Effect of Yb3+ concentration on photoluminescence properties of cubic Gd2O3 phosphor

Infrared Physics & Technology 68 (2015) 92–97

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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared

Effect of Yb3+ concentration on photoluminescence properties of cubic Gd2O3 phosphor Raunak Kumar Tamrakar a,⇑, D.P. Bisen b, Nameeta Brahme b a b

Department of Applied Physics, Bhilai Institute of Technology (Seth Balkrishan Memorial), Near Bhilai House, Durg, C.G. 491001, India School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, C.G. 492010, India

h i g h l i g h t s  Yb

3+

doped Gd2O3 phosphor synthesized by solid state reaction. 3+ doped Gd2O3 phosphor. 3+  Luminescent properties of Yb doped Gd2O3 phosphor. 3+  NIR and visible phenomenon of Yb doped Gd2O3 phosphor. 3+  Yb doped Gd2O3 material which can be used for producing NIR laser diode.

 Structural and morphological behavior of Yb

a r t i c l e

i n f o

Article history: Received 11 August 2014 Available online 18 November 2014 Keywords: Gd2O3:Yb3+ phosphor Solid state synthesis PXRD TEM Cooperative upconversion

a b s t r a c t Yb3+ doped phosphor of Gd2O3 (Gd2O3:Yb3+) have been prepared by solid state reaction method. The structure and the particle size have been determined by X-ray powder diffraction measurements. The average particle size of the phosphor is in between 35 and 50 nm. The particle size and structure of the phosphor was further confirmed by TEM analysis. The visible and NIR luminescence spectra were recorded under the 980 nm laser excitation. The visible upconversion luminescence of Yb3+ ion was due to cooperative luminescence and the presence of rare earth impurity ions. The cooperative upconversion and NIR luminescence spectra as a function of Yb3+ ion concentration were measured and the emission intensity variation with Yb3+ ion concentration was discussed. Yb3+ energy migration quenched the cooperative luminescence of Gd2O3:Yb3+ phosphor with doping level over 5%, while the NIR emission luminescence continuously increases with increasing Yb3+ ion concentration. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, Trivalent Yb3+ doped materials have attracted great interest for applications in high efficiency and high power diode pumped laser systems with advance in high performance laser diode with wavelength between 900 and 1100 nm [1–3]. It has very simple electronic structure which leads to very low quantum defects, reducing thermal loads and preventing undesired effects such as upconversion and excited state absorption and weak concentration quenching, which is suitable for its different applications such as enhanced luminescence efficiency and diode laser systems [4]. The main interest of Yb3+ ion lies in the development of laser diode. Yb3+ activator ion possesses many advantages ⇑ Corresponding author. Tel.: +91 09827850113. E-mail addresses: [email protected], [email protected] (R.K. Tamrakar). http://dx.doi.org/10.1016/j.infrared.2014.10.020 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.

[email protected],

because of its simple energy level diagram [5]. The Yb3+ ion has only two manifolds, the ground state 2F7/2 and upper level 2F5/2, which are separated by about 10,000 cm1. Moreover, it shows a weak non-radioactive transition, large crystal field splitting and long radiative lifetime of the metastable 2F5/2 state. Unfortunately, the quite low absorption cross-section of the Yb3+ ion requires quite high ytterbium concentrations. Another advantage of Yb3+ compared to other dopants such as neodymium is its broadband nature which is very suitable for both tunable and ultrafast lasers [6]. Upconversion is a process where light can be emitted with photon energies higher than those of the light generating the excitation. Cooperative luminescence is one of the mechanism responsible for upconversion. It involves energy transfer processes between different ions. For example two laser ions in a metastable intermediate level interact to excite one ion into a higher lying state while the other one becomes deexcited. The literature revels

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various cooperative upconversion phenomenon, Nakazawa and Shionoya observed this phenomenon in 1970 for Yb3+ in YbPO4 [7]. Malinowski et al. [8], have also reported blue cooperative emission centered at 484 nm in Yb:Gd2O3 [9–11]. Yb3+ ions doped Gadolinium oxide (Gd2O3) is may be used as promising material for solid-state lasers. Gd3+ and Yb3+ trivalent ions have nearly the same ionic radius [12]. Gd2O3 is an excellent host material that is used for various applications, since its properties include chemically stability, high melting point, high thermal stability, low thermal expansion, high thermal conductivity and good transparency to infrared radiation [13–15]. In present work, the results of optical studies on Yb3+ doped Gd2O3 phosphor prepared by solid state reaction method have been discussed. Our interest is primarily concerned with preparation of phosphor, its characterization and mainly its luminescence behavior. We have measured the emission spectra of the prepared phosphor as a function of Yb3+ ion concentration. The influence of Yb3+ ion concentration on both visible and NIR emission is discussed.

3. Results and discussion 3.1. Powder X-ray diffraction (XRD) result The crystalline structure of the prepared phosphor was characterized by X-ray diffraction (XRD) using Bruker D8 Advanced X-ray diffractometer using Cu Ka radiation. The X-rays were produced using a sealed tube and the wavelength of X-ray was 0.154 nm. The X-rays were detected using a fast counting detector based on Silicon strip technology (Bruker Lynx Eye detector). The crystal size was calculated from the XRD pattern using the Scherer’s equation [16].

D ¼ 0:9k=b cos h Here D is the crystalline size of the (hkl) plane, k is the wavelength of the incident X-ray radiation [Cu Ka (0.154056 nm)], b is the full

55

2. Experimental Average Particle size (nm)

50

The Gd2O3:Yb3+ phosphor has been prepared using solid state reaction method [14]. The high purity raw materials gadolinium oxide (Gd2O3) and ytterbium oxide (Yb2O3) have been used. All the raw materials were purchased by sigma Aldrich. The rare earth oxides Yb2O3 and Gd2O3 were weighed according to a specific molar ratio to synthesize Gd2O3:Yb3+ phosphor. These chemicals were weighed and ground into a fine powder by using agate mortar and pestle. After the compounds ground and mixed the mixture was placed in an alumina crucible and heated at 1100 °C for 1 h followed by dry grinding and further heated at 1400 °C for 4 h in a muffle furnace. The sample is allowed to cool at room temperature. To determine the influence of dopant Yb3+ concentration on the luminescence properties, the Gd2O3:Yb3+ samples with different Yb3+ concentration (from 5% to 35%) have been prepared by using above mentioned procedure.

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Fig. 1. XRD patterns of Gd2O3:Yb

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Fig. 2. Relation between particle size and Yb3+ concentration.

(222)

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Percentage

at different concentration of Yb3+ (5–35 mol%).

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width at half maximum (FWHM) in radians, and h is the diffraction angle of the (hkl) plane. All the diffraction peaks are in agreement with those of the JCPDS card no. 43-1040 for Gd2O3 and confirm that the sample have a cubic structure [17]. The XRD patterns for phosphor containing different Yb3+ ion concentration were pre-

sented in Fig. 1. The calculated average particle size was in the range of 35–50 nm. The average particle size increases with increasing Yb3+ ion concentration (Fig. 2). It may be due to the small grain growth of Yb3+ doped Gd2O3 phosphor as comprised with pure Gd2O3 phosphor [15]. The values of average particle size are listed in Table 1.

Table 1 Particle size calculation of Gd2O3:Yb3+.

3.2. FTIR results The formation and purity of the products were further confirmed by FTIR spectroscopy and results are depicted in Fig. 3(a-e). The strong absorption peak near 540 cm1 is associated with the vibration of the Gd–O bond [18]. Further, the absorption peak at 3410 cm1 due to H2O, which normally nanocrystalline materials absorb from the environment due to its high surfaceto-volume ratio. On the basis of FTIR results it can be further inferred that the powders are essentially free from the nitrategroup (2213–2034 cm1). We found no more significant difference in FTIR spectra with increasing Yb3+ %. As similar to PXRD results.

XRD analysis of Gd2O3:Yb3+ Yb3+ (5-35 mol%) 3+

Yb

percentage

2h

FWHM in h

Particle size (nm)

Average particle size (nm)

5%

28.41 47.41

0.22 0.23

36.84 37.30

37.07

10%

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40.52 40.85

40.69

15%

28.39 47.38

0.19 0.2

42.65 42.90

42.77

20%

28.46 47.44

0.18 0.19

45.03 45.16

45.1

25%

28.23 47.82

0.17 0.18

47.65 47.74

47.7

30%

28.53 33.07

0.165 0.165

49.13 49.67

49.4

35%

28.52 47.49

0.16 0.17

50.67 50.49

50.58

3.3. Transmission electron microscope (TEM) results TEM image of the prepared Gd2O3:Yb3+ phosphor was used to characterize the morphology, structure and particle size for the sample. The average crystallite size is around 40–55 nm (Fig. 4a–e). The crystal structure and particle size obtained by the analysis of TEM micrograph are in good agreement with that

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3500

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2500 2000

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Wavenumber (cm-1) Fig. 3. FTIR spectra of Gd2O3:Yb3+ particles at different concentration of Yb3+: (A) 5 mol%, (B) 15 mol%, (C) 25 mol%, (D) 30 mol% and (E) 35 mol%.

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Intensity (Arb Units)

observed by XRD results. HRTEM results indicate that the particle size of the prepared phosphor increases with the increase in Yb3+ ion concentration [3]. Fig. 4(F) shows the SEAD patterns of the Gd2O3:Yb3+ (15 mol%) phosphor. In the XRD analysis, we found many diffraction (hkl) planes; (2 2 2), (4 0 0), (4 4 0) and (6 2 2) planes; this are identical with the SEAD pattern. We therefore conclude that the TEM results are in good agreement with the result of the XRD studies.

3+

Gd 2O3 :Yb (10%)

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4. Optical properties 4.1. Photoluminescence characteristics

0 900

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The emission spectrum of Yb3+:Gd2O3 crystal at room temperature was recorded. The emission spectra of phosphor were found in near infrared region (NIR) (Fig. 5). The NIR spectrum of Gd2O3:Yb3+ phosphor have emission peaks at 1020 nm,1064 nm and 965 nm. The emission peak at 1020 nm is the intense band whereas the other two emission peaks at 1064 nm and 965 nm are comparatively smaller. The NIR emission peaks are due to the transitions between the 2F7/2 excited state and the 2F5/2 ground state for

Fig. 5. NIR emission spectra of Yb3+:Gd2O3.

Yb3+ in Gd2O3. The two energy states splits into seven stark levels, 1–4 levels for the ground 2F5/2 and 5–7 levels for 2F7/2 excited state. The intense peak at 1020 nm attributed to the transition from stark level 5 ? 1, the peaks at lower wavelength 965 nm and at higher

Fig. 4. TEM image of Gd2O3:Yb3+ particles for different concentration of Yb3+: (A) 5 mol%, (B) 15 mol%, (C) 25 mol%, (D) 30 mol% and (E) 35 mol% (F) SEAD pattern.

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wave length 1064 nm are due to transition 6 ? 1 and 5 ? 4 respectively (Fig. 6). The prepared phosphor gives emissions in visible region also. The visible upconversion spectrum of Yb3+:Gd2O3 crystal at room temperature has emission peaks at 364 nm, 489 nm, 545 nm, 590 nm and 624 nm (Fig. 7). The energy level scheme of Yb3+ ion has no energy levels in the visible region it has only the 2F7/2 ground state and the 2F5/2 excited state, have energy difference around 10,000 cm1. Probable mechanism for the visible emissions by Yb3+ doped Gd2O3 phosphor may be explained by using different phenomenon like cooperative upconversion transition, charge transfer transition or presence of other rare earth impurities. The 4f electrons of Yb3+ interact strongly with its surrounding but that interaction is comparatively stronger with neighbouring Yb3+ than other rare earth ions. This interaction phenomenon in known as cooperative transition. This cooperative transition probability which involves the simultaneous radiative relaxation of a pair of excited Yb3+ ions accompanied by the emission of a visible photon may be responsible for the blue emission at 489 nm. The cooperative phenomenon can be represented in the following manner [7]: 



Yb ð2 F5=2 Þ þ Yb ð2 F5=2 Þ ! 2Ybð2 F7=2 Þ þ hm; The photon energy of the cooperative luminescence is nearly exactly twice the energy of the normal (single-ion) luminescence.

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þ IR photon ! Yb

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! ðYb Þ2 F7=2 þ visible photons

Yb Yb

3þ

3þ

The asterisk sign represents the excited state of the corresponding ion. The energy levels positions for Yb3+ and Yb2+ are different. This difference is due to structural distortion due to difference in their ionic radii [20]. Another probable mechanism responsible for visible emission may be due to presence of impurity of some other rare earth ions. Many rare earth ions have approximately similar energy levels, so resonant energy transfer is possible between them. Many rare earth ions like Er3+, Tm3+, Ho3+ etc. have approximately 10,000 cm1 energy range matching with the excited state of Yb3+ ion, therefore energy transfer from excited state of Yb3+ to the excited energy levels of these rare earth ions takes place due to resonant (In Er3+ ion) or non resonant energy transfer [21–23]. Rare earth ions consists impurities of many other rare earth ions because they are chemically related to each other. The emission bands in the visible luminescence spectra centered at 545, 590 and 624 nm may be due to the presence of the Tm3+, Er3+, and Ho3+, etc. impurities in Yb3+:Gd2O3 crystal (Fig. 6). The luminescence at 545 nm may be considered as the upconversion energy transfer from the 2F7/2–2F5/2 Yb3+ absorption transition to the Er3+ 4 I11/2–4F7/2 absorption transition followed by the 4S3/2–4I15/2 emission transition [23–25] or to the Ho3+ 5I6–5S2 absorption transition followed by the 5S2–5I8 emission transition [26–28]. The transition 5 F5–5I8 of Ho3+ ions, 4F9/2–4I15/2 of Er3+ ions or 1G4–3H4 of Tm ions may be responsible for the up-conversion luminescence centered at around 624 nm. The upconversion luminescence of these ions is excited by infrared light through two-or three-step energy transfer from Yb3+ ions in general [3,28]. 4.2. Effect of Yb3+ ion concentration on visible and NIR emission

Fig. 6. Energy level diagram for the NIR transitions.

20

These emissions in visible region may also be correlated with the charge transfer (CT) transition between 2F7/2 and 2F5/2 states of Yb3+. Some Yb2+ doped phosphors gives emission bands in visible region [19]. The process for origin of visible emission of the prepared phosphor is related with a photoinduced change in dopant valence which is associated with a transition of electron from the adjacent oxygen atom. This transition can be represented in following manner:

650

Wavelength (nm) Fig. 7. Visible emission spectra of the Gd2O3:Yb3+ phosphor.

For 1–4 mol% no observable emissions were found in NIR region. After 5 mol% the emission spectra consists emission in IR region in between 960 and 1100 nm (Fig. 8). The NIR emission due to Yb3+ ion increases with increasing Yb3+ ion concentration. No quenching observed for NIR emission after 5 mol% Yb3+ ion concentration. Whereas the intensity of visible upconversion spectra initially increases and then decreases (Fig. 9). The highest cooperative upconversion (CUC) was obtained for 5 mol% doped sample. The low emission for 1–4 mol% doped samples is explained in terms of the formation of only a few pairs of due to low concentration. The decrease in emission for 6 mol% and higher concentration may be due to the formation of cluster, which destroys the formed pair of Yb3+ ion. As Yb3+ concentration increases more pairs are formed reducing the emission of single Yb3+ ions but after some points the cluster formation starts. The cluster formation for large concentration diminishes the emission by quenching both the CUC emission of the formed pairs and the fluorescence of single Yb3+ ion due to nonradiative process such as migration between ions [28,29–31].

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Intensity (Arb Units)

150 125 100

Conflict of interest

5% 10% 15% 20% 25% 30% 35%

(1020)

Their is no conflict of interest. References

(965)

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Wavelength (nm) Fig. 8. Effect of Yb3+ ion concentration on NIR emission spectra of Gd2O3 phosphor.

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Wavelength (nm) Fig. 9. Effect of Yb3+ ion concentration on CUC.

5. Conclusion The Gd2O3:Yb3+ phosphor has been prepared by using conventional solid state reaction method. The prepared phosphor was characterized by using powder XRD, FTIR and TEM analysis method. For optical characterization of the prepared phosphor emission spectra of the phosphors were recorded. The emission spectra consists peaks in NIR region as well as in visible region. The NIR emission peaks are due to transition between 2F7/2 and 2 F5/2 states of Yb3+ ion whereas the peaks in visible region is due to cooperative upconversion phenomenon of Yb3+ ion and the emission bands centered at 545 nm, 590 nm and 624 nm was due to presence of Tm3+, Er3+ and Ho3+, etc. impurities. The emission spectra were recorded as a function of Yb3+ ion concentration. It was observed that after 5 mol% concentrations of Yb3+ ion visible bands have maximum intensity after 5 mol% quenching was observed for cooperative visible luminescence. The NIR emission spectra increases with increasing Yb3+ concentration, no quenching is observed.

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