Aluminum effects on structure and spectroscopic properties of holmium-doped sol–gel silica glasses

Journal of Alloys and Compounds 657 (2016) 478e482

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Aluminum effects on structure and spectroscopic properties of holmium-doped solegel silica glasses Xue Wang a, b, *, Wenbin Xu a, b, Shikai Wang a, **, Chunlei Yu a, Danping Chen a, Lili Hu a, *** a b

Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 September 2015 Received in revised form 12 October 2015 Accepted 17 October 2015 Available online 21 October 2015

A series of Ho3þ-doped silica glasses containing different Al2O3/Ho2O3 mole ratios were prepared successfully using a solegel method combined with high temperature sintering. The X-ray diffraction (XRD), nuclear magnetic resonance (NMR) spectra of 27Al, and X-ray photoelectron spectra (XPS) of O1s, as well as the absorption and fluorescence spectra of the Ho3þ ions in these silica glasses, were recorded and analyzed systematically. The 27Al-NMR spectral studies indicated that Al3þ ions entered the glass structure mainly in sixfold and fivefold co-ordination for the series of solegel HAS glasses with constant 0.2 mol% Ho2O3 content. The NMR and XPS results obtained indicated a relationship between the amounts of Al2O3 and Ho2O3 and the Al coordination. Based on the structure and spectroscopic properties of the Ho3þ-doped silica glasses, it was concluded that the optimum Al2O3/Ho2O3 mole ratio for the Ho3þ/Al3þ co-doped silica glass for a 2-mm laser was 15. © 2015 Elsevier B.V. All rights reserved.

Keywords: Spectroscopy of Ho3þ ion Solegel method 27 Al NMR Structure

1. Introduction In aluminosilicate glasses, it is generally assumed that unless the molar concentration of Al is in excess of that of the charge balancing cations, all of the Al cations have only four oxygen neighbors. However, direct experimental evidence has been reported to the contrary, with the exception of Mg-aluminosilicates. In binary alumina silica and aluminosilicate glasses, with high-field strength modifier cations, AlO5 and/or AlO6 coordinated ions are present [1]. Sinko and Mezei suggested that the Al3þ ions are incorporated into the network of wet gels by the SieOHeAl and/or SieOeAl (OH) bonds, rather than by the SieOeAl bonds, in tetrahedral sites [2]. When the water and OH groups disappear, the Al has a tendency to occupy the octahedral position [3]. Ho-doped materials are usually used to realize a 2.0 mm laser because of the 5I7/5I8 transition, which have received considerable

* Corresponding author. Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (S. Wang), [email protected] (L. Hu). http://dx.doi.org/10.1016/j.jallcom.2015.10.144 0925-8388/© 2015 Elsevier B.V. All rights reserved.

attention because of their valuable applications [4,5]. The request for Ho3þ-doped glasses as high-power and high-repetition laser materials combine desirable environmental stability and high solubility of dopants. Generally, a high Ho3þ doping concentration is necessary to achieve sufficient pump absorption even in a cladding pumped configuration. This reduces the fiber length and increases the threshold for undesirable nonlinear effects in the fiber. In this regard, co-doping with Al3þ ions in silica glass is the most effective way to increase the Ho3þ solubility in silica glass and avoid clustering because of their good “dispersion effect” on holmium ions [6e9]. Although a moderate quantity of Al3þ can decrease the clustering of RE ions in silica glass and enhance its emission intensity, a further addition of Al3þ ions may decrease the emission intensity [10,11]. Previous papers reported that the optimum Al2O3/Ho2O3 mole ratio of Ho3þ/Al3þ co-doped silica glass to realize a 2-mm laser was 15. However, unfortunately, the reason was not given [12]. The effects of aluminum co-doping on the fluorescence and structural properties of rare-earth-doped silica glasses have previously been reported, with most studies based on the plasmatorch chemical vapor deposition method [13], porous silica glass techniques [14] and other traditional glass melting methods. There have been several attempts to prepare aluminum co-doped Tm2O3eSiO2 glass using the solegel process combined with high temperature sintering [15]. Er3þ-doped solegel glass with

X. Wang et al. / Journal of Alloys and Compounds 657 (2016) 478e482 Table 1 Mean compositions of Ho3þ/Al3þ-doped silica glasses (mol%). Sample

Ho2O3

Al2O3

SiO2

Al/Ho

Phase assemblage and remarks

HAS0 HAS1 HAS2 HAS3 HAS4 HAS5 HAS6

0.20 0.20 0.20 0.20 0.20 0.20 0.20

0 1.0 2.0 3.0 4.0 5.0 6.0

99.8 98.8 97.8 96.8 95.8 94.8 93.8

0 5 10 15 20 25 30

Cristobalite, Opacity Glass, Transparent Glass, Transparent Glass, Transparent Glass, Transparent Glass, Transparent Cristobalite, Traces crystallization

NBO/BO+NBO (%)

8

7

6

5 5

10

15

20

25

Al2O3/Ho2O3 Fig. 1. Relative number ratio of NBO as function of Al2O3/Ho2O3 mole ratio.

aluminum co-doping has an enhanced emission spectrum [16]. In the present work, the solegel method was used to fabricate Al3þ/ Ho3þ-codoped silica glasses, and X-ray diffraction (XRD) was used for a comprehensive study of the Al3þ on the solubility of the Ho3þ in the silica glass. The effects of the Al3þco-dopant on the nuclear magnetic resonance (NMR) and O1s X-ray photoelectron spectra (XPS) were also studied to obtain structural information. The absorption and emission spectra were studied to obtain the spectroscopic properties. 2. Experiments Tetraethoxysilane (TEOS), AlCl36H2O and HoCl36H2O were

479

chosen as the precursors. Hydrochloric acid and deionized water were added to sustain the hydrolysis reaction, and the mixtures were stirred 24 h at 30  C to form homogeneous and clear doping solutions. The solutions were heated from 70  C to 1100  C to achieve dried gel powders whose organics were almost decomposed. Then, the powder was sintered at 1750  C for 3 h in a vacuum state to form glass. A series of Ho3þ-doped silica glasses (series HAS) were prepared. The mean glass composition of each sample is listed in Table 1. The Ho2O3 content amounted to 0.2 mol% in series HAS. The samples in series HAS were co-doped with increasing concentrations of only Al2O3 to investigate the effect of the Al3þcontent on the structure and spectroscopic properties. A glass chip was polished to a thickness of 2 mm for the optical property measurements. The density and refractive index of the glass samples were measured with the Archimedes method using distilled water as an immersion liquid and the prism minimum deviation method. The phase assemblage was identified with X-ray diffraction (XRD; D8 Focus, Bruker, Germany) using CueKa radiation. The absorption spectra were recorded by a PerkinElmer Lambda 900 UV/VIS/NIR spectrophotometer in the range of 200e2200 nm. The emission spectra were measured with an Edinburgh FLS920 spectrometer using a 640-nm laser diode as the excitation source. The fluorescence lifetime was measured via pulsed 640-nm LD excitation using the FLSP 920 instrument. NMR measurements were carried out at magnetic field strengths of 11.7 T. 27Al magic angle spinning (MAS)-NMR spectra were acquired at the resonance frequency of 130.3 MHz, using a Bruker Avance III HD 500 MHz NMR spectrometer equipped with a 4 mm MAS-NMR probe operated at a rotor frequency of 12.0 kHz. Typical pulse length is 0.95 ms (30 solid flip angle). And the relaxation delay was 0.2 s. Chemical shifts are referenced to a 1 M aqueous solution of Al(NO3)3. XPS spectra were recorded on K-Alpha (Thermo Fisher Scientific, USA) using a monochromatic Al source (Ka, 1486.6 eV), operating at a constant pass energy of 200.0 eV. All of the measurements were conducted at room temperature. 3. Results and discussion 3.1. Oxygen 1s XPS spectra Table 1 summarizes the phase assemblage results for all the investigated compositions. HAS0 and HAS6 encountered crystallization and the other samples were pure glasses. It is evident that

HAS5

HAS3

HAS1 100

80

60

40

20

0

-20

-40

80

60

40

20

ppm

ppm

(a)

(b)

0

Fig. 2. (a) 27Al MAS NMR spectra of series HAS glasses (b) Deconvolution of 27Al MAS NMR spectrum for the HAS5 glasses, the dotted lines show the components of the overall line shape deconvolution.

480

X. Wang et al. / Journal of Alloys and Compounds 657 (2016) 478e482

Table 2 27 Al Isotropic Chemical Shifts diso and Fractions of the Different Aluminum Sites of the HAS glasses. Sample

% Al4

%A5

%A6

d4iso (ppm)

d5iso (ppm)

d6iso (ppm)

HAS1 HAS3 HAS5

11.8 40.4 14.9

12.7 25.8 23.7

75.5 33.8 61.4

45.9 48.8 45.1

27.1 31.0 26.8

13.9 13.8 13.9

the glass-forming ability depended on the amounts of Al2O3 and Ho2O3. The O1s XPS spectrum can be considered an important tool to distinguish the bridging oxygen from the non-bridging oxygen in the HAS glasses. Considering the charge density, the non-bridging atom peak will be shifted toward a lower binding energy with respect to the bridging oxygen atom peak [17]. Thus, after splitting the O1s spectra into two peaks, the lower bonding peak at ~531 eV was associated with the non-bridging oxygen (NBO), and the higher energy peak at ~533 eV was related to the bridging oxygen (BO). From the O1s spectrum, the NBO and BO peaks were deconvoluted, which gave the variation of NBO and BO with the increase in the Al2O3/Ho2O3 mole ratio [18]. Fig. 1 shows that the relative number ratio of NBO decreases with the increase in the Al2O3/Ho2O3 mole ratio up to 15. Then it increases. The reason will be given in the following structure discussion.

Fig. 2(a) shows the 27Al MAS NMR spectra of the HAS glasses with different Al2O3/Ho2O3 ratios. A main peak is seen at around 13 ppm (the maximum position), but a second shoulder at around 50 ppm and a bump around 0 ppm evidence the presence of two other spectral components. These three peaks are classically assigned to 4-, 5- and 6-fold coordinated aluminum environments (Al(4),Al(5) and Al(6), respectively) [19e21]. The best fit average 27 Al isotropic chemical shift diso and quantitative Al species concentrations were obtained by deconvolution of the experimental 27 Al MAS NMR spectra. The deconvolution of 27Al MAS NMR spectrum of HAS5 that is an example for series HAS glasses are shown in Fig. 2(b). The corresponding 27Al line-shape parameters deduced from such measurements are included in Table 2. Fig. 2 and Table 2 show that the aluminum ions enter the glass predominantly in the sixfold and fivefold coordination for the series of HAS glasses, demonstrating that in every case the aluminum is acting principally as a network modifier. Previous 27Al NMR of

Absorption intensity (a.u)

5

0.2

0.0 200

3

3

5

5

S + F4 5 G F5 G4 5 5F 2 3

400

U6

tc

tm

h

(1020 cm2)

(1020 cm2)

(ms)

(ms)

(%)

11.70 12.13 13.20 9.92 7.44

2.59 2.64 2.69 2.64 1.49

1.19 1.17 1.18 1.09 0.89

13.60 13.87 13.63 14.62 17.05

0.91 1.04 1.19 1.23 1.33

6.69 7.50 8.73 8.41 7.80

160000

H4+ H6( G2)

0.1

U4

(1020 cm2)

The measured absorption spectra of the HAS glasses are similar. The absorption spectrum of the HAS glass ranging from 200 nm to 2200 nm is shown in Fig. 3. Nine noticeable absorption peaks corresponding to transitions from the ground state energy level 5I8 to the higher levels 5I7, 5I6,5F5,5S2 þ 5F4, 5F3,5G6 þ 5F1,3G4,3G5 and 3 H5þ3H6(5G2) are labeled in Fig. 3. The spectroscopic intensity parameters (U2, U4 and U6) of Ho3þ were calculated according to the JuddeOfelt theory [23] and the results are listed in Table 3. Among the three intensity parameters, the U2 parameter reflects the intensity of the hypersensitive transition, whereas the U4 and U6 parameters indicate the overall intensity of the spectral transition [24]. U2 is most sensitive to the local structure and glass composition, which reflects the amount of covalent bonding and the asymmetry of the local environment near the rare-earth site. The large U2 value suggests a lower centrosymmetric coordination environment around the Ho3þ in high silica glass. The value of U2 first increases and then decreases when the Al2O3/Ho2O3 mole ratio increases, indicating that the asymmetry of the local environment

5

0.3 3

U2

lanthanum and Yttrium aluminosilicate studies show that Al3þ ion enters into the glass structure mainly in fourfold co-ordination. However, the tendency of the evolution of Al(4), Al(5) and Al(6) amounts in lanthanum and Yttrium aluminosilicate was consistent with that of our HAS glasses with an increase in the Al2O3/Ln2O3 mole ratio [22]. Table 2 indicates that the population of Al(4) first increases and then decreases with an increase in the Al2O3/Ho2O3 ratio. The maximum population of Al(4) is obtained at HAS3, for which the Al2O3/Ho2O3 mole ratio is 15. Only the network modifier ions could be able to create non-bridging oxygens. Therefore, the 27 Al MAS NMR spectra of the HAS glasses are consistent with the O1s XPS spectrum with an increase in the Al2O3/Ho2O3 ratio.

G6+ F1

0.4

3

HAS1 HAS2 HAS3 HAS4 HAS5

600

5

6

I6

1200

1600

I7

2000

Wavelength (nm) Fig. 3. Room temperature absorption spectrum of Ho3þ in silica glass.

Emission intensity (a.u)

5

Sample

3.3. Absorption spectra of Ho3þ ions in HAS glass

3.2. NMR spectroscopy

0.5

Table 3 JuddeOfelt parameters (Ul), radiative lifetime (tc), measured lifetime (tm) and quantum efficiency (h) for 5I7/5I8 of Ho3þ in HAS glasses.

HAS1 HAS2 HAS3 HAS4 HAS5

120000

80000

40000

0 1800

1900

2000

2100

2200

Wavelength (nm) Fig. 4. Emission spectra of Ho3þ doped HAS glasses with different Al2O3/Ho2O3 mole ratios.

X. Wang et al. / Journal of Alloys and Compounds 657 (2016) 478e482

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Table 4 Values of sem, tm and sem  tm of Al3þ-Ho3þ-codoped silica glasses. Sample

Ho3þ(1020ions cm3)

Ho/Al

sem (1021 cm2)

tm (ms)

sem  tm (1021 cm2 ms)

HAS1 HAS2 HAS3 HAS4 HAS5

0.888 0.880 0.873 0.866 0.858

5 10 15 20 25

3.57 3.63 3.64 3.44 2.91

0.91 1.04 1.19 1.23 1.33

3.25 3.77 4.33 4.23 3.87

near the Ho3þ sites first increases and then decreases with the increasing Al2O3/Ho2O3 mole ratio. The maximum asymmetry of the local environment near the Ho3þ sites is obtained at HAS3, for which the Al2O3/Ho2O3 mole ratio is 15. 3.4. Emission spectra of Ho3þ ions in HAS glass To investigate the Al2O3/Ho2O3 mole ratio dependence of the 2.0-mm emission, the fluorescence spectra of the samples pumped at 640 nm in the 1800e2200 nm wavelength range were measured, along with the fluorescence lifetimes. The results are shown in Fig. 4 and listed Table 3. In Fig. 4, the 1.95-mm fluorescence intensity first increases and then decreases with an increase in the Al2O3/ Ho2O3 mole ratio. The maximum fluorescence peak intensity in the Ho3þ doped HAS glasses is obtained at HAS3, for which the Al2O3/ Ho2O3 mole ratio is 15. The 27Al NMR date for the series of HAS glasses suggest that the population of Al(4) first increases and then decreases with an increase in the Al2O3/Ho2O3 ratio. Florian suggested a preferential localization of RE3þ near the tetra-coordinated aluminum species [22]. A greater Al(4) population produces a better dispersion of the Ho3þ ions. Thus, the Al(4) can increase the Ho3þ solubility and avoid clustering in the glass network there will be a decrease in the luminescence quenching, which will facilitate an increase in the luminescence efficiency. The tendency of the fluorescence intensity was consistent with the population of Al(4) on the whole. The quantum efficiency h for the 5I7 level can be calculated as follows:

h¼

tm  100% tc

where tc is the radiation lifetime calculated using the JuddeOfelt theory, and tm is the measured fluorescence lifetime for the 5I7 level. Table 3 lists the radiation lifetime (tc), measured fluorescence lifetimes (tm), and quantum efficiency (h). We can see from Table 3 that the 1.95-mm quantum efficiency first increases and then decreases with the Al2O3/Ho2O3 mole ratio, and that the maximum fluorescence lifetime in the HAS glasses is also obtained at HAS3. The emission cross section (sem) is important not only for the fundamental understanding of the excitation and depopulation processes, but also for the optimization of laser materials. According to the FuchtbauereLadenburg theory, the emission cross section can be calculated as follows [25]:

sem ¼

l4 Arad lI（l） Z 8pcn2 lI（l）dl

where l is the wavelength, Arad is the spontaneous transition probability, I(l) is the emission spectrum, n is the refractive index, and c is the speed of light in vacuum. Table 4 lists the sem and tm values for the 5I7 / 5I8 transitions of the Ho-doped silica glass. The product of the stimulated emission cross-section sem and measured fluorescence lifetime tm (sem  tm), which is the figure

of merit (FOM) for amplifier gain, is an important parameter to characterize laser materials. In Table 4, the sem  tm values of the Ho3þ-doped silica glasses are listed. The maximum FOM in the Ho3þ-doped HAS glasses is obtained at HAS3, for which the Al2O3/ Ho2O3 mole ratio is 15. This suggests that HAS3 glass in which the Al2O3/Ho2O3 mole ratio is 15 may provide an extended gain bandwidth. According to the structure and spectroscopic properties of the HAS glass, it is concluded that the optimum Al2O3/Ho2O3 mole ratio of the Ho3þ/Al3þ co-doped silica glass for the 2-mm laser is 15. 4. Conclusions In conclusion, we studied the effects of the Al2O3/Ho2O3 ratio on the structure and spectroscopic properties of Ho-doped high silica glasses. The 27Al NMR and oxygen 1s XPS spectra were also studied to obtain the structure of the Ho3þ-doped silica glasses. It was found that the aluminum ions entered the glass predominantly in 6-fold and 5-fold coordination, and the population of Al(4) first increased and then decreased with an increase in the Al2O3/Ho2O3 ratio for the series of HAS glasses. The asymmetry of the local environment near the Ho3þ sites was studied using the spectroscopic intensity parameter U2. The absorption and fluorescence spectra of the Ho3þ ions in these silica glasses were recorded and analyzed systematically. The HAS3 glass had a large stimulated emission cross section (3.64  1021 cm2), good quantum efficiency for the 5I7 level (8.73), long fluorescence lifetime (1.19 ms), and large sem  trad value (4.33  1021 cm2 ms). These results suggest that the optimum Al2O3/Ho2O3 mole ratio of Ho3þ/Al3þ co-doped silica glass for realizing a 2-mm laser is 15. Acknowledgments This research is financially supported by the Chinese National Natural Science Foundation (Grant No. 61405215 and 61505232). The authors would like to thank Mr Jinjun Ren and his student Runli Zhang for their assistance in 27Al NMR test. References [1] J.F. Stebbins, S. Kroeker, S.K. Lee, T.J. Kiczenski, J. Non-Cryst. Solids 275 (2000) 1. [2] K. Sinko, R. Mezei, J. Non-Cryst. Solids 231 (1998) 1. [3] M.G. Ferreira da Silva, J. Non-Cryst. Solids 352 (2006) 807. [4] X. Wang, P. Zhou, X. Wang, H. Xiao, L. Si, High. Power Laser Sci. Eng. 1 (2013) 123. [5] G. Li, Y. Gu, B. Yao, L. Shan, Y. Wang, Chin. Opt. Lett. 11 (2013) 09140401. [6] J. Laegsgaard, Phys. Rev. B Condens. Matter Mater. Phys. 65 (2002) 174114. [7] S. Sen, R. Rakhmatullin, R. Gubaydullin, A. Poppl, Phys. Rev. B Condens. Matter Mater. Phys. 74 (2006) 100201. [8] Liu S, S. Zheng, K. Yang, D. Chen, Chin. Opt. Lett. 13 (2015) 060602. [9] K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, T. Hada, J. Appl. Phys. 59 (1986) 3430. [10] M. Nogami, Y. Abe, J. Appl. Phys. 81 (1997) 6351. [11] C. Wang, G. Zhou, Y. Han, W. Wang, C. Xia, L. Hou, Chin. Opt. Lett. 11 (2013) 061601. [12] X. Wang, L. Hu, S. Wang, L. Zhang, C. Yu, D. Chen, J. Lumin. 166 (2015) 276. [13] K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, T. Hada, J. Appl. Phys. 59 (1986) 3430. [14] Y. Qiao, N. Da, D. Chen, Q. Zhou, J. Qiu, T. Akai, Appl. Phys. B 87 (2007) 717.

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[15] Z. Li, S. Wang, X. Wang, P. Guan, W. Li, M. Wang, et al., Chin. J. Lasers 40 (2013) 6003. [16] Y. Zhou, Y.L. Lam, S.S. Wang, H.L. Liu, C.H. Kam, Y.C. Chan, Appl. Phys. Lett. 71 (1997) 587. [17] V.M.H. Smets, D.M. Krol, Phys. Chem. Glas. 25 (1984) 113. [18] J. Alvarado-Rivera, D.A. Rodrıguez-Carvajal, M.C. Acosta-Enrıquez, M.B. Manzanares-Martınez, E. Alvarez, R. Lozada-Morales, G.C. Dıaz, A. Leon, M.E. Zayas, J. Am. Ceram. Soc. 97 (2014) 3494.

[19] [20] [21] [22] [23] [24] [25]

M. Eden, Annu. Rep. Prog. Chem. Sect. C Phys. Chem. 108 (2012) 177. J. Ren, L. Zhang, H. Eckert, J. Phys. Chem. C 118 (2014) 4906. J.T. Kohly, J.E. Shelby, Y.S. Frye, Phys. Chem. Glas. 33 (1992) 73. P. Florian, N. Sadiki, D. Massiot, J.P. Coutures, J. Phys. Chem. B 111 (2007) 9747. T. Xue, L. Zhang, L. Wen, M. Liao, L. Hu, Chin. Opt. Lett. 13 (2015) 081602. C.K. Jorgensen, R. Reisfeld, J. Less-Common Met. 93 (1983) 107. F. Huang, X. Liu, W. Li, L. Hu, D. Chen, Chin. Opt. Lett. 12 (2014) 051601.

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