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Luminescence properties of nonbridging oxygen hole centers at the silica surface
Journal of Non-Crystalline Solids 355 (2009) 1020–1023
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Luminescence properties of nonbridging oxygen hole centers at the silica surface L. Vaccaro a,b, M. Cannas a,*, V. Radzig c a b c
Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Via Archirafi 36, I-90123 Palermo, Italy Istituto di Biofisica CNR, Area della Ricerca di Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow, Russia
a r t i c l e
i n f o
Article history: Available online 18 May 2009 PACS: 73.20.Hb 78.47.Cd 78.55.Mb 78.68.+m
a b s t r a c t Two variants of the surface-nonbridging oxygen hole center, („Si–O)3Si–O and („Si–O)2(H–O)Si–O, stabilized in porous films of silica nano-particles were investigated by time resolved luminescence excited in the visible and UV spectral range by a tunable laser system. Both defects emit a photoluminescence around 2.0 eV with an excitation spectrum evidencing two maxima at 2.0 and 4.8 eV, this emission decreases by a factor 2 on increasing the temperature from 8 up to 290 K. However, the different local structure influences the emission lineshape, the quantum yield and the decay lifetime. Such peculiarities are discussed on the basis of the symmetry properties of these defects. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Defects Nanoparticles Luminescence Time resolved measurements Silica
1. Introduction Study of nano-meter sized silica particles (nano-silica) is a timely research issue strongly motivated by its relevance in modern nanoscale physics and technology. The very high specific surface area of such systems ( J 102m2/g) favors a large concentration of surface structural defects that play a crucial role in controlling the optical and electrical properties of silica nano-devices [1–4]. One of the most common defects at the silica surface is the oxygen dangling bond or nonbridging oxygen hole center (NBOHC) [5–9] whose structure is denoted by „Si–O, where („) stands for bonds with three oxygen atoms and () indicates an unpaired electron. NBOHC is also found in the irradiated bulk silica [10], where assumes a practical relevance because of its absorption bands that dominate both the visible and the ultraviolet (UV) range, all of them being able to excite a photoluminescence (PL) band around 1.9 eV [11,12]. Despite this wide interest, the identification of the optical transitions with the electronic structure of the NBOHC is still debated in the current literature [5,8,13,14]. In this regard, the investigation of surface-defects stabilized by controlled thermochemical processes is advantageous in comparison with the bulk-defect because their structural properties can be fixed a priori. * Corresponding author. Tel.: +39 09 1623 4298; fax: +39 09 1616 2461. E-mail address: cannas@fisica.unipa.it (M. Cannas). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.11.028
In this work, we report an experimental study on the photoluminescence (PL) properties of two variants of surface-NBOHCs in porous films of silica nano-particles. In agreement with Fig. 1, these variants differ by the structure adjacent to the Si coordination sphere: in the first, („Si–O)3Si–O, each ligand O is bonded to two Si; in the second, („Si–O)2(H–O)Si–O, one of the ligand O atoms is terminated by a H. Our purpose is to examine the influence of these specific structures on the spectroscopic features (spectrum lineshape, lifetime and luminescence quantum yield) thus providing a clue to clarify the origin of the optical transitions associated with the NBOHC. 2. Experimental methods We investigated porous film samples obtained by pressing a highly dispersed AerosilÒ-300, a hydrophilic fumed silica with an average particle size of 7 nm, a pore size of 3–6 nm and a specific surface of 106 cm2/g. A multi-step thermochemical method was applied to stabilize („Si–O)3Si–O and („Si–O)2(H–O)Si–O at the hydroxylated silica surface of our samples hereafter named samples 1 and 2, respectively. The main reactions are: (i) surface hydroxyl groups, „Si–OH, are substituted by methoxy, „Si– OCH3, after treatment in methanol vapor at T = 700 K; (ii) pyrolysis reactions at T P 1050 K firstly cause the transformation, „Si– OCH3 ) Si–H + O„CH2, and (iii) the generation of surface E0 centers by breaking „Si–H; (iv) the treatment in N2O atmosphere
L. Vaccaro et al. / Journal of Non-Crystalline Solids 355 (2009) 1020–1023
1021
Fig. 1. Structure of the two variants of surface-NBOHC; the O* denotes the dangling oxygen atom.
above 750 K leads to the oxygen chemisorptions thus producing the NBOHCs in the sample 1. NBOHCs in the sample 2 are generated after two further steps: (v) the treatment in H2 atmosphere at T = 300 K leads to the recombination of the first NBOHC variant, the made free H atom reacts with the defect („Si–O)2Si@O thus producing an E0 center, („Si–O)2(HO)–Si, with a ligand O atom terminated by a H; (vi) the treatment in N2O atmosphere, similarly to the step (iv), generates the second variant of NBOHC. More details on the manufacturing procedure and on the infrared and EPR measurements used to check the intermediate products are given in Ref. [5]. To avoid any reaction of the surface centers with molecular species, each sample is placed in a pure silica container with a residual He atmosphere of 3–4 mbar. Excitation over the 1.8–6.0 eV spectral range was provided by a VIBRANT OPOTEK optical parametric oscillator pumped by the third harmonic (3.55 eV) of a Nd:YAG laser (pulse width 5 ns, repetition rate of 10 Hz) and equipped with a nonlinear BKBO crystal for the second harmonic generation. The emitted light was spectrally resolved by a grating with 300 grooves mm1 and 500 nm blaze, the spectral slit bandwidth being set to be 3 nm, and acquired by a gated intensified charge coupled device camera (PIMAX Princeton instruments) within a gate window wT delayed of tD with respect to the arrival of laser pulse. All spectra are corrected for the intensity of excitation laser light and for the monochromator dispersion. Temperature was varied in the range 290–8 K by using an Oxford-OptistatCF continuous-flow helium cryostat, equipped with four optical windows and controlled by an Oxford-ITC503 instrument. 3. Results Fig. 2 shows the spectral properties of the luminescence measured at room temperature in the samples 1 and 2, containing the two variants of the surface-NBOHC. The defect („Si–O)3Si–O shows an emission lineshape structured in two sub-bands peaked at 1.92 ± 0.01 eV and 1.99 ± 0.01 eV. The excitation spectrum, measured as the integrated PL intensity, displays two bands: the first asymmetric and centered at 2.02 ± 0.01 eV with FWHM of 0.15 ± 0.02 eV and the second, more intense by a factor of 4, centered at 4.75 ± 0.05 eV with FWHM of 0.8 ± 0.1 eV and superimposed to a component at higher energies Eex > 5.5 eV. The defect („Si–O)2(H–O)Si–O exhibits an emission structureless peaked at 1.98 ± 0.01 eV with FWHM of 0.15 ± 0.01. Also in this case the excitation spectrum consists of two bands, peaked at 2.00 ± 0.01 eV (FWHM = 0.15 ± 0.01) and at 4.75 ± 0.05 eV (FWHM = 0.8 ± 0.1), respectively, the second being more intense by a factor of 15. Fig. 3 is reported the temperature dependence of the PL intensity measured in the two samples under excitation at 4.77 eV. Regardless the defect structure, the emission remains almost constant up to 100 K, after that it decreases by a factor of 2. The PL thermal quenching is fitted by the equation:
IPL ðTÞ ¼
1 1 þ A expðEa =kB TÞ
ð1Þ
Fig. 2. Emission and excitation spectra measured at room temperature in the sample 1 (a) and sample 2 (b).
where A, Ea and kB are the preexponential factor, the activation energy and the Boltzman’s constant, respectively, in the Arrhenius law accounting for the non-radiative processes that lower the luminescence quantum yield. We get the following best-fit parameters: A = 10 ± 3 and Ea = 0.05 ± 0.01 eV in the sample 1, A = 15 ± 3 and Ea = 0.07 ± 0.02 eV in the sample 2, thus proving that a quite similar non-radiative rate acts in both defects. Finally, in Fig. 4 we show the PL decay features derived at room temperature by monitoring the intensity at Eem = 1.99 eV, under excitation at 4.77 eV, with increasing the delay from the laser pulse. Both curves follow a stretched exponential law:
IPL ðt D Þ ¼ exp½ðtD =sÞc
ð2Þ
where s is the lifetime and c is the stretching factor that measures the deviation from a single exponential law consistently with a multiexponential decay whose rates are inhomogeneously distributed. These parameters depend on the defect structure: s = 41.2 ± 0.5 ls, c = 0.76 ± 0.02 in the sample 1; s = 10.5 ± 0.3 ls, c = 0.72 ± 0.02 in the sample 2. We also verified that in both samples the PL decay is weakly dependent on temperature: on cooling from 290 to 8 K, the lifetime increases to s = 52.0 ± 0.5 ls in the sample 1 and s = 12.0 ± 0.3 ls in the sample 2. 4. Discussion The knowledge a priori of the specific structure nearby the silicon coordination sphere allow us to address the observed luminescence features and relate them to the different geometrical properties of each NBOHC variant. The emission around 2.0 eV observed in the two samples is excited by two different channels: the first is due to the direct
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L. Vaccaro et al. / Journal of Non-Crystalline Solids 355 (2009) 1020–1023
Fig. 3. Temperature dependence of the PL intensity detected in the sample 1 (a) and sample 2 (b). Solid lines represent the best-fit curves of Eq. (1).
Fig. 4. Time decay curves of the PL emission detected in the samples 1 and 2 with Eem = 1.99 eV and Eex = 4.77 eV; solid lines represent the best-fit curves of Eq. (2).
excitation at 2.0 eV, the small Stokes-shift being consistent with a very weak electron–phonon coupling; the second is activated at 4.75 eV and the large Stokes-shift involves a non-radiative electronic relaxation. These features bring close similarities with that observed in the bulk-defect as well [6,15]. This evidences that surface- and bulk-NBOHC have a common energetic level scheme and the optical transitions are, therefore, localized within the structural unit „Si–O. The exited state the PL takes place from is commonly associated with a lone pair in both non-bonding 2p orbitals of the dangling oxygen [5,8,13,14,17,18]. In contrast, conflicting models have been put forward to account for the
orbitals originating the excitation bands: at 2.0 eV, that coincides with the orbital terminating the luminescence charge transfer, and at 4.8 eV. In the first [5,8,13,17], the 2.0 and 4.8 eV bands are hypothesized to originate from the Si–O r bonding orbital and from the non-bonding 2p orbitals of the basal oxygen, respectively; in the second [14,18], the origin of the 2.0 and 4.8 eV bands and the above orbitals is reversed. Some spectroscopic features (emission lineshape, time decay, quantum yield efficiency) differ between the two variants of the surface-NBOHC contained in the samples 1 and 2, see Fig. 1, thus pointing out the influence of the structure nearby the Si coordination sphere. The structure („Si–O)3Si–O has a high symmetry, close to C3v, inherited by the generation reaction between an oxygen atom and an axially-symmetric surface E0 -center, „Si [5]. In contrast, due to the O–H bond in the („Si–O)2(H–O)Si–O, the dangling oxygen experiences a different interaction with the basal atoms of the tetrahedron, thus leading to a deviation from the C3v symmetry. In accordance with our results we can deduce that the higher symmetry of the first NBOHC variant compared with the second one induces: (i) a structured PL lineshape with two peaks split by 0.07 eV against a structureless spectrum; (ii) a longer lifetime, s 40 ls against s 10 ls, or equivalently, a smaller oscillator strength of the 2.0 eV absorption, f 5 105 against f 2 104; (iii) a lower ratio between the PL quantum efficiency under UV and visible excitation gUV/gVis 4 against gUV/gVis 15; this difference, taking into account the similarities between the non-radiative rates that control the temperature dependence of PL in the samples 1 and 2, can be attributed to the different excitation efficiency. It is worth noting that, at least the above points, the spectroscopic features of the surface-NBOHC in the sample 2 resemble that observed in the bulk-NBOHC [6,19], which is characterized by a low symmetry as well. The observation of two sub-bands in the sample 1 agrees with the existence of two sub-levels in the electronic structure of the higher symmetry NBOHC. This finding corroborates an earlier prediction based on quantum-chemical calculations [5]. Due to the Jahn-Teller effect, the defect symmetry deviates from C3v to CS thus leading to an energy splitting of the ground state between the levels associated with the 2p orbitals localized on the dangling oxygen; the calculated energy difference lies in the value range 0.04–0.24 eV [5,8,16,17], in agreement with our results. Following this line of reasoning, the lack of the two sub-bands in the PL spectrum of the sample 2 could be ascribed to the lower symmetry of the defect. However, such a hypothesis needs to be checked by further experiments or by computational works. As a fact, we observe that the inhomogeneous broadening, derived by the site-selective detection of the zero-phonon line, results to be larger in the sample 2 (0.006 eV) than in the sample 1 (0.004 eV) [20], thus limiting the spectral resolution of the two sub-bands. Finally, the points (ii) and (iii) evidence the influence of the symmetry on the transition probabilities of the optical bands thus pointing out the geometrical distribution of the molecular orbitals hypothesized to account for them. Whatever the origin of the OA bands at 2.0 and 4.8 eV, the r bonding of Si–O or the 2p non-bonding orbital of basal oxygen, their overlap with the 2p orbitals of the dangling oxygen increases on lowering the defect symmetry.
5. Conclusion Time resolved luminescence under tunable laser excitation were successfully applied to compare the optical properties of two variants of the surface-NBOHC, differing for the local structure nearby the silicon coordination sphere. The obtained results point out that the NBOHC-related emission around 2.0 eV exhibits peculiarities associated with the geometrical structure of the defect: on
L. Vaccaro et al. / Journal of Non-Crystalline Solids 355 (2009) 1020–1023
lowering the symmetry from C3v the PL lineshape around 2.0 eV evolves from two sub-bands to a single band, and the transition probabilities of the absorption bands at 2.0 and 4.8 eV increase. These features are a valid reference for the assignment of the optical transitions to the molecular orbitals of the defect. Acknowledgements The authors would like to thank Prof. R. Boscaino and the group of the Laboratory of Amorphous Materials Physics (Palermo University) for valuable and stimulating discussions. Technical assistance by G. Napoli, F. Testaino and G. Tricomi is also acknowledged. Financial support was received from project P.O.R. Regione Sicilia-Misura 3.15-Sottoazione C. References [1] Yu. D. Glinka, S.-H. Lin, Y.-T. Chen, Phys. Rev. B 62 (2000) 4733. [2] T. Uchino, A. Aboshi, S. Kohara, Y. Ohishi, M. Sakashita, K. Aoki, Phys. Rev. B 69 (2004) 155409. [3] C.M. Carbonaro, P.C. Ricci, A. Anedda, Phys. Rev. B 76 (2007) 125431.
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[4] A. Stesmans, K. Clmer, V.V. Afanasev, Phys. Rev. B 77 (2008) 094130. [5] V. Radzig, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Defects in SiO2 and Related Dielectrics: Science and Technology, Kluwer Academic Publishers, Dordrecht, 2000. [6] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [7] Yu. D. Glinka, S.-H. Lin, Y.-T. Chen, Phys. Rev. B 66 (2002) 035404. [8] L. Giordano, P.V. Sushko, G. Pacchioni, A.L. Shluger, Phys. Rev. B 75 (2007) 024109. [9] V.A. Radzig, Chem. Phys. Rep. 19 (3) (2001) 469. [10] R.A.B. Devine, J. Arndt, Phys. Rev. B 39 (1989) 5132. [11] H. Hosono, K. Kajihara, T. Suzuki, Y. Ikuta, L. Skuja, M. Hirano, Solid State Commun. 122 (2002) 117. [12] L. Vaccaro, M. Cannas, B. Boizot, A. Parlato, J. Non-Cryst. Solids 353 (2007) 586. [13] T. Suzuki, L. Skuja, K. Kajihara, M. Hirano, T. Kamiya, H. Hosono, Phys. Rev. Lett. 90 (2003) 186404. [14] T. Bakos, S.N. Rashkeev, S.T. Pantelides, Phys. Rev. Lett. 91 (2003) 226402. [15] M. Cannas, L. Vaccaro, B. Boizot, J. Non-Cryst. Solids 352 (2006) 203. [16] Z. Hajnal, P. Deák, Th. Köhler, R. Kaschner, Th. Frauenheim, Solid State Commun. 108 (1998) 93. [17] C. Sousa, C. De Graaf, G. Pacchioni, J. Chem. Phys. 114 (2001) 6259. [18] L. Skuja, Solid State Commun. 84 (1992) 613. [19] M. Cannas, L. Vaccaro, R. Boscaino, Nucl. Instrum. Meth. Phys. Res. B 266 (2008) 2945. [20] L. Vaccaro, M. Cannas, V. Radzig, in: Proceedings of Seventh Symposium SiO2 Advanced Dielectrics and Related Devices, St-Etienne, 2008, p. 75.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Luminescence properties of nonbridging oxygen hole centers at the silica surface L. Vaccaro a,b, M. Cannas a,*, V. Radzig c a b c
Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Via Archirafi 36, I-90123 Palermo, Italy Istituto di Biofisica CNR, Area della Ricerca di Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow, Russia
a r t i c l e
i n f o
Article history: Available online 18 May 2009 PACS: 73.20.Hb 78.47.Cd 78.55.Mb 78.68.+m
a b s t r a c t Two variants of the surface-nonbridging oxygen hole center, („Si–O)3Si–O and („Si–O)2(H–O)Si–O, stabilized in porous films of silica nano-particles were investigated by time resolved luminescence excited in the visible and UV spectral range by a tunable laser system. Both defects emit a photoluminescence around 2.0 eV with an excitation spectrum evidencing two maxima at 2.0 and 4.8 eV, this emission decreases by a factor 2 on increasing the temperature from 8 up to 290 K. However, the different local structure influences the emission lineshape, the quantum yield and the decay lifetime. Such peculiarities are discussed on the basis of the symmetry properties of these defects. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Defects Nanoparticles Luminescence Time resolved measurements Silica
1. Introduction Study of nano-meter sized silica particles (nano-silica) is a timely research issue strongly motivated by its relevance in modern nanoscale physics and technology. The very high specific surface area of such systems ( J 102m2/g) favors a large concentration of surface structural defects that play a crucial role in controlling the optical and electrical properties of silica nano-devices [1–4]. One of the most common defects at the silica surface is the oxygen dangling bond or nonbridging oxygen hole center (NBOHC) [5–9] whose structure is denoted by „Si–O, where („) stands for bonds with three oxygen atoms and () indicates an unpaired electron. NBOHC is also found in the irradiated bulk silica [10], where assumes a practical relevance because of its absorption bands that dominate both the visible and the ultraviolet (UV) range, all of them being able to excite a photoluminescence (PL) band around 1.9 eV [11,12]. Despite this wide interest, the identification of the optical transitions with the electronic structure of the NBOHC is still debated in the current literature [5,8,13,14]. In this regard, the investigation of surface-defects stabilized by controlled thermochemical processes is advantageous in comparison with the bulk-defect because their structural properties can be fixed a priori. * Corresponding author. Tel.: +39 09 1623 4298; fax: +39 09 1616 2461. E-mail address: cannas@fisica.unipa.it (M. Cannas). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.11.028
In this work, we report an experimental study on the photoluminescence (PL) properties of two variants of surface-NBOHCs in porous films of silica nano-particles. In agreement with Fig. 1, these variants differ by the structure adjacent to the Si coordination sphere: in the first, („Si–O)3Si–O, each ligand O is bonded to two Si; in the second, („Si–O)2(H–O)Si–O, one of the ligand O atoms is terminated by a H. Our purpose is to examine the influence of these specific structures on the spectroscopic features (spectrum lineshape, lifetime and luminescence quantum yield) thus providing a clue to clarify the origin of the optical transitions associated with the NBOHC. 2. Experimental methods We investigated porous film samples obtained by pressing a highly dispersed AerosilÒ-300, a hydrophilic fumed silica with an average particle size of 7 nm, a pore size of 3–6 nm and a specific surface of 106 cm2/g. A multi-step thermochemical method was applied to stabilize („Si–O)3Si–O and („Si–O)2(H–O)Si–O at the hydroxylated silica surface of our samples hereafter named samples 1 and 2, respectively. The main reactions are: (i) surface hydroxyl groups, „Si–OH, are substituted by methoxy, „Si– OCH3, after treatment in methanol vapor at T = 700 K; (ii) pyrolysis reactions at T P 1050 K firstly cause the transformation, „Si– OCH3 ) Si–H + O„CH2, and (iii) the generation of surface E0 centers by breaking „Si–H; (iv) the treatment in N2O atmosphere
L. Vaccaro et al. / Journal of Non-Crystalline Solids 355 (2009) 1020–1023
1021
Fig. 1. Structure of the two variants of surface-NBOHC; the O* denotes the dangling oxygen atom.
above 750 K leads to the oxygen chemisorptions thus producing the NBOHCs in the sample 1. NBOHCs in the sample 2 are generated after two further steps: (v) the treatment in H2 atmosphere at T = 300 K leads to the recombination of the first NBOHC variant, the made free H atom reacts with the defect („Si–O)2Si@O thus producing an E0 center, („Si–O)2(HO)–Si, with a ligand O atom terminated by a H; (vi) the treatment in N2O atmosphere, similarly to the step (iv), generates the second variant of NBOHC. More details on the manufacturing procedure and on the infrared and EPR measurements used to check the intermediate products are given in Ref. [5]. To avoid any reaction of the surface centers with molecular species, each sample is placed in a pure silica container with a residual He atmosphere of 3–4 mbar. Excitation over the 1.8–6.0 eV spectral range was provided by a VIBRANT OPOTEK optical parametric oscillator pumped by the third harmonic (3.55 eV) of a Nd:YAG laser (pulse width 5 ns, repetition rate of 10 Hz) and equipped with a nonlinear BKBO crystal for the second harmonic generation. The emitted light was spectrally resolved by a grating with 300 grooves mm1 and 500 nm blaze, the spectral slit bandwidth being set to be 3 nm, and acquired by a gated intensified charge coupled device camera (PIMAX Princeton instruments) within a gate window wT delayed of tD with respect to the arrival of laser pulse. All spectra are corrected for the intensity of excitation laser light and for the monochromator dispersion. Temperature was varied in the range 290–8 K by using an Oxford-OptistatCF continuous-flow helium cryostat, equipped with four optical windows and controlled by an Oxford-ITC503 instrument. 3. Results Fig. 2 shows the spectral properties of the luminescence measured at room temperature in the samples 1 and 2, containing the two variants of the surface-NBOHC. The defect („Si–O)3Si–O shows an emission lineshape structured in two sub-bands peaked at 1.92 ± 0.01 eV and 1.99 ± 0.01 eV. The excitation spectrum, measured as the integrated PL intensity, displays two bands: the first asymmetric and centered at 2.02 ± 0.01 eV with FWHM of 0.15 ± 0.02 eV and the second, more intense by a factor of 4, centered at 4.75 ± 0.05 eV with FWHM of 0.8 ± 0.1 eV and superimposed to a component at higher energies Eex > 5.5 eV. The defect („Si–O)2(H–O)Si–O exhibits an emission structureless peaked at 1.98 ± 0.01 eV with FWHM of 0.15 ± 0.01. Also in this case the excitation spectrum consists of two bands, peaked at 2.00 ± 0.01 eV (FWHM = 0.15 ± 0.01) and at 4.75 ± 0.05 eV (FWHM = 0.8 ± 0.1), respectively, the second being more intense by a factor of 15. Fig. 3 is reported the temperature dependence of the PL intensity measured in the two samples under excitation at 4.77 eV. Regardless the defect structure, the emission remains almost constant up to 100 K, after that it decreases by a factor of 2. The PL thermal quenching is fitted by the equation:
IPL ðTÞ ¼
1 1 þ A expðEa =kB TÞ
ð1Þ
Fig. 2. Emission and excitation spectra measured at room temperature in the sample 1 (a) and sample 2 (b).
where A, Ea and kB are the preexponential factor, the activation energy and the Boltzman’s constant, respectively, in the Arrhenius law accounting for the non-radiative processes that lower the luminescence quantum yield. We get the following best-fit parameters: A = 10 ± 3 and Ea = 0.05 ± 0.01 eV in the sample 1, A = 15 ± 3 and Ea = 0.07 ± 0.02 eV in the sample 2, thus proving that a quite similar non-radiative rate acts in both defects. Finally, in Fig. 4 we show the PL decay features derived at room temperature by monitoring the intensity at Eem = 1.99 eV, under excitation at 4.77 eV, with increasing the delay from the laser pulse. Both curves follow a stretched exponential law:
IPL ðt D Þ ¼ exp½ðtD =sÞc
ð2Þ
where s is the lifetime and c is the stretching factor that measures the deviation from a single exponential law consistently with a multiexponential decay whose rates are inhomogeneously distributed. These parameters depend on the defect structure: s = 41.2 ± 0.5 ls, c = 0.76 ± 0.02 in the sample 1; s = 10.5 ± 0.3 ls, c = 0.72 ± 0.02 in the sample 2. We also verified that in both samples the PL decay is weakly dependent on temperature: on cooling from 290 to 8 K, the lifetime increases to s = 52.0 ± 0.5 ls in the sample 1 and s = 12.0 ± 0.3 ls in the sample 2. 4. Discussion The knowledge a priori of the specific structure nearby the silicon coordination sphere allow us to address the observed luminescence features and relate them to the different geometrical properties of each NBOHC variant. The emission around 2.0 eV observed in the two samples is excited by two different channels: the first is due to the direct
1022
L. Vaccaro et al. / Journal of Non-Crystalline Solids 355 (2009) 1020–1023
Fig. 3. Temperature dependence of the PL intensity detected in the sample 1 (a) and sample 2 (b). Solid lines represent the best-fit curves of Eq. (1).
Fig. 4. Time decay curves of the PL emission detected in the samples 1 and 2 with Eem = 1.99 eV and Eex = 4.77 eV; solid lines represent the best-fit curves of Eq. (2).
excitation at 2.0 eV, the small Stokes-shift being consistent with a very weak electron–phonon coupling; the second is activated at 4.75 eV and the large Stokes-shift involves a non-radiative electronic relaxation. These features bring close similarities with that observed in the bulk-defect as well [6,15]. This evidences that surface- and bulk-NBOHC have a common energetic level scheme and the optical transitions are, therefore, localized within the structural unit „Si–O. The exited state the PL takes place from is commonly associated with a lone pair in both non-bonding 2p orbitals of the dangling oxygen [5,8,13,14,17,18]. In contrast, conflicting models have been put forward to account for the
orbitals originating the excitation bands: at 2.0 eV, that coincides with the orbital terminating the luminescence charge transfer, and at 4.8 eV. In the first [5,8,13,17], the 2.0 and 4.8 eV bands are hypothesized to originate from the Si–O r bonding orbital and from the non-bonding 2p orbitals of the basal oxygen, respectively; in the second [14,18], the origin of the 2.0 and 4.8 eV bands and the above orbitals is reversed. Some spectroscopic features (emission lineshape, time decay, quantum yield efficiency) differ between the two variants of the surface-NBOHC contained in the samples 1 and 2, see Fig. 1, thus pointing out the influence of the structure nearby the Si coordination sphere. The structure („Si–O)3Si–O has a high symmetry, close to C3v, inherited by the generation reaction between an oxygen atom and an axially-symmetric surface E0 -center, „Si [5]. In contrast, due to the O–H bond in the („Si–O)2(H–O)Si–O, the dangling oxygen experiences a different interaction with the basal atoms of the tetrahedron, thus leading to a deviation from the C3v symmetry. In accordance with our results we can deduce that the higher symmetry of the first NBOHC variant compared with the second one induces: (i) a structured PL lineshape with two peaks split by 0.07 eV against a structureless spectrum; (ii) a longer lifetime, s 40 ls against s 10 ls, or equivalently, a smaller oscillator strength of the 2.0 eV absorption, f 5 105 against f 2 104; (iii) a lower ratio between the PL quantum efficiency under UV and visible excitation gUV/gVis 4 against gUV/gVis 15; this difference, taking into account the similarities between the non-radiative rates that control the temperature dependence of PL in the samples 1 and 2, can be attributed to the different excitation efficiency. It is worth noting that, at least the above points, the spectroscopic features of the surface-NBOHC in the sample 2 resemble that observed in the bulk-NBOHC [6,19], which is characterized by a low symmetry as well. The observation of two sub-bands in the sample 1 agrees with the existence of two sub-levels in the electronic structure of the higher symmetry NBOHC. This finding corroborates an earlier prediction based on quantum-chemical calculations [5]. Due to the Jahn-Teller effect, the defect symmetry deviates from C3v to CS thus leading to an energy splitting of the ground state between the levels associated with the 2p orbitals localized on the dangling oxygen; the calculated energy difference lies in the value range 0.04–0.24 eV [5,8,16,17], in agreement with our results. Following this line of reasoning, the lack of the two sub-bands in the PL spectrum of the sample 2 could be ascribed to the lower symmetry of the defect. However, such a hypothesis needs to be checked by further experiments or by computational works. As a fact, we observe that the inhomogeneous broadening, derived by the site-selective detection of the zero-phonon line, results to be larger in the sample 2 (0.006 eV) than in the sample 1 (0.004 eV) [20], thus limiting the spectral resolution of the two sub-bands. Finally, the points (ii) and (iii) evidence the influence of the symmetry on the transition probabilities of the optical bands thus pointing out the geometrical distribution of the molecular orbitals hypothesized to account for them. Whatever the origin of the OA bands at 2.0 and 4.8 eV, the r bonding of Si–O or the 2p non-bonding orbital of basal oxygen, their overlap with the 2p orbitals of the dangling oxygen increases on lowering the defect symmetry.
5. Conclusion Time resolved luminescence under tunable laser excitation were successfully applied to compare the optical properties of two variants of the surface-NBOHC, differing for the local structure nearby the silicon coordination sphere. The obtained results point out that the NBOHC-related emission around 2.0 eV exhibits peculiarities associated with the geometrical structure of the defect: on
L. Vaccaro et al. / Journal of Non-Crystalline Solids 355 (2009) 1020–1023
lowering the symmetry from C3v the PL lineshape around 2.0 eV evolves from two sub-bands to a single band, and the transition probabilities of the absorption bands at 2.0 and 4.8 eV increase. These features are a valid reference for the assignment of the optical transitions to the molecular orbitals of the defect. Acknowledgements The authors would like to thank Prof. R. Boscaino and the group of the Laboratory of Amorphous Materials Physics (Palermo University) for valuable and stimulating discussions. Technical assistance by G. Napoli, F. Testaino and G. Tricomi is also acknowledged. Financial support was received from project P.O.R. Regione Sicilia-Misura 3.15-Sottoazione C. References [1] Yu. D. Glinka, S.-H. Lin, Y.-T. Chen, Phys. Rev. B 62 (2000) 4733. [2] T. Uchino, A. Aboshi, S. Kohara, Y. Ohishi, M. Sakashita, K. Aoki, Phys. Rev. B 69 (2004) 155409. [3] C.M. Carbonaro, P.C. Ricci, A. Anedda, Phys. Rev. B 76 (2007) 125431.
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