Bright Visible Luminescence in Silica Nanoparticles - American

ARTICLE pubs.acs.org/JPCC

Bright Visible Luminescence in Silica Nanoparticles Lavinia Vaccaro,*,† Adriana Morana,‡ Viktor Radzig,z and Marco Cannas† †

Dipartimento di Fisica, Universita di Palermo, I-90123 Palermo, Italy Laboratoire H. Curien, UMR CNRS 5516, Universite Jean Monnet, F-42000 St-Etienne, France z N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow, Russia ‡

ABSTRACT: We demonstrate that a porous film of silica nanoparticles emits a bright visible luminescence associated with defects stabilized by oxygen chemisorption at oxygendeficient center sites. Time-resolved spectra excited by a tunable laser allow us to distinguish the luminescence at 1.99 eV, characteristic of the nonbridging oxygen hole center (NBOHC) (tSi
O)3Si
O•, and a fast and a slow emission: the first (lifetime τ ≈ 25 ns) is peaked at 2.27 eV with an excitation spectrum centered at 5.5 eV; the second (τ ≈ 7.5 μs) is peaked at 2.41 eV and is excited around 3.2 and 5.2 eV. Reaction in an air atmosphere leads to the disappearance of the NBOHC luminescence and of the fast band, whereas the slow one remains stable. On the basis of the comparison with previous experimental and computational works, we discuss the role of the silanone SidO and of the dioxasilyrane Si(O2) as the emitting defects.

’ INTRODUCTION The development of nanotechnology has quickly led the research into materials with properties functional to the realization of optical or electrical nanodevices. The milestone of this research area is the discovery of the visible luminescence of porous Si,1 surprising for a not highly emissive material and fundamental for several applications: optoelectronics, display devices, sensors, light detectors, to name a few.2
4 Because of practical requirements, such as radiation hardness, stability in thermal and chemical environments, and nontoxicity, one of the most promising nanostructured systems is the amorphous SiO2, silica,5,6 nowadays the most frequently used optical bulk material due to its exceptional transparency from infrared (IR) up to vacuum ultraviolet (UV).7,8 The miniaturization down to nanoscale introduces intrinsic peculiar properties, such as the visible photoluminescence (PL) under UV excitation,9
16 potentially promising for the use of silica nanoparticles as down converter displays or medical nanoprobes without the need of doping with bright extrinsic fluorophores.6,17
19 However, the knowledge of these favorable characteristics is most of all phenomenological, whereas a thorough understanding of their microscopic origin has not been achieved yet. It is well accepted that a part of the nanoparticles' properties originate from the high surface-to-volume ratio. So far, experimental studies have demonstrated that only two intrinsic defects, luminescent in the visible, can be present both in the bulk and at the surface silica: the nonbridging oxygen hole center (NBOHC) (tSi
O)3Si
O•20,21 and the two-coordinated silicon also named silylene (tSi
O)2Si:.22,23 The origin of all the other PL bands evidenced in nanosilica has been justified by the different structure between nanoparticles and bulk samples that allows the formation of defects peculiar to the surface or to the r 2011 American Chemical Society

surface shell.24,25 This issue is also relevant in connection with the optical properties of oxidized porous Si that can share defects structurally similar, such as the NBOHC, the silylene, or the silanone SidO characterized by a double bond between O and Si.26,27 The most solid experiments that have addressed the surface defect identification are based on optical absorption (OA) and electron paramagnetic resonance (EPR) measurements of silica nanoparticles films where controlled chemical reactions produce defects with a known structure.28
30 Two defects peculiar to high surface silica samples are the silanone (tSi
O)2SidO and the dioxasilyrane (tSi
O)2Si(O2). Radzig30 has pointed out that these defects can be formed by the transformation of the twocoordinated silicon binding to an atomic or molecular oxygen: 1 ð tSi
OÞ2 Si: þ O2 f ð tSi
OÞ2 SidO 2

ð1Þ

ð tSi
OÞ2 Si: þ O2 f ð tSi
OÞ2 SiðO2 Þ

ð2Þ

Both of them are not stable in the ambient atmosphere due to their reaction with some air molecules, such as H2O and CO2. The silanone is unambiguously identified by the IR stretching vibration of the double SidO bond at 1306 cm
1. An OA band at 5.65 ( 0.10 eV has been experimentally correlated to the 1306 cm
1 IR amplitude and, therefore, associated with the same group; the oscillator strength was measured to be f = 0.045 ( 0.010.31,30 Computational works have assigned this UV absorption to a singlet-to-singlet transition: from the ground state S0 to Received: May 10, 2011 Revised: September 1, 2011 Published: September 06, 2011 19476

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The Journal of Physical Chemistry C the lowest excited states S1 and S2, found to be nearly degenerate.32
34 The calculated oscillator strength results are in the range 0.015
0.065,32
34 in agreement with the experimentally measured value. The silanone PL properties in silica have not been observed yet; only calculated values have been predicted: Zyubin et al.33 have assigned two PL bands at 1.6 and 2.31 eV to the transitions S1
S0 and S2
S0; Zwijnenburg et al.34 have assigned a PL in the range 1.48
1.61 eV to the transition S1
S0. In contrast, in porous Si, the PL of silanone-based oxyhydrides has been observed and widely characterized in different variants: OdSi(R)2, OdSi(R)OR, and OdSi(OR)2, where R can be H, CH3, or SiH3.35
37 This defect under excitation around 3.5 eV emits a PL band whose peak energy ranges from 1.6 to 2.3 eV depending on the variant, the large Stokes shift being ascribed to the change of the SidO bond distance. The PL decay is multiexponential and occurs in a microsecond time scale in accordance with a triplet-to-singlet transition. The spectroscopic features of the dioxasilyrane have been indirectly found out by reaction 2, leading to the disappearance of the optical bands of the two-coordinated silicon and the growth of two OA bands: the first centered at 3.2 eV with an oscillator strength of f = 1.35  10
3 and the second centered at 5.14 eV with f = 6.8  10
3.30,38 Computational works30,32,33 have supported those experimental values: the calculated energies of the OA bands were found to be 3 and 5 eV, with oscillator strengths lying in a wide range (10
4
10
2), thus pointing out the weakly allowed character of these transitions. Also for the dioxasilyrane, the PL bands have been assigned on the basis of computational calculations; the predicted energies range from 2.05 to 2.3 eV and from 1.7 to 1.8 eV.33,39 The purpose of the present work is to find experimental proofs of the PL emitted from the silanone and the dioxasilyrane. We have investigated silica nanoparticles where reactions 1 and 2 were induced by controlled thermochemical processes by using time-resolved spectroscopy with a tunable laser source. This approach is advantageous to explore in detail the excitation and the decay properties of the luminescent defects, thus providing a solid reference for the microscopic models proposed in the current literature.

’ EXPERIMENTAL METHODS Our sample is a film (thickness e 0.1 mm, surface of 10  5 mm2) obtained by pressing a highly dispersed Aerosil-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. In agreement with the method described in previous works,29,31 the stabilization of defects was induced by oxidation of thermochemically activated silica. The first step, performed to produce the so-called reactive silica (RSi), consists of a treatment in methanol vapor at T = 700 K that substitutes surface hydroxyl groups tSi
OH with methoxy tSi
OCH3 and a pyrolysis reaction at T g 1050 K. RSi is mainly characterized by a high concentration of surface oxygen deficient centers (ODCs); the main ones are the two-coordinated silicon (tSi
O)2Si: and the paramagnetic three-coordinated silicon (tSi
O)3Si•, well known as the E0 center.23,29 The second step consists of thermo-oxidizing treatments of RSi in O2 and N2O atmospheres at T > 750 K. To avoid further reactions, our sample is placed in a pure silica container with a residual He atmosphere of 3
4 mbar. An additional sample, prepared by the same chemical procedure, has been used

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to study the effects induced by reactions with some molecules present in the ambient atmosphere, such as H2O and CO2. Spectroscopic measurements were performed at room temperature. OA spectra were carried out by a double beam spectrometer (JASCO V-560); the bandwidth was 5 nm. The reported spectra are corrected by subtracting the contribution of the container. Time-resolved PL spectra were excited in the 1.8
5.9 eV spectral range by using an optical parametric oscillator (VIBRANT OPOTEK) pumped by the third harmonic (3.55 eV) of a Nd: YAG laser (pulse width of 5 ns, repetition rate of 10 Hz) and equipped with a nonlinear crystal for the second-harmonic generation. The beam energy density was monitored by a pyroelectric detector and was kept at 160 mJ/cm2 per pulse; the illuminated sample surface had a diameter of 2 mm. The emitted light was spectrally resolved by a grating with 150 grooves mm
1 and a 300 nm blaze; the spectral slit bandwidth was set to be 10 nm. Spectra were acquired by a gated intensified chargecoupled device camera (PIMAX Princeton instruments), sensitive from 1.7 to 6.2 eV, driven by a delay generator that regulates the time acquisition. The main temporal parameters are the gate window, Δt, which is the amplitude of the time window during which the CCD is enabled to reveal the luminescence light, and the delay, TD, which is the temporal shift of the acquisition window with respect to the arrival of the laser pulse. This allows the time-resolved detection, which is the measure of spectra integrated from TD to TD + Δt during the decay of the PL signal after the excitation. To acquire the decay curve, from which the lifetime τ can be estimated, we acquire a set of PL spectra with a fixed Δt (Δt , τ) and TD ranging from zero to several τ, when the luminescence signal is extinguished. All spectra are corrected for the monochromator dispersion. The photoluminescence excitation (PLE) spectra were measured manually point-bypoint by tuning the laser and recording the PL intensity. Each point is normalized to the laser-pulse energy; its value is affected by an uncertainty of ∼10% mainly due to the laser intensity fluctuations.

’ RESULTS As specified above, our sample is obtained by oxidation of RSi. This treatment leads to the oxygen chemisorption at ODC sites and the consequent disappearance of both the E0 center and the two-coordinated silicon. The new optical bands induced by the chemical reactions are described in the following. Figure 1 reports the absorption spectrum of the oxidized RSi sample: it is dominated by a band peaked at 5.45 ( 0.02 eV, and no further bands are observed. The monotonic growth of the absorbance at the lower energies is due to the poor optical quality of the sample leading to a high light scattering. The comparison with the sample after treatment in the ambient atmosphere (dashed line) shows that the UV band disappears. UV laser excitation of the oxidized RSi sample results in a bright luminescence that appears white to the naked eye, as shown in the left photograph of Figure 2. The same figure reports time-resolved PL spectra detected with different Δt and TD under excitation at Eexc = 5.28 eV. The PL spectrum in the visible range measured in the total window (Δt = 10 ms and TD = 5 ns) (Figure 2a) evidences different contributions that can be timeresolved. In the fast window (Δt = 30 ns and TD = 5 ns) (Figure 2b), we observe a band peaked at 2.27 ( 0.02 eV with a full width at half-maximum (fwhm) of 0.64 ( 0.05 eV. In the slow window (Δt = 10 μs and TD = 5 μs) (Figure 2c), we measure two 19477

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Figure 1. OA spectrum of the oxidized RSi sample detected in the range 2.5
6.1 eV. The dashed line shows the modified absorption spectrum after exposure to the ambient atmosphere.

Figure 3. (a) Decay kinetics in the nanosecond time scale of the PL band at 2.27 eV measured under laser excitation at 5.50 eV. (b) Excitation spectrum of the 2.27 eV PL band measured with TD = 5 ns and Δt = 30 ns.

Figure 2. Time-resolved PL spectra of the oxidized RSi sample measured under laser excitation at 5.28 eV (solid lines). The effects induced by exposure to the ambient atmosphere are evidenced by dashed lines. TD and Δt used to measure the PL spectra are reported. Vertical lines evidence the peak position of the fast (2.27 eV) and slow (2.41 eV) components. The photographs show the emitted light from the oxidized RSi sample (left side) and from the air-exposed sample (right side) excited by a laser spot with a diameter of 2 mm.

bands: one peaked at 1.99 ( 0.01 eV with an fwhm of 0.15 ( 0.02 eV and the other centered at 2.41 ( 0.02 eV with an fwhm of

0.50 ( 0.05 eV. After air exposure in the ambient atmosphere, the UV illuminated sample assumes a green color (see right photograph); the effects induced on the emission properties by chemical reactions in the ambient atmosphere are reported in Figure 2 as dashed lines. By inspection of the time-resolved windows, it is possible to evidence that only the slow PL at 2.41 eV is observed; its intensity, within the experimental uncertainty, is comparable to that measured before. At variance, both the slow 1.99 eV PL band and the fast emission at 2.27 eV disappear. In the following, we report the time decay and the excitation features of the three bands measured in the oxidized RSi sample. The emissions at 2.27 and 2.41 eV, nearly coincident in the spectrum, but time-resolved, will be hereafter named fast and slow components, respectively. For each band, we have performed preliminary measurements to determine the temporal interval necessary for the PL signal to extinguish; this fixes the upper limit of TD, whereas Δt is set to be 1/100
1/10 of the same value. To study the fast band, we have carried out time-resolved PL spectra in the nanosecond time scale under laser excitation at 5.50 eV; we set Δt = 1 ns for TD ranging from 0 to 53 ns, Δt = 2 ns for TD ranging from 53 to 133 ns, and Δt = 8 ns for TD ranging from 133 to 277 ns. From these spectra, we have obtained the decay kinetics monitored at Eem = 2.27 eV that is shown in Figure 3a. As evident from the semilogarithmic scale, the PL decay curve deviates from a single-exponential law. This behavior is common to defects in amorphous systems and is consistent with a multiexponential decay with decay constants inhomogeneously distributed. In the present work, we limit to estimate the lifetime τ as the time necessary to reduce the PL intensity to a factor of 1/e and we find τ = 25.0 ( 0.5 ns. The excitation 19478

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Figure 4. (a) Decay kinetics in the microsecond time scale of the PL band at 2.41 eV measured under laser excitation at 5.28 eV. (b) Excitation spectrum of the 2.41 eV PL band measured with TD = 5 μs and Δt = 10 μs.

properties of the fast band have been obtained from PL spectra acquired under laser excitation energy ranging from 4.68 to 5.77 eV. The PLE spectrum monitored at 2.27 eV is reported in Figure 3b; it evidences a band peaked around 5.5 eV with an fwhm of about 0.6 eV. The time decay of the slow band is derived from the timeresolved PL spectra acquired under excitation at 5.28 eV with Δt = 1 μs for TD ranging from 0 to 19 μs and Δt = 2 μs for TD ranging from 19 to 51 μs. The decay kinetics of the PL intensity monitored at Eem = 2.41 eV is shown in Figure 4a: also in this case, the decay curve is not a pure exponential; the lifetime is τ = 7.5 ( 0.5 μs. In Figure 4b, we report the PLE spectrum of the slow band. It has been obtained by measuring the PL spectra by varying the laser excitation from 2.67 to 5.77 eV. The curve evidences a band extending from 2.7 to 3.8 eV and a band in the UV range peaked around 5.2 eV with an fwhm of about 0.9 eV; the intensity ratio between the UV and visible bands is ∼4. Moreover, the excitation profile in the visible seems to be structured in two peaks; however, due to the low signal-to-noise ratio, we do not speculate about the two possible subcomponents. Finally, Figure 5a shows the decay curve of the band at 1.99 eV measured under excitation at 4.77 eV with Δt = 4 μs and TD ranging up to 300 μs. As observed for the other PL bands, the curve is not a single exponential, the lifetime being τ = 45 ( 2 μs. In Figure 5b, we report the PLE spectrum measured in the range extending from 1.85 to 5.90 eV. The curve evidences two bands: the first peaked at 2.03 ( 0.02 eV with an fwhm of 0.16 ( 0.02 eV and the second peaked at 4.77 ( 0.02 eV and with an fwhm of about 0.9 eV, overlapping with a component at higher energies.

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Figure 5. (a) Decay kinetics in the microsecond time scale of the PL band at 1.99 eV measured under laser excitation at 4.77 eV. (b) Excitation spectrum of the 1.99 eV PL band measured with TD = 1 μs and Δt = 1 μs.

’ DISCUSSION The above-reported data show that a PL extending over the visible range is induced by chemical modification of RSi in an oxygen atmosphere. Two spectrally close components can be resolved on the basis of their different time decay: (1) The fast band (τ = 25 ns) is peaked at 2.27 eV, and its PLE is characterized by a band around 5.5 eV that brings close similarities with the absorption spectrum. Both the PL and the OA bands disappear after the sample is exposed to air, thus indicating that the optically active site is located at the surface where it can react with some molecules of the ambient atmosphere. (2) The slow band (τ = 7.5 μs) is peaked at 2.41 eV, and its PLE consists of two bands around 3.2 and 5.2 eV. This PL is stable after air treatment, thus suggesting that the luminescent site is either unreactive or located in a region inaccessible to the air molecules in the ambient atmosphere. We note that no absorption band is observed at the same energy of the excitation peaks in the spectra acquired both before and after air exposure. A third PL band is observed at 1.99 eV; it has a lifetime of τ = 45 μs and can be excited around both 2.0 and 4.8 eV. These spectroscopic features identify the emission of NBOHC on the silica surface, extensively studied in previous works.21,40,41 Consistently, its disappearing after air exposure is due to the reactions of this radical with H2O molecules of the ambient atmosphere. We note that the OA bands related to this defect are not observed in our absorption experiments. The oscillator strength for the OA at 2.0 eV has been measured from the PL lifetime at low temperature, and its value is f = 6  10
5,21 whereas for the 19479

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Figure 6. Diagram of the energy levels and transitions accounting for the fast 2.27 eV PL excited around 5.5 eV (a) and for the slow 2.41 eV PL excited around 3.2 and 5.2 eV (b). Each electronic state is depicted together with the vibrational levels; solid and dashed arrows represent the radiative and nonradiative transitions.

band at 4.8 eV, we roughly estimate f ∼ 2  10
3 by comparing the amplitude of the OA bands.30 These values of f are consistent with weakly allowed transitions; then we can infer that the NBOHC concentration in our sample is not sufficient to allow the observation of its OA bands. As concerns the fast and slow components, we acknowledge that recent studies on silica nanoparticles sintered at T ≈ 1000 °C11,42 have reported a bright white luminescence, peaked around 2.4 eV, that consists of two components with different decay times (∼1
100 and ∼10
1000 μs). Both bands were proposed to originate from UV photoexcited carriers generated from midgap states associated with highly strained t Si
O
Si t bonds peculiar to the structure of nanoparticles. However, this model cannot be applied to the 2.27 and 2.41 eV emissions because, as evidenced by the PLE spectra, they originate from two distinct defects. In this regard, in the following, we deal with the link of these PL with specific surface defects, such as the silanone (tSi
O)2SidO and the dioxasilyrane (tSi
O)2Si(O2), that, as specified above, are stabilized in our sample by reactions 1 and 2. To help our discussion, we sketch in Figure 6 the energy level schemes of the two luminescent defects. The optical properties of the fast component can be described by the two-level diagram of Figure 6a in which the 5.5 eV excitation (or absorption) and the 2.27 eV emission are associated with the transitions occurring between the (0) and (1) levels. The lifetime value measured at room temperature (τ = 25 ns) does not allow one to discern the radiative and nonradiative decay rates; then we can only estimate the upper limit of the oscillator strength (f e 10
1), which is consistent with an allowed transition where (0) and (1) have the same spin. On the one hand, we emphasize the good agreement with previous absorption spectra that demonstrated the link between the UV band and the silanone.30 On the other hand, the OA or PLE peak energies and the oscillator strength, experimentally measured in the present work, corroborate the values predicted by calculations on the same defect.30,32
34 The peak emission at 2.27 eV is consistent with the calculated value of 2.31 eV assigned to the transition S2
S0;33 the large Stokes shift (about 3 eV), spectroscopically due to a high electron
phonon coupling, evidences a significant structural modification in the excited level, such as the elongation of the silanone bond.34,39 We note that the lack of observation of PL in the region of 1.5
1.6 eV, due to the limit of our experimental set up, leaves open the matter of the S1
S0 luminescence signal predicted by Zyubin et al.33 and Zwijnenburg et al.34

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As concerns the optical properties of the slow component, its different excitation f luminescence pathways can be accounted for by the three-level diagram of Figure 6b. The visible excitation, around 3.2 eV, is associated with the transition between (0) and (1), the emission being the inverse transition (1)
(0). The oscillator strength, derived by the lifetime value at room temperature (τ = 7.5 μs), is f e 5  10
4 and evidences the weakly allowed character of this transition. Regardless of the structure of this defect, its concentration does not allow detecting any contribution to the absorption spectrum around 3.2 eV. The UV excitation, around 5.2 eV, is associated with the transition from (0) to (2); from the lack of any OA band at the same energy, we can infer a low value of the oscillator strength. This excitation is followed by a nonradiative relaxation down to (1) and finally by the 2.41 eV radiative emission to (0). At this stage, since the spin character of the levels is not known, we cannot determine if the nonradiative (2)
(1) transition is an internal conversion or an intersystem crossing. Regardless of its nature, the rate is sufficiently fast to inhibit the radiative (2)
(0) transition, which is not experimentally observed. On the basis of the comparison with the literature, we can put forward two interpretative schemes that involve either a variant of the silanone or the dioxasilyrane. In the first case, we relate to the luminescence properties of porous Si where silanonebased oxyhydrides emit slow PL bands decaying in a few microseconds, which can shift from red to green.35
37 In particular, the emissions at higher energies involve a silanone bonded with OCH3 groups. These spectral and decay properties could be consistent with those of the slow band; on the other hand, the treatment in methanol vapor of our samples does not rule out the presence of methoxy and, therefore, silanone-based oxyhydrides. However, the excitation spectrum of the slow band, consisting of two contributions at 3.2 and 5.2 eV, does not agree with the model proposed in the literature for the porous Si that involves only a singlet ground and a triplet excited state, whereas it is consistent with the transitions predicted for the dioxasilyrane. It is worth noting that the PLE profile of Figure 4b, showing two bands at 3.2 and 5.2 eV, is in close agreement with the measured OA spectrum of ref 30 associated with this defect. Moreover, the measured OA peak energies and the oscillator strengths corroborate the calculated values for the S0
S1 and S0
S2 transitions of the same defect.30,32,33,39 An emission at 2.05
2.3 eV has been also predicted33,39 and agrees with our PL spectrum; however, it has been supposed that it can be excited only from S2, in contrast with our scheme of Figure 6b. On the basis of the high reactivity of both silanone and dioxasilyrane with some molecules of the ambient atmosphere, such as H2O and CO2, we can speculate that the slow luminescence site is in a region not accessible to these air molecules, regardless of the models hypothesized above. In this respect, the morphology of oxidized porous Si could give a clue to understanding the location of defects in a subsurface region. This system consists, in fact, of a Si coated with a thin layer of oxide (thickness of a few nanometers), and its visible luminescence is proposed to originate from defects, such as NBOHC or silanonebased oxyhydrides in the Si
SiO2 interface.43,44 As concerns the dioxasilyrane, we point out the lack of experiments that unambiguously demonstrate the presence of this defect in the bulk of silica. Our findings are, therefore, a reference for further investigation on this issue. 19480

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’ CONCLUSIONS The results reported here provide a valid support to clarify the microscopic origin of the bright visible luminescence of silica nanoparticles under UV excitation. This optical feature is related to defects peculiar to high surface samples that are stabilized by oxidation of reactive silica. A fast band (τ = 25 ns) peaked at 2.27 eV is definitely assigned to the silanone, (tSi
O)2SidO, that is localized at the surface. Moreover, we suggest that a slow band (τ = 7.5 μs) peaked at 2.41 eV originates from the dioxasilyrane (tSi
O)2Si(O2). ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank the people of the LAMP group (http://www.fisica.unipa.it//amorphous) at the Department of Physics of the University of Palermo for useful discussions. Partial financial support by POR Sicilia 2000/2006 Misura 3.15-Sottoazione C and technical assistance by G. Napoli and G. Tricomi are acknowledged. V.R. acknowledges the Russian Foundation for Basic Research (grant 10-03-00909a) and the OkhNM Program “Theoretical and Experimental Studies of the Nature of the Chemical Bonds and the Mechanisms of the Most Important Chemical Reactions and Processes” of the Russian Academy of Sciences. ’ REFERENCES (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Searson, P. C.; Macaulay, J. M.; Prokes, S. M. J. Electrochem. Soc. 1992, 139, 3373. (3) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783. (4) Godefroo, S.; Hayne, M.; Jivanescu, M.; Stesmans, A.; Zacharias, M.; Lebedev, O. I.; Tendeloo, G. V.; Moshchalkov, V. V. Nat. Nanotechnol. 2008, 3, 174. (5) Uchino, T. J. Ceram. Soc. Jpn. 2005, 113, 17. (6) Bonacchi, S.; Genovese, D.; Juris, R.; Montalti, M.; Prodi, L.; Rampazzo, E.; Zaccheroni, N. Angew. Chem., Int. Ed. 2011, 50, 4056. (7) Devine, R. A. B., Duraud, J.-P., Dooryhee, E., Eds. Structure and Imperfections in Amorphous and Crystalline Silicon Dioxide; John Wiley & Sons: Chichester, U.K., 2000. (8) Pacchioni, G., Skuja, L., Griscom, D. L., Eds. Defects in SiO2 and Related Dielectrics: Science and Technology; Kluwer Academic Publishers: Boston, MA, 2000. (9) Glinka, Y. D.; Lin, S.-H.; Chen, Y.-T. Appl. Phys. Lett. 1999, 75, 778. (10) Glinka, Y. D.; Lin, S.-H.; Chen, Y.-T. Phys. Rev. B 2002, 66, 035404. (11) Uchino, T.; Yamada, T. Appl. Phys. Lett. 2004, 85, 1164. (12) Uchino, T.; Kurumoto, N.; Sagawa, N. Phys. Rev. B 2006, 73, 233203. (13) Aboshi, A.; Kurumoto, N.; Yamada, T.; Uchino, T. J. Phys. Chem. C 2007, 111, 8483. (14) Inai, S.; Harao, A.; Nishikawa, H. J. Non-Cryst. Solids 2007, 353, 510. (15) Vaccaro, L.; Vaccaro, G.; Agnello, S.; Buscarino, G.; Cannas, M. Solid State Commun. 2010, 150, 2278. (16) Banerjee, S.; Honkote, S.; Datta, A. J. Phys. Chem. C 2011, 115, 1576. (17) Ow, H.; Larson, D. R.; Srivastava, M.; Baird, B. A.; Webb, W. W.; Wiesner, U. Nano Lett. 2005, 5, 113. (18) Wang, L.; Tan, W. Nano Lett. 2006, 6, 84.

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