Wide range excitation of visible luminescence in nanosilica

Solid State Communications 150 (2010) 2278–2280

Contents lists available at ScienceDirect

Solid State Communications journal homepage: www.elsevier.com/locate/ssc

Wide range excitation of visible luminescence in nanosilica L. Vaccaro ∗ , G. Vaccaro, S. Agnello, G. Buscarino, M. Cannas Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Via Archirafi 36, I-90123 Palermo, Italy

article

info

Article history: Received 8 September 2010 Accepted 20 September 2010 by M. Grynberg Available online 25 September 2010 Keywords: A. Nanostructures C. Point defects D. Optical properties E. Luminescence

abstract The visible luminescence of nanometer-sized silica particles (7 nm mean diameter) was investigated using time resolved spectroscopy. This luminescence is characterized by a wide excitation in the visible and ultraviolet range. The emission spectrum is centred at 2.72 eV with a full width at half maximum of 0.70 eV when excited above 3.5 eV, whereas it progressively empties on the high energy side when excited below 3.5 eV. Moreover, the lifetime falls in the ns timescale and decreases on increasing the emission energy. These features are due to the exceptionally broad inhomogeneous distribution of the emitting centres peculiar to the silica nanoparticles. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Since the discovery of visible photoluminescence (PL) of porous silicon [1], a large number of highly emissive materials have been explored in view of their potential use in lighting technologies, lasing [2], displays or sensors [3]. Fumed silica, an aggregate/agglomerate of nanometer sized silica particles, has been highlighted as a choice material in these applications, mainly since it satisfies practical requirements such as stability in thermal and chemical environments, and non toxicity, and it is therefore used to encapsulate luminescent dyes [4,5]. On the other hand, the high specific surface favours the presence of point defects intrinsic to the silica structure that act as visible light emitters. The features of the observed bands strongly depend on the sample history [6–12], such as thermal treatments or irradiation; moreover they appear different from those associated with the well characterized defects in bulk silica [13]. Also for this reason, the peculiarities of the emission centres localized at the surface of nanosilica and their structure are widely debated. For example, a series of experiments carried out on fumed silica after thermal treatment at 300–400 °C has evidenced the enhancement of a luminescence centred at 2.7–2.8 eV under ultraviolet (UV) excitation [10,14,15]. The assignment of this PL is based on the experimental [16] and calculated [17,18,10] values of the absorption energies, but it is not definitively given. On the one hand, it is attributed to the dioxasilirane, = Si(O2 ), on the other, to a metastable defect pair consisting of dioxasilirane, = Si(O2 ), and silylene, = Si•• .

∗

Corresponding author. Tel.: +39 0916234223. E-mail address: [email protected] (L. Vaccaro).

0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.09.025

An exhaustive investigation of the spectroscopic properties of this PL band and of its excitation pathway is mandatory in view of the potential application of fumed silica in specific devices (lamps or probes). In fact, the capability to down convert a wide spectral range into visible light is relevant to the development of new lamps using no toxic sources emitting at higher wavelengths, thus replacing the mercury vapour plasma, emitting around 5–6 eV [3]. In the present work, by using time-resolved spectroscopy with a tunable laser excitation, we demonstrate the exceptionally large inhomogeneity of the visible emitting centres of fumed silica and show that they are excited over a wide range extending from UV to visible. 2. Experimental methods The experiments were carried out on fumed silica particles (Aerosil 300) supplied in powder form by Evonik-Industries. They have a nominal specific surface of 300 ± 30 m2 /g and an average particle diameter of 7 nm. To increase the amount of luminescent material we compacted silica nanoparticles at 300 MPa into pellets of ∼1 mm thickness. Our samples were heated in air at 300 °C for two hours. The purpose of this procedure was twofold [14]: to remove spurious contributions to the UV luminescence spectrum due to the presence of hydrocarbon contaminations or other physisorbed species and to substantially increase the characteristic emission in the visible spectral range that is the object of the present work. Time-resolved PL spectra were performed at room temperature in a standard front-scattering geometry. Pulsed excitation light (pulse width ∼5 ns, repetition rate 10 Hz) was provided by a VIBRANT OPOTEK optical parametric oscillator laser system, pumped by the third harmonic (3.50 eV) of a Nd:YAG laser. The laser photon energy was varied from 2.25 to 5.90 eV; the beam

L. Vaccaro et al. / Solid State Communications 150 (2010) 2278–2280

intensity was monitored by a pyroelectric detector and was kept at ∼0.2 mJ/pulse. The luminescence emitted by the sample was dispersed by a spectrograph (SpectraPro 2300i, PI/Acton, 300 mm focal length) equipped with a grating with 150 grooves/mm, the spectral slit resolution was set to be 20 nm. The detector used an intensified charge coupled device camera driven by a delay generator (PIMAX Princeton instruments) in order to acquire the emitted light only in a time window 1T delayed TD after the arrival of the laser pulse.

2279

a

3. Results Fig. 1 summarizes the luminescence properties of the fumed silica sample on varying the laser excitation energy, Eex . As evidenced in Fig. 1(a) by the contour plot of the combined emission and excitation intensity dependence, I (Eex , Eem ), our sample emits a visible luminescence that can be excited in a wide range extending over visible and UV. Changes of Eex from 5.90 to 3.50 eV cause variations of the PL amplitude, whereas the spectral features are poorly influenced; the emission spectrum is characterized by a broad band covering the whole visible spectral range, peaking at Epeak = 2.72 ± 0.02 eV with a full width at half maximum, hereafter indicated as W , of 0.70 ± 0.03 eV. By contrast, under a lower excitation energy (Eex ≤ 3.50 eV) the spectral features dramatically change: the PL spectrum remains a quite bell-shaped curve, even though it empties of the components at higher energies; as a consequence in the contour plot the profile of I (Eex , Eem ) vanishes in proximity of the bisecting line Eem = Eex . The lineshapes reported in Fig. 1(b) better evidence this effect: on decreasing Eexc the emission peak shifts at lower energies and the width decreases. For example, for Eexc = 2.38 eV we measure Epeak = 2.25 ± 0.02 and W = 0.32 ± 0.04 eV; we note that, on the basis of our experimental data, W could be roughly estimated down to ∼0.1 eV. In Fig. 1(c) we draw the PL amplitude, measured at the peak of each emission spectrum, as a function of the excitation energy (PLE spectrum). Such experimental data demonstrate that the visible PL has two excitation peaks around 5.0 and 3.5 eV; however, we note that the decrease from 3.5eV dovetails with the change of PL lineshape. This finding clarifies the excitation features of this luminescence that cannot be revealed by conventional PLE spectra monitored at fixed Eem ; for example that reported by Uchino et al. [10], measured at Eem = 2.82 eV, ends around 3 eV. In Fig. 2 we report the PL decay features monitored at different emission energies, under excitation at 3.65 eV. This PL decays in the ns timescale thus evidencing that it originates from an orbitally allowed transition. The decay curves deviate from a single exponential law, as evident by the semilogarithmic scale, and are increasingly fast on increasing Eem . As shown in the inset, the lifetime τ , measured as the time necessary to reduce the PL intensity to 1/e, decreases from 4.5 to 3.4 ns with Eem ranging from 2.38 to 3.13 eV. 4. Discussion We hypothesize that the spectral and decay features are the effect of a great inhomogeneity due to the presence of a lot of centres peculiar to silica nanoparticles, whose emission energy is distributed all over the visible spectrum. In fact, the luminescence spectrum is governed by an amplitude factor, absorbed photons A(Eex ) times the quantum yield η ≤ 1, and a lineshape factor given by the convolution between the single-defect (homogeneous) contribution L(E0 − Eem ) and the inhomogeneous distribution w(E0 ) I (Eex , Eem ) = ηA(Eex )

∫

Eu

L(E0 − Eem )w(E0 )dE0

El

where El and Eu are the lower and upper limits of w(E0 ).

(1)

b

c

Fig. 1. (a) Contour plot of the emission spectra collected at various Eex ranging from 2.25 to 5.90 eV with 1T = 200 ns and TD = 3 ns; the dashed line represents Eem = Eex . (b) PL spectra detected under different Eexc indicated by the arrows, the scattered laser light is also evident in the tails at high energies. (c) Excitation energy dependence of the PL amplitude measured at the peak of each spectrum.

The energy level scheme sketched in Fig. 3 helps us to interpret our results by Eq. (1). It includes singlet ground S0 and two higher energy states S1 and S2 , both being able to excite the visible PL associated with the radiative decay from S1 . In this simplified representation the width of the excited S1 and S2 states accounts for the inhomogeneous distribution of the electronic transition from S0 , whereas the vibrational structure is not indicated. Under S0 to S2 UV excitation, Eex ≥ 3.5 eV, all centres distributed within w(E0 ) can contribute to the emission from S1 . The lineshape, given by Eq. (1), does not change and the resulting W depends on the homogeneous Wh and inhomogeneous Win broadening by W = [(Wh )2 + (Win )2 ]1/2 . In contrast, under S0 to S1 visible excitation, Eex ≤ 3.5 eV, only a portion of defects is excited. The progressive narrowing of the PL band on decreasing Eex allows us to fix the upper limit of Wh to be ∼0.1 eV, quite

2280

L. Vaccaro et al. / Solid State Communications 150 (2010) 2278–2280

Fig. 2. Semilog plot of the PL decay monitored at different Eem under excitation at 3.65 eV. The inset shows the measured lifetime as a function of Eem ; the solid line represents the best fit curve discussed in the text.

can be accounted for by radiative and non radiative contributions τ = (kr + knr )−1 . The dependence of the lifetime on Eem is due to kr , in particular when the inhomogeneous broadening is much larger than the homogeneous, condition satisfied for the investigated luminescence, one can approximate kr = 3 γ Eem (Einstein spontaneous emission coefficient) [19]. The good agreement between the best fit curve and the experimental data is evidenced in the inset of Fig. 2, the parameters being γ = (4.6 ± 0.5) × 106 eV−3 s−1 and knr = (1.6 ± 0.2) × 108 s−1 . From these values we can get the luminescence quantum yield η = kr τ ; for examples η ≈ 0.4 at Eem = 2.81 eV. Finally, we remark that the present data could be a benchmark for works dealing with the structural model of this defect. In fact, the huge inhomogeneous broadening spreads out the excitation energies so much that the calculated values so far (3 and 5 eV) [10,17,18] cannot be merely compared with the observed spectrum ranging from 5.9 down to 2.2 eV. Therefore, such inhomogeneity effects have to be included to model the interaction of the defect with the environment so as to definitively clarify the origin of this visible emission in silica nanoparticles. 5. Conclusion

Fig. 3. Electronic energy level scheme accounting for the visible luminescence in silica nanoparticles under UV (1) and visible (2) excitation. The radiative and nonradiative transitions are evidenced by solid and dashed arrows, respectively.

negligible in comparison to Win : (Win /Wh ) ≥ 7. It is worth noting that the inhomogeneity experienced from these defects is much larger than that measured for the well known luminescent defects in bulk and at the surface of silica, such as oxygen deficient centres and nonbridging oxygen hole centres, where Win and Wh are comparable [13,19,20]. This necessarily implies that w(E0 ) is a function slowly variable with respect to L(E0 − Eem ) and we can assume that E0 ranges over all the emission energies from 1.9 to 3.5 eV. Under these conditions, Eq. (1) can be rewritten by: I (Eex , Eem ) = ηA(Eex )w(Eem )

∫

3.5 eV 1.9 eV

L(E0 − Eem )dE0

(2)

that is, when Eex ≥ 3.5 eV the emission lineshape reproduces the inhomogeneous distribution and the PL amplitude (see Fig. 1(c)) is related to the excitation efficiency. When Eex ≤ 3.5 eV, the emission lineshape depends on Eex within S1 in agreement with I (Eex , Eem ) = ηA(Eex )w(Eem )

∫

Eex 1.9 eV

L(E0 − Eem )dE0 .

(3)

Its amplitude is influenced by the decreasing number of emitting centres so that the PLE spectrum below 3.5 eV is not simply proportional to the absorption. The bell-shaped emission curve is the effect of a significant Stokes shift between the excitation and emission, formally included in the homogeneous lineshape. Also the decay findings prove the existence of a mapping between the spectral components inhomogeneously distributed within the PL band, and their lifetime: each excited centre emitting at Eem has a specific τ (Eem ). Generally, the lifetime

Time resolved PL spectra under tunable excitation have been successful in finding out the visible emission properties peculiar to thermally treated silica nanoparticles. The emitting centres are excited over a wide excitation range extending in the UV and visible range and give rise to a fast (ns) luminescence with a huge inhomogeneous broadening from 1.9 to 3.5 eV. These features confer versatility to silica nanoparticles in several optical applications such as down converter displays or non invasive medical nanoprobes excitable by visible light. Acknowledgements The authors are grateful to the group of the Laboratory of Amorphous Materials Physics (Palermo University) (http:// www.fisica.unipa.it//amorphous) for their support and stimulating discussions. Technical assistance by G. Napoli and G. Tricomi is also acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzò, F. Priolo, Nature 408 (2000) 440. W.H. Green, K.P. Le, J. Grey, T.T. Au, M.J. Sailor, Science 276 (1997) 1826. E. Herz, A. Burns, D. Bonner, U. Wiesner, Macromol. Rapid Commun. 30 (2009) 1907. D. Wang, G. Xing, H. Peng, T. Wu, J. Phys. Chem. C 113 (2009) 7065. V.A. Radzig, J. Non Cryst. Solids 239 (1998) 49. Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Appl. Phys. Lett. 75 (1999) 778. N. Chiodini, F. Meinardi, F. Morazzoni, A. Paleari, R. Scotti, D. Di Martino, Appl. Phys. Lett. 76 (2000) 3209. T. Uchino, T. Yamada, Appl. Phys. Lett. 85 (2004) 1164. T. Uchino, N. Kurumoto, N. Sagawa, Phys. Rev. B 73 (2006) 233203. G.Q. Xu, Z.X. Zheng, W.M. Tang, Y.C. Wu, J. Lumin. 126 (2007) 43. L. Vaccaro, M. Cannas, V.A. Radzig, R. Boscaino, Phys. Rev. B 78 (2008) 075421. L. Skuja, M. Hirano, H. Hosono, K. Kajihara, Phys. Status Solidi (c) 2 (2005) 15. A. Aboshi, N. Kurumoto, T. Yamada, T. Uchino, J. Phys. Chem. C 111 (2007) 8483. N. Sagawa, T. Uchino, J. Phys. Chem. C 112 (2008) 4581. V.A. Radzig, Chem. Phys. Reports 14 (1995) 1206. K. Raghavachari, G. Pacchioni, J. Chem. Phys. 114 (2001) 4657. A.S. Zyubin, A.M. Mebel, S.H. Lin, Y.D. Glinka, J. Chem. Phys. 116 (2002) 9889. M. D’Amico, F. Messina, M. Cannas, M. Leone, R. Boscaino, Phys. Rev. B 78 (2008) 014203. L. Vaccaro, M. Cannas, J. Phys.: Condens. Matter 22 (2010) 235801.