- Email: [email protected]

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Judd-Ofelt analysis and temperature dependent upconversion luminescence of Er3 þ /Yb3 þ codoped Gd2(MoO4)3 phosphor Hongyu Lu a, Yachen Gao b, Haoyue Hao a, Guang Shi a, Dongyu Li c, Yinglin Song a, Yuxiao Wang a,n, Xueru Zhang a,n a

Department of Physics, Harbin Institute of Technology, Harbin 150001, China College of Electronic Engineering, Heilongjiang University, Harbin 150080, China c Department of Physics, Lingnan Normal University, Zhanjiang 524048, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 14 January 2016 Received in revised form 28 July 2016 Accepted 3 February 2017 Available online 6 February 2017

Although lanthanide doped luminescent materials have been extensively investigated, a quantitative analysis of how temperature affects upconversion luminescence is still incomplete. The Gd2(MoO4)3:Er3 þ /Yb3 þ phosphor is synthetized by sol-gel method. Based on the absorption spectra of Er3 þ ions, J-O intensity parameters and radiative transition probabilities are computed to estimate the optical properties. In view of ion-phonon interaction, the phonon-assisted energy transfer and multiphonon relaxation are responsible for the temperature dependent luminescence. Additionally, cross relaxation probability for 4I11/2 þ 4I11/2-4I15/2 þ 4F7/2 is determined to be 240 s 1 through quantitative simulation of ion-ion interaction. These meaningful results are of vital values for the ﬁeld of laser crystal and optical temperature sensing. & 2017 Elsevier B.V. All rights reserved.

Keywords: Phonon J-O intensity parameters Energy transfer Multiphonon relaxation Cross relaxation Rare earth

1. Introduction Recently, enormous attention has been paid to lanthanide doped luminescent materials because of their broad applications, such as remote sensing, biological labeling, and display devices [1– 6]. The unique upconversion luminescence depends on intra-4f electronic transition of lanthanide ions, which can convert low energy photons to high energy photons via multiphoton processes. Photon upconversion involves multiple mechanisms, such as ionphoton, ion-ion and ion-phonon interactions. For ion-photon interactions, the Judd-Ofelt (J-O) theory is an effective method to investigate the radiative transition within the 4fN conﬁguration of lanthanide ions based on their optical absorption measurement [7–11]. In view of ion-ion interactions, the energy transfer (ET) and cross relaxation (CR) are extremely sensitive to the distance of lanthanide ions [12–14]. In the case of ion–phonon interactions, phonon-assisted ET and multiphonon relaxation strongly depend on the phonon energy of materials, energy gap (to the adjacent levels) and temperature [15–17]. Among the variety of available phosphors, Gd2(MoO4)3 doping with Er3 þ ions and Yb3 þ ions has n

Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Zhang).

http://dx.doi.org/10.1016/j.jlumin.2017.02.009 0022-2313/& 2017 Elsevier B.V. All rights reserved.

been extensively investigated since not only does Gd2(MoO4)3 have high chemical durability and low phonon energy, but also the Er3 þ ion has abundant energy levels whose emissions almost include the whole visible range [18,19]. Huang et al. investigated radiative transition properties of the Tm3 þ doped Gd2(MoO4)3 crystal using J-O theory and concluded that it could be a promising blue phosphor due to relatively larger radiative transition probability and ﬂuorescence branching ratio [20]. Skrzypczak et al. quantitative analyzed ion-ion interactions (ET and CR processes) of the Nd3 þ doped ceramic using rate equation system [21]. Jin et al. reported the inﬂuence of the phonon on upconversion process for the Yb3 þ and Er3 þ codoped NaYF4 phosphor [22]. To the best of our knowledge, a fundamental understanding of the overall mechanism of upconversion process has not been reported in the literature so far. Here, we quantitatively investigate the upconversion luminescence processes that the population and depopulation of the excited states are responsible for temperature dependent emission in Er3 þ and Yb3 þ codoped Gd2(MoO4)3 phosphor. In addition, the J-O theory is adopted to calculate radiative transition probabilities and predict the material properties. Using a steady-state rate equation system, we develop a comprehensive theoretical model including resonant or phonon-assisted ET, radiative, multiphonon relaxation

H. Lu et al. / Journal of Luminescence 186 (2017) 34–39

35

and CR, which collectively accounts for the variation of luminescent intensity.

2. Experimental The Gd2(MoO4)3:1%Er3 þ /9%Yb3 þ phosphor was prepared using a sol-gel method. Firstly, an appropriate amount of (NH4)6Mo7O24 was dissolved in 15 mL 2-methoxyethanol with 5 mL acetic acid glacial under constant magnetic stirring. 10 mL 2-methoxyethanol solution containing Gd(NO3)3 (1.8 mmol), Yb(NO3)3 (0.18 mmol) and Er(NO3)3 (0.02 mmol) was slowly added into the solution. Subsequently, the citric acid (4.5 mmol) was added to this solution which acts as chelating agent, and then a yellowish transparent solution was obtained by stirring a few minutes. Finally, the wet gel was dried at 60 °С for 8 h, and then the product was sintered 800 °С for 2 h in air atmosphere. The luminescent spectra were measured in the wavelength range 450–650 nm under 980 nm excitation by a Spectrometer (HORIBA Jobin Yvon iHR550). Powder X-ray diffraction (XRD) pattern was recorded using Panalytical Empyrean diffractometer with Cu-Ka radiation (λ ¼1.5406 Å). The absorption spectrum was inspected by a spectrophotometer (Hitachi UV–vis-NIR U4100).

3. Results and discussion The XRD pattern of Gd2(MoO4)3:1%Er3 þ /9%Yb3 þ phosphor is presented in Fig. 1. The pattern reveals that the sample are pure phase, which is well consistent with the standard value of orthorhombic Gd2(MoO4)3 (JCPDS No. 70-1397). The lattice constants are a¼ 10.3881 Å, b¼10.4194 Å, c¼10.7007 Å, α ¼ β ¼ γ ¼90° and the space group is Pba2 [23]. 3.1. Judd-Ofelt analysis The absorption spectrum of Gd2(MoO4)3:Er3 þ /Yb3 þ powder in the Vis-NIR region is shown in Fig. 2. Eight absorption bands arising from the ground (4I15/2 for Er3 þ ) to the upper levels transitions are selected to determine the phenomenological experimental oscillator strengths (ƒexp), which can be determined from the below given expression [24]:

Fig. 2. Absorption spectrum of Gd2(MoO4)3:1%Er3 þ /9%Yb3 þ powder.

(

)

fexp J → J ′ =

mc2 Nπe2

2

c ∫ ε ( v) dν = 2.303m ∫ A ( v ) dν Nπe2cb Δx

4.318 × 10−9 cb Δx

=

∫ A ( v ) dν

(1)

where c (¼ 2.998 1010 cm/s) is the light velocity, N is the Avogadro's number, m (¼ 9.1 10–28 g) is the electron mass, e ( ¼4.8 10 10 esu) is the electron charge, ν (cm 1) is the wavenumber of the absorption band, ε(ν) (L/(mol cm)) is the molar extinction coefﬁcients (ε(ν)¼ 2.303 A(v)/cbΔx), A(ν) is wavenumber dependent absorbance, cb (mol/L) is the amount of substance concentration of Er3 þ , and Δx (cm) is the path-length of the light, respectively. ed The theoretical oscillator strengths of the electric ( f cal ) and md ) dipole transition from the initial J state to the ﬁnal J′ magnetic ( f cal state can be represented as [25]:

2

(

)

ed f cal J → J′ =

2 8π 2mcν ( n + 2) 9n 3h ( 2J + 1)

×

∑

Ωt 4f N (αSL ) J‖U t‖4f N (α′S ′L′) J ′

2

(2)

t = 2,4,6

md

f cal

( J → J ) = 6mch(2νnJ + 1) ′

×

(

fcal J → J ′

)

(

)

4f N ( αSL ) J‖L + 2S‖4f N α′S ′L′ J ′

(

ed = f cal J → J′

)

(

md + f cal J → J′

2

(3)

)

(4) 27

Fig. 1. XRD pattern of Gd2(MoO4)3:1%Er3 þ /9%Yb3 þ phosphor and the standard date of orthorhombic phase Gd2(MoO4)3 (JCPDS No. 70-1397).

2

where n is medium refractive index, h (¼6.63 10 g cm /s) is Plank's constant, J (J′) is the total angular momentum of initial (ﬁnal) state, Ωt is the J-O intensity parameters, and ║U t║2 is the squared reduced matrix elements of the unit tensor operator which is independent of the chemical environment of the ion and was given in Ref. [26], respectively. The wavelength dependent refractive indexes of Gd2(MoO4)3 were reported by Jaque et al. [27]. For powder material of random orientations, the optical anisotropy is averaged. Therefore, the refractive index of Gd2(MoO4)3 powder can be expressed as n¼(2no þne)/3 [28]. The radiative transition probability arising from the J′ to the J state transition is calculated by using the following formulas [29]:

36

H. Lu et al. / Journal of Luminescence 186 (2017) 34–39

Fig. 3. Luminescence decay curve of 4I13/2-4I15/2 transition for Gd2(MoO4)3: Er3 þ /Yb3 þ phosphor under 980 nm excitation.

2

(

)

A ed J ′ → J =

2 64π 4e2v 3 n ( n + 2) ′ 9 3h 2J + 1

(

×

)

(

)

Ωt 4f N α′S ′L′ J ′ ‖U t‖4f N ( αSL ) J

∑

2

t = 2,4,6

(

)

Amd J ′ → J =

4π 2e2hv 3n3

( ) ( α′S L ) J ‖L + 2S‖4f ( αSL) J

3m2c2 2J ′ + 1 × 4f N

(

(5)

)

(

′ ′

)

′

N

(

A J ′ → J = A ed J ′ → J + Amd J ′ → J

2

)

(6)

(7)

where Aed and Amd are electric and magnetic dipole radiative transition probability, respectively. In view of Er3 þ , the transition between 4I13/2 and 4I15/2 involves electric and magnetic dipole transition. The radiative lifetime is related to radiative probability, which can be expressed as [30]:

1/τ = A ed + Amd

(8)

For 4I13/2-4I15/2 transition, radiative lifetime is obtained from the luminescence decay curve (Fig. 3) and determined to be 2.46 ms. In terms of diffuse reﬂectance absorption spectra, the actual path-length (Δx) is unknown. An effective method is used to calculate the three J-O intensity parameters (Ωt), which is reported by Toma et al. [28]. The three J-O intensity parameters (Ωt) can obtain from the least squares ﬁtting between the experimental (ƒexp) and the theoretical (ƒcal) oscillator strengths with the help of Eq. (8). Meanwhile, the relation between three J-O intensity parameters (Ωt) and ﬂuorescence intensity ratio (FIR) (Eq. (9)) is taken into account. As a result, the three J-O intensity parameters (Ωt) are determined to be Ω2 ¼ 11.74 10 20 cm2, 20 2 20 Ω4 ¼8.16 10 cm and Ω6 ¼2.05 10 cm2, respectively. Since the relative population of thermal coupling levels follows the Boltzmann distribution, based on measurement of temperature dependent luminescence, the FIR can be related to the J-O intensity parameters (Ωt) as follows [31]:

FIR =

⎤ ⎡ −ΔEHS ⎤ v 4 ⎡ 0.7158Ω 2 + 0.4138Ω 4 IH = H4 ⎢ + 0.4166⎥ exp ⎢ ⎣ kT ⎥⎦ ⎦ IS 0.2225Ω 6 vS ⎣

(9)

where IH and IS are the integral intensity of luminescence arising

Fig. 4. (a) The upconversion emission spectra of Gd2(MoO4)3:Er3 þ /Yb3 þ phosphor at different temperature (323, 453 and 633 K). (b) Temperature dependent luminescence and FIR.

from 2H11/2 and 4S3/2 to the 4I15/2 transition, νH and νS are the wavenumber of corresponding emission band, k is the Boltzmann constant, T is the absolute temperature, and ΔEHS is the energy gap between the 2H11/2 and 4S3/2 levels, respectively. According to Eq. (9), the theoretical FIR is plotted in Fig. 4(b) as a function of temperature. In addition, the root-mean-square deviation of the experimental and calculated oscillator strengths represents the validity of the J-O intensity parameters, which is deﬁned as [32]:

⎡ N−1 f exp − fcal ⎢ δ rms = ⎢ ∑ N−q ⎣ i=0

(

)

2 ⎤1/2

⎥ ⎥ ⎦

(10)

Table 1 The calculated and experimental oscillator strengths, and radiative transition probabilities for Gd2(MoO4)3: Er3 þ /Yb3 þ phosphor. Transition

Energy (cm 1)

fcal (J, J) ( 10 6)

fexp (J, J) ( 10 6)

A (J, J) (s 1)

4

22,222 20,408 19,083 18,315 15,290 12,531 10,204 6631

1.85 11.24 27.27 2.66 12.42 2.15 3.04 7.14

5.33 11.09 27.18 5.68 11.92 4.24 4.70 6.67

2937 10,831 28,339 (A60) 2857 (A50) 7800 (A40) 1159 (A30) 416 (A20) 406 (A10)

F5/2-4I15/2 F7/2-4I15/2 2 H11/2-4I15/2 4 S3/2-4I15/2 4 F9/2-4I15/2 4 I9/2-4I15/2 4 I11/2-4I15/2 4 I13/2-4I15/2 4

H. Lu et al. / Journal of Luminescence 186 (2017) 34–39

37

Table 2 The J-O intensity parameters (Ωt 10 20 cm2) and quality factor (Ω4/Ω6) of Er3 þ doped typical hosts. Crystal

Ω2

Ω4

Ω6

Ω4/Ω6

Ref.

Er:NaYF4 Er:YAG Er/Yb:GdVO4 Er/Yb:NaY(WO4)2 Er/Yb:LiLa(WO4)2 Er/Yb:Gd2(MoO4)3

2.11 0.45 6.47 18.1 9.03 11.74

1.37 0.98 1.51 2.59 2.02 8.16

1.22 0.62 0.91 1.21 0.59 2.05

1.12 1.58 1.66 2.14 3.42 3.98

[36] [37,38] [37] [39] [39] This work

where N (¼8) is the number of absorption bands analyzed, q (¼ 3) is the number of parameter. The value of δrms is calculated to be 2.40 10 6. The experimental (ƒexp) and the theoretical (ƒcal) oscillator strengths and the radiative transition probability A (J, J) are listed in Table 1, respectively. In general, the Ω2 reﬂects the asymmetry of the local environment and the degree of covalence between rare earth ions and the vicinity ligands [33,34]. The higher the values of Ω2 is, the more asymmetric the ion site and the stronger the covalent chemical bond. The spectroscopic quality factors (Ω4/Ω6) of typical phosphors are summarized in Table 2, which is a signiﬁcant laser characteristic in predicting the stimulated emission [35]. The spectroscopic quality factor of Gd2(MoO4)3: Er3 þ /Yb3 þ crystal is determined to be 3.98, which is much higher than that of YAG host, implying that is a promising laser crystal. 3.2. Temperature dependent theoretical model Under 980 nm excitation, the upconversion spectrum of Gd2(MoO4)3:Er3 þ /Yb3 þ phosphor are shown in Fig. 4(a). The characteristic emissions arising from 2H11/2 and 4S3/2 to 4I15/2 transitions of the Er3 þ ions are observed. Since lanthanide ions can be affected by the surrounding crystal ﬁeld environment according to the crystal ﬁeld theory [40], upconversion emission spectrum splits to several obvious emission peaks. Signiﬁcant changes in the luminescent intensity are found with the temperature rising. In order to investigate more clearly temperature (Fig. 4(b)) dependent behavior, the upconversion process of Er3 þ and Yb3 þ codoped system is explained using the model portrayed in Fig. 5. Firstly, a ﬁrst ET process occurs between the 2F5/2 level (Yb3 þ ) and the 4I15/2 level (Er3 þ ), promoting the latter to the 4I11/2 level [ET1: 2 F5/2(Yb3 þ )þ 4I15/2(Er3 þ )-2F7/2(Yb3 þ )þ 4I11/2(Er3 þ )] [41]. Subsequently, a second ET process occurs between the 2F5/2 level (Yb3 þ ) and the 4I13/2 level (Er3 þ ), which derives from multiphonon relaxation of the 4I11/2 level, promoting the latter to the 4F9/2 level [ET2: 2 F5/2(Yb3 þ )þ 4I13/2(Er3 þ )-2F7/2(Yb3 þ )þ 4F9/2(Er3 þ )]. Meanwhile, the third ET step occurs between the 2F5/2 level (Yb3 þ ) and the 4I11/2 level (Er3 þ ) promoting the latter to the 4F7/2 level which quickly relaxes its population to the 2H11/2 level through nonradiative decay [ET3: 2 F5/2(Yb3 þ )þ 4I11/2(Er3 þ )-2F7/2(Yb3 þ )þ 4F7/2(Er3 þ )] [22]. For above the three ET processes, the energy of corresponding transition is matched. Owing to energy mismatching ( 1100 cm 1), however, the phonon assisted ET process occurs between the 2F5/2 level (Yb3 þ ) and the 4I11/2 level (Er3 þ ) ion promoting the latter to the 2H11/2 level, which is greatly dependent on temperature [ET4: 2 F5/2(Yb3 þ )þ 4I11/2(Er3 þ )-2F7/2(Yb3 þ )þ 2H11/2(Er3 þ )] [42]. The phonon-assisted ET theory is presented by Dexter, which was applied to characterize the ion-ion interactions between these dopants, and the phonon-assisted ET probability is given by [43]

WET ( T ) = W0

e−βΔE ⎡ 1 ⎣

− exp ( −Ephonon/kT ) ⎤⎦

−p

where W0 is the probability of resonant ET,

(11)

β is a host parameter,

Fig. 5. Schematic energy levels diagram of Er3 þ and Yb3 þ ions and upconversion processes under 980 nm excitation.

Ephonon is the phonon energy of Gd2(MoO4)3, ΔE is the energy mismatch, and p is the number of phonon, respectively. For the multiphonon relaxation, each level is taken into account except the 2F5/2 (Yb3 þ ) and 4I13/2 (Er3 þ ) level, since energy gaps between their adjacent levels are extremely large, which means the corresponding multiphonon relaxation is extremely small. The multiphonon relaxation probability can be related to the temperature through [44] −p Wij ( T ) = Wij0 ⎡⎣ 1 − exp ( −Ephonon/kT ) ⎤⎦

(12)

Wij0 = Cm exp ( −αΔEij )

(13)

where Wij0 is multiphonon relaxation probability at T ¼0 K, Cm is a host dependent constant and α is dependent on electron-phonon coupling constant, and ΔEij is energy gaps from i level to j level of Er3 þ , respectively. Due to long lifetimes of the 4I11/2 level, we only consider the most inﬂuential CR process [Er3 þ : 4I11/2 þ 4I11/24 I15/2 þ 4F7/2]. In this model, only optical absorption through Yb3 þ ions and ET from Yb3 þ to Er3 þ are considered, because the absorption cross section of Yb3 þ is an order of magnitude higher than Er3 þ at 980 nm and the high concentration ratio is 9(Yb3 þ ):1(Er3 þ ). Under these assumptions, the sets of phenomenological rate equations which describe the model are listed below as:

dNa P = σab Na − ( WET 1 + WET 2 + WET 3 + WET 4 − Ab ) Nb = 0 dt Sε

(14)

dN1 = W21N2 − WET 2 Nb − A10 N1 = 0 dt

(15)

dN2 = W32 N3 − A20 N2 − W21N2 + ( WET 1 − WET 3 − WET 4 ) Nb dt − 2CR N2 N2 = 0

(16)

38

H. Lu et al. / Journal of Luminescence 186 (2017) 34–39

4. Conclusions

Table 3 Parameters involved in the rate equations, and their sources are also labeled. Symbols

Values

Sources

Symbols

Values

Sources

Ephonon (cm 1) sab (10–25 m2) Ab (s 1) WET1 (s 1) WET2 (s 1) WET3 (s 1)

970 11.7 438 758 118 150

[45] [46] [47] [48] [48] This work

W0 (s 1) β (10 3 cm) Cm (108 s 1) α (10 3 cm) S (10 6 m2) CR (s 1)

120 2.75 4.13 3.45 3.5 240

This This This This This This

work work work work work work

dN3 = W43 N4 − A30 N3 − W32 N3 = 0 dt

(17)

dN4 = W54 N5 − W43 N4 − A 40 N4 + WET 2 Nb = 0 dt

(18)

d ( N5 + N6 ) dt

= ( WET 3 + WET 4 ) Nb − A 60 N6 − A50 N5 − W54 N5 + CR N2 N2 = 0

N6/N5 = 3 × exp ( −ΔE65/kT )

(19)

(20)

N0 + N1 + N2 + N3 + N4 + N5 + N6 = 1

(21)

Na + Nb = 9

(22)

where Aij are the radiation transition probability (calculated by the J-O theory), Ni are the population densities of i level, P is the exciting power, S is the spot size, ε is the exciting photon energy, and CR is the cross relaxation probability between neighbor Er3 þ pairs, respectively. For Eq. (20), the populations of N6 and N5 follow Boltzmann distribution. The Cm, α and β were estimated from typical values observed for similar compounds. The roughly estimated values of resonant ET probability WET3 and W0 were 150 and 120 (s 1), respectively. Using the calculated radiative transition probabilities (Aij), multiphonon relaxation probability (Wij) and other parameters listed in Table 3, temperature dependent luminescence (2H11/2/4S3/2-4I15/2) has been investigated and the theoretical result is shown in Fig. 4(b). It is found that the luminescent intensity arising from 2H11/2 to 4I15/2 transition (525 nm) increases a factor of 2 when the sample was heated from 295 to 490 K, and then continuously decreases in the range of 490–660 K. The electronphonon coupling increases with the temperature rising, resulting in improving multiphonon relaxation probability (Wij) and the phonon-assisted ET probability. The competition between the phonon-assisted ET and multiphonon relaxation accounts for variation of luminescent intensity. The above mentions demonstrate that the phonon-assisted ET makes a major contribution to improving luminescence initially. Further increasing the temperature, however, the multiphonon relaxation becomes dominant, which is responsible for reduction of luminescent intensity. In view of 4S3/2 to 4I15/2 transition (545 nm), the luminescent intensity gradually decreases over the whole temperature range (295–660 K), indicating that multiphonon relaxation is dominant. In addition, the CR probability is determined as 240 s 1 based on the theoretical simulation. The theoretical simulation is well agreement with experimental results, demonstrating that theoretical model and analysis are reasonable.

In summary, we have successfully prepared Gd2(MoO4)3: Er3 þ /Yb3 þ phosphor. Employing the J-O theory, a series of photophysical parameters such as J-O intensity parameters and radiative transition probabilities are calculated and analyzed. The spectroscopic quality factor (Ω4/Ω6 ¼3.98) is found to be much higher than previous reports, indicating that Gd2(MoO4)3:Er3 þ /Yb3 þ crystal is a promising laser crystal. Using a simpliﬁed steady-state rate equation model, the temperature dependent luminescence is quantitatively interpreted by ion–phonon interactions. The variation of luminescence arising from 2H11/2 to 4I15/2 indicates a competitive behavior between the phonon-assisted ET and multiphonon relaxation with the temperature rising. Initially, the phonon-assisted ET made a major contribution to the luminescence increasing, and then the multiphonon relaxation is dominant, which is responsible for the decrease of luminescence. Additionally, for ion-ion interaction, the CR probability ( 240 s 1) is obtained through quantitative simulation data. These results are of signiﬁcant value for understanding temperature dependent upconversion luminescence of Gd2(MoO4)3:Er3 þ /Yb3 þ phosphor.

Acknowledgments This work has been supported by the Grant of National Natural Science Foundation of China (Nos. 11374079, 11474078, 61275117 and 11404283).

References [1] G. Chen, J. Seo, C. Yang, P.N. Prasad, Nanochemistry and nanomaterials for photovoltaics, Chem. Soc. Rev. 42 (2013) 8304–8338. [2] G. Chen, H. Qju, P.N. Prasad, X. Chen, Upconversion nanoparticles: design, nanochemistry, and applications in the ranostics, Chem. Rev. 114 (2014) 5161–5214. [3] Y. Sun, W. Feng, P. Yang, C. Huang, F. Li, The biosafety of lanthanide upconversion nanomaterials, Chem. Soc. Rev. 44 (2015) 1509–1525. [4] W. Zheng, P. Huang, D. Tu, E. Ma, H. Zhu, X. Chen, Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection, Chem. Soc. Rev. 44 (2015) 1379–1415. [5] D. Yang, Z. Hou, Z. Cheng, C. Li, J. Lin, Current advances in lanthanide ion (Ln3 þ )-based upconversion nanomaterials for drug delivery, Chem. Soc. Rev. 44 (2015) 1416–1448. [6] X. Liu, R. Deng, Y. Zhang, Y. Wang, H. Chang, L. Huang, X. Liu, Probing the nature of upconversion nanocrystals: instrumentation matters, Chem. Soc. Rev. 44 (2015) 1479–1508. [7] B. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962) 750. [8] G. Ofelt, Intensities of crystal spectra of rare-earth ions, J. Chem. Phys. 37 (1962) 511–520. [9] M.P. Hehlen, M.G. Brik, K.W. Kramer, 50th anniversary of the Judd-Ofelt theory: an experimentalist's view of the formalism and its application, J. Lumin. 136 (2013) 221–239. [10] R. Saraf, C. Shivakumara, S. Behera, H. Nagabhushana, N. Dhananjaya, Photoluminescence, photocatalysis and Judd-Ofelt analysis of Eu3 þ activated layered BiOCl phosphors, RSC Adv. 5 (2015) 4109–4120. [11] X. Zhang, L. Zhou, Q. Pang, M. Gong, Synthesis, photoluminescence and JuddOfelt analysis of red LiGd5P2O13: eu3 þ phosphors for white LEDs, RSC Adv. 5 (2015) 54622–54628. [12] G. Liu, Advances in the theoretical understanding of photon upconversion in rare-earth activated nanophosphors, Chem. Soc. Rev. 44 (2015) 1635–1652. [13] W. Park, D.W. Lu, S.M. Ahn, Plasmon enhancement of luminescence upconversion, Chem. Soc. Rev. 44 (2015) 2940–2962. [14] K. Zheng, Z. Liu, C. Lv, W. Qin, Temperature sensor based on the UV upconversion luminescence of Gd3 þ in Yb3 þ -Tm3 þ -Gd3 þ codoped NaLuF4 microcrystals, J. Mater. Chem. C 1 (2013) 5502–5507. [15] X. Liu, J. Qiu, Recent advances in energy transfer in bulk and nanoscale luminescent materials: from spectroscopy to applications, Chem. Soc. Rev. 44 (2015) 8714–8746. [16] A. Bednarkiewicz, M. Stefanski, R. Tomala, D. Hreniak, W. Strek, Near infrared absorbing near infrared emitting highly-sensitive luminescent nanothermometer based on Nd3 þ to Yb3 þ energy transfer, Phys. Chem. Chem. Phys. 17 (2015) 24315–24321. [17] X. Bai, H. Song, G. Pan, Y. Lei, T. Wang, X. Ren, S. Lu, B. Dong, Q. Dai, L. Fan, Size-

H. Lu et al. / Journal of Luminescence 186 (2017) 34–39

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

[28] [29] [30]

[31]

[32]

dependent upconversion luminescence in Er3 þ /Yb3 þ -codoped nanocrystalline yttria: saturation and thermal effects, J. Phys. Chem. C 111 (2007) 13611–13617. V.A. Morozov, M.V. Raskina, B.I. Lazoryak, K.W. Meert, K. Korthout, P.F. Smet, D. Poelman, N. Gauquelin, J. Verbeeck, A.M. Abakumov, Crystal structure and luminescent properties of R2–xEux(MoO4)3 (R ¼Gd, Sm) red phosphors, Chem. Mater. 26 (2014) 7124–7136. G.S.R. Raju, E. Pavitra, G.P. Nagaraju, K. Ramesh, B.F. El-Rayes, J.S. Yu, Imaging and curcumin delivery in pancreatic cancer cell lines using PEGylated α-Gd2(MoO4)3 mesoporous particles, Dalton Trans. 43 (2014) 3330–3338. J. Tang, Y. Chen, Y. Lin, Y. Huang, Spectroscopy and photoluminescence of Tm3 þ : β'-Gd2(MoO4)3 crystal, J. Lumin. 138 (2013) 15–18. U. Skrzypczak, C. Pfau, G. Seifert, S. Schweizer, Comprehensive rate equation analysis of upconversion luminescence enhancement due to BaCl2 nanocrystals in neodymium-doped ﬂuorozirconate-based glass ceramics, J. Phys. Chem. C 118 (2014) 13087–13098. J. Zhao, Z. Lu, Y. Yin, C. McRae, J.A. Piper, J.M. Dawes, D. Jin, E.M. Goldys, Upconversion luminescence with tunable lifetime in NaYF4:Yb,Er nanocrystals: role of nanocrystal size, Nanoscale 5 (2013) 944–952. D. Wang, J. Fan, M. Shang, K. Li, Y. Zhang, H. Lian, J. Lin, Pechini-type sol–gel synthesis and multicolor-tunable emission properties of GdY(MoO4)3: Re3 þ (RE ¼Eu, Dy, Sm, Tb) phosphors, Opt. Mater. 51 (2016) 162–170. J. Choi, A. Margaryan, A. Margaryan, F. Shi, Judd–Ofelt analysis of spectroscopic properties of Nd3 þ -doped novel ﬂuorophosphate glass, J. Lumin. 114 (2005) 167–177. G. Monteiro, Y. Li, L.F. Santos, R.M. Almeida, Optical and spectroscopic properties of rare earth-doped (80 x) TeO2–xGeO2–10Nb2O5–10K2O glasses, J. Lumin. 134 (2013) 284–296. D.K. Sardar, J.B. Gruber, B. Zandi, J.A. Hutchinson, C.W. Trussell, Judd-Ofelt analysis of the Er3 þ (4f11) absorption intensities in phosphate glass: Er3 þ , Yb3 þ , J. Appl. Phys. 93 (2003) 2041–2046. D. Jaque, J. Findensein, E. Montoya, J. Capmany, A. Kaminskii, H. Eichler, J. G. Solé, Spectroscopic and laser gain properties of the Nd3 þ : β'-Gd2(MoO4)3 non-linear crystal, J. Phys.: Condens. Matter 12 (2000) 9699. Ş. Georgescu, A. Ştefan, O. Toma, A.-M. Voiculescu, Judd–Ofelt analysis of Ho3 þ doped in ceramic CaSc2O4, J. Lumin. 162 (2015) 174–179. Q. Dong, G. Zhao, D. Cao, J. Chen, Y. Ding, Growth and anisotropic spectral properties of Er: Yalo3 crystal, J. Alloy. Compd. 493 (2010) 661–665. Ş. Georgescu, O. Toma, C. Matei, A.-M. Voiculescu, A. Ştefan, Judd–Ofelt analysis of Tm3 þ in La3Ga5.5Ta0.5O14 ceramic with granular structure, J. Lumin. 157 (2015) 35–38. Y.M. Yang, C. Mi, F.Y. Jiao, X.Y. Su, X.D. Li, L.L. Liu, J. Zhang, F. Yu, Y.Z. Liu, Y. H. Mai, A novel multifunctional upconversion phosphor: Yb3 þ /Er3 þ Codoped La2S3, J. Am. Ceram. Soc. 97 (2014) 1769–1775. X. Guo, H. Guo, L. Fu, L.D. Carlos, R.A.S. Ferreira, L. Sun, R. Deng, H. Zhang, Novel near-infrared luminescent hybrid materials covalently linking with lanthanide Nd(III), Er(III), Yb(III), and Sm(III) complexes via a primary beta-

[33] [34]

[35]

[36]

[37]

[38] [39] [40] [41]

[42]

[43]

[44] [45]

[46]

[47]

[48]

39

diketone ligand: synthesis and photophysical studies, J. Phys. Chem. C 113 (2009) 12538–12545. F. Huang, Y. Ma, W. Li, X. Liu, L. Hu, D. Chen, 2.7 μm emission of high thermally and chemically durable glasses based on AlF3, Sci. Rep. 4 (2014). Y. Guo, M. Li, L. Hu, J. Zhang, Intense 2.7 mm emission and structural origin in Er3 þ -doped bismuthate (Bi2O3-GeO2-Ga2O3-Na2O) glass, Opt. Lett. 37 (2012) 268–270. K. Maheshvaran, S. Arunkumar, V. Sudarsan, V. Natarajan, K. Marimuthu, Structural and luminescence studies on Er3 þ /Yb3 þ co-doped boro-tellurite glasses, J. Alloy. Compd. 561 (2013) 142–150. E.M. Chan, D.J. Gargas, P.J. Schuck, D.J. Milliron, Concentrating and recycling energy in lanthanide codopants for efﬁcient and spectrally pure emission: the case of NaYF4:Er3 þ /Tm3 þ upconverting nanocrystals, J. Phys. Chem. B 116 (2012) 10561–10570. N.F. Zhuang, X.L. Hu, S.K. Gao, B. Zhao, J.L. Chen, J.Z. Chen, Spectral properties and energy transfer of Yb,Er: GdVo4 crystal, Appl. Phys. B-Lasers Opt. 82 (2006) 607–613. W. Zhao, L. Zhang, G. Wang, Growth, thermal and spectral characterization of Er3 þ -doped Li2Gd4(MoO4)7 crystal, J. Cryst. Growth 311 (2009) 2336–2340. X. Huang, G. Wang, Growth and optical characteristics of Er3 þ :LiLa(moo4)2 crystal, J. Alloy. Compd. 475 (2009) 693–697. F. Auzel, Upconversion and anti-stokes processes with f and d ions in solids, Chem. Rev. 104 (2004) 139–173. H. Liu, C.T. Xu, D. Lindgren, H. Xie, D. Thomas, C. Gundlach, S. AnderssonEngels, Balancing power density based quantum yield characterization of upconverting nanoparticles for arbitrary excitation intensities, Nanoscale 5 (2013) 4770–4775. B. Simondi-Teisseire, B. Viana, D. Vivien, A. Lejus, Yb3 þ to Er3 þ energy transfer and rate-equations formalism in the eye safe laser material Yb: Er: Ca2Al2SiO7, Opt. Mater. 6 (1996) 267–274. T. Miyakawa, D. Dexter, Phonon sidebands, multiphonon relaxation of excited states, and phonon-assisted energy transfer between ions in solids, Phys. Rev. B 1 (1970) 2961. F. Auzel, Multiphonon-assisted anti-Stokes and Stokes ﬂuorescence of triply ionized rare-earth ions, Phys. Rev. B 13 (1976) 2809. F.G. Ullman, B. Holden, B. Ganguly, J.R. Hardy, Raman spectrum of gadolinium molybdate above and below the ferroelectric transition, Phys. Rev. B 8 (1973) 2991. F. Wang, F. Song, G. Zhang, Y. Han, Q. Li, C. Ming, J. Tian, Upconversion and pump saturation mechanisms in Er3 þ /Yb3 þ co-doped Y2Ti2O7 nanocrystals, J. Appl. Phys. 115 (2014) 134310. D. Chen, Y. Wang, E. Ma, Y. Yu, F. Liu, Partition, luminescence and energy transfer of Er3 þ /Yb3 þ ions in oxyﬂuoride glass ceramic containing CaF2 nanocrystals, Opt. Mater. 29 (2007) 1693–1699. A. Shyichuk, S.S. Câmara, I.T. Weber, A.N.C. Neto, L.A. Nunes, S. Lis, R.L. Longo, O.L. Malta, Energy transfer upconversion dynamics in YVO4: Yb3 þ , Er3 þ , J. Lumin. 170 (2016) 560–570.