A novel red long lasting phosphorescent (LLP) material

b-Zn3(PO4)2:Mn2+, Sm3+

Jing Wang a, Qiang Su a,b,*, Shubin Wang a

a Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, PR Chinab State Key Laboratory of Optoelectronic Materials and Technology, School of Chemistry and

Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China

Received 7 September 2003; received in revised form 25 September 2004; accepted 26 January 2005

Abstract

A novel red long lasting phosphorescent materials b-Zn3(PO4)2:Mn2+,Sm3+ is firstly synthesized by high-

temperature solid-state reaction. The influence of Sm3+ ions on luminescence and long lasting phosphorescence

properties of Mn2+ in phosphor b-Zn3(PO4)2:Mn2+,Sm3+ are systematically investigated. It is found that the red

phosphorescence (l = 616 nm) performance of Mn2+ ion such as brightness and duration is largely improved when

Sm3+ ion is co-doped into the matrix in which Mn2+ ion acts as luminescent center and Sm3+ ion plays an important

role of electron trap. Thermoluminescence spectrums show that there exists one peak in b-Zn3(PO4)2:Mn2+,Sm3+,

the depth of which is 0.33 eV, and that there are three peaks in b-Zn3(PO4)2:Mn2+, among which the depth of the

lowest temperature peak in b-Zn3(PO4)2:Mn2+ is 0.37 eV. Such differences in the trap depth result in the

improvement of red long lasting phosphorescence of Mn2+ in present matrix.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Optical materials; D. Luminescence; D. Defects

1. Introduction

Since SrAl2O4:Eu2+,Dy3+ with high LLP performance was reported in 1996s by T. Matsuzawa, LLP

materials have attracted much attention because they have large practical and potential applications in

www.elsevier.com/locate/matresbu

Materials Research Bulletin 40 (2005) 590–598

* Corresponding author. Tel.: +86 431 5262208; fax: +86 431 5698041.

E-mail address: cessuq@zsu.edu.cn (Q. Su).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.01.011

many fields, e.g., lighting, display, detection of high energy rays such as UV, X-ray, b-ray, etc, and

multidimensional optical memory and imaging storage [1–5].

Among them, research interests worldwide have been mainly focused on Eu2+ and Dy3+ co-doped

alkali earth aluminates, nowadays well known as commercial blue and green LLP materials. Further-

more, many other efforts have been made to explore more novel systems in which LLP phenomenon

could be observed, and new activators that could demonstrate LLP phenomenon. Nowadays, there are

many compounds in which LLP phenomenon is observed, including aluminates (MAl2O4 (M = Ca, Sr,

Mg and Ba), Sr4Al14O25, BaMgAl10O17, CaYAl3O7 and CaGdAl3O7) [6–10], silicates (Sr2MgSi2O7,

R3MgSi2O8 (M = Ca, Sr and Ba) and MgSiO3) [11–13], aluminosilicates (CaSrAl2SiO7 and Ca2Al2SiO7)

[14], sulfides (ZnS, CaS and Ln2O2S (Ln = Yand Gd)) [15–17] and titanate (CaTiO3) [18], etc. And there

are plenty of activators that can demonstrate LLP phenomenon, including Ce [10,11], Eu [6–13], Mn

[13], Cu [16], Bi [16], Ag [16], Pr [18], and Tb [19]. One important feature of all these activators is that

their valence can be changed [2]. Nowadays, Ce, Pr, Nd, Tb and Dy among trivalent rare earth ions can be

experimentally converted in tetravalent form. The trend of valence changing makes these ions expected

as efficient hole-traps. Among them, Nd3+ and Dy3+ ions are actually proved to be efficient hole-traps in

commercial LLP materials, i.e., CaAl2O4:Eu2+,Nd3+ and SrAl2O4:Eu2+,Dy3+ by measurements of Hall

effect [1,20]. Even though many new LLP materials are developed, the red LLP phenomenon especially

activated by Mn2+ ion remains to be explored.

In this paper, we first report the red LLP (l = 616 nm) phenomenon in phosphate matrix of

b-Zn3(PO4)2:Mn2+,Sm3+ in which Sm3+ ion plays a significant role in providing the proper trap levels.

2. Experimental section

2.1. Preparation

Powder samples were synthesized by high-temperature solid-state reaction. The raw materials were

ZnO (A.G.), (NH4)2HPO4 (A.G.), MnCO3 (A.G.) and Sm2O3 (4N). The concentrations of Mn2+ and Sm3+

ions were, respectively, fixed to 2 mol% of the Zn ions in 3ZnO�P2O5. The mixtures of corresponding raw

materials were thoroughly grounded and then fired at 500 8C for 3 h. After reground, they were sintered at

950 8C in reducing atmospheres for 5 h.

2.2. Characterization

The structure of all samples was analyzed by Rigaku D/max-IIB X-ray powder diffractometer using

Cu Ka1 (l = 1.5405 A) radiation and was coincident with b-Zn3(PO4)2 (JCPDS: 30-1489). Photo-

luminescence (PL) experiments of samples were performed on a HITACHI F-4500 Spectrofluorometer

equipped with monochromator (resolution: 0.2 nm) and 150 W Xe lamp. The LLP emission spectrums

and decay curves of the samples were measured as follows: immediately after irradiated by UV lamp

peaking at 254 nm with a power of 4.07 mW cm�2 for 5 min, the signal was recorded by the

photomultiplier of HITACHI F-4500 Spectrofluorometer. Thermoluminescence (TL) measurements

were done using FJ-427A thermoluminescence-meter (Beijing Nuclear Instrument Factory) at the

temperature range from 273 to 773 K using heating rate of 3, 4 and 5 K s�1.

All measurements except TL spectrum were performed at room temperature.

J. Wang et al. / Materials Research Bulletin 40 (2005) 590–598 591

3. Results and discussion

3.1. Luminescent properties of Zn3(PO4)2:Mn2+0.02 and Zn3(PO4)2:Mn2+

0.02,Sm3+0.02

Excitation and emission spectrums of powder samples are shown in Fig. 1. It is clearly observed that

Mn2+ and Sm3+ co-doped sample shows the same luminescent properties as Mn2+-doped sample except

the luminescent intensity.

Monitored at 616 nm, one broad optical transition band is predominating at 240 nm and assigned to the

charge transfer band (CTB) of Mn2+–O2� [21], but not to the host absorption because the edge of the

optical absorption band for Zn3(PO4)2 is situated at �6.9 eV (180 nm) beyond the range of detection of

our instruments [22]. In the range from 300 to 580 nm, there are more complex bands with lower

intensity, which is attributed to the d–d forbidden transitions of Mn2+. It is well known that the4Eg-4A1g(4G) levels are commonly used as criterion for assignments of Mn2+ ion since they are relatively

less influenced than the other levels by ligand field according to Orgel’s diagram [23]. In present work,

three sharp bands at the wavelength from 400 to 430 nm are assigned to the splitting 4Eg-4A1g(4G) levels.

Then two weak bands at 300–340 nm are components of the 4Eg(4D) levels, and other three bands in the

region from 340 to 380 nm are ascribed to the splitting 4T2g(4D) levels. Finally, three broad bands are

resolved in the region from 430 to 580 nm and assigned as components of the 4T2g(4G), 4T1g(4G) levels.

These designations mentioned above are mainly coincident with those reported by Palumbo and Brown

[24].

In the emission spectrum shown in Fig. 1, one broad band is centered at 616 nm, responding to the

characteristic excitation band of Mn2+ at 418 nm. Under the UV light irradiation, Mn2+ ion is usually

characterized by emitting green or orange-to-red light. It consists of a d–d broad band corresponding to

the transition of Mn2+ from the excited 4T1g state to the ground 6A1g state level. The emission color is

strongly dependent on the coordination environment of Mn2+ in the host lattice, such as coordination

J. Wang et al. / Materials Research Bulletin 40 (2005) 590–598592

Fig. 1. Excitation (Em = 616 nm) and emission (Ex = 418 nm) spectrums of Zn3(PO4)2:Mn2+0.02 (solid line) and

Zn3(PO4)2:Mn2+0.02,Sm3+

0.02 (dash-dot line).

number (CN). The Mn2+ ion emits green light when it is tetrahedrally coordinated (CN = 4) in the lattice

whereas it emits red light in octahedral coordination (CN = 6) [25]. In b-Zn3(PO4)2, there are three non-

equivalent cation sites, i.e., Zn(1), Zn(2) and Zn(3). Zn(1) and Zn(3) are bonded to four and five oxygen

atoms, respectively, whereas Zn(2) is strongly bonded to five oxygen atoms and weakly bonded to

additional oxygen atom O(6) [26]. Therefore, one broad emission band at 616 nm, responding to the

characteristic excitation band of Mn2+ at 418 nm, is assigned to 4T1g(4G) ! 6A1g(6S) transition of Mn2+

in the Zn(2) site with octahedral coordination (CN = 6).

From careful comparison of excitation and emission spectrums between the samples doped with and

without Sm3+ ion, it is observed that introduction of Sm3+ ion decreases the emission intensity of Mn2+ in

b-Zn3(PO4)2. Two reasons can reasonably explain the decrease in emission intensity of Mn2+. On the one

hand, Sm3+ ion competes against Mn2+ ion for the excitation energy, and subsequently emits through its

radiation transitions. On the other hand, Sm3+ ion acts as the electron trap that firstly captures energy

from Mn2+, then stores it in traps, and finally transfer it back to Mn2+ under the thermal stimulation at

room temperature. It is well known that the characteristic 4f ! 4f transition bands of Sm3+ ion mainly

consist of three sharp lines attributed to the 4G5/2 ! 6H5/2, 6H7/2 and 6H9/2 transitions [27]. But in present

sample no characteristic excitation and emission lines of Sm3+ ion are observed. Thus, it is obvious that

Sm3+ ions play a role of electron traps in b-Zn3(PO4)2Mn2+,Sm3+. The following results will largely

support this assignment.

After turnoff and removal of UV lamp used for excitation, the red LLP emission of Mn2+ ion is

clearly observed by naked eyes. Fig. 2 shows LLP emission spectrums of Zn3(PO4)2Mn2+0.02,Sm3+

0.02

within 300 s. The LLP emission is situated at 616 nm, which is completely coincident with PL

emission under steady excitation as shown in Fig. 1. The position of LLP emission does not change as

time passes. The results mentioned above indicate that LLP emission is attributed to the d–d transition

of Mn2+ from the excited state 4T1g(4G) to the ground state 6A1g(6S) level at the site of Zn(2) in present

matrix.

J. Wang et al. / Materials Research Bulletin 40 (2005) 590–598 593

Fig. 2. LLP emission spectrums of the sample Zn2(PO4)3:Mn2+0.02,Sm3+

0.02 at different times after turnoff of irradiation source.

Without co-doping Sm3+ ions, the phosphor Zn3(PO4)2:Mn2+ can also show weak red LLP. Never-

theless, with introduction of Sm3+ ions, such red LLP emission can be visible for about 2 h in the limit of

light perception for naked eyes (0.32 mCd m�2). As shown in Fig. 3, co-doping with Sm3+ ions largely

improves the LLP performance of Mn2+ such as brightness and duration. These results indicate and

confirm our previous assignment that Sm3+ ions act as electron trap. This assignment explains the

decrease in the emission intensity of Mn2+ under steady excitation, and simultaneously accounts for the

increase in the LLP emission intensity of Mn2+ in Zn3(PO4)2:Mn2+,Sm3+ as shown in Fig. 1 and Fig. 3. In

addition, these decay curves shown in Fig. 3 can be roughly divided into two parts, i.e., the fast process

and slow process.

3.2. Defects properties of b-Zn3(PO4)2:Mn2+0.02 and Zn3(PO4)2:Mn2+

0.02, Sm3+0.02

The glow curves of Zn3(PO4)2:Mn2+0.02 and Zn3(PO4)2:Mn2+

0.02,Sm3+0.02 are shown in Fig. 4. For

Zn3(PO4)2:Mn2+0.02, three peaks are clearly observed, which are denoted as defects A–C. Defect A is

predominating at 410 K. The defects B and C are situated at 469 and 528 K, respectively. These peaks

indicate that there are mainly three kinds of traps with different depths in Zn3(PO4)2:Mn2+, which may be

due to intrinsic defects in the matrix [28]. With the introduction of Sm3+ ions, a novel peak is

predominating at 383 K and suppresses those peaks related to intrinsic defects in higher temperature

region. Therefore, this new peak at 383 K, we suppose, is related to Sm3+ ions.

In the present sample, Sm3+ ion is expected as an electron trap since it has the trend to attract electron

to be converted into divalent form. Actually, the similar assignment of the role of Sm3+ is also supposed in

the field of photo-stimulated luminescence (PSL), in the process of which electron-transferring from

activators to traps (Sm) is called the ‘‘write’’ process and the reverse transferring called the ‘‘read’’

process [29].

J. Wang et al. / Materials Research Bulletin 40 (2005) 590–598594

Fig. 3. Decay curves of Zn3(PO4)2:Mn2+0.02 (solid line) and Zn3(PO4)2:Mn2+

0.02,Sm3+0.02 (dash-dot line) monitored at 616 nm

immediately after the removal of UV (254 nm, 4.07 mW cm�2) irradiation.

Based on the one-trap/one-center model, Levy developed the ‘‘General One-Trap’’ (GOT) expression

for TL emission

ITL ¼ ns exp � Et

kT

� �1 � ðN � nÞsn

ðN � nÞsn þ msmn

� �(1)

where n is the concentration of electrons filled in the discrete trap, and m is the concentration of the holes

in the discrete recombination center; N is the concentration of electron trap, and Et is the energy of a

discrete electron trap; s is the pre-exponential factor well known as the ‘‘attempt-to-escape’’ frequency,

and k is Boltzmann constant. And smn and sn are the recombination cross-section for the free electrons

and the capture cross-section for the retrapping of free electrons, respectively, T is temperature. The

ration msmn=ðN � nÞsn is the ratio of recombination probability to the retrapping probability. Therefore,

the term in square brackets in Eq. (1) is the probability that the thermally released electrons will not be

retrapped.

Applying the assumption that the retrapping is negligible during the thermal excitation period, i.e.,

msmn ðN � nÞsn, the GOT equation becomes

ITL ¼ ns exp � Et

kT

� �(2)

Integrating from t = 0 to t, using a constant heating rate, b = dT/dt, yielding the well-known Randall–

Wilkins first-order expression for the function ITL Tð Þ, namely

ITL ¼ n0s exp � Et

kT

� �exp � s

b

� �Z T

T0

exp �Et

ku

� �du

� �(3)

where n0 is the initial value of n at t = 0, T0 is the initial temperature and u is a dummy vari-

able representing temperature [30]. By differentiating Eq. (3) with respect to T and equating to zero,

J. Wang et al. / Materials Research Bulletin 40 (2005) 590–598 595

Fig. 4. The normalized TL spectrums of Zn3(PO4)2:Mn2+0.02 (solid line) and Zn3(PO4)2:Mn2+

0.02,Sm3+0.02 (dash-dot line) at the

temperature range from 273 to 773 K in the heating rate of 3 K s�1.

the Eq. (3) becomes

E

kT2m

¼ s

bexp � E

kTm

� �(4)

From Eq. (4), Hoogenstraaten had shown that the peak temperature Tm is related to E, and proposed a

method for measuring E by the equation

lnT2

m

b

� �¼ E

kTmþ ln

E

ks

� �(5)

Thus, the plot between ln ðT2m=bÞ against 1/Tm is linear having a slope E/k and intercept ln ðE=ksÞ. And

both E and s can be easily obtained. Apart from being simple, the method for determining the value of E

and s has the advantage of being insensitive to retrapping effects and thermal quenching. But it is just fit

for the prominent peak [31].

Therefore, we just apply the method above to the predominating peak induced by Sm3+ ion and by the

defect A, which, respectively, exists in the lower temperature region and also have the relative smaller

thermal activation energy [32]. Fig. 5 shows the relationship of ln ðT2m=bÞ versus 1/Tm. The results of Fig. 5

are summarized in Table 1. For the sample Zn3(PO4)2:Mn2+0.02, E=k ¼ 4316, and ln ðE=ksÞ ¼ 0:29.

Therefore, E = 0.37 eVand s = 3.23 103 s�1. For the sample Zn3(PO4)2:Mn2+0.02,Sm3+

0.02, E=k ¼ 3879,

and lnðE=ksÞ ¼ 0:66. Thus, E = 0.33 eV and s = 2.01 103 s�1.

Based on the results mentioned above, it can be seen that the differences of the samples doped with and

without Sm3+ ions in phosphorescence properties are directly related to the differences of these two

samples in the defects properties. When only the predominating traps induced by Sm3+ and defect A are

concerned with, the mechanism of red LLP phenomenon in Zn3(PO4)2:Mn2+, Zn3(PO4)2:Mn2+,Sm3+ can

simply interpreted as follows.

J. Wang et al. / Materials Research Bulletin 40 (2005) 590–598596

Fig. 5. The plot between ln ðT2m=bÞ against 1=Tm. (The filled square for experiment data; the solid line for deconvoluted line.)

Under UVexcitation, electrons and holes are generated. Subsequently, the holes are captured by Mn2+

ions that result in the excited state of Mn2+ ions, (Mn2+)* [2,33], and the electrons are captured at the

electron traps. Finally, electrons trapped at the defects are thermally released and recombine with

(Mn2+)* at room temperature, resulting in phenomenon of red LLP of Mn2+. In this process, the depth of

electron trap is one important feature that is responsible for the differences of red LLP performance of the

samples doped with and without Sm3+ [34]. In the sample doped with Sm3+, the depth of Sm3+-induced

trap is 0.33 eV, which is shallower than that of defect A, 0.37 eV. That is to say that the thermal activation

energy for the releasing of electrons captured at the Sm3+-induced trap is smaller than that for defect A. In

Zn3(PO4)2:Mn2+, the trap depth for defect A is deeper so that the electrons are strongly bound at the traps.

Therefore, poor red LLP performance of b-Zn3(PO4)2:Mn2+ is observed. On the contrary, the trap depth

for Sm3+-induced defect in Zn3(PO4)2:Mn2+,Sm3+ is shallower so that the electrons are relatively easier

to be released from the trap and then contribute to the red LLP performance of Mn2+. That results in the

improved red LLP performance of Mn2+.

Acknowledgement

We were very grateful to State Key Project of Basic Research (G1998061312) of China for financial

support.

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Table 1

The experiment results of thermoluminescence spectrums

(bi; Tim) E=k ln ðE=ksÞ

(b1; T1m) (b2; T

2m) (b3; T

3m)

Zn3(PO4)2:Mn2+ (3, 410) (4, 411) (5, 423) 4316 0.29

Zn3(PO4)2:Mn2+,Sm3+ (3, 383) (4, 391) (5, 400) 3879 0.66

bi (K s�1): the heating rate, Tim (K): the peak temperature of the defect A and Sm3+-induced trap in the heating rate of bi.

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