© Copyright 1998 American Chemical Society
JUNE 23, 1998 VOLUME 14, NUMBER 13
Letters Characterization of Photoluminescence from a Material Made by Interaction of (3-Aminopropyl)triethoxysilane with Acetic Acid Vlasoula Bekiari and Panagiotis Lianos* Engineering Science Department, University of Patras, 26500 Patras, Greece Received January 5, 1998. In Final Form: April 6, 1998 A new photoluminescent material has been synthesized by the interaction of (3-aminopropyl)triethoxysilane with acetic acid under oxygen-free conditions. The photophysical properties of this material in bulk form, colloidal solution, and thin films have been studied by absorption, excitation, steady-state fluorescence, time-resolved fluorescence, and Fourier transform infrared spectroscopy. These studies showed the existence of two distinct luminescent species emitting bright blue or yellow photoluminescence.
1. Introduction Silicon alkoxides are the most popular precursors for inorganic polymerization by the so-called sol-gel method. This method consists of controlled hydrolysis of an alkoxide followed by condensation through -O-Si-O- network formation.1 Particular attention has been paid to (3aminopropyl)triethoxysilane (APTS). This aminosilane precursor is used for pretreatment of hydroxyl-bearing substrates, aiming to improve adsorption and immobilization of macromolecules of biological interest.2 However, a recent publication, showed that alkoxysilanes, including APTS, are very promising as precursors for the fabrication of new and interesting luminescent materials.3 A solgel transition can be obtained not only in the presence but also in the absence of water. In the latter case, an alkoxide can be made to react with an organic acid in the absence of oxygen.3 An inorganic polymerization does take place incorporating carbon atoms into the silica network.3 * To whom correspondence may be addressed. Tel.: 30-61997587. FAX: 30-61-997803. e-mail:
[email protected]. (1) Segal, D. Chemical Synthesis of Advanced Ceramic Materials; Cambridge University Press: Cambridge, MA, 1989. (2) (a) Hu, J.; Wang, M.; Weier, H.-U. G.; Frantz, P.; Kolbe, W.; Ogletree, D. F.; Salmeron, M. Langmuir 1996, 12, 1697. (b) Moon, J. H.; Shin, J. W.; Kim, Y. K.; Park, J. W. Langmuir 1996, 12, 4621. (3) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science 1997, 276, 1826.
Carbon impurities are considered to be the origin of photoluminescence in these materials. In this respect, it has been found that impregnation of atmospheric carbon into porous silica also leads to photoluminescence.4 In the present work we exclusively study APTS interaction with acetic acid and we present the photophysical properties of the ensuing material, in bulk form, solution, and thin films. 2. Experimental Section (3-Aminopropyl)triethoxysilane (APTS, Aldrich), ethyl alcohol (spectrophotometric grade, Aldrich), and glacial acetic acid (100%, Merck) were used as received. A detailed procedure for the preparation of this material is the following:3 APTS was introduced in a closed container and was deoxygenated by the freeze-pump-thaw method. Acetic acid was then introduced and mixed with APTS at the molar ratio APTS/acid ) 1/3. The mixture was stirred for 1/2 h and the clear liquid was allowed to gel. The whole procedure was carried out at 25 °C. After 10 days the container was opened to the air and a clear yellowish viscous liquid was obtained. This liquid was then poured into a plastic cuvette covered with perforated aluminum foil and left to dry at 50 °C for 10 more days. A clear, deep yellow gel, soluble in water or ethanol was thus obtained. (4) Canham, L. T.; Loni, A.; Calcott, P. D. J.; Simons, A. J.; Reeves, C.; Houlton, M. R.; Newey, J. P.; Nash, K. J.; Cox, T. I. Thin Solid Films 1996, 276, 112.
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Figure 1. Absorption (1), excitation (2, 4), and photoluminescence spectra (3, 5) of bulk gel. Excitation spectra were recorded at the luminescence maxima. The solution in ethanol was used for the formation of thin films by spin-coating. The films were deposited by spin-coating on glass slides, previously cleaned in sulfochromic solution. Absorption measurements were made with a Carry 1E spectrophotometer, Fourier transform (FTIR) measurements with a Magna Nicolet 550 IR spectrophotometer, excitation and fluorescence measurements with a home-assembled spectrofluorometer using Oriel parts, and time-resolved fluorescence measurements with the photon-counting technique, using a homemade hydrogen flash and ORTEC electronics.
Results and Discussion Absorption, excitation, and luminescence spectra have been recorded with the gel in the bulk form and are shown in Figure 1. The gel strongly absorbed light in the ultraviolet region, but a structureless absorption with onset at ∼550 nm was also observed in the visible region. The gel gave a bright blue photoluminescence (PL) with near-UV excitation. An even brighter yellow photoluminescence was also observed with longer-wavelength excitation. The yellow PL was actually peaking at 560 nm with a sharp excitation peak at 520 nm and two weaker and broad ones at 450 and 370 nm. The blue PL consisted of two peaks (420 and 460 nm), the ratio of which changed during the curing period of the gel. It is very hard to measure fluorescence quantum yield of a solid material, but we managed to obtain approximate values of both blue and yellow PL by following (with slight modifications) a previously published technique.5 A standard fluorescence cuvette containing the gel was fit into a cavity with highly reflective walls. Front face excitation and observation have been made. Light was collected by an optical fiber. The scattered light was recorded at the excitation wavelength both in the presence and in the absence of gel (i.e., empty cuvette). Under identical conditions, we also recorded the fluorescence spectrum of the gel. The area of the fluorescence band was taken to represent the number of emitted photons.5 The difference between the areas of the two reflection bands at the excitation wavelength was taken to represent the number of absorbed photons. The obtained values for (5) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. J. Phys. Chem. 1974, 78, 2229.
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Figure 2. Thin film excitation (1) and photoluminescence spectrum (3) and absorption spectrum (2) of an ethanolic solution. Excitation spectrum was recorded at the maximum of luminescence.
the PL quantum yield, i.e., the ratio of the emitted over the absorbed photons, were 0.21 and 0.12 for the blue and yellow PL, respectively. These values are obviously measured with a lot of error, particularly, for the yellow PL, where the extensive overlap between absorption and emission, that is, the extensive self-absorption, leads to underestimated values for the PL quantum yield. Nevertheless, they characterize systems with important PL emission capacity. The gel readily forms a clear solution when dissolved in ethanol or water. The absorption spectrum of an ethanolic solution, shown in Figure 2, resembled that of the bulk gel, but it was also distinguished by a sharp absorption peak at 280 nm. Only blue PL was detected with solutions. The structure of the blue PL spectrum of the solution was similar with that of the gel. The corresponding spectra of thin films, obtained by spin-coating of the ethanolic solution on glass slides, are also shown in Figure 2. Both emission and excitation spectra were recorded by front-face excitation. Similar to the bulk gel, the film had a strong absorption in the UV region and an onset in the visible region. Films gave only blue structureless PL. Excited-state lifetime was 9.9 ns for the blue PL (excitation, 370 nm; emission, 450 nm) and 5.8 ns for the yellow PL (excitation, 450 or 500 nm; emission, 560 nm) for all samples studied. Transmission Fourier transform infrared (FTIR) spectra of pure APTS and of the gel synthesized from APTS and acetic acid are shown in Figure 3. In both cases, the material was sandwiched between two KBr disks. The corresponding spectra are similar and display strong absorption bands, charactreristic of hydroxyl groups (1), amide (4), aliphatic functionalities (5), and the Si-O and Si-C bands (6, 7). For the silicate material we have additional absorptions (2, 3), which indicate the presence of the CdO bond. The characteristic absence of extensive hydroxyl absorption in the silicate gel is obviously due to polymerization in the absence of water. On the contrary, pure APTS contains water absorbed from the ambient atmosphere. The important modifications of the Si-C absorption band in the silicate material (7), already existing in pure APTS (due to the aminopropyl group), may be taken to represent the expected introduction of carbon into the Si-O-Si network. The present IR data do not testify to any complexation for the amino group.
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Figure 3. Transmission FTIR spectra of APTS (- - -) and of the silicate gel made from APTS and acetic acid (s).
To explain the above results, we have adopted models previously presented by other authors.3,4,6 It is known that porous silica absorbs carbon from the atmosphere, and it then becomes photoluminescent producing blue PL.4 This phenomenon stems from the introduction of carbon impurities4,6 in the -O-Si-O- network by forming -OC-O- or -Si-C- bonds. Blue PL is also emitted by materials made by using nonsubstituted alkoxysilanes (i.e., not bearing an amino group).3 This fact has also been verified in our laboratory. We then believe that blue PL originates from carbon impurities without involvement of the amino group. The origin of the yellow PL is not as clear as the blue one. We can only speculate over three possible mechanisms: (1) Silicon nanocrystals emit red PL due to quantum confinement effects.7,8 Presence of silicon nanocrystals in the present material is, however, (6) Sendova-Vassileva, M.; Tzenov, N.; Dimova-Malinonska, D.; Marinova, Ts.; Krastev, V. Thin Solid Films 1996, 276, 318.
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excluded for two reasons: their emission appears at longer wavelengths (above 600 nm) and their excited-state lifetime is 3 orders of magnitude longer8 (several microseconds vs 5.8 ns). (2) The substitution of an alkoxy group by a propylamino group deprives silicon from a fourth -O-Si-O- polymerization possibility. This may create vacancies in the polymer network that may act as luminescence centers. Such a possibility is not excluded, but it has yet to be found in other substituents besides the propylamino group. (3) The lone electron pair in the amino group may participate in electronic transitions to carbonimpurity states located within the large SiO2 band gap. This last possibility seems the most plausible of the three, in view of the fine structure of the excitation and yellow PL spectra. The sharp peaks observed in these spectra are indicative of discrete transitions. This yellow PL disappeared in solutions or in the ensuing thin films. Whatever the origin of the yellow PL may be, it is obviously sensitive to the physical state of the material. These questions are further studied in our laboratory. Conclusions Interaction of alkoxysilanes with organic acids in the absence of oxygen creates photoluminescent materials by introducing carbon impurities in the silica network.3 By using APTS and acetic acid, a material is made that photoluminescences at two distinct colors, when excited around 400 and 500 nm. Acknowledgment. We are grateful to Dr. Petrides and Dr. Pistolis for the IR measurements. We acknowledge financial aid from the program ΠΕΝΕ∆ of ΓΓΕΤ. LA980038D (7) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (8) Kontkiewicz, A. J.; Kontkiewicz, A. M.; Siejka, J.; Sen, S.; Nowak, G.; Hoff, A. M.; Sakthivel, P.; Ahmed, K.; Mukherjee, P.; Witanachchi, S.; Lagowski, J. Appl. Phys. Lett. 1994, 65, 1436.
