Structure and Luminescence Properties of Eu3+-Doped Cubic Mesoporous Silica Thin Films
Eu3+ ions-doped cubic mesoporous silica thin films with a thickness of about 205 nm were prepared on silicon and glass substrates using triblock copolymer as a structure-directing agent using sol–gel spin-coating and calcination processes. X-ray diffraction and transmission electron microscopy analysis show that the mesoporous silica thin films have a highly ordered body-centered cubic mesoporous structure. High Eu3+ ion loading and high temperature calcination do not destroy the ordered cubic mesoporous structure of the mesoporous silica thin films. Photoluminescence spectra show two characteristic emission peaks corresponding to the transitions of5D0-7F1 and 5D0-7F2 of Eu3+ ions located in low symmetry sites in mesoporous silica thin films. With the Eu/Si molar ratio increasing to 3.41%, the luminescence intensity of the Eu3+ ions-doped mesoporous silica thin films increases linearly with increasing Eu3+ concentration.
KeywordsLuminescence Eu3+ doping Cubic mesoporous Thin film Sol–gel
Preparation of mesoporous silica thin films (MTFs) by evaporation-induced self-assembly took its origins from the pioneer work by Mobil on surfactant-templated materials . The study on MTFs was reported by Ozin’s and Brinker’s groups [2, 3]. Since then, a variety of mesoporous thin films have attracted a great deal of attention because of their controllable structures and compositions as well as their potential applications as catalyst supports , optical devices [4, 5], sensors , low-k-dielectrics , and membranes . MTFs have large surface areas, ordered porous structures, and uniform pore size distributions and can serve as excellent hosts for homogeneous distribution of functional guest species without aggregation. Up to date, many studies were focused on fabricating the hybrid materials with improved performance by introducing functional guest species into the pores of mesoporous silica thin films [9, 10, 11]. It was reported that well-ordered mesoporous titania thin films can act as a host matrix for luminescence rare earth ions [12, 13, 14], and Eu could be doped up to 8.0% Eu/Ti molar ratio without luminescence quenching occurring . Cubic mesoporous titania thin film doped with Cr3+ exhibited superior visible light photocatalytic activity .
The special luminescence properties of lanthanide ion-doped nanomaterials with various inorganic hosts have found applications in many fields including displays, optical telecommunications, lasers, infrared-to-visible-light up-conversion, and optoelectronic devices. Rare earth ions-doped glasses have been extensively studied as potential materials for various optical devices. Luminescence properties of europium ions doped in oxide glasses using a sol–gel process have been investigated during the last decade [16, 17, 18] because of their usefulness in various applications due to its sharp, near-monochromatic emission lines. Furthermore, the Eu3+ ion is a sensitive optical probe for the dopant site environment or symmetry because of its particular luminescence spectrum . Currently, the luminescence efficiency of Eu3+ ions in sol–gel host materials is limited due to concentration fluorescence quenching originated from the aggregation and cluster formation of Eu3+ ions and due to hydroxyl quenching resulted from the presence of residual water, solvents, and hydroxyl groups. These two problems must be solved to achieve practical optical devices based on Eu3+ ions-doped sol–gel glasses. The solubility of Eu in silica glass is very limited. The high Eu3+ concentration results in fluorescence quenching. Practically, fluorescence quenching is evident for Eu content higher than 1% (molar ratio) in silica glass . In comparison with bulk silica, ordered mesoporous silica provides a unique structure; the luminescence of Eu3+ ions can be used to probe the chemical environment of the Eu3+ ions. In addition, ordered mesoporous silica thin films with large surface areas may load relatively high content of Eu3+ ions without the aggregation of Eu3+ ions, because they can provide enough non-network oxygen species from amorphous silica walls and inner surface areas to coordinate and charge compensate the Eu3+ ions . Furthermore, the mesoporous silica thin films have a low cost, good thermal, and moisture stability. Therefore, the development of mesoporous silica thin film as an inorganic host matrix for europium ions is of importance.
In the present work, we prepared Eu3+ ions-doped cubic mesoporous silica thin films and studied their structure and luminescence properties. The results reveal that the thin films keep well-ordered mesoporous structures, even with high Eu3+ ion loading and calcined at high temperature. Photoluminescence spectra show two characteristic emission peaks of Eu3+ ions as emission centers. The Eu3+ content of Eu/Si molar ratio up to 3.41% could be doped in mesoporous silica thin film without fluorescence quenching occurring. Our study presents an interesting optical application of an all-inorganic mesoporous material.
In a typical synthesis, Eu3+ ions-doped ordered cubic mesoporous silica thin films were coated on glass substrates and crystal silicon wafers by spin-coating via evaporation-induced self-assembly . The precursor for the coating sol solution was prepared in the following procedure. The polymeric silica sol was prepared by stirring a mixture of 2.08 g tetraethoxy silane (TEOS), 6 g ethanol (EtOH), and 1.8 g 0.7 M dilute hydrochloric acid (HCl) at room temperature for 1 h. Then, the silica sol was added into a solution containing 8 g ethanol and 0.67 g triblock copolymer HO(CH2CH2O)106[CH2CH(CH3)O]70(CH2CH2O)106H (abbreviated as EO106PO70EO106, Pluronic F-127, Aldrich). After stirring a few minutes, an amount of europium nitrate [Eu(NO3)3·6H2O] was added to the mixed solution. Finally, a clear solution with the molar ratio of TEOS:F127:H2O:HCl:EtOH:Eu(NO3)3 = 1:0.005:10:0.13:30:x (x ranging from 0.005 to 0.06) was obtained after 2-h continuous stirring at room temperature. Then, the coating solution was spin-coated on cleaned glass substrates or silicon wafers rotating at a speed of 2,000 rpm for 10 s. The films were stored for 48 h at room temperature and then calcined at different temperatures for 4 h in air at a heating rate of 1°C/min to obtain Eu3+ ions-doped mesoporous silica thin films.
The content of Eu in the Eu3+-doped mesoporous silica thin film was analyzed using an IRIS Advantage inductively coupled plasma atomic emission spectrometry (ICP-AES). Differential thermal analysis (DTA) and thermogravimetry (TG) measurements were taken on a Pyris Diamond thermoanalyzer in air at a heating rate of 20°C/min. Low-angle and wide-angle X-ray diffraction (XRD) measurements were taken on a Rigaku D/Max-2400 X-ray diffractometer using Cu Kα radiation in θ-2θ scan mode. High-resolution transmission electron microscope (HRTEM) analysis and energy dispersive spectroscopy (EDS) measurements were taken using a Philips Tecnai F30 FEG-TEM electron microscope operated at 300 kV. The specimens for the TEM observations were prepared by removing the films from the substrates using a blade and suspending them in ethanol. This suspension solution was then dropped on a holey carbon film supported by a copper grid. Cross-section scanning electron microscope (SEM) observations of the films were conducted on a Hitachi S4800 field emission SEM at an accelerating voltage of 5 kV. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of the Eu3+-doped cubic mesoporous silica films deposited on single crystal silicon wafers were recorded on a Nicolet Nexus 670 FT-IR spectrometer using 4 cm −1 resolution and 60 scans. Room temperature photoluminescence spectra of the Eu3+-doped cubic mesoporous silica films deposited on single crystal silicon wafers were recorded on a FLS-920T fluorescence spectrophotometer using Xe 900 (450-W xenon arc lamp) as the light source using an excitation wavelength of 246 nm. The slit was 0.2 nm for the excitation spectra and 2.0 nm for the emission spectra. The step was 0.5 nm, and the dwell time was 0.2 s.
Results and Discussion
Structure of Eu3+ Ions-Doped Mesoporous Silica Thin Films
The Eu/Si molar ratios of the Eu3+-doped mesoporous silica thin films estimated from the solution compositions and determined by the ICP-AES analysis
Eu/Si molar ratios estimated from the solution compositions (%)
Eu/Si molar ratios determined by the ICP-AES analysis (%)
As we know, the parity intra 4f transition of the rare earth ions is forbidden in a symmetrical crystal field. However, if the crystal field is destroyed, the intra 4f transition of the rare earth ions is allowed, and its characteristic luminescence occurs. The transition of5D0 to7F1 is a magnetic dipole allowed, and its intensity shows very little variation with the bonding environment around Eu3+ ions. It is a dominant transition for the Eu3+ions in an environment with an inversion symmetry. The transition of5D0 to7F2 is an electric dipole-allowed transition and is hypersensitive to the variation in the bonding environment of the Eu3+ ions. It becomes allowed when the bonding environment is distorted to non-inversion symmetries. As shown in Fig. 11, the emission intensity of the 5D0-7F1 transition is much weaker than that of the 5D0-7F2 transition which corresponds to a well-known behavior of the Eu3+ ions located in low symmetry sites . From this point of view, the Eu3+ ions doped in the mesoporous silica thin films should be present in an environment with a low symmetry. In addition, the emission intensity of the Eu3+ ions-doped mesoporous silica thin films increases with increasing calcination temperature. It is well known that a lot of hydroxyl groups, residual adsorbed water, and organic groups exist in the mesoporous silica thin films fabricated by a sol–gel technique (as discussed earlier in the FTIR analysis). The presence of hydroxyl groups, residual adsorbed water, and organic groups can lower the luminescence efficiency of the Eu3+ ions through a non-radiative phonon quenching mechanism . As discussed previously, the FTIR analysis reveals that with increasing calcination temperature, the content of hydroxyl groups, residual adsorbed water, and organic groups in the films decreases or disappears. Therefore, more efficient luminescence centers form in the films, which can increase the emission intensity.
The ordered mesoporous silica thin films doped with the Eu3+ ions were prepared using non-ionic triblock copolymer surfactant as a structure-directing agent by sol–gel spin-coating and calcination process. The films with the thickness of about 205 nm are continuous, smooth, and dense without cracks and have ordered body-centered cubic structure of the Im 3 m space group. Although the Eu3+ ion loading is up to an Eu/Si molar ratio of 5.24% and the calcination temperature is as high as 600°C, the body-centered cubic mesoporous structure of the films is well retained. The photoluminescence spectra show two main emission peaks attributed to the 5D0-7F1 and 5D0-7F2 transitions of the Eu3+ions located in low symmetry sites. Furthermore, the intensity of the 5D0-7F2 transition increases with increasing calcination temperature. The 615-nm emission intensity increases almost linearly with the Eu/Si molar ratio increasing from 0.44 to 3.41%.
The work was supported by the International S&T Cooperation Program (ISCP) under 2008DFA50340, MOST, the Specialized Research Foundation for the Doctoral Programs (20070730022), MOE, China, and the National Natural Science Foundation of China (50872046).
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