Trapped Molecular and Ionic Species in Poled Borosilicate Glasses

Article pubs.acs.org/JPCC

Trapped Molecular and Ionic Species in Poled Borosilicate Glasses: Toward a Rationalized Description of Thermal Poling in Glasses Tatiana Cremoux,† Marc Dussauze,*,† Evelyne Fargin,‡ Thierry Cardinal,‡ David Talaga,† Frédéric Adamietz,† and Vincent Rodriguez† †

Universite Bordeaux, Institut des Sciences Moléculaires, CNRS UMR 5255, F-33400 Talence, France Universite Bordeaux, ICMCB, CNRS UPR 9048, F-33600 Pessac, France

‡

ABSTRACT: Molecular and ionic species of nitrogen oxides have been injected, formed, and then trapped within a borosilicate glass during a thermal poling process. This original observation denotes a new aspect of poling mechanisms for ionic glasses. For each poled borosilicate glasses studied, compositional, structural, and second harmonic generation profiles of their subanodic polarized layers have been characterized. A description of the space charge implementation process, involving interactions between charged and chemically active species formed both within plasma discharges at the glass/electrode interface and within the polarized glass matrix, is proposed.

1. INTRODUCTION Thermal poling of a glass slide is a very simple treatment which has been developed for anodic bonding in electronic application1−4 and widely studied for photonic applications since a second order optical response can be generated through an EFISH process (Electric Field Induced Second Harmonic) by the implementation of a high static electric field inside the glassy matrix.5−15 Polarization mechanisms in glasses are dictated by charge rearrangement processes which are triggered by a depletion of mobile cations formed in a micrometer-sized layer below the anode surface.8,12 Several recent studies on thermal poling in ionic glasses have focused their attention on structural rearrangements induced within this cation-depleted layer.15−21 It has been demonstrated that charge compensation mechanisms occurring after the departure of the mobile cations during the space charge implementation is one key factor to control both the chemical structure of the poled glass and the amplitude of the entrapped static electric field.15,21 In thermal poling of oxide ionic glasses, these compensation mechanisms can be classified in two categories. The first one addresses a field assisted ion exchange process when the anode is an open electrode. In that case injected ions come from the surrounding atmosphere (generally H3O+) or from the ionization of a metallic electrode like silver for example.19,22,23 The second category addresses the motion of negative charge carriers from the glass network. In that case, two possibilities have been proposed in the literature, namely electronic conductivity or anionic conductivities by release of oxygen from the polarized glass network.19,20,24−30 Recently, we have proposed a common process linked to the oxidation of oxygen anions which release an electron and form molecular oxygen.20 Actually a unified description of the poling mechanisms remains difficult since each charge carrier motion is highly dependent on the glass composition. In this work, we have correlated structural, compositional, and optical characterization of a poled ionic borosilicate glass © 2014 American Chemical Society

matrix. Very original structural rearrangements have been observed involving the injection of nitrogen oxide species from the atmosphere into the poled glass matrix. In situ spectral measurements of the plasma formed at the anode/glass interface were carried out to achieve a description of the poling mechanisms.

2. EXPERIMENTAL DETAILS Commercial DURAN borosilicate glass of composition 81% SiO2−13%B2O3−4%Na2O−1%K2O−1%Al2O3 (weight %) were poled under air or N2 atmospheric pressure at a temperature of 300 °C and under 2 kV during 30 min. To avoid anodic bonding an unpolished silicon wafer was used at the anode side. Note that in these poling conditions a light emission originated from plasma discharges formed at the silicon anode/glass interface can be clearly observed by eye. To analyze spectrally the plasma emission our experimental setup was based on a modified confocal μ-Raman HR800 (Horiba/ Jobin-Yvon) instrument using a 600 grooves·cm−1 grating for a typical resolution of 0.5 nm. To collect the light as close as possible to the silicon electrode edges, we used a UV−visible Cassegrains objective (×25). The emission spectrum was recorded in the 300−440 nm range. The IR spectra were recorded on a Nexus 670 spectrophotometer (Thermo Optek) equipped with a DTGS detector and a germanium coated KBr beam splitter or hybrid FIR beam splitter. A total of 200 scans were averaged with a resolution of 4 cm−1. The spectrometer was purged with dry air to minimize atmospheric CO2 and water vapor. Reflectance experiments were performed using an external reflection attachment (Graesby, Specac) at an angle of incidence of 12°. Absorbance Received: October 11, 2013 Revised: January 16, 2014 Published: January 21, 2014 3716

dx.doi.org/10.1021/jp4101015 | J. Phys. Chem. C 2014, 118, 3716−3723

The Journal of Physical Chemistry C

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spectra were calculated from the reflectance spectra by the Kramers−Kronig analysis.31,32 A coupled micro-Raman/micro-Second Harmonic Generation (SHG) imaging technique has been used in the backscattering mode to gain a direct link between physical properties and the local structure.19,33 The setup is based on a modified μ-Raman HR800 (Horiba/Jobin−Yvon) instrument equipped with two laser sources (picosecond laser at 1064 nm -hyper-Raman and SHG, CW laser at 532 nm - Raman). Confocal microscopy and motorized stages (X, Y, Z) allow 3D investigation. The μ-Raman spectra were recorded with a typical resolution of 2.5 cm−1 in the backscattering geometry at room temperature. More details about the μ-spectrometer setup and its ability are given elsewhere.34 Electron Dispersive Spectroscopy (EDS) analyses have been done with a Cameca SX 100 instrument. After metallization, composition profiles have been measured on the cross section of the poled samples.

Figure 2. Absorption coefficient spectra obtained by Kramers−Kronig transformation of infrared reflectance measurements measured on the anode side of polarized borosilicate glasses under air and N2.

3. RESULTS The composition profile measured by EDS analysis on the cross section of the air-poled borosilicate glass is shown in Figure 1.

range is attributed to the asymmetric stretching vibrations of BØ3 groups.35−37 At lower frequencies in the 900−1000 cm−1 spectral range the asymmetric stretching vibration of the [BØ4]− tetrahedral is expected.38 Peaks at frequencies lower than 900 cm−1 are associated with the bending modes of various structural borosilicate entities and notably the Si−O−Si bridges peaking at 460 cm−1. After the poling process, spectral variations are similar in both poling under N2 and air. They concern a clear increase of the asymmetric stretching vibrations of BØ3 groups and a decrease within the spectral range attributed to borate tetrahedral entities. The structural changes observed in the infrared spectra measured after poling at the anode side have already been observed by Moncke et al.39 and attributed to a transformation reaction of the borate tetrahedral into triangular units which can be correlated to the depletion in sodium shown in Figure 1: NaBØ4 → BØ3 + Na + + 1/2O2 −

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In a second step, coupled μ-Raman/μ-SHG recording (typically in steps of tenth of micrometers) has been carried out on the cross section of the polarized glasses to extract detailed structural and electric field depth profiles from the anode surface toward the bulk of the glass. Our methodology consists of (i) localizing the internal static electric field implemented during the poling treatment using μ-SHG and (ii) recording the corresponding μ-Raman map exactly at the same position. Such a protocol warrants direct correlations at the micrometer scale between nonlinear optical (NLO) responses (i.e., electric field location) and vibrational/structural features. In Figures 3 A and 4 A, two SHG profiles that give the space charge location are presented. For the poling under air (Figure 3A), the SHG signal is located in a 4.5 μm thick layer beneath the anode surface located between 8.5 to13 μm. That result, indicating that the space charge is observed well below the anode surface for a poling done under air, has been already observed by Ann et al. and modeled by Petrov et al. in the case of an open anode poling configuration.40−42 When poling under a N2 atmosphere is considered (Figure 4A), the SHG active layer is 2 μm thick and its maximum intensity is located just beneath the anode surface at 1 μm. This sharp space charge location close to the anode surface has been also observed in soda lime glass.19

Figure 1. Sodium, potassium, and aluminum composition in weight percent measured by EDS on the cross section of a poled borosilicate glass under air. Note that small discrepancies as compared to the nominal composition should be related to the geometry of the experimental setup as a function of the cross section orientation.

The concentrations of silicon (not shown) and aluminum atoms remain constant, whereas complete sodium depletion is observed up to 12.5 μm beneath the anode surface. Concerning the potassium profile, the first 6 μm thick layer is completely depleted, followed by an increase of twice the original potassium concentration forming a stacking layer from 6 to 13 μm under the anode surface. To characterize the effect of the treatment on the borosilicate structure, we have used infrared (IR) spectroscopy in reflection mode at the anode surface of the poled glass. Note that only the first micrometers at the anode surface can been probed using an IR specular reflectance technique. As shown in Figure 2, the IR spectra of the borosilicate glass are composed of strong absorption in the 900−1500 cm−1 range linked to asymmetric stretching of SiO and BO bonds. One can notably discern the most intense band picking at 1090 cm−1 which originates mostly from the asymmetric stretching of the fully polymerized silicate tetrahedral (SiØ4 where Ø denotes a bridging oxygen). At higher frequencies, the large band in the 1250−1500 cm−1 3717



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first 14 μm from the anode surface (Figure 5). All Raman spectra recorded far from that area were identical to the initial

Figure 3. (A) SHG profile and (B) Raman intensity profiles measured on the cross section of a Duran glass polarized under air.

Figure 5. Raman spectra measured on the cross section of a Duran glass polarized under air. The different Z values correspond to the depth under the anode surface. The symbols *, #, and o are linked to the spectral assignments summarized in Table 1.

glass spectrum. The Raman spectrum of the initial borosilicate glass contains mainly one broad and asymmetric band at 455 cm−1 attributed to the symmetric stretching of Si−O−Si bridges and a smaller band at 805 cm−1 linked to borate rings in the borosilicate glassy structure.39,43 Noteworthy, any modification of the borate network may not be accurately evidenced by Raman spectroscopy because of its low Raman cross section and also low concentration in the glass composition. Spectral modifications induced by the poling under air can be depicted as follow: (i) for the first 6 μm thick layer from the anode surface, the Raman bands assigned to the silicate network do not vary significantly except for an additional band that appears at ca. 3500 cm−1 assigned to the stretching vibrations of OH groups; (ii) in an intermediate 8 μm thick layer located between 6 and 14 μm beneath the anode surface, new features occur in the Raman spectrum. First, a very sharp and strong band appears at low frequency (260 cm−1). Another new spectral feature comes from a broad and asymmetric band occurring at 2220 cm−1. Other new spectral features concern numerous sharp bands but with lower intensities at 615, 754, 809, 1290, 1323, 1381, 1508, 1880, and 2075 cm−1. These bands correspond to nitrogen oxide species trapped inside the layer during the poling process under air, mostly NO2 and N2O4. All the vibrational modes expected from a NO2/N2O4 mixture are observed in that second intermediate layer (see

Figure 4. (A) SHG profile and (B) Raman intensity profile linked to the stretching vibration of molecular oxygen measured on the cross section of a Duran glass polarized under N2.

For the poling done under air, two main spectral modifications have been observed by μ-Raman within the 3718



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(3500 cm−1) as well as those of modes attributed to N2O4 (260 cm−1) and NO+ (2220 cm−1). The Raman profiles show quite a rather sharp borderline between a hydroxyl reach glassy layer, located just beneath the anode surface with no SHG signal, and the next layer where many nitrogen oxide species coexist with an imbedded internal electric field. The Raman spectra observed on the cross section of the poled glass treated under a N2 atmosphere are gathered in Figure 6. The poled layer directly beneath the anode surface is

Table1 and Figure 5). It is well-known that NO2 molecules produce by association the stable dimer dinitrogen tetroxide, Table 1. Spectral Assignments of Raman Modes Linked to Nitrogen Oxide Species Observed within the Air-Polarized Borosilcate Glasses signs (Figure 5) *

species N2O4

frequency (cm−1) 260 809 1381

#

NO2

754 1323 1508 2075

o

NO (nitroso group) NO+ (nitrosonium cation) −NO2 (from asymmetric N2O3 or N2O3+)

1880 2220 600 1290

mode ν3 N−N stretching ν2 NO2 sym. bending ν1 NO2 sym. stretching ν2 NO2 sym. bending ν1 NO2 sym. stretching 2* ν2 ν1+ ν2 sym. stretching sym. stretching ν2 NO2 sym. bending ν1 NO2 sym. stretching

Figure 6. Raman spectra measured on the cross section of a Duran glass polarized under N2. The different Z values correspond to the depth under the anode surface.

−1

N2O4. Hence the very strong Raman band at 260 cm , assigned to the N−N symmetric stretching of N2O4, appears at low frequency because of the long (1.75 Å) and weak N−N bond.44−47 All characteristic Raman bands of nitrogen dioxide NO2 are observed in Figure 4, but their respective intensities are quite weak in comparison to N2O4 ones probably due to a lower content. In addition to these NO2/N2O4 spectral signatures, several other Raman bands are observed, particularly a strong, large, and asymmetric peak at 2220 cm−1 assigned to the stretching mode of the nitrosonium cation, NO+. The Raman signature of NO+ has been previously clearly identified, for example, in N2O4 amorphous solids,45,46 or in a solution of N2O4 in sulfuric or nitric acid.48 Note that the solid ionic form of dinitrogen tetroxide is composed of the NO+ and NO3− ion pair. Here in that intermediate layer, we do not observe Raman modes from the anion NO3− (check for example the absence of the ν1 mode of the NO3− anion expected ca. 1045 cm−1). Hence the formation of NO+ cations inside the polarized glass matrix cannot be explained by the simple ionization of dinitrogen tetroxide molecules. Three other weak intensity Raman modes at 625, 1290, and 1880 cm−1 should also be discussed. At 1880 cm−1, we expect the symmetric stretching of a nitroso functional group NO. The nitroso group may be associated with NO2, as observed in N2O3 for example.49,50 We may also refer to nearly finished work on the oxide and oxyacide forms of dinitrogen tetroxide:48 cations as N2O3+ could also be expected from interactions between NO2 and the nitrosonium cations. Thus, we suggest the two modes at 625 and 1290 cm−1correspond to the symmetric bending and stretching modes, respectively, of the nitro (NO2) part of the asymmetric form of N2O3. For comparison with the SHG profile, spatial profiles of three characteristic Raman bands induced by the poling procedure under air are reported in Figure 3B, namely the Raman intensity depth profiles of the O−H bonds stretching mode

characterized by a new sharp and intense Raman band at 1550 cm−1 assigned to molecular oxygen. In addition, similarly to the poling done under air we observe a band at 2220 cm−1 and a sharp peak at 1325 cm−1 assigned to the nitrosonium NO+ cation and the symmetric stretching of NO2, respectively. When normalizing the nitrogen oxide Raman intensities to the silicate glass band at 460 cm−1, the concentration of NO+ and NO2 is estimated to be 15 times weaker in the N2-poled glass than in the air-poled sample. Nevertheless, that approximation may overestimate nitrogen oxide concentrations since no Raman signature of the dimer N2O4 can be evidenced in the N2-poled borosilicate glass. A Raman depth profile of the molecular O2 is shown in Figure 4 for comparison with the SHG profile. All the spectral changes appear in a 3.5 μm thick layer just below the anode surface. The Raman intensity profile of molecular oxygen shows a maximum peaking at 1.2 μm (±0.2 μm) below the anode surface corresponding well with the location of the SHG response. In order to better understand the poling mechanisms which could explain how nitrogen oxide species could be formed and trapped during the poling treatment, we have characterized the plasma emission induced during the poling treatment. The emission spectrum collected close to the anode surface within the 300−440 nm spectral range is presented in Figure 7A. This emission is dominated by the second positive system (SPS) electronic transition of N2 (C3Πu→ B3Πg) which is typically observed for “cold” nonequilibrium plasma in nitrogen or air at atmospheric pressure generated within a corona or a dielectric barrier discharge (DBD) for example.51−54 In addition to the predominant N2(C3Πu→ B3Πg) transition visible with several of its vibrational bands (Δv = +1, 0, −1, −2, −3, and −4), other smaller contributions can be observed from the OH (A2∑+→ 3719



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Figure 8. Time evolution of electrical current measured on the high voltage supply of the poling setup and light emission from the plasma measured at the anode glass surface.

matrix. To this aim, one should make the link between (i) physical and chemical processes within the gas discharge at the anode/glass interface and (ii) the displacement of numerous charged entities within the glassy matrix during the implementation of the space charge. In the present case, the gas gap between the n-doped silicon anode electrode and the glass surface is due to the roughness of the silicon used to avoid anodic bonding.3 To better understand the chronology of the main processes involved within the treatment, one can refer to the temporal evolution of both global electrical current and light intensity from the plasma emission. By comparing these two curves in Figure 8, the light emission intensity is delayed for at least 1 min as compared to the electrical current. It denotes that, at the first moment of treatment, the main contribution to the poling current should be mainly due to charge motions inside the glass matrix. Such charge displacement is notably illustrated in Figures 1 and 2 which clearly depict (i) a mobile cation depletion (Na+, K+) and (ii) structural rearrangements at the origin of complex negative charge motions which will be discussed later in this paper. Finally, the overall charge motions in the glass matrix will induce an accumulation of negative charge on the glass surface. This should increase significantly the potential on the dielectric surface and the electric field within the gas gap. When a breakdown field strength is reached in the gas gap, plasma gas discharges ignite. Such low pressure plasma should first induce an electron conduction (avalanche) which will excite neutral atoms and molecules at the origin of the light emission shown in Figure 7. Simultaneously positive cations will be formed and accelerated by the high local field of the plasma and accumulated on the glass surface. According to the early work of Knewstubb and Tickner, in an air gas discharge, the main cations expected to be formed and then accelerated toward the cathode surface of the gas gap (i.e., the glass surface) are H3O+, N 2 +, O 2 +, and NO +.57,58 Considering an open anode configuration, all these cationic species are suspected to be injected within the glass matrix similarly as an electric fielddriven ion exchanged process. It is well established within the poled glass community that hydronium cations can be injected during the polarization process, and the Raman data have clearly shown its incorporation inside the borosilicate poled glass matrix within a layer of 5 μm just below the anode (Figure 3B). In contrast, obviously we could not find any literature

Figure 7. Emission spectrum from the gas discharge at the anode glass surface collected within the spectral range 300−440 nm. This emission is dominated by the second positive system (SPS) electronic transition of N2(C3Πu→ B3Πg).

X2Π) at 309 nm and N2+(B2∑u+→X2∑g+) at 391 nm as shown in Figure 7C and D.52,53 Note that a strong contribution from NO (A2∑+→X2Π) is also expected below 300 nm but could not be measured with our experimental setup.52 One should notice that if such an emission is well-known in all the works related to plasma induced by glow or streamer discharges, similar observations have been reported by Moura et al. for a glass poling process.55,56 The emission spectrum was similar but recorded with a much lower spectral resolution, and it was not assigned to N2 electronic transition. Nevertheless, Moura et al. pointed out for the first time a clear correlation between the intensity of the plasma emission measured during the poling treatment and the second order optical responses of the polarized glass. The time dependence of both electrical current measured on the high voltage source and light emission intensity recorded at the anode/glass interface during a poling treatment done at 300 °C with 2 kV during 30 min in air are presented in Figure 8. As soon as the applied voltage reaches 2 kV, the electrical current shows a classical exponential decrease from a maximum value of 0.8 mA until it reaches a plateau of 3.10−2 mA at the end of the treatment. The maximum of light emission intensity shows a delay of 60 s as compared to the electrical current. The decrease of intensity observed for the light emission does not follow an exponential decrease behavior and is much slower than that for the electrical current. The delay observed between the two curves to reach half of their maximum value is 235 s.

4. DISCUSSION Now we aim to propose a global mechanism for the poling treatment which can explain the formation of nitrogen oxide molecular species trapped inside the polarized borosilicate glass 3720



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referring to a field-driven injection of nitrogen or nitrogen oxide cations in a glass matrix. To explain the presence of NO2/ N2O4 species, we may also consider the chemical reactivity in order to explain the formation of such neutral molecules trapped within the poled glass matrix. To discuss this new aspect of thermal poling mechanisms, we will first compare the two poling conditions done respectively under N2 and air. In a pure nitrogen atmosphere, one should expect only N2+ (and at a less extend N+) cations to be projected onto the glass surface. As shown in Figure 6, a very small contribution from NO2 and NO+ cations can be detected in the Raman spectra of the polarized glass, but the main effects observed can be summarized as a strong space charge located in a 2 μm thick layer right beneath the glass surface, the location of which corresponds to an accumulation of molecular oxygen (see Figure 4B). Such behavior is similar to previous results concerning polarized glasses in an argon atmosphere.19 In that configuration, considered as a blocking anode electrode, we have proposed that structural rearrangements of the glass matrix could induced a release of oxygen anions which are in a second step oxidized to form a neutral dioxygen molecule.20 This oxidation process has to be followed by an electronic conduction to release the negative charge toward the anode: O2 − →

1 O2 + 2e− 2

Figure 9. Profile comparison of (i) sodium and potassium concentration measured by EDS, (ii) SHG, and (iii) Raman intensities linked to hydroxyl group and nitrogen oxide species. All these data were recorded at the same position on the cross section of a Duran glass polarized under air.

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Finally, since a thermal poling treatment under N 2 atmosphere is almost similar to the use of an argon atmosphere, it clearly denotes that N2+ or N+ cations are not preferentially injected in the glass during the poling treatment. Focusing now on the data of the air-poled borosilicate glass, Raman measurements have clearly shown two distinct layers, one rich in the hydroxyl group just below the glass surface and one rich in nitrogen oxides starting 5 μm below the glass surface. It is worthy to point out that the nitrosonium cation NO+ not only is observed inside the glass matrix but also is one of the predominant cations formed during the ionization of a gas containing both N2 and O2.58 Thus, one hypothesis to consider could be the injection of NO+ cations during the poling treatment. Nevertheless, it is necessary to notice that such a large injection of nitrogen oxides has never been observed in polarized silicate, phosphate, borate, or germanate oxide glasses, which should denote a real particularity of the borosilicate glass structure allowing both injection and ionic conduction of NO+ cations. Let us now analyze the spatial distribution of the different entities observed in the air-poled anodic layers. In Figure 9, we have summarized depth profiles of EDS measurements (from Figure 1) for Na+ and K+ concentrations, SHG intensity, and Raman band intensities linked to hydroxyl groups, nitrosonium cations, and the N2O4 dimer (from Figure 3). Both hydroxyl and nitrogen oxide Raman bands are observed within the Na depletion layer. The hydroxyl rich layer corresponds to the sodium and potassium whole depletion zone. The increase of nitrogen oxide concentrations start 5 μm below the anode surface which corresponds well with both the potassium stacking layer and the SHG signal location. When the respective locations of N2O4 and NO+ cations as compared to the SHG signal are considered, the concentration profiles of N2O4 and NO+ are clearly asymmetric and their apparent maxima are shifted from a distance of 3 μm. The maximum of the SHG signal is observed right in between the two nitrogen oxide entity profiles. Interestingly, these three depth profiles

appear to form a symmetric figure centered on the maximum of the second harmonic generation response, i.e., of the embedded static electric field. The two main parameters governing these concentration depth profiles are (i) their respective mobilities and (ii) some chemical affinity or reactivity. Concerning mobile cations, if we refer to the theoretical calculations done by Petrov et al., we can try to correlate the spatial distribution of the charge carrier with the embedded electric field location (i.e., location of the SHG signal) to estimate their mobility.42 Petrov et al. have notably demonstrated that the location of the electric field is correlated to the difference of mobility between the penetrating charge carriers and the mobile cations originally present in the glass. Let us consider that, for the air-poled borosilicate glass, we have mainly two injected charge carriers H3O+ and NO+ and two types of mobile cations, one with a high mobility Na+ and the other one with a slower mobility K+. The hydroxyl groups are located inside the poled glass layer just below the anode and well above the space charge. In accordance with all previous reports, either theoretical or experimental, it shows that hydronium ions have the slowest mobility which generally scales about 3 orders of magnitude lower than the sodium cation mobility does. Another interesting result concerns the location of nitrosonium cations for which the maximum of the concentration profile is located beneath both the embedded electric field and the K+ cation stacking layer. This points out two important remarks: (i) the mobility of NO+ during the poling treatment should be higher than the mobility of potassium cations, and as a consequence (ii) NO+ injection and conduction during the poling process play a crucial role in the final location of the space charge. Knowing the respective mobility of each positive charge carrier involved during the polarization process, we will now discuss the possible chemical reactivity leading to the formation 3721



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Notes

of the neutral NO2/N2O4 entities. First we should as a reminder mention that N2O4 and NO+ cation Raman profiles form a symmetric figure centered on the maximum of the internal electric field. For the N2-poling treatment, we have shown that negative charge motions are linked to an oxidation reaction of oxygen anions originating from the glass network (see eq 2). Molecular oxygen could not be detected during the air-poling conditions, but the borate network rearrangement characterized by IR remains the same under both atmospheres. Thus, one can suspect that, upon NO+ conduction within the poled glass matrix, parallel redox reactions of O2− and NO+ could occur: ⎧ 2− 1 ⎪O → O2 + 2e− 2 ⎨ ⎪ + ⎩ NO + 1e− → NO

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Région Aquitaine (Project: 20121101025) and by Agence Nationale de la Recherche (Project PolarChem ANR-2010 JCJC-0806 01 and Project Felins ANR-2010 blan94603).

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Then, the well-known high reactivity of nitric oxide with oxygen could reach to the formation of NO2 molecules: NO +

1 O2 → NO2 2

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Finally, to explain the formation of NO2/N2O4 molecules in the poled borosilicate glass matrix, one should consider thermal poling as a solid state electrochemical process involving coupled redox reactions between anionic and cationic species. This redox reactivity is dictated by the electric field implementation which explains the respective locations of nitrogen oxide species symmetrically to the second harmonic generation profile. One should note that the role of the borosilicate matrix on both the high injection and high mobility of NO+ is yet unclear. Nevertheless one should consider the particularity of the DURAN glass composition which is very close to the boric oxide anomaly. Modified borate glasses along this anomaly exhibit a distinct extremum for numerous properties.39,59 In such a glass matrix, all the structural entities are interconnected through iono-covalent bonding. To charge compensate modifier elements such as sodium or potassium cations, the conversion of trigonal BØ3 groups into charged BØ 4 − tetrahedra occurs (Ø denotes an oxygen atom bridging to a boron or a silicon). Such a particularity of this two-glass former system will have to be taken into account in future investigations.

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CONCLUSION Using a classical thermal poling treatment on a specific borosilicate glass composition, a large amount of dinitrogen tetroxide has been formed and trapped in the polarized glass matrix. By comparing the space charge implementation and the structural rearrangements induced by poling treatments done under different atmospheres, we have proposed a global mechanism involving (i) injection of NO+ ions formed within gas discharges at the anode/glass interface and (ii) redox reactions leading to the formation of neutral nitrogen oxide molecular species. These results point out several new aspects of glass polarization processes for which interactions between glass chemistry and physics/chemistry of gas discharge should be taken in account.

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Trapped Molecular and Ionic Species in Poled Borosilicate Glasses

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