Effects of Non-4f States on Pr3+ Luminescence in Phosphors

LIU Feng, WANG Xiao-jun

(1. College of Engineering, Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA;2. Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA)*Corresponding Author, E-mail: xwang@georgiasouthern.edu



Effects of Non-4f States on Pr3+Luminescence in Phosphors

LIU Feng1, WANG Xiao-jun2*

(1.CollegeofEngineering,DepartmentofPhysicsandAstronomy,UniversityofGeorgia,Athens,GA30602,USA;2.DepartmentofPhysics,GeorgiaSouthernUniversity,Statesboro,GA30460,USA)
*CorrespondingAuthor,E-mail:xwang@georgiasouthern.edu

Different emission wavelengths of Pr3+ion, from the ultraviolet to the infrared, may result from the effects of non-4f states in phosphors. Such non-4f states generally refer to the Pr3+4f5d state, exciton-like state or charge-transfer state. Here, we present a brief review on Pr3+luminescence by the means of emission spectral measurement in several representative phosphors. Several interesting spectral phenomena are reported and reviewed, including the effects of impurity-trapped exciton state and charge-transfer state on the luminescence quenching in the Pr3+-activated phosphors. We expect this review is beneficial to the readers to better understand their experimental findings, and to inspire them to design new and improved phosphor systems.

Pr3+;1S0emission;1D2emission; impurity-trapped exciton; charge-transfer state; infrared

1 Introduction

Trivalent praseodymium ion (Pr3+) has been extensively studied over the past decades. Much research was devoted toward the search for Pr3+-activated phosphors as photon cascade emission materials[1-3], red luminescent materials[4-6], or efficient scintillators[7-8].

Pr3+is characterized by a 4f2shell, which consists of 13 multiplets. The luminescence behavior of Pr3+varies considerably in different host lattices[9]. The intra-configurational 4f2→4f2emissions feature with sharp lines in the spectra, typically as follows: ultra-violet (UV) emission from the1S0level, blue, green, and yellow emissions from the3P0level, and red/infrared emissions from the1D2level. In addition, a Pr3+4f electron may be excited into Pr3+5d orbital, giving a 4f5d configuration. Under certain conditions, inter-configurational 4f5d→4f2broadband emission dominates. The energy of the 4f5d is sensitive to the lattice environment, so that the 4f5d→4f2transitions may exhibit different emission wavelengths (usually in the UV spectral region).

The purpose of the present paper is to briefly review the luminescence spectral aspect of Pr3+in phosphors. For clear illustration, we present several representative Pr3+-activated phosphors, which exhibit photoluminescence emissions from the1S0,3P0,1D2, and the lowest 4f5d levels, respectively. The effects of non-4f states (e.g., Pr3+4f5d state, exciton-like state, or charge-transfer state) on the Pr3+luminescence are addressed in the following sections, including an introduction on the fundamental background knowledge (section 2) and the overview of the emission spectral appearances in the phosphors (section 3). Although there are other important physical mechanisms (such as multiphonon relaxation or crossing relaxation), which may also account for the performances of Pr3+luminescence in some phosphors, we do not intend to discuss them here. Some relevant references that the readers may consult can be found elsewhere[10-12].

2 Fundamental

According to the energy level scheme of Pr3+shown in Fig.1(a), the energy gap between the1S0level and the next 4f2electronic manifold (3P2) is around 24 000 cm-1. Such a large energy gap prevents the non-radiative quenching of the1S0state through the other 4f2states. Therefore, upon effective excitation, if there is no non-4f level locates between the1S0and3P2levels, emissions from the1S0are expected. In many phosphors, however, the lowest crystal-field level of the 4f5d configuration is below the1S0level. As a consequence, the1S0emission is non-radiatively quenched by the 4f5d state. Upon excitation into the 4f5d configuration, the photon emissions probably occur from the3P0level, the1D2level, or the lowest 4f5d level. The different Pr3+emissions depend strongly on the host, in which some external states (e.g., exciton-like state[13]or charge-transfer state[6]) play an important role to determine the emission channels.

A configurational coordinate diagram of Fig.1(b) illustrates the essential influence of the external states location on the emission characteristics of Pr3+. For clarity, only1S0,3P0,1D2and3H4levels of the 4f2configuration, as well as the lowest 4f5d level, are drawn as solid-line parabolas. The dashed-line parabolas ES-h and ES-l respectively represent the high-energy and low-energy positions of the possible external states in Pr3+-activated phosphors. The external states may non-radiatively quench the lowest 4f5d level, feeding the emitting3P0level (e.g., the case of parabola ES-h) or1D2level (e.g., the case of parabola ES-l). Alternatively, if the external states are absent in the Pr3+-activated phosphors, 4f5d→4f2emission may occur.

Fig.1 (a) Energy level scheme of Pr3+4f2configuration. (b) Configurational coordinate diagram illustrating the important role of non-4f states in Pr3+luminescence. The external states at high-energy and low-energy are indicated as ES-h and ES-l, respectively. Arrows 1, 2, 3 represent the electron transfer crossing from the non-4f state parabolas to the 4f2state parabolas.

3 Results and Discussion

3.11S0Emissions

1S0is the highest 4f2state of Pr3+, locating at around 47 000 cm-1in phosphors. Upon excitation on the1S0emitting state, spin selection rules favor transitions to the singlet 4f2states, such as1G4,1D2and1I6. Especially the transition to the1I6will make photon cascade emissions possible.

Cascade emission of Pr3+has now been demonstrated in a variety of fluoride and oxide materials[1-3]. As an example, Fig.2(a) presents room-temperature emission spectrum of SrAl12O19∶Pr3+upon 200 nm excitation. The emission lines at 217, 255, 275, 340, 402 nm are assigned to optical transitions from the1S0level to the3H4,3F4,1G4,1D2and1I6levels, respectively. The1S0→1I6transition is followed by the second photon emission from the3P0level (e.g.,3P0→3H4transition at 485 nm). The stepwise emission processes are very efficient and can be used to realize the UV to visible quantum cutting, although the1S0→1I6transition falls in deep violet[14-15].

It is worth noting that the vibrational fine structure is always absent in the1S0emission spectrum. The absence of emission fine structure may be ascribed to the effect of the close-lying 4f5d state. That is, compared with the general 4fNstate of trivalent rare-earth ions, the high-lying1S0emitting state acquires more 4f5d opposite-parity character through remarkable wave-function mixing[16-17], in the sense that the1S0wave-function becomes more delocalized.

3.23P0Emissions

In some Pr3+-activated phosphors, upon excitation into the 4f5d configuration, the3P0emissions occur. According to Fig.1(b), the population of the3P0emitting state may result from the intersystem crossing between a high-lying non-4f excited state parabola and the3P0parabola. Such non-4f states may be the lowest 4f5d (e.g., process (1) in Fig.1(b)) or an external state (e.g., exciton-like state or charge-transfer state, process (2) in Fig.1(b)).

Fig.2 4f2→4f2emission spectra of Pr3+in phosphors at 300 K. (a) Emission from the1S0in SrAl12O19∶Pr3+upon 200 nm excitation. (b) Emission from the3P0in Gd3Ga5O12∶Pr3+upon 254 nm excitation. (c) Emission from the1D2in CaTiO3∶Pr3+upon 254 nm excitation.

Fig.2(b) shows that, under 254 nm UV illuminagives rise to the3P0emission. This room-temperature emission spectrum consists of emission lines at 484 nm (3P0→3H4), 559 nm (→3H5), 616 nm (→3H6), 656 nm (→3F2), 710 nm (→3F3), 738 nm (→3F4), and 925 nm (→1G4). Color mixing of these spectral lines results in a visual greenish-white luminescence in the material. In the present case, we think the emitting3P0level is fed by an impurity-trapped exciton state (ES-h in Fig.1(b)), which results from the photoionization of Pr3+in the compound containing d10ion (Ga3+)[18].

3.31D2Emissions

According to previous reports on Pr3+-activated phosphors, the1D2emissions are dominating in some transition-metal complex compounds like titantes, vanadates, or niobates[5]. An electron transfer model has been proposed by Boutinaudetal. to explain the relevant physical process[6]. It is assumed that the Pr3+ions have a tendency of oxidation to Pr4+upon excitation in such phosphors. The excited 4f electron is transferred to the transition-metal ion with d0configuration (i.e., Ti4+(3d0), V5+(3d0), or Nb5+(4d0)), forming a Pr3+-to-metal charge transfer state, which may non-radiatively quench the3P0level. Subsequently the electron is transferred back to the emitting1D2levelviaintersystem crossing (process (3) in Fig. 1(b)).

Fig. 2(c) shows that in CaTiO3∶Pr3+the1D2emissions are in the red and infrared, dominating with the transitions to3H4(610 nm),3H6(890 nm) and3F4(1 040 nm). It should be noted that, prior studies on the1D2emissions focused primarily on the red emission, but overlooked the infrared emission components. In our opinion， the infrared luminescence deserves more attention, since the infrared technique is promising for many advanced applications ranging from infrared night-vision surveillance to biomedical imaging probe[19-20].

3.44f5d→4f2Emissions

Besides the intra-configurational 4f2→4f2transition, if the lowest 4f5d level is at an appropriate energy, and there is no other external state between the lowest 4f5d and the lower-lying1D2level, Pr3+ion has an alternative way to emit,i.e., the inter-configurational 4f5d→4f2emissions.

In the case of Y3Al5O12∶Pr3+, the room-temperature emission spectrum is dominated by broadband 4f5d→4f2transitions in the UV region[21], as shown in Fig. 3. In addition to the broadband emissions, sharp line emissions from the3P0level occur at room temperature. We attributed the3P0emission to a thermally activated crossing over the energy barrier of the lowest 4f5d state (process (1) in Fig.1(b)). In the sense that the3P0emission will be remarkably suppressed at low-temperature. Such a conclusion is examined by a 77 K emission spectral measurement, as shown in Fig.3. Note that, compared with the Y3Al5O12∶Pr3+, the absence of 4f5d→4f2emissions in the Gd3Ga5O12∶Pr3+(Fig.2(b)) is due to the ionization quenching of the lowest 4f5d state.

Fig.3 4f5d→4f2emission spectra of Y3Al5O12∶Pr3+, recorded at 300 K and 77 K, respectively. The excitation wavelength is 254 nm.

In addition, the lowest state of 4f5d is also dependent on the temperature and concentration, as shown in the case of SrAl12O19∶Pr3+, where the onset of 4f5d band absorption shifts towards the blue by more than 5 nm as the temperature decreases from 300 to 77 K[22].

3.5Complete Quenching of Pr3+Luminescence

The Pr3+ion shows 4f2→4f2or 4f5d→4f2emissions in garnets containing Gd3+(e.g., Gd3Ga5O12in Fig. 2(b)) and Y3+(e.g., Y3Al5O12in Fig. 3), but the Pr3+luminescence is absent in ytterbium aluminium garnet (i.e., Yb3Al5O12).

Room-temperature emission spectra of Y3-xYbx-Al5O12∶Pr3+are presented in Fig. 4. Along with the increase of Yb3+content in the phosphor system (x=0, 0.3, 1.5, 3), both the 4f5d→4f2and 4f2→4f2emission intensities decrease gradually till to complete disappearance. Some studies argued that, in the Pr3+and Yb3+co-doped phosphors, Pr3+-Yb3+energy transfer should be responsible for the quenching of Pr3+emission[23-27]. However, the proposed energy transfer model cannot explain the quenching of 4f5d→4f2emissions in our case (Fig. 4), since the 4f5d emitting state does not play a role in such energy transfer process. The details of the luminescence quenching mechanism are insufficiently known. In our opinion, the simultaneous presence of Yb3+and Pr3+may yield a low-energy charge-transfer state(not shown in Fig.1), which may account for the quenching phenomenon of Pr3+luminescence.

Fig.4 Emission spectra of Pr3+-doped Y3-xYbxAl5O12(x=0, 0.3, 1.5， 3) at 300 K. The excitation wavelength is 254 nm. No emission signal of Pr3+can be detected in Yb3Al5O12∶Pr3+.

4 Conclusion

Possible emission ways of Pr3+in phosphors, as

well as the corresponding physical insights, are presented. Several specific items are worth addressing:

(1) In SrAl12O19∶Pr3+, 4f-5d wave-function mixing contributes significantly to the distinct luminescence performance of the1S0state.

(2) In Gd3Ga5O12∶Pr3+, the impurity-trapped exciton state is considered to be responsible for the population of the emitting3P0level.

16例术后均获得随访，随访时间12～36个月，平均(22.6±3.41)个月。全部病例末次随访时均能正常参与日常运动，未发生再次脱位。全部病例未出现关节内或浅表感染、深静脉血栓等并发症。

(3) In CaTiO3∶Pr3+, the Pr4+-Ti3+charge-transfer state feeds the emitting1D2level. In addition, the1D2infrared emission phenomenon deserves more attention.

(4) In Y3Al5O12∶Pr3+, the intensity ratio between the 4f5d and the3P0emissions is temperature dependent.

(5) In Yb3Al5O12∶Pr3+, a low-energy charge-transfer state, which may be caused by the simultaneous presence of Yb3+and Pr3+, may quench the Pr3+luminescence.

Acknowledgements

The authors wish to thank Professors S. H. Huang and R. S. Meltzer for helpful discussion.

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刘峰(1978-)，男，吉林长春人，2007年于中科院长春光机所获凝聚态物理专业博士学位，2007年开始在美国佐治亚大学工作至今，现作为学术研究员任职于工学院。主要从事发光物理和发光材料的研究，尤其是对新颖荧光材料的谱学表征和光存储材料的物理功能调控，成果发表于PhysicalReviewLetters和NatureMaterials等学术刊物。

E-mail: fliu@uga.edu 王笑军(1958-)，男，吉林舒兰人，美国南佐治亚大学物理系教授，1992年于美国佐治亚大学获物理博士学位，1992—1995年在俄克拉荷马州立大学及加州大学尔湾分校做博士后研究，1995年在美国南佐治亚大学物理系工作至今。主要从事发光物理和发光材料的研究。发表学术论文及著作章节200余篇(章)。现为国际发光大会IPC委员，LSA、MRB杂志编辑，发光学报、中国稀土学报编委等。

E-mail: xwang@georgiasouthern.edu

2016-10-24；

2016-11-27

基质中非4f组态的电子态对Pr3+离子发光的影响

刘 峰1, 王笑军2*

(1.CollegeofEngineering,DepartmentofPhysicsandAstronomy,UniversityofGeorgia,Athens,GA30602,USA;
2.DepartmentofPhysics,GeorgiaSouthernUniversity,Statesboro,GA30460,USA)

三价镨离子(Pr3+)是一种备受关注的稀土发光离子。学者们在过去几十年里对其发光性质进行了大量的理论和实验研究。在不同的基质材料中，由于非4f组态的电子态与Pr3+离子发光能级相互作用，Pr3+离子可以展现从紫外到红外波段的不同特征的光发射。影响Pr3+离子发光的这些电子态可能源于4f5d激发组态、电荷迁移态或类激子态。本文中，我们以几种具有代表性的发光材料为例，简短地总结和评述了Pr3+离子发光的不同谱形；也尝试解释了几个新颖的实验现象，例如：杂质束缚激子态和电荷迁移态对发光的猝灭影响。我们希望这些相关概念和谱学结果的整理有助于读者更好地理解一些实验上的发光现象，并为设计发光材料提供新的思路。

Pr3+;1S0发射;1D2发射; 杂质束缚激子态; 电荷迁移态; 红外发光

1000-7032(2017)01-0001-06

O482.31 Document code： A

10.3788/fgxb20173801.0001