J. Phys. Chem. B 2005, 109, 1239-1242
1239
Time Resolved Ultraviolet Photoluminescence of Mesoporous Silica Alberto Anedda, Carlo M. Carbonaro,* Francesca Clemente, Riccardo Corpino, and P. Carlo Ricci Dipartimento di Fisica, UniVersita` di Cagliari and INFM UdR Cagliari, Cittadella UniVersitaria, I-09042 Monserrato (Ca), Italy ReceiVed: June 30, 2004; In Final Form: September 8, 2004
We studied the optical properties of sol-gel synthesized porous silica excited by synchrotron radiation in the 4-10 eV range. The spectral and temporal characteristics of the ultraviolet photoluminescence at about 3.7 eV are reported. The UV emission results from the contribution of two different centers: the first one centered at 3.7 eV with a decay time of 2.0 ns and the second one peaked at 3.9 eV with a decay time of 20 ns. We propose to assign the observed luminescence to different interacting surface silanols.
I. Introduction Sol-gel derived porous silica (PS) is a promising material for several technological applications: owing to its open and highly interconnected spongelike porous structure, it can be filled with photosensitive materials or conveniently used as a host or template for polymers, catalysts, and nanoparticles.1-5 Moreover, oxygen sensors based on the photoluminescence (PL) properties of silica aerogels have been recently developed.6 The mild characteristics of sol-gel synthesis allow one to tailor the porosity in terms of pore size, distribution, morphology, and interconnection, supplying an extremely versatile material.7,8 In the past few years, the optical activity of nanosized silica has been extensively investigated due to the similarities with the PL properties of porous silicon:9-15 indeed, the characterization of the PS-PL can help to clarify the origin of the luminescence in porous silicon and the PS emissions in the ultraviolet (UV)-visible range can be interesting for applications in the optoelectronic field. It is presently accepted that the PL observed in porous silica originates from surface centers, as the efficiency and emission range are affected by the chemical and physical structure of the PS surface.9-15 The absorption spectrum of porous silica shows a main absorption band in the 4.7-6.5 eV range.16 Absorption bands at about 5.0 and 5.9 eV have been observed in high surface silica and are related to paramagnetic Si-OH and Si-H defects.17 By exciting at 5.6 eV, mesoporous silica monoliths display a composite emission in the 2.5-4.5 eV range.10,16,18 Among the contributions, the 3.7 eV component has been recently associated with OH related surface defects.14,18 In this paper, we present an investigation of the spectral and temporal characteristics of the UV-PL band by means of synchrotron radiation. The reported data show that the UV emission results from two different contributions with peaks at 3.7 and 3.9 eV and decay times of 2.0 and 20 ns, respectively. By combining the present data with previously reported Raman scattering evidences,19 we propose to assign the investigated emissions to two different kinds of OH related surface defects. II. Experimental Methods Measurements were performed on sol-gel synthesized porous silica monoliths produced by Geltech Inc. (United States). * Corresponding author. E-mail:
[email protected], phone: +390706754823, fax: +39070510171.
Investigated samples have a pore diameter distribution sharply peaked at 3.2 nm (5% of standard deviation), a pore volume of 0.488 cm3 g-1, a specific surface area of 594 m2 g-1, and a density of about 1.2 g cm-3.20 PL and PLE measurements were carried out with pulsed excitation light using the synchrotron radiation (SR) at the SUPERLUMI experimental station on the I beamline of the HASYLAB synchrotron laboratories at Desy (Hamburg). The PL signal was dispersed by a 0.5 m Czerny-Turner monochromator and detected in the 1.5-5.0 eV energy range with a photomultiplier (Valvo XP2020Q). The emission bandwidth was 16 nm. The PLE measurements were performed in the 4-10 eV energy range with 0.3 nm of bandwidth. Excitation spectra were corrected for the spectral efficiency of the excitation source. Unless differently specified (see text below), the PL and PLE spectra were recorded under multibunch operation and detected with an integral time window of 190 ns correlated to the SR pulses. Decay times in the nanosecond domain were gathered under single bunch operation, using 1024 channels to scan the 192 ns interval time between adjacent pulses (pulse width of 0.2 ns).21 III. Results It has been shown that PS displays an UV-PL at about 3.7 eV with the main excitation channel centered at 5.5 eV.11 In Figure 1, we report the excitation spectrum of the emission at 3.65 eV in the UV and vacuum-UV range. Beside the narrow excitation channel at 5.5 eV with a full width at half-maximum (fwhm) of 0.55 eV, we observe two less efficient excitation bands around 5.0 and above 6.0 eV. The emission spectrum excited at a few excitation energies within the excitation spectrum (4.96, 5.64, and 6.53 eV) is reported in Figure 2; PL amplitudes are arbitrarily normalized to their maximum for the sake of clarity. Two emission bands are detected: the first one centered around 3.7 eV and the second one peaked around 2.8 eV. It can be observed that the peak position of the main PL band depends on the excitation energy: by heuristically resolving the emission spectrum with two Gaussian bands, the UV band peaks at 3.75 eV when exciting at 4.96 eV, red shifts to 3.70 eV, increasing the excitation energy to 5.64 eV, and blue shifts to 3.89 eV as the excitation energy is set to 6.53 eV. The fwhm keeps almost
10.1021/jp0471397 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/31/2004
1240 J. Phys. Chem. B, Vol. 109, No. 3, 2005
Anedda et al.
Figure 1. Excitation spectrum of the 3.65 eV PL.
Figure 3. PL excited at 5.64 (a) and 6.53 eV (b) at different delay times and temporal windows (see text).
Figure 2. PL at different excitation energies.
constant: 0.61, 0.68, and 0.68 eV at 4.96, 5.64, and 6.53 eV, respectively. In a similar way, the blue PL peaks at 3.03 eV when the excitation is 4.96 eV, red shifts to 2.84 eV when the excitation is 5.64 eV, and blue shifts to 3.13 eV once the excitation is 6.53 eV. We note that the contribution of the blue band with respect to the UV-PL is larger in the spectrum excited at 4.96 eV. To better isolate the spectral and temporal properties of the UV-PL band, we focused our investigation on the data gathered with the 5.64 and 6.53 eV excitation energies where the blue band contributes to the PL spectrum less efficiently.
The temporal properties of the UV-PL band were investigated by collecting the emission spectrum with two different time windows at two delay times from the excitation pulse, that is with a 20 ns time window 1.5 ns after the excitation pulse and with a 150 ns time window 40 ns after the excitation pulse. In Figure 3a, we report the PL signal excited at 5.64 eV and collected with the two different time windows. It can be observed that the position of the emission peak changes as the delay time increases: it is centered at 3.73 eV 1.5 ns after the excitation pulse with the time window of 20 ns and shifts to 3.60 eV once the delay time is set to 40 ns and the time window is set to 150 ns. This is an indication of the presence of two different emissions with different spectral and temporal characteristics. A similar analysis with different and delayed time windows of the PL gathered under 6.53 eV excitation was carried out, and it is displayed in Figure 3b. The reported spectra indicate that the UV emission excited at 6.53 eV has a decay time on the order of a few nanoseconds: indeed, the spectrum recorded with the 150 ns time window 40 ns after the excitation pulse displays a very small contribution to the UV-PL. The PL decay time spectra are reported in Figure 4. The data were recorded by monitoring the PL at 3.65 eV excited at 5.64 and 6.53 eV. As evidenced by the semilogarithmic scale, the decay kinetics do not follow a single exponential law. At variance, the decay time excited at 5.64 eV can be fitted with two exponential decays of 2.0 and 20 ns respectively (square correlation factor g 0.97, estimated error 5%). The PL decay time excited at 6.53 eV can be successfully fitted with the previous exponential decays with the addition of a baseline because of the lower
Ultraviolet Photoluminescence of Mesoporous Silica
Figure 4. 3.65 eV PL decay time excited at 5.64 and 6.53 eV.
signal-to-noise ratio. We note that the intensity ratio of the two exponential decays is about 3:1 in the first case and 20:1 in the second one. IV. Discussion The reported results indicate that the UV-PL at 3.7 eV is due to the contribution of two different emitting centers. The investigation of the PL spectra excited at different excitation energies and the analysis of the decay times with a combination of two exponential decays allow one to spectrally and temporally resolve the observed emission spectra. By changing the excitation energy from 5.63 to 6.53 eV, we observed a blue shift of the emission peak from 3.7 to 3.9 eV, as suggested by the heuristic deconvolution of the PL spectra by Gaussian bands (Figure 2). The temporal analysis of the PL excited at 5.63 eV (Figure 3) is evidence that the emission peak red shifts as the delay time from the excitation pulse increases. In addition, the result of the fitting procedure of the decay time of the same PL (Figure 4) temporally resolves two emissions with lifetimes of 2.0 and 20 ns, calling for singlet-singlet transitions. By combining both the results, it comes out that the emission contribution at higher energy (at about 3.9 eV) is characterized by a shorter lifetime (2.0 ns) and the emission contribution at lower energy (at about 3.7 eV) is characterized by a longer lifetime (20 ns). The temporal analysis of the spectra excited at 6.53 eV supports the previous interpretation; in addition, the changes in the intensity ratio of the two contributions confirm the data gathered with different and delayed time windows (Figure 3) and indicate that the PL with a 20 ns decay time is less efficiently excited at higher energies. We will now discuss the attribution of the reported experimental evidence to specific centers. It has been shown that the UV-PL band is due to a surface defect11 and that the emission recorded under 5.6 eV excitation is enhanced once PS is exposed to a hydrating atmosphere.14,18 The OH dependence of the observed PL indicates the interacting silanol species (Si-OH) as possible candidates for the UV-PL band. Indeed, absorption bands in the 5.0-6.0 eV range have been associated with H and OH related defects.17 In addition, few theoretical works have indicated H related surface species as good candidates for the PL observed in porous silicon and PS.22,23 In a recent paper, we studied by Raman spectroscopy the surface of PS, looking for the presence of silanol groups at the surface of the material and water embedded into the pore and bonded to the surface of PS through hydrogen bonding.19 The PS surface is indeed covered by different types of Si-OH groups that can be divided into isolated and interacting silanols depending on their interac-
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1241 tion with neighbors. Two types of interacting silanols can be found on the surface of PS: the first one characterized by a Raman band peaked at 3525 cm-1 and the second one by a Raman band peaked at 3658 cm-1. We will call them Interacting Silanol-1 (IS-1) and Interacting Silanol-2 (IS-2) in the following discussion. In particular, these vibrational bands have been related to SiO-H stretching of surface silanols hydrogen bonded to molecular water (IS-1, 3510-3540 cm-1) and to mutually H-bonded Si-OH stretching of surface hydroxyl (IS-2, 3660 cm-1).7,24 Furthermore, the 3520 and 3660 cm-1 bands have been ascribed to stronger and weaker hydrogen bonding in interacting OH species.7 We then propose that the PL features observed could be associated with singlet-singlet transitions of these two kinds of IS where the different surrounding environment can account both for the spectral and temporal differences. We finally note that the relative content of OH species in PS depends on porosity; that is, smaller pore samples have a larger relative content of IS-1 with respect to that of IS-2.19 The investigation of the PL properties as a function of porosity will allow one to verify the proposed attribution and to associate a specific interacting silanol (IS-1 or IS-2) with one of the reported emission bands. V. Conclusions We have presented the investigation of the UV-PL observed in mesoporous silica when excited in the UV and vacuum UV. By means of synchrotron radiation, we have spectrally and temporally resolved the contribution of two different emitting centers. Two singlet-singlet transitions at about 3.7 and 3.9 eV with lifetimes of 2.0 and 20 ns when excited at 5.63 eV have been ascribed to surface interacting silanols. Acknowledgment. We thank M. Kirm of the G. Zimmerer group and A. Paleari for the SR experimental time at DESY (Hamburg). This study has been supported by a national research project (PRIN2002) of MIUR (Ministero dell’Istruzione, dell’Universita` e della Ricerca) and by INFM (Istituto Nazionale per la Fisica della Materia) of Italy. References and Notes (1) Popov, V.; Roizin, Y. O.; Rysiakiewick-Pasek, E.; Marczuk, K. Opt. Mater. 1993, 2, 249. (2) Shafer, M. W.; Awschalom, D. D.; Warnock, J.; Ruben, G. J. Appl. Phys. 1987, 61, 4339. (3) Wirnsberg, G.; Stucky, G. D. Chem. Phys. Chem. 2000, 1, 89. (4) Huang, M. H.; Choudrey, A.; Yang, P. Chem. Commun. 2000, 12, 1063. (5) Schlotting, F.; Textor, M.; Georgi, U.; Roewer, G. J. Mater. Sci. Lett. 1999, 18, 599. (6) McDonagh, C.; Bowe, P.; Mongey, K.; MacCraith, B. D. J. NonCryst. Solids 2002, 306, 138. (7) Brinker, J.; Sherer, W. G. Sol-Gel Science: the Physics and Chemistry of Sol-Gel Processing; San Diego Academic Press 1990. (8) Hench, L. L.; West, J. K. Chem. ReV. 1990, 90, 33. (9) Qin, G. G.; Lin, J.; Duan, J. Q.; Yao, G. Q. Appl. Phys. Lett. 1996, 69, 1689. (10) Glinka, Y. D.; Lin, S.-H.; Hwang, L. P.; Chen, Y.-T. Appl. Phys. Lett. 2000, 77, 3968. (11) Chiodini, N.; Meinardi, F.; Morazzoni, F.; Paleari, A.; Scotti, R.; Di Martino, D. Appl. Phys. Lett. 2000, 76, 3209. (12) Glinka, Y. D.; Naumenko, S. N.; Ogenko, V. M.; Chuiko, A. A. Opt. Spectrosc. (USSR) 1992, 71, 250. (13) Glinka, Y. D.; Lin, S.-H.; Chen, Y.-T. Phys. ReV. B 2000, 62, 4733. (14) Yao, B.; Shi, H.; Zhang, X.; Zhang, L. Appl. Phys. Lett. 2001, 78, 174. (15) Glinka, Y. D.; Lin, S.-H.; Chen, Y.-T. Appl. Phys. Lett. 1999, 75, 778. (16) Anedda, A.; Carbonaro, C. M.; Clemente, F.; Corpino, R.; Raga, F.; Serpi, A. J. Non-Cryst. Solids 2003, 322, 95.
1242 J. Phys. Chem. B, Vol. 109, No. 3, 2005 (17) Radzig, V. A. Defects in SiO2 and related Dielectrics: Science and Technology; Pacchioni, G., Skuja, L., Griscom, D. L., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; p 339. (18) Anedda, A.; Carbonaro, C. M.; Clemente, F.; Corpino, R.; Grandi, S.; Mustarelli, P.; Magistris, A. J. Non-Cryst. Solids 2003, 322, 68. (19) Anedda, A.; Carbonaro, C. M.; Clemente, F.; Corpino, R.; Ricci, P. C. J. Phys. Chem. B 2003, 107, 13661.
Anedda et al. (20) Technical report; Geltech Inc. United States. (21) Zimmerer, G. Nucl. Instrum. Methods Phys. Res., Sect. A 1991, 308, 178. (22) Gole, J. L.; Dixon, D. A. Phys. ReV. B 1998, 57, 12002. (23) Zyubin, A. S.; Mebel, A. M.; Lin, S. H.; Glinka, Y. D. J. Chem. Phys. 2002, 116, 9889. (24) Davis, K. M.; Tomozawa, M. J. Non-Cryst. Solids 1996, 201, 177.
