Enhanced green upconversion luminescence in Ho3+ and Yb3+ codoped Y2O3 ceramics with Gd3+ ions

Journal of Luminescence 143 (2013) 388–392

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Enhanced green upconversion luminescence in Ho3+ and Yb3+ codoped Y2O3 ceramics with Gd3+ ions Ying Yu a,n, Dawei Qi a, Hua Zhao b a b

Department of Physics, Northeast Forestry University, 150001 Harbin, PR China School of Materials Science and Engineering, Harbin Institute of Technology, 150001 Harbin, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 7 April 2012 Received in revised form 8 May 2013 Accepted 30 May 2013 Available online 8 June 2013

A detailed investigation on the effect of Gd3+ ions on the upconversion (UC) luminescence of Y2O3:Ho3 + /Yb3+ was performed. The green UC emission intensity was obviously enhanced along with an increase in the relative content of Gd3+ ions from 0 mol% to 70 mol%, with a corresponding decrease in the Y3+ content from 91.9 mol% to 21.9 mol%. X-ray diffraction and decay time investigations confirmed that tridoping with Gd3+ ions can influence the local crystal field of the Y2O3 host lattice. Theoretical calculations illustrated that the enhancement in the green UC emission resulted from the induction of the Gd3+ ions, which increased the lifetimes in the intermediate 2F5/2 (Yb) and 5I6 (Ho) states, reduced the highest phonon cutoff energy of Y2O3 and dissociated the Yb3+ and Ho3+ ion clusters in the bulk ceramics. & 2013 Elsevier B.V. All rights reserved.

Keywords: UC Enhance Rare-earth Energy transfer

1. Introduction Trivalent rare-earth (RE)-doped inorganic upconversion (UC) materials have drawn extensive attention for their potential applications in volumetric displays [1–3], biolabeling [4–6], biosensors [7], DNA detection [8], and photodynamic therapy [9,10]. Among the various developed UC luminescent materials, REdoped yttrium oxide (Y2O3) exhibits the intriguing chemical and optical properties of a host lattice, such as high thermal conductivity and expansion coefficient, broad transparency range of 0.23 μm to 0.8 μm, and relatively low phonon energy (maximum: 600 cm−1) [11–13]. Two different types of substitutional sites in cubic Y2O3 are found [14]. Three sites are point-group symmetry C2 and one site is point-group symmetry C3i. Given that C3i sites have inversion symmetry, electric-dipole transitions are forbidden. Thus, the observed spectrum has been attributed to ions in the sites with C2 symmetry. However, the application of Y2O3:Ln3+ is still constrained because of its insufficient intensity [15–18]. Researchers have tried various methods to improve UC efficiency substantially, such as concentrating RE ions into transparent vitroceramic instantly in a glass matrix [19,20], growing a shell on nanocrystals [21], and reducing grain size [22]. Various studies have indicated that Ln3+ ions are particularly sensitive to their environment [23–25].

n

Corresponding author. Tel.: +86 451 82190652. E-mail addresses: [email protected] (Y. Yu), [email protected] (D. Qi). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.05.019

Chen and Huang found that Li+ and Sc3+ ions can be used to tailor the host lattice local crystal field and increase UC luminescence [23,26,27]. Gd3+ ions have larger cationic radii than Y3+ ions and can displace the Y3+ lattice easily, which can modify the symmetry of the crystal field in the lattice [28]. Therefore, we infer that the Gd3+ ions can be tridoped in Y2O3:Ho3+/Yb3+ bulk ceramics to alter the local environment and increase UC luminescence. We report an enhancement of the green UC emission intensity in Y2O3:Ho3+/Yb3+ bulk ceramics by tridoping with Gd3+ ions. The systematic experimental investigations of the effect of Gd3+ ions using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, the lifetimes of the intermediate 5I6 (Ho3+) and 2F5/2 (Yb3+) states, and the mechanism of the UC emissions are presented. Theoretical calculations have been performed to interpret the observed enhancement in the UC emissions. 2. Experimental Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, and 1.4) bulk ceramics were synthesized using the following route [29]. Yttrium oxide (Y2O3, 99.99%), ytterbium oxide (Yb2O3, 99.99%), holmium oxide (H2O3, 99.95%) and gadolinium oxide (Gd2O3, 99.99%) were dissolved in nitric acid. After the solution was dried, the corresponding nitrates were obtained. Yttrium nitrate, ytterbium nitrate, holmium nitrate, and gadolinium nitrate with corresponding mole ratio of cations were then completely dissolved in deionized water by stirring at a constant rate. Subsequently, citric acid was

Y. Yu et al. / Journal of Luminescence 143 (2013) 388–392

added into the solution with a 1:3 mol ratio of (Y+Gd+Ho+Yb) to citric acid. After complete dissolution, the pH of the solution was adjusted to 6.0 by the addition of ammonium hydroxide. The resulting solution was dried at 120 1C for 24 h until it was transformed into a black bulk, which was further calcined at 800 1C for 2 h. The calcined powders were pressed into 1 mm-thick smooth and flat disks, which then were sintered at 1300 1C for 24 h to become ceramics. The XRD of the samples were recorded on a Rigaku D/maxγB diffractometer using Cu Kα radiation (λ ¼ 0:15418 nm). FTIR spectra were measured with a Lambda 7600 FTIR spectrometer via the potassium bromide (KBr) pellet technique, in which 1 mg of the sample was diluted by approximately 100 mg KBr powder. The visible UC emission spectra were measured under a power-tunable 976 nm laser diode (Hi-Tech Optoelectronics Co. Ltd., Beijing) excitation and detected with a lens-coupled monochromator of 3 nm spectral resolution (Zolix Instruments Co. Ltd., Beijing) attached to a photomultiplier tube (Hamamatsu CR131). The photomultiplier tube (Hamamatsu CR131) was replaced by a near-infrared (NIR) sensitive InGaAs photodiode (Thorlabs, DET 4101M) when the decay times of 1030 and 1210 nm emissions were measured. These decay times were measured using square-wave modulation of the electric current input to the 976 nm diode laser and by recording the signals via a Tektronix TDS 5052 digital oscilloscope with a lock-in preamplifier (Stanford Research System Model SR830 DSP) employing a chopping rate of 3000 rps. The integrating sphere (Labsphere RT-060-SF) was used to measure the absolute UC emission efficiency. All experiments were performed at room temperature.

389

625 nm to 700 nm) correspond to the 5F4/5S2-5I8 and 5F5-5I8 transitions of the Ho3+ ions, respectively. As shown in Fig. 1, the green UC emission intensity increases by approximately 1.8-, 3.0-, 3.3-, and 3.6-fold when the relative content of the Gd3+ ions changes from 8 mol% to 20, 50, and 70 mol%, respectively. A fixed excitation power of 375 mW was used for all samples to determine accurately the relative increase in the intensity of the UC emission. Compared with the green UC emission, a slightly enhanced intensity of red UC emission was also observed with Gd3+ ion concentrations from 0 mol% to 70 mol%. The FTIR transmission spectra of Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, 1.4) bulk ceramics are shown in Fig. 3. As shown in Fig. 3, the absorption band around 570 cm−1 and 450 cm−1 are assigned to the Y–O vibration of cubic Y2O3 [30], and the highest phonon energy of Y2O3 changes to lower phonon cutoff energy by doping with Gd3+ ions. The UC efficiency is regulated by nonradiative processes, which are dependent on the highest phonon cutoff energy in the materials. The lower phonon cutoff energy can increase the UC emissions of bulk ceramics by hindering nonradiative relaxations [24]. The FTIR transmission spectra demonstrated that the UC emission efficiency of Y2O3:Ho3+/Yb3+ can enhance by doping with Gd3+ ions. The pumping power dependence of the fluorescent emissions was investigated to better understand the UC mechanism. For an unsaturated UC process, the number of photons that are necessary to populate the upper emitting state can be obtained by the following relation [31]: I f ∝P n

3. Results and discussion The main diffraction peaks of the XRD patterns of the Y1.838 −xGdxYb0.16Ho0.002O3 (x¼ 0, 0.16, 0.4, 1, 1.4) samples are illustrated in Fig. 1. All bulk ceramics are of cubic structure, corresponding well to the standard pattern of Y2O3 (JCPDS 41 1105). This condition suggests that tridoping Gd3+ ions do not change the crystal phase of the bulk ceramics. The main diffraction peaks shift toward smaller angles for Gd3+ ion concentrations of 0 mol% to 70 mol%. This phenomenon indicates that the Gd3+ ions have been doped into the Y2O3 lattice, and the host lattice dimension expands after Gd3+ ion doping. The XRD spectra demonstrated that the substitution of Y3+ ions can tailor the local crystal field around the Ho3+ ions, which is expected to tailor their radiative parameters and affect their anti-Stokes luminescence. The green and red UC emission spectra of Y1.838−xGdxYb0.16Ho0.002O3 (x¼ 0, 0.16, 0.4, 1, 1.4) bulk ceramics are illustrated in Fig. 2. The two distinct emission bands (530 nm to 600 nm and

ð1Þ

where If is the fluorescent intensity, P is the pump laser power, and n is the number of laser photons required. The pump power dependence of the green and red UC emissions in Y1.678Gd0.16Yb0.16Ho0.002O3 bulk ceramic is shown in Fig. 4. The n values are easily evaluated from the slope of the linear fit. As shown in Fig. 4, the n value of the red emission is about 2, consistent with the results reported before [29]. It indicates that a two-photon process is involved in populating the 5F5 state. The n value of the green emission is 2.47, indicating that a two-photon process and a threephoton process coordinate to populate the 5F4/5S2 states. The proposed UC mechanism is described in the energy diagram shown in Fig. 5. In the Ho3+ and Yb3+ ions codoped system, the ground- and excited-state absorptions processes of the Ho3+ ions are neglected because Yb3+ ions have a much larger absorption cross-section and efficiency energy transfer to Ho3+ ions [1]. Two consecutive energy transfer (ET1 and ET2) processes from Yb3+ to Ho3+ and a nonradiative process from 5F2,3 (Ho)

2.5

(f)

70 Gd

2.0

(e)

50 Gd

1.5

(d)

20 Gd

1.0

(c)

8 Gd

0.5

(b)

0 Gd

0.0 27

×3.6

Intensity (a.u.)

Intensity (a.u.)

3.0

JCPDS

(a) 28

29

30

31

2θ(degree) Fig. 1. (a) The standard pattern of JCPDS 05 0574. (b) to (f) The measured XRD spectra of Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, 1.4). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

20

×3.3 ×3.0

10

0

×1.8

0 Gd 8 Gd 20 Gd 50 Gd 70 Gd

525 550 575 600 625 650 675 700 Wavelength (nm)

Fig. 2. The green and red UC emissions in Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, 1.4) bulk ceramics under diode laser excitation of 976 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

390

Y. Yu et al. / Journal of Luminescence 143 (2013) 388–392

2.5

Transmission (a.u.)

160 140

2.4

120

n value

100 80 60 40

2.3

2.2

20 0 700

650

600

550

500

Wavenumber

450

400

Intensity (a.u.)

Fig. 3. Measured FTIR transmission spectra of Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, 1.4) bulk ceramics. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Green Red

240

5

=1.9

slop

7

=2.4

slop 260

2.1

(cm-1)

280 300 320 340 360 380 400

0

10

20

30

40

50

60

Fig. 6. The n values of green UC emissions as a function of Gd3+ ions under excitation of 976 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

relaxation process from 5F2,3 (Ho) to 5F4/5S2 (Ho) states are hindered by further doping with Gd3+ ions. This conclusion is consistent with the result of FTIR analysis above and gives evidence that the nonradiative process from 5F2,3 states to 5 F4/5S2 states can be neglected when doped with higher Gd3 + -ion concentrations. To theoretically account for the enhancement of the green and red UC emissions, the steady-state equations are given. In the following discussion, we just consider the condition of higher Gd3 + -ion concentrations to simplify the steady-state equations. IsNYb0 =hν þ WN 4 N Yb0 −W 0 N 0 N Yb1 −W 1 N 1 N Yb1 −W 2 N 2 N Yb1 −N Yb1 =τYb ¼ 0; ð2Þ

Pump Power (mW) Fig. 4. The typical pump power dependence of the green and red UC emissions in Y1.678Gd0.16Yb0.16Ho0.002O3 bulk ceramic. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Simplified energy-level diagram of the Ho3+ and Yb3+ ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

states to 5F4/5S2 (Ho) states are needed to promote the 5F4/5S2 states, generating the green UC emission. The nonradiative from 5 I6 (Ho) state to 5I7 (Ho) state and the emission from 5F4/5S2 states to 5I7 state processes can populate the 5I7 state, which is further excited to the 5F5 (Ho) state (using ET3) for the red UC emission. Additionally, the nonradiative from the 5F4/5S2 states to the 5F5 state process also populates the 5F5 state. The n values of the green UC emissions as a function of Gd3 + -ion concentrations are shown in Fig. 6. As shown in Fig. 6, the n values of the green UC emissions decrease and are closer to 2 with increasing of Gd3+ ions. It indicates that the nonradiative

70

Gd3+ concentration (mol%)

WN 4 N Yb0 þ W 0 N 0 N Yb1 −W 2 N 2 N Yb1 −N 2 =τ2 ¼ 0;

ð3Þ

W 2 N2 NYb1 −WN 4 N Yb0 −W CR N 0 N 4 −N 4 =τ4 ¼ 0;

ð4Þ

R4 N 4 þ W non2 N 2 þ W CR N0 N 4 −W 1 N 1 N Yb1 −N 1 =τ1 ¼ 0;

ð5Þ

W 1 N1 NYb1 þ W non4 N 4 −N3 =τ3 ¼ 0;

ð6Þ

1=τ4 ¼ W non4 þ 1=τrad4 þ WN Yb0 þ W CR N 0 ;

ð7Þ

βgreen ≡τ4 =τrad4

ð8Þ

where NYb0 and NYb1 (τYb) are the population densities (decay time) of Yb3+ ions in the ground and excited states, respectively. N0 (W0), N1 (W1, τ1), N2 (W2, τ2), N3 (τ3) and N4 (τ4) are the population densities (ET rates from excited Yb3+ ions, decay time) of the 5I8, 5I7, 5I6, 5F5 and 5F4/5S2 states of Ho3+ ions, respectively. W is the efficiency of the energy back transfer (EBT) process [5F4/5S2(Ho)+2F7/2(Yb)-5I6(Ho)+2F5/2(Yb)], WCR is the efficiency of the cross-relaxation (CR) process [5F4/5S2(Ho)+5I8(Ho)-5I4(Ho) +5I7(Ho)], s is the absorption cross-section of the Yb3+ ions, I denotes the laser intensity, and ν is the laser frequency. Wnon2 and Wnon4 represent the nonradiative decay rate from 5I6 to 5I7 and 5 F4/5S2 to 5F5 states, respectively. R4 is the radiative decay rate from 5F4/5S2 to 5I7 states. τrad4 and βgreen are the radiative lifetime and the luminescent population ratio of the 5F4/5S2 states, respectively. The WN4NYb0 process is neglected in the following discussions because its contribution to NYb1 is much lower than that induced by the laser population [32]. The nonradiative rate from 5 I6 to 5I7 state is higher than the radiative decay rate of the 5F4/5S2 state to the 5I7 state and the CR process, for the long lifetime of 5I7 state. Additionally, the ET rates are supposed to be much lower than the decay rates [33]. Thus, the ET rates may be neglected. According to Eqs. (2)–(8), we can easily obtain

Y. Yu et al. / Journal of Luminescence 143 (2013) 388–392

N Yb1 ¼ sIN Yb0 τYb =hυ∝I;

ð9Þ

N 3 ¼ W non2 W 0 W 1 N 0 N2Yb1 τ1 τ2 τ3 ∝I 2 ;

ð10Þ

N 4 ¼ W 0 W 2 N 0 N 2Yb1 τ2 τ4 ∝I 2 ;

ð11Þ 2

I green ¼ βgreen s2 W 0 W 2 N 0 N 2Yb0 I 2 τ2 τ2Yb =h υ2 ;

ð12Þ

I red ¼ ð1=βgreen −1ÞI green

ð13Þ

Eq. (13) is directly derived from our recent work in Ref. [23]. As illustrated in Eq. (12), the green UC fluorescent intensity strongly depends on the lifetime of the 5I6 interstate and the lifetime of the 2F5/2 state (τ2Yb τ2 ), the absorption cross-section (s2 ), the concentration of the Yb3+ ions (N 2Yb0 ), the laser pump power 2 density [I 2 =ðhνÞ ], the luminescent population ratio in the 5F4/5S2 states (βgreen), the Ho3+ concentration (N0), and the ET rates of ET1 and ET2 (W0W2). The same pump power density was utilized to compare the UC emission intensity, which implies a constant 2 parameter I 2 =ðhνÞ for all the samples. Furthermore, the same ion concentrations of 0.1 mol% Ho3+ and 8 mol% Yb3+ ions are employed for all the samples, which suggests the same s2 , N 2Yb0 , N0, and W0W2 parameters. Therefore, the parameters that can lead to changes in the UC emission intensity are (τ2Yb τ2 ) and βgreen. To discuss the influence of the lifetimes (τ2Yb τ2 ), the decay times of the 2F5/2(Yb)-2F7/2(Yb) (1030 nm) and 5I6(Ho)-5I8(Ho) (1210 nm) transitions in Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, 1.4) bulk ceramics are displayed in Fig. 7. The contrasted IR spectra are shown in Fig. 8. For easy comparison, these experimental decay times in Fig. 7 are compiled in Table 1. The decay times of the 5I6-5I8 and 2F5/2-2F7/2 transitions increased with Gd3+ ions from 0 mol% to 70 mol%, respectively. The intensity

391

of the UC emissions increased. However, the different increased intensities of the green and red UC emissions for all the Gd3+ ion concentrations were not evident. To understand this phenomenon, the influence of parameter βgreen is discussed. According to Eqs. (7) and (8), the parameter βgreen directly relates to the EBT [5F4/5S2(Ho)+2F7/2(Yb)-5I6(Ho) +2F5/2(Yb)] process, which can be observed when doped with 8 mol% Yb3+ ions [34], and the CR [5F4/5S2(Ho)+5I8(Ho)-5I4(Ho) +5I7(Ho)] process [35]. The host lattice dimension expands after Gd3+ ion doping from the analyze of XRD, which is expected to enlarge the distance between the Yb3+–Ho3+ and Ho3+–Ho3+ ion clusters [26,28]. This condition indicates that the EBT and CR processes are hindered by doping the Gd3+ ions. The hindrance of the EBT and CR processes can result in the increase in parameter βgreen that consequently reduces the coefficient (1/βgreen−1) in Eq. (13). This phenomenon explains why the green UC emission intensity is enhanced more than the red UC emission. Additionally, the absolute green UC efficiency of Y0.438Gd1.4 Yb0.16Ho0.002O3 was investigated. In order to measure the absolute UC emission efficiency, an integrating sphere was used. The calibrated spectra of Y0.438Gd1.4Yb0.16Ho0.002O3 bulk ceramic under the pump-power density of 54.8 W/cm2 are shown in Fig. 9. The curve marked by “a” corresponds to the experiment procedure that the sample is placed inside the sphere and laser light beam is directed on to the sample. The curve marked by “b” is the experiment procedure that the sample is placed inside the sphere and laser light beam is directed on to the sphere wall. The curve marked by “c” corresponds to the experiment procedure that the sphere is empty and laser light alone is detected by the spectrometer [36]. According to the method described by Mello [36], the green UC emission power-conversion efficiency η of Y0.438Gd1.4 Yb0.16Ho0.002O3 was calculated by the calibrated spectra and is equal to 0.34. Therefore, the absolute green UC emission efficiency of Y0.438Gd1.4Yb0.16Ho0.002O3 is equal to 6.2  10−3 cm2/W, which is

Fig. 8. The IR fluorescence spectra of 5I6(Ho)-5I8(Ho) and 2F5/2(Yb)-2F7/2(Yb) transitions in Y1.838Yb0.16Ho0.002O3 bulk ceramic under laser excitation of 976 nm.

Table 1 Lifetimes of the 5I6 state of Ho3+ ions and the 2F5/2 state of Yb3+ ions for Y2O3 tridoped with various Gd3+ ion concentrations. Gd3+concentration

Fig. 7. (a) and (b) are the decay times of the 5I6(Ho)-5I8(Ho) and 2F5/2(Yb)-2F7/ 2(Yb) transitions in Y1.838−xGdxYb0.16Ho0.002O3 (x ¼0, 0.16, 0.4, 1, 1.4) bulk ceramics, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(mol%)

τYb(μs)(2F5/2)

τ2(μs) (5I6)

τYb2τ2(107μs3)

0 8 20 50 70

338(5) 522(5) 531(5) 606(5) 656(5)

425(5) 694(5) 709(5) 819(5) 877(5)

5.0(0.5) 18.5(0.5) 19.9(0.5) 30.0(0.5) 37.7(0.5)

Y. Yu et al. / Journal of Luminescence 143 (2013) 388–392

Intensity (a.u.)

392

24 22 a 20 b 18 c 16 14 12 10 8 6 4 2 0 500 600 700 800 950 Wavelength (nm)

1000

Fig. 9. Calibrated spectra of Y0.438Gd1.4Yb0.16Ho0.002O3 bulk ceramic at the pumppower density of 54.8 W/cm2. The curves marked by a, b, c correspond to different experiment procedures described by Mello et al. [36]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

lager than that of the fluorohafnate glass doping with Yb3+–Ho3+ (8.4  10−4 cm2/W) [37]. 4. Conclusion The green UC emission at 550 nm was enhanced with the relative content of Gd3+ ions from 0 mol% to 70 mol% doped in Ho3 + /Yb3+-codoped Y2O3 bulk ceramics. The observed enhancement rate of the green UC emission was 3.6-fold. The lower phonon cutoff energy and prolonged lifetimes of the 5I6 (Ho) and 2F5/2 (Yb) states enhanced the green and red UC emission intensities. The different increased intensity ratios of the green and red UC emissions were caused by the Ho3+–Yb3+ and Ho3+–Ho3+ ion cluster dissociation induced by the Gd3+ ions. Thus, Gd3+ ions might be advantageous sensibilizers for Ho3+ ions. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities of China (Grant No. DL12BB15). References [1] J.F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K.W. Krämer, C. Reinhard, H.U. Güdel, Opt. Mater. 27 (2005) 1111. [2] S. Sivakumar, F.C.J.M. van Veggel, P. Stanley May, J. Am. Chem. Soc. 129 (2007) 620.

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