ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 268–271 www.elsevier.com/locate/jlumin
Optical properties of BBOT-doped silica films prepared via sol–gel processing Junling Wang, Zhiqun He, Huaxiang Mao, Yufan Du, Yongsheng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China Available online 13 March 2006
Abstract Optical and photoluminescent properties of an 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene (BBOT) silica films are reported here. The materials were prepared via a sol–gel process in combination with dioxane as a co-solvent. The doped silica can be uniformly dispersed as a visibly transparent film, which displays bright blue photoluminescence from the film. Excited state of BBOT was found to have an increased polarity as a bathochromic shift was observed when the solvent polarity increased. This acted as a probe to monitor the interactions within the BBOT-doped silica matrices. A strong interaction was found between the silica and the BBOT molecules through the Stoke’s shift and the broadening in the spectra. However, the shift in photoluminescence as increased in concentrations was due to the aggregation or micro-crystallization of the dopant. No distinct concentration quenching was observed in the concentration range studied. This investigation demonstrated that the silica prepared via sol–gel process might be used as an excellent host for organic lightemitting materials. This opens a new route to prepare novel materials in combination with the excellent stability of inorganic materials and the tunability of the organics. r 2006 Elsevier B.V. All rights reserved. Keywords: Sol–gel; Silica; BBOT; Photoluminescence; Spectral shift
1. Introduction Composites of dye-doped solid matrices have been investigated in recent years. They can be used as laser materials, nonlinear optical materials, and light concentrators in solar cells because of their photostability. Although organic dye molecules have been successfully incorporated into silica matrices by the sol–gel process [1–2], dyes of only a few classes has been reported due to the great difficulty in dissolution process. In this paper, our work has been focused on a new composite system, which comprises a blue dye, 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene (BBOT)-doped silica glass. BBOT was a well-known organic brightener in dye industry and was also reported as an electron transporting material alone or components in organic electroluminescent devices [3]. So far no report has been found regarding the use of BBOT in silica matrices to the best of our knowledge. A process has Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933.
E-mail address:
[email protected] (Z. He). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.137
been developed to prepare an optically transparent and blue emitting silica matrix. Dioxane was selected as a cosolvent in a sol–gel process. High concentration, 102 M of blue BBOT-doped silica was successfully prepared. Optical properties of the doped silica matrices have been investigated. 2. Experimental In the preparation of sol–gel matrices a hydrolysis reaction was involved. Starting materials includes tetraethyoxysiliane (TEOS, AR), dioxane (AR), HCl (AR), BBOT (FISONS). Firstly, HCl was added to a mixture of TEOS and dioxane. The molar ratio of TEOS/dioxane/ H2O is 3:12:4. Concentration of BBOT in the silica host was defined by the concentration of the dye dissolved in the final sol. The sol had been aged for 1 day before preparation of films. BBOT-doped thin-film specimens were prepared on quartz slides by spin coating at a spin rate of 7500 rpm. Thick films were cast onto a substrate (2 cm 2 cm 0.1 cm). Both used quartz slides as the
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substrates in film preparation. The films were then dried in an oven. IR spectrum was measured using a Magna 750 IR spectrometer. Surface morphologies of the films were observed in a HITACHI S-4300 scanning electronic microscope (SEM). UV–VIS absorption spectra of the specimen were performed using a SHIMADZ UV-3101PC UV–VIS–NIR scanning spectrophotometer. Fluorescence spectra were recorded on a SPEX Fluorolog-3 spectrophotometer at room temperature while the spectral resolution was set at 3 nm. Film thickness was determined via an Ambios Technology XP-2 surface profiler. Standard 1 cm path length cuvette was used for solution specimen. The thickness of the thin film was about 150 nm and the thick films are 5 mm. 3. Results and discussions 3.1. Properties of silica matrix IR spectrum of the silica gel prepared shows a broad absorption band between 3000 and 3600 cm1 with peak at about 3446 cm1 which is associated with the fundamental valence vibrations of the hydroxyl groups within the silanol or water. This band was, generally, the result of the following superimposed valence modes: (a) free SiO–H at the surface of the silica gel at 3750 cm1; (b) hydrogen bonded surface and internal SiO–H at 3660 cm1; and (c) absorbed water at 3500–3400 cm1 [4,5]. The peaks at 1080, 793, 454 cm1 are due to asymmetric, symmetric and bending modes of silicon dioxide, respectively [2,6]. In addition, a weak absorption peak at about 580 cm1 was also observed which are associated with the bending of the Si–O–Si linkage in HO–(SiO3/2) [2,7]. Therefore, an estimate could be made that only a little amount of residual Si–OH groups in the silica gel based on IR observation.
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SEM image (Fig. 1) of films as prepared from undoped silica shows a smooth surface profile observed even at 120,000 magnifications. It was shown that the method was able to produce a high quality film. Excellent optical transmission was found from silica films prepared by our sol–gel method in the range of 200–700 nm. Therefore, the silica matrices thus prepared can be used as an inert host for optically active materials and they will not contribute to the spectra in this range. 3.2. Absorption spectra Fig. 2 shows the absorption spectra of the BBOT in different environments. It generally shows unsymmetrical triple peaks. The central maximum absorptions in TEOS, dioxane and cycloxane solutions were 372, 372 and 374 nm, respectively. There was no distinct change in absorption peaks for different concentrations. The central absorption in thin silica films prepared was around 377 nm, slightly shifted to the red compared to those from solution absorptions. In addition, the absorption spectra of the film specimen became broadened (Fig. 2) when compared to those from solutions. It reflects an increased interaction to the BBOT molecules and will be further discussed later. Similar observation was explained as the chemical reaction with residual water and the silica-cage wall [8]. 3.3. Emission spectra Photoluminescent emission of BBOT is located in the range of 400–460 nm. A typical emission from a dilute cycloxane solution has a structure with a center peak at 426 nm and two shoulders at 401 and 454 nm respectively. The emission from the solution specimen was similar
Absorption
f e d
TEOS, 10-5M Dioxane, 10-5M
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350
400
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Fig. 1. SEM image at 120,000 magnifications from a silica film prepared using a sol–gel method.
Fig. 2. Absorption spectra of BBOT in solutions (as indicated) and in thin silica films ((d) 3 103 M, (e) 6 103 M, (f) 1 102 M) (The curves are normalized and shifted on the absorbance axis for clarity.)
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to that reported [9]. Take the 0–00 vibrational peaks of the excitation and emission, a small Stoke’s shift about 0.05 eV of the BBOT was observed in cycloxane solution. The Stoke’s shift changed slightly as the solvents changed. It was about 0.10 eV in TEOS and 0.11 eV in dioxane solutions, due mainly to the slight red shift in their emission peaks (not shown). As these two solvents have larger polarities than cycloxane, the shifts indicate an increase in polarity of the excited states in BBOT molecule. Self-absorption to the high-energy shoulder was observed while the BBOT concentrations increased to 1 105 M. (Fig. 3, inset). The center emission peaked at the same wavelengths for the concentration range studied. Photoluminescence from BBOT-doped silica films was much more complicated. A similar triple peak with poor peak separation was observed. In additional to a similar self-absorption at high-energy shoulder observed in their high doping level, its center emission at a low dopant concentration peaked at 438 nm, a 10–12 nm bathochromic shift compared to that from solutions such as TEOS or cycloxane. The Stoke’s shift was also increased to 0.14 eV. The emission intensities increased rapidly in general when the dopant concentrations increased along with a further bathochromic shift of the center emissions from 438 (105 M) to 446 nm (102 M). Photoluminescence from relatively high dopant concentrations was much brighter than the solution specimens, either measured with an instrument or observed visually. No distinct concentration quenching was observed within the concentration range studied. This demonstrated that sol–gel prepared silica was an excellent host material to hold dye molecules such as BBOT.
5
Intensity (a.u.)
4
455 nm g 446 nm
3
fh-
2
f-
1
edcba,ah-
0 350
400
438 nm 450
500
550
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Wavelength (nm) Fig. 3. Emission spectra from BBOT-doped silica thin films: (a) 1 105 M, (b) 8 104 M, (c) 1 103 M, (d) 3 103 M, (e) 6 103 M, (f) 1 102 M, (g) pure BBOT powder, where ah and fh are the corresponding thick films at the same dopant concentration. The inset is the photoluminescent spectra of BBOT in cycloxane at different concentrations as indicated. (The curves are normalized and shifted sequentially on the intensity axis for clarity.)
It was realized from above observation that the sol–gel prepared silica matrix was not a completely inert host. The shifts of the spectra as well as the spectral broadenings all indicated some kind of strong interaction existed in the doped matrices. Silica matrix thus prepared is generally considered to have a porous structure [1]. The ‘‘dissolved’’ BBOT molecules may be hold inside ‘‘cage’’ of the silica. The silica acts as a solid solvent to the dopant molecule and causes a strong interaction between the dopant and the silica. An increase in the Stoke’s shift of the BBOT-doped silica films reflects a further reduction in energy of the molecular excited state. It is understandable that as silica ‘cage’ in gel might contain various amount of Si–OH groups, which could play an active role to trap BBOT molecules, enhance the polarization effect and provide a stable environment for the dopant. This can be estimated from the IR spectrum. Kosower Z value, a polarity parameter has been established to compare solvent polarities quantitatively. The Kosower Z value of Si–OH group and Si–O–Si group has been assigned as Z ¼ 88 [8], which is between those of water (Z ¼ 94:6) and MeOH (Z ¼ 83:6). This supports our suggestion, i.e. the interaction between the polar silica ‘cage’ and the BBOT molecule caused a decrease in the excited state energy, which resulted in an increase in Stoke’s shift. This interaction was also found in the small shift in absorption spectra BBOT-doped silica as compared to solution ones. In contrast to the solution specimens, BBOT-doped silica matrices also showed a gradual shift in emission center to a longer wavelength as the dopant concentration was increased. The center emission peaks varied from 438 nm at 1 105 M dopant concentration to 440 nm of 8 104 and 1 103 M and finally stayed at 446 from 3 103 to 1 102 M concentration. To order to understand this, an emission spectrum from a pure BBOT powder was also compared (Fig. 3, curve g). It was shown in Fig. 3 that, at high dopant concentrations, the emission seemed to be very similar to that from its pure powder form, especially in the thick specimen such as that at 1 102 M (Fig. 3, curve fh). This indicated that the shift and the change of spectra against the concentrations could come from the dopant – dopant interaction due to the formation of molecular aggregation or microcrystallization. This was confirmed from a further experiment in an optical microscope. It is therefore reasonable that the emissions from lowly doped films were very similar to those from solutions as the aggregation was minimized. Many discussions in the literatures also presented similar spectra changes without comparing to that from the pure sample. It could be easily misled by their proposal to the interaction between the dye molecules and the residual water and/or the silica-cage walls, in terms of sensitivity to the polarity and the hydrophobicity of the molecular environment [8]. The additional broadening of the spectra may be understood from the cage effect of the silica. A major difference between the solution and the solid was the restriction to the molecular movement. The molecular
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rotations and vibrations were further constrained by the solid matrices. It might cause to slow down their relaxations and therefore to broaden the energy band.
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process might be used as an excellent host for organic lightemitting materials. It opens a new route to prepare novel materials in combination with the excellent stability of inorganic materials and the tunability of the organics.
4. Conclusions Blue light-emitting silica were prepared via sol–gel process in combination with dioxane as a co-solvent and 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene (BBOT) as a dopant. A bathochromic shift of absorption (5 nm) and photoluminescence (20 nm) spectra from BBOT in silica matrix were observed as compared to those in solutions due to the polarity of the silica matrices. The central emission peaks of the BBOT-doped silica matrices also shifted to the red as the concentrations increased as a result of the dopant–dopant interaction due to the aggregation or micro-crystallization of the dopant. A distinct broadening of emission peaks, different from its solution emissions, was observed within all solid films studied. This is attributed to the effects of the polarity and rigidity of the ‘cage’ in the silica host on the vibration energies of the dye molecules. No distinct concentration quenching was observed within the concentrations studied. This investigation demonstrated that the silica prepared via sol–gel
Acknowledgement This work was funded by the NSFC project (nos. 20344002 and 10434030), the state key program for basic research of China (no. 2003CB314707).
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