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A Novel Example of X-Ray-Radiation-Induced Chemical Reduction of an Aromatic Nitro-Group-Containing Thin Film on SiO2 to an Aromatic Amine Film

A Novel Example of X-Ray-Radiation-Induced Chemical Reduction of an Aromatic Nitro-Group-Containing Thin Film on SiO2 to an Aromatic Amine Film
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  884  ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  DOI: 10.1002/cphc.200300699  CHEMPHYSCHEM  2003  , 4, 884 A Novel Example of X-Ray-Radiation-Induced Chemical Reduction of anAromatic Nitro-Group-ContainingThin Film on SiO 2  to an AromaticAmine Film Paula Mendes, [a] Maura Belloni, [a] Mark Ashworth, [b] Chris Hardy, [b] Kirill Nikitin, [c] Donald Fitzmaurice, [c] Kevin Critchley, [d] Stephen Evans, [d] and Jon Preece* [a] KEYWORDS: chemical modification  ¥  organic film  ¥  nanostructures  ¥  nitrogroup  ¥  SiO 2 ¥  X-ray photoelectron spectroscopy Introduction Self-assembled monolayers (SAMs) have attracted much interestsince they allow one to deliberately derivatise metal (e. g. gold)and semiconductor (e. g. silicon) surfaces with a chemically well-defined organic ultrathin film, in a relatively facile and reliablemanner. [1, 2] SAMs have many potential applications in moderntechnology, including chemical sensing, [3±5] control of surfaceproperties such as wettability and friction, [6, 7] corrosion protec-tion [8] and, in recent times, the nanofabrication of electroniccomponents. [9] Among the most well-known and studied SAMs are thiols ongold, [1, 2, 10, 11] and organosilanes on silicon. [12±14] In comparison tothiols, silanes that contain reactive functional groups areexperimentally more difficult to form. In part, this difficultyarises from the water that is required for the formation of siloxane-anchored SAM, which also promotes the formation of oligomeric siloxanes. These oligomeric siloxanes then physisorbto the surface, forming a multilayer structure. [12, 13] Moreover,these multilayers require lengthy washing procedures to removethem; this involves alkaline detergents and even mechanicalabrasion of the monolayer surface with a soft brush or 100%The four values of these quantities are indeed very close. If, inarbitrary units, their average is 1.0, then we obtain the scatteredvalues 1.33, 0.95, 0.85 and 0.87 for NG  Ne, Ar, Kr and Xe,respectively.The motions of Xe parallel to the COC plane of DME are verysoft, with force constants of about 0.07 Nm  1 . The behaviour of Xe in DME±Xe is quite similar to that in Xe±oxirane. The valuesof the dissociation energies (306 and 325 cm  1 , respectively) arecomparable within the experimental errors. Experimental Section The design of the Stark free-jet absorption millimeter-wave spec-trometer used in this study has been described previously. [15, 16] Theaccuracy of the frequency measurements is estimated to be about0.05 MHz.The complex was formed by expanding a mixture of 2% of DME inxenon at a pressure of about 0.8 bar through a nozzle with adiameter of 0.13mm to about 5  10  3 mbar. An estimated ™rota-tional∫ temperature of about 5±10 K was reached. Xenon 99.997%was supplied by Rivoira, and DME by Linde. The University of Bologna (funds for special topics), the Ministerodell   Universita¡ e della Ricerca Scientifica (MURST) and the ConsiglioNazionale delle Ricerche (C.N.R.) are acknowledged for financial support. [1] S. E. Novick, Bibliography of Rotational Spectra of Weakly BoundComplexes,  2001 , available at[2]  Atomic and Molecular Beam Methods ,  Vols. I and II   (Ed.: G. Scoles), OxfordUniversity Press, Oxford,  1988 .[3] Th. Brupbacher, J. Makarewicz, A. Bauder,  J. Chem. Phys.  1994 ,  101 , 9736±9746.[4] B. Velino, P. G. Favero, W. Caminati,  J. Chem. Phys.  2002 ,  117  , 5688±5691.[5] P. Ottaviani, A. Maris, W. Caminati, Y. Tatamitani, Y. Suzuki, T. Ogata, J. L.Alonso,  Chem. Phys. Lett.  2002 ,  361 , 341±348.[6] A. Maris, W. Caminati,  J. Chem. Phys.  2003 ,  118 , 1649±1652.[7] B. Velino, S. Melandri, W. Caminati, unpublished results.[8] F. J. Lovas, K. Lutz, H. Dreizler,  J. Phys. Chem. Ref. Data 1979 ,  8 , 1051±1093.[9] J.K. G.Watson in  Vibrational Spectra and Structure ,  Vol. 6  (Ed.: J. R. Durig),Elsevier, New York/Amsterdam,  1977 , pp. 1±89.[10] J. Kraitchman,  Am. J. Phys.  1953 ,  21 , 17±25.[11] See, forexample, R. P. A.Bettens, R. M. Spycher,A.Bauder, Mol. Phys. 1995 , 86 , 487±511.[12] W. Caminati, P. G. Favero, S. Melandri,R. Meyer,  Chem. Phys. Lett. 1997 ,  268 ,393±400.[13] D. J. Millen,  Can. J. Chem. 1985 , 63 ,1477±1479; W. G. Read, E. J. Campbell,G. Henderson,  J. Chem. Phys.  1983 ,  78 , 3501±3508.[14] R. Meyer,  J. Mol. Spectrosc.  1979 ,  276 , 266±300.[15] S. Melandri, W. Caminati, L. B. Favero, A. Millemaggi, P. G. Favero,  J. Mol.Struct.  1995 ,  352 / 353 , 253±258.[16] S. Melandri, G. Maccaferri, A. Maris, A. Millemaggi, W. Caminati, P. G.Favero,  Chem. Phys. Lett.  1996 ,  261 , 267±271.Received: February 10, 2003 [Z698][a]  Dr. J. Preece, Dr. P. Mendes, Dr. M. Belloni School of Chemical Sciences, University of BirminghamEdgbaston, Birmingham B15 2TT (UK)Fax: (   44) 121±414±4403E-mail:  [b]  Dr. M. Ashworth, C. Hardy School of Engineering, University of BirminghamEdgbaston, Birmingham B15 2TT (UK) [c]  Dr. K. Nikitin, Prof. D. FitzmauriceNanochemistry Group, Department of Chemistry University College DublinBelfield, Dublin 4 (Ireland) [d]  K. Critchley, Prof. S. EvansDepartment of Physics and Astronomy, University of LeedsLeeds LS2 9JT (UK)  CHEMPHYSCHEM  2003  , 4, 885  ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  885 cotton cloth. [15, 16] However, herein we take advantage of suchmultilayer structures.Among other techniques to analyse SAM structure, X-rayphotoelectron spectroscopy (XPS) has been widely used toobtain, through core-level analysis, information on the forma-tion, composition, and X-ray-induced damage and modificationof SAMs. [1, 2] At present, it is reasonably well-accepted that it isnot the incident X-ray photons, but the subsequent primary andsecondary electrons emitted from the surface that play thedominant part in the damage/modification of the organicfilm. [17±19] Recently, several efforts have been devoted to thestudy of the damage and/or modification of SAMs produced byX-rays. [17±26] Rieke et al. [20] observed that both X-rays and electron-beamexposure cause damage on Br-terminated and CH 3 -terminatedalkylsilane SAMs on silicon, respectively. However, while X-raysdamage on Br-terminated films resulted in the removal of the Brwithout significant loss in the amount of C (the hydrocarbonchain is not cleaved), the electron beam was found to damagethe CH 3 -terminated alkylsilane SAM, with loss of some of thehydrocarbon monolayer.Frydman et al. [21] carried out an extensive XPS and Fouriertransform infrared spectroscopy (FTIR) study of some represen-tative alkylsilane (e.g., CH 3 -, and COOH-terminated) SAMs onvarious substrates (silicon, quartz, glass, ZnSe). Significant differ-ences were observed on the rate and damage of the variousSAMs/substrate systems when irradiated with X-ray photoelec-trons. The nature and extent of the induced damage was shownto depend on the nature of the terminal functional groups. Forinstance, in CH 3 -terminated SAMs on Si, the irradiation isresponsible only for a small loss in the C (1 s) peak after 152minutes of exposure, while in COOH-terminated SAMs on Si, ahigher decrease of the total C is observed after 100 minutes, duemainly to the loss of the surface carboxylic acid functionality. Itwas suggested that the X-ray irradiation-induced damage in theCH 3 -terminated SAMs occurs primarily through dehydration,cross-linking, branching, double-bond formation and rearrange-ments of the hydrocarbon chain without loss of C, while in theCOOH-terminated SAMs the damage consists mainly of thecleavage of the terminal carboxylic fragment and its evapora-tion, which accounts for the decrease of total C.In related studies, Park and co-workers [22±24] used a synchro-tron radiation source for soft X-ray irradiation on various nitro-substituted aromatic imine monolayers on silicon. The authorsfound that soft X-ray irradiation promotes a selective cleavage of nitro groups in the various nitro-substituted aromatic iminemonolayers, leaving the phenyl ring intact in the layer.However, this should be contrasted with some studies carriedout on nitro-terminated thiols deposited on silver and goldsubstrates. [11, 27] Kim and co-workers [27] studied visible laser lightirradiation effects on nitro-terminated monolayers on silver. Theresults indicated the conversion of the nitro group into an aminefunctionality by laser irradiation, preserving the overall structuralintegrity of the monolayer. Grunze and co-workers [11] investigat-ed NO 2 -terminated biphenyl SAMs on gold substrates. Theyreported the transformation of surface nitro groups to aminegroups by low energy electron irradiation while the underlyingaromatic layer is dehydrogenated and cross-linked. They sug-gested that the hydrogen atoms required for reduction of thenitro groups are generated by the secondary electron-induceddissociation of the C  H bonds in the biphenyl units.This led us to reconsider the results of Park and co-work-ers, [22±24] as we thought that the NO 2  group may very well havebeen converted into NH 2 . XPS spectra in their studies did indeedshow the binding energy at 406 eV of the N (1 s) NO 2  moietydecreasing, but any conversion into the NH 2  group, which has aN (1 s) binding energy at 399 eV, [28] would have been hidden bythebinding energy ofthe N (1 s) peak ofan imine functionality at400 eV,whichwas alsopresentin theSAMs. Inthestudy reportedhere, we use 3-(4-nitrophenoxy)-propyltrimethoxysilane(NPPTMS), in which no imine is present, as the model systemused toinvestigate whether theX-raysource does indeed leadtothe chemical modification of NO 2  to NH 2 .Due to the low signal intensity of the elements in SAMs duringXPS analysis, prolonged acquisition times are required in orderto obtain a higher signal-to-noise ratio. However, during thisextended period of measurement, molecular components of SAMs may undergo substantial chemical and structural trans-formations as a result of the irradiation. Therefore, as a first stepin our study for demonstrating the conversion of the NO 2  intothe NH 2  group of organic thin films on silicon by XPS irradiation,we formed multilayer films of NPPTMS with subsequentenhanced resolution of binding energy peaks, compared withultrathin films (i.e., SAMs), that is, higher signal-to-noise forshorter X-ray exposure times. Results and Discussion In order to investigate the conversion of NO 2  into NH 2  by X-rayirradiation in thin organic films, NPPTMS was designed andsynthesised (Scheme 1). NPPTMS is characterised by three main Scheme 1.  Reaction steps for the synthesis of 3-(4-nitrophenoxy)-propyltrime-thoxysilane (NPPTMS). functionalities: 1) the trimethoxysilane, 2) an aromatic ring and3) a terminal NO 2  group. The trimethoxysilane moiety isnecessary for chemisorption on the silicon substrate and foroligomerisation to form the oligomeric siloxane which physisorbto the surface. The aromatic ring has been introduced in thestructure to improve the order and the close packing of themultilayers via   ±   stacking interaction. [29] Additionally, thearomatic unit might play a crucial role in the reduction of theNO 2  to NH 2 , as source of hydrogen atoms. [11] As previouslymentioned, e-beam [11] lithography has shown that an NO 2  group  886  ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  CHEMPHYSCHEM  2003  , 4, 886 can be reduced to NH 2  in systems based on a nitrophenylenemoiety (i.e., where the NO 2  group is directly attached to aphenylene aromatic ring). Therefore, in our work we have alsointroduced a nitrophenylene moiety, since we considered itimportant to maintain the molecular electronic and stericproperties of the NO 2 .The synthesis of NPPTMS, illustrated in Scheme 1, involvesinitial formation of the 4-nitrophenolate as the tetramethylam-monium salt followed by  O -alkylation with bromo-propyl-trimethoxysilane. NPPTMS was characterised by 1 H and  13 C NMR and mass spectrometry (MS). The NMRspectra indicated that the product was 83% pure anddid not contain any bromo-propyl-trimethoxysilane.Attempts at further purification of NPPTMS by chroma-tography and distillation were not successful. Similardifficulties have been reported in the literature. [1, 30] Therefore, the crude sample was used without furtherpurification.Multilayer films of NPPTMS were formed as describedin the Experimental Section (Scheme 2). Ellipsometryshowed that the average film thickness was 27  2 ä.Contact angle measurements provide additional infor-mation about the self-assembled film of NPPTMS. Theclean bare silicon substrates displayed low advancingand receding H 2 O contact angles (  a  and   r   10  ). Thelow H 2 O contact angles on clean bare silicon areindicative of a highly hydroxylated, clean and smoothwafer surface prior to modification. Upon multilayer filmformation, these   a  and   r  increased to 62  8 and 47  10  ,respectively, in good agreement with literature data for NO 2 -terminated SAMs (lit.   a  64  1 and   r  59  1  ). [31] NPPTMS multilayer films were then exposed to X-ray irradi-ation for 447 minutes, collecting data every 6 minutes. Clearly,the spectrum presented in Figure 1a (3 min) shows the N (1 s)binding energy at 405.6 eV, characteristic of the NO 2  moiety (lit.405.5 eV). [28, 32] As time progresses, a second peak at 399.6 eVappears, which would correspond to the binding energy of anNH 2  functionality (lit. 399 eV). [28, 32] Furthermore, the spectrarepresented in Figure 1a±e at different irradiation times of 3, 97,163, 273 and 447 minutes, show the decrease in the peak intensity of the N (1 s) binding energy of the NO 2  group with theconcomitant increase of the peak area of N (1 s) binding energyat 399.6 eV. [33] At longer irradiation times (447 minutes), almost all NO 2 groups under irradiation were converted into NH 2  groups(Figure 1e). Park and co-workers [22±24] also observed the disap-pearance of NO 2  peak for a nitrobenzaldimine SAM onsilicon. [22±24] However, maybe due to coincidental binding energyof the imine and amine functionalities, the authors did notreport the concomitant appearance of the NH 2  binding energy.Figure 2 shows the evolution of N (1 s) peak areas for NH 2 , NO 2 and the summation of NH 2  and NO 2  within the film as a functionof X-ray exposure time. Itshouldbe noted that thetotal nitrogenpeak area (NO 2  and NH 2  regions) appears to decrease slightlywith prolonged irradiation times (by 18% after 447 mincontinuous irradiation), which suggests irradiation-inducedpartial desorption of N-containing molecular fragments.Analogous X-ray irradiation experiments were carried out in afresh film of NPPTMS in order to evaluate the evolution of theintensities of O (1 s), C (1 s) and Si (2p) peaks as a function of theirradiation time (Figure 3a±c). For these elements, the intensityof the XPS signals is invariant with the time of irradiation, whichsuggests that the carbon skeleton, including thearomatic ring, isnot affected by X-ray irradiation. A decrease in the O (1 s) peak intensity would be expected in concert with the conversion of  Figure 1.  XPS N (1 s) spectra of a multilayer film of NPPTMS on Si/SiO 2  taken at five timeintervals of continuous exposure to X-ray irradiation: after a) 3; b) 97; c) 163; d) 273;e) 447 min. Scheme 2.  Schematic representation of the formation and modification of multilayers of self-assembling silanes, with NPPTMS as the basic monolayer building unit.  CHEMPHYSCHEM  2003  , 4, 887  ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  887 nitro into an amine functionality. This invariability on O (1 s) peak may result from the oxygen present in the silicon oxide layer,which overwhelms that of the NO 2  group.In order to validate the conclusion that the new bindingenergy at 399.6 eV was indeed a result of conversion of the NO 2 into NH 2 , a fresh film of NPPTMS was prepared and subjected toreductive conditions to convert the NO 2  into an NH 2  terminuschemically as described in the Experimental Section. This samplewas exposed to X-ray irradiation for 221 minutes with the XPSspectra recorded at 13, 96, 146, and 196 minutes, as illustrated inFigure 4a±d, respectively. The first point to note is that there isno peak corresponding to the N (1 s) binding energy of the NO 2 moiety. The second point to note is that there is a bindingenergy peak at 399.8 eV, which is within 0.2 eV of the value inFigure 1, which was assigned to the the NH 2  moiety. Thirdly,inspection of the area under the binding energy peaks revealsthat there is no significant change and thus the chemicalreduced SAM is stable to X-ray irradiation and it is not removedfrom the surface by the irradiation. Conclusions It has been observed, for the first time, that during the X-rayirradiation of multilayer films of NPPTMS the NO 2  terminalfunctionality is reduced to aNH 2  group. This conclusion is arrivedat by comparing the values of the binding energies of the NO 2 and NH 2  groups: the binding energy of the NO 2  group decreasesas that of the NH 2  group concomitantly increases. Additionally,under longer exposure times of X-ray irradiation on NO 2 -terminated multilayer films, there is no significant decrease of the total N (1 s) peak intensity (NO 2  and NH 2  regions). Therefore,we conclude that the NO 2 -terminated multilayer film is mainlyreduced to NH 2 , but is not further degraded or removed fromthe substrate by the X-ray irradiation or secondary electrons.These results indicate that X-ray irradiation in conjunctionwith a proximity mask could be useful for patterning surfaceswhich contain the NO 2  group, in the wider context of the designof new materials targeted at fabricating nanoscale structures.The use of X-rays and proximity masks isa particularly interesting Figure 3.  XPS spectra of a) O (1 s) regions taken at 43 (    ), 108 (    ), 173 (    ), 272(    ), 337 (    ), 402 (    ) and 500 min (    ) of continuous exposure to X-ray irradiation;b) C (1 s) regions taken at 63 (    ), 128 (    ), 193 (    ), 292 (    ), 357 (    ), 422 (    ) and 520 min (    ) of continuous exposure to X-ray irradiation; and c) Si (2p) regionstaken at 86 (    ), 151 (    ), 216 (    ), 314 (    ), 379 (    ), 444 (    ) and 533 min (    ) of continuous exposure to X-ray irradiation. Figure 2.  Evolution of the N (1 s) peak intensity area for the nitro group (    ),amine group (    ) and summation of nitro and amine groups (    ) during progressive X-ray irradiation.  Figure 4.  XPS N (1 s) spectra of a chemically reduced multilayer film of NPPTMSon Si/SiO 2  taken at variable time intervals during 221 minutes of continuousexposure to X-ray irradiation: after a) 13; b) 96; c) 146; d) 196 min.  888  ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  CHEMPHYSCHEM  2003  , 4, 888 route to creating nanostructures, as the parallel nature lendsitself to being a scaleable methodology for the creation of nanostructures with lateral dimensions of 70 nm at the presenttime, [34] in contrast to e-beam lithography which at present is aserial process, but offers enhanced lateral resolution of sub-10 nm.Currently, we are investigating the reduction of SAMs and thinfilms of other oxygenated functional groups by X-ray irradiation. Experimental Section Chemicals: Commercially available chemicals and solvents werepurchased from Fluorochem, BDH and Aldrich, and used as suchwithout further purification unless otherwise stated. Anhydrousacetonitrile was dried over CaH 2  and distilled under a N 2  or Aratmosphere.Substrates: Silicon (Si/SiO 2 ) wafers were purchased from VirginiaSemiconductor Inc., and they were p-doped (dopant B), prime grade,single-side-polished, orientation   111  , with resistivity 11±21  cm and 100-nm thick thermally grown oxide on both sides.These silicon wafers were cut into approximately 1-cm 2 pieces, andcleaned according to the following method. [1, 35, 36] The silicon waferswere immersed in piranha solution (conc. H 2 SO 4 : 20% H 2 O 2  7:3) at90±100  C for 45±60 minutes  (caution: piranha solution reactsviolently with organic materials and should be handled carefully).  Oncecooled, the substrates were rinsed with Ultra High Pure (UHP) H 2 O(Resistivity  18 M  cm), and then sonicated in an RCA solution (UHPH 2 O: 30% H 2 O 2 : NH 4 OH  5:1:1) for 1 h at room temperature. Finally,the substrates were rinsed repeatedly with UHP H 2 O and stored inUHP H 2 O before use.NMR:  1 H NMR spectra were recorded on a Bruker AC300(300.13 MHz) spectrometer.  13 C NMR spectra were recorded on aBruker AV300 (100.61 MHz) spectrometer at room temperature andusing  1 H-decoupled mode.Mass Spectrometry (MS): Electrospray Mass Spectrometry (ESMS) wasperformed on a Micromass LCT Time of Flight (TOF) using methanolas the running solvent.Tetramethylammonium 4-nitrophenolate: 4-Nitrophenol (1.37 g,9.86 mmol) was added at room temperature to a solution of tetramethylammonium hydroxide hydrate (1.75 g, 9.67 mmol) inMeOH (40 mL), forming the salt immediately. The solvent wasremoved in vacuo. The crude residue was recrystallised from 10%acetone in ethyl acetate to afford the product as a yellow solid(2.01 g, 95%).  1 H NMR (300 MHz, CH 3 CN):    3.45 (12H, s, CH 3 ), 5.96(2H, d,  J  o  9.56, ArH), 7.79 ppm (2H, d,  J  o  9.56, ArH); ESMS  m /  z  :NO 2 C 6 H 4 O  138, 74 [(CH 3 ) 4 N  ].3-(4-Nitrophenoxy)-propyltrimethoxysilane: 3-Bromopropyl trime-thoxysilane (0.870 g, 3.6 mmol) was added to a solution of tetrame-thylammonium 4-nitrophenolate (0.848 g, 4 mmol) in anhydrousCH 3 CN (4 mL), and the mixture was stirred at 40  C for 3.5 h, duringwhich a white precipitate (Me 4 NBr) formed. The solvent wasremoved in vacuo. The residue was dissolved in CH 2 Cl 2  (50 mL) andwashed with aqueous K  2 CO 3  (2 M , 2  25 mL). The solution was dried(MgSO 4 ), filtered and a drop of acetic acid was added in order toprevent the base-catalysed polymerisation process. The solvent wasevaporated in vacuo to yield the compound (900 mg, 83% crude) asa pale yellow waxy material.  1 H NMR (300 MHz, CDCl 3 )    0.76±0.83(2H, m, CH 2 ), 1.88±1.99 (2H, m, CH 2 ), 3.64 (9H, s, OCH 3 ), 4.02 (2H, t,  J   6.45, OCH 2 ), 6.93 (2H, dd,  J  o  9.18,  J   p  2.58, ArH), 8.18 ppm (2H,d,  J  o  9.18, ArH);  13 C NMR (100 MHz, CDCl 3 )    5.21 (1 C, CH 2 ), 22.45(1 C, CH 2 ), 50.58 (3 C, OCH 3 ), 70.47 (1 C, CH 2 ), 114.40 (2 C, ArCH),125.89 (2 C, ArCH), 141.34 (2 C, ArC), 167.14 ppm (2 C, ArC); ESMS  m /  z  (%) 324 (M  Na  , 100), 301 (M  , 30).Film preparation: Multilayer films of NPPTMS were formed byimmersing cleaned silicon substrates into a 5 m M  solution of NPPTMS(15 mg, 0.01 mmol) in anhydrous CHCl 3  (10 mL) for 2 h. Uponremoval from the solution, the substrate were thoroughly rinsedwith CHCl 3 , EtOH and ultrahigh purity H 2 O, then sonicated in CHCl 3 for 5 min and dried with a stream of N 2 .Chemical reduction: Chemical reduction of the multilayer film of NPPTMS sample was performed by immersing the substrate in a0.1 m M  solution (10 mL) of SnCl 2  in anhydrous EtOH for 3 h at 50  C. [11] The sample was then sonicated successively for 5 min in 5 M  HCl,1 m M  EDTA (ethylenediaminetetraacetic acid), and anhydrous CHCl 3 ,and dried under a stream of N 2 .Ellipsometry: The thickness of the deposited multilayer film wasdetermined by multiwavelength spectroscopic ellipsometry. A Jobin-Yvon UVISEL ellipsometer with a He±Ne laser light source was usedfor the measurements. The angle of incidence was fixed at 70   andthe compensator was set at   45.0  . A wavelength range of 220±820 nm was used. The ellipsometric parameters,    and   , weredetermined for both the bare clean substrate and the self-assembledfilm. The DeltaPsi software was employed to determine the thicknessvalues using a cauchy oscillator model. The thickness reported is theaverage of six measurements taken on the same sample.Contact angle: Contact angles were determined by the sessile dropmethod using a home-built contact-angle apparatus, equipped witha charged-coupled-device (CCD) camera that is attached to apersonal computer for video capture. To measure advancing (  a ) orreceding (  r ) contact angles, the angle was measured as a micro-syringe was used to add liquid to or remove liquid quasistaticallyfrom the drop. The drop is shown as a live video image on the PCscreen and recorded in computer for future analysis of the images.The acquisition rate was 4 frames per second. Stored images of droplet were analyzed by using software from FTä. Contact angleswere determined from an average of five different measurements oneach sample.XPS measurements: XPS measurements were performed in a VGEscalab MkII Scanning Auger Microscope, equipped with a conven-tional hemisphericalsector analyser andcontrolledby aVGX900 datasystem. XPS experiments were carried out using a Mg K    source(1253.6 eV) operated at 15 kV and 20    A. O (1 s), C (1 s) and Si (2p)spectra were recorded using a pass energy of 20 eV, while N (1 s)regions were obtained using a pass energy of 50eV. The energy scaleof the spectrometer was calibrated to the Ag 3d 5/2  peak at 368.3 eV.The binding energy scale was calibrated to 284.6 eV for the main C(1 s) feature. High-resolution peak fitting was performed using themanufacturers' software. This work was supported by the European Community under Grant No. HPRN-CT-2000±00028. [1] A. Ulman,  An Introduction to Ultrathin Organic Films , Academic PressLimited, UK,  1991 .[2] A. Ulman,  Chem. Rev.  1996 ,  95 , 1533±1554.[3] L. Sun, L.J. Kepley, R. M. Crooks,  Langmuir   1992 ,  8 , 2101±2103.[4] H. C. Yang, D. L. Dermody, C. Xu, A. J. Ricco, R. M. Crooks,  Langmuir   1996 , 12 , 726±735.
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