Highly Ordered “Defect-Free” Self-Assembled Hybrid Films with a Tetragonal Mesostructure

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One-pot self-assembled hybrid films were synthesized by the cohydrolysis of methyltriethoxysilane and tetraethoxysilane and deposited via dip-coating. The films show a high “defect-free” mesophase organization that extends throughout the film

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  Highly Ordered “Defect-Free” Self-Assembled Hybrid Filmswith a Tetragonal Mesostructure Paolo Falcaro, † Stefano Costacurta, † Giovanni Mattei, ‡ Heinz Amenitsch, § Augusto Marcelli, | Mariangela Cestelli Guidi, | Massimo Piccinini, | Alessandro Nucara, ⊥ Luca Malfatti, ∇  Tongjit Kidchob, ∇  and Plinio Innocenzi* , ∇  Contribution from the Dipartimento di Ingegneria Meccanica, Settore Materiali,Uni V  ersita `  di Pado V  a, Via Marzolo 9, 35131 Pado V  a, Italy, Dipartimento di Fisica “GalileoGalilei”, Uni V  ersita `  di Pado V  a, Via Marzolo 8, 35131 Pado V  a, Italy, Institute of Biophysics and  X-ray Structure Research, Austrian Academy of Sciences, Schmiedelstrasse 6, A-8042 Graz, Austria, Laboratori Nazionali di Frascati s  INFN, Via E. Fermi 40, 00044 Frascati, Italy, Dipartimento di Fisica, Uni V  ersita `  di Roma “La Sapienza”, P. le A. Moro 2, 00185 Roma, Italy,and Laboratorio di Scienza dei Materiali e Nanotecnologie, Nanoworld Institute, Dipartimentodi Architettura e Pianificazione, Uni V  ersita `  di Sassari, Palazzo Pou Salid, Piazza Duomo 6,07041 Alghero, Sassari, Italy Received November 30, 2004; E-mail: plinio@uniss.it Abstract:   One-pot self-assembled hybrid films were synthesized by the cohydrolysis of methyltriethoxysilaneand tetraethoxysilane and deposited via dip-coating. The films show a high “defect-free” mesophaseorganization that extends throughout the film thickness and for domains of a micrometer scale, as shownby scanning transmission electron microscopy. We have defined these films defect-free to describe thehigh degree of order that is achieved without defects in the pore organization, such as dislocations ofpores or stacking faults. A novel mesophase, which is tetragonal  I  4  /mmm   (space group), is observed inthe films. This phase evolves but retains the same symmetry throughout a wide range of temperatures ofcalcination. The thermal stability and the structural changes as a function of the calcination temperaturehave been studied by small-angle X-ray scattering, scanning transmission electron microscopy, and Fouriertransform infrared spectroscopy. In situ Fourier transform infrared spectroscopy employing synchrotronradiation has been used to study the kinetics of film formation during the deposition. The experimentshave shown that the slower kinetics of silica species can explain the high degree of organization of themesostructure. Introduction Mesoporous materials have potential applications in micro-electronics as ultralow- k   dielectric materials and in photonicsas low refractive index materials. Spin- and dip-coated hybridmaterials containing methyl groups have been prepared in thepast few years. Research on mesostructured silica and otherinorganic oxides has grown steadily in recent years; mesoporousmaterials have been synthesized as powders, monoliths, andfilms. 1 Several studies, in particular, have focused on oxide andhybrid organic - inorganic films obtained through evaporation-induced self-assembly 2,3 (EISA), as well as on understandingmesostructure development. 4 The next step toward the fabrica-tion of mesoporous devices is the achievement of a precisecontrol of the properties of the material such as surface areaand pore accessibility, as well as a high and reproduciblemesostructural order. Novel devices based on mesostructuredfilms are currently under development: for instance, forelectrochemical and optical sensors, 5 ultralow- k   materials formicroelectronics and low refractive index materials. 6 - 8 An important challenge to the chemical synthesis of organizedporous films is the possibility to reach a degree of order that isextended throughout the thickness of the films and for a regionof several centimeters. Up to now several authors have disclosedthe potentialities of self-assembly to obtain mesostructured films † Dipartimento di Ingegneria Meccanica, Settore Materiali, Universita`di Padova. ‡ Dipartimento di Fisica “Galileo Galilei”, Universita` di Padova. § Austrian Academy of Sciences. | Laboratori Nazionali di Frascati s INFN. ⊥ Universita` di Roma “La Sapienza”. ∇  Universita` di Sassari. (1) Soler-Illia, G. J. de A. A.; Sanchez C.; Lebeau B.; Patarin, J.  Chem. Re V  . 2002 ,  102 , 4093.(2) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan H.  Ad  V  . Mater.  1999 ,  11 , 579.(3) Grosso, D.; Boissie`re, C.; Smarsly, B.; Brezesinski, T.; Pinna, N.; Albouy,P. A.; Amenitsch, H.; Antonietti, M.; Sanchez, C.  Nat. Mater.  2004 ,  3 ,787.(4) (a) Grosso, D.; Babonneau, F.; Soler-Illia, G. J. A. A.; Albouy, P. A.;Amenitsch, H.  Chem. Commun.  2002 , 748. (b) Soler-Illia, G. J. A. A.;Crepaldi, E. L.; Grosso, D.; Durand, D.; Sanchez, C.  Chem. Commun.  2002 ,2298. (c) Cagnol, F.; Grosso, D.; Soler-Illia, G. J. A. A.; Crepaldi, E. L.;Babonneau, F.; Amenitsch, H.; Sanchez, C.  J. Mater. Chem.  2003 ,  13 , 61.(5) Wirnsberger, G.; Scott, B. J.; Stucky G. D.  Chem. Commun.  2001 , 119.(6) Jain, A.; Rogojevic, S.; Ponoth, S.; Agarwal, N.; Matthew, I.; Gill, W. N.;Persans, P.; Tomozawa, M.; Plawsky, J. L.; Simonyi, E.  Thin Solid Films 2001 ,  398  - 399 , 513.(7) Baskaran, S.; Liu, J.; Domansky, K.; Kohler, N.; Li, X.; Coyle, C.; Fryxell,G. E.; Suntharampillai, T.; Williford, R. E.  Ad  V  . Mater.  2000 ,  12 , 291.(8) Balkenende, A. R.; de Theije, F. K.; Kriege, J. C.  Ad  V  . Mater.  2003 ,  15 ,139. Published on Web 02/26/2005 3838  9  J. AM. CHEM. SOC. 2005 ,  127  , 3838 - 3846  10.1021/ja0427956 CCC: $30.25 © 2005 American Chemical Society  with different compositions, but only very recently the attentionhas been focused on the possibility of preparing orderedmaterials whose order can resemble that of ideal crystallinestructures. It is, in fact, very important for a practical applicationof mesoporous films that the order is extended on a larger scalethan local domains of submicrometric scale. In general, theorganization is shown to be reached in terms of organizeddomains, similar to grains in crystalline materials. 9 - 10 Miyataet al. 11 have recently obtained EISA silica films with a single-crystalline mesoporous 3D-hexagonal structure maintained overa scale of centimeters. This high level of organization has beenreached through epitaxial growth of a self-organized mixtureof silicon alkoxide and organic templates on a rubbing-treatedpolyimide layer. However, the presence of an organic interlayerbetween the film and the substrate can undermine the mechanicaland thermal stability of the film.In other cases defects in the porous organization that can becompared to atomic dislocations in crystalline solids areobserved. 16 It is important, however, to reach a full “defect-free” organization within all of the film to satisfy most of therequirements for new devices in nanotechnologies.The mesostructural instability of this type of material,especially when exposed to the combined action of heat,pressure. and water, is another critical feature. In a previouswork, 12 mesostructured silica thin films and monoliths stableup to 950  ° C were synthesized using block copolymers (e.g.,Pluronic F127) and performing suitable thermal post-treatments.A full removal of the silanols on the pore surface was achievedafter thermal calcination at 750  ° C, without significant loss of pore organization. Dehydroxylation is a fundamental step in thepreparation of this material, because the presence of OH groupsmakes the environment hydrophilic, causing the material toadsorb large amounts of water, bonded to the pore surface,resulting in a decrease in material performance. 13 Different strategies have been employed to synthesizemesoporous films with a hydrophobic surface, most of whichhave been selected keeping in mind that ultralow- k   dielectricmaterials can represent a fundamental breakthrough for this newclass of materials. In the majority of these methods, methylgroups have been used to form hydrophobic hybrid mesoporousfilms.The preparations basically differ in the methods by whichmethyl groups are introduced to functionalize the surface andmake it hydrophobic. Three main methods can be taken intoconsideration: postpreparation grafting, one-pot co-self-as-sembly, and vapor infiltration. Though several slight modifica-tions have been proposed by different authors, all of thesyntheses can be somehow classified within these groups. Themost common approach to remove the silanols and make thematerial hydrophobic is to react the residual silanols in thermallycalcined silica films with hexamethyldisilazane (HMDS) ortrimethylchlorosilazane (TMCS) in a postpreparation treatment(grafting methods). Previously, vapor-phase reaction of HDMShas been shown to be effective in removing the silanols andtherefore lowering the effective dielectric constant. 14 Otheralternative routes have been proposed, such as in situ deriva-tization of the precursor solution with TMCS, 15 self-assemblyusing diblock copolymers and MTES to obtain pore isolation, 16 cohydrolysis and self-assembly of methyltriethoxysilane (MTES) - tetraethyl orthosilicate (TEOS) mixtures (one-pot EISA), 4,14,8 and MTES vapor infiltration techniques. 17 In this work we have synthesized hybrid mesostructured filmsprepared by cohydrolysis and self-assembly of an optimizedMTES - TEOS mixture with the purpose to reach a large orderin the pore organization without “point defects”. The synthesisthat we report results in a high organization of the films, whichappear defect-free. We have used these films to introduce arigorous methodology, based on TEM and SAXS analysisfollowed by a computer simulation, to identify phases in thesemesoporous films. By this approach we were able to identify anew tetragonal mesophase in the final material. We have alsoused in situ analytical techniques, such as SAXS and infraredspectroscopy using synchrotron radiation, to investigate andexplain the high organization that we have observed incomparison to that of other mesoporous systems. Experimental Section See the Supporting Information. Results and Discussion Identification of the Mesophases . The EISA of thin filmstypically yields organized porous structures whose porosity liesin the 2 - 50 nm range (mesopores). Generally, the identificationof the mesophase is not straightforward, since 1-D X-rayanalysis does not allow an easy indexing of the diffractionpatterns. SAXS techniques using synchrotron light are, therefore,effectively applied to achieve a better identification of theorganized phases. In situ SAXS techniques have been widelyused to study the mechanisms of mesophase formation in filmssynthesized via EISA, where phase transformation is inducedby solvent evaporation. 18 In addition, a phase transformationin mesostructured films is induced by the post-depositionthermal treatment, performed to achieve the removal of thetemplating agent and the condensation of pore walls. This lattereffect has been generally recognized by several authors; 10,19 however, a clear identification of the involved phase transitionsis seldom reported. 10 This is related to the difficult interpretationof SAXS patterns and to the lack of a codified procedure toachieve phase identification in the case of mesostructured films.We have employed GI-SAXS to record four diffractionimages, corresponding to samples thermally calcined at 60, 200,400, and 600  ° C (Figure 1). The images were collected by the (9) Klotz, M.; Albouy, P. A.; Ayral, A.; Me´nager, C.; Grosso, D.; Van derLee, A.; Cabuil, V.; Babonneau, F.; Guizard, C.  Chem. Mater.  2000 ,  12 ,1721.(10) Besson, S.; Ricolleau, C.; Gacoin, T.; Jacquiod, C.; Boilot, J.-P.  Microporous Mesoporous Mater.  2003 ,  60 , 43.(11) Miyata, H.; Suzuki, T.; Fukuoka, A.; Sawada, T.; Watanabe, M.; Noma,T.; Takada, K.; Mukaide, T.; Kuroda, K.  Nat. Mater.  2004 ,  3 , 651.(12) Falcaro, P.; Grosso, D.; Amenistch, H.; Innocenzi, P.  J. Phys. Chem. B 2004 ,  108  , 10942.(13) Maex, K.; Baklanov, M. R.; Shamiryan, D.; Iacopi, F.; Brongersma, S. H.;Yanovitskaya, Z. S.  J. Appl. Phys.  2003 ,  93 , 8793.(14) Pai, R. A.; Humayun, R.; Schulber, T.; Sengupta, A.; Sun, J.-N.; Watkins,J. J.  Science  2004 ,  303 , 507.(15) Yang, C.-M.; Cho, A.-T.; Pan, F.-M.; Tsai, T.-G., Chao, K.-J.  Ad  V  . Mater. 2001 ,  13 , 1099.(16) (a) Yu, K.; Wu, X.; Brinker, C. J.; Ripmeester, J.  Langmuir   2003 ,  19 , 7282 . (b) Wu, X.; Yu, K.; Brinker, C. J.; Ripmeester, J.  Langmuir   2003 ,  19 ,7289.(17) Tanaka, S.; Kaihara, J.; Nishiyama, N.; Oku, Y.; Egashira, Y.; Ueyama,K.  Langmuir   2004 ,  20 , 3780.(18) Grosso, D.; Babonneau, F.; Sanchez, C.; Soler-Illia, G. J. A. A.; Crepaldi,E. L.; Albouy, P. A.; Amenitsch, H.; Balkenende, A. R.; Brunet-Bruneau,A.  J. Sol-Gel Sci. Technol.  2003 ,  26  , 561.(19) Grosso, D.; Soler-Illia, G.; Babonneau F.; Sanchez C.; Albouy P.; Brunet-Bruneau A.; Balkenende, A.  Ad  V  . Mater.  2001 ,  13 , 14. Hybrid Films with a Tetragonal Mesostructure   A R T I C L E S J. AM. CHEM. SOC.  9  VOL. 127, NO. 11, 2005  3839  CCD detector and processed using the FIT2D program (A. P.Hammersley/ESRF). 20 To analyze the GI-SAXS images, wehave introduced some corrections by FIT2D; in particular theintensities of the images were normalized, the spatial distortionwas corrected, and instrumental errors were subtracted (back-ground noise and dark current). The intensities of the images,however, cannot be quantitatively compared because in somesamples different instrumental amplifications were used to avoidsignal saturation.To identify the mesostructure, we have used the GI-SAXSimage at 200  ° C (Figure 1b), because the several detecteddiffraction spots should allow, in principle, an easier attributionof the phase with respect to the samples that exhibit a lowernumber of diffraction spots (Figure 1a,c,d). The spots have beenobtained from images taken with different exposition times andthen merged into a low saturation level diffraction image usingtheir srcinal positions. The reader can notice the more regularshape of the added spots.A support to start the identification of the phase through asimulation of the diffraction spots was given by the fast Fouriertransform (FFT) of the scanning mode TEM (STEM) imagesof the sample calcined at 200  ° C (Figure 2), which suggestedthat the structure is body-centered tetragonal. The Fouriertransform of the bright-field and dark-field, cross-section TEMimage and the corresponding indexation along the [110]direction are shown in Figure 2.To support the hypothesis of a tetragonal mesophase, we havesimulated the GI-SAXS diffraction patterns of the 200 ° C sampleby the CMPR program (B. Toby, NIST) 21 using an  I  4/  mmm structure (space group). The simulated patterns well reproducedthe GI-SAXS spots: Figure 3 shows the enlarged GI-SAXSimage of the sample calcined at 200  ° C, together with thesimulated image by CMPR. The GI-SAXS diffraction spotsappear in the image as black bold points, while the simulatedspots in agreement with the experimental GI-SAXS spots appearas red dots. Some red hollow circles indicate the simulated spotsthat belong to higher diffraction orders and are not observed inour GI-SAXS images. Some distortion effects are evident inthe diffraction pattern: in fact some spots (marked by violethollow squares) appear translated from their theoretical posi-tions; we attribute this to refraction effects that may occur duringscattering when the angle between the incident and scatteredwaves at the vacuum/sample interface is large. This phenomenonstrongly influences only low-order Bragg peaks. Some extraspots (green hollow circles in Figure 3) are observable in otherworks (see, for example, Miyata et al. 11 ), but no interpretationhas been provided so far. They seem to be caused bysimultaneous refraction and reflection at the film/substrateinterface, followed by diffraction. These extra spots are morevisible at low diffraction orders. The comparison between thesimulated spectra and those experimentally obtained gives agood correspondence (Figure 3), verifying the tetragonal sym-metry and allowing spot indexing. Applying the same procedure,we have also identified the mesophase in the samples uponcalcination at 400 and 600  ° C as tetragonal  I  4/  mmm .A general phenomenon observed in EISA-produced films isa phase transformation of the film mesophase during thermalshrinkage. This transformation is given by the contraction of the films in the direction orthogonal to the substrate. In severalcases 4 the low-temperature mesophase, i.e., before the thermalshrinkage, has been resolved as a body-centered cubic structure,  Im 3 m , arranged with the (110) face parallel to the substrate. 12 If the  Im 3 m  cubic cells of the as-deposited film were orientedin this way, the thermal contraction along the direction normalto the substrate would have led to an orthorhombic symmetryinstead of tetragonal. On the other hand, the tetragonal structureobserved here requires the starting mesophase to have an equalor a higher symmetry. In our case, therefore, it is necessary tosuppose that the as-deposited mesophase has a different in-planardisposition on the substrate: the condition of coplanarity of the(001) face with the substrate implies the contraction of the  c axis only, in accordance with the supposed phase contraction.The simulated image obtained by CMPR, generated on the basisof previous assumptions of an  I  4/  mmm  phase, was in goodagreement also with the GI-SAXS image of the sample treatedat 60  ° C.To support this suggested transition, we have also calculatedthe lattice constants of the different samples. These werecalculated by means of an algorithm (eq 1),implemented in Turbo Pascal programming language, whichminimizes  S  2 ( a , c ), which is the sum of the squares of thedifferences between the experimental and theoretical spots,expressed in  x ,  y  positions, as a function of the lattice parameters a  and  c  (with  h ,  k  ,  l  crystallographic indexes fixed by symmetry).The use of this procedure is justified by the fact that thepositions of the cell axes with respect to the substrate are (20) Available at www.esrf.fr/computing/expg/subgroups/data analysis/ FIT2D/index.html.(21) http://www.ncnr.nist.gov/programs/crystallography/software/cmpr/. Figure 1.  GI-SAXS images of the mesostructured films upon thermalcalcination at (a) 60  ° C, (b) 200  ° C, (c) 400  ° C, and (d) 600  ° C. S  2 ( a , c ) ) ∑ i [  x i -  x th ( a , h i , k  i )] 2 + ∑ i [  y i -  y th ( c , l i )] 2 ) S  2 ( a ) + S  2 ( c ) (1)  x th )   ( h i 2 + k  i 2 ) a (2)  y th ) l i c A R T I C L E S  Falcaro et al. 3840 J. AM. CHEM. SOC.  9  VOL. 127, NO. 11, 2005  univocally defined. In our case only the condition of coplanarityof the (001) face with the substrate can imply the contractionof the  c  axis only: this allowed us to decouple the contributionsof   a  )  b  and  c  with respect to the  x  and  y  axes of the 2-DGI-SAXS patterns. From a simple error propagation it is possibleto demonstrate that the percent error on  a  and  c  is the same asthe error on the spot measurement in the  x ,  y  pattern.To summarize, the GI-SAXS and TEM data show a tetragonalmesophase with  I  4/  mmm  symmetry (space group) at tempera-tures between 60 and 200 ° C. The decrease in the lattice constant c  that is observed with the increase of the thermal treatmentcan be explained by a shrinkage caused by the thermalcontraction affecting the cell parameter parallel to the directionof contraction. The phase transition between body-centeredtetragonal (  I  4/  mmm ) with  a < c  to a structure having the samesymmetry but with  a > c  is illustrated in Figure 4. The tetragonalcell is oriented with the (001) plane parallel to the substrate ( c axis normal to the substrate); after the thermal treatment, adecrease of the cell parameters takes place, markedly in the [001]direction perpendicular to the substrate, causing the shrinkageof the structure along this direction. This effect has beenpreviously reported in the case of silica mesostructured filmsobtained with TEOS as the single silica source 4,9,22 and can beextended to the MTES hybrid films produced here. Thedifference in the composition of the mesophases in these twocases implies a fundamental difference in symmetry and in thethermal evolution. Thus, the presence of the methyl groupsseems to play a fundamental role during the self-assemblyprocess, modifying the packing of micelles inside the as-deposited film.The decrease of the cell parameters with increasing calcinationtemperatures (Figure 5) is in accordance with the phase transitionmodel suggested here: the  a  and  b  axes do not vary significantlywithin the experimental error, while the  c  axis shows a decreasefrom 23.4 nm at 60  ° C to 14.3 nm at 600  ° C, corresponding toa shrinkage of 39%. In a previous work  12 mesostructured silicafilms were obtained using TEOS as the silica precursor andPluronics F127 as the templating agent, and the deposition wasperformed in the same experimental conditions as those (22) Doshi, D. A.; Gibaud, A.; Goletto, V.; Lu, M.; Gerung, H.; Ocko, B.; Han,S. M.; Brinker, C. J.  J. Am. Chem. Soc.  2003 ,  125 , 11646. Figure 2.  (a) Bright-field cross-section TEM image, (b) its Fourier transform, and (c) the correspondent indexation along the [110] direction. (d) Dark-fieldcross-section TEM image that is rotated 90 °  with respect to the image in Figure 6a, (e) its Fourier transform, and (f) the correspondent indexation along the[110] direction. Figure 3.  Comparison between the GI-SAXS image of a sample calcinedat 200  ° C and the computer-simulated image obtained by the CMPRprogram. Hybrid Films with a Tetragonal Mesostructure   A R T I C L E S J. AM. CHEM. SOC.  9  VOL. 127, NO. 11, 2005  3841  employed in this work (RH, speed rate, thermal treatment). Thecalculated percent contraction of the lattice parameter perpen-dicular to the substrate was 34%.Shrinkage following thermal treatment in MTES-derivedmesostructured films has been observed previously by otherauthors, without, however, identification of a similar phase. 23 The high degree of order reached in the MTES-derived self-assembled mesostructured films can be directly observed byTEM images. A bright-field cross-section TEM image is shownin Figure 6a. The silicon substrate appears as a dark layer inthe bottom part of the figure; the thickness of the film is 370nm ( ( 5 nm). The film appears highly ordered with ellipticalpores, due to the thermally induced shrinkage along the directionnormal to the substrate. In the dark-field cross-section STEMimages (Figure 6b) of a sample treated at 350 ° C, the structuralorder and the elliptical shape (enlargement in Figure 6c) areclearly observed. The pore dimensions measured from the TEMimages are 8.8 nm ( σ  )( 0.9 nm) along the major axis, parallelto the surface, and 4.8 nm ( σ  )( 0.5 nm) along the minor axis,perpendicular to the substrate, with a 1.8 nm ( σ   ) ( 0.3 nm)ratio between the longer and shorter axes. Kinetics of Film Formation during EISA . The formationof an organized mesophase results from the balance of differentcompetitive kinetic processes: inorganic polycondensation andphase separation or organization of the template. 24 In silicasystems at low pH ( ∼ 2), the condensation rates are slow enoughto allow the formation of ordered phases during self-assembly.Speeding up of the silica reactivity gives, in general, a lowerdegree of organization, in accordance with the ideal order forself-assembly of the kinetic constants involved in the process: 25 While from a theoretical point of view the relationshipbetween kinetic processes and order is well assessed, there is alack of experimental evidence in self-assembled films tocorrelate the kinetics of inorganic polycondensation with micelleorganization into a final organized phase. The difficulty inperforming a reliable set of experiments is, in fact, a seriouslimitation. In situ experiments were done by SAXS; however,they give information only on phase formation, but not onpolycondensation kinetics during EISA. An interesting pos-sibility is to apply vibrational spectroscopies, such as FTIR “insitu” during EISA. An example of this kind of study has beenreported by Doshi et al., 22 but the results are not very informativebecause the scale of time employed is too big to allow aninvestigation of film formation. We have used infrared syn-chrotron radiation to perform “real time” in situ analysis duringEISA. The main advantage of the experimental configurationthat we have used is that we can record the spectra intransmission, with a good signal-noise ratio, even on a shorttime scale with the possibility to focus the IR beam in a smallersample region. We have also coupled this analysis with in situSAXS experiments in transmission during dip-coating. The insitu conditions of analysis between IR and SAXS experimentsare different because we measured EISA in cast (IR) or dip-coated (SAXS) films, but since we have a correlated set of thesame samples, a comparison of the trends was highly informa-tive.As previously pointed out and observed by other researchers,the MTES - TEOS system gives a better and easier organizationduring EISA with respect to other systems such as silica, titania 3 ,zirconia, or hybrid compositions. 26 We have also observedduring our experiments that self-assembly with MTES - TEOSis highly reproducible and less sensitive to variations indeposition parameters such as relative humidity or withdrawalspeed. Figure 7a shows the FTIR absorption transmission spectrarecorded in situ during film formation in an MTES - TEOS (23) De Theije, F. K.; Balkenende, A. R.; Verheijen, M. A.; Baklanov, M. R.;Mogilnikov, K. P.; Furukawa, Y.  J. Phys. Chem. B  2003 ,  107  , 4280.(24) Soler-Illia, G. J. de A. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C.;  Curr.Opin. Colloid Interface Sci.  2003 ,  8  , 109.(25) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T.E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D.  Chem. Mater.  1994 ,  6  , 1176.(26) Grosso, D.; Boissie`re, C.; Smarsly, B.; Brezesinski, T.; Pinna, N.; Albouy,P. A.; Amenitsch, H.; Antonietti, M.; Sanchez, C.  Nat. Mater.  2004 ,  3 ,787. Figure 4.  Representative pictures of the contraction of   c  for an  I  4/  mmm  mesostructure from 60 to 600  ° C.  c  axis normal to the substrate. Figure 5.  Variation of   a  (circles) and  c  (squares) cell parameters as afunction of the thermal treatment. The lines are a guide for the eyes. k  inter > k  org > k  inorg  (3) A R T I C L E S  Falcaro et al. 3842 J. AM. CHEM. SOC.  9  VOL. 127, NO. 11, 2005
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