF:2%Ce3+

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Fig. 4 CIE chromaticity diagram of CLPSF:2%Ce3+ (Point 1), CLPSF:2%Mn2+ (Point

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2), CLPSF:2%Ce3+,yMn2+ (1% ≤ y ≤ 4%) (Points 3-6 are for y = 0, 1%, 2%, 3% and 4%, respectively), CLPSF:2%Ce3+,zTb3+ (Points 7-10 are for z = 1%, 3%, 5% and 7%,

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respectively) and CLPSF:2%Ce3+,1%Mn2+,zTb3+ (Points 11-13 are for z = 0.5%, 1% and 2%, respectively)

Fig. 5 (a) Emission spectra of CLPSF:2%Ce3+,yMn2+ (0 ≤ y ≤ 4%), inset shows the emission intensities of Ce3+ and Mn2+ as a function of Mn2+ concentration; (b) decay curves of CLPSF:2%Ce3+,yMn2+ (0 ≤ y ≤ 4%); (c) excitation spectra of CLPSF:2%Ce3+,2%Mn2+ by monitoring 361 and 590 nm; (d) dependences of IS0/IS of Ce3+ on CMn2+6/3, CMn2+8/3 and CMn2+10/3 Fig. 6 (a) Emission spectra of CLPSF:2%Ce3+,zTb3+ (0 ≤ z ≤ 7%); (b) excitation spectra of CLPSF:2%Ce3+,3%Tb3+ by monitoring 361 and 541 nm; (c) decay curves

Journal Pre-proof of CLPSF:2%Ce3+,zTb3+ (0 ≤ z ≤ 7%) Fig. 7 (a) Emission spectra of CLPSF:2%Ce2+,1%Mn2+,zTb3+ (0 ≤ z ≤ 2%); (b) excitation spectrum of CLPSF:2%Ce2+,1%Mn2+,0.5%Tb3+ Fig.

8

Emission

spectra

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(a)

CLPSF:2%Ce3+,2%Mn2+

and

(b)

CLPSF:2%Ce3+,4%Mn2+ under various temperatures; dependences of ICe/IMn on

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temperature for (c) CLPSF:2%Ce3+,2%Mn2+ and (d) CLPSF:2%Ce3+,4%Mn2+; (e) absolute and (f) relative sensitivities as a function of temperature for

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CLPSF:2%Ce3+,2%Mn2+ (y = 2% and 4%)

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Fig. 9 Emission spectra of CLPSF:2%Ce3+,3%Tb3+ excited at 300 nm under various

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temperatures, inset shows the dependence of the integrated intensity on temperature

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Fig. 10 Emission spectra of CLPSF:2%Ce3+,3%Tb3+,0.5%Mn2+ excited at 300 nm

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temperature

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under various temperatures, inset shows the dependence of the integrated intensity on

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Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a

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conflict of interest in connection with the work submitted.

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Author Contribution Jia Zhang analyzed the experimental data and wrote the paper. Yurong Shi carried out the spectral measurement.

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Songsong An prepared the samples.

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Graphical abstract

Highlights

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> Bifunctional Ca9La(PO4)5SiO4F2:Ce3+/Tb3+/Mn2+ phosphors were designed in this work.

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> High sensor sensitivities were obtained in the Ce3+-Mn2+ codoped phosphors.

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> High thermal stability was obtained in Ce3+-Tb3+ and Ce3+-Tb3+-Mn2+ doped

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phosphors.

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thermometer.

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> The present phosphors have potential applications in LEDs and optical

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