Interaction of Surface Trap States and Defect Pair of Photoluminescent

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Interaction of Surface Trap States and Defect Pair of Photoluminescent Silica Nanostructures with H2O2 and Solvents Subhasree Banerjee, Sukumar Honkote, and Anindya Datta* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India

bS Supporting Information ABSTRACT: The photoluminescence (PL) of silica nanostructures has been found to be affected remarkably by organic solvents and hydrogen peroxide. The absorption and PL emission as well as excitation spectra show a variation in the position of the spectral maxima, with the change in the solvent polarity. The most interesting effect of solvents is observed in the PL decays. The decays, which are biexponential in polar media, are single exponential in nonpolar solvents. This observation indicates the absence or destabilization of surface related trap states in nonpolar media. On the other hand, these states seem to be stabilized in polar solvents, as the long component of the PL decay is further enhanced in the dispersions in polar solvents. Such enhancement occurs, most likely, because of hydrogen bonding between the silanol groups and the solvent molecules. PL quenching is observed upon addition of H2O2. Surprisingly, this is accompanied by an enhancement in the long component of the decay. This pair of apparently contradictory observations indicates the stabilization of surface trap states with H2O2 and at the same time, formation of new, nonluminescent defect centers by a reaction between the photoluminescent defect pair and H2O2.

’ INTRODUCTION Amorphous silica has several important surface related applications in the field of catalysis, surface derivatization, and chromatography.1,2 Surface hydroxylating Si-OH (silanol) chemistry plays a major role in such applications. The different kinds of surface Si-OH groups include isolated Si-OH, vicinal HO-SiOSi-OH, geminal HO-Si-OH, and H-bonded Si-OH 3 3 3 OH-Si.3 Hydrogen bonding as well as non-hydrogen bonding sites occur on silica surface. Such sites consist of geminal OH groups, isolated (free) OH groups, nonhydroxylic adsorption sites, and surface complexes.4 Along with Si-OH groups, the Si-O-Si groups present in the surface of silica also participate in the hydrogen bond formation. Surface defects include the nonbridging oxygen hole center, NBOHC (tSiO•) and the E0 (tSi•) center, for silica samples calcined above 673 K. Edge-sharing tetrahedron and silanone (SidO) groups are also reported to be present at the silica surface. Bulk silica comprises of SiO4 units joined tetrahedrally to form a rigid network of ring structures consisting of silicon and oxygen atoms in alternate positions. A defect formation occurs due to the absence or dislocation of one or more of these atoms. Such dislocation often takes place at the time of formation of the silica structure.5,6 Besides, extrinsic defects can also be created by r 2011 American Chemical Society

methods of electron irradiation,7 laser irradiation,8 thermal treatment,7,9 or ion implantation.10 Defects present in silica can be classified into two general categories: those associated with an oxygen excess and those that are oxygen-deficient. Superoxide (tSi-O-O•)6 and peroxide defects (tSi-O-O-Sit)11 are examples of oxygen excess defects that can be created by oxygen ion implantation in the interstitial sites. Such implantation may occur in neutral,12 positive,13 and negative oxidation states.14 Ozone implantation into the interstitial sites also gives rise to these kinds of defects.15 The more abundant oxygen deficient defects are comprised of relaxed, nonrelaxed, and neutral oxygen vacancies, Si-Si bond, and combinations of E0 centers and nonbridging oxygen hole centers that lead to the formation of self-trapped excitons (STE).16 Some of these defect centers are responsible for the photoluminescence (PL) of amorphous silica. Several PL bands are reported in different spectral regions. The focus of the present work is on the blue band (∼450 nm). This band is often attributed to the STE formed by the breakage of a Si-O bond Received: September 18, 2010 Revised: December 18, 2010 Published: January 12, 2011 1576

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The Journal of Physical Chemistry C Scheme 1. Formation of the Defect Pairs Responsible for the Blue PL in Amorphous Silica20

and contains an E0 (tSi•) and nonbridging oxygen hole center (NBOHC) (tSi-O•).17-19 Uchino and co-workers have proposed a different, new model of the defect pair consisting of dioxasilirane and silylene to be responsible for the blue PL (Scheme 1).20,21 The short component of the biexponential PL decay has been attributed to the germane emission center. The long component of the blue PL is dependent on the surface morphology. It has been attributed to surface trap states.21 In a recent paper, we have had a similar observation in silica nanotubes and nanodisks.22 Subsequently, biexponential decays have been reported for other silica nanostructures.23 Very recently, Uchino and co-workers have performed a detailed study on the PL properties of silica based hybrid organic-inorganic materials with various degree of cross-linking.24,25 Each silicon atom should be connected to at least two oxygen atoms in the form of siloxane bond and/or OH groups.24 They have also observed that an increased degree of cross-linking decreases the PL intensity. The condensation process between two silanol groups are kinetically hindered by the presence of organic group providing spatial separation between the silanol groups. The degree of cross-linking also causes the PL decay almost pure exponential with a faster decay time. The slower component has originated due to radiative transition from highly strained siloxane bond and/or dangling bond or silanol groups acting as metastable defect pairs.25 This is a very plausible model, and several experimental results support it. However, the jury is still out on this issue, and other models cannot be ruled out at this time. It is possible, for example, that the longer component is due to delayed PL, involving two states: one bright and the other dark. The blue PL of amorphous silica disappears upon aging but can be restored by annealing or purging with oxygen or nitrogen gas.7,22 It is sensitive to the electron beam irradiation,7 specimen temperature,7,26 laser irradiation,18 and ion implantation.27 An earlier study has reported the effect of surface interactions, especially hydrogen bond formation, on the rotational and vibrational relaxation of the small molecules in the pores of the hydroxylated silica.28 With this background, the solvent dependence of PL of silica nanostructures has been investigated in the present study, in an attempt to ascertain the contribution of surface properties on the blue PL. The focus here is on the perturbation of the surface related states or defects, due to the chemical environment of the nanostructure, as manifested in the PL properties. Besides the organic solvents, the effect of H2O2 on the PL has also been studied, as it can also form hydrogen bonds.29 Moreover, H2O2 is a strong oxidizing reagent. It can react with the defect states present in the surface of solid structures. Such studies have previously been performed on ZnO thin film, whose surface gets modified by addition of H2O2, resulting in the production of oxide radicals, which penetrate into the film.30 These oxide radicals either occupy the interstitial sites to form oxygen interstitials Oi or fill the Zn vacancies to form antisite oxygen OZn defects. These new defect centers are found to be responsible for the enhancement of the green PL of the ZnO film after addition of H2O2 solution.

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’ MATERIALS AND METHODS Silica nanostructures are prepared by a reverse microemulsion mediated sol-gel technique, from aerosol OT microemulsions in heptane, with 17 M FeCl3 present in the water pool.22,31 Nanotubes (SNT) and nanodisks (SND) are prepared from microemulsions with w0 = 2 and 22, respectively.22 Silica nanoparticles (SNP) are prepared by St€ober's method,32 in which an aliquot (5.0 mL) of Si(OC2H5)4 (tetraethyl orthosilicate, TEOS) is added to 30.0 mL of absolute ethanol. The mixture is sonicated for 10 min at room temperature. In the next step 1 mL of Millipore water is added followed by a sonication for 1.5 h. Finally, 2 mL of NH3 is added and sonicated for 3 h. The product is allowed to stand for 1 h, for gelation. Then, it is dried at 70 °C for 24 h. The SNTs and SNPs have been imaged by field emission electron microscope (JEOL JEM-2100F). The size of nanoparticle is ca. 150 nm. The nanodisks are imaged by scanning electron microscope (SEM, Hitachi, S-3400N). All of the solvents of spectroscopic grade from Spectrochem, Mumbai, India are distilled immediately prior to use. The 30% H2O2 from Merck, India, has been used as such. The steady state absorption and PL spectra have been recorded on a JASCO V530 spectrophotometer and a Varian Cary Eclipse fluorimeter, respectively. For the PL spectra, λex = 360 nm, and bandwidth = 5 nm. PL decays are recorded in a time-correlated single photon counting (TCSPC) system, from IBH, U.K., with λex = 341 nm. The full width at half-maximum of the instrument response function is 800 ps. The PL decays are collected with emission polarizer at a magic angle of 54.7° and is analyzed by using IBH DAS 6.2 software.33,34 The data are fitted, using the iterative reconvolution method, to a sum of exponentials: X ai expð - t=τi Þ IðtÞ ¼ Ið0Þ ð1Þ i

where I(t) and I(0) are the PL intensities at time t and zero after excitation by the pulse of light. ai is the amplitude of the ith component. Stern-Volmer plots have been constructed for experiments involving PL quenching. Here, the data are fitted to the equation:35 I0 ¼ 1 þ Ksv ½Q  I

ð2Þ

where I0 and I are the intensities of the PL data at λem = 410, 430, and 460 nm in the absence and presence of H2O2, respectively. [Q] is the concentration of the quencher. Ksv is the Stern-Volmer constant.

’ RESULTS AND DISCUSSION The addition of H2O2 to SNP causes the quenching of PL intensity, and the spectra get slightly red-shifted (Figure 1A). The volume-corrected Stern-Volmer plot is linear (Figure 1C). The Stern-Volmer constants, obtained from the slopes of the plots for λem = 430 and 410 nm, are almost same and much higher than that of 460 nm (Table 1). The excitation spectrum for H2O2-treated SNP is different from that of SNP before treating with the reagent (Figure 1B). Identical observations are obtained in SNTs (Figure S2 of the Supporting Information, SI) and SNDs (Figure S3, SI) as well. The explanation of this observation may be proposed as follows: H2O2 is a strong oxidizing reagent. It can react chemically with the defect pair and generate emissive or nonemissive centers. The small but perceptible shift 1577

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Figure 1. (A) PL spectra of an ethanolic dispersion of SNP (solid line) get red-shifted upon the addition of H2O2 (dashed line). (B) Excitation spectra (λem = 460 nm) before (solid line) and after the addition of H2O2 solution to the ethanolic dispersion of SNP (dashed line). (C) Stern-Volmer plot of I0/I against the concentration of H2O2 at different emission wavelengths of (i) 410 nm (4), (ii) 430 nm (O), (iii) 460 nm (-b-).

Figure 2. (A) PL decays of silica nanoparticle dispersed in ethanol before and after the adddtion of 14.21 mM H2O2 solution at λex = 341 nm and λem = 460 nm. (B) PL decays of the ethanolic dispersion of SNP, SND, adn SNT after the addition of 14.21 mM H2O2. λex = 341 nm, λem = 460 nm in all cases.

Table 2. Temporal Parameters of PL of SNP Different Concentrations of H2O2 at λem = 460 nm and λex = 360 nma

Table 1. Values of Stern-Volmer Constants for SNP, SNT, and SND at Three Different λem Valuesa

a

Ksv (L mol-1)

Ksv (L mol-1)

Ksv (L mol-1)

sample

at λem = 410 nm

at λem = 430 nm

at λem = 460 nm

SNP

0.070

0.055

0.025

SNT

0.061

0.054

0.023

SND

0.060

0.047

0.020

Ksv is the Stern-Volmer quenching constant.

in the excitation spectrum is brought about by H2O2, along with the quenching of PL indicate the formation of a new species, with a less stronger emission. As will be discussed later, such quenching and shift in excitation spectra are not observed with other polar or nonpolar solvents. A second way in which H2O2 can affect the PL is through the formation of a stable cage-like structure on the solvation shell of silica by hydrogen bonding with H2O2. It may be worthwhile to note here the earlier work on Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) study on the H2O2 treated silica gel, which indicates a formation of H-bond between H2O2 and hydrophilic silica surface.36 These cage-like structures can act as emissive surface trap states. The evidence for the formation of such cage-like structure is obtained in the shift of the PL maximum to lower energy and the small but perceptible change in the excitation spectrum. This is the factor that is likely to be responsible for the increase in the lifetime of the longer component of the PL decay of silica. Time-resolved PL studies have been performed in an attempt to gain a greater insight into the effect of H2O2 of PL properties of silica nanostructures. The PL decays of SNTs and SNDs are biexponential, as stated earlier.22 They remain biexponential upon the addition of H2O2. The short component remains almost constant, while the longer lifetime increases (Figure 2 and Table 2 for SNP, Figure S4A, Table S1 of

[H2O2]/mM

a1

τ1(ns)

a2

τ2 (ns)

χ2

0

0.97

1.31

0.03

5.30

1.10

14

0.90

1.36

0.10

6.30

1.12

a

The decays are fitted with a biexponential function: I(t) = I(0)[a1 exp(-t/τ1) þ a2 exp(-t/τ2)], where τ1 and τ2 are the two lifetimes and a1 and a2 are their amplitudes. a1 þ a2 = 1. λex = 341 nm.

the SI for SNT and Figure S4B, Table S2 of the SI for SND). The relative contributions vary slightly (Figure 2A, Table 2). The near invariance of the short component indicates that the emissive reaction center does not change. What is likely to be happening is that H2O2 destroys one or both of the components of the defect pair to form nonemissive defects. However, there is no doubt about the fact that the longer time decays are slower upon addition of H2O2 (Figure 2) Thus, the contribution of trap states appears to increase with the addition of H2O2. It is difficult to assign meaning to each of the four parameters in the PL decays, as the decays are dominated by the shorter component, while the longer component, although weak, is definitely present. The products a1τ1 and a2τ2 denote the net contribution of the two components to the steady state fluorescence intensities. A comparison of these terms reveals that the contribution of the longer component increases with the addition of H2O2. This observation strengthens the contention of stabilization of the surface trap states by H2O2, presumably by hydrogen bonding.36 The change in the surface morphology-related long component is found to follow the order: SNP > SND > SNT (Figure 2B). This may be rationalized by the fact that smaller spherical nanoparticles have much more accessible surface than SNT and SND. So the surface of SNP is affected to a greater extent, in comparison with SNT and SND. Thus, hydrogen peroxide seems to be affecting the PL of silica nanostructures in two different ways: by reaction with the defect pair, leading to the formation of a nonluminescent center, and by stabilization of the surface states by hydrogen bonding. It is difficult to establish the nature of the product of the reaction 1578

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Figure 4. Comparison of PL decays of silica nanoparticle dispersed (i) in methanol, (ii) acetonitrile, and (iii) hexane at λex = 341 nm, λem = 460 nm in all cases. Figure 3. Peak-normalized PL spectra of silica nanoparticle dispersed in isooctane (solid line), tetrahydrofuran (dashed line), and methanol (dashed and dotted line) at λex = 360 nm.

between the defect center and H2O2. Attempts to record the FTIR and EPR spectra of the nanostructures, post H2O2 treatment, have been unsuccessful, are most likely because of insufficient abundance of the defect centers. The marked effect of H2O2 on the PL intensity and lifetimes has prompted us to explore the effect of organic solvents on the PL of the silica nanostructures. The PL spectra of SNP exhibit small but significant shifts in the PL maxima, upon changing the dispersion medium (Figures 3 and S5, SI). More importantly, the vibronic structure of the bands becomes more prominent in nonpolar solvents. The absorption (Figure S6, SI) and excitation spectra (Figure S7, SI) also exhibit solvent dependence. There are two possible explanations: The emissive defect pair is localized. It is analogous to a fluorescent molecule tagged to the surface of silica and may be compared with some such molecule, like nile red.37 In nonpolar solvents, the vibronic structure prevails, as there is no specific interaction of the fluorophore with the solvent molecules, and the vibronic states are well-defined. In polar solvents, however, interaction with the solvent molecules has a smoothening effect, and invariably, the spectra are smooth and structureless. The situation in silica nanostructures is similar. There could be another explanation. Silica nanostructures are amorphous in nature. So, unlike in crystals, the defect centers are localized in different environments. In all probabilities, there is a site-to-site distribution of structural parameters like bond lengths and bond angles. The statistical distribution of structural parameters causes an inhomogeneity effect in the optical properties of the defects, including inhomogeneous broadening of the optical band.38 This could be the reason for the “vibronic” structures of the PL band. It may be expected that the emissive surface trap states formed by the interaction with the molecules of polar solvents through dipole-dipole interactions and in the case of protic solvents, through H-bonds, can overwhelm the intrinsic inhomogeneity in the nanostructure and smoothen out the spectra to some extent, thus causing the “vibronic” structure to decrease. Nonpolar solvents, however, can have no such effect, and so in such solvents, the vibronic structure remains more prominent. There is no straightforward correlation between the spectral properties of these silica nanostructures and the polarity or viscosity of the solvents. However, the solvent dependence of PL decays of silica nanoparticles is more systematic (Figures 4 and S8, SI). The usual biexponential decays are observed in dispersions in polar solvents, but the PL decays are monoexponential in nature, in nonpolar solvents. The lifetime is about 1 ns

Table 3. PL Lifetimes and Their Relative Amplitudes of Silica Nanoparticle Dispersed in Different Solvents at λex = 341 nm and λem = 460 nm nature of the ε

solvent n-hexane

1.9

n-heptane

1.9

solvent

a1

nonpolar

τ1 (ns)

2.1

0.97

2.4

1.22

4.8 7.5

0.08

2.00

0.99

toluene tetrahydrofuran

τ2 (ns)

1.01

isooctane chloroform

a2

1.48 polar aprotic

0.92

1.14

(THF) dichloromethane (DCM)

9.1

1.33

acetone

21

0.99

1.22

0.01

3.75

acetonitrile

37.5

0.95

1.26

0.05

3.10

dimethylformamide

38

0.92

1.34

0.08

2.66

(DMF) isopropanol

18

0.96

1.32

0.04

6.71

n-propanol

20

polar protic

0.97

1.33

0.03

3.59

ethanol methanol

30 33

0.97 0.91

1.35 1.37

0.03 0.09

6.80 2.87

(Figures 4 and S8, SI, Table 3). This lifetime increases gradually with the increase of polarity of the solvent. In polar aprotic solvents, the decays are bimodal (Figures 4 and S8, SI, Table3). The short component of 1-1.3 remains the major one, but the long component gradually sets in. In protic solvents, the decays are biexponential with a higher value of the longer lifetime (Figures 4 and S8, SI, Table 3). Like polar aprotic solvents, the short component remains the dominating species in polar protic solvents and is ∼1.3 ns. Thus, the short component occurs in all microenvironment and has very little dependence on the solvent polarity parameter. It is the long component that makes a higher contribution in polar media and even more so, in protic media (Figures 4 and S8, SI, Table 3). According to the Uchino model the long component is due to surface traps. In the light of this model, it appears that the surface trap states are stabilized by the interaction with the solvent molecules in polar media and especially by hydrogen bonding with alcohols. The stabilization leads to the increase in the long lifetimes. The short component is not affected very significantly by the solvents, indicating an absence of such interactions between the defect pair and the solvent molecules. Unlike H2O2, organic solvents do not quench the PL of the silica nanostructures. 1579

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The Journal of Physical Chemistry C The PL decay traces of SNP, SNT, and SND are almost identical (Figure S10, SI). The long components of PL decays of all three morphologies differ very slightly in polar solvents (Figure S10A-C, SI) and are almost the same, in case of nonpolar solvents (Figure S10D and E, SI). The addition of polar protic ethanol to the dispersion of SNP in isooctane (Figure S11, SI), carbon tetrachloride (Figure S12, SI), and hexane (Figure S13, SI) shifts the PL spectra of SNP to the higher wavelength. Corresponding PL decays change from monoexponential to biexponential. However, the reverse addition, that is, the addition of nonpolar solvent to the ethanolic dispersion does not change the biexponential behavior to monoexponential behavior. This reflects the inability of the nonpolar solvent molecules to displace the polar protic solvent molecules sticking to the silica surface. It may be recalled here that organic solvent molecules cannot react chemically with the defect pair unlike H2O2. Therefore, no quenching of PL is observed by the addition of solvents. They play a single role: that of stabilizing the surface trap states. Polar protic as well as aprotic solvent molecules can form surface trap state with the surface of silica through dipole-dipole interaction and hydrogen bonding. An evidence of such stabilization is obtained in the fact that the addition of polar protic solvent to a nonpolar dispersion causes the PL maxima to shift toward lower energies. The longer component of the PL decay appears as well, indicating the formation of a radiative trap state. This kind of interaction is absent in the case of nonpolar solvent molecules as they cannot bind with the surface trap state with strong dipolar interaction or H-bonding. So the addition of nonpolar solvent molecules to a polar protic dispersion fails to bring any change neither in the steady state PL spectra nor in the PL decay. It is also possible that the polar solvents interact with the emission centers and convert them into nonemissive centers. Such nonemissive centers can act as surface trap states and further contribute to the PL process.

’ CONCLUSION The PL of silica nanostructures is found to be affected strongly by organic solvents and H2O2. The long component of PL decay, assigned to surface traps by Uchino and co-workers, gets longer and contributes more to the PL in polar and especially protic media. Such stabilization may be expected to occur for emissive surface states. On the other hand, in nonpolar media, where the surface states cannot be stabilized by such interactions with the medium, the long component is not even there. This observation strengthens the contention that it is associated with surface traps. Besides, the longer lifetime becomes even longer upon the addition of H2O2, further confirming the role of hydrogen bonds in stabilizing the surface traps. Interestingly, the PL intensity is quenched by H2O2, even though the longer component increases and the shorter one remains constant. Thus, it appears that the defect center responsible for PL is reactive to H2O2. To conclude, the defect center and the surface trap states are found to be vulnerable to chemical inputs, those which change their immediate environment and more drastically, those which can modify the defect center chemically. This observation could have implication in the future design of optoelectronic applications of silica that would use such input for the purpose of sensing or actuation. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM micrographs, size distribution of SNP, SEM micrographs, additional PL spectra and decays, Stern-Volmer plots, table of PL lifetimes, absorption

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spectra, and excitation spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ91 22 2576 7149. Fax: þ91 22 2570 3480. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by the CSIR project number 01(2277)/08/EMR-II. S.B. thanks CSIR for a Senior Research Fellowship. The SEM images have been recorded at the Department of Metallurgy and Materials Science, IIT Bombay. The authors thank Dr. S. L. Kamath for the SEM images. The TEM images have been recorded at CRNTS, IIT Bombay. ’ REFERENCES (1) Li, Z.; Johnson, M. C.; Sun, M.; Ryan, E. T.; Earl, D. J.; Maichen, W.; Martin, J. I.; Li, S.; Lew, C. M.; Wang, J.; Deem, M. W.; Davis, M. E.; Yan, Y. Angew. Chem., Int. Ed. 2006, 45, 6329. (2) Pan, V. H.; Tao, T.; Zhou, J.-W.; Maciel, G. E. J. Phys. Chem. B 1999, 103, 6930. (3) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. Rev. 1994, 94, 2095. (4) Bernstein, T.; Michel, D.; Pfeifer, H.; Fink, P. J. Colloid Interface Sci. 1981, 84, 310. (5) Fiori, C.; Devine, R. Phys. Rev. B 1986, 33, 2972. (6) Griscom, D. L.; Friebele, E. J. Phys. Rev. B 1986, 34, 7524. (7) Shang, N. G.; Vetter, U.; Gerhards, I.; Hofs€ass, H.; Ronning, C.; Seibt, M. Nanotechnology 2006, 17, 3215. (8) Nishikawa, H.; Nakamura, R.; Tohmon, R.; Ohki, Y.; Sakurai, Y.; Nagasawa, K.; Hama, Y. Phys. Rev. B 1990, 41, 7828. (9) Lee, J.-W.; Tomozawa, M.; MacCrone, R. K. J. Non-Cryst. Solids 2007, 354, 1509. (10) Seol, K. S.; Ohki, Y.; Nishikawa, H.; Takiyama, M.; Hama, Y. J. Appl. Phys. 1996, 80, 6444. (11) Edwards, A. H.; Fowler, W. B. Phys. Rev. B 1982, 26, 6649. (12) Bakos, T.; Rashkeev, S. N.; Pantelides, S. T. Phys. Rev. B 2004, 69, 195206. (13) Morimoto, Y.; Weeks, R. A.; Barnesn, A. V.; Tolk, N. H.; Zuhr, R. A. J. Non-Cryst. Solids 1996, 203, 62. (14) Jin, Y.-G.; Chang., K. J. Phys. Rev. Lett. 2001, 86, 1793. (15) Skuja, L.; Hirano, M.; Hosono, H. Phys. Rev. Lett. 2000, 84, 302. (16) Skuja, L. J. Non-Cryst. Solids 1998, 239, 16. (17) Glinka, Y. D.; Lin, S.-H.; Huang, L.-P.; Chen, Y.-T.; Tolk, N. H. Phys. Rev. B 2001, 64, 085421. (18) Fukata, N.; Yamamoto, Y.; Murakami, K.; Hase, M.; Kitajima, M. Physica B 2003, 340-342, 986. (19) Beigi, S. I.; Louie, S. G. Phys. Rev. Lett. 2005, 95, 156401. (20) Uchino, T.; Kurumoto, N.; Sagawa, N. Phys. Rev. B 2006, 73, 233203. (21) Nakazaki, Y.; Fujita, K.; Tanaka, K.; Uchino, T. J. Phys. Chem. C 2008, 112, 10878. (22) Banerjee, S.; Datta, A. Langmuir 2010, 26, 1172. (23) Lei, S.; Zhang, J.; Wang, J.; Huang, J. Langmuir 2010, 26, 4288. (24) Nishimura, A.; Sagawa, N.; Uchino, T. J. Phys. Chem. C 2009, 113, 4260. (25) Nishimura, A.; Harda, S.; Uchino, T. J. Phys. Chem. C 2010, 114, 8568. (26) Sagawa, N.; Uchino, T. J. Phys. Chem. C 2008, 112, 4581. (27) Nogami, M.; Abe, Y. Appl. Phys. Lett. 1994, 65, 2545. (28) Nikiel, L.; Hopkins, B.; Zerda, T. W. J. Phys. Chem. 1990, 94, 7458. 1580

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