Highly Efficient Near-IR Photoluminescence of Er3+ Immobilized in Mesoporous SBA-15
© The Author(s) 2010
Received: 23 February 2010
Accepted: 4 August 2010
Published: 24 August 2010
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© The Author(s) 2010
Received: 23 February 2010
Accepted: 4 August 2010
Published: 24 August 2010
SiO2 mesoporous molecular sieve SBA-15 with the incorporation of erbium ions is studied as a novel type of nanoscopic composite photoluminescent material in this paper. To enhance the photoluminescence efficiency, two schemes have been used for the incorporation of Er3+ where (1) Er3+ is ligated with bis-(perfluoromethylsulfonyl)-aminate (PMS) forming Er(PMS)x-SBA-15 and (2) Yb3+ is codoped with Er3+ forming Yb-Er-SBA-15. As high as 11.17 × 10−21cm2 of fluorescent cross section at 1534 nm and 88 nm of “effective bandwidth” have been gained. It is a 29.3% boost in fluorescent cross section compared to what has been obtained in conventional silica. The upconversion coefficient in Yb-Er-SBA-15 is relatively small compared to that in other ordinary glass hosts. The increased fluorescent cross section and lowered upconversion coefficient could benefit for the high-gain optical amplifier. Finally, the Judd–Ofelt theory has also been used for the analyses of the optical spectra of Er(PMS)x-SBA-15.
Lanthanide ion Er3+ has usually been immobilized in disordered host materials like silicas and aluminosilicates for applications in optical communications. In recent years, micelle-templated silicas and aluminosilicates have attracted great attentions as hosts with ordered mesopores and micropores for their potential for better optical properties [1, 2]. Among them, mesoporous silica has been regarded as an ideal candidate due to its appealing textural properties, appreciable thermal and hydrothermal stability, tunable pore size, and alignment, while microporous aluminosilicate zeolite exhibits good features of highly crystalline framework, ordered sub-nanopores with pore diameter ranging from 1 to 20 Å, and high hydrophobicity. In particular, mesoporous silica SBA-15 synthesized using Zhao’s method has highly ordered hexagonal mesopores with parallel channels and adjustable pore size in the range of 5–30 nm . Lanthanide ions have been reported to be immobilized in the mesoporous silica like MCM-41, MCM-48, and SBA-15 [4–6] or microporous aluminosilicates like faujasite-type zeolites [7, 8].
In general, the 4f–4f transitions are electric dipole forbidden for the free lanthanide ions. After the incorporation of lanthanide ions into host lattices, the electric-dipole transitions induced by odd-parity terms in the local field become weakly allowed, although their strength is still weak. Hence, usually efficient photoluminescence of lanthanide ions cannot be obtained from their direct incorporation into mesoporous silicas or microporous aluminosilicates. To date, two approaches have been used to enhance the photoluminescent efficiency. One is based on the work of Wada and the coworkers , in which a low vibrational environment by excluding the high vibrational bonds such as C–H and O–H from the surroundings of lanthanide ions has been adopted. Lanthanide ion Nd3+ is ligated with bis-(perfluoromethylsulfonyl)aminate (PMS) to form a low vibrational ligand Nd(PMS)x. This approach has been proved to be effective to the Nd3+ complex captured in zeolite nanostructure , but not yet to other lanthanide ions.
The other makes use of the antenna effect (or sensitization process) . Lanthanide ions are incorporated into organic chromophores to form the lanthanide complexes that are covalently linked to the inner walls of the mesoporous silica’s pores. The absorption coefficients of organic chromophores are considered to be orders of magnitude higher than that of lanthanide ions. And organic chromophores are able to control and even enhance the photophysical properties of lanthanide ions. However, the usage of organic chromophores is often constrained to mesoporous materials because their molecules are usually too large to the pores of microporous materials. So far, appropriate organic chromophores all incorporated in mesoporous silicas have been found to some lanthanide ions, such as Nd3+, Yb3+, and Eu3+[4–6], but not yet to Er3+ since its emission in mesoporous silicas is still weak and does not exhibit the saddle-shaped characteristic spectra [5, 6].
Similarly to the sensitization of organic chromophores to lanthanide ions, another lanthanide ion Yb3+ can be codoped with the luminescent center Er3+ to sensitize Er3+ since state 2 F 5/2 of Yb3+ is in the similar energy level with state 4 I 11/2 of Er3+ and the absorption of 2 F 5/2 around 980 nm is much stronger and broader than that of 4 I 11/2. It is also noted that codoping of Yb3+ is an effective way to enhance the photoluminescence in microporous aluminosilicates since the large molecules of organic chromophores cannot enter into the small pores of microporous aluminosilicates. Such sensitization of Yb3+ to Er3+ has been widely used in disordered silicas and aluminosilicates, but not yet in mesoporous silica and microporous aluminosilicates.
In addition, since the nonhomogeneous distribution of immobilized Er3+ is the reason causing clustering which results in cross relaxations and degrades the photoluminescence, codoping Yb3+ to form low vibrational Er(PMS)x complexes and the walls of mesopores and cages in SBA-15 can play the roles of dispersing Er3+, providing a more homogeneous distribution for Er3+ ions and therefore suppressing the cross relaxation processes.
In this paper, a study was performed on mesoporous SBA-15 with Er3+ incorporation. Two approaches have been adopted to enhance the photoluminescence of Er3+ ions in which (1) Er3+ is ligated with PMS forming Er(PMS)x-SBA-15 and (2) Yb3+ is codoped forming Yb-Er-SBA-15. The highly efficient near-IR emission of Er3+ has been observed.
SBA-15 was hydrothermally synthesized in an acidic medium using triblock copolymer P123 as template and tetraethyl orthosilicate (TEOS) as silica source. P123 (24 g) was first dissolved in deionized water (630 mL). TEOS (51 g) and 37wt% HCl (140 mL) were then added into above aqueous solution to form a synthetic gel after 24-h stirring. The gel was then heated in a Teflon-lined autoclave under static conditions at 100°C for 20 h. The product was gathered by filtration and washed with deionized water. SBA-15 was then obtained after drying at 100°C and calcinating in air at 550°C for 5 h to burn off the template .
SBA-15 was stirred with 30 mL of 0.0312 g, i.e. 0.0075 mol/L, Er(Ac)3·4H2O solution, into which 0.0673 g, 0.1121 g, 0.1794 g, or 0.2425 g of Yb2(CO3)3·4H2O was added to carry out co-impregnation of Er and Yb species. The molar ratio of Er3+ to Yb3+ was 1:3,1:5,1:8, and 1:10, respectively. After drying at 60°C overnight, the sample impregnated with both Er3+ and Yb3+ was obtained, Yb-Er-SBA-15. Meanwhile, the (SiO)xEr or (SiO)xYb species were also formed inside the mesopores due to the existence of large amount of SiOH on the surface of inner pores.
The inductively coupled plasma (ICP) measurement was carried out for the Er3+ and Yb3+ contents on Thermo IRIS Intrepid II XSP atomic emission spectrometer. Small-angle X-ray diffraction patterns were recorded with a Germany Bruker D8 Advance diffractometer using Cu Kα radiation (40 kV, 200 mA) at a step width of 0.01°. Nitrogen (N2) adsorption–desorption isotherms were measured at 77 K on a Quantachrome Autosorb-3B instrument after the samples were outgassed at 473 K in vacuum at least for 10 h prior to investigation. SEM and TEM images were measured on a Hitachi S-4800 scanning electron microscope and a JEOL JEM-2010 transmission electron microscope, respectively. EDS spectra were obtained on an EMAX. The absorption spectra were measured on Perkin–Elmer Lambda 900 UV/VIS/NIR spectrometer. The emission spectra were recorded on Jobin-Yvon Fluolog-3 fluorescence spectrometer equipped with a 980 nm picosecond laser diode (LD) from HaiDer Company as excitation source. Refractive index measurement was done on a SC620 elliptical polarization spectrometer.
The Er3+ contents in SBA-15 were obtained from ICP measurement as 3.17, 6.34, and 9.51wt%, which correspond to the concentration of 9.03 × 1019, 1.81 × 1020, and 2.71 × 1020ions/cm3, respectively. Due to the high porosity and larger specific surface area (>700 m2/g) in SBA-15 system the obtained Er3+ contents in weight percent are relatively larger than that in conventional silica (ca. 300 m2/g).
In addition, except a primary emission peak at 1531.8 nm as usual, an obvious subsidiary peak splits out at 1563.0 nm and three small subsidiary peaks turn up at 1509.4, 1491.0, and 1468.5 nm, respectively. This is an obvious difference to the emission of Er3+ in bulk glass materials  where all subsidiary peaks are smoothly linked to the primary peak and almost cannot be distinguished individually. In general, when the particle size of a material is reduced to the nano range, the quantum size effects may arise with the presence of splitting of spectra and shifting of spectrum peaks . The splitting of the subsidiary emission peaks in Figs. 8 and 9 demonstrates the existence of quantum size effects in Er(PMS)x-SBA-15. It can also be discerned that at Er3+ concentration 1.81 × 1020ions/cm3 and 2.71 × 1020ions/cm3, the FWHM bandwidths are 20 and 53 nm, respectively. The split of the subsidiary peak is the reason corresponding to these narrower FWHM bandwidths.
In Fig. 12, although the too high emission peak corresponds to a not too wide FWHM bandwidth 45 nm (1519–1564 nm) at 5.59 × 10−21cm2, if the left subsidiary peak is taken into account, the “effective bandwidth” could be 88 nm (1483–1570 nm) in total at cross section 4.03 × 10−21cm2. This cross section is close to half of the maximum emission cross section 3.95 × 10−21cm2 in conventional silica where the FWHM is 35 nm usually. This means when one obtains an amplifier gain (proportional to the emission cross section) which equals to the gain of a commercial erbium-doped fiber amplifier (EDFA) made from conventional silica, one can obtain a much broader bandwidth 88 nm in Yb-Er-SBA-15. Therefore, Yb-Er-SBA-15 is promise to the applications of both high output lasers and broadband amplifiers.
where C is homogeneous upconversion coefficient. A 21 R is spontaneous emission rate between states 4 I 13/2 and 4 I 15/2. R 13 is pumping rate of 980 nm laser. N = 2.71 × 1020ions/cm3 is total Er3+ concentration.
Comparison of upconversion coefficients in different host materials
The upconversion effect, induced by the aggregation of Er3+ ions and causing the gain reduction, is more serious in the densely Er3+ doped case. To avoid this, it is important to separate Er3+ ions. In Yb-Er-SBA-15, two mechanisms correspond to the separation of Er3+ ions. They are (1) walls of mesopores and cages for those Er3+ ions located in different mesopores and cages; (2) Yb3+ ions for those Er3+ ions located in the same mesopore or cage. These two mechanisms explain why upconversion effect in Yb-Er-SBA-15 is relatively weak.
The Judd–Ofelt theory is usually used to evaluate the transition probability of rare-earth ions in various environments and to calculate the spectroscopic parameters . It has been shown that for glass materials the Judd–Ofelt parameters are of dependence to the local structure in the vicinity of rare-earth ions and to the basicity of rare-earth sites. Such dependence is useful in estimating the emission properties of rare-earth-doped glass . The Judd–Ofelt theory can also be used to study the mesoporous silica with a well-ordered pore arrangement and symmetry because it has similarity in material composition with fully disordered silica though different in structure.
Comparison of Ω t (t = 2, 4, 6)(×10−20cm2) of Er3+ in some host materials
Germanate glass 
Fluorophosphate glass 
Silicate glass 
Aluminate glass 
Fluoride glass 
Phosphate glass 
Tellurite glass 
Bismuth-based glass 
It has been reported that Ω 2 is closely related to the hypersensitive transitions 2 H 11/2 ← 4 I 15/2 and 4 G 11/2 ← 4 I 15/2 for Er3+ ions , namely, a stronger hypersensitive transition corresponds to a larger value of Ω 2. Jørgensen and Judd  also reported that the hypersensitivity of certain lines in the spectra of rare-earth ions arises from the inhomogeneity of the environment of rare-earth ions and the most striking effect is expected for highly polarized and asymmetric environment around rare-earth ions. In our case, SBA-15 contains many highly ordered and symmetric mesopores, although its silica walls are amorphous. No doubt, SBA-15 is with a higher content of orderliness in comparison with those fully disordered and asymmetric glass materials in Table 2. This contributes to relatively weaker hypersensitive transitions 2 H 11/2 ← 4 I 15/2 and 4 G 11/2 ← 4 I 15/2 for Er3+ ions in Er(PMS)x-SBA-15, and furthermore results in a smaller value of Ω 2 in Table 2. Our experiments (see Fig. 7) also show that the hypersensitive transitions 2 H 11/2 ← 4 I 15/2 and 4 G 11/2 ← 4 I 15/2 in Er(PMS)x-SBA-15 are less intense than those in Er-SBA-15. This indicates that the shrinkage of SBA-15 sieve framework introduced by PMS (see Fig. 2) somewhat destroys the mesostructure order of SBA-15 background.
In addition, similar results can be obtained for Yb-Er-SBA-15 for upconversion coefficient and Judd–Ofelt parameters.
Er(PMS)x functionalized mesoporous SBA-15 and Yb3+/Er3+-codoped SBA-15 have been fabricated and characterized. Both of these two complexes exhibit intense near-IR luminescence with large peak emission cross section σem = 10.9 × 10−21cm2 and σem = 11.17 × 10−21cm2, respectively. Compared with the peak emission cross section σem = 7.9 × 10−21cm2 of Er3+ in conventional silica, the above results have 27.5 and 29.3% increase, respectively. This is attributed to the low-vibration environment created by PMS or efficient sensitization of Yb3+ to Er3+. The effective separation of Er3+ ions, obtained from the walls and cages of mesopores, and the ligating with PMS or codoped Yb3+ ions, makes the upconversion effect relatively weak. Although Er(PMS)x-SBA-15 does not have extremely attractive bandwidth for the amplifier application in optical communications, Yb-Er-SBA-15 has 88 nm broad “effective bandwidth”. The high emission cross section and broad “effective bandwidth” makes them good candidates for the applications of high output lasers or broadband amplifiers.
The authors thank Professor Chunhua Yan from School of Chemistry, Beijing University for helpful discussion. The authors also thank Liqiong An from Shanghai Institute of Ceramics, Chinese Academy of Sciences, Meiying Huang and Shunguang Li from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and Zhigao Hu from East China Normal University for some measurements. This research was financially supported by the Science and Technology Commission of Shanghai under grants 05JC14069 and 09XD1401500, National Fundamental Research Program of China (973 Program) under grant 2006CB921100, the NSFC of China (20925310), and Specialized Research Fund for the Doctoral Program of Higher Education (20070269023).
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